Microsoft Power Point - Hmm1414c3_part2

3.2 BIOLOGICAL MEMBRANES

3.2.1 Properties of Cell Membranes • Separates living cell from its nonliving surroundings. • 8 nm thick. • Selectively permeable - allows some substances to cross more easily than others. HMM/SCM1414

3.2.2 Fluid Mosaic Model • Singer and Nicolson (1972) - the plasma membrane is a mosaic of proteins dispersed within the lipid bilayer, with only the hydrophilic regions exposed to water.

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Hydrophilic region of protein

Phospholipid bilayer

Hydrophobic region of protein

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• Plasma membrane is a continuous, fluid, double layer of phospholipids, the lipid bilayer. • Phospholipids & most other membrane constituents are amphipathic molecules have hydrophobic regions & hydrophilic regions.
Hydrophobic tails face inside of bilayer.
Hydrophilic head faces exterior (extracellular fluid) and interior (cytosol). HMM/SCM1414

WATER Hydrophilic head Hydrophobic tail

WATER HMM/SCM1414

• Proteins - embedded in bilayer or associated with cytoplasmic or extracellular face. • Carbohydrates - linked to proteins (glycoproteins) or lipids (glycolipids) only on extracellular side. • Cholesterol - lies within membrane.

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• Membrane molecules held in place by weak hydrophobic interactions. • Most lipids & some proteins drift laterally. • Rarely flip-flop from one layer to the other.

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Lateral movement (~107 times per second) Movement of phospholipids

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Flip-flop (~ once per month)

• Many larger membrane proteins drift within the phospholipid bilayer. • Proteins are much larger than lipids and move more slowly. • Other proteins are anchored to cytoskeleton.

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3.2.3 Membrane Components a) Membrane proteins • •

Amphipathic. Determine most of membrane’s specific functions

•

Two groups:

i.

Peripheral proteins
Not embedded but loosely bound to surface of protein. HMM/SCM1414

Fibers of extracellular matrix (ECM)

Glycoprotein

Carbohydrate

Glycolipid EXTRACELLULAR SIDE OF MEMBRANE

Cholesterol Microfilaments of cytoskeleton

Peripheral proteins

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Integral protein

CYTOPLASMIC SIDE OF MEMBRANE

ii. Integral proteins
Penetrate hydrophobic core, often completely as transmembrane proteins. proteins.
Hydrophobic segments consist of stretches of nonnon-polar amino acids, coiled into α-helices.
Hydrophilic segments have hydrophilic nonnon-helical amino acids.

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EXTRACELLULAR SIDE

N-terminus

C-terminus

CYTOPLASMIC SIDE

α Helix HMM/SCM1414

Six major functions of protein 1. 2. 3. 4. 5. 6.

Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

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Signal Enzymes

Receptor

ATP

Transport

Enzymatic activity

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Signal transduction

Glycoprotein

Cell-cell recognition

Intercellular joining

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Attachment to the cytoskeleton and extracellular matrix (ECM)

(b) Carbohydrates • Branched oligosaccharides with < 15 sugar units. • Two types: 1. Glycolipids 2. Glycoproteins. Glycoproteins • Oligosaccharides on external side of membrane vary from species to species, from individual to individual, and from cell type to cell type within same individual. • This variation distinguishes each cell type. HMM/SCM1414

•

•

Carbohydrates on plasma membrane surface enables cell-cell recognition  Ability of a cell to distinguish one type of neighboring cell from another by binding to surface molecules. Importance: 1) Sorting and organizing cells into tissues and organs. 2) Basis for rejection of foreign cells by immune system.

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(c) Cholesterol • Interdigitates between phospholipids. • Enhances mechanical stability & flexibility of membrane, and making it less permeable to water-soluble substances. • In animal cell membranes

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3.3 Movement Across Membranes •

•

Hydrophobic molecules (hydrocarbons, CO2, & O2) dissolve in lipid bilayer & cross easily. Hydrophobic core of membrane impedes passage of ions and polar molecules (water, glucose & other sugars) - cross membrane with difficulty.

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•

Ions and polar molecules cross bilayer through transport proteins: proteins i. Channel proteins: proteins Have hydrophilic channel for passage of certain molecules or ions.  Example, passage of water through membrane facilitated by channel proteins known as aquaporins. aquaporins. ii. Carrier proteins: proteins Bind to molecules & change shape to shuttle them across membrane. HMM/SCM1414

3.3.1 Passive Transport

a) Diffusion  Spontaneous tendency of molecules of any substance to move down its concentration gradient from a more concentrated to a less concentrated area.

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• Individual molecule moves randomly. • But diffusion of a population of molecules exhibit a net movement in one direction. • At dynamic equilibrium, as many molecules cross one way as cross in the other direction. • Each substance diffuses independent of the concentration gradients of other substances.

