Chapter 7A Tour of the Cell…!

How We Study Cells• Light Microscope: visible light passes through the specimen

and then through glass lenses. The lenses refract the light to where the specimen is magnified as it is projected into the eye– Most sub cellular structures, or organelles, are too small to be

resolved by the light microscope• Electron Microscope: Does not use light. Instead focuses a

beam of electrons through the specimen or onto its surface– Transmission Electron Microscope(TEM):scientist use mainly to

study the internal ultra structure of cells. It aims an electron beam through a thin section of the specimen.

– Scanning Electron Microscope(SEM): Useful for detailed study of the surface of the specimen

Cell biologists can isolate organelles to study their functions

• The goal of cell fractionation is to take cells apart, separating the major organelles so that their functions can be studied– The instrument used to fractionate cells is the

centrifuge: a merry-go-round for test tubes that can spin at various speeds.• Fractionation begins with homogenization-the

disruption of cells

Prokaryotic and eukaryotic cells differ in size and complexity

• All cells have a plasma membrane. Within the membrane is a semifluid substance called cytosol

• All cells contain: Chromosomes: which carry genes in the form of

DNARibosomes: make proteins according to

instructions from the genes

Prokaryotic Cells

• Does not contain a nucleus• DNA is concentrated in the nucleoid with no

region to separate it from the rest of the cell• The entire region between the nucleus and

the plasma membrane is called the cytoplasm, only in this type of cell

Eukaryotic Cells

• Larger than Prokaryotic cells• Contains a nucleus• Chromosomes are located in the nucleus• Within the cytoplasm in a eukaryotic cell,

suspended in cytosol, are a variety of membrane-bound organelles of specialized form and function

The Plasma Membrane• Its function is to serve as a selective barrier that

allows sufficient passage of oxygen, nutrients, and wastes to service the entire volume if the cell.– A eukaryotic cell has extensive and elaborately arranged

internal membranes. These membranes also participate directly in the cell’s metabolism; and many enzymes are built right into the membranes.

– In general, biological membranes consist of a double layer of phospholipids and other lipids, embedded in this lipid bilayer or attached to its surfaces are diverse proteins

– Each type of membrane has a unique composition of lipids and proteins suited to that membranes specific functions

The Nucleus and Ribosomes • The nucleolus does not divide and is inside the nucleus. It is a mass

of densely stained granules and fibers adjoining part of the chromatin.

• In the nucleolus a special type of RNA is synthesized and assembled with proteins imported from the cytoplasm into the main components of ribosomes called ribosomal sub units

• The nuclear envelope encloses the nucleus, separating its contents from the cytoplasm. It is a double membrane and is perforated by pores.

• Except at the pores, the nuclear side of the envelope is lined by the nuclear lamina which is a netlike array of protein filaments that maintain the shape of the nucleus.

• Within the nucleus, the DNA is organized along with proteins into a fibrous material called chromatin

• The thin chromatin fibers coil up, becoming think enough to be discerned as separate structures called chromosomes

• Ribosomes are composed of two sub units and are the organelles that carry out protein synthesis.

• Ribosomes build proteins in two cytoplasmic locales.– Free ribosomes are suspended in the cytosol

• Most free ribosomes will function in the cytosol– Bonded ribosomes are attached to the outside of the

endoplasmic reticulum or nuclear envelope • Generally make proteins that are destined either for insertion into

membranes , for packaging within certain organelles such as lysosomes, or for export from the cell(secretion)

• Bound and free ribosomes are structurally identical and can alternate between the two roles

The Endomembrane System

• Includes vesicles such as nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various kinds of vacuoles, and the plasma membrane

The Endoplasmic Reticulum

• The ER consists of a network of membranous tubules and sacs called cisternae

• Two types of ER– Smooth ER: lacks ribosomes. Important to the synthesis of

lipids, metabolism of carbohydrates and detoxification of drugs and poisons. It also pumps calcium ions from the cytosol into the cisternal space.

– Rough ER: contains ribosomes. Makes secretory proteins . It grows in place by adding proteins and phospholipids. It also makes its own membrane phospholipids; enzymes built into the ER membrane assemble phospholipids from precursors in the cytosol.

The Golgi apparatus• After leaving the ER, many transport vesicles

travel to the Golgi Apparatus.– Here its is thought of as the center for manufacturing,

warehousing, sorting, and shipping– Consists of flattened membranous sacs- cisternae,

which has a distinct polarity on each side(poles)• Proteins and phospholipids of membranes may

be altered• Manufactures certain macromolecules by itself.

