CELL MEMBRANE STRUCTURE & FUNCTIONS

PASSIVE DIFFUSION, OSMOSIS, FACILITATED DIFFUSION AND

ACTIVE TRANSPORT

Hydrophilichead

WATER

Hydrophobictail

WATER

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

(a) Movement of phospholipids

(b) Membrane fluidity

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

Cell Membrane Structures

• Recognition Factors• Antenna that capture required

solutes/hormnones• Amphipathic nature of the phospholipids and

its consequences• Integral Proteins

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

MEMBRANE PROTEINS

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

Importance

1. Acquire molecules & Ions

2.Transport into and out of cell through membrane

3.Transport WITHIN the cell

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

Cell Membrane Function

Problems and Solutions1. Relative concentrations

a. Passive Transportb. Active Transport

2. Lipid bilayers are impermeable to most essential molecules and ions

a. permeable to:

Cell membrane function, con’t

b. impermeable to:---

DIFFUSION

SIMPLE DIFFUSION REQUIRES A CONCENTRATION GRADIENT

FACILITATED DIFFUSION REQUIRES A CONCENTRATION GRADIENT AND A PROTEIN

TRANSPORTER

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

(b) Diffusion of two solutes

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

Relate to Lab: Glucose, Starch. NaCl, Proitein

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

ACTIVE TRANSPORT

REQUIRES A PROTEIN TRANSPORTER AND ATP ENERGY

Active transport:

ATP

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

REVIEW Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Osmosis

• Passive Diffusion of WaterDissociation of ions in solutionAquaporinsOsmotic concentrationFree EnergyWater Potential

ENVIRONMENT 99% water

Cell Interior:

Less than 1% solute

Tonicity of cell IN COMPARISON TO the environment:

The cell is ________in comparison to the environment. Hence, water will move from _____________________ to ____________.

The results:

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Cell Environment

Osmotic or Solute Potential -

Free Energy

Osmotic Potential

Osmotic Pressure

Water Potential Ψ

Environment with greater than 10% SOLUTE (less than 90 % water)

Cell with less than 1 % solute

Cell with less than 1 % SOLUTE (freshwater cell

with more than 99% water)

Environment:0.01 M sucrose

0.01 M glucose

0.01 M fructose

“Cell”

0.03 M sucrose

0.02 M glucose

Lowerconcentrationof solute (sugar)

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Additional transport Mechanisms

• Bulk Transport• Endocytosis

– Phagocytosis– Pinocytosis

• Receptor-mediated endocytosis

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedpit

Ligand

Coatprotein

Plasmamembrane

0.25 µm

Coatedvesicle

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)