Transport across cell membranes and water potential

What is diffusion ?

The net movement of particles from an are go high concentration to an area of lower concentration. It is a passive process- no energy is needed for it .

What is concentration gradient ?

The path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient.

What can diffuse across cell membranes ?

• Small , non-polar molecule can easily diffuse across phospholipids

• E.g: Oxygen, Carbon dioxide

How does water move across plasma membranes ?

• Osmosis

• Although it is polar , molecules are small enough to fit between the phospholipids and diffuse across plasma membranes

What 4 factors affect rate of diffusion ?

1. Concentration gradient - higher =faster

2. Thickness of exchange surface - thinner = shorter distance particles travel = faster

3. Surface area - larger=faster

4. Temperature- hotter=more kinetic energy=faster

How can you investigate diffusion in model cells ?

1. Make agar jelly either dilute sodium hydroxide and phenolthalein (A pH indicator pink = alkaline , colourless= acidic )

2. Add dilute hydrochloric acid to a beaker and place a few cubes of jelly inside

3. Cubes turn colourless as acid diffuses into cubes and neutralises the sodium hydroxide

How do you investigate how surface area affects rate of diffusion ?

1. Cut different sizes of agar jelly cubes ( sodium hydroxide and thenolthelein) and calculate surface area to volume ratio for each

2. Time how long it takes for each cube to go colourless whilst in the same concentration of hydrochloric acid

3. Largest surface area to volume ratio = fastest

How do you investigate how concentration gradient affects rate of diffusion ?

1. Create test tubes with different concentrations of hydrochloric acid inside

2. Add equally sized agar jelly cubes ( sodium hydroxide and thenolthylein) into each and time how long it takes each cube to turn colourless

3. Highest concentration = fastest

How do you investigate how temperature affects the rate of diffusion ?

1. Prepare boiling tubes with same concentration of hydrochloric acid in water baths at varying temperatures . Don’t make temperature above 65^oC or the agar jelly will start to melt .

2. Put an equal sized cube of agar jelly ( sodium hydroxide and thenothylein ) into each test tube and time how long it takes for each tube to go colourless.

3. Hottest = fastest

What is facilitated diffusion ?

• Carrier proteins or channel proteins are used to diffuse molecules in the cell membrane when larger molecules (E.g : amino acids ), ions or polar molecules can’t diffuse directly through the phospholipid bilayer.

• Passive process- no energy used

• Particles move down a concentration gradient - high to low concentration

When are carrier proteins used in facilitated diffusion and how do they work ?

• They move large molecules into or out of a cell down a concentration gradient

• Different carrier proteins facilitate the diffusion of different large molecules

1. Large molecule attaches to carrier protein in the membrane

2. Protein changes shape

3. Releases the molecule on the other side of the membrane

When are channel proteins used in facilitated diffusion ?

• Form pores in the membrane for charged particles to diffuse through (down a concentration gradient )

• Different channel proteins facilitate the diffusion of different charged particles

What is active transport ?

Uses energy to move molecules and ions across plasma membranes , against a concentration gradient . (Low to high concentration ). This involves carrier proteins .

How does active transport work ?

1. A molecule attaches to a carrier protein

2. Protein changes shape

3. Molecule released on other side of the plasma membrane

• Only difference between active transport and facilitated diffusion of ions is that the energy comes from ATP- to move the solute against the concentration gradient

How do cells take in very large substances ?

• Endocytosis- for substances that are too big to be taken in by carrier proteins (E.g : Proteins)

1. Cell surrounds substance with a section of its plasma membrane

2. The membrane pinches off to form a vesicle inside the cell which contains the ingested substance

• It is an active process so needs ATP for energy

• E.g : White blood cells take in pathogens by endocytosis so they can destroy them

How do cells secrete substances ?

Exocytosis- substances like lipids that are produced by the cell need to be removed

1. Vesicle pinches off from Golgi apparatus and moves towards plasma membrane

2. Vesicle fuses with plasma membrane

3. Contents released outside of the cell or inserted into the plasma membrane

Active process - uses ATP for energy source

What is osmosis ?

Diffusion of water molecules across a partially permeable membrane down a water potential gradient .

Water molecules move from an area of high water potential (lots of water molecules ) to an area of lower water potential (less water molecules )

What is water potential ?

Potential of water molecules to diffuse in or out of a solution.

Water has the highest water potential (0) , all other solutions have a lower water potential. The less water molecules / the more solutes , the lower the water potential is.

What is a hypotonic solution ?

Solution with higher water potential that the cell

What is an isotonic solution ?

Solution with the same water potential as the cell

What is a hypertonic solution ?

Solution with a lower water potential that the cell

How does water potential affect osmosis in animal cells ?

• Hypotonic solution : Net movement of water into cell - cell bursts, as no cell wall to stabilise it

• Isotonic solution : Water moves equally into and out of cell , no net movement - cell stays same

• Hypertonic solution : Net movement of water out of cell - cell shrinks

How does water potential affect osmosis in plant cells ?

• Hypotonic solution - Net movement into cell , vacuole swells , vacuole and cytoplasm push against cell wall - cell become turgid

• Isotonic solution - water moves into and out of cell equally , no net movement - cell stays same

• Hypertonic solution - Net movement of water out of cell- cell becomes flaccid . Cytoplasm and membrane pull away from cell wall (plasmolysis )

How can you investigate water potential ?

1. Complete a serial dilution to get 0.0 M to 1.0M solutions at 0.2M intervals ( Higher concentration = lower water potential )

2. Use a cork boater to create equally sized potato pieces

3. Measure starting mass of each piece of potato

4. Leave one piece in each solution for a set time

5. Pat them dry gently with a towel and reweigh them

6. Calculate percentage change in mass for each piece of potato in different concentrations

7. Repeat 3 times and calculate mean percentage change in mass

8. Plot results on a graph

9. Where the percentage change in mass = 0 , this is where water potential of sucrose = water potential of cells / same amount of sucrose in solution and potato. The higher the sucrose concentration