Chapter 5B: Membrane Structure & Function 1. Membrane Structure 2. Diffusion 3. Membrane Transport
1. Membrane Structure
Plasma Membrane All cells are enclosed by a plasma membrane a selectively permeable barrier to outside Prokaryotic cell Eukaryotic cell Membrane
What are Membranes made of? Hydrophilic head phosphate group Biological membranes consist mainly of phospholipids, with some cholesterol (in animal cell membranes) and a variety of membrane proteins Symbol cholesterol Hydrophobic tails
Phospholipids form a Lipid Bilayer all biological membranes are a lipid bilayer Polar heads face out Hydrophobic tails face in
(extracellular environment) Phospholipid Bilayer with some Cholesterol and Proteins protein phospholipid microfilaments of cytoskeleton cholesterol
The Lipid Bilayer is Fluid Phospholipids, cholesterol, membrane proteins can move freely in the bilayer consistency of the bilayer is like a viscous oil degree of fluidity depends on: 1) temperature 2) types of fatty tails (saturated vs unsaturated) 3) amount of cholesterol
2. Diffusion
Diffusion Diffusion is the spontaneous movement of a substance from higher to lower concentration molecules dissolved in liquid move randomly over time the net effect is equal dispersion of the molecules (provided there is no barrier) aka moving down concentration gradient
Osmosis is the Diffusion of Water solute molecule lower concentration of solute H 2 O higher concentration of solute equal concentration of solute water undergoes a net flow from high to low concentration selectively permeable membrane water molecule net flow of water solute molecule with cluster of water molecules has powerful effects if a barrier is semipermeable large molecules can t diffuse, so water keeps diffusing in
Osmosis can cause cells to swell, shrivel Isotonic solution Hypotonic solution Hypertonic solution Animal cell (A) normal (B) lysed (C) shriveled plasma membrane Plant cell (D) flaccid (E) turgid (F) shriveled (plasmolyzed) Water will diffuse to where it is less concentrated!
3. Membrane Transport
Small-scale Transport Cells accomplish membrane transport on a small scale (molecule by molecule) in 3 basic ways: 1) passive transport (simple diffusion) diffusion directly through the membrane bilayer 2) facilitated diffusion diffusion with the help of specific membrane proteins 3) active transport movement from low to high concentration requires special membrane proteins and energy
Passive Transport across a Membrane small, uncharged molecules can diffuse across a lipid bilayer (membrane) without help e.g. O 2 CO 2 H 2 O
Facilitated Diffusion via Protein Channels allow the diffusion of small polar or charged molecules small molecule each channel is specific for particular ion or molecule protein channel creates a pore that allows only 1 type of molecule to pass
Facilitated Diffusion with the help of Transport Proteins Specific transport proteins bind & transfer specific molecules from high to low concentration transport proteins change shape upon binding of the molecule and release it on other side of membrane
Active Transport Substances can be moved from low to high concentration across membranes via specific protein pumps (requires a source of energy such as ATP)
Large-scale Transport Cells accomplish membrane transport on a large scale (in bulk) in 2 basic ways: 1) exocytosis release of material packaged in membrane vesicles to the outside of a cell 2) endocytosis ingestion of large objects or large amounts of material by enclosing within a membrane vesicle: PINOCYTOSIS PHAGOCYTOSIS RECEPTOR-MEDIATED ENDOCYTOSIS
Exocytosis Fluid outside cell Vesicle Protein Cytoplasm A general process for releasing material from a cell (e.g., release of neurotransmitters).
Pinocytosis Pinocytosis Capture of extracellular fluid in vesicles. Plasma membrane Vesicle Plasma membrane ingestion of fluid = PINOCYTOSIS ( cell drinking )
Phagocytosis EXTRACELLULAR FLUID Phagocytosis ( cell eating ) Capture of large extracellular particles in vesicles. Pseudopodium CYTOPLASM Food being ingested Food or other particle Food vacuole how many single-celled organisms feed (e.g., amoeba) how cells of the immune system destroy invaders
Receptor-mediated Endocytosis Capture of very specific substances in vesicles. Receptor-mediated endocytosis Plasma membrane Receptor Coat protein Coated vesicle Coated pit Specific molecule Coated pit Material bound to receptor proteins receptors bind specific substances, enclosed in vesicles
Key Terms for Chapter 5B plasma membrane, lipid bilayer diffusion, osmosis isotonic, hypertonic, hypotonic passive transport (simple diffusion) facilitated diffusion, active transport transport proteins, protein channels, pumps exocytosis, endocytosis, receptor-mediated end. vesicle, pinocytosis, phagocytosis