Diffusion, Osmosis and Active Transport

Diffusion is the spreading out of materials from a high concentrated area to a low concentrated area. Diffusions happens in both solutions and gases. The bigger the concentration gradient, the faster the diffusion rate. A higher temperature will also give a faster diffusion rate because the particles would have more energy, and so move around faster. Dissolved substances move in and out of cells by diffusion. However, only small molecules like oxygen, glucose, amino acids and water can diffuse through the cell membrane. The larger the surface area of the cell membrane, the faster the diffusion rate, because more particles can pass through at once.

Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration. A partially permeable membrane is a membrane with small holes in it which allows only tiny molecules like water to pass through them. The water molecules actually pass both ways through the membrane during osmosis as water molecules move around randomly all the time.

1) Cut up a potato into identical cylinders and place them in some beakers with different sugar solutions. One should be pure water and the other should be highly concentrated.

2) Measure the mass of the cylinders and then leave the cylinders in the beaker for approximately 24 hours.

3) Take the potatoes out of the beaker and measure their masses again after you dry them using a paper towel.

Active transport is the movement of molecules across a cell membrance against a concentration gradient. This allows a plant to absorb minerals from a very dilute solution, against a concentration gradient. Active transport requires respiration and energy to make it work. It also happens in humans. 

Cells can use diffusion to take in substances they need and get rid of waste products. For example, Oxygen and Carbon Dioxide are transferred between cells and the environment during gas exchange. The difficulty for an organism to exchange substances with the environment depends on the organism's surface area to volume ratio. 

Exchange surfaces are adapted to maximise effectiveness. They have a thin membrane, so substances only have a short distance to diffuse. They have a large surface area so lots of substances can diffuse at once. Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly. Gas exchange surfaces in animals are often ventilated too.

Gas exchange happens in the lungs. The job of the lungs is to transfer oxygen to the blood and to remove carbon dioxide from it. To do this, the lungs contain millions of air sacs called alveoli where gas exchange takes place. The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have an enormous surface area, a moist lining for dissolving gases, very thin walls and a good blood supply. 

The inside of the small intestine is covered in millions of tiny projections called villi. They enormously increase the surface area of the small intestines so that digested food is absorbed much more quickly into the bloodstream. The villi have a single layer of surface cells and a very good blood supply to assist quick absorption.

Carbon dioxide diffuses into the air spaces within the leaf and then it diffuses into the cells where photosynthesis happens. The leaf's structure is adapted so that this can happen easily. The underneath of a leaf is an exchange surface. It is covered in stomata which carbon dioxide diffuses in from. Oxygen and water vapour also diffuse out through the stomata. The size of the stomata is controlled by its guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. The flattened shape of the leaf increases the area of this exchange surface so that it is more effective. 

The gills are the gas exchange surface in fish. Water enters the fish through the mouth and passes out through the gills. As this happens, oxygen diffuses from the water into the blood in the gills. Each gill is made up of lots of thin plates called gill filaments which increases its surface area. This is covered in tiny structures called lamellae which have lots of capillaries.