Diffusion

Both plants and animals rely on the process of diffusion to ensure useful substances are taken in and waste products are removed.

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

The rate of diffusion can be increased by increasing the temperature, increasing the concentration , and increasing the surface area

Temperature

Increasing the surface area makes particles move faster as they gain kinetic energy so spread out quicker until they reach an equilibrium

Surface area

The larger the surface are the faster the rate of diffusion as there is more space for more particles to diffuse across at the same time

Concentration gradient

The difference between the concentration is the number of molecules on either side of the membrane. If there is a big difference we say there is a high concentration gradient. If there is a small difference we say there is a low concentration gradient.

The cell membrane has tiny holes through which small particles can pass by diffusion, we say that the membrane is partially permeable. Diffusion is a passive process (it doesn’t require any energy for it to happen)

Surface area - Volume ratio

Surface area = area of 1 face (of shape) * the amount of faces

Volume = length ** width * height*

The ratio is set out as surface area : volume

The greater the surface area to volume ratio the more efficient diffusion is. Singular celled organisms have a large surface area to volume ratio maximising diffusion. Large multicellular organisms have a low surface area to volume ratio so need specialised exchange surfaces to make the process more efficient.

Using a cube :

As the length of the side increases the surface area to volume ratio increases

Specialised exchange systems

• Alveoli - Found in the lungs

• Leaves and root hair cells - In plants

• Villi - Found in the small intestine

• Gills - Found on fish (take oxygen out of the water)

Osmosis

Osmosis is the movement of water molecules across a partially permeable membrane from an area of high water concentration to an area of low water concentration until it reaches an equilibrium.

Solutions and osmosis

Hypotonic solution - water moves into the cell, cell membrane pushes up against the cell wall and it becomes turgid

Isotonic solution - water moves out of the cell, the vacuole stops pushing out against the cell membrane and cell wall , the cell becomes flaccid

Hypertonic solution - if water continues to move out, the cell membrane pulls away from the cell wall it is plasmolised

Active transport

Active transport is the movement of substances from an area of low concentration to an area of high concentration going against the concentration gradient across a partially permeable membrane (it requires energy from respiration and involves carrier proteins in the cell membrane)

Active transport allows mineral ions, such as nitrates to be absorbed into plant root hairs from very dilute solutions in the soil.

Root hair cells are adapted to take in water and mineral ions by:

Lots of tiny hairs produce a large surface are

Tiny hairs are 1 cell thick for short movement pathway

Lots of mitochondria releasing energy for active transport

Active transport in humans

Whilst the majority of digested food molecules are absorbed into the bloodstream by diffusion, glucose sometimes has to be moved from the villi (small intestine) into the blood stream by active transport. (villi are small folds in the small instestine)