Chapter 5 Membrane Structure and Function. Copyright © The McGraw-Hill Companies, Inc. Permission...

Chapter 5

Membrane Structure and Function

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2.7 mm

nuclearenvelope

nucleolus

plasma membrane

organelles

© Professors P. Motta & T. Naguro/Science Photo Library/Photo Researchers, Inc.

CO 5

• Cell or plasma membrane

• Phospholipid bilayer with embedded proteins

• Amphipathic molecule- having both hydrophilic (water-loving) region and hydrophobic (water-fearing) region

• Proteins are scattered and can vary from cell to cell

• Integral proteins which usually span the membrane

• Some protrude from one surface, others from both surfaces

• Hydrophobic region within phospholipid membrane

• Hydrophilic region protruding beyond membrane

• Peripheral proteins occur only on cytoplasmic side

• Only animal cells have extracellular matrix

• Proteins can be held in place by attachments of the cytoskeleton (inside) and fibers of the extracellular matrix (outside)


Outside

Inside

plasma membrane

glycolipid

glycoprotein

integral protein

cholesterol

peripheral protein

filaments of cytoskeleton

phospholipidbilayer

extracellular matrix (ECM)

hydrophilicheads

hydrophobictails

CarbohydrateChain

Fig. 5.1

hydrophobicregion

peripheralproteins

cholesterol

integralprotein

hydrophilicregions


Page 87

• Fluid-Mosaic Model

• Membrane is fluid because of lipid with the consistency of olive oil at body temperature

• The greater the concentration of unsaturated fatty acid residues the more fluid is the bilayer

• This fluidity means cells are pliable

• Also prevents membranes from solidifying when temperatures drop

• Cholesterol molecules in membrane affects fluidity

• At low temperature prevents freezing by keeping phospholipid tails apart

• Proteins are bound to ECM and move little

• Phospholipids and proteins can have attached carbohydrate chains on outer surface making the membrane asymmetrical

• Called glycolipids and glycoproteins

• Glycocalyx or “sugar coat” of animal cells protects, helps adhesion between cells, receives signal molecules, and allows cell-to-cell recognition


Fig. 5.3a

Channel Protein:Allows a particularmolecule or ion tocross the plasmamembrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl –)channel; a thickmucus collects inairways and inpancreatic and liver ducts.

a.

b.

Carrier Protein:

Selectively interacts

with a specific

molecule or ion so

that it can cross the

plasma membrane.

The inability of some

persons to use

energy for sodium-

potassium (Na+–K+)

transport has been

suggested as the

cause of their obesity.


Fig. 5.3b


Cell RecognitionProtein:The MHC (majorhistocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHCglycoproteins areattacked by white bloodcells responsible forimmunity.

c.

Fig. 5.3c


Receptor Protein:Is shaped in such away that a specificmolecule can bind toit. Pygmies are short,not because they donot produce enoughgrowth hormone, butbecause their plasmamembrane growthhormone receptorsare faulty and cannotinteract with growthhormone.

d.

Fig. 5.3d

Enzymatic Protein:Catalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase;sodium (Na+) and waterleave intestinal cells, andthe individual may diefrom severe diarrhea.

e.


Fig. 5.3e

Junction Proteins:

Tight junctions join

cells so that a tissue

can fulfill a function, as

when a tissue pinches

off the neural tube

during development.

Without this

cooperation between

cells, an animal

embryo would have no

nervous system.f.


Fig. 5.3f

signalingmolecule

receptoractivation

unactivatedreceptorprotein

nuclearenvelope

b.

a. egg embryo newborn

plasmamembrane

Targetedprotein:

Cellularresponse:

enzyme

generegulatory

proteinNucleusCytoplasm

Altered shapeor movementof cell

1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway.

2. Transduction pathway: Series of relay proteins that ends when a protein is activated.

3. Response: Targeted protein(s) bring about the response(s) noted.

Alteredmetabolismor a functionof cell

Altered geneexpression andthe amount ofa cell protein


(Human egg): © Anatomical Travelogue/Photo Researchers, Inc.; (Embryo): © Neil Harding/Stone/Getty Images; (Baby): © Photodisc Collection/Getty Images

structuralprotein

Fig. 5A

• Cell membranes are differentially or selectively permeable

• Passage of molecules into and out of cells:

• Diffusion from high concentration to low concentration without the use of energy

• Facilitated diffusion from high concentration to low concentration without the use of energy, but requires the help of a carrier molecule

• Active transport from low concentration to high concentration with the use of energy

• Bulk transport toward inside or outside of cell using vesicles


macromolecule

H2O

nonchargedmolecules

charged molecules and ions

protein

phospholipidmolecule

– +

–+

Fig. 5.4

• Aquaporins are channel proteins that allow water to cross a membrane very quickly

• A solution contains a solute, usually a solid, and a solvent, usually a liquid

• Diffusion is movement of molecules and ions through a solution from high to low concentration to come to an equalibrium

• Diffusion rates are affected by temperature, pressure, electrical currents, and molecular size


time time

a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results

crystal dye

Fig. 5.5

• Diffusion


capillaryalveolus

bronchiole

oxygen

O2

O2

O2 O2

O2O2

O2

O2

O2

O2

O2

O2

Fig. 5.6

• Gas exchange in lungs

Fig. 5.7

• Osmosis is diffusion of water across a selectively permeable membraneCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

a.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

10%

5%

<10%

>5%

solute

differentiallypermeablemembrane

water

b.

c.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

beaker

thistletube

• Osmotic pressure is pressure that develops in a system due to osmosis

• Isotonic means water concentration is equal on both sides of the membrane

• 0.9% solution of NaCl is isotonic to red blood cells

• Hypotonic means solution has lower concentration of solutes in fluid outside the cell

• Causes turgor pressure in plant cells

• Hypertonic means solution has higher concentration of solutes in fluid outside the cell

• Causes red blood cells to shrink or crenate

• Causes plasmolysis (wilting) in plant cells


Animalcells

Plantcells

plasmamembrane

chloroplast

nucleus

cellwall

plasmamembrane

In an isotonic solution, there is nonet movement of water.

In a hypotonic solution, vacuolesfill with water, turgor pressuredevelops, and chloroplasts areseen next to the cell wall.

In a hypertonic solution, vacuoleslose water, the cytoplasm shrinks(plasmolysis), and chloroplastsare seen in the center of the cell.

In a hypotonic solution, watermainly enters the cell, which mayburst (lysis).

In an isotonic solution, there is nonet movement of water.

In a hypertonic solution, watermainly leaves the cell, whichshrivels (crenation).

nucleus

centralvacuole

Fig. 5.8


solute

Outside

Inside

plasma membrane

carrier protein

Fig. 5.9

• Faciliated transport

• Proteins involved in active transport are often called pumps

• They use energy to move molecules against the concentration gradient

• Sodium-potassium pump is associated with nerve and muscle cells

• Moves sodium ions to outside of cell

• Moves potassium ions to inside of cell

carrier protein

6. Change in shape results and causes carrier to release 2 K+

inside the cell.

1. Carrier has a shape that allows it to take up 3 Na+.

4. Carrier has a shape that allows it to take up 2 K+.

2. ATP is split, and phosphate group attaches to carrier.

3. Change in shape results and causes carrier to release 3 Na+

outside the cell.

5. Phosphate group is released from carrier.

Outside

Inside

ATP

K+

P

P

P

P

Na+

Na+

Na+

Na+

Na+

Na+

Na+ Na+

Na+

Na+

Na +

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na +

Na +

Na +

Na +

Na+

Na+

Na+

Na+

Na +


K+

K+

K+

K+K+ K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+

K+ K+

K+

K+

ADP

Fig. 5.10

• During exocytosis, a vesicle fuses with the plasma membrane as secretion occurs

• Golgi apparatus (body) produces the vesicles that contain materials produced in the cell for export to other parts of the body


plasma membrane

Inside

Outside

secretory vesicle

Fig. 5.11

• During endocytosis, cells take in substances by vesicle formation as part of the plasma membrane invaginates

• Phagocytosis is taking in large things like a food particle or another cell

• White blood cells in the immune system move like amoeba and take in invading forms and debris by phagocytosis

• Pinocytosis is taking in a liquid or small particle

• Receptor-mediated endocytosis is pinocytosis for a specific molecule like a vitamin, peptide hormone, or lipoprotein

pseudopod

paramecium

vacuole forming

vesiclesforming

coated pit

coatedvesicle

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

receptor protein

coated pit

c. Receptor-mediated endocytosis

vesicle

0.5 mm

399.9 mm


(Top): © Eric Grave/Phototake; (Center): © Don W. Fawcett/Photo Researchers, Inc.; (Bottom, both): Courtesy Mark Bretscher

Fig. 5.12

• Extracellular structures are built from materials produced and secreted by the cell

• Extracellular matrix is proteins and polysaccharides around the cell

• Collagen (resists stretching), elastin (give resilience)

• Fibronectin is an adhesive protein that binds to integrin that hooks to the cytoskeleton

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Fig. 5.13

Outside (extracellular matrix)

Inside (cytoplasm)

actin filament

fibronectin

proteoglycan

integrin

elastin

collagen

• Junctions between cells:

• There are three types:

• Adhesion junctions mechanically attach adjacent cells

• Come in two types:

• In desmosomes, internal cytoplasmic plaques attach to cytoskeleton in each cell which are joined by intercellular filaments

• Sturdy but flexible sheet of cells in stretchable organs like heart, stomach, and bladder.

• In hemidesmosomes, a single point of attachment connects the cytoskeletons of adjacent cells

Tight junctions even more closely join adjacent cells when plasma membranes attach to each other in a zipperlike fastening

Found in organs like intestine and kidneys where leakage between cells is not welcome

• Gap junctions allow cells to communicate

• Two identical membrane channels join

• Allow small molecules and ions to pass between them

• Important in heart (cardiac) and smooth muscles where the flow of ions is necessary for cells to contract as a unit

c. Gap junctionb. Tight junctiona. Adhesion junction

membrane channels

intercellular space

plasma membranes

plasmamembranes

intercellularspace

tight junctionproteins

intercellularspace

filaments ofcytoskeleton

cytoplasmicplaque

intercellularfilaments

plasmamembranes


100 nm 50 nm 20 nm

a: From Douglas E. Kelly, Journal of Cell Biology 28 (1966): 51. Reproduced by copyright permission of The Rockefeller University Press; b: © David M. Phillips/Visuals Unlimited; c: Courtesy Camillo Peracchia, M.D.

Fig. 5.14

• Adhesion junctions, Tight junctions, and Gap junctions

• Plant cell membranes are surrounded by cell walls

• The living cells are connected by plasmodesmata

• They contain numerous narrow, membrane-lined channels passing through cell walls

• Allows water and small solutes to pass freely from cell to cell


cell wall

plasmodesmata

cell wall

Cell 1 Cell 2

plasma membrane

cell wall cell wall

cytoplasm

plasma membrane

cytoplasm

middle lamella

plasmodesmata

0.3 mm

© E.H. Newcomb/Biological Photo Service

Fig. 5.15

Chapter 5 Membrane Structure and Function. Copyright © The McGraw-Hill Companies, Inc. Permission...

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