Biology B3 unit 3

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These notes don't yet include Yeast, fermentation, biogas and yoghurt and cheese making. they will however be coming soon.

Diffusion-along concentration gradient

osmosis -along concentration gradient (only involves water through a partially permeable membrane)

active transport- against concentration gradient (LOW to HIGH concentration) uses energy released in respiration  to move substances into cells via a transport protein in the cell membrane. 

Gas Exchange- body needs a constant supply of oxygen. Breathing bring oxygen into lungs and removes carbon dioxide.

Alveoli in lungs make gas exchange as efficient as possible:-

  • large surface area
  • moist surface to allow gases to diffuse more easily
  • rich blood supply (capillaries) which maintains high concentration gradient 
  • this layer of cells between alveoli and capillaries so substance only ave to travel a short distance.

Exchange in gut-your food is broken down in your gut to form simple sugars (glucose, amino acids, fatty acids and glycerol) which are absorbed into blood stream by a combination of diffusion and active transport. These molecules provide fuel for respiration. Food molecules need to be broken down into a soluble form so that the can dissolve in water and diffusion can take place. they move into the blood along a steep concentration gradient. 

The small intestion is folded into thousands of projections known as villi which increase the surface area for diffusion. There is also a rich blood supply to maintain a steep concentration gradient (makes diffusion rapid and efficient) Glucose is absorbed via active transport so no food is wasted. 

Fish cant get oxgen directly from water because of their scales os they have evolved gills. Gills are made up of many thin layers of tissue with a rich blood supply. Some fish have to keep moving to keep water moving over their gills. Their gills dont work in air because they stick together.

Frogs have very moist skin, so that is where most gas exchange takes place. but they alos have a pair of very simple lungs.

Insects need alot of oxygen for heir muscles because they are so active. Gas exchange happens through spiracles on their sides. These lead to a system of tubes that go right inside the cells. Most exchange happens in the tracheoles. There is no blood supply in an insect.

Exchange in plants- plants rely on diffusion for photosynthesis, osmosis to get water from the soil and active transport to obtain minerals from the soil. 

Photosynthesis carbon dioxide + water ---> oxygen + glucose (+energy)

Light is a limiting factor of photosynthesis therefor photosynthesis only takes place during daylight. Carbon dioxide is obtained through by diffusion into the leaves. Leaves provide a large surface are for diffusion to take place. There are many large air spaces inside the leaves to allow air to come in contact with as many cells as possible.

Leaf cells constantly lose water by evaporation. Water is vital for plants to maintain the shape of cells for photosynthesis. If carbon dioxide can diffuse easily in and out of cells so can water. Leaves are covered…



This really helped me, thanks! (Y)

Matthew Dawber

Good notes, thanks!

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