Van der Waals interactions keep the hydrophobic region in the

lipid bilayer stable

Liposomes

Planar artificial bilayers

What substances cross the lipid bilayer?

Permeability

• the tendency of a structure to allow a given substance to pass across.

• Why is permeability important?– Certain molecules or ions can pass through a lipid

bilayer more readily than others – The internal environment of the vesicle can

become different from the outside.

• Lipid bilayers are highly selective– Selective permeability means that some

substances cross a membrane more easily than others substances can.

• Small nonpolar molecules move across bilayers quickly.

• Large molecules and charged substances cross the membrane slowly.– Sometimes they don’t cross at all.

How fast can some molecules cross the membrane?

How fast can some molecules cross the membrane?

Large molecules and charged molecules can’t pass through thenonpolar hydrophobic tails of the lipid bilayer.

What characteristics of phospholipids affect the permeability of the cell

membrane?

• Type of hydrocarbons– Saturated– Unsaturated– Poly-unsaturated

• The length of the hydrocarbon chain

In a lipid bilayer membrane with unsaturated phospholipids, the hydrophobic

part is less tight

Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in

length.

• Membranes compose of phospholipids with long saturated tails are more stiff and less permeable. – The reason is that the interaction between

the tails are stronger.

Fluidity of the lipids depends on the characteristics of their

hydrocarbon chain.

What is another lipid that will affect the permeability of the

cell membrane?

• amount of cholesterol affects the permeability of the cell membrane

You make a phospholipid bilayer with short, saturated hydrocarbon tails. You measure the permeability of this membrane to oxygen. You then double the length of the hydrocarbon tails, and re-measure membrane permeability. You then double the length of the hydrocarbon tails again, and make a third measurement of membrane permeability. You graph membrane permeability as a function of hydrocarbon tail length. Which of the graphs below best represents the data you expect?

A B C D

1 2 3 4

9%2%2%

86%

1. A

2. B

3. C

4. D

How cholesterol affects the permeability of the

membrane

The phospholipid bilayer and the movement of molecules

and ions across the cell membrane

Diffusion, osmosis, tonicity.

The phospholipid bilayer is a dynamic structure

• The movement and fluidity of the bilayer is affected by temperature and the composition of the membrane.

Which of the following molecules would be most likely to passively diffuse through

the plasma membrane of a cell?

K+, Cl-, Na+)

A. Glucose B. Testosterone C. Ions D. Aspartic Acid

1 2 3 4

13%

24%

8%

56%

1. A

2. B

3. C

4. D

How do molecules move across the membrane?

• Solvent- the substance in which the solute dissolves; it is usually found in a larger quantity.

• Solute- the component in a solution which changes state upon dissolving; it is found in a smaller quantity.

• Concentration gradient- the difference in solute concentration between two areas

• Diffusion- spontaneous movement of molecules from an area of high concentration to low concentration (i.e., down a concentration gradient).

Diffusion

Both sides of the membrane are similar after diffusion.

Osmosis

• Is the diffusion of water across a selectively permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration).

Some important terms

• Hypotonic- In comparing two solutions, referring to the one with the lower solute concentration (thus, higher water concentration).

• Hypertonic- In comparing two solutions, referring to the one with the higher solute concentration (thus, lower water concentration).

• Isotonic- Having the same solute concentration (thus, same water concentration) as another solution.

Osmosis

Effect of tonicity on the cell

Which of the following statements correctly describes the normal

tonicity conditions for typical plant and animal cells?

1. The animal cell is in a hypotonic solution, and the plant cell is in an isotonic solution.

2. The animal cell is in an isotonic solution, and the plant cell is in a hypertonic solution.

3. The animal cell is in a hypertonic solution, and the plant cell is in an isotonic solution.

4. The animal cell is in an isotonic solution, and the plant cell is in a hypotonic solution.

5. The animal cell is in a hypertonic solution, and the plant cell is in a hypotonic solution.

A cell whose cytoplasm has a concentration of 0.02 molar glucose is placed in a test tube of water containing 0.05 molar glucose. 1) Which of the following terms describes the tonicity of the external solution relative to the cytoplasm of the cell? 2) towards which direction will the net movement of water occur?

1. Hypertonic, towards the outside of the cell

2. Hypertonic, towards the inside of the cell

3. Hypotonic, towards the outside of the cell

4. Hypotonic, towards the inside of the cell

5. Isotonic, the net movement of water will be zero

Celery stalks that are immersed in fresh water for several hours become stiff and hard. Similar stalks left in a salt solution become limp and soft. From this we can

deduce that the cells of the celery stalks are

1. hypotonic to both fresh water and the salt solution.

2. hypertonic to both fresh water and the salt solution.

3. hypertonic to fresh water but hypotonic to the salt solution.

4. hypotonic to fresh water but hypertonic to the salt solution.

5. isotonic with fresh water but hypotonic to the salt solution.

Membrane proteins and the fluid mosaic model

Membrane Proteins

• Proteins can be amphipathic

• Because their R-groups range from highly non-polar to highly polar.

• The nonpolar amino acids will be stable in the hydrophobic region

• The polar or charge amino acids will be stable alongside the polar heads.

The role of proteins in the movement of ions and other

molecules.

Facilitated diffusion

How do membrane proteins affect ions and molecules?

• Researchers have identified three broad groups of transmembrane proteins.– Channel proteins– Transporters – pumps

Let’s see again what happens to ions in the membrane bilayer

Membrane channel protein are highly selective and highly regulated

Membrane channel protein are highly selective and highly regulated. (second example)

Passive and facilitated diffusion

• Passive transport:– is produced by diffusion along an

electrochemical gradient.– doesn’t require an expenditure of energy.

• Facilitated diffusion:– is the passive transport of substances that

otherwise would cross a membrane.

Carrier proteins and facilitated diffusion

• Facilitated diffusion can occur through channel proteins or through carrier proteins.

• Carrier proteins are also know as transporters.– They change their shape during the

process of facilitated diffusion.

Active Transport

• The movement of substance across a cell membrane against the concentration of an electrochemical gradient.

– with an expenditure of energy (ATP)– specific transport molecules

When a person eats iodine-rich foods, the body sends most of the iodine to the thyroid gland. Which of the following best describes the transport of iodine to cells in the thyroid?

1. Movement with a concentration gradient requiring no energy or assistance

2. Movement with a concentration gradient, requiring no energy, but requiring facilitation through a channel or by a protein

3. Movement against a gradient, requiring active transportation using energy