Additional Biology Higher Tier Notes
All living things are made up of cells. The structures of different types of cells are
related to their functions.
• Most human cells like most other animal cells have the following
− a nucleus which controls the activities of the cell
− cytoplasm in which most of the chemical reactions take place
− a cell membrane which controls the passage of substances in
and out of the cell
− mitochondria, which is where most energy is released in
− ribosomes, which is where protein synthesis occurs.
• Plant cells also have a cell wall which strengthens the cell. Plant
cells often have:
− chloroplasts which absorb light energy to make food
− a permanent vacuole filled with cell sap.
• The chemical reactions inside cells are controlled by enzymes.
• Cells may be specialised to carry out a particular function.
To get into or out of cells, dissolved substances have to cross the cell membranes.
• Dissolved substances can move into and out of cells by diffusion
• Diffusion is the spreading of the particles of a gas, or of any
substance in solution, resulting in a net movement from a region
where they are of a higher concentration. The greater the
difference in concentration, the faster the rate of diffusion.
Oxygen required for respiration passes through cell membranes by
• Water often moves across boundaries by osmosis. Osmosis is the
diffusion of water from a dilute to a more concentrated solution
through a partially permeable membrane that allows the passage of
• Differences in the concentrations of the solutions inside and
outside a cell cause water to move into or out of the cell by
Green plants use light energy to make their own food. They obtain the raw
materials they need to make this food from the air and the soil.
• Photosynthesis is summarised by the equation:
carbon dioxide + water (+ light energy) → glucose + oxygen
• During photosynthesis:
− light energy is absorbed by a green substance called
chlorophyll which is found in chloroplasts in some plant cells
− this energy is used by converting carbon dioxide and water
into sugar (glucose)
− oxygen is released as a by-product.
• The rate of photosynthesis may be limited by:
− low temperature
− shortage of carbon dioxide
− shortage of light.
• Light, temperature and the availability of carbon dioxide interact
and in practice any one of them may be the factor that limits
• The glucose produced in photosynthesis may be converted into
insoluble starch for storage. Plant cells use some of the glucose
produced during photosynthesis for respiration.
• Plant roots absorb mineral salts including nitrates needed for
healthy growth. For healthy growth plants need mineral ions
− nitrate – for producing amino acids which are then used to
− magnesium – which is needed for chlorophyll production.
• The symptoms shown by plants growing…