Membranes

Chapter 5

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Membrane Structure

• Phospholipid bilayer

• Globular proteins inserted in the bilayer

• Fluid mosiac model –proteins float in or

on the fluid lipid bilayer like boats on a

pond

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• Cellular membranes have 4 components

1. Phospholipid bilayer

• Flexible matrix, barrier to permeability

2. Transmembrane proteins

• Integral membrane proteins

3. Interior protein network

• Peripheral membrane proteins

4. Cell surface markers

• Glycoproteins and glycolipids

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• Both transmission electron microscope (TEM) and

scanning (SEM) used to study membranes

• One method to embed specimen in resin

– 1!m shavings

• Freeze-fracture visualizes inside of

membrane

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Phospholipids

• Structure consists of

– Glycerol

– 2 fatty acids• Nonpolar and hydrophobic (“water-fearing”)

– Phosphate group• Polar and hydrophilic (“water-loving”)

• Spontaneously forms a bilayer

– Fatty acids are on the inside

– Phosphate groups are on both surfaces

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• Bilayers are fluid

• Individualphospholipids andunanchoredproteins can movethrough themembrane

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• Environmental influences

– Saturated fatty acids make the membraneless fluid than unsaturated fatty acids

• “Kinks” introduced by the double bonds keep them

from packing tightly

• Most membranes also contain sterols such as

cholesterol, which can either increase or

decrease membrane fluidity, depending on the

temperature

– Warm temperatures make the membranemore fluid than cold temperatures

• Cold tolerance in bacteria due to fatty aciddesaturases

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Membrane Proteins

• Various functions:

1. Transporters

2. Enzymes

3. Cell-surface receptors

4. Cell-surface identity markers

5. Cell-to-cell adhesion proteins

6. Attachments to the cytoskeleton

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• Integral membrane proteins

– Span the lipid bilayer (transmembrane

proteins)

• Nonpolar regions embedded in the interior of the

bilayer

• Polar regions protrude from both sides of the

bilayer

– Transmembrane domain

• Spans the lipid bilayer

• Hydrophobic amino acids arranged in " helices

Membrane Proteins

• Bacteriorhodopsin has 7 transmembrane

domains forming a structure within the

membrane through which protons pass

during the light-driven pumping of protons

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Membrane Proteins

• Pores

– Nonpolar regions within a transmembrane

protein can create a pore through the

membrane

– Cylinder of ! sheets in the protein secondary

structure called a !-barrel

• Interior is polar and allows water and small polar

molecules to pass through the membrane

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Passive Transport

• Diffusion is movement of molecules from

high concentration to low concentration

– Will continue until equilibrium is reached

• Passive transport is movement of

molecules through the membrane in which

– No energy is required

– Molecules move in response to a

concentration gradient

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• Barrier to crossing a biological membrane

is the hydrophobic interior

– Nonpolar molecules will move until the

concentration is equal on both sides

• O2, CO2, steroid hormones, fat-soluble vitamins

– Limited permeability to polar molecules and

ions

• Na+, K+, Cl-, amino acids, sugars

Selectively Permeable Membrane

• Facilitated diffusion

– Molecules may move through proteins

– From higher to lower concentration

– Passive transport

– Channel proteins

• Hydrophilic channel when open

– Carrier proteins

• Bind specifically to molecules they assist

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Selectively Permeable Membrane

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Channel proteins

• Ion channels

– Allow the passage of ions

– Gated channels – open or close in response

to stimulus (chemical or electrical)

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Carrier proteins

• Can transport ions and other solutes, such

as sugars and amino acids

• Requires a concentration difference

across the membrane

• Must bind to the molecule they transport

– Saturation – rate of transport limited by

number of transporters

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Osmosis

• Cytoplasm of the cell is an aqueous

solution

– Water is solvent

– Dissolved substances are solutes

• Osmosis – diffusion of water across a

membrane toward a higher solute

concentration

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Tonicity

• Isotonic solution has the same solute

concentration on both sides of the

membrane

• Hypertonic solution has a higher solute

concentration

• Hypotonic solution has a lower solute

concentration

• Aquaporins facilitate osmosis

Osmotic pressure

• Cell in a hypotonic solution gains water causing

cell to swell – creates pressure

• If membrane strong enough, cell reaches

counterbalance of osmotic pressure driving

water in with hydrostatic pressure driving

water out

– Cell wall of prokaryotes, fungi, plants, protists

• If membrane is not strong, may lyse (burst)

– Animal cells must be in isotonic environments

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Maintaining osmotic balance

• Some cells use extrusion in which water is

ejected through contractile vacuoles

• Isosmotic regulation involves keeping cells

isotonic with their environment

– Marine organisms adjust internal concentration to

match sea water

– Terrestrial animals circulate isotonic fluid

• Plant cells use turgor pressure to push the cell

membrane against the cell wall and keep the cell

rigid

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Active Transport

• Requires energy – ATP

• Moves substances from low to high

concentration

• Requires the use of highly selective

carrier proteins

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• Carrier proteins used in active transport

include

– Uniporters – move one molecule at a time

– Symporters – move two molecules in the

same direction

– Antiporters – move two molecules in

opposite directions

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Sodium–potassium (Na+–K+) pump

• Uses an antiporter to move 3 Na+ out of

the cell and 2 K+ into the cell

– Against their concentration gradient

• ATP energy is used to change the

conformation of the carrier protein

• Affinity of the carrier protein for either Na+

or K+ changes so the ions can be carried

across the membrane

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Coupled transport

• Uses ATP indirectly

• Uses the energy released when a

molecule moves by diffusion to supply

energy to active transport of a different

molecule

• Symporter is used

• Glucose–Na+ symporter captures the

energy from Na+ diffusion to move glucose

against a concentration gradient

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Bulk Transport

• Endocytosis

– Movement of substances into the cell

– Phagocytosis – cell takes in particulate matter

– Pinocytosis – cell takes in only fluid

– Receptor-mediated endocytosis – specific

molecules are taken in after they bind to a receptor

• Exocytosis

– Movement of substances out of cell

• Requires energy

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• In the human genetic disease familial hypercholesterolemia, the LDL

receptors lack tails, so they are never fastened in the clathrin-coated

pits and as a result, do not trigger vesicle formation. The cholesterol

stays in the bloodstream of affected individuals, accumulating as

plaques inside arteries and leading to heart attacks.

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• Exocytosis

– Movement of materials out of the cell

– Used in plants to export cell wall material

– Used in animals to secrete hormones,

neurotransmitters, digestive enzymes

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Diffusion

Requires no energy

Passive transport

Higher solute concentration

Facilitateddiffusion

Osmosis

Higher waterconcentration

Higher soluteconcentration

Requires energy

Active transport

Solute

Water

Lower soluteconcentration

Lower waterconcentration

Lower soluteconcentration