Slide1 l.jpg
This presentation is the property of its rightful owner.
Sponsored Links
1 / 42

Cell Membrane Structure and Function PowerPoint PPT Presentation


  • 118 Views
  • Uploaded on
  • Presentation posted in: General

Cell Membrane Structure and Function. Membranes and Cell Transport. All cells are surrounded by a plasma membrane. Cell membranes are composed of a lipid bilayer with globular proteins embedded in the bilayer.

Download Presentation

Cell Membrane Structure and Function

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Slide1 l.jpg

Cell Membrane Structure and Function


Membranes and cell transport l.jpg

Membranes and Cell Transport

  • All cells are surrounded by a plasma membrane.

  • Cell membranes are composed of a lipid bilayer with globular proteins embedded in the bilayer.

  • On the external surface, carbohydrate groups join with lipids to form glycolipids, and with proteins to form glycoproteins. These function as cell identity markers.


Fluid mosaic model l.jpg

Fluid Mosaic Model

Extracellular fluid

Glycoprotein

Glycolipid

Carbohydrate

Cholesterol

Transmembrane

proteins

Peripheral

protein

Cytoplasm

Filaments of

cytoskeleton

  • In 1972, S. Singer and G. Nicolson proposed the Fluid Mosaic Model of membrane structure


Phospholipids l.jpg

Phospholipids

Choline

Hydrophilic head

Phosphate

Glycerol

Hydrophobic tails

Fatty acids

Hydrophilic

head

Hydrophobic

tails

Space-filling model

Structural formula

Phospholipid symbol

  • In phospholipids, two of the –OH groups on glycerol are joined to fatty acids. The third –OH joins to a phosphate group which joins, in turn, to another polar group of atoms.

  • The phosphate and polar groups are hydrophilic (polar head) while the hydrocarbon chains of the 2 fatty acids are hydrophobic (nonpolar tails).


Phospholipids5 l.jpg

Phospholipids

  • Glycerol

  • Two fatty acids

  • Phosphate group

Hydrophilic

heads

ECF WATER

Hydrophobic

tails

ICF WATER


Phospholipid bilayer l.jpg

Phospholipid Bilayer

Polar

hydro-philic

heads

Nonpolar

hydro-phobic

tails

Polar

hydro-philic

heads

  • Mainly 2 layers of phospholipids; the non-polar tails point inward and the polar heads are on the surface.

  • Contains cholesterol in animal cells.

  • Is fluid, allowing proteins to move around within the bilayer.


The fluidity of membranes l.jpg

The Fluidity of Membranes

Lateral movement

(~107 times per second)

Flip-flop

(~ once per month)

  • Membrane molecules are held in place by relatively weak hydrophobic interactions.

  • Most of the lipids and some proteins drift laterally in the plane of the membrane, but rarely flip-flop from one phospholipid layer to the other.

  • Membrane fluidity is influenced by temperature. As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids pack more closely.

  • Membrane fluidity is also influenced by its components. Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids because the kinks in the unsaturated fatty acid tails at the locations of the double bonds prevent tight packing.


Membrane components l.jpg

Membrane Components

Cholesterol

  • Steroid Cholesterol

    • Wedged between phospholipid molecules in the plasma membrane of animal cells.

    • At warm temperatures (such as 37°C), cholesterol restrains the movement of phospholipids and reduces fluidity.

    • At cool temperatures, it maintains fluidity by preventing tight packing.

    • Thus, cholesterol acts as a “temperature buffer” for the membrane, resisting changes in membrane fluidity as temperature changes.


Membrane components9 l.jpg

Membrane Components

Fibers of extracellular

matrix (ECM)

EXTRACELLULAR

SIDE

Glycoprotein

N-terminus

Carbohydrate

Glycolipid

Microfilaments

of cytoskeleton

C-terminus

Peripheral

protein

Cholesterol

Integral

protein

CYTOPLASMIC

SIDE

a Helix

  • Membrane carbohydrates

    • Interact with the surface molecules of other cells, facilitating cell-cell recognition

    • Cell-cell recognition is a cell’s ability to distinguish one type of neighboring cell from another

  • Membrane Proteins

    • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

    • Peripheral proteins are appendages loosely bound to the surface of the membrane

    • Integral proteins penetrate the hydrophobic core of the lipid bilayer

    • Many are transmembrane proteins, completely spanning the membrane


Functions of cell membranes l.jpg

Functions of Cell Membranes

  • Regulate the passage of substance into and out of cells and between cell organelles and cytosol

  • Detect chemical messengers arriving at the surface

  • Link adjacent cells together by membrane junctions

  • Anchor cells to the extracellular matrix


6 major functions of membrane proteins l.jpg

6 Major Functions Of Membrane Proteins

ATP

Enzymes

Signal

Receptor

  • 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.


Slide12 l.jpg

Glyco-

protein

6 Major Functions Of Membrane Proteins

4.

