Membrane Structure & Function Notes

About Cell Membranes

1.All cells have a cell membrane

2.Functions: a.Controls what enters

and exits the cellb.Maintain an internal

balance (homeostasis)c.Provides protection and

support for the cell TEM picture of a real cell membrane.

3.Structure of cell membrane

Lipid Bilayer -2 layers of phospholipidsa.Phosphate head is polar,

hydrophilicb.Fatty acid tails non-polar

hydrophobicc.Proteins embedded in

membraned. Cholesterol give flexibility/

fluidity

About Cell Membranes (continued)

Phospholipid

Lipid Bilayer

Proteins

Membrane movement animation

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers

Fluid Mosaic Model of the

cell membrane

(pieces that move)

http://www.youtube.com/watch?NR=1&v=moPJkCbKjBs&feature=endscreen

1.“Framework” proteins = Integerins; span membrane, attach to cytoskeleton on inside & E.C.M on outside

2. Cell Recognition Glycoproteins(carb surface markers = ID cells by species/cell type/etc.)

3. Enzymes – carry out rxns

Membrane Proteins

4. Receptors – messenger molecules (hormones) bind to; relay “message” & trigger rxns inside cell (signal transduction)

5.Transport Proteins – channels for large, /polar molecules/ions

6.Junctions – connect cells

Functions of Membrane Proteins

Framework

Junctions

Pores (holes) in membrane allow some molecules in & keeps other out.

Structure helps it be selective (form/fxn)

Selectively Permeable

Pores

Outside of cell

Inside of cell (cytoplasm)

Lipid Bilayer

Proteins

Transport Protein Phospholipids

Carbohydratechains

Structure of the Cell Membrane

Go to Section: Animations

of membrane structure

Cholesterol in membrane keeps membrane fluid.)

Membrane TransportPass freelySmall, nonpolar molecules (O2, CO2) – DIFFUSION

Need help of TRANSPORT PROTEINS

Polar molecules (glucose)

Larger molecules (starches)

Charged ions (Na+, K+

Aquaporins: transport proteins for water (can diffuse, but slow).

Types of Cellular Transport• Passive Transport

cell doesn’t use energy1. Diffusion

2. Facilitated Diffusion

3. Osmosis (water)

• Active Transport

cell uses energy1. Protein Pumps

2. Endocytosis

3. Exocytosis

high

low

This is gonna be

hard work!!

high

low

Weeee!!!

•Animations of Active Transport & Passive

Transport

Passive Transport• cell uses no energy

• molecules move randomly

• Molecules spread out from an area of high concentration to an area of low concentration.

• (HighLow)• Three types:

3 Types of Passive Transport

1. Diffusion

2. Facilitative Diffusion – diffusion with the help of transport proteins

3. Osmosis – diffusion of water

Passive Transport: 1. Diffusion

1. Diffusion: random movement of particles from an area of high concentration to an area of low concentration.

(High to Low)• Diffusion continues until all

molecules are evenly spaced (equilibrium is reached)-Note: molecules will still move around but no NET movement.

http://bio.winona.edu/berg/Free.htm

Simple Diffusion Animation

2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane a.Transport Proteins are

specific – they “select” only certain molecules to cross the membrane

b.Transports larger or charged molecules

Facilitated diffusion (Channel Protein)

Diffusion (Lipid

Bilayer)

Passive Transport: 2. Facilitated Diffusion

Carrier Protein

A B

• http://bio.winona.edu/berg/Free.htm

High Concentration

Low Concentration

Cell Membrane

Glucosemolecules

Proteinchannel

Passive Transport:

2. Facilitated Diffusion

Go to Section:

Transport Protein

Through a

Cellular Transport From a-High

Low

• Channel Proteins animations

• 3.Osmosis: diffusion of water through a selectively permeable membrane

• Water moves from high to low concentrations •Water moves freely

through pore space or through aquaporins.

•Solute (green) to large to move across.

Osmosis animationPassive Transport: 3. Osmosis

Active Transport•cell uses energy

•actively moves molecules to where they are needed

•Movement from an area of low concentration to an area of high concentration

•(Low High)

•Three Types:

Types of Active Transport

1. Protein Pumps -transport proteins that require energy to do work

•Example: Sodium / Potassium Pumps are important in nerve responses.

