Cell biology module 1

SARDARHUSSAIN,,AssistantProfessor,Biotechnology,[email protected]

LECTURE NOTES IN CELL BIOLOGY Module 1: cellular organization of organisms

Historical perspective of cell biology, cell as structure and functional unit of living organisms, cellular organization in prokaryotes and eukaryotes, compartmentalization of cell, cell

fractionation ,cell membrane and permeability

Cell Biology notes by, Sardar Hussain, Asst. Prof. GSC, CTA.

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The cell theory, or cell doctrine, states that all organisms are composed of similar units of organization,

called cells. The concept was formally articulated in 1839 by Schleiden & Schwann and has remained as the

foundation of modern biology. The idea predates other great paradigms of biology including Darwin’s theory of

evolution (1859), Mendel’s laws of inheritance (1865), and the establishment of comparative biochemistry

(1940).

First Cells Seen in Cork

While the invention of the telescope made the Cosmos accessible to human observation, the microsope

opened up smaller worlds, showing what living forms were composed of. The cell was first discovered and

named by Robert Hooke in 1665. He remarked that it looked strangely similar to cellula or small rooms which

monks inhabited, thus deriving the name. However what Hooke actually saw was the dead cell walls of plant

cells (cork) as it appeared under the microscope. Hooke’s description of these cells was published in

Micrographia . The cell walls observed by Hooke gave no indication of the nucleus and other organelles found

in most living cells. The first man to witness a live cell under a microscope was Antonvan Leeuwenhoek, who in

1674 described the algae Spirogyra. Van Leeuwenhoek probably also saw bacteria.

Formulation of the Cell Theory

In 1838, Theodor Schwann and Matthias Schleiden were enjoying after-dinner coffee and talking about

their studies on cells. It has been suggested that when Schwann heard Schleiden describe plant cells with

nuclei, he was struck by the similarity of these plant cells to cells he had observed in animal tissues. The two

scientists went immediately to Schwann’s lab to look at his slides. Schwann published his book on animal and

plant cells (Schwann 1839) the next year, a treatise devoid of acknowledgments of anyone else’s contribution,

including that of Schleiden (1838). He summarized his observations into three conclusions about cells:

The cell is the unit of structure, physiology, and organization in living things.

The cell retains a dual existence as a distinct entity and a building block in the construction of

organisms.

Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).

We know today that the first two tenets are correct, but the third is clearly wrong. The correct interpretation

of cell formation by division was finally promoted by others and formally enunciated in Rudolph Virchow’s

powerful dictum, Omnis cellula e cellula , “All cells only arise from pre-existing cells”.

Modern Cell Theory

All known living things are made up of cells.

The cell is structural & functional unit of all living things.

All cells come from pre-existing cells by division. (Spontaneous Generation does not occur).

Cells contains hereditary information which is passed from cell to cell during cell division.

All cells are basically the same in chemical composition.

All energy flow (metabolism & biochemistry) of life occurs within cells.

Historical perspectives of Cell Biology

http://inventors.about.com/library/inventors/blroberthooke.htm

http://www.ucmp.berkeley.edu/history/leeuwenhoek.html


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As with the rapid growth of molecular biology in the mid-20th century, cell biology research exploded in

the 1950’s. It became possible to maintain, grow, and manipulate cells outside of living organisms. The first

continuous cell line to be so cultured was in 1951 by George Otto Gey and coworkers, derived from cervical

cancer cells taken from Henrietta Lacks, who died from her cancer in 1951. The cell line, which was eventually

referred to as HeLa cells, have been the watershed in studying cell biology in the way that the structure of DNA

was the significant breakthrough of molecular biology.

In an avalanche of progress in the study of cells, the coming decade included the characterization of the

minimal media requirements for cells and development of sterile cell culture techniques. It was also aided by

the prior advances in electron microscopy, and later advances such as development of transfection methods,

discovery of green fluorescent protein in jellyfish, and discovery of small interfering RNA (siRNA), among

others.

A Timeline

1595 – Jansen credited with 1st compound microscope

1655 – Hooke described ‘cells’ in cork.

1674 – Leeuwenhoek discovered protozoa. He saw bacteria some 9 years later.

1833 – Brown descibed the cell nucleus in cells of the orchid.

1838 – Schleiden and Schwann proposed cell theory.

1840 – Albrecht von Roelliker realized that sperm cells and egg cells are also cells.

1856 – N. Pringsheim observed how a sperm cell penetrated an egg cell.

1858 – Rudolf Virchow (physician, pathologist and anthropologist) expounds his famous conclusion: omnis

cellula e cellula , that is cells develop only from existing cells [cells come from preexisting cells]

1857 – Kolliker described mitochondria.

1879 – Flemming described chromosome behavior during mitosis.

1883 – Germ cells are haploid, chromosome theory of heredity.

1898 – Golgi described the golgi apparatus.

1938 – Behrens used differential centrifugation to separate nuclei from cytoplasm.

1939 – Siemens produced the first commercial transmission electron microscope.

1952 – Gey and coworkers established a continuous human cell line.

1955 – Eagle systematically defined the nutritional needs of animal cells in culture.

1957 – Meselson, Stahl and Vinograd developed density gradient centrifugation in cesium chloride solutions for

separating nucleic acids.

1965 – Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial

scanning electron microscope.

1976 – Sato and colleagues publish papers showing that different cell lines require different mixtures of

hormones and growth factors in serum-free media.

1981 – Transgenic mice and fruit flies are produced. Mouse embryonic stem cell line established.

1995 – Tsien identifies mutant of GFP with enhanced spectral properties

1998 – Mice are cloned from somatic cells.

1999 – Hamilton and Baulcombe discover siRNA as part of post-transcriptional gene silencing (PTGS) in

plants

http://bitesizebio.com/2007/10/24/history-of-molecular-biology/

http://en.wikipedia.org/wiki/HeLa


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References:

Landmark Papers in Cell Biology: Selected Research Articles Celebrating Forty Years of The American

Society for Cell Biology. 2000. Cold Spring Harbor Laboratory Press.

Mazzarello P. A unifying concept: the history of cell theory. Nat Cell Biol. 1999. 1(1):E13-5.

