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Biology · Transport In and Out of Cells

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Biology · AQA Paper 1

Transport In and Out of Cells

Diffusion

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.

  • Diffusion The spreading out of particles of a gas, or of a substance in solution, resulting in a net movement from an area of…
  • Concentration The amount of a substance in a given volume.
  • Concentration gradient The difference in concentration between two places.
Many particles on the left of a membrane and few on the right, with an arrow showing net movement from left to right.
Particles move across the membrane from the side with more of them to the side with fewer.

Following oxygen from the air to a muscle cell

  • Breathing in Air in the lungs has a high concentration of oxygen.
  • Into the blood Oxygen diffuses from the air in the alveoli into the blood.
  • Round the body The blood carries the oxygen to the muscles.
  • Into the cells The muscle cells use oxygen up, so its concentration in them is low.

Surface Area to Volume Ratio and Exchange Surfaces

A big organism has a small surface area compared with its volume, so it needs exchange surfaces and a transport system.

  • Surface area to volume ratio The surface area of an object divided by its volume. It gets smaller as the object gets bigger.
  • Surface area The total area of the outside of an object.
  • Volume The amount of space inside an object.
Three cubes with sides 1 cm, 2 cm and 3 cm, with their surface areas, volumes and surface area to volume ratios of 6 to 1, 3 to 1 and 2 to 1.
As the cube gets bigger, the ratio gets smaller.

Single-celled and multicellular organisms

  • Single-celled organism Large surface area to volume ratio; Every part of the cell is close to the outside; No transport system or special exchange surface needed
  • Multicellular organism Small surface area to volume ratio; The body surface alone could not supply the cells deep inside; Diffusion over such long distances is far too slow

Osmosis

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

  • Osmosis The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
  • Partially permeable membrane A membrane that lets some substances, such as water, pass through but not others, such as large dissolved molecules.
  • Dilute solution A solution with a low concentration of dissolved substance.
A beaker with a dilute solution on the left and a concentrated solution on the right, separated by a membrane, with an arrow showing water moving from left to right.
Water moves through the membrane from the dilute side to the concentrated side. The sugar molecules are too big to cross.
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Biology · AQA Paper 1

Transport In and Out of Cells continued

Investigating Osmosis in Plant Tissue

Where the line on the graph crosses zero, the solution has the same concentration as the potato cells.

  • Independent variable The variable that is deliberately changed in an investigation.
  • Dependent variable The variable that is measured in an investigation.
  • Control variable A variable that is kept the same so that the test is fair.
Six steps of the potato investigation: cutting identical cylinders, weighing them, putting them in solutions of different concentration, waiting, blotting dry and reweighing, and calculating.
The practical in six steps.

The method

  • Cut the potato Use a cork borer to cut cylinders of potato.
  • Measure the starting mass Dry each cylinder and measure its mass on a balance. Record it.
  • Set up the solutions Put the same volume of each sugar solution into labelled test…
  • Add one cylinder to each tube Make sure each is covered with solution.

Active Transport

Active transport moves substances from a more dilute solution to a more concentrated solution, and it needs energy from respiration.

  • Active transport The movement of substances from a more dilute solution to a more concentrated solution, against a concentration gradient.
  • Against the concentration gradient From a lower concentration to a higher concentration.
  • Respiration The process in cells that releases energy from glucose.
Ions moved by a carrier protein from a lower concentration outside a cell to a higher concentration inside, using energy from a mitochondrion.
Active transport moves substances against the concentration gradient, using energy from respiration.

Diffusion and active transport

  • Diffusion From a higher to a lower concentration; Down the concentration gradient; No energy needed from the cell
  • Active transport From a lower to a higher concentration; Against the concentration gradient; Needs energy from respiration