CHAPTER 7A TOUR OF THE CELL

Cytology: science/study of cells

Light microscopy resolving power~ measure of clarity

Electron microscopy

TEM ~ electron beam to study cell ultrastructure (internal anatomy)

SEM ~ electron beam to study cell surfaces Cell fractionation ~ cell separation; organelle study Ultracentrifuge ~ cell fractionation; 130,000rpm

A cell is a living unit greater than the sum of its parts

• While the cell has many structures that have specific functions, they must work together.

• Cell Theory– Cells are the basic unit of structure and function– All cell arise from other cells– All living things are made of one or more cells

Cell Types: ProkaryoticDomains: Bacteria and Archaea

Nucleoid: DNA

concentration

No membrane bound

organellesRibosomes:protein synthesis

Plasma membrane: (all

cells); semi-permeable

Cytoplasm/cytosol(all cells)

Cell types: EukaryoticDomains: Protist, fungi, Plants, and Animals

Nucleus:membrane enclosed organelle containing chromosomes

Membrane bound organelles of specialized form and function

Generally larger than prokaryotic cells

Cell Size

As cell size increases, the surface area to volume ratio decreases (as well as the % Diffusion)

Rates of chemical exchange may then be inadequate for cell size

Cell size, therefore, remains small

Nucleus

Genetic material… chromatin Chromosomes Nucleolus: rRNA; ribosome synthesis

Nuclear Envelope: double membrane with pores

mRNA~ protein synthesis

Ribosomes

Protein manufacture Types: a) free cytosol;protein function in cell

b) bound: on ER; proteins function in membranes, organelles and export

Endoplasmic reticulum(ER) Continuous with nuclear envelope Smooth ER

no ribosomes Synthesis of lipids Metabolism of carbohydrates Detoxification of drugs &poisons

Rough ER With ribosomes Synthesis of secretory proteins

(glycoproteins) Membrane production

The Endomembrane System

The Golgi apparatusER products are modified, stored, and then

shipped to either: lysosomes, central vacuole, plasma membrane

Cisternae: flattened membranous sacsTrans face(shipping) & cis face (receiving)Transport vesicles

Lysosomes

Sac of hydrolytic

enzymes; digestion of

macromoleculesPhagocytosis Autophagy: recycle cell’s

own organic material

Tay-Sachs disease~

lipid digestions disorder

VacuolesMembrane-bound

sacs(larger than vesicles)Food (phagocytosis)Contractile (pump excess

water)Central (storage in plants

as well as lysosomal functions) Tonoplast membrane

Other Membranous Organelles

1. Mitochondria and chloroplasts are the main

energy transformers of cells• Both organelles have small quantities of DNA that direct the

synthesis of the polypeptides produced by their internal

ribosomes.

• Mitochondria and chloroplasts grow and reproduce as

semiautonomous organelles.

2. Peroxisomes generate and degrade H2O2 in

performing various metabolic functions• What enzyme breaks down H2O2?

Mitochondria Site of cellular respiration have a smooth outer membrane

and a highly folded inner membrane, the cristae inner membrane encloses the mitochondrial matrix,

a fluid-filled space with DNA, ribosomes, and enzymes.

chloroplasts

found in plants, and eukaryotic algae (protista)site of photosynthesis.Inside the innermost membrane is a fluid-

filled space, the stroma, in which float membranous sacs, the thylakoids.

Peroxisomes

• generate and degrade H2O2 in performing various metabolic functions

• bounded by a single membrane.• They form not from the endomembrane system,

but by incorporation of proteins and lipids from the cytosol.

The Cytoskeleton

• Providing structural support to the cell, the cytoskeleton also functions in cell motility and regulation

There are three main types of fibers in the cytoskeleton:

microtubules, microfilaments, and intermediate filaments.

Microtubules

• the thickest fibers, are hollow rods about 25 microns in diameter.

• They move chromosomes during cell division. • Another function is

as tracks that guide motor proteins carrying organelles to their destination.

cilia and flagella.

• Microtubules are the central structural support

• Cilia usually occur in large numbers on the cell surface.

• There are usually just one or a few flagella per cell

cilia and flagella.

• A flagellum has an undulatory movement

cilia and flagella.

• Cilia move more like oars with alternating power and recovery strokes.

• have the same ultrastructure.

cilia and flagella

Microfilaments

• the thinnest class of the

cytoskeletal fibers,

are solid rods of the globular protein

actin.

designed to resist tension

• form a three-dimensional

network just inside

the plasma membrane.

Microfilaments

• In muscle cells, thousands of actin filaments are arranged parallel to one another.

• Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers

Microfilaments

• In other cells, these actin-myosin aggregates are less organized but still cause localized contraction

•Pseudopodia, cellular extensions, extend and contract through the reversible assembly and contraction of actin subunits into microfilaments.

• In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming.

Microfilaments

Intermediate filaments,

• more permanent fixtures

of the cytoskeleton than

are the other two classes• reinforce cell shape • and fix organelle location.

Cell Surfaces and Junctions

1. Plant cells are encased by cell walls

2. The extracellular matrix (ECM) of animal cells

functions in support, adhesion, movement, and

regulation

3. Intercellular junctions help integrate cells into higher

levels of structure and function

4. The cell is a living unit greater than the sum of its parts

Plant cells are encased by cell walls

• The cell wall, found in prokaryotes, fungi, and some protists, has multiple functions.

• In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water.

• It also supports the plant against the force of gravity.

A mature cell wall consists of a primary cell wall, a middle lamella with sticky polysaccharides that holds cell together, and layers of secondary cell

wall.

The extracellular matrix (ECM) of animal cells functions in support,

adhesion, movement, and regulation• In many cells, fibronectins in the ECM

connect to integrins, intrinsic membrane proteins.

. Intracellular junctions help integrate cells into higher levels of

structure and function• Plant cells are perforated with

plasmodesmata, channels allowing cysotol to pass between cells.

Animal have 3 main types of intercellular links: tight junctions, desmosomes, and gap

junctions• In tight junctions, membranes of adjacent cells

are fused, forming continuous belts around cells.– This prevents leakage of extracellular fluid.

Desmosomes (or anchoring junctions) fasten cells together into strong sheets,

much like rivets.

• Gap junctions (or communicating junctions) provide cytoplasmic channels between adjacent cells.

Microtubules

• In many cells, microtubules grow out from a centrosome near the nucleus.

• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring.

• During cell division the centrioles replicate.