1.10 - Active Transport

What active transport is

• Active transport is the movement of particles across a cell membrane against a concentration gradient - from an area of lower concentration to an area of higher concentration.
• It requires energy transferred by respiration to make it happen.

Active transport is the opposite of diffusion

• In diffusion, particles move down the concentration gradient on their own, and no energy is needed.
• Active transport moves particles the other way - up the gradient - so it cannot happen by itself.
• It has to be driven by energy from respiration. This energy is carried by a molecule called ATP, which the cell uses to power the transport.

Carrier proteins do the moving

• Carrier proteins are embedded in the cell membrane.
• They pick up the particles on one side and move them across to the other side, against the gradient.

Active transport in plant roots

• Plants need mineral ions from the soil to grow healthily. They absorb them through their root hair cells.
• The mineral ions include nitrate ions, which the plant uses to make proteins.

Why diffusion won't work

• The concentration of mineral ions is usually higher inside the root hair cells than in the soil around them.
• If the ions followed diffusion, they'd move the wrong way - out of the cells and into the soil.
• So the root hair cells use active transport to move the mineral ions in, against the concentration gradient.
• This lets the plant absorb minerals even from a very dilute soil solution, which is essential for growth.
• Active transport needs energy, which comes from respiration (carried by ATP).

Root hairs give a large surface area

• Each root hair is a long, finger-like extension of the cell that sticks out into the soil.
• A root is covered in millions of them.
• This gives the plant a large surface area for absorbing mineral ions by active transport, and water by osmosis.

Active transport in animals

• Active transport isn't just a plant process - animals use it too.
• It lets animals move substances against a concentration gradient, where diffusion alone wouldn't work.
• Two key examples are absorbing nutrients from the gut and reabsorbing useful substances in the kidney.

Absorbing nutrients in the gut

• Digested nutrients such as glucose and amino acids usually pass into the blood by diffusion, because there's a higher concentration of them in the gut than in the blood.
• Sometimes the concentration of nutrients is lower in the gut than in the blood, so the concentration gradient is the wrong way for diffusion.
• Active transport then takes the nutrients into the blood against that gradient, so the body can absorb the last of its food rather than leave it behind - essential to stop us starving.
• The cells lining the gut have many mitochondria to supply the energy from respiration that active transport needs.

Reabsorbing glucose in the kidney

• As the kidney filters the blood, all of the glucose is reabsorbed by active transport, against the concentration gradient.
• This means none is lost in the urine, so it can be used in respiration.