CELL STRUCTURE AND FUNCTION

1. What is life?

2. The living world

3. Early cells

4. Prokaryotes

5. Prokaryote cell features

6. Eukaryote cells

7. Functions of Eukaryote organelles

1. WHAT IS LIFE?• Life is complex and dynamic – composed of carbon-based

molecules• Life is organised and self-sustaining – composed of biomolecules• Life is cellular • Life is information-based – genes• Life adapts and evolves – mutations

All living things share four processes: Growth: an increase in size Reproduction: an increase in number Responsiveness: an ability to react to environmental stimuli Metabolism: controlled chemical reactions In addition, all living organisms share a cellular structure

2. The Living World• A protocell could have contained only RNA to function

as both genetic material and enzymes.

• First protocells were heterotrophs using ATP as energy and carrying out a form of fermentation.

Domains of Life on Earth: 3 domains1. ARCHAEA: Halophiles and Thermophiles2. BACTERIA: Cyanobacteria and Heterotrophic bacteria3. EUKARYA: Flagellates, Fungi, Plants and Animals

3. Early cells• Bacteria and Archaea are termed as

PROKARYOTES –organisms whose DNA is not enclosed in a nucleus of the cell.

• EUKARYOTIC cells are aerobic and arose 2.1 billion years ago. They contain nuclei and organelles.

• PLANTS appeared on land (mud flats) during the ‘Paleozoic’ period, about 440 million years ago. They provided food for higher animals to evolve

3.1 Cells

Cells are fundamental units of biology, and building block of all organisms

Organisms range from:

Unicellular: 1 cell = 1 organism e.g.

Paramecium & Amoeba

Multicellular: 1036 cells = 1 organism, different cells for different functions, exhibit division of labor e.g. muscle, skeletal, immune, lungs, epithelium, etc...)

4. ProkaryotesProkaryotes are single-celled microorganisms

characterized by:• the lack of a membrane-bound nucleus and • membrane bound organelles.

There are two domains of prokaryote:

1. Eubacteria / Bacteria

2. Archaebacteria/Archaea

4.1 Differences between bacteria and archae

Eubacteria have cell walls composed of peptidoglycan, Archaebacteria have cell walls composed of various different substances.

Eubacteria have ester-linked straight-chain membrane lipids (fatty acids). Archaebacteria have ether-linked branched-chain member lipids.

Eubacteria and Archaebacteria have differences in their DNA replication and transcription systems that suggest independent elaboration in these two groups

Bacteria translation apparatus inhibited by antibiotics (e.g. streptomycin, tetracycline etc.). Archaea not affected by antibiotics.

5. Prokaryote cell features• Has five essential structural components:

1.genome (DNA)

2.ribosomes

3.cell membrane

4.cell wall

5.surface layer

• Structurally, a prokaryotic cell has three architectural regions:

1.appendages (flagella and pili)

2.cell envelope (capsule, cell wall , plasma membrane))

3.cytoplasm region (cell genome (DNA) and ribosomes.

5.1 Prokaryote cell

5.2 External structures of prokaryotes (p.60-65, Bauman)

The external structures of prokaryotic cells include glycocalyces, flagella, fimbriae, and pili

Glycocalyces (Ch 3, Bauman)• A glycocalyx is a gelatinous, sticky substance that

surrounds the outside of the cell. • When the glycocalyx of a prokaryote is firmly attached to

the cell surface, it is called a capsule. • When loose and water-soluble, it is called a slime layer.

• Both types protect the cell from dessication, and both increase the cell’s ability to cause disease.

5.2 contd. (Flagella)• The structures responsible for cell motility include flagella, long

extensions from the cell surface and glycocalyx that propel a cell through its environment.

• Bacterial flagella are composed of a filament, a hook, and a basal body. Flagella covering the cell are termed peritrichous flagella and those found at the ends of a cell are termed polar flagella. (refer p.62, Bauman)

• Endoflagella are the special flagella of spirochetes that spiral tightly around the cell instead of protruding into the environment.

• Flagella enable bacterial cells to move clockwise or counterclockwise, in a series of runs and tumbles.

• Via taxis, flagella move the cell toward or away from stimuli such as chemicals (chemotaxis) or light (phototaxis).

