The Plasma Membrane

Membrane Transport

Figure 5.1

The Fluid Mosaic Model

• Phospholipids- the main “fabric”– amphipathic= they have both hydrophilic AND a

hydrophobic regions• Proteins- embedded in the phospholipid

membrane– Also amphipathic– Determine the function of the membrane– Proteins are not distributed randomly or evenly,

but rather according to function

How fluid is fluid?

• The membrane is held together my hydrophobic interactions-weaker than covalent bonds

• Constant lateral movement• Proteins larger than lipids therefore move

more slowly

Viscosity

• A measure of a fluid’s resistance to flow; how “thick” or “sticky” it is

• Due to molecular makeup and internal friction• Honey is more viscous than water

What determines a membrane’s viscosity?

• Hydrocarbon tails on its phospholipids– Saturated- more viscous– Unsaturated- less viscous, more fluid

• Temperature– Decrease in temp more viscous; may eventually

solidify – Increase in temp less viscous; too fluid, cannot

support proteins and their function

What determines a membrane’s viscosity?

• Cholesterol- helps membranes resist changes in fluidity with changes in temperature – High temps- restricts movement of phospholipids– Low temps- prevents phospholipids from packing

together

• Evolution– Membrane composition evolves to meet specific

environmental needs• Cold

Figure 5.5

Fluid

Unsaturated tails preventpacking.

Cholesterol

Viscous

Saturated tails packtogether.

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol reducesmembrane fluidity atmoderate temperatures,but at low temperatureshinders solidification.

What determines a membrane’s viscosity?

• Evolution– Membrane composition evolves to meet specific

environmental needs• Cold water fish• Archea that live at 90°C (194°F)• Some alter their composition seasonally

Membrane Proteins and Their Functions

• The proteins within the phospholipid bilayer determine the function of the membrane.

• Different cells different membrane proteins• Different organelles with a specific cell

different membrane proteins

Two Major Types of Proteins

• Integral

• Peripheral

• Can you see the difference?

Figure 5.2

Glyco-protein

Glycolipid

Fibers of extra-cellular matrix (ECM)

Carbohydrate

Cholesterol

Microfilamentsof cytoskeleton

Peripheralproteins Integral

protein

EXTRACELLULARSIDE OF

MEMBRANE

CYTOPLASMIC SIDE OF MEMBRANE

Integral

• Penetrates the membrane– Transmembrane- through to both surfaces– Partially embedded- only exposed on one surface

• The embedded portions have hydrophobic amino acids, often in an α helix

• Some have hydrophilic channels through them to allow for passage of substances through the membrane

Figure 5.6

N-terminus

C-terminus

helixCYTOPLASMICSIDE

EXTRACELLULARSIDE

Peripheral

• Not embedded• Bound to either surface

– Extracellular matrix (outside)– Cytoskeletal elements (inside)

• Provide extra support for the membrane

Figure 5.2

Glyco-protein

Glycolipid

Fibers of extra-cellular matrix (ECM)

Carbohydrate

Cholesterol

Microfilamentsof cytoskeleton

Peripheralproteins Integral

protein

EXTRACELLULARSIDE OF

MEMBRANE

CYTOPLASMIC SIDE OF MEMBRANE

6 Major Functions of Plasma Membrane Proteins

1. Transport2. Enzymatic activity3. Attachment to the cytoskeleton and ECM4. Cell-cell recognition5. Intercellular joining6. Signal transduction

Figure 5.7

Signalingmolecule

(a) Transport (b) Enzymatic activity

ATP

(c) Attachment to thecytoskeleton and extra-cellular matrix (ECM)

Receptor

(f) Signal transduction(e) Intercellular joining(d) Cell-cell recognition

Glyco-protein

Enzymes

Membrane Carbohydrates• Cell-cell recognition• Can be covalent bound to either lipids or

proteins on the extracellular side of the membrane– Glycoproteins– Glycolipids

• Act as markers to distinguish cells– Ex. ABO blood types

Membrane Synthesis

• Proteins and lipids- ER • Carbohydrates added –Golgi

Selective permeability

• 2 aspects of “selectivity”– The membrane takes up some small ions and

molecules, but not others– Substances that are allowed through, do so at

different rates

• How does the membrane accomplish this selectivity?

Figure 5.3

Hydrophobictail

Hydrophilichead

WATER

WATER

Form Follows Function

• Nonpolar substances= hydrophobic– Cross easily– Ex. Hydrocarbons, CO2 ,O2

• Ions & polar substances= hydrophilic– Hard to pass– Ex. Glucose, H2O, Na+, Cl-

– Ions especially have a hard time as they tend to be surrounded by a “shell” of water molecules

Transport Proteins

•Channel proteins vs. carrier proteins

– Channel proteins create a channel through which hydrophilic substances may pass. Ex. Aquaporins

– Carrier proteins hold onto substances, change shape and redeposit them on the other side

Figure 5.14

[K] high

EXTRACELLULARFLUID

CYTOPLASM

[Na] low

[K] low[Na] high

ADP1 2

3

45

6

Directionality of transport• Controlled by

– Passive transport• Diffusion• Osmosis• Facilitated diffusion

– Active transport• Ion pumps, membrane potential• Cotransport

– Bulk transport• Exocytosis• Endocytosis

Active transport

• Moves substances against their gradient; from an area of low concentration to one of high concentration

• Requires energy- supplied by ATP• Allows cells to maintain a different

environment inside vs. outside the cell

An example is the sodium- potassium pump

Figure 5.14a

[K] high

EXTRACELLULARFLUID

CYTOPLASM

[Na] low

[K] low

[Na] high

21 Na binding stimulatesphosphorylation by ATP.

Cytoplasmic Na bindsto the sodium-potassiumpump. The affinity for Na

is high when the proteinhas this shape.

ADP

The new shape has ahigh affinity for K, whichbinds on the extracellularside and triggers releaseof the phosphate group.

Figure 5.14b

43 Phosphorylation leadsto a change in proteinshape, reducing its affinityfor Na, which is releasedoutside.

Figure 5.14c

65 Loss of the phosphategroup restores the protein’soriginal shape, which has alower affinity for K.

K is released; affinityfor Na is high again, andthe cycle repeats.

Ion pumps maintain voltage across membranes

• Membrane potential= the voltage across a membrane

• Cytoplasmic side relatively negative• Creates electrical potential energy that drives

passive transport of cations into the cell and anions out

• Electrochemical gradient= chemical (concentration gradient) and electrical forces that drive diffusion across membranes

Main electrogenic pumps

• Animals-– Sodium-potassium pump

• Plants-– Proton pump

Figure 5.16

EXTRACELLULARFLUID

CYTOPLASM

Proton pump

Cotransport

• A process by which one protein transports 2 molecules or ions at a time. It uses the diffusion of solute to force the other against it’s gradient.

• It does not use ATP directly, but often is coupled with an ion pump that does use ATP

Figure 5.17

Sucrose

Proton pump

Sucrose-H

cotransporterDiffusion of H

Sucrose

Bulk Transport

• Exocytosis• Endocytosis

– Phagocytosis– Pinocytosis– Receptor-mediated endocytosis

Figure 5.18

Phagocytosis Pinocytosis Receptor-MediatedEndocytosis

ReceptorPlasmamembrane

Coatprotein

Coatedpit

CoatedvesicleFood

vacuole

“Food”or otherparticle

CYTOPLASM

Pseudopodium

Solutes

EXTRACELLULARFLUID