Chapter 4

Membrane Structure and Function

Plasma Membrane

Fluid Mosaic Model• Cholesterol– Reduces the amount of stuff allowed to enter a cell

• Phospholipids – This is the lipid bilayer

• Glycolipids – A carbohydrate that is attached to a phospholipid

• Proteins– Can help move things in and out of the cell

• Glycoproteins– A carbohydrate that is attached to a protein

• The carbohydrates that come out of the plasma membrane allow cells to recognize each other

• If a cell is not recognized the body tries to kill that foreign cell

Permeability of Plasma Membrane• Plasma membrane is selectively permeable

Selective Permeability

• Selective permeability- Some things are let into the cell while some things remain outside of the cell

Concentration Gradient• There is a greater concentration of small balls on one

side than the other. This means that it will try to reach an equilibrium.

• A gradient means that there is more on one side and gets less and less as you move to the other side

Diffusion• Diffusion is the movement of molecules from

a higher concentration to a lower concentration

Time

Semi-Permeable

• Diffusion– This occurs in our lungs.– High concentration of oxygen air enters lungs and

O2 diffuses into our blood, which has a lower concentration of oxygen

Osmosis• Osmosis is the diffusion of water• Water moves from a high concentration to a low

concentration

• Water moves to the right because there is a difference in concentration

• Osmotic pressure is the pressure that develops in a system because of osmosis

Factors that will affect the rate of Diffusion

1. Temperature higher temp. the faster the rate of diffusion

2. Size of molecule the smaller the molecule then the faster the rate of diffusion

3. Charge of molecule charged molecules have difficulty moving through the cell

4. Concentration Gradient the larger the difference in concentration the faster the diffusion

5. Pressure Gradient If there is pressure on one side then the faster the diffusion

What happens to a cell in a solution• Tonicity – concentration of the solution

compared to the concentration inside the cell• Isotonic – The concentration of the solution is equal to that of

inside the cell• Hypotonic – Solution has a lower concentration of solute (higher

concentration of water) than inside the cell• Hypertonic – Solution has higher concentration of solute than

inside the cell

What happens to a cell in solution

• Isotonic solution– The cell remains the same. No changes

• Hypotonic solution– The cell swells and may undergo lysis

• Hypertonic solution– The cell loses water and shrivels. Or undergoes

crenation

Which is which?

Hypertonic Isotonic Hypotonic

Facilitated Transport

• This is for molecules that are too big to diffuse through the plasma membrane

• Proteins help them move• It stills move with the concentration gradient• It uses NO ENERGY

Diffusion Facilitated Transport

• Facilitated Transport• Diffusion• Osmosis

None of these use EnergySo therefore they are called “Passive Transport”

Active Transport

• This uses energy– ATP ADP

• It moves molecules and ions against a concentration gradient

• An example of active transport is the sodium potassium pump– Sodium is moved to the outside and potassium is

moved to the inside of the cell

Endocytosis and Exocytosis

• These processes are used for materials that are too big to pass through the plasma membrane by a protein transport

• Endocytosis– The cell engulfs material to bring it into the cell– Two types

• Phagocytosis• Pinocytosis

• Exocytosis– Moves material to the outside of the cell– This allows cell to secrete molecules

Endocytosis• Phagocytosis– A type of endocytosis that engulfs large items. Like

cells or bacteria– The vesicle that forms afterwards combines with

lysosomes so digestion can occur

Endocytosis

• Pinocytosis– This is when very small particles are engulfed by

the membrane– Receptor mediated endocytosis is a type of

pinocytosis that uses receptor proteins to help initiate pinocytosis