The Structure and Transport of Materials across - Cell

Membranes

The cell regulates its internal environment. Its internal environment is very different from the external environment that it is bathed in. It uses cell walls and cell membrane to control its environment. The membrane is semi-permeable (or selectively permeable) meaning some molecules can cross the membrane and other cannot.

Most cells except for animal cells have cell walls. They are composed of carbohydrates depending on the kingdom. Cell walls are dead and are used to prevent the cells from bursting and give them structures. Cell walls are porous and allow most everything to cross- The cell membrane is the barrier that regulates what "get into and out of" the cell.

All cells have membranes surrounding them and many of their organelles. The basic structure is a bilayer of phospholipids with proteins inserted in the layer. This model is the fluid mosaic layer of cell membranes.

All cells have membranes surrounding them and many of their organelles. The basic structure is a bilayer of phospholipids with proteins inserted in the layer. This model is the fluid mosaic layer of cell membranes. The charge phosphate group is the hydrophilic head and the hydrocarbon tails are hydrophobic tails.

The cell membrane has two layers of phospholipids as shown below. The hydrophilic heads are facing an aqueous environment and the hydrophobic tails are facing one another

Very seldom in illustrating phospholipids, are all the atoms shown. Instead they are illustrated as such-

The phospholipids are capable of horizontal movement but very little vertical movement. Proteins can also exhibit lateral movement.

When the bilayer contains more unsaturated phospholipids, the membrane becomes more fluid. When preparing for winter, many organisms incorporate more unsaturated phospholipids.

A word about cholesterol - It is found in the cell membrane of animals but not plants. It functions in the following:

1. It can weakly bind to hydrocarbon tails making it more difficult for smaller molecules to cross membrane.

2. If the phospholipids are saturated, it prevents them from being packed too closely, making the membrane more fluid.

3. However - if the phospholipids are unsaturated there are kinks in the tails and the cholesterol molecules can fill in and anchor them.

Proteins are "stuck" in the membrane like a mosaic. They can be lying on the surface in an integral part of the membrane. Proteins can be on just the surface (extrinsic) or embedded in the membrane (intrinsic). It is the different proteins that are responsible for the uniqueness of different membranes (plasma, eukaryotic, prokaryotic, organelle etc.)

Function of Membrane Proteins

1. Transport Proteins or permeases transport molecules across the membrane. (Aquaporins are channels used to move water across)

2. Enzyme-Some proteins in the membrane may expose their active site to speed up a chemical reaction.

3. Receptor site-Ex. Insulin never goes into a cell but binds to a receptor site on the cell membrane

4. Cell to cell recognition.

5. Intercellular joining

6. Attachment to the cytoskeleton and extracellular matrix (ECM only in animal cells)

The plasma membrane is increased and decreased with the interaction of vesicles. Vesicles that are bringing material to the membrane to be secreted will increase the surface area of the plasma membrane. Through the process of endocytosis, the surface area of the cell membrane can be decreased.

The cell membrane is said to be semi-permeable or selectively permeable because certain molecules can cross the membrane and others can not. Those that can cross the membrane are said to be permeable and those that cannot are said to be impermeable

Passive diffusion- the molecules (CO2, O2, etc) involved are permeable to the membrane. Predicting the net movement is as if there were no membrane at all. The molecules move from high to low concentration.

In the first system, molecule A is more concentrated on side 2 than side 1. Molecule A is permeable to the membrane. The net movement of molecule A is from side 2-->1. At some point in time the concentration of molecule A on side 1 equals that of A on side 2. Equilibrium is reached. The movement of molecule A across the membrane is equal in both directions.

The net movement is from the side from the side with the high concentration of A to the side with the low concentration of A.

Facilitated diffusion- smaller molecules(glucose, amino acids etc.) are impermeable to the membrane. Because of that, a transport protein is needed to transport the molecule across the membrane. Predicting the net movement is like passive diffusion. The net movement of the molecules move from an area of high concentration to an area of low concentration.

The protein carriers (permeases) may take the form of a channel that allow certain molecules with a certain shape to enter the cytoplasm and others may take the form of a carries protein that changes conformational shape to move the molecules across. Facilitated diffusion and passive diffusion are also called passive transport as no energy in the form of ATP is needed.

Active Transport- molecules are impermeable to the membrane and the molecules are moving against a concentration gradient (from low to high) Because of that, a transport protein and ATP is needed.

First example is an electrogenic pump which is active transport that is goes against a concentra-tion gradient AND a charge gradient. The second example is cotransport involving an electrogenic pump and then the build up of H+ ions drives the cotransport of sucrose across the membrane by facilitated diffusion.

Osmosis-is the special case of the movement of water. When things are impermeable to the membrane and not in equilibrium, water will move across the membrane to help reach equilibrium. "Water likes to dilute".

Hypertonic-An excess of solutes in the surrounding water that are unable to cross the membrane. Extreme- Submerging cells in syrup. These cells tend to lose water to the environment until the concentration of solutes inside equals that outside the cell. Then the movement of water will be same in both directions. Plasmolysis is the plasma membrane pulling away from plant cell walls.

Hypotonic-A solution that has fewer solutes. Extreme- Submerging cells in pure water. Water crosses into the cell until the concentration of solutes inside equals that outside the cell, however, animal cell may not be able to accommodate the water and will burst. Plant cells become turgid because of a back pressure from the cell wall. Then the movement of water will be same in both directions.

Isotonic- Two solutions that has equal amounts of solutes on both sides of the membrane. Water moves across the membrane at the same rate.

Endocytosis is the movement of larger particles into the cell by use of membrane vesicles. Three types

Phagocytosis-cell eating

Pinocytosis-cell drinking

Receptor mediated- Receptors on the outside of cell membrane allows for the attachment of a particular molecule. When a certain number of receptor sites are filled, endocytosis occurs.

Exocytosis- is the movement of larger particles out of the cell by membrane vesicles