nvc bio 120 lect 7 membrane - napa valley college … 120 lectures/nvc... · chapter 7 - membranes...

9/20/2011

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Chapter 7 - Membranes Outline

I. Biological membranesII. Plasma membrane

A. Structure and functionB. ProteinsC. Transport

III. Cell to cell contact

Fig. 4.6

Biological membranes

Membranes all have a similar structure

Plasma membrane surrounds the cell – this membrane controls what enters and exits the cell - exhibits selective permeability

There are many membrane bound organelles within the cell. The membranes that surround these organelles have the same basic structure

Functions of the Plasma Membrane

1. Regulates passage of materials

2. Participates in biochemical reactions

3. Receives information about environment – signal transduction

4. Communicates with other cells

membranes are fluid mosaics of lipids and proteins

Phospholipids are the most abundant lipid in the plasma membrane

Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

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

© 2011 Pearson Education, Inc.

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• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

Plasma Membrane

Bilayer

Mixture of phospholipids, sterols (cholesterol), and proteins

Phospholipids arrange to have the polar head on the outside and the non-polar, lipid portion on the inside

Proteins arranged on surfaces, some form channels

Figure 7.2

Hydrophilichead

Hydrophobictail

WATER

WATER

Figure 7.3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

The Fluidity of Membranes

Phospholipids in the plasma membrane can move within the bilayer

Most of the lipids, and some proteins, drift laterally

Rarely does a molecule flip-flop transversely across the membrane


Figure 7.7

Membrane proteins

Mouse cellHuman cell Hybrid cell

Mixed proteinsafter 1 hour

RESULTS

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Figure 7.6

Lateral movement occurs107 times per second.

Flip-flopping across the membraneis rare ( once per month).

Figure 7.5

Glyco-protein

Carbohydrate

Glycolipid

Microfilamentsof cytoskeleton

EXTRACELLULARSIDE OFMEMBRANE

CYTOPLASMIC SIDEOF MEMBRANE

Integralprotein

Peripheralproteins

Cholesterol

Fibers of extra-cellular matrix (ECM)

Membrane Fluidity

As temperatures cool, membranes switch from a fluid state to a solid state

The temperature at which a membrane solidifies depends on the types of lipids

Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

Membranes must be fluid to work properly; they are usually about as fluid as salad oil


The steroid cholesterol has different effects on membrane fluidity at different temperatures

At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

At cool temperatures, it maintains fluidity by preventing tight packing


Membrane Fluidity – Cholesterol’s Role

Figure 7.8

Fluid

Unsaturated hydrocarbontails

Viscous

Saturated hydrocarbon tails

(a) Unsaturated versus saturated hydrocarbon tails

(b) Cholesterol within the animalcell membrane

Cholesterol

Plasma Membrane – Lipid Rafts

Cholesterol aids in maintaining the fluidity of the membrane and making “lipid rafts”

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Figure 7.5

Glyco-protein

Carbohydrate

Glycolipid

Microfilamentsof cytoskeleton

EXTRACELLULARSIDE OFMEMBRANE

CYTOPLASMIC SIDEOF MEMBRANE

Integralprotein

Peripheralproteins

Cholesterol

Fibers of extra-cellular matrix (ECM)

Five main components in membrane

1. Phospholipid bilayer – controls what passes through membrane

2. Cholesterol – maintains proper fluidity of membrane

3. Proteins – transport, support, communication, recognition, enzymatic, junctions

4. Glycoproteins - form binding sites and function in cell recognition

5. Glycolipids - form binding sites and function in cell recognition

Phospholipids

Amphipathic - have both hydrophilic and hydrophobic regions

Phosphate head (hydrophilic) Fatty acid tail (hydrophobic)

Glycoproteins and Glycolipids

Short chains of carbohydrates (usually less than 15 sugars) attached to proteins or lipids

Glycoproteins Glycolipids

Cell to Cell Interactions

Indentity Markers:

Glycolipids – Identify tissue types

Glycoproteins – Identify cells as “our cells”. MHC proteins – major histocompatibility complex proteins

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Six major functions of membrane proteins Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular

matrix (ECM)


Membrane Proteins - Functions Figure 7.10

Enzymes

Signaling molecule

Receptor

Signal transduction

Glyco-protein

ATP

(a) Transport (b) Enzymatic activity (c) Signal transduction

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment tothe cytoskeletonand extracellularmatrix (ECM)

Figure 7.10a

Enzymes

Signaling molecule

Receptor

Signal transductionATP

(a) Transport (b) Enzymatic activity (c) Signal transduction

Figure 7.10b

Glyco-protein

(d) Cell-cell recognition (e) Intercellular joining (f) Attachment tothe cytoskeletonand extracellularmatrix (ECM)

Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside faces

The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus


Membrane Proteins - Production

Proteins that will be associated with the inner surface of the membrane are manufactured by free floating ribosomes

Proteins that will be associated with the outer surface of the membrane are manufactured by ribosomes on the rough endoplasmic reticulum.

