Plasma Membrane Rap

Chapter 7Membrane Structure and

Function

① Phospholipids② Cholesterol③ Embedded Proteins④ Glycoproteins⑤ Glycolipids

Makeup of the PM

CARBOHYDRATES

most abundant lipid in the plasma membraneamphipathic molecules, containing

hydrophobic and hydrophilic regionsJ. Singer & G. Nicolson (1972) proposed

that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water (fluid mosaic model)

1) Phospholipids

Plasma Membrane Video

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer

Inside of cytoplasmic layer

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Cytoplasmic layerPlasma membrane

Phospholipids Cont’d

Phospholipids can move within the bilayer

As temperatures cool, membranes become more solid

Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids

Membranes must be fluid to work properly; they are usually about as fluid as salad oil

Phospholipids Cont’d

Ring structure reduces membrane fluidity at moderate temperatures by reducing phospholipid movementlow temperatures hinders solidification by disrupting the

regular packing of phospholipids.warm temperatures (such as 37°C), cholesterol restrains

movement of phospholipids

Cholesterol is known as “temperature buffer”.

Cholesterol

(c) Cholesterol within the animal cell membrane

2) Cholesterol

collage of different proteins embedded in the fluid matrix of the lipid bilayerProteins determine most of the membrane’s

specific functionsThe two sides of a membrane have

different protein and lipid compositions.

3) Proteins

• Peripheral proteins are bound to the surface of the membrane Not embedded• Loosely bound to

surface of membrane

• Integral proteins penetrate the hydrophobic core• transmembrane proteins span

the membrane• hydrophobic regions consist of

one or more stretches of nonpolar amino acids, often coiled into alpha helices

• Hydrophilic regions exposed to inside/outside of cell

Proteins Cont’d

Transport proteins allow passage of large hydrophilic (polar) substances and ions across the membrane (FACILITATED DIFFUSION)1) channel proteins have a hydrophilic channel

that certain molecules or ions can use as a tunnelChannel proteins called aquaporins facilitate the

passage of water

2) carrier proteins bind to molecules and change shape to shuttle them across the membraneA transport protein is specific for the substance it

moves

Proteins Cont’d

Small, uncharged polar and nonpolar molecules can freely pass through a cell membraneHydrophobic (nonpolar) molecules, such as

hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidlyHydrocarbons, CO2 and O2

Polar molecules, such as sugars, do not cross the membrane easilyC6H12O6, or charged molecules

The Permeability of the Lipid Bilayer

Six major functionsTransportEnzymatic

activitySignal

transductionCell-cell

recognitionIntercellular

joiningAttachment to

the cytoskeleton and extracellular matrix (ECM)

Proteins Cont’d

The Role of Membrane Carbohydrates in Cell-Cell Recognition

Cells recognize each other by binding to surface molecules, usually carbohydrates

Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Carbohydrates are the third major component of plasma membranes found on the exterior surface of cells

bound either to proteins (forming glycoproteins)or bound to lipids (forming glycolipids)

Chains can be either straight or branchedForm specialized sites on the cell surface that allow

cells to recognize each other. unique patterns that allow the cell to be recognized

allows the immune system to differentiate between body cells (called “self”) and foreign cells or tissues (called “non-self”).

Similar types of glycoproteins and glycolipids are found on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them.

4) Glycoproteins & 5) Glycolipids

Glycoproteins & Glycolipids Cont’d

Substances diffuse down their concentration gradient (the difference in concentration of a substance from one area to another)From an area of high concentration to low

concentrationNo work must be done to move substances down

the concentration gradientO2 gets into cells this way for cellular respiration

Diffusion is the tendency for molecules to spread out evenly into the available space

At dynamic equilibrium, as many molecules cross one way as cross in the other directionThis is not static – even at equilibrium molecules are still

moving.

Passive Transport:

Molecules of dye

Fig. 7-11a

Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

(b) Diffusion of two solutes

Fig. 7-11b

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

The diffusion of one solute is unaffected by the diffusion of another solute.

Osmosis is the diffusion of water across a selectively permeable membraneThe direction of osmosis is determined only by a

difference in total solute concentration. Water diffuses across a membrane from the

region of lower solute concentration to the region of higher solute concentrationor you can think [high water] to [low water]

Passive Transport Cont’d

Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Tonicity is the ability of a solution to cause a cell to gain or lose waterIsotonic solution: Solute concentration is the

same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Passive Transport Cont’d

Fig. 7-13

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

Hypertonic or hypotonic environments create osmotic problems for organisms

Osmoregulation, the control of water balance, is a necessary adaptation for life in such environmentsEx) The protist Paramecium, which is hypertonic

to its pond water environment, has a contractile vacuole that acts as a pump

Passive Transport Cont’d

Cell walls of plants, bacteria, fungi, algae, and some archaea help maintain water balanceplant cell in a hypotonic solution cell swells

turgid (firm)plant cell and its surroundings are isotonic

there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

Plant cell in hypertonic environment cells lose water; eventually, the membrane pulls

away from the wall, a usually lethal effect called plasmolysis

Passive Transport Cont’d

In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membraneMost transport proteins are very specific2 types of transport proteins

