Ch 5 PowerPoint Biology 201

Chapter 05

Lecture and Animation Outline

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MembranesChapter 5

Membrane Structure

• Phospholipids arranged in a bilayer• Globular proteins inserted in the lipid

bilayer• Fluid mosaic model – mosaic of proteins

floats in or on the fluid lipid bilayer like boats on a pond

N+(CH3)3

CH2H2C C

O OC CCH2 CH2

CH2 CH2

CH2 CHCH2 CHCH2 CH2

CH2 CH2

CH3 CH3

b. Space-filling modela. Formula c. Iconb. Space-filling modela. Formula c. Icon

GlycoproteinExtracellular

matrix protein Glycolipid

Cholesterol

Glycoprotein

Integralproteins

Actin filamentsof cytoskeleton

Intermediatefilamentsof cytoskeleton

PeripheralproteinPeripheralprotein

Glycolipid

Cholesterol

Glycoprotein

Integralproteins

Actin filamentsof cytoskeleton

Intermediatefilamentsof cytoskeleton

• Cellular membranes have 4 components1. Phospholipid bilayer

• Flexible matrix, barrier to permeability

2. Transmembrane proteins• Integral membrane proteins

3. Interior protein network• Peripheral or Intracellular membrane proteins

4. Cell surface markers• Glycoproteins and glycolipids

• Both transmission electron microscope (TEM) and scanning (SEM) used to study membranes

• One method to embed specimen in epoxy– 1µm shavings– TEM shows layers

Plasma membrane of cell 1

Cell 1

Cell 2

25 nm © Dr. Donald Fawcett/ Photo Researchers, Inc.

• Freeze-fracture visualizes inside of membrane

Medium

2. The cell often fractures through the interior, hydrophobic area of the lipid bilayer, splitting the plasma membrane into two layers.

3. The plasma membrane separates such that proteins and other embedded membrane structures remain within one or the other layers of the membrane.

4. The exposed membrane is coated with platinum, which forms a replica of the membrane. The underlying membrane is dissolved away, and the replica is then viewed with electron microscopy .

Fracturedupper halfof lipid bilayer

Exposed lowerhalf of lipid bilayer

Exposedlower half oflipid bilayer

External surfaceof plasma membrane

0.15 µm

1. A cell frozen in medium is cracked with a knife blade.

Phospholipids

• Structure consists of– Glycerol – a 3-carbon polyalcohol– 2 fatty acids attached to the glycerol

• Nonpolar and hydrophobic (“water-fearing”)– Phosphate group attached to the glycerol

• Polar and hydrophilic (“water-loving”)

• Spontaneously forms a bilayer– Fatty acids are on the inside– Phosphate groups are on both surfaces

Cell 1

Cell 2

Polar hydrophilic heads

Extracellular fluid

Intracellular fluid (cytosol)

Nonpolarhydrophobic tails

• Bilayers are fluid• Hydrogen bonding

of water holds the 2 layers together

• Individual phospholipids and unanchored proteins can move through the membrane

Mousecell

Fusecells

Intermixedmembrane proteins

Allow time formixing to occur

Humancell

• Environmental influences on fluidity– Saturated fatty acids make the membrane less

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 membrane more fluid than cold temperatures

• Cold tolerance in bacteria due to fatty acid desaturases

Membrane Proteins

• Various functions:1. Transporters2. Enzymes3. Cell-surface receptors4. Cell-surface identity markers5. Cell-to-cell adhesion proteins6. Attachments to the cytoskeleton

Enzyme Cell surface receptor

Cell surface identity marker Cell-to-cell adhesion Attachment to the cytoskeleton

Outsidecell

Insidecell

Transporter

Structure relates to function

• Diverse functions arise from the diverse structures of membrane proteins

• Have common structural features related to their role as membrane proteins

• Peripheral proteins– Anchoring molecules attach membrane

protein to surface

• Anchoring molecules are modified lipids with1. Nonpolar regions that insert into the internal portion

of the lipid bilayer 2. Chemical bonding domains that link directly to

proteins

Protein anchored tophospholipid

• Integral membrane proteins– Span the lipid bilayer (transmembrane

proteins)• Nonpolar regions of the protein are embedded in

the interior of the bilayer• Polar regions of the protein protrude from both

sides of the bilayer– Transmembrane domain

• Spans the lipid bilayer• Hydrophobic amino acids arranged in α helices

Membrane Proteins

• Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain

18a. b.