Cell-cell recognition. Some glyco-proteins serve as

identification tags that are specifically recognized

by other cells.

5.

Intercellular joining. Membrane proteins of adjacent cells

may hook together in various kinds of junctions, such as

gap junctions or tight junctions

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

6.


Slide13 l.jpg

Functions of Plasma Membrane Proteins

Outside

Plasma

membrane

Inside

Transporter

Enzyme

Cell surface

receptor

Cell surface identity

marker

Attachment to the

cytoskeleton

Cell adhesion


Membrane transport l.jpg

Membrane Transport

  • The plasma membrane is the boundary that separates the living cell from its nonliving surroundings

  • In order to survive, A cell must exchange materials with its surroundings, a process controlled by the plasma membrane

  • Materials must enter and leave the cell through the plasma membrane.

  • Membrane structure results in selective permeability, it allows some substances to cross it more easily than others


Membrane transport15 l.jpg

Membrane Transport

  • The plasma membrane exhibits selective permeability - It allows some substances to cross it more easily than others


Passive transport l.jpg

Passive Transport

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

  • 4 types

    • Simple diffusion

    • Dialysis

    • Osmosis

    • Facilitated diffusion


Kinetic theory of matter l.jpg

Kinetic Theory of Matter

  • All atoms and molecules are in constant random motion. (Energy of motion is called kinetic energy.)

  • The higher the temperature, the faster the atoms and molecules move.

  • We detect this motion as heat.

  • All motion theoretically stops at absolute zero.


Solutions and transport l.jpg

Solutions and Transport

  • Solution – homogeneous mixture of two or more components

    • Solvent – dissolving medium

    • Solutes – components in smaller quantities within a solution

  • Intracellular fluid – nucleoplasm and cytosol

  • Extracellular fluid

    • Interstitial fluid – fluid on the exterior of the cell within tissues

    • Plasma – fluid component of blood


Diffusion l.jpg

Diffusion

Random movement leads to net movement down a concentration gradient

Lump

of sugar

Water

No net movement at equilibrium

  • The net movement of a substance from an area of higher concentration to an area of lower concentration - down a concentration gradient

  • Caused by the constant random motion of all atoms and molecules

  • Movement of individual atoms & molecules is random, but each substance moves down its own concentration gradient.


Diffusion across a membrane l.jpg

Diffusion Across a Membrane

Equilibrium

Net diffusion

Net diffusion

  • The membrane has pores large enough for the molecules to pass through.

  • Random movement of the 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

  • This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions.


Diffusion across a membrane21 l.jpg

Diffusion Across a Membrane

  • Two different solutes are separated by a membrane that is permeable to both

  • Each solute diffuses down its own concentration gradient.

  • There will be a net diffusion of the purple molecules toward the left, even though the total solute concentration was initially greater on the left side

Equilibrium

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Net diffusion


The permeability of the lipid bilayer l.jpg

The Permeability of the Lipid Bilayer

  • Permeability Factors

    • Lipid solubility

    • Size

    • Charge

    • Presence of channels and transporters

  • Hydrophobic molecules are lipid soluble and can pass through the membrane rapidly

  • Polar molecules do not cross the membrane rapidly

  • Transport proteins allow passage of hydrophilic substances across the membrane


Passive transport processes l.jpg

Passive Transport Processes

  • 3 special types of diffusion that involve movement of materials across a semipermeable membrane

  • Dialysis/selective diffusion of solutes

    • Lipid-soluble materials

    • Small molecules that can pass through membrane pores unassisted

  • Facilitated diffusion - substances require a protein carrier for passive transport

  • Osmosis – simple diffusion of water


Osmosis l.jpg

Osmosis

  • Diffusion of the solvent across a semipermeable membrane.

  • In living systems the solvent is always water, so biologists generally define osmosis as the diffusion of water across a semipermeable membrane:


Osmosis25 l.jpg

Osmosis

Lower

concentration

of solute (sugar)

Higher

concentration

of sugar

Same concentration

of sugar

Selectively

permeable mem-

brane: sugar mole-

cules cannot pass

through pores, but

water molecules can

Water molecules

cluster around

sugar molecules

More free water

molecules (higher

concentration)

Fewer free water

molecules (lower

concentration)

Osmosis

Water moves from an area of higher

free water concentration to an area

of lower free water concentration


Osmotic pressure l.jpg

Osmotic Pressure

  • Osmotic pressure of a solution is the pressure needed to keep it in equilibrium with pure H20.

  • The higher the concentration of solutes in a solution, the higher its osmotic pressure.