Cell Size and Shape

The shapes of cells are quite varied with some, such as neurons, being longer than they are wide and

others, such as parenchyma (a common type of plant cell) and erythrocytes (red blood cells) being

equidimensional. Some cells are encased in a rigid wall, which constrains their shape, while others have a

flexible cell membrane (and no rigid cell wall).

The size of cells is also related to their functions. Eggs (or to use the latin word, ova) are very large,

often being the largest cells an organism produces. The large size of many eggs is related to the process of

development that occurs after the egg is fertilized, when the contents of the egg (now termed a zygote) are used

in a rapid series of cellular divisions, each requiring tremendous amounts of energy that is available in the

zygote cells. Later in life the energy must be acquired, but at first a sort of inheritance/trust fund of energy is

used.

Cell size is limited. As cell size increases, it takes longer for material to diffuse from the cell membrane to the

interior of the cell. Surface area-to-volume ratio: as a cell increases in size, the volume increases 10x faster

than the surface area

Cells range in size from small bacteria to large, unfertilized eggs laid by birds and dinosaurs. The

realtive size ranges of biological things is shown in Figure 1. In science we use the metric system for

measuring. Here are some measurements and convesrions that will aid your understanding of biology.

1 meter = 100 cm = 1,000 mm = 1,000,000 µm = 1,000,000,000 nm

1 centimenter (cm) = 1/100 meter = 10 mm

1 millimeter (mm) = 1/1000 meter = 1/10 cm

http://www.amazon.com/exec/obidos/ASIN/0879696028/aconcernedsci-20

http://www.nature.com/ncb/journal/v1/n1/abs/ncb0599_E13.html

https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossN.html#neurons

https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossPQ.html#parenchyma

https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossE.html#erythrocytes

https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossO.html#ovum

https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookglossWXYZ.html#zygote


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1 micrometer (µm) = 1/1,000,000 meter = 1/10,000 cm

1 nanometer (nm) = 1/1,000,000,000 meter = 1/10,000,000 cm

Figure 1. Sizes of viruses, cells, and organisms. Images from Purves et al., Life: The Science of Biology, 4th

Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with

permission.

All living organisms (bacteria, blue green algae, plants and animals) have cellular organization and

may contain one or many cells. The organisms with only one cell in their body are called unicellular organisms

(bacteria, blue green algae, some algae, Protozoa, etc.). The organisms having many cells in their body are

called multicellular organisms (fungi, most plants and animals). Any living organism may contain only one type

of cell either

A. Prokaryotic cells;

B. Eukaryotic cells.

Cellular organization in prokaryotes and eukaryotes

http://www.sinauer.com/

http://www.whfreeman.com/


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The terms prokaryotic and eukaryotic were suggested by Hans Ris in the 1960’s. This classification is

based on their complexcity. Further based on the kingdom into which they may fall i.e the plant or the animal

kingdom, plant and animal cells bear many differences. These will be studied in detail in the upcoming sections.

Prokaryotic cells

Prokaryote means before nucleus in Greek. They include all cells which lack nucleus and other

membrane bound organelles. Mycoplasma, virus, bacteria and cyanobacteria or blue-green algae are

prokaryotes.

Most prokaryotes range between 1 μm to 10 μm, but they can vary in size from 0.2 μm to 750 μm

(Thiomargarita namibiensis). They belong to two taxonomic domains which are the bacteria and the archaea.

Most prokaryotes are unicellular, exceptions being myxobacteria which have multicellular stages in their life

cycles. They are membrane bound mostly unicellular organisms lacking any internal membrane bound

organelles. A typical prokaryotic cell is schematically illustrated in Figure 1. Though prokaryotes lack cell

organelles they harbor few internal structures, such as the cytoskeletons, ribosomes, which translate mRNA to

proteins. Membranous organelles are known in some groups of prokaryotes, such as vacuoles or membrane

systems devoted to special metabolic properties, e.g., photosynthesis or chemolithotrophy. In addition, some

species also contain protein-enclosed microcompartments, which have distinct physiological roles

(carboxysomes or gas vacuoles).

Figure 1: Schematic diagram of a prokaryotic cell

The individual structures depicted in Figure 1 are as follows

Flagella: It is a long, whip-like protrusion found in most prokaryotes that aids in cellular locomotion. Besides

its main function of locomotion it also often functions as a sensory organelle, being sensitive to chemicals and

temperatures outside the cell.

Capsule: The capsule is found in some bacterial cells, this additional outer covering protects the cell when it is

engulfed by phagocytes and by viruses, assists in retaining moisture, and helps the cell adhere to surfaces and

nutrients. The capsule is found most commonly among Gram-negative bacteria. Escherichia coli, Klebsiella

pneumoniae Haemophilus influenzae, Pseudomonas aeruginosa and Salmonella are some examples Gram-


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negative bacteria possessing capsules. Whereas examples of Gram positive bacteria are Bacillus megaterium,

Streptococcus pneumoniae, Streptococcus pyogenes.

Cell wall: Cell wall is the outermost layer of most cells that protects the bacterial cell and gives it shape. One

exception is Mycoplasma which lacks cell wall. Bacterial cell walls are made of peptidoglycan which is made

from polysaccharide chains cross-linked by unusual peptides containing D-amino acids. Bacterial cell walls are

different from the cell walls of plants and fungi which are made of cellulose and chitin, respectively. The cell

wall of bacteria is also distinct from that of Archaea, which do not contain peptidoglycan.

The cell wall is essential to the survival of many bacteria. The antibiotic penicillin is able to kill bacteria

by preventing the cross-linking of peptidoglycan and this causes the cell wall to weaken and lyse. Lysozyme

enzyme can also damage bacterial cell walls.

There are broadly speaking two different types of cell wall in bacteria, called Gram-positive and Gram-

negative. The names originate from the reaction of cells to the Gram stain, a test long-employed for the

classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of

peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting

of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and

lipoproteins. These differences in structure can produce differences in property as antibiotic susceptibility. For

example vancomycin can kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens,

such as Pseudomonas aeruginosa or Haemophilus influenzae

Cell membrane: Cell membrane surrounds the cell's cytoplasm and regulates the flow of substances in and out

of the cell. It will be discussed in detail in one of the coming chapters.

Cytoplasm: The cytoplasm of a cell is a fluid in nature that fills the cell and is composed mainly of 80% water

that also contains enzymes, salts, cell organelles, and various organic molecules. The details will be discussed

in forthcoming chapter.

Ribosomes: Ribosomes are the organelles of the cell responsible for protein synthesis. Details of ribosomes will

be explained in coming chapter.

Nucleiod Region: The nucleoid region is possessed by a prokaryotic bacterial cell. It is the area of the

cytoplasm that contains the bacterial DNA molecule.

Plasmids: The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in

1952. A plasmid is a DNA molecule (mostly in bacteria) that is separate from, and can replicate independently

of, the chromosomal DNA. They are double-stranded and circular. Plasmids usually occur naturally in

bacteria, but are sometimes found in eukaryotic organisms. Their sizes vary from 1 to over 1,000 kbp. The

number of identical plasmids in a single cell can range anywhere from one to thousands under some

circumstances and it is represented by the copy number. Plasmids can be considered mobile because they are

often associated with conjugation, a mechanism of horizontal gene transfer. Plasmids that can coexist within a

bacterium are said to be compatible. Plasmids which cannot coexist are said to be incompatible and after a few

generations are lost from the cell. Plasmids that encode their own transfer between bacteria are termed

conjugative. Non-conjugative plasmids do not have these transfer genes but can be carried along by

conjugative plasmids via a mobilisation site. Functionally they carry genes that code for a wide range of

metabolic activities, enabling their host bacteria to degrade pollutant compounds, and produce antibacterial

proteins. They can also harbour genes for virulence that help to increase pathogenicity of bacteria causing


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diseases such as plague, dysentery, anthrax and tetanus. They are also responsible for the spread of antibiotic

resistance genes that ultimately have an impact on the treatment of Pili.

Pili: Pili are hair-like structures on the surface of the cell that help attach to other bacterial cells. Shorter pili

called fimbriae help bacteria attach to various surfaces. A pilus is typically 6 to 7 nm in diameter. The types of

pili are Conjugative pili and Type IV pili. Conjugative pili allow the transfer of DNA between bacteria, in the

process of bacterial conjugation. Some pili, called type IV pili, generate motile forces.

Morphology of prokaryotic cells

Prokaryotic cells have various shapes; the four basic shapes are (Figure 3):

• Cocci - spherical

• Bacilli - rod-shaped

• Spirochaete - spiral-shaped

• Vibrio - comma-shape

Eukaryotic cells

Plant cells are eukaryotic cells that differ in several key aspects from the cells of other eukaryotic organisms.

Their distinctive features include the following organelles:

1. Vacuole: It is present at the centre and is water-filled volume enclosed by a membrane known as the

tonoplast. The function is to maintain the cell's turgor, pressure by controlling movement of molecules between

the cytosol and sap, stores useful material and digests waste proteins and organelles.

2. Cell Wall: It is the extracellular structure surrounding plasma membrane. The cell wall is composed of

cellulose, hemicellulose, pectin and in many cases lignin, is secreted by the protoplast on the outside of the cell

membrane. This contrasts with the cell walls of fungi (which are made of chitin), and of bacteria, which are

made of peptidoglycan. An important function of the cell wall is that it controls turgity. The cell wall is divided

into the primary cell wall and the secondary cell wall. The Primary cell wall: extremely elastic and the

secondary cell wall forms around primary cell wall after growth are complete.

3. Plasmodesmata: Pores in the primary cell wall through which the plasmalemma and endoplasmic reticulum

of adjacent cells are continuous.

4. Plastids: The plastids are chloroplasts, which contain chlorophyll and the biochemical systems for light

harvesting and photosynthesis. A typical plant cell (e.g., in the palisade layer of a leaf) might contain as many

as 50 chloroplasts. The other plastids are amyloplasts specialized for starch storage, elaioplasts specialized for

fat storage, and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have

a genome encoding 37 genes, plastids have their own genomes of about 100–120 unique genes and, it is

presumed, arose as prokaryotic endosymbionts living in the cells of an early eukaryotic ancestor of the land

plants and algae


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A typical plant cell

Plant cell types

Parenchyma cells: These are living cells that have diverse functions ranging from storage and support to

photosynthesis and phloem loading (transfer cells). Apart from the xylem and phloem in its vascular bundles,

leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized

for light penetration and focusing or regulation of gas exchange, but others are among the least specialized

cells in plant tissue, and may remain totipotent, capable of dividing to produce new populations of

undifferentiated cells, throughout their lives. Parenchyma cells have thin, permeable primary walls enabling the

transport of small molecules between them, and their cytoplasm is responsible for a wide range of biochemical

functions such as nectar secretion, or the manufacture of secondary products that discourage herbivory.

Parenchyma cells that contain many chloroplasts and are concerned primarily with photosynthesis are called

chlorenchyma cells. Others, such as the majority of the parenchyma cells in potato tubers and the seed

cotyledons of legumes, have a storage function

Collenchyma cells: Collenchyma cells (Figure 2b) are alive at maturity and have only a primary wall. These

cells mature from meristem derivatives that initially resemble parenchyma, but differences quickly become

apparent. Plastids do not develop, and the secretory apparatus (ER and Golgi) proliferates to secrete

additional primary wall. The wall is most commonly thickest at the corners, where three or more cells come in

contact, and thinnest where only two cells come in contact, though other arrangements of the wall thickening

are possible. Pectin and hemicellulose are the dominant constituents of collenchyma cell walls of dicotyledon

angiosperms, which may contain as little as 20% of cellulose in Petasites. Collenchyma cells are typically quite

elongated, and may divide transversely to give a septate appearance. The role of this cell type is to support the

plant in axes still growing in length, and to confer flexibility and tensile strength on tissues. The primary wall

lacks lignin that would make it tough and rigid, so this cell type provides what could be called plastic support –

support that can hold a young stem or petiole into the air, but in cells that can be stretched as the cells around


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them elongate. Stretchable support (without elastic snap-back) is a good way to describe what collenchyma

does. Parts of the strings in celery are collenchymas

Sclerenchyma cells: Sclerenchyma cells (from the Greek skleros, hard) are hard and tough cells with a

function in mechanical support. They are of two broad types – sclereids or stone cells and fibres. The cells

develop an extensive secondary cell wall that is laid down on the inside of the primary cell wall. The secondary

wall is impregnated with lignin, making it hard and impermeable to water. Thus, these cells cannot survive for

long' as they cannot exchange sufficient material to maintain active metabolism. Sclerenchyma cells are

typically dead at functional maturity, and the cytoplasm is missing, leaving an empty central cavity.

parenchyma collenchyma sclerenchyma

Animal cells: An animal cell is a form of eukaryotic cell that makes up many tissues in animals. Figure 7

depicts a typical animal cell. The animal cell is distinct from other eukaryotes, most notably plant cells, as they

lack cell walls and chloroplasts, and they have smaller vacuoles. Due to the lack of a rigid cell wall, animal

cells can adopt a variety of shapes, and a phagocytic cell can even engulf other structures. There are many

different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.


