Chapter 7 Membrane Structure

and Function

Plasma Membrane

The membrane at the boundary of every cell.

Functions as a selective barrier for the passage of materials in and out of cells.

Membrane Composition

phospholipids Proteins Cholesterol Question:

How are the materials arranged?

Phospholipid Bilayer

Phospholipids Hydrophilic

heads Hydrophobic

tails

Membrane Models

Fluid Mosaic Model 1972

New model to fit the new evidence with membranes.

Fluid Mosaic Model

The way the phospholipids and proteins behave in a membrane.

“Fluid”

Refers to the phospholipid bilayer.

Molecules are not bonded together, so are free to shift.

Must remain "fluid" for membranes to function.

“Mosaic”

Proteins: float in a sea of phospholipids.

Proteins form a collage or mosaic pattern that shifts over time.

Protein Function in Membranes

Transport. Enzymatic activity. Receptor sites for signals. Cell adhesion. Cell-cell recognition. Attachment to the cytoskeleton.

Types of Membrane Proteins

Integral - inserted into the phospholipid bilayer.

Peripheral - are attached to the membrane surface.

I or P?(mark these on the list above)

Transport –Channels and pumps (Integral)

Enzymes- I or P Receptors- P or sometimes I Cell adhesion- P or I Cell to cell recognition- P

(glycoproteins) Attachment to cytoskeleton- I

Question?

How do the integral proteins stick to the membrane?

By the solubility of their amino acids.

Non-polar will be in the hydrophobic area

Hydrophilic Amino Acids

Hydrophobic Amino Acids

Hydrophilic Amino Acids

Membranes are Bifacial

Inside layer is different than outer layer

The proteins have specific orientations.

Carbohydrates are found only on the outer surface.

Carbohydrates

Membrane Carbohydrates

Branched oligosaccharides form glycophospholipids and glycoproteins on external surface.

Function – cell to cell recognition

Cholesterol

Keeps membranes the right fluidity level Not too flimsy Not too rigid

End of Part 1 Create a Paper Model of a

membrane

Question

How do materials get across a cell's membrane?

Problems

phospholipid bilayer is hydrophobic. Hydrophilic materials don't cross easily.

Large molecules don't cross easily. Too big to get through the membrane.

Mechanisms

1. Passive Transport

2. Active Transport

Passive Transport

Movement across membranes that does NOT require cellular energy.

Types of Passive Transport

1. Diffusion

2. Osmosis

3. Facilitated Diffusion

Diffusion

The net movement of atoms, ions or molecules down a concentration gradient.

Movement is from: High Low

Equilibrium

When the concentration is equal on both sides.

There is no net movement of materials.

Factors that Affect Diffusion

1. Concentration

2. Temperature

3. Pressure

4. Particle size

5. Mixing

Osmosis

Diffusion of water. Water moving from an area of

its high concentration to an area of its low concentration.

No cell energy is used.

Tonicity

The concentration of water relative to a cell. 1. Isotonic (same)

2. Hypotonic (below)

3. Hypertonic (above)

Isotonic

Cell and water are equal in solute concentration.

No net movement of water in or out of the cell.

No change in cell size.

Hypotonic

Cell's water is lower than the outside water (more solutes).

Water moves into the cell. Cell swells, may burst or the

cell is turgid.

Hypertonic

Cell's water is higher than the outside water (less solutes)

Water moves out of the cell. Cell shrinks or plasmolysis

occurs.

Facilitated Diffusion

RequiresTransport protein that helps materials through the cell membrane.

Doesn't require energy (ATP).

Aquaporins

Newly found channels for osmosis.

GFP labeled Aquaporins

Active Transport

Movement across membranes that DOES require cellular energy.

Types of Active Transport

1. Carrier-Mediated

2. Endocytosis

3. Exocytosis

Carrier-Mediated Transport

General term for the active transport of materials into cells AGAINST the concentration gradient.

Movement is: low high

Examples

1. Na+- K+ pump

2. Electrogenic or H+ pumps

3. Cotransport

Na+- K+ pump

Moves Na+ ions out of cells while moving K+ ions in.

Electrogenic or H+ pumps

Also called Proton pumps. Create voltages across

membranes for other cell processes.

Used by plants, fungi and bacteria.

Cotransport

Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane.

Example - Sucrose transport

Exocytosis

Moves bulk material out of cells.

Example - secretion of enzymes.

Endocytosis

Moves bulk materials into cells.

Several types known.

Types

1. Pinocytosis - liquids

2. Phagocytosis – solids

Carbohydrates

Forming vesicles

Summary

Know membrane structure. Be able to discuss the various

methods by which cells move materials through membranes.

Be able to solve problems in osmosis.