(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 molecules 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 of the epithelial cells 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 tubules 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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Key points about active transport

Based on the syllabus-specifications for students taking the UK AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10 biology).

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What Is Active Transport?

Active transport is the movement of substances against the concentration gradient - from an area of low concentration to high concentration - using energy from respiration.

• Unlike diffusion or osmosis, active transport requires energy in the form of ATP.

• It uses carrier proteins embedded in the cell membrane.

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Why Is Active Transport Needed?

Cells often need to absorb substances that are:

• Already in higher concentration inside the cell (going against the gradient),

• In low concentration outside, yet still essential for cell function.

Key Situations:

Example	Why It Needs Active Transport
Root hair cells in plants	Absorbing mineral ions (e.g. nitrates) from soil where they are in lower concentration than inside the root.
Small intestine cells	Absorbing all glucose and amino acids from the gut, even when their concentration is higher inside the cell.
Kidneys	Reabsorbing useful substances (e.g. glucose) back into the blood from the filtrate.

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How Does Active Transport Work?

Step-by-Step Process:

1. A specific molecule (e.g. glucose or an ion) binds to a carrier protein in the membrane.

2. Energy from ATP is used to change the shape of the protein.

3. The molecule is pushed across the membrane into the cell (or out, depending on the direction needed).

4. The carrier protein returns to its original shape, ready for another molecule.

This is highly selective — only specific substances are transported.

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Energy Source: ATP

• Active transport depends on energy from aerobic respiration.

• That’s why cells involved in active transport (e.g. root hair cells, epithelial cells of the intestine) contain many mitochondria to release energy.

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Exam Tips

• Clearly state that active transport moves substances against the concentration gradient.

• Emphasize that it requires energy (ATP) and uses carrier proteins.

• Give real biological examples — root hair cells and intestines are favourites!

• Don’t confuse active transport with diffusion or osmosis — these are passive.

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Keywords, phrases and learning objectives for this part on transport in organisms

Explain active transport of soluble molecules and ions and why it is needed?

How does active transport work? e.g. why needed in gut for digestion and plant roots absorbing vital nutrients.

• Know and understand that substances are sometimes absorbed against a concentration gradient.

  • This means transfer occurs in the opposite direction to the natural direction of diffusion and osmosis.

  • Know that this requires the use of energy from respiration and this process is called active transport.

  • Know that active transport enables cells to absorb ions from very dilute solutions.

  • Active transport is required to absorb nutrients 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 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 blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

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

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