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The proteins destined to be on the outer surface of the plasma membrane have a sugar chain is added to these proteins transmembrane integral outer surface peripherial proteins

These proteins are glycoproteins.

The sugar chain is added in the lumen of the RER

Figure 7.12

Transmembraneglycoproteins

ERER lumen

Glycolipid

Plasma membrane:Cytoplasmic faceExtracellular face

Secretoryprotein

Golgiapparatus

Vesicle

Transmembraneglycoprotein Secreted

protein

Membraneglycolipid

Membrane Proteins

Peripheral membrane proteins Not embedded in the cell membrane, either

found inner or the outer surface of the membrane

Integral membrane proteins Embedded in the cell membrane. Some completely cross the membrane =

transmembrane proteins

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Membrane Proteins - Peripheral

Peripheral membrane proteins

anchored to a phospholipid in one layer of the membrane

possess nonpolar regions that are inserted in the lipid bilayer

are free to move throughout one layer of the bilayer

35 36

Membrane Proteins - Intergral

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

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Integral Proteins

Integral proteins penetrate the hydrophobic core

Integral proteins that span the membrane are called transmembrane proteins

The hydrophobic regions of an integral protein consist of one or more stretches of nonpolaramino acids, often coiled into alpha helices


Figure 7.9

N-terminus

helix

C-terminus

EXTRACELLULARSIDE

CYTOPLASMICSIDE

39 40

Membrane Proteins - integral

Extensive polar regions within a transmembrane protein can create a pore through the membrane.

–pleated sheets in the protein secondary structure form a cylinder called a -barrel

-barrel interior is polar and allows water and small polar molecules to pass through the membrane

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Types of membrane proteins

1. Spectrins – give shape to the cell, form a scaffold.

2. Clatherins – line coated pits to facilitate receptor mediated endocytosis

3. Adhesion proteins – adhere cells to each other or to surface Integrins and cadherins – proteins that anchor

the cell to a substrate or cell to cell, they also attach to microfilaments inside the cell, also can play a role in signal transduction

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Types of membrane proteins

4. Cell Surface Receptors – transmit messages from outside the cell to inside the cell = signal transduction

5. Cell Surface identity markers – proteins and glycoproteins characteristic for that cell type

6. Junction proteins – form channels between cells (Gap junctions)

7. Transport proteins – Carrier and transport proteins. Transport of items across the cell membrane Aquaporins – Transport water, important in

kidneys and in certain bacteria Ion channels

8. Enzymes – catalyze reactions

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Proteins that anchor the cell to a substrate, they also attach to microfilaments inside the cell

1. Spectrins2. Receptors3. Integrins4. Enzymes

Junction

Rec

eptor

s

Integrin

s

33% 33%33%

Passage Through the Cell Membrane

The cell membrane is selectively permeable or semi-permeable.

This means some molecules can pass but not all molecules.

The Permeability of the Lipid Bilayer

Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly

Polar molecules, such as sugars, do not cross the membrane easily


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What can freely pass through the membrane

1. Hydrophobic compounds (non-polar)2. Gases – oxygen, carbon dioxide3. Very small uncharged polar compounds

(Water, glycerol) – but they will pass slowly

What can not freely pass through the membrane

1. Ions2. Most hydrophillic compounds 3. Charged compounds

Transport Proteins

Transport proteins allow passage of hydrophilic substances across the membrane

Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

Channel proteins called aquaporins facilitate the passage of water


Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane

A transport protein is specific for the substance it moves


Passive transport is diffusion of a substance across a membrane with no energy investment

Diffusion is the tendency for molecules to spread out evenly into the available space

Although each molecule moves randomly, diffusion of a population of molecules may be directional

At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other


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Animation: Membrane SelectivityRight-click slide / select “Play”


Animation: DiffusionRight-click slide / select “Play”

Figure 7.13a

Molecules of dyeMembrane (cross section)

WATER

(a) Diffusion of one solute

Net diffusion Net diffusion Equilibrium

Figure 7.13b

(b) Diffusion of two solutes

Net diffusion Net diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium

Movement through the membrane

If you add molecules to water they will disperse until it is equally distributed in the water = diffusion

Equilibrium is reached when the molecules are equally distributed

Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases

No work must be done to move substances down the concentration gradient

The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen


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Movement through the membrane

Concentration gradient

Molecules will go from higher concentration to lower concentration.