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Cell organelles in animal cell

Cell membrane: Plasma membrane is the thin layer of protein and fat that surrounds the cell, but is inside the

cell wall. The cell membrane is semipermeable, allowing selective substances to pass into the cell and blocking

others.

Nucleus: They are spherical body containing many organelles, including the nucleolus. The nucleus controls

many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The

nucleus is surrounded by the nuclear membrane and possesses the nucleolus which is an organelle within the

nucleus - it is where ribosomal RNA is produced.

Golgi apparatus: It is a flattened, layered, sac-like organelle involved in packaging proteins and carbohydrates

into membrane-bound vesicles for export from the cell.

Ribosome and Endoplasmic reticulum: Ribosomes are small organelles composed of RNA-rich cytoplasmic

granules that are sites of protein synthesis and Endoplasmic reticulum are the sites of protein maturation and

they can be divided into the following types:

a. Rough endoplasmic reticulum: These are a vast system of interconnected, membranous, infolded and

convoluted sacks that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear

membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transport

materials through the cell and produces proteins in sacks called cisternae (which are sent to the Golgi body, or

inserted into the cell membrane).

b. Smooth endoplasmic reticulum: These are a vast system of interconnected, membranous, infolded and

convoluted tubes that are located in the cell's cytoplasm (the ER is continuous with the outer nuclear

membrane). The space within the ER is called the ER lumen. Smooth ER transport materials through the cell. It

contains enzymes and produces and digests lipids (fats) and membrane proteins; smooth ER buds off from

rough ER, moving the newly-made proteins and lipids to the Golgi body and membranes.

Mitochondria: These are spherical to rod-shaped organelles with a double membrane. The inner membrane is

infolded many times, forming a series of projections (called cristae). The mitochondrion converts the energy

stored in glucose into ATP (adenosine triphosphate) for the cell.

Lysosome: Lysosomes are cellular organelles that contain the hydrolase enzymes which breaks down waste

materials and cellular debris. They can be described as the stomach of the cell. They are found in animal cells,

while in yeast and plants the same roles are performed by lytic vacuoles.Lysosomes digest excess or worn-out

organelles, food particles, and engulf viruses or bacteria. The membrane around a lysosome allows the

digestive enzymes to work at the 4.5 pH they require. Lysosomes fuse with vacuoles and dispense their enzymes

into the vacuoles, digesting their contents. They are created by the addition of hydrolytic enzymes to early

endosomes from the Golgi apparatus.

Centrosome: They are small body located near the nucleus and has a dense center and radiating tubules. The

centrosomes are the destination where microtubules are made. During mitosis, the centrosome divides and the

two parts move to opposite sides of the dividing cell. Unlike the centrosomes in animal cells, plant cell

centrosomes do not have centrioles.

Peroxisome

Peroxisomes are organelles that contain oxidative enzymes, such as D-amino acid oxidase, ureate oxidase, and

catalase. They may resemble a lysosome, however, they are not formed in the Golgi complex. Peroxisomes are

distinguished by a crystalline structure inside a sac which also contains amorphous gray material. They are


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self-replicating, like the mitochondria. Components accumulate at a given site and they can be assembled into a

peroxisome. Peroxisomes function to rid the body of toxic substances like hydrogen peroxide, or other

metabolites. They are a major site of oxygen utilization and are numerous in the liver where toxic byproducts

accumulate.

Vacuoles and vesicles

Vacuoles are single-membrane organelles that are essentially part of the outside that is located within the cell.

The single membrane is known in plant cells as a tonoplast. Many organisms will use vacuoles as storage

areas. Vesicles are much smaller than vacuoles and function in transporting materials both within and to the

outside of the cell.

Differences between plant and animal cells

1. Animal cells are generally small in size. Plant cells are larger than animal cells.

2. Cell wall is absent. The plasma membrane of plant cells is

Surrounded by a rigid cell wall of cellulose.

3. Except the protozoan Euglena no animal cell

possesses plastids. Plastids are present.

4. Vacuoles in animal cells are many and small. Most mature plant cells have a large central sap

vacuole.

5. Animal cells have a single highly complex Golgi

Plant cells have many simpler units of and prominent

Golgi apparatus. Apparatus, called dictyosomes.

6. Animal cells have centrosome and centrioles. Plant cells lack centrosome and centrioles

Interesting Facts:

1. There are anywhere from 75 to 100 trillion cells in the human body.

2. There are more bacterial cells in the body than human cells.

3. Thiomargarita namibiensis is the largest bacterium ever discovered, found in the ocean

Sediments of the continental shelf of Namibia and can be seen through the naked eye.

4. An unfertilized Ostrich egg is the largest single cell.

5. The smallest cell is a type of bacteria known as mycoplasma. Its diameter is 0.001 mm. 6. The Longest Cell in

your body is the motor neuron cell, which is located in the spinal cord, near the central nervous system.

Cellular compartments in cell biology comprise all of the closed parts within the cytosol of a eukaryotic

cell, usually surrounded by a single or double lipid layer membrane. These compartments are often, but not

always, defined as membrane enclosed regions. The formation of cellular compartments is called

compartmentalization.

Both organelles, the mitochondria and chloroplasts (in photosynthetic organisms), are compartments that are

believed to be of endosymbiotic origin. Other compartments such as peroxisomes, lysosomes, the endoplasmic

Compartmentalization of eukaryotic cells


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reticulum, the cell nucleus or the Golgi apparatus are not of endosymbiotic origin. Smaller elements like

vesicles, and sometimes even microtubules can also be counted as compartments.

