BIOLOGY

                            Movement of Substances 

(a) define diffusion and discuss its importance in nutrient uptake and gaseous exchange in plants and humans 
Diffusion in the movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
It is important as the cells of the lungs exchange oxygen and carbon dioxide through diffusion. Digested food substances such as amino acids and glucose diffuse through the villi into the blood capillaries.
Plant cells take in oxygen and remove carbon dioxide through diffusion. 

(b) define osmosis and discuss the effects of osmosis on plant and animal tissues 
Osmosis is the movement of water molecules from a solution of higher water potential to a solution of lower water potential, across a partially permeable membrane. 
Animal cells will shrink and have little spikes appear on the cell surfacae membrane when placed in a solution of low water potential. The crenated cell will become dehydrated and eventually die.
When placed in a solution of high water potential, they will swell and burst.

Water enters the plant cell by osmosis. As water enters the cell, the vacuole increases in size and pushes the cell contents against the cellulose cell walls. The cell wall is strong and relatively inelastic. It prevents over-expansion of the cell by exerting an opposing pressure as water enters the cell. This prevents the entry of more water, The plant expands and becomes turgid. The turgor pressure keeps the cell turgid. Turgor helps to maintain the shape of soft tissues in plant and keep them firm and erect. The surrounding cells pushed against each other creating support for the plant.
• It also causes some flowers to open during the day and close at night. 
• The changes in turgor of cells in the small swellings at the base of the leaflets of mimosa plant causes them to fold when they are touched. 
• The closing and opening of the stomata is due to the changes in turgor of the guard cells.

Plasmolysis causes tissues to become limp or flaccid. Cells will be kille dif they remain plasmolysed for too long. 


(c) define active transport and discuss its importance as an energy-consuming process by which substances are transported against a concentration gradient, as in ion uptake by root hairs and uptake of glucose by cells in the villi 
Active transport is the movement of substances against a concentration gradient, from a region of lower concentration to a region of higher concentration. It is an energy-consuming process.

Dissolved mineral salts is absorbed by active transport through the root hair cells and glucose and amino acids by cells of the small intestine.