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topic 3 igcse biology


The cell membrane and cell wall control what substances enter and exit the cell. Molecules
such as glucose and proteins move into the cell for use in metabolic reactions and storage.
Whereas waste products such as carbon dioxide and lactic acid are transported out into the
blood to be excreted from the body.

There are 3 physiological processes that help in movement of substances in and out of the cell. they include Diffusion, Osmosis and Active transport.

Diffusion is the net movement of particles from an area of high concentration to an area of
low concentration down the concentration gradient, as a result of their random movement.
The energy for diffusion comes from the kinetic energy of the molecules.
Solutes and gases, such as carbon dioxide and oxygen, are able to diffuse in and out of cells
across the cell membrane. This is important as these substances are crucial to metabolic
reactions which occur within the cell, for example respiration and photosynthesis. Without
them, the processes would not occur, and the cell would die.

Diffusion always takes place down a concentration gradient, that means that the particles that diffuse try to spread evenly in all spaces, so it moves from where it’s very concentrated to where it’s not concentrated.

Substances move into and out of cells by diffusion through the cell membrane.

The importance of diffusion of gases and solutes:


  • (Animals) Necessary for gas exchange in all living organisms (O2 in, CO2 out)
  • (Plants) Necessary for obtaining Carbon Dioxide and releasing oxygen during photosynthesis.

Solutes (liquids)

  • Dissolved salts diffuse through root hair cell.
  • Absorption of dissolved food material in many organisms, like amoeba, bacteria and fungi is carried out through diffusion.
  • Some digested food material is absorbed by diffusion

Water as a solvent

  • Plants cannot obtain minerals unless they are dissolved in water
  • Enzymes and hormones cannot be secreted unless they are dissolved in water
  • Excretory products cannot be excreted unless they are dissolved in water.

Factors affecting rate of diffusion:
● Surface area – As the surface area increases, the rate of diffusion increases. This is
because there is more space available for the substances to diffuse through.
● Temperature – As temperature increases, the rate of diffusion increases. This is because
the molecules gain kinetic energy and thus move faster.
● Concentration gradient – As the concentration gradient increases, rate of diffusion
● Diffusion distance – A greater diffusion distance slows the rate of diffusion as molecules
must travel further.
Osmosis is the net movement of water molecules from an area of higher water potential to an
area of lower water potential through a partially permeable membrane.
Key terms:

● Turgid – cells are described as turgid when they are swollen due to a high-water content.
● Turgor pressure – The pressure on the cell wall from the cell membrane pushing upon it.
This is a result of the cell becoming turgid as water moves into the cell via osmosis.

● Flaccid – Occurs when water moves out of the cell via osmosis. The cell shrinks but the
cell membrane does not peel away from the cell wall. If more water leaves the cell, it
becomes plasmolyzed.
● Plasmolysis – Occurs when there is too little water in cells. In plant cells, the cell
membrane peels away from the cell wall.
Water moves in and out of cells through the cell membrane via osmosis. Water is important to
provide support for the cell structure through maintaining the turgor pressure. It also provides
a medium in which metabolic reactions occur. Water has a high specific heat capacity, thus
acts as a temperature buffer. This is important as it maintains the optimum temperature for
enzyme reactions.
Water particles diffuse from regions of high-water potential to regions of low water potential,
i.e. they move from dilute solutions to concentrated solutions. When the cell is more
concentrated than the surrounding cells, water molecules diffuse into the cell via osmosis,
making it turgid. When it is less concentrated than the surrounding cells, water molecules will
leave the cell, making it flaccid and leading to plasmolysis. This effect can be investigated by
placing cells in solutions of different concentrations.

A dilute solution means it has lots of water molecules, and a high water potential.

A concentrated solution has few water molecules and low water potential.

  • Water moves in and out of cells by osmosis through the cell membrane.
  • It is important that the cells in an animal’s body are surrounded by a liquid which has the same concentration as the liquid inside the cells.
  • Tissue fluid: the liquid outside the cells.
  • Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.

 If a plant cell is placed in distilled water, water molecules will move from the distilled water to the cell, the cell swells up and becomes turgid but it will never burst because plant cells are surrounded by cell walls, which are made of cellulose and is elastic, it will stretch but never break. The vacuole is exerting turgor pressure on the elastic cell wall.

If we place a plant cell in a concentrated salt solution with low water potential, water will move from the cell to the solution causing the cell to become plasmolysed.

The importance of osmosis:

Plants gain water through osmosis in their roots from the soil. Without a water potential gradient,water will be loss from the roots. Plant cells contain vacuoles, which, if not full with water, will cause the cell to become flaccid. If all the cells in a leaf become flaccid, the whole leaf will become flaccid, hence causing the plant to wilt. Plant cells therefore need water to remain turgid and keep firm.

If an animal cell surrounded with a high water potential, osmosis will take place, and if the water is not expelled some way or another, the cell will burst (a process called haemolysis in red blood cells). This is because an animal cell doesn’t have a cell wall to keep it strong. If an animal cell is surrounded with low water potential, the water in the cytoplasm will diffuse outwards, causing the cell to shrink (crenated)

Active transport
Active transport is the movement of molecules against a concentration gradient using energy
from respiration. Molecules are actively transported from regions of low concentrations to
regions of higher concentration.
Carrier proteins:
Carrier proteins facilitate active transport. They are embedded in the cell membrane and allow
passage through it. Molecules from the side with lower concentration bind to the carrier
protein. The carrier protein then changes shape using energy from respiration. This forces the
molecule to move through the membrane to the side with high concentration, where it is
Examples of active transport:
● Uptake of ions by root hair cells – plants take up ions such as nitrates and magnesium
from the soil via root hair cells. The concentration of ions in the root hair cell is greater

than the concentration of ions in the soil. Energy from respiration is therefore used to
transport ions into the cell against the concentration gradient.
● Uptake of glucose – glucose is taken up in the small intestine and kidney tubules.
Glucose moves against the concentration gradient through carrier proteins.

Examples of active transport include:

  • uptake of glucose by epithelial cells in the villi of the small intestine (and by kidney tubules)
  • uptake of ions from soil water by root hair cells in plants

Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.

  • Substance combines with carrier protein molecule
  • Carrier transports substances across membrane using energy from respiration
  • Substance released into cell