Cytoskeleton Functions Mechanical support Maintain shape Movement of the cell Movement of things inside the cell (motor molecule – carry things on “rails”)

Microtubules – hollow rods, walls made of tubulin – globular protein Structures Microtubules – hollow rods, walls made of tubulin – globular protein Serves as tracks for motor molecules Make up centrioles & basal bodies (9 + 0) Interior of cilia & flagella (9 + 2) Built to resist compression Microfilaments – solid rods, built from actin – globular protein Bear tension – pulling Maintain cell shape With myosin, causes muscle contraction, Pinching in of cleavage furrow Amoeboid movement (pseuodopods) Intermediate filaments – more permanent Made from fibrous proteins Scaffold for attachment of organelles, cell shape that is fairly permanent

Plasma Membrane – Fluid Mosaic Model Phospholipid bilayer – hydrophobic interactions hold phospholipids and proteins together Plasma membrane is selectively permeable (semi-permeable)

Cholesterol – stabilizes membranes at moderate temps and hinders solidification at low temperatures (animal cell membranes) Carbohydrates – attached to lipids (glycolipids) or proteins (glycoproteins) Serve as ID for cells so they can be recognized

Proteins – Integral proteins - have hydrophobic sides to hold them in position Peripheral proteins – attached to hydrophilic part of membrane – often attached to integral proteins Functions of proteins Transport Enzyme Receptor for hormones or other chemical messengers Junction between cells (tight junctions, desmosomes, etc.) Glycoproteins are ID tags Attachments to cytoskeleton & extracellular matrix

Transport Across Membranes Passive Transport Diffusion – Net movement of molecules by their own kinetic energy from an area of high conc. to low conc. (down concentration gradient) Continues until equilibrium is reached Molecules that can do this (oxygen, carbon dioxide, alcohols (lipid soluble)

Osmosis – diffusion of water across membrane Hypertonic solution (more solute) Hypotonic solution (less solute) Isotonic solution (equal solute conc.) Many cells live in isotonic environments Protists in hypotonic environment may have contractile vacuoles to excrete water Freshwater fish excrete water through kidneys Plant cells have cell walls that prevent lysing (bursting) of cells. The pressure the membranes exert against the cell walls is turgor pressure. This pressure keeps the green parts of the plant turgid. Lose turgor pressure – they become flaccid Lose more water - plasmolysis

Facilitated diffusion – transport protein either provides channel or binds and bends to allow molecules through (faster than they would otherwise) – channels may be gated

Active Transport – expenditure of energy to transport against a gradient Sodium-Potassium pump - creates a concentration and electrical charge difference 3 Na+ out and 2 K+ in Glucose pumping into storage cells (liver) Proton pump (H+ ions) – they are pumped to one side of the membrane & create electrochemical gradient. The H+ may diffuse back through specific protein channels.

Endocytosis & Exocytosis Exocytosis – vesicle from Golgi fuses with plasma membrane & releases contents Secretion of hormones, release of neurotransmitters, release of cell wall material, release of digestive enzymes Endocytosis – vesicle forming & bringing substances inward Phagocytosis – pseudopodia engulf visible particles, lysosomes fuse & digest the particle Pinocytosis – pseudopodia engulf liquid