Many polysaccharides secreted by cells are Golgi products

Lysosomes • A lysosome is a membrane-bound sac of hydrolytic enzymes that

the cell uses to digest macromolecules. – some can hydrolyze proteins, polysaccharides, fats, and nucleic acids-

all the major classes of macromolecules.• These enzymes work best in an acidic environment, at about pH 5• Is a space where the cell can digest macromolecules safely, without

the general destruction that would occur if hydrolytic enzymes roamed at large.

• Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing

• Lysosomes also use their hydrolytic enzymes to recycle the cells own organic material, a process called autophagy– This helps the cell continually renew itself

Vacuoles

• Are larger than vesicles but is still a membrane-bound sac within the cell.

• Contractile vacuoles: pump excess water out of the cells of freshwater fish

• Central vacuole: is enclosed by a membrane called tonoplast.

• finishhhhh

Other Membranous Organelles

• In eukaryotic cells , mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work.

• Mitochondria are the sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen.

• Chloroplasts, found only in plants and algae, are the sites of photosynthesis

• Mitochondria and chloroplasts are semiautonomous organelles that grow and reproduce within the cell

mitochondria• Are found in nearly all eukaryotic cells including those of plants,

animals, fungi, and protists• The number of mitochondria in a cell is correlated with the cell’s

level of metabolic activity.• Is enclosed by two membranes, each a phospholipid bilayer with a

unique collection of embedded proteins. The outer membrane is smooth but the inner membrane is convoluted, with infolding called cristae.

• The inner membrane divides the mitochondrion into two internal compartments. – The first is the intermembrane space, the narrow region between the

inner and outer membranes – The second is the mitochondrial matrix, which is enclosed by the inner

membrane

chloroplasts• Is a family member closely related plant organelles called plastids

– Amyoplasts are colorless plastids that store starch (amylose)– Chromoplasts have pigments that fruits that give fruits and flowers

their hues– Chloroplasts contain the green pigment chlorophyll, along with the

enzymes and other molecules that function In the photosynthetic production of sugar.

• The contents of a chloroplast are partitioned from the cytosol by an envelope consisting of two membranes separated by a very narrow intermembrane space– Inside the chloroplasts is another membranous system in the form of

flattened sacs called thylakoids. Each stack of thylakoids is called granum. The fluid outside the thylakoids is the stroma , which contains the chloroplast DNA and ribosomes as well as many enzymes.

peroxisomes

• Is a specialized metabolic compartment bounded by a single membrane.

• They contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide as a by-product– Some peroxisomes use oxygen to break fatty acids down

into smaller molecules that can then by transported to mitochondria as fuel for cellular respiration.

• Peroxisomes do not bud from the endomembrane system. They grow by incorporating proteins and lipids made in the cytosol, and they increase in number by splitting in two when they reach a certain size.

THE CYTOSKELETON• Gives mechanical support to the cell and maintains it

shape. – Is especially important for animal cells because they lack

walls• Cell motility(changes in location and movements of

parts of the cell) generally requires the interaction of the cytoskeleton with proteins called motor molecules– Motor molecules of the cytoskeleton bring about the

movements of cilia and flagella by allowing components of the cytoskeleton to slide past each other

• Main types of fibers that make up the cytoskeleton are– Microtubules –the thickest of the three types– Microfilaments-also called actin filaments; are the thinnest– Intermediate filaments-fibers with diameters in a middle

range

microtubules• Are found in the cytoplasm of all eukaryotic cells• The wall of the hollow tube is constructed from a

globular protein called tubulin.– Each tubulin molecule is a dimer consisting of two slightly

different polypeptide subunits, a-tubulin and B-tubulin.– A molecule grows in length by adding tubulin dimes to its

ends.– Microtubules can be disassembled and their tubulin used

to build microtubules elsewhere in the cell• Microtubules shape and support the cell and also serve as tracks

along which organelles equipped with motor molecules• Microtubules are also responsible for the separation of

chromosomes during cell division

centrosomoes and centrioles• In many cells, microtubules grow out from a

centrosome-a region often located near the nucleus– These microtubules function as compression-resisting

girders of the cytoskeleton.• Within the centrosome of an animal cell are a pair of

centrioles-each composed of nine sets of triplet microtubules arranged in a ring– Before a cell divides, the centrioles replicate

• Although centrioles may help organize microtubule assembly, they are not essential for this function in all eukaryotes– Centrosomes of most plants lack centrioles altogether

cilia and flagella• In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella and cilia-which

are locomotor appendages that protrude from some cells– Many unicellular eukaryotic organisms are propelled through water such as the sperm of animals, algae, and

some plants• Cilia usually occur in large numbers on the cell surface

– Flagella are the same diameter as cilia but are longer in length and are usually limited to just one or a few per cell

