Biology - Processes of Life (B4)

Revision cards for units 4, 5 and 6 in Biology, Triple Science. (OCR 21st Century Notes)


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  • Created by: Sabiha
  • Created on: 31-10-12 12:07

Photosynthesis and Respiration

Photosynthesis: process in which green plants make their own food.

  • During this process, light energy is converted into chemical energy.
  • End product of photosynthesis is glucose.
  • Word equation: Carbon dioxide + water → glucose + oxygen.
  • Symbol equation: 6CO + 6H O → C H  O + 6O 

Respiration: process which all organisms release energy from food.

  • Word equation: Glucose + oxygen → carbon dioxide + water.
  • Symbol equation: H  O  + 6O  → 6CO + 6H O.


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Rates of photosynthesis & limitations

Factors that limit the rate of photosynthesis:

  • Temperature.
  • Carbon dioxide.
  • Light intensity.

Carbon dioxide

There isn't enough carbon dioxide in the air (0.04%) so plant growers increase the level in greenhouses. As carbon dioxide levels increase, so does the rate of photosynthesis.

Light intensity

As light intensity increases, so does the rate of photosynthesis.


A rise in temperature increases the rate of photosynthesis, up to a certain point. The rate decreases because of the effect of temperature on enzyme activity.

Limiting factor: the thing that is limiting the rate of photosynthesis and the graph levels off.

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Enzymes: chemicals that speed up chemical reactions & are proteins made of long chains of amino acids.

Substrates: chemicals that enzymes work on.

Products: chemicals produced in the reaction.

Essential things needed for enzymes to work:

  • It's 3-D shape.
  • Specific pH level.
  • Optimum temperature (37°).

Active site: part of the enzyme that has special shape for substrate to fit into (lock & key)

After substrate locks into active site, chemical reaction happens & products are released.

As temperature increases, chemical reaction rate increases. If the temperature is too high, the active site changes shape and becomes permanent, so enzyme is denatured.

Enzymes aren't living things so they can't die, they just stop working (become denatured).

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Glucose: making it and using it

Word equation for photosynthesis: carbon dioxide + water → glucose + oxygen.

Symbol equation for photosynthesis: 6CO  + 6H O → C H  O + 6O

Sunlight: absorbed by chlorophyll (green chemical) which is found in chloroplasts.

Glucose (product of photosynthesis) is:

  • Converted into chemicals needed for growth.
  • Converted into starch needed for storage.
  • Used in respiration to release energy.

Glucose from photosynthesis and nitrates taken up by plants roots: used to synthesise amino acids. (Amino acids are used to make proteins in plant, animal, and microbial cells.)

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The 'Cell'

Cell is surrounded by cell membrane (controls what goes in & out of leaves cells).

The cell membrane lets gases and water to pass in and out but blocks other chemicals.

Cell membrane is surrounded by cell wall which is made of cellulose and it lets water and other chemicals pass through easily.

Cytoplasm: where chemical reactions occur.

Nucleus contains DNA (stores genetic code). 

Genetic code: carries info the cell uses to make enzymes and proteins.

Cytoplasm contains mitochondria (contain enzymes requires for release of energy by aerobic respiration. (Respiration using oxygen).

Cell wall isn't found in animal cells. Found in plants. Also micro-organisms but doesn't contain cellulose.

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Diffusion: movement of chemicals from high to low concentration.

Diffusion is passive because it doesn't need energy, it just happens.


  • Water is taken up by roots.
  • Carbon dioxide enters leaves by diffusion.
  • Oxygen leaves the plants leaves by diffusion.

Diffusion happens because of random movements of molecules.

This movement doesn't stop when concentrations of molecules in two areas are equal; there's just no overall movement.

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Osmosis is movement of water from area of high concentration to area of low concentration.

Osmosis is diffusion involving water.

Osmosis happens when chemicals are separated by a partially permeable membrane.

Partially permeable membranes: let some substances pass through them, but not others.

Cell membranes: let water and gases pass through, but keeps chemicals out.

Examples of osmosis:

  • Movement of water into plants roots from the soil.
  • Movement of water across the roots.
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Active transport

Active transport: process which dissolved molecules move across a cell membrane from lower to higher concentrations.

In order for particles move against concentration gradients, input of energy from the cell is required.

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

Example of active transport in plants:

Minerals taken up by roots are used to make chemicals essential to cells, for example nitrogen taken up as nitrates is used to make proteins.

Nitrates are normally in a higher concentration in plants cells than the soil, so root cells can't take up nitrates by diffusion.

Root cells use active transport to take up nitrates instead.

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Effects of light on plant growth

Ecologists need to use do these things when investigating the effects of light intensity on plant growth:

  • Use an identification key to identify the plants they find.
  • Measure how common the plants are.
  • Use a light meter to measure the light intensity.

Ecologists investigate the abundance & distribution of plants:

  • At different locations in an area & make a comparison.
  • In different areas & make a comparison.

A sample is usually taken using a quadrat (a metal/wooden frame area).

A quadrat is put on the ground:

  • Several times so an average is taken & the number per metre squared can be calculated.
  • At random in a location, e.g throwing, if there's no obvious change in the distribution of plants in the area.
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Aerobic respiration

Aerobic respiration: respiration using oxygen.

Word equation for respiration: glucose + oxygen → carbon dioxide + water (+ energy released)

Symbol equation for respiration: C H  O  + 6O  → 6CO  + 6H O (+ energy released)

Respiration is a series of chemical reactions in cells that release energy by breaking down large food molecules. It occurs in every cell in the body.

Chemical used for respiration: glucose.

Aerobic respiration takes place in animal and plant cells, and some micro-organisms.

Plants use food produced by photosynthesis for respiration and active transport. Excess glucose is stored as starch.

Each step in respiration needs its own enzyme and releases small amounts of energy at each step.

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Anaerobic respiration

Anaerobic respiration: respiration without oxygen.

Some organisms can still obtain energy with a lack or without oxygen by anaerobic respiration.

it occurs in:

  • Human muscle cells during ******** exercise.
  • Plant roots in waterlogged soil.
  • Bacteria in deep wounds.

Anaerobic respiration releases much more energy than anaerobic respiration.

Word equation for anaerobic respiration in animal cells and some bacteria is:

Glucose → lactic acid + energy

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Fermentation: type of anaerobic respiration used by some micro-organisms

Word equation for fermentation: glucose → ethanol (alcohol) + carbon dioxide + energy

(Important in production of alcohol and bread).

Ethanol: product from fermentation of yeast. It's useful to us.

Bubbles of carbon dioxide make bread rise and alcohol sparkle.

In bread, alcohol (ethanol), is evaporated as it cooks.

Anaerobic respiration: also important in bio-gas generation.

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