Sodium Potassium Pumps (Active Transport using proteins)

Protein changes shape to move molecules: this requires energy!

Active Transport in NeuronsSodium-Potassium Pumphttp://www.sumanasinc.com/webcontent/animations/content/electricalsignaling.html

Sodium Potassium Pumps (Active Transport using proteins)

Sodium-Potassium PumpSodium Potassium Pumps (Active Transport using proteins)

Cells use energy in the form of ATP to pump Na+ and K+ ions against gradient.

1.3 Na+ ions bind to protein pump (based on shape). 2.ATP phosphorylates protein pump (Breaks bond b/w phosphate groups. Pi binds to protein pump and causes the pump to change its shape – this costs energy)3.Pump changes shape & releases 3 Na+ outside of cell4.2 K+ bind to activated pump (phosphorylated)5.Phosphate is released; pump returns to original shape; 2K+ ions released in cell

End Result of Na+/K+ pump

1.Maintain concentration gradients of Na+ and K+

3 Na+ out; 2 K+ in

High Na+ outside cell (wants to reenter; but cell is not permeable to it). Na+ pumped from [low] [high]

High K+ inside cell (wants to leave and will leak out if concentration gradient becomes imbalanced. K+ pumped from [low] [high]

2. Inside of cell is (-) relative to outside of cell (+). Resting potential inside of cell is -70 mV3. When neurons receive “stimulation” special channels will open that allow Na+ in this change in charge is the basis for electrical impulse transmission along neurons!

Sodium-Potassium Pump

http://biomhs.blogspot.com/2012/07/nerve-impulse.html

1. When neurons receive “stimulation” (heat, pressure, binding of neurotransmitters);

2. Na+ rushes into the cell (cell membrane is DEPOLARIZED and ACTION POTENTIAL is triggered. This is an electrical impulse!

3. K+ leaks out (membrane is more permeable to K+) to restore membrane to resting potential (Polarized).

Types of Active Transport

2. Endocytosis: taking bulky material into a cell • Uses energy• Cell membrane in-folds

around food particle– “Cell eating”(phagocytosis); – “Cell Drinking” (pinocytosis=

dissolved substances)• forms food vacuole & digests

food• This is how white blood cells

eat bacteria!

Types of Active Transport2.

Endocytosis: receptor – mediated

Take in specific molecules (complimentary shape to surface receptors)

Ex: take in cholesterol needed for membrane

Types of Active Transport

3. Exocytosis: Forces material out of cell in bulk• membrane surrounding the

material fuses with cell membrane

• Cell changes shape – requires energy

• EX: Hormones or wastes released from cell

Endocytosis & Exocytosis

animations

Effects of Osmosis on Life

• Osmosis- diffusion of water through a selectively permeable membrane

• Water is so small and there is so much of it the cell can’t control it’s movement through the cell membrane.

Hypotonic SolutionHypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water)

Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis)!

• Osmosis Animations for isotonic, hypertonic, and hypotonic

solutions

Hypertonic SolutionHypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water)

Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis)!

• Osmosis Animations for isotonic, hypertonic, and hypotonic solutions

Isotonic SolutionIsotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell.

Result: Water moves equally in both directions and the cell remains same size! (Dynamic Equilibrium)

• Osmosis Animations for

isotonic, hypertonic, and

hypotonic solutions

What type of solution are these cells in?

A CB

Hypertonic Isotonic Hypotonic

How Organisms Deal with Osmotic Pressure?

• Paramecium (protist) removing excess water video

•Bacteria and plants have cell walls that prevent them from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure.

•A protist like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding.

•Salt water fish pump salt out of their specialized gills so they do not dehydrate.

•Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water.

Video Clips

• http://www.youtube.com/watch?v=JaCCKPyE6I4   - cytoplasmic streaming of chloroplasts and plasmolysis of Elodea cells

•  • http://www.youtube.com/watch?v=lzDlGl3b4is – plasmolysis in

onion cells

http://www.youtube.com/watch?v=gYbt7hhIxPo – plasmolysis in red onion cells (goes with the one below showing water reentering the cells)

•  • http://www.youtube.com/watch?v=5uGroik-6QE&feature=related –

cytolysis of plasmolyzed red onion cells