Transport Through Membranes

Diffusion - movement of molecules from a region of high concentration of that molecule to a region of lower concentration of that molecule down a concentration gradient.

Molecules and ions in a gas or liquid posses kinetic energy that keeps them moving, diffusion is dependent on kinetic energy.

When overall there is no longer a net movement of molecules the molecules are said to have reached equilibrium. Most living organisms have systems in place to ensure equilibrium is never reached (thus the conc. gradient is maintained).

Factors that affect rate of diffusion

• Temperature - inc. in temperature inc. kinetic energy of the molecules.
• Concentration Gradient 
• Stirring/Moving - inc. movement of molecules thus rate of diffusion
• Surface Area - diffusion occurs more rapidly if there is a greater surface area to diffuse across. 
• Distance/Thickness - diffusion is slowed down by long diffusion paths
• Size of molecule - smaller molecules diffuse more quickly than larger ones 

Simple Diffusion 

• Lipid based molecules - the phospholipid bilayer is made of lipids.
• Very small molecules and ions - can fit through the bilayer, really smaller polar molecules can use this form of diffusion.

Facilitated Diffusion 

• Ions like sodium and calcium ions use channel proteins which are essentially pores in the membrane that have a shape specific to a certain ion.
• Larger molecules like glucose and amino acids use carrier proteins. Carrier proteins can change shape to allow the molecule through to the other side of the membrane. Like channel proteins shape is specific.

Different membranes can have different carrier and channel proteins to control which substances enter the cell.

Active Transport - movement of molecules or ions across membranes, which uses ATP to drive protein carriers within the membrane.

Moves against concentration gradient and carry specific molecules one way across a membrane, which allows molecules to be accumulated in specific areas. This process is much faster than diffusion.

Examples - magnesium ions in the soil, needed to produce chlorophyll, need active transport. Calcium ions needed to make our muscles contract. 

Endo/exocytosis - Process of taking materials into or out of a cell by surrounding them with part of the plasma membrane which then pinches off to form a vesicle. It is an active transport requiring ATP.

Examples - hormones like insulin, white blood cells and plants carry & transport the material needed to make cell walls using endocytosis.

**Spare terms 

Endo - inwards, exo - outwards, phago - solid, pino - liquid.

Osmosis - movement of water molecules from a region of higher water potential to a region of lower water potenial across a partially permeable membrane.

Partially permeable membrane - membrane that will allow some molecules to pass throufh but will not allow other molecules to pass through.

Water potential is a measure of the tendancy of water molecules to diffuse from one place to another, it always moved from a high concentration of 'free' (those not clustered around a dissolved molecule) water molecules to region of lower concentration. 

As with diffusion, osmosis will continue to occur until equilibrium (same water potential on both sides of the membrane) is reached.

Units of measuring water potential (kPa - kiloPascals)

Pure water has the highest water potential = 0kPa

A dilute solution (small amount of solute dissolved) = -10kPa

A concentrated solution (large amount of solute dissolved) = -500kPa

The concentration of cells is lower than that of pure water because of the substances dissolved in the cytoplasm. Animal cells also have a vacuole which contains dissolved substances.

Cells + High Water Potential.

• Forms a water potential gradient from outside to inside the cell. Water will move down a concentration gradient into the cells via osmosis. 
• Plant cells will become turgid because the cell wall will provide a counter pressure that prevents further osmosis (regardless of a water potential gradient).
• Animal cell becomes haemolysed (bursts)

Cells + Low Water Potential 

• Water potential gradient from inside to outside the cell forms. Water will move out of the cell, down a concentration gradient, via osmosis. 
• The plant plasma membrane will begin to pull away from the cell wall because the vacuole and cytoplasm have shrunk so much. The cell is said to be plasmolysed. 
• Animal cell shrinks and appears wrinkled, and is said to be crenated.