Membrane-Bound:NucleusMitochondriaPeroxisomesLysosomes Endoplasmic Reticulum Golgi apparatus, etc. Nonmembranous:CytoskeletonCentrioles Ribosomes

*Can also be attached to cytoskeleton

ER

Hydrophilichead Hydrophobic tail INTERSTITIAL FLUID CYTOSOL Phospholipid bilayer Hydrophilic region of protein Hydrophobic region of protein

FluidViscous Unsaturated hydrocarbon tails with kinks Saturated hydro- Carbon tails

It keeps the membrane from becoming too solid in colder temperatures, or too fluid in warmer temperatures. Cholesterol

collage of different proteins embedded A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

EXTRACELLULAR SIDE Peripheral proteins Are appendages loosely bound to the surface of the membrane

Membrane structure results in selective permeability A cell must exchange materials with its surroundings, but is highly regulated by the membrane. Hydrophobic molecules Are lipid soluble and can pass through the membrane rapidly Polar molecules Cross the membrane slowly

Passive TransportActive Transport No ATP requiredEnergy required (often ATP) Must go with, or down the concentration gradient (from high to low) Can go against, or up the concentration gradient (from low to high) Simple diffusion (small, uncharged)Primary Active: Uses chemical energy (ATP) Facilitated diffusion (with proteins— channel or carrier) Secondary Active: Uses electrochemical gradient Osmosis (diffusion of water) Water can pass through bilayer on own OR Through aquaporin (channel) proteins

Net loss of water from cell Animal cell will: Plasmolyze (RBCs: “crenate”) No net movement of water Animal cells will: Stay the same shape & size Net gain of water into the cell Animal cells will: Lyse (explode)

EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. Can transport the solute in either direction (net movement down concentration gradient)

Passive transport. Substances diffuse spontaneously down concentration gradients, crossing membrane with no expenditure of energy. Rate of diffusion can be increased by transport proteins in the membrane. Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP. Simple DiffusionFacilitated Diffusion ATP Channel Protein Carrier Protein Protein Pump

Prominent Example: Sodium/ Potassium Pump

P P i Na+ binding stimulates phosphorylation by ATP (ATP hydrolysis). Na + Three cytoplasmic Na + ions bind to sodium-potassium pump protein. K + is released and Na + sites are receptive again; ATP binds; the cycle repeats. Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. Two extracellular K + ions bind to the protein, triggering release of the phosphate group. Loss of the phosphate restores the protein’s original conformation. CYTOPLASM [Na + ] low [K + ] high Na + P ATP Na + P ADP K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ [Na + ] high [K + ] low P P EXTRACELLULAR FLUID

Proton Pump EXTRACELLULAR FLUID + H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Proton pump ATP CYTOPLASM – – – – – + EXTRACELLULAR FLUID

In exocytosis Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents In endocytosis The cell takes in macromolecules by forming new vesicles from the plasma membrane

In phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it. Packaged into vacuole Vacuole fuses with lysosome to digest the material In pinocytosis, the cell “gulps” droplets of extracellular fluid into tiny vesicles. (They want the solutes.) Nonspecific PHAGOCYTOSIS

0.25 µm RECEPTOR-MEDIATED ENDOCYTOSIS Receptor Ligand Coat protein Coated pit Coated vesicle A coated pit and a coated vesicle formed during receptor- mediated endocytosis (TEMs). Plasma membrane Coat protein Specific substances can be acquired in bulk Specific receptor sites exposed on outside of membrane. Extracellular substances (ligands) bind to these receptors, and the “coated pit” forms a vesicle. After this ingested material is liberated from the vesicle, the receptors are recycled.

Tight Junctions Desmosomes Gap Junctions Impermeable junctionsAnchoring junctions bind to adjacent cells like Velcro Allow for intercellular communication Prevent molecules from passing through intercellular space Form internal tension-reducing network of fibers; plaques on surface of membrane attach to protein filaments Allow ions and small molecules to pass through channels formed by connexon protein cylinders Example: Lining of the digestive tractFound in tissues subject to stress like skin; heart muscle Found in electrically excitable tissue (heart; smooth muscle) to synchronize