Wednesday January 27, 1999
Announcements: Test on Friday

Lecture notes:

Cell membranes are composed of lipids and proteins.

They form a Bilayer with hydrophilic heads facing outward and hydrophobic tail facing inward.

The bilayer is a hydrophobic barrier between the cytoplasm of the cell and the extracellular fluid.

Membranes are fluid because of:
1. Rapid movement of molecules in each layer
2. Cholesterol (in animals, phytosterol in plants)
3. Unsaturated fats

Membrane proteins carry out most membrane functions. They can appear on the surface of the bilayer or be embedded into or through the membrane.

Membrane Lipids:

Most are phospholipids (3C backbone)
Some cholesterol in animal (phytosterol in plants)
Some sphingolipids (4C backbone)

Functions of membrane lipids: structure and hydrophobic barrier

Membrane Proteins:

Carry out most membrane functions

Channel Proteins
These provide transport for water soluble molecules.
The channel protein has a "hole" in it that allows certain types of molecules to pass.

Active Transport Proteins
These proteins require energy to move molecules against a concentration gradient. Etc..across a cell membrane

Electron Transfer Proteins
These proteins move electrons from one protein to another. They are important in producing energy for the cell.

Recognition Proteins
These proteins aid in tissue formation.
They are important in the interactions between cells.
Many of these proteins have polysaccharides which aid in recognition

Receptor Proteins
These proteins function as molecular switches to turn on functions in the cell.
Different cell types have different receptors.

Diffusion: random movement of like molecules along a concentration gradient form an area of greater concentration to one of lesser concentration.

The rate of diffusion depends on: the concentration gradient
temperature
molecular size
pressure gradient

The greatest volume of substances pass across the cell membrane by diffusion.

Only very small molecules can freely diffuse across cell membrane. O₂, CO₂, H₂O.

Osmosis: passive movements of water across a membrane in response to solute concentration, a pressure gradient or both.

Isotonic solution: has same concentration inside and outside the cell.

Hypotonic solution: has a lower concentration in the solution than inside the cell.

Hypertonic solution: has a higher concentration in the solution than inside the cell.

• Turgor pressure in plant cells can resist osmosis.

Facilitated Diffusion or Passive Transport- (no energy expenditure)

1. Can move only with the concentration gradient
2. A channel protein is required (passive transport protein)
3. A hydrophilic group in the channel protein binds to the molecule to be moved.
4. This triggers a change in shape in the channel protein and "kicks" the molecule through the membrane.

1. Proteins involved in active transport change shape, too.
2. This change requires energy.
3. They are highly sensitive in what they transport.
4. They transport ions (+or-) in or out of a cell or organelle.
5. They transport large molecules in and out against a concentration gradient.

Exocytosis - moves substances out of a cell. (Cell products such as insulin or digestive enzymes)

1. A secretory vesicle pinches off from the Golgi.
2. The vesicle goes to the plasma membrane.
3. The vesicle fuses with the plasma membrane.
4. It releases its contents to the outside.

Endocytosis - moves substances into the cell.

1. The plasma membrane encloses a particle near the cell surface.
2. A vesicle is formed that moves to the cytoplasm.
3. Phagocytosis is endocytosis of "solid" materials (cell eating)
4. Pinocytosis is endocytosis of liquids.(cell drinking)