• Flagella has an undulating motion that generates force in the same direction as the flagellum’s axis– Cilia works more like oars, with alternating power and recovery strokes generating force in a direction

perpendicular to the cilium’s axis• Both cilia and flagella have a core of microtubules sheathed in an extension of the plasma membrane• Nine doublets of microtubules, the members of each pair sharing part of their walls, are arranged in a ring. In the

center of the ring are two single microtubules– This arrangement, referred to as the “9+2” pattern , is found in nearly all eukaryotic flagella and cilia.– Flexible “wheels” of protein, evenly spaced along the length of the cilium or flagellum ,connect the outer

doublets to each other and to the two central microtubules.• Each outer doublet also has pairs of side-arms spaced along its length and reaching toward the neighboring

doublet; these are motor molecules.– The microtubule assembly of a cilium or flagellum is anchored in the cell by a basal body, which is

structurally identical to a centriole.• In many animals (including humans), the basal body of the fertilizing sperm’s flagellum enters the egg

and becomes a centriole• The motor molecules extending from each microtubule doublet to the next are made of a large protein called

dynein-which are responsible for the bending movements of cilia and flagella– A dynein arm performs a complex cycle of movements caused by changes in the confirmation of the

protein, with ATP providing energy for these changes

Microfilaments (Actin filaments)• Are solid rods about 7nm in diameter• Are also called actin filaments because they are built from molecules of

actin, a globular protein• A microfilament seems to be present in all eukaryotic cells• Thousands of actin filaments are arranged parallel to one another along

the length of a muscle cell, interdigitated with thicker filaments made of a protein called myosin.– It acts as a motor molecule of projections (arms) that “walk” along the actin

filaments.– Contraction of the muscle cell results from the actin and myosin filaments

sliding past one another, shortening the cell.– Localized contraction brought about by actin and myosin also plays a role in

amoeboid movement, in which a cell crawls along a surface by extending and flowing into cellular extensions called pseudopodia. • Pseudopodia extend and contract through reversible assembly of actin sub units into

microfilaments and of microfilaments into networks that convert cytoplasm from sol to gel.

• In plants, both actin-myosin interactions and sol-gel transformations brought about by actin may be involved in cytoplasmic streaming-a circular flow of cytoplasm within cells

Intermediate filaments • Is larger than the diameter of microfilaments but

smaller that that of microtubules• Each type of intermediate filament is constructed from

a different molecular subunit belonging to a family of proteins called keratins.– Are more permanent fixtures of cells that are

microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell

• Other intermediate filaments make up the nuclear lamina that lines the interior of the nuclear envelope. – The long extensions(axons) of nerve cells that

transmit impulses are strengthened by one class of intermediate filament.

Cell Surfaces and Junctions• The cell wall in plants protects the cell, maintains

its shape, and prevents excessive uptake of water.

• The exact chemical composition of the wall is consistent– Microfibrils made of the polysaccharide cellulose are

embedded in a matrix of other polysaccharides and protein

• A young plant cell first secrets a relatively thin and flexible wall called the primary cell wall

• Between the primary walls of the adjacent is the middle lamella-a thin layer rich in sticky polysaccharides call pectins.– The middle lamella glues the cells together.

Extracellular matrix (ECM)• Main ingredients of the ECM are glycoprotein secreted by

the cells– The most abundant glycoprotein in the ECM of most animal cells

is collagen• Collagen accounts for about half of the total protein in the human

body• The collagen fibers are embedded in a network woven from

proteoglycans- which are glycoprotein of another class– Are especially rich in carbohydrates.

• Some cells are attached to the ECM by still other kinds of glycoprotein, most commonly fibronections– Fibronections bind to receptor proteins called integrins that are

built into the plasma membrane.• Integrins span the membrane and bind on the cytoplasmic side to

microfilaments of the cytoskeleton. Thus, integrins are in a position to transmit changes in the ECM to the cytoskeleton, and vice versa

• Communicating with a cell through integrins, the ECM can regulate a cell’s behavior

Intercellular junctions

• Neighboring cells often adhere, interact, and communicate through special patches of direct physical contact.

• Plant cell walls are perforated with channels called plasmodesmata– Cytosol passes through the plasmodesmata and

connect the living contents of adjacent cells.– Water and small solutes can pass freely from cell

to cell

Continued..• In animals, there are three main types of intercellular

junction: – Tight junction

• The membranes of neighboring cells are actually fused. Forming continuous belts around the cells, these junctions prevent leakage of extracellular fluid across a layer of epithelial cells

– Desmosomes• Also called anchoring junctions. Function like rivets, fastening cells

together into strong sheets. Intermediate filaments made of sturdy protein keratin reinforce desmosomes.

– Gap junctions• Also called communication junctions. They provide cytoplasmic

channels between adjacent animal cells. Special membrane proteins surround each pore which is wide enough for salt ions, sugars, amino acids, and other small molecules to pass.

• Are especially common in animal embryos, in which chemical communication between cells is essential for development