Chapter 7: Membrane

Structure and Function

Essential Knowledge

2.b.1 – Cell membranes are selectively permeable due to their structure (7.1 & 7.2).

2.b.2 – Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes (7.3-7.5).

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. Called a semi-permeable membrane (regulates

crossing of materials)

Membrane Composition Lipids

Most numerous Phospholipids and

cholesterol Proteins

Very large (in size) Peripheral and

integral Carbohydrates

Historical Cell Membrane Models

Davson-Danielli Model 1935 Lipid bilayer Proteins coat the surfaces Sometimes called the

“sandwich” model Evidence:

Biochemical work TEM pictures showed

double line

Accepted until 1960s

Problems w/ Davson model Not all membranes in a cell were the same How could the proteins stay in place?

Protein placement was confusing Result - the model was questioned and

tested by scientific process

Fluid Mosaic Model 1972

Current/New model to fit the new evidence

Example of “Science as a Process” Refers to the way the lipids and

proteins behave in a membrane

“Fluid” Refers to the lipid bilayer Molecules are not bonded together, so are

free to shift. Must remain "fluid" for membranes to

function. Fluid = dynamic, changing Cell membrane will remain fluid until temperature

drops to extreme levels

Ways to keep the membrane “fluid”

Lipid changes or shifts: Plants:

Cold hardening (shift to unsaturated fatty acids – remember unsaturated fats are kinked in shape)

Animals: Hibernating - Cholesterol amount increases – eat more

fatty foods right BEFORE they hibernate

Sat = NO double bonds

Unsat = double bonds

“Mosaic”

Proteins: float in a sea of lipids Proteins form a collage or mosaic pattern

that shifts over time

Evidence for fluid-mosaic TEM pictures of

fractured membranes

Cell fusion studies Tagging of

membrane proteins by antibodies

MEMORIZE THIS!!!

Protein Function in Membranes

Main function: Determine cell’s specific function

Other functions: Transport Enzymatic activity Receptor sites for signals (hormones) Cell adhesion Cell-cell recognition (immunity) Attachment to the cytoskeleton (cilia and

flagella)

2 types of transport:a) Channelb) Carrier

Types of Membrane Proteins

1) Integral Inserted into the lipid bilayer Go through ENTIRE bilayer

2) Peripheral Not embedded in the lipid bilayer Attached to the membrane surface

Question? How do the integral proteins stick to the

membrane? By the solubility of their amino acids Protein folding/structure type

Hydrophilic Amino Acids

Hydrophobic Amino Acids

Hydrophilic Amino Acids

Membranes are Bifacial The lipid composition of the two layers is

different The proteins have specific orientations. Carbohydrates are found only on the outer surface

Membrane Carbohydrates Branched oligosaccharides form glycolipids

and glycoproteins on external surface Glyco = carbohydrate

Made from modification of existing cellular molecules

Glycolipids = Lipids+carbohydrates Glycoproteins = Proteins + carbohydrates Function: recognition of "self" vs "other”

Carbohydrates

1

3

4

2

3

2

3

Questions

How do materials get across a cell's membrane? Do they use energy/no energy? Do the molecules move against/with

concentration gradient? REMEMBER: Cell membrane is

regulatory membrane (semi-permeable)

Problems with using cell membrane to move materials 1) Lipid bilayer is hydrophobic

Hydrophilic materials don't cross easily Ex: ions, H2O, polar molecules

Hydrophobic materials will cross easily Ex: CO2, O2, hydrocarbons

2) Large molecules don't cross easily Too big to get through the membrane (without

assistance or through the use of energy) Proteins play HUGE role in getting certain

molecules across (hydrophilic)

Two Mechanisms for Movement

1. Passive Transport2. Active Transport

* Both involve concentration gradients (movement of molecules from areas of high/low concentrations)

Passive Transport

Movement across membranes that does NOT require cellular energy

Types: Diffusion Osmosis Facilitated Diffusion

Diffusion

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

Ex: smells crossing room Movement is from: High Low

Diffusion movie

Equilibrium

When the concentration is equal on both sides

There is no net movement of materials Molecules are constantly in motion (don’t stop

moving!) However, CONCENTRATION STAYS THE SAME!!!