Single transmembrane domain

Many transmembrane domains

• Pores– Extensive 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

β-pleated sheets

Passive Transport

• Passive transport is movement of molecules through the membrane in which– No energy is required– Molecules move in response to a

concentration gradient• Diffusion is movement of molecules from

high concentration to low concentration– Will continue until the concentration is the

same in all regions

a. b. c. d.

• Major barrier to crossing a biological membrane is the hydrophobic interior that repels polar molecules but not nonpolar molecules– Nonpolar molecules will move until the

concentration is equal on both sides– Limited permeability to small polar molecules– Very limited permeability to larger polar

molecules and ions

• Facilitated diffusion– Molecules that cannot cross membrane easily

may move through proteins– Move from higher to lower concentration– Channel proteins

• Hydrophilic channel when open– Carrier proteins

• Bind specifically to molecules they assist

• Membrane is selectively permeable

Channel proteins

• Ion channels – Allow the passage of ions– Gated channels – open or close in response

to stimulus (chemical or electrical)– 3 conditions determine direction

• Relative concentration on either side of membrane• Voltage differences across membrane• Gated channels – channel open or closed

Extracellular fluid Extracellular fluid

Cytoplasm

Carrier proteins

• Can help transport both ions and other solutes, such as some 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

Extracellular fluid

Cytoplasm

Osmosis

• Cytoplasm of the cell is an aqueous solution– Water is solvent– Dissolved substances are solutes

• Osmosis – net diffusion of water across a membrane toward a higher solute concentration

Urea molecule

Semipermeablemembrane

Watermolecules

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Osmotic concentration

• When 2 solutions have different osmotic concentrations– Hypertonic solution has a higher solute concentration– Hypotonic solution has a lower solute concentration

• When two solutions have the same osmotic concentration, the solutions are isotonic

• Aquaporins facilitate osmosis

Osmotic pressure

• Force needed to stop osmotic flow• 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 burst– Animal cells must be in isotonic environments

Shriveled cells Normal cells

Flaccid cell Normal turgid cell

HypertonicSolution

IsotonicSolution

HypotonicSolution

Cells swell andeventually burst

Cell body shrinksfrom cell wall

5 µm 5 µm 5 µm

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

Active Transport

• Requires energy – ATP is used directly or indirectly to fuel active transport

• Moves substances from low to high concentration

• Requires the use of highly selective carrier proteins

• 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– Terms can also be used to describe facilitated

diffusion carriers

Sodium–potassium (Na+–K+) pump

• Direct use of ATP for active transport• 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

6. Dephosphorylation of protein triggers change back to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats.

1. Carrier in membrane binds intracellular sodium.

2. ATP phosphorylates protein with bound sodium.

5. Binding of potassium causes dephosphorylation of protein.

4. This conformation has higher affinity for K+. Extracellular potassium binds to exposed sites.

3. Phosphorylation causes conformational change in protein, reducing its affinity for Na+. The Na+

then diffuses out.

Extracellular

Intracellular

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

Na+/ K+ pump

Glucose

Outsideof cell

ADP + Pi

Insideof cell

Coupledtransportprotein

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

Cytoplasm

Solute

Bacterialcells

Plasmamembrane

a. Phagocytosis

Plasmamembrane

b. Pinocytosis

• 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.

Receptor protein

Coated pit

Clathrin Coated vesicle

Target molecule

c. Receptor-mediated endocytosis

(both): © Reproduced with permission from M.M. Perry and A.B. Gilbert, “Yolk transport in the ovarian follicle of the hen (Gallus domesticus): lipoprotein-like particles at the periphery of the oocyte in the rapid growth phase,” Journal of Cell Science, 39:257-72, October 1979. © The Company of Biologists

• Exocytosis – Discharge of materials out of the cell– Used in plants to export cell wall material– Used in animals to secrete hormones,

neurotransmitters, digestive enzymes

Plasma membrane

Secretory vesicle

Secretory product

Cytoplasm

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