Biology AQA Additional: Cells and Photosynthesis

A summary of everything you need to know about cells and photosynthesis, in my opinion :)

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What's inside a cell?

All cells, plant and animal, contain:

  • Ribosomes: where protein synthesis occurs
  • Mitochondria: where the chemical reaction of respiration takes place
  • Nucleus: contains the genetic material that controls how the cell combines amino acids in a specific order to manufacture proteins
  • Cytoplasm:contains enzymes that control cells reactions
  • Cell membrane: controls what substances enter and leave the cells and holds the cells together

Plants cells also contain:

  • Vacuole: contains cell sap: a solution of sugars and salts that keeps the cells turgid (full of water)
  • Chloroplasts: contain chlorophyll where photosynthesis occurs
  • Cell wall: cellulose structure that supports and strengthens the cell
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How are some cells specialised?

Palisade Leaf Cell

  • Lots of chloroplasts packed at the top of the cell so they're near the sunlight for photosynthesis
  • Tall, giving them a large surface are so plenty of carbon dioxide can be absorbed for photosynthesis
  • Thin and rectangular, so lots can be packed together

Guard Cell

  • Kidney shape to open and close the stomata
  • When they're filled with water they are turgid and plump so the stomata open and gases can exchange for photosynthesis
  • When there's less water they become flaccid and wilt so stomata close and less water vapour escapes
  • Sensitive to light so stomata close at night to prevent water vapour escaping and save it for photosynthesis during the day
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More cell specialisation...

Red blood cell

  • Concave: large surface area for oxygen diffusion and can easily pass through capillaries
  • Packed with haemoglobin to absorb oxygen
  • No nucleus so more room for haemoglobin


  • Long tail and streamlined head to improve swimming ability
  • Mitochondria packed in head to provide energy
  • Enzymes in head digest egg cell membrane

Egg cell

  • Huge food reserves for embryo
  • Membrane changes structure when sperm enters to there's the right amount of DNA
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Diffusion is the gradual net movement of particles from an area of high concentration to low concentration. It only occurs in liquids and gases because the particles are free to move.

Cell membranes allow diffusion:

  • Starches and proteins can't pass through
  • Glucose and amino acids can pass through

... because of their size in relation to the gaps in the cell membrane

The rate of diffusion depends on:

  • Concentration gradient
  • Distance
  • Surface area
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Osmosis is the movement of water particles across a semi-permeable membrane from an area of high water concentration to low water concentration.

Partially permeable means only tiny molecules can fit through.

Water enters cells through the process of osmosis.

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6CO2 + 6H2O -> C6H12O6 + 6O2

Carbon dioxide + Water -> Glucose + Oxygen

You need:

  • Chlorophyll: found in chloroplasts
  • Water: found in soil
  • Carbon dioxide: found in air
  • Light: from the sun
  • Warmth: from the sun

But it can't be too hot otherwise enzymes get denatured so the structure of the active site changes and it no longer fits the substrate particle ... a long-winded way of saying it goes kaput!

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What do plants use glucose for?

  • Respiration: to produce energy
  • Fruits: animals eat fruits and disperse seeds ... I won't go into detail...
  • Cellulose: to make cell walls
  • To combine with nitrates to make amino acids and therefore protein
  • Starch: an insoluble energy reserve for when there's less photosynthesis
  • Turn into fats/lipids to store in seeds
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