Membranes and Molecular Transport

Dr. Steele

1. Maintain integrity of the cell

2. Control Entry and Exit

3. Responsible for Compartmentalization

Describe the general characteristics of cell membranes

Cell membranes maintain integrity of the cell

General characteristic of cell membranes that defines the border of a cell and acts as a barrier between the inside and outside of the cell

Cell membranes control entry and exit

General characteristic of the cell membrane that requires it to be selectively permeable

Cell membranes are responsible for compartmentalization

General characteristic of cell membranes that separate different parts of the cell

The phospholipid bilayer contains hydrophobic, non polar "tails" and hydrophilic, polar "heads". the fatty acid tails face towards each other and hydrophilic heads face the outside and inside of the cell since it is mostly water in both places

Describe the phospholipid bilayer structure of a cell membrane

Phosphate

To create the cell membrane structure, you remove a fatty acid tail and add a __ portion

Function

Protein roles are dependent on __

Integral proteins

Cell membrane proteins that penetrate into the phospholipid bilayer and act as passages for substances to pass through the membrane - form pores, channels, and carriers in the cell membrane, transduce signals

Peripheral proteins

Cell membrane proteins that are located on the outside of the phospholipid bilayer and many times need to be in contact with water - include receptors, enzymes, cell surface proteins, and cellular adhesion molecules

Receptors

Peripheral proteins which respond to extracellular signals

Enzymes

Peripheral proteins that catalyze chemical reactions

Cell surface proteins

Peripheral proteins that establish self from foreign bodies - not static

Cellular adhesion molecules

Peripheral proteins that enable cells to stick to each other - not static

Cholesterol adds stability to cell membranes and maintains structure during change in the environment

Describe the role of cholesterol in the cell membrane

Concentration gradient

The difference in concentration that drives diffusion

Non-polar substances like gases and lipids do not have any differences in charge or electron distribution, making them non-polar. Since the inside of the phospholipid bilayer is also non polar, those molecules to pass through easily

Explain membrane permeability to non-polar substances

Polar substances such as carbohydrates, proteins, and charged ions that have electrons that are not shared equally. Since those substances are polar and the inside of the bilayer is non polar, they need channels to allow them to pass through - most polar molecules are not lipid soluble

Explain membrane permeability to polar substances

Diffusion, facilitated diffusion, osmosis

What are the 3 physical (passive) processes we discussed that explain how substances move through the cell membrane

Diffusion

Physical (passive) process that includes movement with/down a concentration gradient as molecules randomly move and eventually redistribute - does not need a membrane/barrier - affected by distance, size of concentration gradient, and temperature

Facilitated diffusion

Physical (passive) process that includes uses of integral proteins to facilitate/organize diffusion - is still passive because it moves with the concentration gradient - requires a specialized transport protein such as channel or carrier proteins

Osmosis

Physical (passive) process described as the diffusion of water across a barrier/membrane - requires a selectively permeable membrane to water, and the water moves with its gradient - also dependent on osmotic pressure

Water is a very small molecule, so it can diffuse

Why can water (a polar molecule) diffuse across the cell membrane?

Channel proteins

These proteins allow the passage of molecules with the concentration gradient, can be gated which determines entrance and exit since this isn't always available - responds to ligand, voltage, or mechanical force

Carrier proteins

Proteins that bring larger molecules down the concentration gradient in conformation shape change - polar molecules need these

Osmotic pressure

The ability to lift a volume of water

Water creates its own pressure which is based on solute concentration. - the greater the solute concentration, the greater the osmotic pressure, the lower the solute concentration, the lower the osmotic pressure

Explain osmotic pressure

Water tends to move by osmosis toward solutions of greater osmotic pressure (more concentrated solute)

In what direction does water in osmosis tend to move?

Hydrostatic forces create pressure in the capillaries and force the molecules small enough to exit through filtration out. Filtration is dependent on size of the molecule and whether or not it can be forced through a porous membrane

Explain how substances move out of capillaries by filtration

Filtration is dependent on size of the molecule and whether or not they can be forced through a porous membrane

What is filtration dependent on?