  • Tonicity is the ability of a solution to cause a cell to gain or lose water – based on the concentration of solutes


Tonicity l.jpg

Tonicity

Hypotonic solution

Hypertonic solution

Isotonic solution

H2O

H2O

H2O

H2O

Normal

Shriveled

Lysed

  • If 2 solutions have equal [solutes], they are called isotonic

  • If one has a higher [solute], and lower [solvent], is hypertonic

  • The one with a lower [solute], and higher [solvent], is hypotonic


Water balance in cells with walls l.jpg

Water Balance In Cells With Walls

(b)

Plant cell. Plant cells

are turgid (firm) and

generally healthiest in

a hypotonic environ-

ment, where the

uptake of water is

eventually balanced

by the elastic wall

pushing back on the

cell.

H2O

H2O

H2O

H2O

Turgid (normal)

Flaccid

Plasmolyzed


My definition of osmosis l.jpg

My definition of Osmosis

  • Osmosis is the diffusion of water across a semi-permeable membrane from a hypotonic solution to a hypertonic solution


Facilitated diffusion l.jpg

Facilitated Diffusion

EXTRACELLULAR

FLUID

Solute

Carrier protein

Channel protein

Solute

CYTOPLASM

  • Diffusion of solutes through a semipermeable membrane with the help of special transport proteins i.e. large polar molecules and ions that cannot pass through phospholipid bilayer.

  • Two types of transport proteins can help ions and large polar molecules diffuse through cell membranes:

    • Channel proteins – provide a narrow channel for the substance to pass through.

    • Carrier proteins – physically bind to the substance on one side of membrane and release it on the other.


Facilitated diffusion31 l.jpg

Facilitated Diffusion

  • Specific – each channel or carrier transports certain ions or molecules only

  • Passive – direction of net movement is always down the concentration gradient

  • Saturates – once all transport proteins are in use, rate of diffusion cannot be increased further


Active transport l.jpg

Active Transport

  • Uses energy (from ATP) to move a substance against its natural tendency e.g. up a concentration gradient.

  • Requires the use of carrier proteins (transport proteins that physically bind to the substance being transported).

  • 2 types:

    • Membrane pump (protein-mediated active transport)

    • Coupled transport (cotransport).


Membrane pump l.jpg

Membrane Pump

  • A carrier protein uses energy from ATP to move a substance across a membrane, up its concentration gradient:


The sodium potassium pump l.jpg

The Sodium-potassium Pump

  • One type of active transport system

[Na+] high

[K+] low

Na+

Na+

1. Cytoplasmic Na+ binds to the sodium-potassium pump.

2. Na+ binding stimulates

phosphorylation by ATP.

Na+

Na+

EXTRACELLULAR

FLUID

Na+

ATP

[Na+] low

[K+] high

P

Na+

ADP

CYTOPLASM

Na+

Na+

6. K+ is released and Na+

sites are receptive again;

the cycle repeats.

3. Phosphorylation causes the

protein to change its conformation, expelling Na+ to the outside.

Na+

K+

P

K+

5. Loss of the phosphate

restores the protein’s

original conformation.

K+

4. Extracellular K+ binds to the

protein, triggering release of the

Phosphate group.

K+

K+

K+

P i

P

P i


Coupled transport l.jpg

Coupled transport

  • 2 stages:

    • Carrier protein uses ATP to move a substance across the membrane against its concentration gradient. Storing energy.

    • Coupled transport protein allows the substance to move down its concentration gradient using the stored energy to move a second substance up its concentration gradient:


Review passive and active transport compared l.jpg

Review: Passive And Active Transport Compared

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.

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.


Bulk transport l.jpg

Bulk Transport

  • Allows small particles, or groups of molecules to enter or leave a cell without actually passing through the membrane.

  • 2 mechanisms of bulk transport: endocytosis and exocytosis.


Endocytosis l.jpg

Endocytosis

  • The plasma membrane envelops small particles or fluid, then seals on itself to form a vesicle or vacuole which enters the cell:

    • Phagocytosis

    • Pinocytosis

    • Receptor-Mediated Endocytosis -


Three types of endocytosis l.jpg

Three Types Of Endocytosis

PHAGOCYTOSIS

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.

EXTRACELLULAR

FLUID

1 µm

CYTOPLASM

Pseudopodium

Pseudopodium

of amoeba

“Food” or

other particle

Bacterium

Food

vacuole

Food vacuole

An amoeba engulfing a bacterium via

phagocytosis (TEM).

PINOCYTOSIS

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, pinocytosisis nonspecific in the substances it transports.

0.5 µm

Plasma

membrane

Pinocytosis vesicles

forming (arrows) in

a cell lining a small

blood vessel (TEM).

Vesicle


Process of phagocytosis l.jpg

Process of Phagocytosis


Receptor mediated endocytosis l.jpg

Receptor-mediated Endocytosis

Coat protein

Receptor

Coated

vesicle

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.

Coated

pit

Ligand

Coat

protein

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis

(TEMs).

Plasma

membrane

0.25 µm


Exocytosis l.jpg

Exocytosis

  • The reverse of endocytosis

  • During this process, the membrane of a vesicle fuses with the plasma membrane and its contents are released outside the cell:


  • Login