Chapter 7Membrane Structure and Function

CELL MEMBRANE:

Basics:

Regulates flow in and out of the cell

Composed of phospholipids and proteins -some carbohydrates and lipids

SELECTIVELY PERMEABLE

- allows some things across more easily than others

Structure varies from one cell type to another

determines function of the membrane

Membrane Structure

Cellular membranes are fluid mosaics of lipids and proteins

Phospholipids

Are the most abundant lipid in the plasma membrane

Are amphipathic, containing both hydrophobic and hydrophilic regions

Organized in a bilayer

Hydrophilic

head

Hydrophobic

tail

WATER

WATER

Membrane Structure

States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

Proteins are either integral or peripheral

Integral – embedded in the membrane and traverse the phospholipid bilayer

- transmembranal

Peripheral – on the surface

- on both the inner and outer surfaces

- surfaces differ – membrane is bifacial or asymmetric

Singer and Nicolson Model Proposed that membrane proteins are

dispersed and individually inserted into the phospholipid bilayer

Figure 7.3

Phospholipid

bilayer

Hydrophobic

region

of protein

Hydrophobic region of protein

Freeze-fracture studies of the plasma membrane

Supported the fluid mosaic model of membrane structure

• A cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

Coat the split layers with a metal, usually gold or silver, and examine it under an electron scanning microscope

APPLICATION A cell membrane can be split into its two layers, revealing the ultrastructure of the membrane’s interior.

TECHNIQUE

Extracellular

layer

Proteins

Cytoplasmic layer

Knife

Plasma

membrane

These SEMs show membrane proteins (the “bumps”) in the two layers,

demonstrating that proteins are embedded in the phospholipid bilayer. RESULTS

Chemical Nature of Bilayer

Amphipathic Parts Match Up

- non-polar portions of proteins embed in the hydrophobic tails of the bilayer

- polar portions of proteins embed in the hydrophilic heads or jut into the interior or exterior of the cell

The Fluidity of Membranes

Membrane parts are held together by hydrophobic interactions - weak bonds so phospholipids move around laterally(Side to side)

very rare for a phospholipid to flip-flopfrom one side of the membrane to the other

The Fluidity of Membranes

Figure 7.5 A

Lateral movement

(~107 times per second) Flip-flop(~ once per month)

(a) Movement of phospholipids

Movement of Molecules

phospholipids move quickly

Slows with temperature decrease until freezes

Unsaturated tails - more flowing

Saturated tails - more viscous

Figure 7.5 B

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-

Carbon tails

(b) Membrane fluidity

Role of cholesterol

wedges between phospholipids and stabilizes structure

hinders close packing decreases freezing point of membrane

Figure 7.5 (c) Cholesterol within the animal cell membrane

Cholesterol

Proteins move slowly

larger

anchored to the cytoskeleton

some travel along the cytoskeleton

some immobile

Movement of Proteins in the Plasma Membrane

EXPERIMENT Researchers labeled the plasma mambrane proteins of a mouse

cell and a human cell with two different markers and fused the cells. Using a microscope,

they observed the markers on the hybrid cell.

Membrane proteins

Mouse cell

Human cell

Hybrid cell

Mixed

proteins

after

1 hour

RESULTS

CONCLUSION The mixing of the mouse and human membrane proteins

indicates that at least some membrane proteins move sideways within the plane

of the plasma membrane.Figure 7.6

+

TYPES OF STRUCTURES ATTACHED TO MEMBRANES1) Proteins:

Integral - transmembranal - interior and exterior end

Peripheral - surface - on outside attached to membrane but not imbedded

- attached to extracellular matrix and cytoskelton

2) Carbohydrates –

glycocalyx

glycoproteins

glycolipids

FUNCTIONS:

cell stickyness

cell communication and recognition

Figure 7.7

Glycoprotein

Carbohydrate

Microfilaments

of cytoskeleton Cholesterol Peripheral

proteinIntegral

proteinCYTOPLASMIC SIDE

OF MEMBRANE

EXTRACELLULAR

SIDE OF

MEMBRANE

Glycolipid

Membrane Proteins and Their Functions

Fibers of

extracellular

matrix (ECM)

Integral proteins

Penetrate the hydrophobic core of the lipid bilayer

Are often transmembrane proteins, completely spanning the membrane

EXTRACELLULAR

SIDE

Figure 7.8

N-terminus

C-terminus

a HelixCYTOPLASMIC

SIDE

Peripheral proteins

Are appendages loosely bound to the surface of the membrane

An overview of six major functions of membrane proteins

Figure 7.9

Transport. (left) A protein that spans the membrane

may provide a hydrophilic channel across the

membrane that is selective for a particular solute.