It was thought that compartmentalization is not found in prokaryotic cells. But the discovery of carboxysomes

and many other metabolosomes revealed that prokaryotic cells are capable of making compartmentalized

structures, though these are in most cases not surrounded by a lipid bilayer, but of pure proteinaceous built.

Types:

In general there are 4 main cellular compartments, they are:

1.The nuclear compartment comprising the nucleus

2.The intercisternal space which comprises the space between the membranes of the endoplasmic

reticulum (which is continuous with the nuclear envelope)

3.Organelles (the mitochondrion in all eukaryotes and the plastid in phototrophic eukaryotes)

4.The cytosol

Functions - Compartments have three main roles.

One is to establish physical boundaries for biological processes that enables the cell to carry out

different metabolic activities at the same time. This may include keeping certain biomolecules within a region,

or keeping other molecules outside. Within the membrane-bound compartments, different intracellular pH,

different enzyme systems, and other differences are isolated from other organelles and cytosol. With

mitochondria, the cytosol has an oxidizing environment which converts NADHto NAD+. With these cases, the

compartmentalization is physical.

Another is to generate a specific micro-environment to spatially or temporally regulate a biological

process. As an example, a yeast vacuole is normally acidified by proton transporters on the membrane.

A third role is to establish specific locations or cellular addresses for which processes should occur. For

example, a transcription factor may be directed to a nucleus, where it can promote transcription of certain

genes. In terms of protein synthesis, the necessary organelles are relatively near one another. The nucleolus

within the nuclear envelope is the location of ribosome synthesis. The destination of synthesized ribosomes for

protein translation is rough endoplasmic reticulum (rough ER), which is connected to and shares the same

membrane with the nucleus. The Golgi body is also near the rough ER for packaging and redistributing.

Likewise, intracellular compartmentalization allows specific sites of related eukaryotic cell functions isolated

from other processes and therefore efficient.

Establishment Often, cellular compartments are defined by membrane enclosure. These membranes

provide physical barriers to biomolecules. Transport across these barriers is often controlled in order to

maintain the optimal concentration of biomolecules within and outside of the compartment

Eukaryotic cells are complex and contain many kinds of membrane organelles. For instance, they

contain nuclei, mitochondria, vacuoles etc. Two methods exist to study the organelles in more detail. The first

is by using a variety of techniques to visualize the nuclei while still inside the cells by microscopy (i.e. cell

staining and immunofluorescence). The second method involves suspending the cells in solution, and breaking

them open (lysing the cells). Then the various organelles are then separated from each other by centrifugation,

which then allows them to be used for further study. It is this second method that we will use in this laboratory.

Cell fractionation


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A procedure called cell fractionation is used to break open the cells and separate the various

organelles. To perform cell fractionation, we first will suspend our cells in solution, and then we break open

the cells, or lyse them. This will release the organelles inside into solution. Next, we can separate the

organelles by centrifuging our solution. By using centrifugation, we can easily separate the various organelles,

since the various organelles are of different mass, and density (for instance, nuclei are significantly heavier

than mitochondria etc.). During centrifugation, different organelles will pellet at the bottom at specific speeds

based on the mass and densities of the organelles. For instance, the heavier (larger) the organelles, the less

velocity is needed to pellet the organelle. The lighter (smaller) the organelle, centrifugation must occur at a

greater velocity to pellet the organelle.

Therefore, if we want to separate nuclei from mitochondria, we will centrifuge at low speed. At low

speed, the nuclei will pellet at the bottom, while the mitochondria will stay suspended in the solution. The left

over solution after centrifugation is called the supernatant, and will contain lighter organelles (i.e.

mitochondria and dissolved proteins). If we then want to separate the mitochondria from the rest of the

supernatant, we can centrifuge at the appropriate speed that would pellet the mitochondria, and then remove

the resulting supernatant.

If we first centrifuged or cell suspension at the speed appropriate to pellet mitochondria, we would

bring the mitochondria to the bottom of the tube. However, we would pellet everything that is heavier than the

mitochondria (i.e. nuclei etc.). Therefore, in order to get nuclei separated from mitochondria, we must

centrifuge at the lower speed first to obtain the nuclei, and centrifuge the supernatant at the higher speed to

collect the mitochondria. This type of separation protocol is called differential centrifugation. In this

procedure, it is possible to separate the organelles to purification because objects a similar size to our desired

organelles will also pellet at the same speeds. However, these objects are significantly less concentrated in our

suspension. Therefore, our pellets will are enriched for the organelle we are attempting to isolate.

Each pellet that is isolated can then be resuspended by adding solution, and can be called a fraction (for

instance, the resuspended nuclear pellet is considered the nuclear fraction). Additionally, a sample of the

original suspension is considered a fraction, and is called the crude fraction, as it contains all the organelles

and soluble proteins. Lastly, a sample of the final supernatant is also considered a fraction and is called the

soluble fraction, and contains everything that was not pelleted by centrifugation.

Cell Fractionation means separating different parts and organelles of a cell, so that they can be studied

in detail. All the processes of cell metabolism (such as respiration or photosynthesis) have been studied in this

way. The most common method of fractionating cells is to use differential centrifugation.A more sophisticated

separation can be performed by density gradient centrifugation. In this, the cell-free extract is centrifuged in a

dense solution (such as sucrose or caesium chloride). The fractions don't pellet, but instead separate out into

layers with the densest fractions near the bottom of the tube. The desired layer can then be pipetted off. This is

the technique used in the Meselson-Stahl experiment, and it is also used to separate the two types of ribosomes.

The terms 70S and 80S refer to their positions in a density gradient

How is subcellular farctionation done?


14

1. Cut tissue (e.g. liver, heart, leaf, etc) in ice-cold

isotonic buffer. Cold to stop enzyme reactions, isotonic to

stop osmosis, and buffer to stop pH changes.

2. Grind tissue in a blender to break open cells.

3. Filter. This removes insoluble tissue (e.g. fat,

connective tissue, plant cell walls, etc). This filtrate is not

called a cell-free extract, and is capable of carrying out

most of the normal cell reactions.