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Movement through the membraneOsmosis

The cell membrane is semipermeable

The cytoplasm is mainly water (solvent) and molecules and ions are dissolved in it (solutes)

Water will travel across the membrane to try to restore the balance of solutes = Osmosis

Effects of Osmosis on Water Balance

Osmosis is the diffusion of water across a selectively permeable membrane

Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides


Figure 7.14 Lowerconcentrationof solute (sugar)

Higher concentrationof solute

Sugarmolecule

H2O

Same concentrationof solute

Selectivelypermeablemembrane

Osmosis

Water Balance of Cells Without Walls

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water


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Figure 7.15

Hypotonicsolution

Osmosis

Isotonicsolution

Hypertonicsolution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2OCell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

Hypertonic or hypotonic environments create osmotic problems for organisms

Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments

The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump


Figure 7.16

Contractile vacuole50 m

Water Balance of Cells with Walls

Cell walls help maintain water balance

A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt


In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Video: Plasmolysis

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If you put a cell in very salty water, would water enter or leave the cell?

1. Enter2. Leave

Enter

Leave

50%50%

Can Calcium (Ca2+) pass freely through the membrane?

1. Yes2. No

Yes No

50%50%

Can glucose pass freely through the membrane?

1. Yes2. No

Yes No

50%50%

Transporting molecules across the membrane

Passive transport – molecules travel across using a concentration gradient

1. Simple diffusion2. Facilitated diffusion

Active Transport – requires energy, usually in the form of ATP, to move molecules against a concentration gradient

Passive Transport: 1. Simple diffusion

Simple diffusion: molecules that can freely passthrough the membrane are controlled by concentration gradient

They will travel from a high concentration to a low concentration

Gases like oxygen and CO2

Very small molecules that are not charged (H2O, glycerol) hydrophobic (non-polar) molecules (steroids)

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Passive Transport: Facilitated diffusion Passive Transport: 2. Facilitated diffusion

Facilitated diffusion: Aided by a transport protein, still controlled by concentration gradient, does not require energy

Molecules use this type of transport if they can’t pass freely through the membrane, but travels from high concentration to low concentration

Hydrophilic molecules like glucose and polar amino acids Ions or charged compounds

Passive Transport: Facilitated diffusion Figure 7.17EXTRACELLULARFLUID

CYTOPLASM

Channel protein Solute

SoluteCarrier protein

(a) A channelprotein

(b) A carrier protein

Passive Transport Active Transport

Sometimes our cells want to move a molecule across the membrane but against the concentration gradient. The cell wants more of the solute on one side of the membrane.

The cell will use energy to maintain the higher concentration. The energy is usually in the form of ATP

Example: Used to transport ions

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Types of Active Transport Proteins

1. Uniporters – transport a single type of molecule

2. Symporter – transport two types of molecules in the same direction

3. Antiporter – transport two molecules in the opposite direction

Active Transport - Example

Sodium-potassium pump Energy is used to transport sodium ions to the

outside of a cell in exchange for potassium ion which enter the cell. Transport proteins are used Two potassium ions enter in exchange for three

sodium ions that leave the cell Used in nerve cells = neurons

How Ion Pumps Maintain Membrane Potential

Membrane potential is the voltage difference across a membrane

Voltage is created by differences in the distribution of positive and negative ions across a membrane

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

Animation: Active TransportRight-click slide / select “Play”

Figure 7.18-1

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1

Figure 7.18-2

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1 2

Na

Na

Na

P ATP

ADP

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Figure 7.18-3

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1 2 3

Na

Na

Na

NaNa

Na

P PATP

ADP

Figure 7.18-4

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1 2 3

4

Na

Na

Na

NaNa

Na

K

K

P P

PP i

ATP

ADP

Figure 7.18-5

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1 2 3

45

Na

Na

Na

NaNa

Na

K

K

K

K

P P

PP i

ATP

ADP

Figure 7.18-6

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na1 2 3

456

Na

Na

Na

NaNa

Na

K

K

K

K

K

K

P P

PP i

ATP

ADP

Figure 7.19 Passive transport Active transport

Diffusion Facilitated diffusion ATP

If a transport protein is used to move glucose using a concentration gradient this is called:

1. Simple diffusion2. Facilitated diffusion3. Active transport

Simple

diffusio

n Fac

ilitate

d diffu

sion A

ctive t

ransport

33% 33%33%

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Bulk Transport

When the cell needs to transport larger things they can use vesicles to transport things in and out of the cell.