(right) Other transport proteins shuttle a substance

from one side to the other by changing shape. Some

of these proteins hydrolyze ATP as an energy ssource

to actively pump substances across the membrane.

Enzymatic activity. A protein built into the membrane

may be an enzyme with its active site exposed to

substances in the adjacent solution. In some cases,

several enzymes in a membrane are organized as

a team that carries out sequential steps of a

metabolic pathway.

Signal transduction. A membrane protein may have

a binding site with a specific shape that fits the shape

of a chemical messenger, such as a hormone. The

external messenger (signal) may cause a

conformational change in the protein (receptor) that

relays the message to the inside of the cell.

(a)

(b)

(c)

ATP

Enzymes

Signal

Receptor

Cell-cell recognition. Some glyco-proteins serve as

identification tags that are specifically recognized

by other cells.

Intercellular joining. Membrane proteins of adjacent cells

may hook together in various kinds of junctions, such as

gap junctions or tight junctions (see Figure 6.31).

Attachment to the cytoskeleton and extracellular matrix

(ECM). Microfilaments or other elements of the

cytoskeleton may be bonded to membrane proteins,

a function that helps maintain cell shape and stabilizes

the location of certain membrane proteins. Proteins that

adhere to the ECM can coordinate extracellular and

intracellular changes (see Figure 6.29).

(d)

(e)

(f)

Glyco-

protein

Figure 7.9

Synthesis and Sidedness of Membranes

Secreted proteins are on the inside of secretory vesicles

Proteins on the outside of the cell membraneare on the inside of a secretory vesicle.

Proteins on the inside of the cell membraneare on the outside of the secretory vesicle.

ER

Figure 7.10

Transmembrane

glycoproteins

Secretory

protein

Glycolipid

Golgi

apparatus

Vesicle

Transmembrane

glycoprotein

Membrane glycolipid

Plasma membrane:

Cytoplasmic face

Extracellular face

Secreted

protein

4

1

2

3

Traffic Across Membranes

movement of materials is dependent upon the make up of the membrane

certain substances move more easily

certain substances move more quickly

The Permeability of the Lipid BilayerHydrophobic molecules

Are lipid soluble and can pass through the membrane rapidly

Polar molecules

Do not cross the membrane rapidly

Transports:

hydrocarbons

carbon dioxide

oxygen

small polar molecules with a neutral charge (H2O)

Impedes:

ions

polar molecule

large, uncharged polar molecules - glucose

Transport Proteins:

Create channels for ions and polar molecules to pass through the lipid bilayer

Selectively transport like enzymes - substrate specific

each protein transports one specific molecule

Methods:

hydrophilic channel

bind substance and move it across the bilayer

Types of Transport Across MembranePassive vs. Active

Passive: does not require energy

downhill process

moves from high concentration to low concentration with the concentration gradient (comparison of concentration levels across a membrane)

Active: Requires energy

uphill process

moves from low concentration to high concentration

against the concentration gradient

requires transport proteins

Types of Passive Transport

Diffusion

Osmosis

Facilitated Diffusion

Diffusion

based on thermal kinetics – particles move throughout a space until they are distributed evenly

movement of particles from high concentration to low concentration until net flow is zero

NET FLOW: change in concentrations due to flow across membrane -

each substance moves independently of the others

continues until equilibrium is reached

Diffusion

Figure 7.11 A

Diffusion of one solute. The membrane

has pores large enough for molecules

of dye to pass through. Random

movement of dye molecules will cause

some to pass through the pores; this

will happen more often on the side

with more molecules. The dye diffuses

from where it is more concentrated

to where it is less concentrated

(called diffusing down a concentration

gradient). This leads to a dynamic

equilibrium: The solute molecules

continue to cross the membrane,

but at equal rates in both directions.

Molecules of dye Membrane (cross section)

Net diffusion Net diffusion Equilibrium

(a)

Figure 7.11 B

Diffusion of two solutes. Solutions of

two different dyes are separated by a

membrane that is permeable to both.

Each dye diffuses down its own concen-

tration gradient. There will be a net

diffusion of the purple dye toward the

left, even though the total solute

concentration was initially greater on

the left side.