4. Centrifuge filtrate at low speed

(1 000 x g for 10 min)

5. Centrifuge supernatant at medium speed

(10 000 x g for 30 min)

6. Centrifuge supernatant at high speed

(100 000 x g for 1 hour)

7. Centrifuge supernatant at very high speed

(300 000 x g for 3 hours)

8. Supernatant is now organelle-free cytoplasm


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2. Cell membrane and permeability

Overview: Life at the Edge

The plasma membrane separates the living cell from its nonliving surroundings.

This thin barrier, 8 nm thick, controls traffic into and out of the cell.

Like all biological membranes, the plasma membrane is selectively permeable, allowing some substances to

cross more easily than others.

Concept 2.1 Cellular membranes are fluid mosaics of lipids and proteins

The main macromolecules in membranes are lipids and proteins, but carbohydrates are also important.

The most abundant lipids are phospholipids.

Phospholipids and most other membrane constituents are amphipathic molecules.

Amphipathic molecules have both hydrophobic regions and hydrophilic regions.

The arrangement of phospholipids and proteins in biological membranes is described by the fluid mosaic

model.

Membrane models have evolved to fit new data.

Models of membranes were developed long before membranes were first seen with electron microscopes in the

1950s.

In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids

and proteins.

In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a phospholipid bilayer two molecules

thick.

The molecules in the bilayer are arranged such that the hydrophobic fatty acid tails are sheltered from water

while the hydrophilic phosphate groups interact with water.

Actual membranes adhere more strongly to water than do artificial membranes composed only of

phospholipids.

One suggestion was that proteins on the surface of the membrane increased adhesion.

In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between

two layers of globular proteins.

Early images from electron microscopes seemed to support the Davson-Danielli model, and until the 1960s, it

was widely accepted as the structure of the plasma membrane and internal membranes.

Further investigation revealed two problems.

First, not all membranes were alike. Membranes differ in thickness, appearance when stained, and percentage

of proteins.

Membranes with different functions differ in chemical composition and structure.

Second, measurements showed that membrane proteins are not very soluble in water.

Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions.

If membrane proteins were at the membrane surface, their hydrophobic regions would be in contact with water.

In 1972, S. J. Singer and G. Nicolson presented a revised model that proposed that the membrane proteins are

dispersed and individually inserted into the phospholipid bilayer.


16

AP Biology

Membrane is a collage of proteins & other molecules

embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with

water, and the hydrophobic regions are in a no aqueous environment within the membrane.

A specialized preparation technique, freeze-fracture, splits a membrane along the middle of the phospholipid

bilayer.

When a freeze-fracture preparation is viewed with an electron microscope, protein particles are interspersed in

a smooth matrix, supporting the fluid mosaic model.

Membranes are fluid.

Membrane molecules are held in place by relatively weak hydrophobic interactions.

Most of the lipids and some proteins drift laterally in the plane of the membrane, but rarely flip-flop from one

phospholipid layer to the other.

The lateral movements of phospholipids are rapid, about 2 microns per second. A phospholipid can travel the

length of a typical bacterial cell in 1 second.

Many larger membrane proteins drift within the phospholipid bilayer, although they move more slowly than the

phospholipids.

Some proteins move in a very directed manner, perhaps guided or driven by motor proteins attached to the

cytoskeleton.

Other proteins never move and are anchored to the cytoskeleton.

Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a

solid state as the phospholipids pack more closely.

Membrane fluidity is also influenced by its components. Membranes rich in unsaturated fatty acids are more

fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails at the

locations of the double bonds prevent tight packing.

The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells.


17

At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids and reduces

fluidity.

At cool temperatures, it maintains fluidity by preventing tight packing.

Thus, cholesterol acts as a “temperature buffer” for the membrane, resisting changes in membrane fluidity as

temperature changes.

To work properly with active enzymes and appropriate permeability, membranes must be about as fluid as

salad oil.

Cells can alter the lipid composition of membranes to compensate for changes in fluidity caused by changing

temperatures.

For example, cold-adapted organisms such as winter wheat increase the percentage of unsaturated

phospholipids in their membranes in the autumn.

This prevents membranes from solidifying during winter.

Membranes are mosaics of structure and function.

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

Proteins determine most of the membrane’s specific functions.

The plasma membrane and the membranes of the various organelles each have unique collections of proteins.

There are two major populations of membrane proteins.

Peripheral proteins are not embedded in the lipid bilayer at all.

Instead, they are loosely bound to the surface of the protein, often connected to integral proteins.

Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane

(as transmembrane proteins).

The hydrophobic regions embedded in the membrane’s core consist of stretches of nonpolar amino acids, often

coiled into alpha helices.

Where integral proteins are in contact with the aqueous environment, they have hydrophilic regions of amino

acids.

On the cytoplasmic side of the membrane, some membrane proteins connect to the cytoskeleton.

On the exterior side of the membrane, some membrane proteins attach to the fibers of the extracellular matrix.

The proteins of the plasma membrane have six major functions:

Transport of specific solutes into or out of cells.

Enzymatic activity, sometimes catalyzing one of a number of steps of a metabolic pathway.

Signal transduction, relaying hormonal messages to the cell.

Cell-cell recognition, allowing other proteins to attach two adjacent cells together.

Intercellular joining of adjacent cells with gap or tight junctions.

Attachment to the cytoskeleton and extracellular matrix, maintaining cell shape and stabilizing the location of

certain membrane proteins.

Membrane carbohydrates are important for cell-cell recognition.

The plasma membrane plays the key role in cell-cell recognition.

Cell-cell recognition, the ability of a cell to distinguish one type of neighboring cell from another, is crucial to

the functioning of an organism.

This attribute is important in the sorting and organization of cells into tissues and organs during development.

It is also the basis for rejection of foreign cells by the immune system.


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Cells recognize other cells by binding to surface molecules, often carbohydrates, on the plasma membrane.

Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units.

They may be covalently bonded to lipids, forming glycolipids, or more commonly to proteins, forming

glycoproteins.

The oligosaccharides on the external side of the plasma membrane vary from species to species, from individual

to individual, and even from cell type to cell type within the same individual.

This variation distinguishes each cell type.

The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

Membranes have distinctive inside and outside faces.

