Cell Membranes Biological Barriers Gate Keepers

Biological Membranes composition –phospholipids & other membrane lipids (~50% by mass) –various proteins (~50% by mass)

Cross section of phospholipid bilayer Figure 5.2

Biological Membranes functions –phospholipid bilayer cell, organelle boundary barrier to hydrophilic compounds fluid medium for membrane proteins The Fluid Mosaic Model

Biological Membranes functions –phospholipid bilayer cell, organelle boundary barrier to hydrophilic compounds fluid medium for membrane proteins –proteins provide selective permeability process materials, energy & information

Biological Membranes functions –carbohydrates oligosaccharides signaling molecules on outer surface attached to proteins, lipids added in ER, Golgi few monomers, distinct branching patterns

Biological Membranes variations –lipids fatty acid composition determines fluidity –short unsaturated –> more fluid –long, saturated –> less fluid –composition changes with conditions

integral protein Figure 5.4

Biological Membranes variations –proteins integral (embedded), or peripheral (associated) asymmetrical distribution –inner & outer layer compositions differ

Freeze- Fracture Technique to study integral membrane proteins Figure 5.3

one type of protein reversibly binds red sponge cells Figure 5.5

Biological Membranes cell adhesion –membrane proteins bind adjacent cells impermanent permanent

tight junctions prevent leaks, protein migration gap junctions form small hydrophilic channels Figure 5.6

tight junction gap junction connexons desmosomes Figure 5.6

Membrane Transport Processes passive transport - diffusion –properties of diffusion in solution each molecule moves randomly diffusion is net directional movement –from higher concentration to lower concentration –independent of other particles =>Down a Concentration Gradient<=

diffusion: net directional movement Figure 5.7

Membrane Transport Processes properties of diffusion in solution –rapid over short distances –organelle length ~ 1 millisecond –centimeter > 1 hour –meter years

Membrane Transport Processes Osmosis –diffusion of solvent across a membrane from higher concentration to lower concentration (of solvent) = down a concentration gradient two solutions divided by a membrane –isotonic –hypertonic & hypotonic

solutions: hyper, iso, hypotonic Figure 5.8

Membrane Transport Processes simple diffusion across a membrane –direction & rate determined by concentration gradient facilitated diffusion across a membrane –direction determined by concentration gradient –rate determined by concentration gradient, and availability of channel or carrier proteins

diffusion through a gated channel protein Figure 5.9

diffusion through a carrier protein Figure 5.11

uniport, symport, antiport Figure 5.12

direct active antiport system Figure 5.13

Membrane Transport Processes active transport –moves particles up a concentration gradient –involves carrier proteins uniport: one solute, one direction symport: two solutes, same direction antiport: two solutes, opposite directions –requires energy direct indirect (secondary active transport)

indirect active symport system Figure 5.14

Membrane Transport Processes endocytosis imports macromolecules –plasma membrane folds inward, encloses particles –infolding forms a vesicle

import, export at the plasma membrane Figure 5.15

Membrane Transport Processes endocytosis imports macromolecules –phagocytosis - engulfs entire cells –pinocytosis - nonspecific uptake of small particles –receptor-mediated endocytosis highly specific uptake of small particles –external receptor proteins in pits –internal protein, clathrin, coats the infolding membrane

receptor-mediated endocytosis 1. receptors bind target molecules 2. clathrin coats the inside of the membrane 3 & 4. a vesicle, surrounded by clathrin, contains the target molecule Figure 5.16

a cell responds to information about its environment Figure 5.17 membranes are required for efficient energy production some chemical pathways require “anchored” enzymes

Membrane Transport Processes other membrane functions –information processing signal transduction –energy transformation photochemically driven red-ox driven –organizing enzymatic pathways

Membrane Transport Processes membrane maintenance –transport vesicles become part of target membranes

dynamic membrane activity