(b)

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

Osmosis- special case of diffusion - APPLIES TO WATER ONLY

movement of water across the membrane to balance solute concentration

movement of water continues directionally until solute concentrations are equal

Based on relative comparisons of solute concentration (tonicity)

hypertonic: solution with more solute

hypotonic: solution with less solute

isotonic: solutions with equal concentrations

Movement of water: 2 ways to say the same thing:

from least solute to higher solute

from high water concentration to low water concentration

Figure 7.12

Net Flow

Water Balance of Cells Without Walls

Isotonic

The concentration of solutes is the same as it is inside the cell

There will be no net movement of water

Hypertonic

The concentration of solutes is greater than it is inside the cell

The cell will lose water

Hypotonic

The concentration of solutes is less than it is inside the cell

The cell will gain water

Water balance in cells without walls

Figure 7.13

H2O H2O H2O H2O

Lysed Normal Shriveled

Identify the Tonicity

Water balance in cells without walls

Figure 7.13

Hypotonic solution Isotonic solution Hypertonic solution

H2OH2O H2O H2O

Lysed Normal Shriveled

Water Balance of Cells with Walls

Hypertonic environment

- More solute outside of cell

- Cell loses water

- Becomes plasmolyzed

Water Balance of Cells with Walls

Isotonic

Same level of solute

Net flow of water is zero

No internal pressure – plant cells have no support – flaccid

Plants wilt

Water Balance of Cells with Walls

Hypotonic Environment

More solute inside than out

Positive flow of water into the cell

plant cell is turgid

healthy state in most plants

Cells don’t rupture because of strength of the cell wall

Combination of osmotic pressure and the pressure of the walls is called the cell’s water potential

Water balance in cells with walls

H2OH2OH2OH2O

Figure 7.13

Identify the Tonicity

Water balance in cells with walls

H2OH2OH2OH2O

Turgid (normal) Flaccid Plasmolyzed

Figure 7.13

Hypotonic solution Isotonic solution Hypertonic solution

Water Potential

determines the flow of water

Water flows from high water potential to low water potential

High pressure from the walls raises the water potential (tendency to lose water)

High levels of solute in the cell lower water potential (tendency to gain water)

Water flows into/out of the cell until the water potential reaches zero –pressure from the cell walls prevents any more water from entering the cell.

Water Potential Equation

Ψ = ΨP + Ψπ

ΨP = Pressure Potential

Ψπ = Osmotic Potential

Ψπ = − MiRT or Ψπ = − CiRT

M or C = osmotic molar concentration

R = pressure constant (Gas Laws)

T = temperature (K)

i = ionization constant (with plants the solute is mainly sucrose which has an ionization constant of 1 because sucrose does not ionize)

Facilitated Diffusion

Figure 7.15

EXTRACELLULAR

FLUID

Channel proteinSolute

CYTOPLASM

A channel protein (purple) has a channel through which

water molecules or a specific solute can pass.

(a)

Facilitated Diffusion: Passive Transport Aided by Proteins

Transmembranal proteins speed the movement of molecules across the plasma membrane

passive - no energy required - moves with the concentration gradient

Regulation of Facilitated Diffusion:

protein saturation

can only transport so much at once

proteins are specialized for a specific substance

protein inhibition

similar molecules to normal transport substance bind to protein and block path of transport

THESE ARE ONLY PHYSICAL CHANGES NOT CATALYZED REACTIONS

Mechanism of Transport

Conformational changes - binding and release of substance causes change in protein - moves substance across membrane

gated channels - stimulus causes protein to open and allow the flow of the substance - the stimulus is not the substance transported

Figure 7.15

Carrier proteinSolute

A carrier protein alternates between two conformations, moving a

solute across the membrane as the shape of the protein changes.

The protein can transport the solute in either direction, with the net

movement being down the concentration gradient of the solute.

(b)

Active Transport

pumping of solutes against the concentration gradient

moving uphill - requires ATP or a membrane potential

-membrane potential: electrical charge built up across the cell membrane due to unequal balance of anions and/or cations -

Types of Active Transport

Use of ATP:

phosphorylation reaction - adds phosphate to transport molecule and causes a conformational change

EX: sodium-potassium pump

Na/K Ion Pump

Generates an ELECTROCHEMICAL GRADIENT

Unbalanced concentration of ions

creates a new dynamic for diffusion -ions now move from high concentration and high positive charge

important for Nerve Impulses

Maintenance of Membrane Potential by Ion Pumps

Types of Pumps:

electrogenic pump - pumping of various ions - animal cells

proton pump - pumping H+ - plants, fungi, bacteria

Electrogenic pump

Figure 7.18

EXTRACELLULAR

FLUID

+

H+

H+

H+

H+

H+

H+Proton pump

ATP

CYTOPLASM

+

+

+

+–

–

–

–

–

+

COTRANSPORT

movement of one type of ion out allows for the movement of another substance inwhen the ion diffuses back in through a cotransport protein