Membranes have distinct inside and outside faces. The two layers may differ in lipid composition. Each protein

in the membrane has a directional orientation in the membrane.

The asymmetrical orientation of proteins, lipids and associated carbohydrates begins during the synthesis of

membrane in the ER and Golgi apparatus.

Membrane lipids and proteins are synthesized in the endoplasmic reticulum. Carbohydrates are added to

proteins in the ER, and the resulting glycoproteins are further modified in the Golgi apparatus. Glycolipids are

also produced in the Golgi apparatus.

When a vesicle fuses with the plasma membrane, the outside layer of the vesicle becomes continuous with the

inside layer of the plasma membrane. In that way, molecules that originate on the inside face of the ER end up

on the outside face of the plasma membrane.

Concept 2.2 Membrane structure results in selective permeability

A steady traffic of small molecules and ions moves across the plasma membrane in both directions.

For example, sugars, amino acids, and other nutrients enter a muscle cell, and metabolic waste products leave.

The cell absorbs oxygen and expels carbon dioxide.

It also regulates concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl?, by shuttling them across

the membrane.

However, substances do not move across the barrier indiscriminately; membranes are selectively permeable.

The plasma membrane allows the cell to take up many varieties of small molecules and ions and exclude others.

Substances that move through the membrane do so at different rates.

Movement of a molecule through a membrane depends on the interaction of the molecule with the hydrophobic

core of the membrane.

Hydrophobic molecules, such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.

The hydrophobic core of the membrane impedes the direct passage of ions and polar molecules, which cross the

membrane with difficulty.

This includes small molecules, such as water, and larger molecules, such as glucose and other sugars.

An ion, whether a charged atom or molecule, and its surrounding shell of water also has difficulty penetrating

the hydrophobic core.

Proteins assist and regulate the transport of ions and polar molecules.

Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span

the membrane.

Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can

use as a tunnel through the membrane.


19

For example, the passage of water through the membrane can be greatly facilitated by channel proteins known

as aquaporins.

Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the

membrane.

Each transport protein is specific as to the substances that it will translocate.

For example, the glucose transport protein in the liver will carry glucose into the cell but will not transport

fructose, its structural isomer.

Concept 2.3 Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion is the tendency of molecules of any substance to spread out in the available space.

Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules.

Movements of individual molecules are random.

However, movement of a population of molecules may be directional.

Imagine a permeable membrane separating a solution with dye molecules from pure water. If the membrane

has microscopic pores that are large enough, dye molecules will cross the barrier randomly.

The net movement of dye molecules across the membrane will continue until both sides have equal

concentrations of the dye.

At this dynamic equilibrium, as many molecules cross one way as cross in the other direction.

In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less

concentrated, down its concentration gradient.

No work must be done to move substances down the concentration gradient.

Diffusion is a spontaneous process that decreases free energy and increases entropy by creating a randomized

mixture.

Each substance diffuses down its own concentration gradient, independent of the concentration gradients of

other substances.

The diffusion of a substance across a biological membrane is passive transport because it requires no energy

from the cell to make it happen.

The concentration gradient itself represents potential energy and drives diffusion.

Because membranes are selectively permeable, the interactions of the molecules with the membrane play a role

in the diffusion rate.

Diffusion of molecules of limited permeability through the lipid bilayer may be assisted by transport proteins.

Osmosis is the passive transport of water.

Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of

ions from one to the other.

The solution with the higher concentration of solutes is hypertonic relative to the other solution.

The solution with the lower concentration of solutes is hypotonic relative to the other solution.

These are comparative terms.

Tap water is hypertonic compared to distilled water but hypotonic compared to seawater.

Solutions with equal solute concentrations are isotonic.

Imagine that two sugar solutions differing in concentration are separated by a membrane that will allow water

through, but not sugar.

The hypertonic solution has a lower water concentration than the hypotonic solution.


20

More of the water molecules in the hypertonic solution are bound up in hydration shells around the sugar

molecules, leaving fewer unbound water molecules.

Unbound water molecules will move from the hypotonic solution, where they are abundant, to the hypertonic

solution, where they are rarer. Net movement of water continues until the solutions are isotonic.

The diffusion of water across a selectively permeable membrane is called osmosis.

The direction of osmosis is determined only by a difference in total solute concentration.

The kinds of solutes in the solutions do not matter.

This makes sense because the total solute concentration is an indicator of the abundance of bound water

molecules (and, therefore, of free water molecules).

When two solutions are isotonic, water molecules move at equal rates from one to the other, with no net

osmosis.

The movement of water by osmosis is crucial to living organisms.

Cell survival depends on balancing water uptake and loss.

An animal cell (or other cell without a cell wall) immersed in an isotonic environment experiences no net

movement of water across its plasma membrane.

Water molecules move across the membrane but at the same rate in both directions.

The volume of the cell is stable.

The same cell in a hypertonic environment will lose water, shrivel, and probably die.

A cell in a hypotonic solution will gain water, swell, and burst.

For organisms living in an isotonic environment (for example, many marine invertebrates), osmosis is not a

problem.

The cells of most land animals are bathed in extracellular fluid that is isotonic to the cells.

Organisms without rigid walls have osmotic problems in either a hypertonic or hypotonic environment and

must have adaptations for osmoregulation, the control of water balance, to maintain their internal environment.

For example, Paramecium, a protist, is hypertonic to the pond water in which it lives.

In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the

Paramecium cell.

To solve this problem, Paramecium cells have a specialized organelle, the contractile vacuole, which functions

as a bilge pump to force water out of the cell.

The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell’s water balance.

A plant cell in a hypotonic solution will swell until the elastic cell wall opposes further uptake.

At this point the cell is turgid (very firm), a healthy state for most plant cells.

Turgid cells contribute to the mechanical support of the plant.

If a plant cell and its surroundings are isotonic, there is no movement of water into the cell. The cell becomes

flaccid (limp), and the plant may wilt.

The cell wall provides no advantages when a plant cell is immersed in a hypertonic solution. As the plant cell

loses water, its volume shrinks. Eventually, the plasma membrane pulls away from the wall. This plasmolysis is

usually lethal.

Specific proteins facilitate passive transport of water and selected solutes.

