Biology (UP Slide 11 only) - Diffusion and Osmosis

• Plasma membranes are a barrier to most substances
• This allows materials to be concentrated inside cells, exlcuded from cells or simply separated from the environment.
• This is compartmentalisation and it is essential for life. It enables reactions to take place that would otherwise be impossible. Eukaryotic cells also compartmentalise materials inside organelles.

Osmosis is the diffusion of free-moving water molecules from an area of high water potenial to an area of low water potential through a partially permeable membrane

Water potenial is measured in kPa. Pure water is 0kPa. The cell cytoplasm has dissolved sugars and salts, so the water potential is more negative. A solution with a high water potential has a *large number of free-moving water molecules. Represented by Greek letter psi. /

• Hypotonic: one of two solutions with more water and less solute (more positive water potential). Cells in this solution will expand, as there is high WP outside cell.
• Hypertonic: one of two solutions with more solute and less water (more negative water potential). Cells in this solution will shrink as there is lower WP outside cell.
• Isotonic: one of two solutions with the same water potential and same amount of solute.

Animal cells:

• Crentation: too much water leaving the cell, damaging and shrinking them
• Haemolysis: too much water entering the cell, so it bursts

Plant cells:

• Plasmolysis: too much water leaving cell, damaging and shrinking them
• Flaccid: Normal, in isotonic solution
• Turgid: too much water entering the cell, so it bursts

Animal cells:

• Hypotonic solution: Haemolysed; moves from high WP to low; free moving water molecules move from the solution into the cell
• Hypertonic solution: Crenated; High WP to low; free moving water molecules move from the cell into the solution

Plant cells:

• Hypotonic solution: Turgid; moves from high WP to low; free moving water molecules move from solution into cell
• Hypertonic solution: Plasmolysed; moves from high WP to low; free moving water molecules move from the cell to the solution

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, until evenly distrubuted. It is a passive process.

Facilitated diffusion is exactly the same as simple diffusion, except it needs the help of a protein channel or a carrier protein.

1. Particles are in high concentration. High concentration gradient. Diffusion is rapid at this point. Movement of particles is random and they collide with each other (and container). There is only a net movement of particles in one direction (from high to low)

2. Through random movement, particles have spread out little. Movement of particles in both directions, but overall a net movement in one direction (high to low). Smaller concentration gradient. Diffusion begins to slow.

3. Particles are now evenly distributed. There is no concentration gradient and no diffusion. Particles continue to move, but there is the same probability of them moving each way.

4. Particles remain distributed. Individual particles move, but overall number on each side remains the same. They are in dynamic equilibrium.

• Size of the molecule: less KE is needed to move a small molecule/ion, so at the same temp, a small molecule will move faster, increasing rate of diffusion
• Difference in the concentration gradient: The bigger the difference in concentration gradient, the greater the probability of molecules/ions colliding over the region and increasing rate of diffusion. This will also affect the speed at which the molecules/ions move
• Distance to travel: If the speed is the same, but the distance is smaller e.g. thin membrane, then the time taken to move will be quicker and increase rate of diffusion
• Temperature: Molecules have more KE at higher temperatures, so there will be more collisions, increasing the rate
• Surface Area: The more space there is for diffusion to occur, the faster it can happen as there are more opportunities for diffusion to happen.

There are limitations to simple diffusion across membranes: the molecule must be lipid-soulube/non-polar so it can be passed through the hydrophobic tails of the membrane, and must be small so that it can fit. If the molecule is water-soluble/polar or large, it must be moved by facilitated diffusion.

Protein Channels

• These form water-filled hydrophilic channels across the membrane.
• They therefore allow specific water-soluble ions to pass through
• Channels are selective, and only open if a particular ion is present. If not they remain closed to control entry/exit of ions.
• The ions bind with the protein, so it changes shape in a way that closes it to one side of the membrane and opens it to the other side.

Carrier Proteins:

• Alternative form of facilitated diffusion. These proteins span the membrane
• When a molecule specific to protein is present e.g. glucose, it binds with the protein
• This causes the protein to change shape in such a way that the molecule is released to the inside of the membrane.
• No external energy needed
• Move from high to low concentration using their own kinetic energy

Active transport is the process by which a cell takes up a substance against a concentration gradient (from low to high, or from high to higher) using ATP and carrier proteins.

It uses protein carrier molecules, but also requires energy. This comes in the form of a molecule called ATP produced during respiration.

Cells in which lots of active transport takes place, e.g. epithelial cells in small intestine, have many mitochondria to produce ATP for active transport.

See kerboodle textbook and schoology and notes