Membrane Structure and Function

Chapter 7A. P. Biology

Plasma Membrane• Also called the plasmalemma.• Plasma Membrane =

Phospholipid Bilayer + Transmembrane Proteins + Supporting Fibers + Glycoproteins and Glycolipids

Scientists studying the plasma membrane– Reasoned that it must be a phospholipid

bilayer

Figure 7.2

HydrophilicheadHydrophobictail

WATER

WATER

Phospholipid Bilayer• Glycerol + 2 Fatty Acids +

Phosphorylated Alcohol = Phospholipid• Hydrophilic or Polar Region =

Phosphate• Hydrophobic or Nonpolar region = Fatty

Acids

The Davson-Danielli sandwich model of membrane structure

–Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers.

–Was supported by electron microscope pictures of membranes

In 1972, Singer and Nicolson– Proposed that membrane proteins are dispersed

and individually inserted into the phospholipid bilayer

Figure 7.3

Phospholipidbilayer

Hydrophobic region of protein

Hydrophobic region of protein

Freeze-fracture studies of the plasma membrane– Supported the fluid mosaic model of membrane structure

Figure 7.4

A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

Extracellular layer Cytoplasmic layer

APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior.

TECHNIQUE

Extracellularlayer

Proteins

Cytoplasmic layer

Knife

Plasmamembrane

These SEMs show membrane proteins (the “bumps”) in the two layers, demonstrating that proteins are embedded in the phospholipid bilayer.

RESULTS

Lipid Bilayer• Nonpolar interior prevents passage of

water-soluble, polar compounds.• Only very small, uncharged molecules

like O2 and H2O can enter through the lipid bilayer.

• Also, allows nonpolar compounds to freely enter.

The Fluidity of Membranes• Phospholipids in the plasma membrane

– Can move within the bilayer

Figure 7.5 A

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

(a) Movement of phospholipids

The type of hydrocarbon tails in phospholipids– Affects the fluidity of the plasma membrane

Figure 7.5 B

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-Carbon tails

(b) Membrane fluidity

The steroid cholesterol– Has different effects on membrane fluidity at

different temperatures

Figure 7.5 (c) Cholesterol within the animal cell membrane

Cholesterol

Figure 7.7

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton Cholesterol Peripheral

proteinIntegral

proteinCYTOPLASMIC SIDE

OF MEMBRANE

EXTRACELLULAR

SIDE OFMEMBRANE

Glycolipid

Membrane Proteins and Their Functions• A membrane

– Is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

Fibers of

extracellularmatrix (ECM)

Lipid Bilayer is Fluid• Fluid = Moving, Dynamic.• Each lipid can rotate, move laterally• Fluidity depends on temperature and

type of fatty acid used.• Unsaturated fatty acids are more fluid.• Fluid Mosaic Model

Transmembrane Proteins (Integral Proteins)

• Part of the protein that extends through the bilayer is nonpolar (several nonpolar amino acids in this region).

• Usually is an alpha helix or beta barrel.• Used to anchor protein in the membrane.• Beta-barrels = form a pore and are called

a porin protein.

Integral proteins– Penetrate the hydrophobic core of the lipid bilayer– Are often transmembrane proteins, completely

spanning the membrane

EXTRACELLULARSIDE

Figure 7.8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

Extracellular Side

• An overview of six major functions of membrane proteins

Figure 7.9

Transport

Enzymatic activity

Signal transduction

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

Cell-cell recognition

Intercellular joining

Attachment to the cytoskeletonand extracellular matrix(ECM).

(d)

(e)

(f)

Glyco-protein

Figure 7.9

Transmembrane Proteins• Channels = passive transport of molecules across

membrane.• Carriers = transport of molecules against the gradient.• Receptors = transmit information into the cell.• Cell Adhesion Proteins = connect cells to each

other.• Cytoskeleton Attachment Proteins = to attach actin.

Membrane Receptors Conduct Signals

Integral Proteins Laterally Diffuse in the Membrane

Movement Across the Membrane

• Diffusion = random motion of molecules that causes a net movement from areas of high concentration to areas of low concentration.

• Osmosis = diffusion of water across a selectively permeable membrane.

Passive Diffusion

Factors that Affect the Direction of Diffusion

• The concentration gradient; High Low.

• Temperature; High heat Low Heat.

• Pressure; High Pressure Low Pressure

Factors that Affect the Rate of Diffusion

• The steepness of the gradient.

• The molecular weight of the solute.

Concentrations• Osmotic concentrations = concentrations of all

solutes in a solution.• If unequal concentrations…

Hyperosmotic = solution with the higher solute concentration.Hypoosmotic = solution with the lower solute concentration.Isosmotic = solutions with the same osmotic or solute concentration.

Plasmolyzes

Crenate

Osmotic Pressure• If a cell’s cytoplasm is hyperosmotic to

the extracellular fluid, then water diffuses into the cell and it swells. Pressure of the cytoplasm pushing out against the membrane- hydrostatic pressure.

• Osmotic pressure is the pressure needed to stop the osmotic movement of water across a membrane.

How do living things maintain osmotic balance?