5.2 contd. (Fimbriae and Pili)

Fimbriae are short, sticky, proteinaceous, nonmotile extensions of some bacteria that help cells adhere to one another and to substances in the environment. They serve an important function in biofilms, slimy masses of bacteria adhering to a surface.

Pili (also called conjugation pili) are hollow, nonmotile tubes of a protein called pilin that connect some prokaryotic cells. Typically, only one or two are present per cell. They join two bacterial cells and mediate the movement of DNA from one cell to another, a process called conjugation.

5.3 Cell wall of prokaryotes (p. 65-69)

• Most prokaryotic cells are surrounded by a cell wall (not found in eukaryotes) that provides structure and protection from osmotic forces. Cell walls are composed of polysaccharide chains.

• Walls composed of peptidoglycan, a complex polysaccharide composed of two alternating sugars called acetylglucosamine (NAG) and N-acetylmuranic acid (NAM)

• Gram-positive cells have thick layers of peptidoglycan; stained cells appear purple under magnification.

• Gram-negative cells have only a thin layer of peptidoglycan, outside of which is a membrane containing lipopolysaccharide (LPS); Gram-negativecells appear pink.

5.4 Prokaryotic Cytoplasmic Membranes (pp. B. 69–73)

• The cytoplasmic membrane is a double-layered structure, called a phospholipid bilayer, composed of molecules with hydrophobic lipid tails and hydrophilic phosphate heads.

• The selectively permeable cytoplasmic membrane, not only,separates the contents of the cell from the outside environment, but also controls the contents of the cell allowing some substances to cross it while preventing the crossing of others.

• Although impermeable to most substances, its proteins act as pores, channels, or carriers to allow or facilitate the transport of substances the cell needs.

• The relative concentration of chemicals (concentration gradients) inside and outside the cell and of the corresponding electrical charges, or voltage (electrical gradients), create an overall electrochemical gradient across the membrane.

• A cytoplasmic membrane uses the energy inherent in its electrochemical gradient to transport substances into or out of the cell using passive process (diffusion and osmosis) and active process (ATP utilization and translocation)

5.5 Cytoplasm of Prokaryotes

• Cytoplasm is the gelatinous, elastic material inside a cell. It is composed of cytosol, inclusions, ribosomes, and in many cells a cytoskeleton. Some bacterial cells produce internal, resistant, dormant forms called endospores. Endospores can survive under harsh conditions, making them a concern to food processors and health care professionals.

• The liquid portion of the cytoplasm is called cytosol. It is mostly water, plus dissolved and suspended substances such as ions, carbohydrates, proteins, lipids, and wastes. The cytosol of prokaryotes also contains the cell’s DNA in a region called the nucleoid.

5.5 Cytoplasm contd.

• Deposits called inclusions may be found within the cytosol of prokaryotes. These may be reserve deposits of lipids, starch, or other chemicals

• Nonmembranous Organelles: ribosomes and the cytoskeleton.

a) Ribosomes are the sites of protein synthesis in cells. They are composed of protein and ribosomal RNA (rRNA).

b) The cytoskeleton is an internal network of fibers that play a role forming a cell’s basic shape. Spherical prokaryotes appear to lack cytoskeletons.

6. EUKARYOTIC CELLS• Endosymbiotic theory: mitochondria and chloroplast have

70S ribosomes, circular DNA, and 2 membranes suggesting that the ancestors of these organelles were prokaryotic cells that were internalized by other prokaryotes and then lost the ability to exist outside their host – thus forming early eukaryotes

• Eukaryotic cells are larger than prokaryotes.

• They have a variety of internal membranes and structures, they are:1.Organelles 2.cytoskeleton composed of microtubules, microfilaments

and intermediate filaments

• Eukaryotic DNA is composed of several linear bundles called chromosomes

6.1 Eukaryotic cell

6.1.1 Animal cell

6.1.2 Plant cell

6.3 External Structures of Eukaryotic Cells (p.77 B.)• Eukaryotic cells have many external structures similar to those of

prokaryotes, as well as some unique features (flagella, cilia, and glycocalyces for attachment [in animals and protozoan cells lacking cell walls])

• Eukaryotic flagella are within the cytoplasmic membrane.

• The shaft of a eukaryotic flagellum is composed of molecules of a globular protein called tubulin arranged in chains to form hollow microtubules arranged in nine pairs around a central two.

