Active Transport

Sometimes, a cell needs to move a substance from an area of lower concentration to an area of higher concentration. In other words, it needs to move a substance against its concentration gradient.

For example, the diagram below shows a cell with a high concentration of glucose in its cytoplasm. It is surrounded by a solution which has a low concentration of glucose. The cell needs to take in glucose from the surrounding solution.

Diagram of a cell floating in a solution. Small green hexagons represent glucose molecules. The glucose concentration is much higher inside the cell than outside (as indicated by the fact that the glucose molecules are much more closely packed together). One of the glucose molecules is labelled "Glucose molecule".

The glucose concentration is higher inside the cell than outside the cell. However, the cell needs to absorb glucose from its surroundings.

It a situation like this, diffusion would not benefit the cell. When substances move by diffusion, they always move down the concentration gradient. Therefore, if glucose was allowed to move through the cell membrane by diffusion, it would actually move out of the cell, towards the area of lower concentration.

Therefore, cells have an alternative way of moving substances, called active transport. Active transport moves a substance against its concentration gradient - from the area of lower concentration to the area of higher concentration.

Diagram showing glucose being actively transported into a cell. The diagram shows a cell floating in a solution. There are small green hexagons representing glucose molecules. The glucose concentration is higher inside the cell than outside the cell. There are red arrows showing glucose moving into the cell. One of these arrows is labelled "Active transport".

The cell uses active transport to move glucose against its concentration gradient.

Unlike diffusion, which happens spontaneously due the random movement of particles, active transport is something that the cell has to do itself. When a cell carries out active transport it moves a substance in the opposite direction to the one it would move in if it was allowed to diffuse.

Because active transport moves substances in the opposite direction to the one they would move in spontaneously, the cell needs to use energy in order to carry it out. Cells carry out active transport using energy that has been released from food molecules through the process of respiration.

Diagram showing the use of energy in active transport. The diagram shows a zoomed in section of one corner of a cell. A glucose molecule is being actively transported into the cell. Wavy orange arrows show energy going into the process of active transport. One of them is labelled, "Energy from respiration used to carry out active transport".

In active transport, the cell uses energy from respiration to move a substance against its concentration gradient.

Active transport is defined as follows:

Active transport is the net movement of a substance from an area of lower concentration to an area of higher concentration (against the concentration gradient), using energy from respiration.

Root hair cells use active transport to absorb mineral ions

Root hair cells are cells in the roots of plants, which absorb water and mineral ions from the soil.

Because root hair cells absorb mineral ions, they tend to end up with a high concentration of mineral ions in their cytoplasm.

On the other hand, the concentration of mineral ions in the soil tends to be much lower.

Diagram of a root hair cell with soil around it. There are small circles representing mineral ions. The mineral ions are much more closely packed together in the cell than they are in the soil. The cell is labelled, "High mineral ion concentration in cell". The soil is labelled, "Low mineral ion concentration in soil".

A root hair cell's cytoplasm usually has a higher mineral ion concentration than the soil around it.

Therefore, if mineral ions were allowed to simply move by diffusion, they would move out of the root hair cells and into the soil (because they would move down the concentration gradient). This would be bad for the plant because it needs mineral ions for healthy growth.

Therefore, root hair cells use active transport to force mineral ions to move against their concentration gradient - from the soil (which is the area of lower concentration) into the cytoplasm of the cells (which is the area of higher concentration).

Diagram of a root hair cell absorbing mineral ions from the soil. A root hair cell is shown with soil around it. There are small circles representing mineral ions. The mineral ion concentration is much higher inside the cell than in the soil. There are arrows showing mineral ions moving into the cell by active transport. One of these arrows is labelled "Active Transport".

Root hair cells use active transport to absorb mineral ions from the soil.

Animals use active transport to absorb sugar from the gut into the blood

As an animal digests a meal, sugar molecules from the food need to be absorbed into the blood from the gut.

However, the blood has a higher sugar concentration than the gut does. Therefore, if the sugar molecules were allowed to move by diffusion, they would move out of the blood and into the gut.

Therefore, active transport is used to move sugar molecules from the gut into the blood.

Diagram of active transport in the gut. At the bottom of the diagram there is a blood vessel running horizontally. Above this, there is a row of cells which make up the lining of the gut. Above this is the inside of the gut (the gut lumen). There are small hexagons representing sugar molecules. They are much more closely packed together in the blood vessel than in the gut. The gut is labelled, "Gut. Low sugar concentration". The blood vessel is labelled, "Blood vessel, high sugar concentration". There are arrows showing the movement of sugar from gut to the blood vessel. One of these is labelled "Active transport".

Active transport is used to absorb sugar molecules from the gut into the blood stream.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.

1/2

What is active transport?

2/2

Give two examples of situations where active transport is used.

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