Factors that Affect Diffusion

1. Concentrationa) Of solute or solvent/s

2. Temperature3. Pressure4. Particle size

a) Smaller size = quicker movement

5. Mixing a) More = faster diffusion

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 Passive transport Relies upon tonicity of solutions (both

internally and externally)

Tonicity

The concentration of water relative to a cell. 1. Isotonic (same)2. Hypotonic (below)

a) The hypotonic solution has a solute concentration BELOW that of the cell

3. Hypertonic (above) Tonicity Tutorial

Isotonic Isosmotic solution Cell and water/solution are equal in solute

concentration No net movement of water in or out of the

cell Water still MOVES, but concentration of solutes

stays the same!!! RESULT: No change in cell size Ex: Marine mammals

Hypotonic Hypoosmotic solution Cell's water is lower than the outside water

(more solutes) Water moves into the cell RESULT: Cell swells, may burst or the cell is

turgid Ex: place raisin in water (raisin will swell, because

water rushes in to attempt to EQUAL out concentrations of solutes and water)

Ex: place egg in vinegar/water solution

Hypertonic Hyperosmotic solution Cell's water is higher inside than the outside

water (less solutes) Water moves out of the cell RESULT: Cell shrinks or plasmolysis occurs

Ex: Placing RBC in salt solution (RBC shrivels to attempt to push water out of cell to = solute concentrations)

Ex: Placing egg in corn syrup Ex: onion cell w/ salt solution added

Osmosis movie

Facilitated Diffusion Transport protein that helps materials through

the cell membrane Polar molecules and ions USE this!!!

Doesn't require energy (ATP) Still passive transport

Works on a downhill concentration gradient

Facilitated Diff movie

Channel proteins Carrier proteins

Active Transport Movement across membranes that DOES

require cellular energy Uses ALL carrier proteins Allows cells to differ in solute concentration

Why? Important in Ps, Rs and hormones Types:

Carrier-mediated Endocytosis Exocytosis

Key terms for Active Transport

Membrane potential: Voltage across membrane (opposite charges and

concentration of them) – likelihood charges will pass across the membrane

Electrochemical gradient: Electro – change in charge Chemical – change in concentration

Carrier-Mediated Transport

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

Movement is: low high Examples:

Na+ - K+ pump Electrogenic/H+ pump Cotransport

Na+- K+ pump Moves Na+ ions out of cells while moving K+

ions in Occurs in animals Sodium ions increase outside cell while

potassium ions increase inside cell

Electrogenic or H+ pumps Also called Proton pumps. Create voltages (change of charge) across

membranes for other cell processes Ps and Rs

Used by plants, fungi and bacteria. Transports H+ OUT of cell This change in charge (or change in voltage)

allows for storage of ATP Used in later rxns

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

This is how plants transport food/sugar to non-photosynthetic organs (like the roots)

Outside cell

Inside cell

Exocytosis

Moves bulk material out of cells Uses Golgi vesicles to do this Example:

Secretion of enzymes Hormone movement Secretion of insulin by pancreas

Endocytosis

Moves bulk materials into cells Cell forms new transport vesicles (from parts of

cell membrane) Types of Endocytosis:

1. Pinocytosis – move liquids 2. Phagocytosis – move solids 3. Receptor Mediated - uses receptors to "catch"

specific kinds of molecules.

Summary Recognize the Davson-Danielli model of cell membranes. Identify the components and structure of the fluid mosaic

model of cell membranes. Identify methods that keep cell membranes fluid. Identify methods that cells use for transporting small

molecules across membranes. Recognize the conditions that regulate osmosis and

tonicity in cells. Identify methods that cells use for transporting large

molecules across membranes. Be able to solve problems in osmosis – pgs. 140 and 141

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