The Movement of Substance

Substance move by diffusion, osmosis and avtive transport.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. 

Osmosis is the diffusion of water from a dilute soultion to a concentrated solution through a partially permeable membrane.

Active transport is the absorbtion of particles against the concentration gradient, using energy from respiration. 

Cell Membranes:

• Dissolved substances can move in and out of cells by diffusion.
• Only small molecules can diffuse through cell membranes, like oxygen, amino acids, glucose and water.
• Big molecules like sugar and proteins can't fit throught the gaps in the cell membrane.
• The particels move both ways, due to their random movement, but there is an overall net movement into the region with a lower concentration.

Leaves:

• The structure of a leaf is adapted to let gases diffuse in and out of cells easily
• Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. 
• The underneath of the leaf is an exchange surface
• It's covered in stomata which carbon dioxide diffuses in through
• Oxygen (produced in photosynthesis) and water vapour diffuses out through the stomata
• The flattened shape of the leaf and internal air spaces increase the area of this exchange so that it's more effective.

Osmosis is the diffusion of water from a dilute soultion to a concentrated solution through a partially permeable membrane.

The water particles move both ways, due to their random movement, but there is an overall net flow of water into the region with fewer water molecules.

Differences in the concentrations of the solutions inside and outside a cell causes water to move into and out of the cell by osmosis.

Osmosis in Humans:

Tissue fluid which surrounds and supplies the bodies cells with everything they need usually has a different concentration to the fluid inside the cell, so water will move into or out of the cell by osmosis.

Short of water:                                           Lots of water:

• Solution inside is more concentrated       Solution inside is more dilute
• Solution outside is more dilute                Solution outside is more concentrated
• Water moves into the cell by osmosis     Water moves out by osmosis

• Method:

1) Cut potato into identical cylinders

2) Fill different beakers with different concentrations of sugar solutions. One should be pure water, another should b a very concentrated sugar solution.

3) Measure the lenght of the potato cylinders and add a few to each beaker. Leave for half an hour

4) Take out potato cylinders and measure lengths.

• Results:
• If the cylinders have drawn in water by osmosis, they'll be a bit longer.
• If water has been drawn out of the cylinders by osmosis, they'll have shrunk a bit

• Variables:
• Dependent = cylinder length, Independent = the concentration of the sugar solution.
• All other variables must be contolled for it to be a fair test. e.g. volume of solution/time/temp./type of sugar used etc.

Root Hair Cells: 

• Root hair cells take in minerals using active transport
• The concentration of minerals is usually higher in the root hair cell than in the soil around it. 
• Active transport allows the plant to absorb minearls from a very dilute soultion, against a concentration gradient.
• This is essential for growth
• But active transport needs energy from respiration to make it work.

The Gut:

• Active transport is used in the gut when there is a low concentration of nutrients in the gut, but a high concentration of nutrients in the blood.
• Active transport allows nutrients to be taken into the blood, against a concentration gradient
• This is essential for growth
• But active transport needs energy from respiration to make it work.

The Kidney:

• During the process of reabsorbtion in the kidney, active transport is needed to enable sugar, ions and water to be reaborbed into the blood, against a concentration gradient. 
• This is essential for growth
• But active transport needs energy from respiration to make it work.