• Some oceanic eukaryotes adjust internal [solutes]- they are isosmotic.

• Animals – circulate an isosmotic fluid around their cells. Must constantly monitor the fluid’s [solute] Ex. Humans secrete albumin into the plasma to match the body cells.

• Protozoa- are hyperosmotic, so use extrusion to remove excess water; may have special organelles-contractile vacuoles.

• Plants- are hyperosmotic, but do not circulate an isosmotic solution; are usually under osmotic pressure- turgor pressure-presses the plasma membrane against the cell wall.

• Water balance in cells with walls

Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.

(b)

H2OH2OH2OH2O

Turgid (normal) Flaccid Plasmolyzed

Figure 7.13

Bulk Movement through Membranes

• Endocytosis- the cytoskeleton extends the membrane outward toward food particles. Bulk transport into cell.

• Extended membrane encircles the particle, fuses with itself, and contracts.

• Forms a vesicle around particle.

Three Kinds of Endocytosis• Phagocytosis - “cell eating”- large, amounts of

organic material;white blood cells and protists.• Pinocytosis- “cell drinking”- liquid material

brought into cell; mammalian ova and follicle cells.

• Receptor-mediated Endocytosis- use receptors in the membrane for specific transport into cell.

Receptor-Mediated Endocytosis• Have indentations on the plasma membrane.• Indentations = are clathrin-coated pits.• Pits have receptor proteins on the extracellular side

= trigger• When receptor binds to target molecule, clathrin

proteins on the cytoplasmic side begins endocytosis.

• Forms a clathrin -coated vesicle. • Very specific.

Receptor-mediated Endocytosis

Exocytosis• The release of material from vesicles

at the cell surface.• Examples: protists using a

contractile vacuole to release water, gland cells secreting hormones, neurons releasing neurotransmitters.

Secretion Vesicles

Exocytosis and Neurons

Problems with Bulk Transport

• Endocytosis and Exocytosis are energy-intensive.

• Not highly selective.

Channel \Proteins– Provide corridors that allow a specific

molecule or ion to cross the membrane

Figure 7.15

EXTRACELLULARFLUID

Channel proteinSolute

CYTOPLASM

A channel protein (purple) has a channel through which water molecules or a specific solute can pass.

(a)

Selectively Permeable Transport• Channels- proteins in the cell

membrane that transport specific ions into and out of the cell.

• Water-filled pores span the membrane. • Ions do not interact with the channel

protein. • Diffusion is passive, [high] --> [low].

Channels are somewhat selective

Selectively Permeable Transport

• Carriers- proteins that transport specific ions, sugars, and amino acids into and out of the cell.

• The proteins facilitate movement by binding to the solute-facilitated diffusion.

• The proteins bind to the solute on one side of the membrane and release them on the other.

Uniport Facilitated Diffusion

Facilitated Diffusion• It is specific, only certain

molecules transported by a given carrier.

• It is passive, net movement is [high]-->[low].

• It may become saturated if all protein carriers are occupied.

Active Transport• A method of transporting specific ions,

sugars, amino acids, nucleotides against conc. gradient.

• Involves protein carriers in the membrane and energy (ATP).

• How cells accumulate molecules internally.

Active Transport

The Sodium-Potassium PumpIs one type of active transport system

An Example of Active Transport• Sodium-Potassium Pump- Active

transport of Na+ and K+ ions.• Normally, inside the cell:

the [Na+] is low the [K+] is high

• The cell maintains this by actively pumping Na+ out and K+ in.

Sodium-Potassium Pump• The protein uses ATP as an energy source for

this movement against the gradient [low]--> [high].

• See Fig. 6.21, p. 135 for how.• Uses ~1/3 of all ATP in resting cell.• This pump can transport 300 Na+ ions/second.• All animals use it.

Cotransport: active transport driven by a concentration gradient

Figure 7.19

Proton pump

Sucrose-H+

cotransporter

Diffusionof H+

Sucrose

ATP H+

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+–

–

–

–

–

–

Cotransport and Countertransport• Many amino acids and sugars are

transported into the cell through coupled channels.

• Their active transport is coupled with the movement of Na+ inside the cell.

• [Na+] high --> [Na+] low into cell.• [Amino acid] low--> [Amino acid] high.

An Electrogenic Pump– Is a transport protein that generates the voltage

across a membrane

Figure 7.18

EXTRACELLULARFLUID

+

H+

H+

H+

H+

H+

H+Proton pump

ATP

CYTOPLASM

+

+

+

+–

–

–

–

–

+

The Proton Pump• A transmembrane protein that moves H+

against their concentration gradient, from [low] --> [high] outside of cells or organelles.

• Example: mitochondria move H+ across the inner membrane during electron transport.

• Energy to power this pump comes from NADH and FADH molecules.

Proton Pump• The proton pump moves H+ out of the matrix,

through the inner membrane.• ATP synthase channels H+ back into the matrix,

DOWN the gradient. PROVIDES ENERGY! • ATP synthesis is coupled to H+ movement.• Almost all of the energy for cells is made this

way.

Electron Transport Chain