• The basal body also has microtubules, but in triplets with no central pair. Eukaryotic flagella have no hook, and do not extend outside the cell. Rather than rotating, eukaryotic flagella undulate rhythmically to push or pull the cell through the medium. They do exhibit taxis.

• Some eukaryotic cells are covered with cilia, which have the same structure as eukaryotic flagella but are much shorter and more numerous. Their rhythmic beating propels single-celled eukaryotes through their environment.

Image of flagellum

6.4 Eukaryotic Cell Walls and CytoplasmicMembranes (B.pp. 77–80)

• The eukaryotic cells of fungi, algae, plants, and some protozoa lack glycocalyces;

• Cell wall composed of polysaccharides provides protection from the environment. It also provides shape and support against osmotic pressure.

• The cell walls of plants are composed of cellulose, whereas fungal cell walls are composed of chitin or other polysaccharides.

• Algal cell walls contain agar, carrageenan, algin, or other chemicals.

• All eukaryotic cells have cytoplasmic membranes. Like bacterial membranes, they are a fluid mosaic of phospholipids and proteins. Unlike

6.4.1 Transportation (endocytosis)

• Some eukaryotic cells transport substances into the cytoplasm via endocytosis, which is an active process requiring the expenditure of energy by the cell.

• In endocytosis, pseudopodia—moveable extensions of the cytoplasm and membrane of the cell—surround a substance and move it into the cell.

• When solids are brought into the cell, endocytosis is called phagocytosis. The incorporation of liquids is called pinocytosis

6.5 Cytoplasm of Eukaryotes

• The cytoplasm of eukaryotes is more complex than that of prokaryotes, containing a few nonmembranous and numerous membranous organelles.

• Three nonmembranous organelles are found in eukaryotes: ribosomes, a cytoskeleton, and centrioles

• Eukaryotic ribosomes are 80S and are found within the cytosol (liquid portion of cytoplasm)

• The cytoskeleton is extensive, and composed of both fibers and tubules. It acts to anchor organelles and functions in cytoplasmic streaming and in movement of organelles within the cytosol.

• Centrioles are composed of nine triplets of tubulin microtubules. Centrosomes play a role in mitosis, cytokinesis, and in the formation of flagella and cilia.

6.5.1 Membranous Organelles• Nucleus: - is enclosed in a double membrane and

communicates with the surrounding cytosol via numerous nuclear pores. Within the nucleus is the DNA providing the cell with its unique characteristics.

• Nucleolus: The nucleolus produces ribosomes• Cytosol: The cytosol is the "soup" within which all the

other cell organelles reside and where most of the cellular metabolism occurs. -The cytosol is full of proteins that control cell metabolism including signal transduction pathways, glycolysis, intracellular receptors, and transcription factors.

• Cytoplasm: This is a collective term for the cytosol plus the organelles suspended within the cytosol.

Membranous Organelles

• Mitochondria: Mitochondria provide the energy to a cell. They are the power centers of the cell. Mitochondria are membrane-bound organelles (double membrane).

• Vacuole: A vacuole is a membrane-bound sac that plays roles in intracellular digestion and the release of cellular waste products.

• Lysosome: -contain hydrolytic enzymes necessary for intracellular digestion. They are common in animal cells, but rare in plant cells. Hydrolytic enzymes of plant cells are more often found in the vacuole

Diagram of mitochondrion

SECTIONS OF MITOCHONDRION

• The different sections are: outer membrane; intermembrane space; inner membrane and matrix

• The Outer Membrane: This membrane contains a great number of large transport proteins, which allows for large molecules to enter with ease. This membrane includes proteins that can convert lipid substrates into forms that can be used by the matrix.

• The Intermembrane Space: This space contains enzymes that use ATP to phosphorylate other nucleotides.

• The Inner Membrane: This membrane is highly convoluted, forming many folds called cristae. This serves to greatly increase the surface area, allowing more work to be done is a smaller space.

• It contains three major proteins – (1). the proteins that carry out the oxidation reactions of the respiratory chain, (2).2). an enzyme complex called ATP synthetase which makes ATP, and (3). transport proteins which regulate the transfer of molecules into and out of the matrix. This is where the oxidation phosphorylation takes place.

The Matrix of Mitochondrion

• The Matrix: The Kreb Cycle takes place here.

• It also contains: (1) several copies of the mitochondrial DNA genome, (2) special mitochondrial ribosomes, (3) tRNAs, (4) and various enzymes required for the expression of the mitochondrial gene.

