Diffusion

• Diffusion is the gradual movement of particles from places where there are lots to where there are fewer

• Technical Definition: the ‘net movement of particles from an area of high concentration to an area of low concentration’

• Its a passive process - no energy required.

• Happens in both liquids and gases - the particles there are free to move about randomly

• The simplest type of diffusion is when different gases diffuse through each other, eg: the smell of perfume through a room.

The bigger the difference in concentration, the faster the diffusion rate

• Things diffuse through cell membranes - selectively permeable ones. They hold the cell together, but let things in and out as well.

• Substances can move in and out of cells via diffusion, active transport, and osmosis

• Only very small molecules can diffuse through cell membranes though, things like glucose, amino acids, water and oxygen. Bigger molecules like starch and **proteins **cannot fit through.

• Like diffusion in air, particles flow through the cell membrane from a higher concentration to a lower one.

• Although, its entirely random, so it does go both ways, but if one side has more particles, there’s a net (overall) movement from that side

Osmosis

• Technical definition: ‘Osmosis is the net movement of water molecules across a partially permeable membrane from a high concentration of water to a low concentration of water’

• Selectively permeable membrane - just one with very small holes. Only tiny molecules (eg: water) can pass through. The cell membrane is a partially permeable membrane

• Water molecules pass both ways through the membrane during osmosis. This happens because water molecules move randomly all the time

• But, because there are more water molecules on one side then the other, there’s a steady net flow into the side of the membrane with less water, eg: into a sucrose solution. This means the sucrose solution gets more dilute, as the water acts like it’s trying to ‘even up’ the concentration on either side of the membrane.

• Tissue fluid surrounds the cells in the body - it’s just water with oxygen, glucose and other things dissolved in it. It’s squeezed out of the blood capillaries to supply the cells with everything they need

• The tissue fluid will usually have a different concentration to the fluid inside a cell. This means that water will either move into the cell from the tissue fluid, or out of the cell, both via osmosis.

• If a cell is short of water, the solution inside will become v concentrated. This means the solution outside the cell is more dilute, so water will move into the cell by osmosis (because although the cell is more concentrated, there is a less concentration of water)

• If a cell has lots of water, the solution inside will be more dilute, so water will move out of the cell and into the fluid, again, via osmosis. (for the opposite reason, there is a higher concentration of water in the dilute cell, so it will move out into the concentrated fluid - think squash!)

Diffusion + Osmosis Experiments

• Experiment One

  • Make up some agar jelly with Phenolphthalein (a pH indicator, pink in alkaline solutions, and colourless in acidic ones) and Dilute Sodium Hydroxide, your jelly should be pink.

  • Put some Dilute Hydrochloric Acid in a beaker. Cut a few cubes from the jelly and add them to the beaker of acid as well.

  • you leave the cubes for a while, they’ll eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide

• Experiment Two

  • Cut up a potato into identical cylinders and get some with different sugar solutions in them: one should be pure water, another should be a very concentrated sugar solution, have a couple more with varied concentrations between those two in them. Measure the length of the potato cylinders then, leave a few cylinders in each beaker for half an hour or so. Then take them out and measure them again.

  • If the cylinders have taken in water via osmosis, they should be longer. The potato cylinders in the pure water should’ve done this. If the water has moved out, they’ll be smaller - like the ones in the concentrated sugar solution.

• Experiment three

  • Put some visking tubing over the edge of a thistle funnel. Then pour some sugar solution down the glass tube into the thistle funnel.

  • Put the thistle funnel into a beaker of pure water - measure where the sugar solution comes up to on the glass tube.

  • Leave this overnight, then measure where the solution is in the glass tube. Water should be drawn through the visking tubing by osmosis and this will force the solution up the glass tube (the thistle funnel has an open bottom where the visking tubing goes)

Active transport

• Definition: ‘Active Transport is the movement of particles against a concentration (i.e: from a lower concentration to a higher concentration) using energy released during respiration’

• Active transport is used to move substances in and out of cells. For example, active transport is used in the digestive system when there is a low concentration of nutrients in the gut, but a high concentration of nutrients in the blood

  • When there’s a high concentration of nutrients in the gut they diffuse naturally into the blood.

  • But sometimes it’s the opposite: there’s a low concentration of nutrients in the gut, and a higher one in the blood. The concentration gradient is the wrong way. If the nutrients followed the rules of diffusion, they should go the other way.

  • Active transport allows nutrients to be taken into the blood, despite the fact that the concentration gradient is the wrong way. This is essential to stop us from starving. However, active transport needs energy from respiration to work

• The rates of diffusion, osmosis and active transport vary. They’re affected by several factors:

  • Imagine cells are cubes: The rate of diffusion, osmosis and active transport is higher in cubes (cells) with a larger surface area to volume ratio.

  • The smaller cube has a larger surface area to volume ratio (Imagine you have two cubes, one with each face having an area of 2cm², and another with each face having an area of 3cm². The 2cm²’s surface area is 2 x 2 x 6 (base x height x number of sides) and it’s volume is 2 x 2 x 2 (base x height x width). This equates to a surface area to volume ratio of 24:8, or 3:1. Now, the bigger one’s surface area to volume ration is 54:21, or 2:1. Hence, the smaller cube has a larger surface area to volume ratio)

  • This means the substances would move in and out of the cube/cell faster.

  • If substances only have a short distance to move, then they’ll move in and out of cells faster. Cell membranes, for example, are very thin.

  • As the particles in a substance get warmer, then they’ll have more energy, so they’ll move faster. This means as temp increases, substances move in and out faster

  • Substances move in and out faster the larger the concentration gradient. This only increases the rate of diffusion and osmosis though - concentration gradients do not affect active transport.