Chapter 7~ A 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

•SEM~ electron beam to study cell surfaces

Cell fractionation~ cell separation; organelle study

Ultracentrifuges~ cell fractionation; 130,000 rpm

Cell Theory (Schleiden/Schwann)

All organisms made of cells.

Cells are the unit of structure and function in organisms.

Cells come from pre-existing cells through cell division

Exceptions:

Muscle cells~larger, multinucleate

Fungi~hyphae: hairlike structures, multinucleate

Extracellular material (bone, dentine, matrix)

Unicellular organisms

Cell Types: Prokaryotic

Nucleoid: DNA concentration (naked DNA)

No organelles with membranes

Ribosomes: protein synthesis

Mesosome: extra folds of plasma membrane for ATP production; may function in cell division

Plasma membrane (all cells); semi-permeable

Cytoplasm/cytosol (all cells)

Prokaryotes

All bacteria (monerans) [eubacteria and archaebacteria

Variations:

Photosynthesis (blue-green bacteria)

Nitrogen fixation/conversion (nitrogen cycling)

Fermentation (yogurt production)

Chemosynthesis (hydrothermal vents)

Cell size

As cell size increases, the surface area to volume ratio decreases

Rates of chemical exchange may then be inadequate for cell size

Cell size, therefore, remains small

Eukaryotes

Protists (unicellular), fungi, plants, animals

Contain membrane-bound organelles (mitochondria, chloroplasts, lysosomes, endoplasmic reticulum, golgi complex

Typically larger than prokaryotic cells

Have a true nucleus with chromosomal DNA (chromatin)

Nucleus

Genetic material... (genome) •chromatin •chromosomes •nucleolus: rRNA; ribosome synthesis

Double membrane envelope with pores

Protein synthesis (mRNA)

Ribosomes

Protein manufacture

Free •cytosol; •protein function in cell

Bound •endoplasmic reticulum; •membranes, organelles, and export

Endomembrane system, I

Endoplasmic reticulum (ER)

Continuous with nuclear envelope

Smooth ER •no ribosomes; •synthesis of lipids,

•metabolism of carbohydrates; •detoxification of drugs and poisons

Rough ER •with ribosomes; •synthesis of secretory proteins (glycoproteins), membrane production

Endomembrane system, II

Golgi apparatus •ER products are modified, stored, and then shipped

Cisternae: flattened membranous sacs

trans face (shipping) & cis face (receiving)

Transport vesicles

Endomembrane Function

Endomembrane system, III

Lysosomes •sac of hydrolytic enzymes; digestion of macromolecules

Phagocytosis

Autophagy: recycle cell’s own organic material

Tay-Sachs disease~ lipid-digestion disorder

Endomembrane system, IV

Vacuoles

•membrane-bound sacs (larger than vesicles)

Food (phagocytosis)

Contractile (pump excess water)

Central (storage in plants) •tonoplast membrane

Other membranous organelles, I

Mitochondria

(ATP production)

• quantity in cell correlated with metabolic activity;

•cellular respiration;

•double membranous (phospholipid);

•cristae/matrix;

•intermembrane space; •contain own DNA (maternal inheritance)

Spaces allow for concentration of substances on one side of membrane (chemiosmosis) à energy storage

Other membranous organelles, II

Chloroplast (photosynthesis)

•type of plastid; •double membranous; •thylakoids (flattened disks); •grana (stacked thylakoids);

•stroma (fluid surrounding grana); •own DNA

Plastids

Found in plant cells

Specialized functions in plants

Amyloplast – stores starch (leucoplast)

Chromoplast – accessory photosynthetic pigments (xanthophyll, carotenoids, phycocyanin), responsible for colors of leaves in fall

Chloroplast – contains chlorophyll, major photosynthetic organelle

Peroxisomes

Single membrane

Produce hydrogen peroxide in cells

Metabolism of fatty acids; detoxification of alcohol (liver)

Hydrogen peroxide then converted to water (catalase)

The Cytoskeleton

Fibrous network in cytoplasm

Support, cell motility, biochemical regulation

Microtubules : •thickest; •tubulin protein; •shape, support, transport, chromosome separation (mitosis)

Microfilaments : •thinnest; •actin protein filaments; •motility, cell division, shape

Intermediate filaments : middle diameter; •keratin; •shape, nucleus anchorage

Centrosomes/centrioles

Centrosome: region near nucleus (eukaryotic cells)

Centrioles: 9 sets of triplet microtubules in a ring; used in cell replication; only in animal cells

9 x 3 microtubules

Cilia/flagella

Locomotive appendages

Ultrastructure: “9+2” •9 doublets of microtubules in a ring •2 single microtubules in center •connected by radial spokes •anchored by basal body •dynein protein

Cell surfaces & junctions

Cell wall:

•not in animal cells

•protection, shape, regulation

Plant cell:

•primary cell wall produced first

•middle lamella of pectin (polysaccharide); holds cells together (pectinase – enzyme helps extract more juice from fruit)

•some plants, a secondary cell wall; strong durable matrix; wood (between plasma membrane and primary wall)

Extracellular matrix (ECM)

Glycoproteins:

• proteins covalently bonded to carbohydrate

Collagen (50% of protein in human body)

•embedded in proteoglycan (another glycoprotein-95% carbohydrate)

Fibronectins •bind to receptor proteins in plasma membrane called integrins (cell communication?)

Intracellular junctions

PLANTS:

Plasmodesmata: ~cell wall perforations; water and solute passage in plants

ANIMALS:

Tight junctions ~ fusion of neighboring cells; prevents leakage between cells

Desmosomes ~ riveted, anchoring junction; strong sheets of cells

Gap junctions ~ cytoplasmic channels; allows passage of materials or current between cells