AP Bio - Unit 2 Part 3: Cell Membrane & Transport

Cell Membrane

Plasma membrane is selectively permeable, Fluid Mosaic Model

Selectively Permeable

Allows some substances to cross more easily than others

Fluid Mosaic Model

Fluid; membrane held together by weak interactions, each molecule acts independently | Mosaic; phospholipids, proteins, carbs

Phospholipids

Bilayer, Amphipathic = hydrophilic head, hydrophobic tail, Hydrophobic barrier; keeps hydrophilic molecules out

Membrane fluidity - Low temps

phospholipids w/unsaturated tails (kinks prevent close packing)

Membrane fluidity - Cholesterol resists changes by

limits fluidity at high temps, hinders close packing at low temps

Membrane fluidity - Adaptations

bacteria in hot springs (unusual lipids); winter wheat (🡩 unsaturated phospholipids)

Membrane Integral Proteins

Embedded in membrane, Determined by freeze fracture, Transmembrane with hydrophilic heads/tails and hydrophobic middles, Involved in transmembrane transport and communication

Membrane Peripheral Proteins

Extracellular or cytoplasmic sides of membrane, NOT embedded, Held in place by the cytoskeleton or ECM, Provides stronger framework

Carbohydrates

Function; cell-cell recognition; developing organisms, Glycolipids, glycoproteins, Eg. blood transfusions are type-specific

How is it selectively permeable

Small molecules (mostly nonpolar) cross easily (hydrocarbons, hydrophobic molecules, CO2, O2, while Hydrophobic core prevents passage of ions, large polar molecules

Passive Transport

NO ENERGY needed, Diffusion down concentration gradient (high 🡪 low concentration), Eg. hydrocarbons, CO2, O2, H2O

Osmosis

diffusion of H2O

External environments can be

hypotonic, isotonic or hypertonic compared to internal environments of cell

Osmoregulation

Control solute & water balance Contractile vacuole, Eg. paramecium caudatum – freshwater protist

Contractile vacuole

“bilge pump” forces out fresh water as it enters by osmosis

Water Potential

Water moves from solutions with a high water potential (hypotonic) to areas of low water potential (hypertonic).

Facilitated Diffusion

Transport proteins (channel or carrier proteins) help hydrophilic substance cross, (1) Provide hydrophilic channel or (2) loosely bind/carry molecule across, Eg. ions, polar molecules (H2O, glucose)

Aquaporin

channel protein that allows passage of H2O

Active Transport

Requires ENERGY (ATP), Proteins transport substances against concentration gradient (low 🡪 high conc.), Eg. Na+/K+ pump, proton pump

Electrogenic Pumps

generate voltage across membrane

Na+/K+ Pump (electrogenic pump)

Pump Na+ out, K+ into cell, Nerve transmission

Proton Pump (electrogenic pump)

Push protons (H+) across membrane, Eg. mitochondria (ATP production)

Cotransport

membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other, Eg. sucrose-H+ cotransporter (sugar-loading in plants)

Passive (vs. Active Transport)

Little or no Energy, High 🡪 low concentrations, DOWN the concentration gradient, eg. diffusion, osmosis, facilitated diffusion (w/transport protein)

Active Transport (vs. Passive transport)

Requires Energy (ATP), Low 🡪 high concentrations, AGAINST the concentration gradient, eg. pumps, exo/endocytosis

Bulk Transport

Transport of proteins, polysaccharides, large molecules, eg. Endocytosis, Exocytosis

Endocytosis

take in macromolecules, form new vesicles

Exocytosis

vesicles fuse with cell membrane, expel contents

Types of Endocytosis

Phagocytosis, Pinocytosis, Receptor-Mediated Endocytosis

Phagocytosis

“cellular eating” - solids

Pinocytosis

“cellular drinking” - fluids

Receptor-Mediated Endocytosis

Ligands bind to specific receptors on cell surface