CHLOROPLAST IN PLANTS• Chloroplast: This organelle contains the plant cell's

chlorophyll responsible for the plant's green color. Structurally it is very similar to the mitochondrion except it is larger than the mitochondrion, not folded into cristae, and not used for electron transportIt contains:

• 1. A permeable outer membrane; • 2. A less permeable inner membrane; • 3. Inter membrane space; • 4. A third membrane containing the light-absorbing

system, the electron transport chain, and ATP synthetase, that forms a series of flattened discs, called the thylakoids

Image of chloroplast

ORGANELLES (GOLGI AND CELL MEMBRANE)

• Golgi:

-The Golgi apparatus is a single membrane-bound structure.

-It is actually a stack of membrane-bound vesicles that are important in packaging macromolecules for transport elsewhere in the cell.

• Cell Membrane: – Every cell is enclosed in a membrane, a

double layer of phospholipids (lipid bilayer)

CYTOSKELETON

• Cytoskeleton: – Helps to maintain cell shape.

– The primary importance of the cytoskeleton is in cell motility.

– Provides a supporting structure for the internal movement of cell organelles, as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton.

– It is composed of proteinaceous fibers:1. Microtubules (diameter – 25 nm)

2. Actin filaments (microfilaments), diameter, 5-7nm

3. Intermediate fibers (diameter – 10 nm)

7. SIMILARITIES BETWEEN EUKARYOTES AND PROKARYOTES

• They both have DNA as their genetic material.  

• They are both membrane bound.

• They both have ribosomes .

• They have similar basic metabolism .

• They are both amazingly diverse in forms

7.1 Differentiating Prokaryotes and Eukaryotes

NUCLEUSEukaryotes have a nucleus and membrane-boundorganelles , while prokaryotes do not. DNA• The DNA of prokaryotes floats freely around the cell; DNA of eukaryotes is

held within its nucleus. • Prokaryotes also differ from eukaryotes in that they contain only a single

loop of DNA stored in an area named the nucleoid.• Prokaryotic DNA also lacks the proteins found in eukaryotic DNA .SIZEProkaryotes are usually much smaller than eukaryotic cells. Eukaryotic cells

are, on average, ten times the sizeof prokaryotic cells.CELL WALLProkaryotes have cell wall composed of peptidoglycan (a single large polymer

of amino acid and sugar). Cell wall of eukaryotes is not made up of this polymer.

Differentiating Prokaryotes and Eukaryotes (contd.)

SUPPORTIn Eukaryotes provided by cytoskeleton;

none in ProkaryotesPROTEIN SYNTHESISIn Eukaryotes (animals) Rough Endoplasmic

Reticulum (Rough ER) is involvedIn Prokaryotes ribosomes are involved FAT SYNTHESIS In Eukaryotes – Smooth ER involvedNo fat synthesis in Prokaryotes

Differentiating Prokaryotes and Eukaryotes (contd.)

ENERGY PRODUCTIONIn Eukaryotes – chloroplasts (plants);

mitochondrion (Kreb’s cycle)In Prokaryotes – chlorophyll (if present) but has no

covering or chloroplast; no mitochondrion and Kreb’s cycle replaced by fermentation

ENERGY DIGESTION Lysosomes involved in aging process of cell in

EukaryotesNo lysosomes in Prokaryotes

Differentiating Prokaryotes and Eukaryotes (contd.)

MOVEMENT In Eukaryotes – cilia, flagella and pseudopod

movementIn Prokaryotes – flagella of different structure

involved in locomotionREPRODUCTION - DNA controlIn Eukaryotes – DNA in chromosomes inside

nucleusIn Prokaryotes – DNA in single strand and floating

freely without a nucleus

8. Review Questions

1. Differentiate among:i. pili, ii. fimbriae, iii. prokaryotic flagella, iv. eukaryotic flagella, v. and cilia using sketches and descriptive labels

2. Contrast prokaryotic and eukaryotic cells with reference to:i. Sizeii. Presence of membrane-bound organellesiii. Structure of flagellaiv. Chemicals in cell wallsv. Type of ribosomes

3. Compare and contrast 3 types of passive transport across a cell membrane

4. What factors may prevent a molecule from moving across a cell membrane

5. Describe the ‘endosymbiotic theory’