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Net diffusion Net diffusion

Net diffusion Net diffusion

Diffusion of two solutes

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Equilibrium Equilibrium

• No work done to move substances down concentration gradient. • Diffusion of substance across biological membrane is passive transport - requires no energy from the cell.

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Factors determining rate of diffusion 1) Concentration gradient - the steeper the gradient, the faster the rate of diffusion. 2) Surface area across which substance is diffusing - the greater the surface area, the faster the rate of diffusion. 3) Distance over which substance has to diffuse (the diffusion distance) - the greater the diffusion distance, the slower the rate of diffusion. HMM/SCM1414

• The rate of diffusion is directly proportional to the concentration difference and surface area, and inversely proportional to the diffusion distance.

Rate of diffusion

Surface x Concentration area distance ∝ Diffusion distance

• This is known as Fick’s Law.

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•

Diffusion through membrane (barrier) affected by: i. Nature of membrane, for example, its permeability. ii. Size and type of molecule or ion diffusing through it.

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b)

Osmosis

Diffusion of water through a semipermeable membrane from a solution with a low solute concentration (high water potential) to a solution with a higher solute concentration (low water potential) until there is an equal concentration (water potential) on both sides of the membrane. • Direction of osmosis determined only by a difference in total solute concentration HMM/SCM1414

• Tendency of water molecules to move across membrane depends on: i. Solute concentration. ii. Pressure on each side of membrane. • Water potential (ψ ψ) - combined effect of solute concentration and pressure in a solution.  Units in kilopascals (kPa)

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• Water potential of a solution is the tendency for water to diffuse out of it. • Water diffuses from a high water potential to a low water potential, down its water potential gradient. • By definition, pure water at atmospheric pressure has a water potential of zero.

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Effect of concentration on water potential and osmosis: • Adding solute to pure water will decrease its water potential - becomes negative. • The more solute is added, the lower (more negative) the water potential. • Example: • 17 g sucrose/dm3 of water = -130 kPa • 34 g of sucrose/ dm3 of water = -260 kPa. • Effect of solute concentration is called solute potential (Ψs). • Value of Ψs is always negative. HMM/SCM1414

Lower concentration of solute (sugar)

Higher concentration of sugar

Same concentration of sugar

H2O

Selectively permeable membrane: sugar molecules cannot pass through pores, but water molecules can

Low solute conc.

High solute conc.

High water conc.

Low water conc,

High water potential

Low water potential Osmosis

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The effect of pressure on water potential and osmosis: • Increasing the pressure would increase the tendency of water to diffuse out of the membrane.

High water potential

Low water potential

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Increased pressure

High water potential

Low water potential

There is a substantial movement of water in osmosis

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High water potential

Low water potential

Pressure applied on this side

There is decreased osmosis or osmosis is stopped or even completely reversed

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• Effect of pressure on a solution is called the pressure potential (Ψ Ψp). • Value of Ψp usually positive.  In plants, this is the force of the cell wall pushing inwards on contents of cells (cytoplasm) when water enters cell by osmosis.  In animals this may be due, for example, to high blood pressure in glomerulus of the kidney.

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Water potential = Solute potential + Pressure potential

ψ

=

ψS

+

ψP

(usually negative) (usually negative) (usually positive)

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Effect of osmosis on plant cells • Plant cells generally have lower water potential than that of their surroundings. Due to presence of solutes in fluid within vacuole (cell sap). • Plasma membrane & tonoplast surrounding vacuole are both partially permeable, letting water through but not solutes. • Cell wall permeable to both water & solutes. HMM/SCM1414

• Refer to handout for effect of osmosis on plant cells. • Incipient plasmolysis – point when cytoplasm just starts pulling away from cell wall. • Full plasmolysis – when cytoplasm has completely withdrawn from cell wall except at plasmodesmata.

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Effect of osmosis on animal cells • Refer to handout. • Animals without rigid cell walls have osmotic problems in environment with low or high water potential. • To maintain their internal environment, they must have adaptations for osmoregulation. • Paramecium, which has low water potential compared to its pond water environment, has a contractile vacuole that acts as a pump. HMM/SCM1414

Filling vacuole

Contracting vacuole

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50 µm

50 µm

Worked examples on the water potential concept Example 1 • A plant cell with a water potential of –700 kPa is immersed in a sucrose solution whose water potential is –350 kPa. In which direction will water flow? Answer • Water will flow from the sucrose solution into the cell. This is because the water potential of the cell is lower than (i.e. more negative than) the sucrose solution, and there is a net flow of water from a region of higher water potential to a region of lower potential, i.e., down the water potential gradients. HMM/SCM1414