Channel proteins and carrier proteins

Passive Transport Cont’d: Facilitated Diffusion

Channel proteins provide corridors that allow a specific molecule or ion to cross the membraneAquaporins, for facilitated diffusion of water –

speed up osmosisIon channels that open or close in response to

a stimulus (gated channels)

Passive Transport Cont’d: Facilitated Diffusion via Channel Proteins

Carrier proteins transport substances out of or into the cell by facilitated diffusion and active transport.Each designed to recognize only ONE substance or

ONE group of very similar substances. Diffusion of sugars, amino acids, nucleoside. Uptake of glucose. Transportation of salts, glucose, and amino acids

Passive Transport Cont’d: Facilitated Diffusion via Carrier Proteins

undergo a subtle change in shape that translocates the solute-binding site across the membrane

Substances diffuse against their concentration gradientFrom an area of low concentration to high

concentrationWork must be done to move substances from low

to highperformed by specific proteins embedded in the

membranesAllows cells to maintain concentration gradients that

differ from their surroundingsEx) sodium-potassium pump

Active Transport

Figure 5-14 Mechanism of the Na+-K+-ATPase

ADP

ATP

ATPase isphosphorylated

with Pi from ATP.

Protein changesconformation.

ICF

ECF

Protein changesconformation.

3 Na+ fromICF bind

3 Na+ releasedinto ECF

2 K+ fromECF bind

2 K+ releasedinto ICF

1

25

4 3

Membrane potential is the voltage difference across a membraneVoltage is created by differences in the

distribution of positive and negative ionsCells have a net negative internal charge

Active Transport Cont’d:Ion Pumps

Figure 5-31c Potassium equilibrium potential

Concentrationgradient

Electricalgradient

(c)

Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:A chemical force (the ion’s concentration

gradient)An electrical force (the effect of the membrane

potential on the ion’s movement)

An electrogenic pump is a transport protein that generates voltage across a membraneEX: Sodium-Potassium pump – major electrogenic

pump of animals3Na+out 2K+ in = overall1 positive charge to

the extracellular fluidThe main electrogenic pump of plants, fungi,

and bacteria is a proton pumpThese pumps generate voltage across

membranes which stores energy for use for the cell

Large molecules cross the membrane in bulk via vesiclespolysaccharides and proteins

Bulk transport requires energy

Active Transport Cont’d:Bulk transport: exocytosis and endocytosis

Transport vesicles migrate to the membrane, fuse with it, and release their contents

Many secretory cells use exocytosis to export their productsPancreatic beta cells releasing insulinNeurons releasing neurotransmitersPlants making cell walls

Exocytosis

Cell takes in macromolecules by forming vesicles from the plasma membranereversal of exocytosis, involving different

proteinsThere are three types of endocytosis:

Phagocytosis (“cellular eating”)Pinocytosis (“cellular drinking”)Receptor-mediated endocytosis

Endocytosis

Figure 5-20 Phagocytosis

Bacterium

Lysosome

Phagocyte

The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.

The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.

The phagosome containingthe bacterium separatesfrom the cell membrane andmoves into the cytoplasm.

The phagosome fuses withlysosomes containing digestive enzymes.

The bacterium is killedand digested within thevesicle.

1

2

3

4

5

In phagocytosis (cellular eating) a cell engulfs a particle in a vacuoleThe vacuole fuses with a lysosome to digest the

particleIn pinocytosis,( cellular drinking) molecules are

taken up when extracellular fluid is “gulped” into tiny vesicles

In receptor-mediated endocytosis, (picky eater) binding of ligands to receptors triggers vesicle formationA ligand is any molecule that binds specifically

to a receptor site of another molecule

Figure 5-21 Receptor-mediated endocytosis and exocytosis

Ligand binds to membrane receptor.

Clathrin-coatedpit

Receptor

Extracellular fluid

Intracellular fluid

To lysosome orGolgi complex

Receptor-ligand migrates to clathrin-coated pit.

Endocytosis

Vesicle losesclathrin coat.

Ligands go to lysosomesor Golgi for processing.

Transport vesiclewith receptors moves to the cell membrane.

Transport vesicleand cell membranefuse (membranerecycling).

Exocytosis

Clathrin

Endosome

1

2

3

4

5

6

7

8

9

Receptorsand ligands separate.

SUMMARY OF TRANSPORT

1. Define the following terms: amphipathic molecules, aquaporins, diffusion

2. Explain how membrane fluidity is influenced by temperature and membrane composition

3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions

You should now be able to:

4. Explain how transport proteins facilitate diffusion

5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps

6. Explain how large molecules are transported across a cell membrane

You should now be able to: