4.1 Plasma Membrane Structure and · PDF file4.1 Plasma Membrane ... • Carrier proteins...

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4.1 Plasma Membrane

Structure and Function

• The plasma membrane separates the internal environment of the cell from its external

environment.

• It regulates the entrance and exit of molecules

into and out of the cell.

• The steady internal environment maintained is

called homeostasis.

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4.1 Plasma Membrane


• Phospholipid bilayer with embedded proteins

– Hydrophilic (water-loving) polar heads

• Face inside and outside of cell (water present)

– Hydrophobic (water-fearing) nonpolar tails

• Face each other, away from water

– Cholesterol (animal cells) controls excess fluidity

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4.1 Plasma Membrane


• Membrane proteins throughout membrane

may be:

– Peripheral proteins – associated with only one side of membrane

– Integral proteins – span the membrane

• Can protrude from one or both sides

• Embedded within the membrane

• Able to move laterally

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4.1 Plasma Membrane

Structure and Function• Both phospholipids and protein can have

attached carbohydrate chains.

• Glycolipids are lipids attached to

carbohydrates.

• Glycoproteins are proteins attached to

carbohydrates.

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Outside

Inside

plasma membrane

glycolipid

glycoprotein

integral protein

cholesterol

peripheral protein

filaments of cytoskeleton

hydrophobictails

hydrophilicheads

phospholipidbilayer

carbohydratechain

Figure 4.1

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Functions of Membrane Proteins

• Channel proteins are involved in the passage

of solutes through the membrane.

– Substances simply move across the membrane.

– Some may contain a gate that must be opened in response to a signal.

• Carrier proteins allow the passage of a solute

by combing with it and help it move across the membrane.

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• Cell recognition proteins are glycoproteins.

– These proteins help the body recognize when it is being invaded by pathogens.

• Receptor proteins have a shape that allows

a specific molecule to bind.

– The binding causes the receptor to change

shape to initiate a cellular response.

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• Enzymatic proteins carry out metabolic

reactions directly.

– Example: the proteins of the electron transport chain, which carry out the final steps of aerobic respiration

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4.1 Plasma Membrane

Structure and Function• 5 Membrane Protein Functions


Channel ProteinAllows a particularmolecule or ion tocross the plasma

membrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl–)channel; a thick

mucus collects in airways and inpancreatic and liver ducts.

Carrier ProteinSelectively interactswith a specificmolecule or ion so

that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types

of the body . Different carriers respond differently to blood levels of glucose.

b.a.

Figure 4.2

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c.

Cell RecognitionProtein The MHC(major histocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHC glycoproteins are attacked by whiteblood cells responsiblefor immunity.

d. e.

Enzymatic ProteinCatalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase, whicheventually leads tosevere diarrhea.

Receptor ProteinShaped in such a waythat a specificmolecule can bind toit. Some types ofdwarfism result notbecause the bodydoes not produceenough growthhormone, but becausethe plasma membranegrowth hormonereceptors are faultyand cannot interactwith growth hormone.

Figure 4.2

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Channel Protein

Allows a particular

molecule or ion to

cross the plasmamembrane freely .

Cystic fibrosis, an

inherited disorder,

is caused by afaulty chloride (Cl–)

channel; a thick

mucus collects in

airways and inpancreatic and

liver ducts.

Carrier Protein

Selectively interacts

with a specific

molecule or ion sothat it can cross the

plasma membrane.

The family of GLUT

carriers transfers glucose in and out of

the various cell types

of the body . Different

carriers respond differently to blood

levels of glucose.

b. c.

Cell Recognition

Protein The MHC

(major histocompatibility

complex) glycoproteinsare different for each

person, so organ

transplants are difficult

to achieve. Cells withforeign MHC

glycoproteins are

attacked by white

blood cells responsiblefor immunity.

d. e.

Enzymatic Protein

Catalyzes a specific

reaction. The membrane

protein, adenylatecyclase, is involved in

ATP metabolism. Cholera

bacteria release a toxin

that interferes with theproper functioning of

adenylate cyclase, which

eventually leads to

severe diarrhea.

Receptor Protein

Shaped in such a way

that a specific

molecule can bind toit. Some types of

dwarfism result not

because the body

does not produceenough growth

hormone, but because

the plasma membrane

growth hormonereceptors are faulty

and cannot interact

with growth hormone.

a.

Figure 4.2

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4.2 Permeability of the Plasma

Membrane

• The plasma membrane can regulate the passage of molecules into and out of the cell

because it is selectively permeable.

• Which molecules can freely cross the

membrane and which may require carrier proteins and/or energy depends on

– Size

– Nature of molecule – polarity, charge

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Membrane

• Small, uncharged molecules freely cross

membrane

– Examples: CO2, O2, glycerol, and alcohol

– Slip in between the hydrophilic heads and pass through hydrophobic tails

– Driven by the concentration gradient

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Membrane

• Concentration gradient

– More of a substance on one side of the

membrane

– Going “down” a concentration gradient

• From an area of higher to lower concentration

– Going “up” a concentration gradient

• From an area of lower to higher concentration

• Requires input of energy

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Membrane

• Water which is polar would not be expected

to readily cross the membrane.

– Aquaporins are special channels that allow water to cross the membrane.

– Aquaporins are present in the majority of cells.

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Membrane

• Large molecules, ions, and charged molecules are unable to freely cross the membrane, but

can cross the membrane via

– Channel proteins forming a pore through the membrane

– Carrier proteins that are specific for substance

they transport

– Vesicle formation in endocytosis or exocytosis

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macromolecule

H2O

protein

+

+-

-charged molecules

and ions

phospholipidmolecule

nonchargedmolecules


Figure 4.3

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Passage of Molecules Into and

Out of the Cell

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Diffusion and Osmosis

• Diffusion

– Movement of molecules from an area of higher

to lower concentration

• Down a concentration gradient

• Occurs until equilibrium is reached

– For example, when a crystal of dye is placed in water

the dye and water molecules move about until

equilibrium occurs

– Solution contains a solute (solid) and a solvent(liquid)

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• Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction


b. Diffusion of water and dye molecules c. Equal distribution of molecules resultsa. Crystal of dye is placed in the water

crystal dye

time time

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• Gases can

diffuse through a membrane

• Oxygen and carbon dioxide

enter and exit

this way


capillaryalveolus

bronchiole

oxygen

O2

O2 O2

O2

O2

O2

O2O2

O2

O2

O2

O2

Figure 4.5

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Diffusion and Osmosis

• Several factors influence the rate of

diffusion

– Temperature

• As temperature increases, the rate of diffusion increases.

– Pressure

– Electrical currents

– Molecular size

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Osmosis

• diffusion of water across a differentially

permeable membrane

– Diffusion always occurs from higher to lower concentration.

– Osmotic pressure is the pressure that

develops in a system due to osmosis.

• The greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction.

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• Membrane is not permeable to solute


a.

10%

5%

< 10%

> 5%

solutewater

b.

c.

beaker

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

more water (lowerpercentage of solute)

less water (higherpercentage of solute)

differentiallypermeablemembrane

thistletube

Figure 4.6

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Osmosis

• Isotonic: the solute concentration is equal inside and outside of a cell

• Hypotonic: a solution has a lower solute concentration than the inside of a cell

• Hypertonic: a solution has a higher solute concentration than the inside of a cell

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• Isotonic

– No net gain or loss of water

– 0.9% NaCl

• Hypotonic

– Cell gains water

– Cytolysis – hemolysis

• Hypertonic

– Cell loses water

– Crenation


nucleus

6.6 µm 6.6 µm 6.6 µm

Animal cells

plasmamembrane

In an isotonic solution, there is no netmovement of water .

In a hypotonic solution, water enters the cell,which may burst (lysis).

In a hypertonic solution, water leaves thecell, which shrivels (crenation).

© David M. Phillips/Photo Researchers, Inc.

Figure 4.7

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• Isotonic

– No net gain or loss of water

• Hypotonic

– Cell gains water

– Turgor pressure keeps plant erect – cell wall

• Hypertonic

– Cell loses water

– Plasmolysis


chloroplast

nucleus

25 µm 40 µm25 µm

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

In a hypotonic solution, the central vacuolefills with water, turgor pressure develops, andchloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loseswater, the cytoplasm shrinks (plasmolysis), andchloroplasts are seen in the center of the cell.

centralvacuole

cellwall

plasmamembrane

Plant cells

(bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold

Figure 4.7

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Plant cells

chloroplast

nucleus

nucleus

6.6 µm 6.6 µm 6.6 µm

25 µm 40 µm25 µm

plasmamembrane

In an isotonic solution, there is no netmovement of water .

In a hypotonic solution, water enters the cell,which may burst (lysis).

In a hypertonic solution, water leaves thecell, which shrivels (crenation).

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

In a hypotonic solution, the central vacuolefills with water , turgor pressure develops, andchloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loseswater, the cytoplasm shrinks (plasmolysis), andchloroplasts are seen in the center of the cell.

centralvacuole

cellwall plasma

membrane

Animal cells

(all top): © David M. Phillips/Photo Researchers, Inc.; (bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold

Figure 4.7

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Transport by Carrier Proteins

• The plasma membrane impedes the passage of all but few substances.

• Substances enter or exit cells because of carrier proteins.

• Carrier proteins are specific.

– Combine with a molecule or ion to be transported across the membrane

– Change shape to move molecules across membranes

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Transport by Carrier Proteins

• Carrier proteins are required for

– Facilitated Transport

– Active Transport

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Facilitated Transport

• Facilitated transport explains the passage

of molecules such as glucose or amino acids.

– Neither molecule is lipid-soluble.

– Reversible combination and transport occurs.

– Like diffusion, ATP is not required because molecules are transported down their concentration gradient.

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Facilitated Transport

• Small molecules that are not lipid-soluble

• Molecules follow the concentration gradient

• Energy is not required

Inside

plasma

membranecarrier

protein

solute

Outside Figure 4.8

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Active Transport

• Active Transport

– Molecules or ions combine with carrier proteins.

• Often called pumps

– Molecules move against the concentration

gradient

• Entering or leaving cell

• Accumulate either inside or outside the cell

– Energy and carrier proteins are required.

• Usually ATP is used

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Active Transport

• Proteins in active transport are referred to as pumps.

• Proteins use energy to move molecules against the concentration gradient.

• Na+/K+ pump is especially important for nerve and muscle cells –it moves Na+ out and K+ into

cells.

• The carrier changes shape after phosphate

attaches, and then again after it detaches.

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Sodium-Potassium PumpCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

K+

Inside

carrierprotein

OutsideK+

K+

K+

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

Figure 4.9

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K+

P

ADPATP

K+ K+

K+

2. ATP is split, and phosphategroup attaches to carrier.

Figure 4.9

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K+

K+K+

K+

P

3. Change in shape results andcauses carrier to release 3 Na+

outside the cell.

Figure 4.9

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K+

K+

K+

K+

P

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


Figure 4.9

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K+

K+

K+

K+

P

5. Phosphate group is releasedfrom carrier.

Figure 4.9

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K+

K+

K+K+

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

inside the cell. Figure 4.9

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K+

K+

K+

K+

K+

K+K+

K+

K +

K+

K+

K+

K+

K+

K+

K+

K+K+

P

P

P

P

Inside

6. Change in shape results and

causes carrier to release 2 K+

inside the cell.

carrier

proteinOutsideK+

K+

K+

ADPATP

K+ K+

K+

3. Change in shape results and

causes carrier to release 3 Na+

outside the cell.

2. ATP is split, and phosphate

group attaches to carrier.

4. Carrier has a shape that

allows it to take up 2 K+.

5. Phosphate group is released

from carrier.

1. Carrier has a shape that allows

it to take up 3 Na+.


Figure 4.9

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Bulk Transport

• Macromolecules are transported into or out of cells by vesicle formation.– Macromolecules are too large to be transported

by carrier proteins.

– Energy is required to form vesicles.

– Vesicle formation is called membrane-assisted

transport.

• Exocytosis – exit from cell

• Endocytosis – enter into cell

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Exocytosis

– The vesicle fuses with plasma membrane as secretion occurs.

– The vesicle membrane becomes part of plasma

membrane.

– Cells of particular organs are specialized to produce and export molecules.

• Pancreatic cells release insulin or enzymes.

• Anterior pituitary cells release growth hormone.

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plasma membrane

Inside

Outside

secretoryvesicle

Figure 4.10

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Endocytosis

– Cells take in substances by vesicle formation.

• Part of the plasma membrane invaginates to envelop the substance.

• The membrane then pinches off to form an intracellular vesicle.

– Three types of endocytosis

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis

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Endocytosis

• Phagocytosis: large, particulate matter such as “food” molecules or viruses or whole cells

– Amoeba and macrophages

• Pinocytosis: liquids and small particles dissolved in liquid

– Certain blood cells or plant root cells

• Receptor Mediated Endocytosis: a type of

pinocytosis that involves a coated pit

– Certain placental cells

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paramecium

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

coated pit

c. Receptor-mediated endocytosis

399.9 µm

vesicle

vacuole

forming

pseudopod

of amoeba

0.5 µm

vesicles

forming

coated

vesiclecoated

pit

receptor

protein

a(right): © Eric Grave/Phototake; b(right): © Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark BretscherFigure 4.11

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4.3 Modifications of Cell Surfaces

• Cells live and interact with external environment.

• Extracellular environment is made of large molecules produced by nearby cells.

• Materials are deposited by secretion.

• Plants, prokaryotes, fungi are surrounded by

cell walls.

• Animals have more varied extracellular

environments that can change.

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Cell Surfaces in Animals

• Animal cells have two different types of cell

surfaces.

– Extracellular matrix outside of cells

– Junctions that occur between cells

• Both can associate with the cytoskeleton and contribute to cell-to-cell communication

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Extracellular Matrix

• A meshwork of proteins and polysaccharides closely associated with cells that produced them

• Common structural proteins in ECM

– Collagen resists stretching

– Elastin provide resilience to ECM

– Fibronectin is an adhesive protein that links integrin

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Extracellular Matrix

• Polysaccharides made of amino sugars in

ECM attach to proteins called proteoglycans

– Proteoglycans attach to a long, centrally placed polysaccharide.

• Resist compression of ECM

• Assist cell signaling by regulating the passage of molecules through ECM to plasma membrane

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Junctions Between Cells

• Cell surfaces in certain tissues of animals

– Junctions Between Cells

• Adhesion Junctions

– Intercellular filaments between cells

• Tight Junctions

– Form impermeable barriers between cells

• Gap Junctions

– Plasma membrane channels are joined (allows

communication)

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Plant Cell Walls

• All plant cells have a cell wall.

– It contains cellulose as the main component.

– Pectins allow the walls to stretch as cells grow.

– Noncellulose polysaccharides harden the wall as the cell matures.

– Pectin is abundant in the middle lamella, a

layer of adhesive substances that holds cells together.

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Plant Cell Walls

• Plasmodesmata are narrow channels that penetrate the cell wall to connect adjacent cells.

• Each channel contains a strand of cytoplasm.

• Cytoplasm allows exchange of materials between cells.

– Only water and small solutes pass freely.

• Cytoplasm connects all the cells within a plant.

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