- Osmosis is the diffusion of water
- Water diffuses from a dilute concentration to a more concentrated solution through a partially permeable membrane
- Water moves into or out of the cells by osmosis through the cell membrane
Remember: osmosis only refers to the movement of water
- Active transport allows cells to take in substances against a concentration gradient.
- Active transport uses energy from respiration
- Ions, from dilute solutions and other substances such as sugar can be absorbed by active transport
- Glucose can be reabsorbed in the kidney tubules by active transport.
Remember : active transport requires energy; diffusion and osmosis do not.
The sports drink dilemma
- Most soft drinks contain:
- water to replace water lost when sweating
- sugar to replace sugar used for energy release in exercise
- mineral ions to replace those lost in sweating
- For normal levels of exercise, water is likely to be as effective as a sports drink
- The drinks are designed to help balance the concentration of body fluids and the concentration inside cells. If the drink concentration matches the body fluids the sports solution is called isotonic
Exchanging materials - the lungs
- Large and complex organisms need exchange surfaces to obtain all the food and oxygen they need.
- Efficient exchange surfaces have a large surface area, thin walls or a short diffusion path and an efficient transport system
- The lungs contain the gaseous exchange surface. The surface area is increased by the alveoli (air sacs)
- The lungs are ventilated to maintain a steep diffusion gradient
- Oxygen diffuses into the capillaries surrounding the alveoli and carbon dioxide back out into the lungs to be breathed out
Ventilating the lungs
- The lungs contain the exchange surface of the breathing system.
- The lungs are situated in the thorax, inside the ribcage and above the diaphragm, which separates the lungs from the abdomen
- When we breathe in:
- The intercostal muscles contract
- The ribcage moves up and out and the diaphragm flattens
- The volume of the thorax increases
- The pressure in the throax decreases and air goes in
- When we breathe out:
- The intercostal muscles relax
- The ribcage moves down and in and the diaphragm becomes domed
- The volume of the thorax decreases
- The pressure increases and air is forced out
Artificial breathing aids
There are many reasons why someone cannot get enough oxygen into their bloodstream:
- If the alveoli are damaged, the surface area is reduced
- If the tubes to the lungs are narrowed, less air can be moved through them
- If the person is paralysed their muscles will not work to pull the ribcage up and out
Several types of breathing aid have been developed:
- The 'iron lung' was used for people with polio who were paralysed. When air was drawn out of the iron lung the person's chest moved out and they breathed in. The vacuum which was formed inside the cylinder created a negative pressure. When air was forced into the cylinder it created pressure on the chest and air was forced out of the lungs.
- Breathing aids use positive pressure, bags of air linked to masks force air down the trachea,
- Positive pressure aids are smaller, easier to manage in the home and can be linked to computers for control.
Exchange in the gut
- The villi are an efficient exchange surface for the absorption of the soluble products of digestion by diffusion or active transport.
- Villi have a large surface area and have a good blood supply.
- Villi have a thin wall (only one cell thick) so there is only a short distance for diffusion to take place.
Exchange in plants
- Gases diffuse in and out of the stomata. The size of the stomata is controlled by guard cells. The gases that diffuse are:
- Oxygen - needed for respiration and is a waste product of photosynthesis
- Carbon dioxide - needed for photosynthesis and waste product of respiration
- Leaves are flat and thin with internal air spaces to increase the surface area for diffusion of gases
- Root hair cells increase the surface area of roots
- Water is lost from the leaves by evaporation
- The water vapour escapes the leaf through the stomata when they open up to allow carbon dioxide into the leaf.
- Wilting can protect the leaf from excessive water loss.
- Conditons which increase the rate of evaporation will increase the rate of transpiration.