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 The Fluid MosaicFluid Mosaic Model 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 proteinFigure 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 movementFigure 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, hypotonicFigure 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 proteinFigure 5.9

diffusion through a carrier proteinFigure 5.11

uniport, symport, antiportFigure 5.12

direct active antiport systemFigure 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 systemFigure 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

endocytosis1. receptors bind target molecules2. clathrin coats the inside of the membrane

3 & 4. a vesicle, surrounded by clathrin, contains the target moleculeFigure 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