Worked examples on the water potential concept Example 2 • A plant cell has a solute potential of –240 kPa and a pressure potential of 350 kPa. What is the water potential of the cell? Answer Water potential = Solute potential + Pressure potential (Ψ) = (Ψs) + (Ψp) = -1240 + 350 = -890 kPa HMM/SCM1414

Worked examples on the water potential concept • Example 3: • A plasmolysed cell is found to have a solute potential of –960 kPa. What is the water potential of the cell? • Answer • In a plasmolysed cell, pressure potential (Ψp) is 0. • Therefore, Ψ = Ψs = -960 kPa

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Worked examples on the water potential concept • Example 4: • Two plant cells, A and B, are next to each other in a tissue. The water potential of cell A is –700 kPa, and the water potential of cell B is –550 kPa. In which direction will water flow – from A to B, or from B to A? • Answer • Water will flow from B to A. This is because water flows down a water potential gradient. HMM/SCM1414

c) Facilitated Diffusion • Passive movement of molecules down their concentration gradient via transport proteins.

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• i.

Two types of transport proteins: Channel proteins  Some provide hydrophilic corridors for passage of specific molecules or ions.  Example, aquaporins,, facilitate diffusion of water.  Many ion channels function as gated channels - open or close depending on presence or absence of a chemical or physical stimulus.

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EXTRACELLULAR FLUID

Solute

Channel protein

CYTOPLASM

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ii. Carrier proteins  Some proteins translocate the solute-binding site and solute across the membrane as the transport protein changes shape.

Solute

Carrier protein

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3.3.2 Active Transport – Sodium Pump and Coupled Transport  Movement of a substance across a biological membrane against its concentration gradient or electrochemical gradient with the help of energy input (ATP) and specific transport proteins. • Example: sodium-potassium pump

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EXTRACELLULAR [Na+] high FLUID [K+] low

Na+ Na+ Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM

[Na+] low [K+] high

Na+

Cytoplasmic Na+ bonds to the sodium-potassium pump

P

ATP P

ADP

Na+ binding stimulates phosphorylation by ATP.

Phosphorylation causes the protein to change its conformation, expelling Na+ to the outside.

K+

K+ K+

K+ K+

P

K+

P

Extracellular K+ binds to the protein, triggering release of the phosphate group.

Loss of the phosphate restores the protein’s original conformation.

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K+ is released and Na+ sites are receptive again; the cycle repeats.

Passive transport

Active transport

ATP Diffusion

Facilitated diffusion

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Maintenance of Membrane Potential by Ion Pumps • Membrane potential - voltage difference across a membrane. • Voltage difference is due to separation of opposite charges. • Cytoplasm is negative in charge compared to extracellular fluid. Due to unequal distribution of cations & anions on opposite sides of membrane. HMM/SCM1414

• Membrane potential favors passive transport of cations into cell and anions out of cell. • Two combined forces, the electrochemical gradient,, drive diffusion of ions across a membrane. 1)Chemical force: an ion’s concentration gradient. 2)Electrical force: effect of membrane potential on ion’s movement.

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• Special transport proteins, the electrogenic pumps,, generate voltage gradient across a membrane. • Example:  Sodium-potassium pump in animal cells.  Proton pump in plants, fungi, & bacteria.

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–

–

ATP

EXTRACELLULAR FLUID

+

+

H+ H+

Proton pump H+ –

+

H+ H+

–

+

CYTOPLASM H+ –

+

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Cotransport • The coupling of the diffusion of one substance down its concentration gradient to the transfer of another against its concentration gradient. • Transport protein may move two substances in the:
Same direction – symport carriers.
Opposite directions – antiport carriers. carriers

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• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell. –

+

H+

ATP –

H+

+ H+

Proton pump

H+ –

+

–

+

Sucrose-H+ cotransporter – –

H+ H+

Diffusion of H+ H+

+ + HMM/SCM1414

Sucrose

3.3.3 Bulk Transport • Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins • Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles

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Exocytosis  The cellular secretion of macromolecules by fusion of vesicles with plasma membrane. • Transport vesicles migrate to membrane, fuse with it, and release their contents • Many secretory cells use exocytosis to export their products

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ER Transmembrane glycoproteins Secretory protein Glycolipid Golgi apparatus

Vesicle

Plasma membrane: Cytoplasmic face Extracellular face Secreted protein

Transmembrane glycoprotein

Plasma membrane:

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Endocytosis  The cellular uptake of macromolecules and particulate substances by localized regions of plasma membrane that surround the substances and pinch off to form an intracellular vesicle. • Endocytosis is a reversal of exocytosis, involving different proteins

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Three types of endocytosis: 1) Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole. 2) Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid. 3) Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation.

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RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor

Coated vesicle

Coated pit Ligand A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs).

Coat protein

Plasma membrane 0.25 µm HMM/SCM1414