UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 23/05/2023 [SEARCH]

Transport: 7. Active transport explained, why is it needed? how does it work?

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(7) Active transport of soluble molecules and ions is explained

You need to explain how this happens having seen that in osmosis only water passes through a partially permeable membrane.

However, cells cannot just rely on diffusion and osmosis for the input (e.g. nutrients) and output (e.g. waste) of substances.

What is 'active transport'

Active transport differs from spontaneous diffusion where there is a net transfer of substances down a concentration gradient in two ways.

(i) The transport of materials works against the concentration gradient - opposing the normal diffusion direction.

(ii) Active transport requires a net energy input involving other molecular systems to transport material through a membrane against the concentration gradient.

The mechanisms of active transport are complex but the process involves protein carrier molecules in cell membranes to convey the nitrate ions into the root cells,.

Active transport is the movement of particles across a membrane against a concentration gradient.

So, sometimes, substances are absorbed by cells against a concentration gradient - a net transfer against the normal diffusion gradient action is required.

This means transfer occurs in the opposite direction to the natural direction of the diffusion gradient.

e.g. active transport enables cells to absorb ions from very dilute solutions.

BUT, this movement of chemicals across a cell membrane against a natural diffusion gradient, requires the use of energy from respiration (via ATP molecules) and the overall process is called active transport.

Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration.

Active transport is very complicated system

BUT, 'basically' it uses transport molecules and ions within the membrane structure and the process powered by ATP from respiration.

By this means soluble nutrient molecules/ions can pass through the membrane into cells.

 

Examples of active transport

The gut and digestion

The villi in the small intestine absorb glucose and other nutrients from the gut and transfer them into the bloodstream.

The diagram illustrates the movement of molecules (green spheres) being moved through the membrane of the gut from the gut into the bloodstream, in the opposite direction to the natural diffusion gradient.

(The blue circles represent water molecules - solvent medium.)

The red circles represent the relatively large red blood cells, which are too large to get through the membrane, so staying in the bloodstream, to be joined by nutrient molecules (green circles) and ions via active transport.

Active transport is required to absorb nutrients (green circles) like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

If the concentrations of nutrients (e.g. sugars, amino acids) in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

If the concentration gradient flow is in the direction of the blood stream (higher) to the gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

So active transport enables the gut to move nutrients like into the blood even though the natural concentration gradient (diffusion gradient) is the wrong way round.

Glucose can be transferred into the blood stream, even if its concentration is higher in the blood stream, and so conveyed to cells for respiration.

Other examples where active transport is essential in animals

Cells in the kidney reabsorb sodium ions from urine - sodium ions are needed for many biochemical processes in the body.

Fish in seawater have cells in the gills than can transfer salt back into the more salty sea water.

Similarly, crocodiles have salt glands in their tongue that can transfer excess salt from their bodies back into the water.

These are two good animal examples of adaptations to their environment.

The thyroid glands have cells that can concentrate iodine, against the diffusion gradient, to make the important hormone thyroxine.

 

For more on the gut and other examples see

Surfaces for the exchange of substances in animal organisms

 

Active transport in plants

Active transport is used in the absorption of nitrates and other ions by plant roots.

 It is essential plant roots can absorb vital nutrients for the health and growth of a plant.

For details see

Transport and gas exchange in plants, transpiration, absorption of nutrients etc.


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