Exocytosis: moving things out of the cell Endocytosis: moving things into the cell

Used to transport macromolecules including: whole cells (bacteria), cholesterol, and proteins


Animation: ExocytosisRight-click slide / select “Play”

Endocytosis

In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

Endocytosis is a reversal of exocytosis, involving different proteins

There are three types of endocytosis Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis


Endocytosis

Phagocytosis – when cells transport large particles and cells (bacteria) into the cell using vesicles

Pinocytosis – when cells transport fluid into the cell using vesicles

Receptor-mediated endocytosis – molecules attach to a receptor, and are then transported into the cell. The molecules that specifically bind to a receptor are called ligands


Animation: Exocytosis and Endocytosis IntroductionRight-click slide / select “Play”

In phagocytosis a cell engulfs a particle in a vacuole

The vacuole fuses with a lysosome to digest the particle


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Animation: PhagocytosisRight-click slide / select “Play”

In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles



Animation: PinocytosisRight-click slide / select “Play”

receptor-mediated endocytosis

In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation

A ligand is any molecule that binds specifically to a receptor site of another molecule



Animation: Receptor-Mediated EndocytosisRight-click slide / select “Play”

Figure 7.22

Solutes

Pseudopodium

“Food” orother particle

Foodvacuole

CYTOPLASM

Plasmamembrane

Vesicle

Receptor

Ligand

Coat proteins

Coatedpit

Coatedvesicle

EXTRACELLULARFLUID

Phagocytosis Pinocytosis Receptor-Mediated Endocytosis

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Figure 7.22a

Pseudopodium

Solutes

“Food”or otherparticle

Foodvacuole

CYTOPLASM

EXTRACELLULARFLUID

Pseudopodiumof amoeba

BacteriumFood vacuole

An amoeba engulfing a bacteriumvia phagocytosis (TEM).

Phagocytosis

1 m

Figure 7.22b

Pinocytosis vesicles formingin a cell lining a small bloodvessel (TEM).

Plasma membrane

Vesicle

0.5

m

Pinocytosis

Figure 7.22c

Top: A coated pit. Bottom: Acoated vesicle forming duringreceptor-mediated endocytosis(TEMs).

Receptor

0.25

m

Receptor-Mediated Endocytosis

Ligand

Coat proteins

Coatedpit

Coatedvesicle

Coatproteins

Plasma membrane

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Receptor-mediated endocytosis

HIV (Human Immunodeficiency Virus) entering a cell. HIV infects a number of cells in the body, though the main target is the lymphocyte cell, which is a type of T-cell. Once the HIV virus has infected the host cell it then replicates itself with thousands of copies. Image 1 of 3 in series. TEM X120,000.

Credit: © Dr. Hans Gelderblom/Visuals Unlimited

229674 HIV (Human Immunodeficiency Virus) being surrounded by a cell. HIV infects a number of cells in the body, though the main target is the lymphocyte cell, which is a type of T-cell. Once the HIV virus has infected the host cell it then replicates itself with thousands of copies. Image 2 of 3 in series. TEM X120,000.


229675

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HIV (Human Immunodeficiency Virus) in a cell. HIV infects a number of cells in the body, though the main target is the lymphocyte cell, which is a type of T-cell. Once the HIV virus has infected the host cell it then replicates itself with thousands of copies. Image 3 of 3 in series. TEM X120,000.


229676

when cells transport fluid into the cell using vesicles this is called:

1. Phagocytosis2. Pinocytosis3. Exocytosis

Phagoc

ytosis

Pinocy

tosis

Exocy

tosis

33% 33%33%

Important Concepts

Reading Chapter 6 Know the vocabulary from this lecture

What are the functions of the plasma membrane

Identify the main components in membrane and their functions. Be able to draw a membrane.

Be able to identify what can pass freely through a membrane and what can’t pass freely

What happens to cells in isotonic, hypertonic, and hypertonic situations

Important Concepts

Be able to describe how things are transported across membranes – know all the mechanisms

What are the types of membrane proteins, what are the secondary structures of intergral membrane proteins

What are the different cell to cell junctions, their functions and locations where there are found in high concentration

Important Concepts

Understand how things are transported across membranes – know the different mechanisms

Know all the types of membrane proteins and their functions

Know the different cell to cell junctions, their functions and locations where there are found in high concentration

nvc bio 120 lect 7 membrane - napa valley college … 120 lectures/nvc... · chapter 7 - membranes...

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