Edexcel Biology Gcse - Transport In Cells Part 2

The overall effect of something when all additions and deductions are calculated

The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration

Also called semi-permeable. A partially permeable membrane allows water and other small molecules to pass through, but not larger molecules such as starch

This is the movement of molecules down a concentration gradient without the need for additional energy, eg diffusion and osmosis

Range is the difference between the highest and lowest values in a set of data

A specialised cell that increases the surface area of the root epidermis to improve the uptake of water and minerals

The dissolved substance in a solution

Mixture formed by a solute and a solvent

The liquid in which the solute dissolves to form a solution

A disaccharide made from glucose and fructose. It is used as table sugar

Fluid which is derived from blood plasma that passes through the walls of capillaries

Enlarged and swollen with water. Having turgor. Description of a plant cell in which the vacuole has swollen due to water gain by osmosis

Particles in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher concentration, they will therefore move from this region to where they are in a lower concentration. This is called diffusion.

It is important to remember that the particles:

will move in both directions, but there will be a net movement from high to low concentration

will end up evenly spread throughout the liquid or gas but will continue to move

If a crystal of a coloured chemical, eg potassium manganate(VII), is placed in water, the particles spread out and mix with the water particles. The process is:

1) the potassium manganate(VII) has dissolved

2) the potassium manganate(VII) is the solute

3) the water is the solvent

4) the mixture that results is the solution

The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

1) Concentration gradient:

The greater the difference in concentration, the quicker the rate of diffusion

2) Temperature:

The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly

3) Surface area of the cell membrane separating different regions:

The greater the surface area, the faster the rate of diffusion

For a bacterium, substances diffuse into and out of the bacterial cell across its surface. Once inside, because of the bacterium's size, substances will need to diffuse 1 μm or less to where they are needed.

For simple multicellular organisms, such as small plants such as mosses, substances diffuse into the leaves and simple roots over their surface. Again, once inside the plant, they don’t need to move far. Substances move into and around the moss plants by diffusion and osmosis.

Simple organisms therefore take in substances over their body surface. Their needs are determined by their volume. As organisms increase in size, their surface area does not increase at the same rate as their volume. For example, the surface area to volume ratio of a puppy is several times greater than that of an adult dog.

Dogs lose heat over their body surface. Puppies have a larger surface area to volume ratio than adult dogs, so will lose heat more readily.

Method:

1) Cut a 1 cm3 cube of agar. The agar should be made using 0.1 M sodium hydroxide and coloured with phenolphthalein indicator to produce pink-coloured alkaline agar.

2) Place a 1 cm3 cube of agar into a 0.5 M solution of hydrochloric acid for two minutes.

3) Remove the cube and wash it in water to stop the reaction.

4) Cut the cube in half and measure the distance in millimeters that the acid has caused the agar to become colourless from the outside, inwards towards the centre.

5) Repeat the experiment a further two times and calculate a mean value.

6) Repeat using 1, 1.5 and 2 M solutions of hydrochloric acid.

Risks:

-Hydrochloric acid is corrosive. If it touches skin it should be washed off.

-Goggles should be worn at all times.

Osmosis is the diffusion of water molecules, from a region where they are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane.

A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.

Osmosis refers to the movement of water molecules only.

When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. There will be no net movement of water molecules.

Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar.

Cells contain dilute solutions of ions, sugars and amino acids. The cell membrane is partially permeable. Water will move into and out of cells by osmosis.