Proton pump

Sucrose-H+

cotransporter

Diffusion

of H+

Sucrose

ATP H+

H+

H+

H+

H+

H+

H+

+

+

+

+

+

+–

–

–

–

–

–

Other Types of Active Transport

Use of Energized Electrons To Generate Chemical Gradients

Ex: Kreb’s Cycle and Light Dependent Reaction of Photosynthesis – Drives the production of ATP

Electron Transport Chain In the Kreb’s Cycle and Photosynthesis

Passive and active transport compared

Figure 7.17

Passive transport. Substances diffuse spontaneously

down their concentration gradients, crossing a

membrane with no expenditure of energy by the cell.

The rate of diffusion can be greatly increased by transport

proteins in the membrane.

Active transport. Some transport proteins

act as pumps, moving substances across a

membrane against their concentration

gradients. Energy for this work is usually

supplied by ATP.

Diffusion. Hydrophobic

molecules and (at a slow

rate) very small uncharged

polar molecules can diffuse

through the lipid bilayer.

Facilitated diffusion. Many

hydrophilic substances diffuse

through membranes with the

assistance of transport proteins,

either channel or carrier proteins.

ATP

BULK MOVEMENT

active transport that requires change in the cell membrane

movement of large molecules (proteins and polysaccharides)

Exocytosis

- movement of materials out of the cell transport vesicle fuses with cell membrane dumping contents into extracellular fluid

Examples: insulin

neurotransmitters

glycocalyx

Endocytosis

- movement of materials into the cell

results in the formation of a vacuole or vesicle

Three types

Phagocytosis

Pinocytosis

Receptor Mediated Endocytosis

1) Phagocytosis - cellular eating

cell membrane surrounds and engulfs large particles of food - becomes a food vacuole to be fused with lysosome

2) Pinocytosis - cellular drinking

cell membrane draws in extra cellular fluid and forms a small vesicle

– Non-discriminatory process

EXTRACELLULAR

FLUIDPseudopodium

CYTOPLASM

“Food” or

other particle

Food

vacuole

1 µm

Pseudopodium

of amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacterium via

phagocytosis (TEM).

PINOCYTOSIS

Pinocytosis vesicles

forming (arrows) in

a cell lining a small

blood vessel (TEM).

0.5 µm

In pinocytosis, the cell

“gulps” droplets of

extracellular fluid into tiny

vesicles. It is not the fluid

itself that is needed by the

cell, but the molecules

dissolved in the droplet.

Because any and all

included solutes are taken

into the cell, pinocytosis

is nonspecific in the

substances it transports.

Plasma

membrane

Vesicle

In phagocytosis, a cell

engulfs a particle by

Wrapping pseudopodia

around it and packaging

it within a membrane-

enclosed sac large

enough to be classified

as a vacuole. The

particle is digested after

the vacuole fuses with a

lysosome containing

hydrolytic enzymes.

Three types of endocytosis

Figure 7.20

PHAGOCYTOSIS

3) Receptor Mediated Endocytosis -specific contents are taken into cell -regulated by surface proteins that recognize specific substances called LIGANDS

receptor proteins usually gathered in one area of the cell membrane (coated pits) -concentrates the gathering effort so cell can absorb concentrated amounts of a specific substance

Receptor Mediated Endocytosis Animation

0.25 µm

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor

Ligand

Coat protein

Coated

pit

Coated

vesicle

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis

(TEMs).

Plasma

membrane

Coat

protein

Receptor-mediated endocytosis enables the

cell to acquire bulk quantities of specific

substances, even though those substances

may not be very concentrated in the

extracellular fluid. Embedded in the

membrane are proteins with

specific receptor sites exposed to

the extracellular fluid. The receptor

proteins are usually already clustered

in regions of the membrane called coated

pits, which are lined on their cytoplasmic

side by a fuzzy layer of coat proteins.

Extracellular substances (ligands) bind

to these receptors. When binding occurs,

the coated pit forms a vesicle containing the

ligand molecules. Notice that there are

relatively more bound molecules (purple)

inside the vesicle, other molecules

(green) are also present. After this ingested

material is liberated from the vesicle, the

receptors are recycled to the plasma

membrane by the same vesicle.

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