Many polar molecules and ions that are normally impeded by the lipid bilayer of the membrane diffuse

passively with the help of transport proteins that span the membrane.


21

The passive movement of molecules down their concentration gradient via transport proteins is called

facilitated diffusion.

Two types of transport proteins facilitate the movement of molecules or ions across membranes: channel

proteins and carrier proteins.

Some channel proteins simply provide hydrophilic corridors for the passage of specific molecules or ions.

For example, water channel proteins, aquaporins, greatly facilitate the diffusion of water.

Many ion channels function as gated channels. These channels open or close depending on the presence or

absence of a chemical or physical stimulus.

If chemical, the stimulus is a substance other than the one to be transported.

For example, stimulation of a receiving neuron by specific neurotransmitters opens gated channels to allow

sodium ions into the cell.

When the neurotransmitters are not present, the channels are closed.

Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and

solute across the membrane as the transport protein changes shape.

These shape changes may be triggered by the binding and release of the transported molecule.

In certain inherited diseases, specific transport systems may be defective or absent.

Cystinuria is a human disease characterized by the absence of a protein that transports cysteine and other

amino acids across the membranes of kidney cells.

An individual with cystinuria develops painful kidney stones as amino acids accumulate and crystallize in the

kidneys.

Concept 2.4 Active transport uses energy to move solutes against their gradients

Some transport proteins can move solutes across membranes against their concentration gradient, from the side

where they are less concentrated to the side where they are more concentrated.

This active transport requires the cell to expend metabolic energy.


22

Active transport enables a cell to maintain its internal concentrations of small molecules that would otherwise

diffuse across the membrane.

Active transport is performed by specific proteins embedded in the membranes.

ATP supplies the energy for most active transport.

ATP can power active transport by transferring a phosphate group from ATP (forming ADP) to the transport

protein.

This may induce a conformational change in the transport protein, translocating the solute across the

membrane.

The sodium-potassium pump actively maintains the gradient of sodium ions (Na+) and potassium ions (K+)

across the plasma membrane of animal cells.

Typically, K+ concentration is low outside an animal cell and high inside the cell, while Na+ concentration is

high outside an animal cell and low inside the cell.

The sodium-potassium pump maintains these concentration gradients, using the energy of one ATP to pump

three Na+ out and two K+ in.

Some ion pumps generate voltage across membranes.

All cells maintain a voltage across their plasma membranes.

Voltage is electrical potential energy due to the separation of opposite charges.

The cytoplasm of a cell is negative in charge compared to the extracellular fluid because of an unequal

distribution of cations and anions on opposite sides of the membrane.

The voltage across a membrane is called a membrane potential, and ranges from? 50 to? 200 millivolts (mV).

The inside of the cell is negative compared to the outside.

The membrane potential acts like a battery.

The membrane potential favors the passive transport of cations into the cell and anions out of the cell.

Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a

membrane.

One is a chemical force based on an ion’s concentration gradient.

The other is an electrical force based on the effect of the membrane potential on the ion’s movement.

An ion does not simply diffuse down its concentration gradient but diffuses down its electrochemical gradient.


23

For example, there is a higher concentration of Na+ outside a resting nerve cell than inside.

When the neuron is stimulated, a gated channel opens and Na+ diffuse into the cell down their electrochemical

gradient. The diffusion of Na+ is driven by their concentration gradient and by the attraction of cations to the

negative side of the membrane.

Special transport proteins, electrogenic pumps, generate the voltage gradient across a membrane.

The sodium-potassium pump in animals restores the electrochemical gradient not only by the active transport of

Na+ and K+, setting up a concentration gradient, but because it pumps two K+ inside for every three Na+ that

it moves out, setting up a voltage across the membrane.

The sodium-potassium pump is the major electrogenic pump of animal cells.

In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of

the cell.

Proton pumps in the cristae of mitochondria and the thylakoids of chloroplasts concentrate H+ behind

membranes.

These electrogenic pumps store energy that can be accessed for cellular work.

In cotransport, a membrane protein couples the transport of two solutes.

A single ATP-powered pump that transports one solute can indirectly drive the active transport of several other

solutes in a mechanism called cotransport.

As the solute that has been actively transported diffuses back passively through a transport protein, its

movement can be coupled with the active transport of another substance against its concentration gradient.

Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive the active transport of

amino acids, sugars, and other nutrients into the cell.

One specific transport protein couples the diffusion of protons out of the cell and the transport of sucrose into

the cell. Plants use the mechanism of sucrose-proton cotransport to load sucrose into specialized cells in the

veins of leaves for distribution to nonphotosynthetic organs such as roots.

Concept 2.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis


24

Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins.

Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles.

During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the

plasma membrane.

When the two membranes come in contact, the bilayers fuse and spill the contents to the outside.

Many secretory cells use exocytosis to export their products.

During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the

plasma membrane.

Endocytosis is a reversal of exocytosis, although different proteins are involved in the two processes.

A small area of the plasma membrane sinks inward to form a pocket.

As the pocket deepens, it pinches in to form a vesicle containing the material that had been outside the cell.

There are three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and

receptor-mediated endocytosis.

In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and packaging it in a large

vacuole.

The contents of the vacuole are digested when the vacuole fuses with a lysosome.

In pinocytosis, a cell creates a vesicle around a droplet of extracellular fluid. All included solutes are taken into

the cell in this nonspecific process.

Receptor-mediated endocytosis allows greater specificity, transporting only certain substances.

This process is triggered when extracellular substances, or ligands, bind to special receptors on the membrane

surface. The receptor proteins are clustered in regions of the membrane called coated pits, which are lined on

their cytoplasmic side by a layer of coat proteins.

Binding of ligands to receptors triggers the formation of a vesicle by the coated pit, bringing the bound

substances into the cell.

Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low

concentrations in the environment.

Human cells use this process to take in cholesterol for use in the synthesis of membranes and as a precursor for

the synthesis of steroids.

Cholesterol travels in the blood in low-density lipoproteins (LDL), complexes of protein and lipid.

These lipoproteins act as ligands to bind to LDL receptors and enter the cell by endocytosis.

In an inherited disease called familial hypercholesterolemia, the LDL receptors are defective, leading to an

accumulation of LDL and cholesterol in the blood.

This contributes to early atherosclerosis.