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AQA GCSE Biology – Unit 2 summary notes

AQA

GCSE Biology

Unit 2

Summary Notes


12.1 Animal and Plant Cells

Cells

  • Cells are the smallest unit of life.
  • All living things are made of cells.
  • Most human cells, like most other animal cells, have the following parts:
  • nucleus
  • cytoplasm
  • cell membrane
  • mitochondria
  • ribosomes

  • Plant cells also have:
  • cell wall
  • chloroplasts
  • permanent vacuole

What do these structures do?

  • Nucleus – controls the activities of the cell.
  • Cytoplasm – where most of the chemical reactions take place.
  • Cell membrane - controls the passage of substances in and out of the cell.
  • Mitochondria - where most energy is released in respiration.
  • Ribosomes -  where protein synthesis occurs.
  • Cell wall - strengthens plant cells.
  • Chloroplasts - absorb light energy to make food in plant cells.
  • Permanent vacuole - filled with cell sap in plant cells.

Other facts:

  • The chemical reactions inside cells are controlled by enzymes.

Examples:

  • Enzymes for respiration are in the mitochondria.
  • Enzymes for photosynthesis are in the chloroplasts.
  • Enzymes for protein synthesis are on the ribosomes.

  • Cells may be specialised to carry out a particular function.

Examples:

Tissues, organs and systems

  • A tissue is a group of similar cells carrying out a particular function.
  • An organ is a group of different tissues carrying out a particular function.
  • A system is a group of different organs carrying out a particular function.

Examples:


12.2 Movement into and out of cells

  • To get into or out of cells, dissolved substances have to cross the cell membranes.
  • Solutes = particles in solution eg glucose, sodium ions, chloride ions.
  • Solvent = liquid in which the particles are dissolved eg water.
  • Solute and solvent molecules move around randomly.
  • Solutes can move into and out of cells by diffusion and osmosis.  

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.
  • Oxygen required for respiration passes through cell membranes by diffusion.
  • The greater the difference in concentration, the faster the rate of diffusion.

Osmosis

  • 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 water molecules.
  • Differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.
  • If there is a higher solute concentration on one side of a membrane, water will move in that direction.

Osmosis in animal cells:

If animal cells are placed in a solution that has a higher solute concentration than the cytoplasm, then water will leave the cell by osmosis, until it shrinks and dies.

If animal cells are placed in a solution that has a lower solute concentration than the cytoplasm, then water will enter the cell by osmosis until it bursts.

This is why it is vital that we maintain the concentration of our body fluids at an equal solute concentration to our cells’ cytoplasm.

Osmosis in plant cells:

If plant cells are placed in a solution that has a higher solute concentration than the cytoplasm, then water will leave the cell by osmosis, and the cell membrane separates from the cell wall (plasmolysis).  This will cause a plant to wilt.

If plant cells are placed in a solution that has a lower solute concentration than the cytoplasm, then water will enter the cell by osmosis until it is fully turgid, and the cell wall prevents any more water entering.  This is important in enabling plants to remain upright.


12.3 Photosynthesis

Photosynthesis

  • Photo = light
  • Synthesis = making of (glucose)
  • Photosynthesis =  making glucose using light

Complex definition:

  • A process in green plants that produces complex organic chemicals (glucose) from simple inorganic chemicals (carbon dioxide and water) using light energy.

More detail:

  • Light energy is absorbed by a green substance called chlorophyll.
  • Plants look green because chlorophyll reflects green wavelengths of light.
  • The plant then uses this energy for photosynthesis.
  • Light energy is used to convert carbon dioxide and water into sugar (glucose).
  • Oxygen is released as a by-product.

Word equation:

                                                     chlorophyll

Carbon Dioxide  +  Water                                    Glucose  +  Oxygen

                                                         Light

Chemical equation (do not need to learn!):

    6CO2      +       6H2O                                                 C6H12O6   +    6O2

Where does photosynthesis happen?

  • Leaves are the main site of photosynthesis.
  • Photosynthesis mainly in occurs in the mesophyll cells.
  • These cells contain lots of chloroplasts.
  • Chloroplasts contain chlorophyll.
  • Two types of mesophyll cells:
  • Palisade cells absorb most light.
  • Spongy cells absorb the rest.

A palisade mesophyll cell:

Factors that limit the rate of photosynthesis

  1. Temperature

A low temperature will limit the rate as the molecules will move less and therefore the reaction happens slower

  1. Carbon dioxide

A shortage of CO2 will limit the rate as fewer molecules will be available for the reaction.

  1. Light intensity

A shortage of light means there is less energy to power the reaction.

Limiting factors explained:

  •   If one of these factors is closest to its minimum value it will limit the rate.
  •   Increasing this factor will increase the rate.
  •   The rate will continue to increase until another factor becomes limiting.
  •   Any further increase in the original factor will now not increase the rate.
  •   With no limiting factors, increasing a factor above a certain level will not increase the rate.  All chlorophyll molecules are being used.

Farming practices

  • Farmers artificially manipulate the environment in which they grow plants.
  • They grow plants in greenhouses or in polythene tunnels.
  • They can control the temperature in greenhouses using heaters and ventilation.
  • They can artificially increase the carbon dioxide levels.
  • They can control the light using fluorescent lamps.
  • By doing all of this, their plants grow faster and certain plants can be grown in this country out of their natural growth season.  Eg tomatoes can be grown all year round.
  • Therefore, they increase their profits.
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How do plants use glucose?

  • 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.

Why do plants need minerals?

  • Plant roots absorb mineral salts including nitrates needed for healthy growth.
  • For healthy growth plants need mineral ions including:
  • nitrate - for producing amino acids which are then used to form proteins.
  • magnesium - which is needed for chlorophyll production.
  • The symptoms shown by plants growing in conditions where mineral ions are deficient include:
    ...

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***** An excellent set of summary notes. Some of the notes are a little bit basic but are good for getting the framework necessary for a more deeper understanding. With the carbon cycle, it is important to realise it is not a balanced cycle anymore due to humans burning fossil fuels and releasing carbon dioxide into the atmosphere. Also, when you discuss diabetes be aware that there are two types of this disease both with different causes ( I have put more detail in the text on this subject). Finally with the pyramids of numbers and pyramids of biomass, the pyramids of biomass usually give a more accurate reflection of an ecosystem but it is difficult to measure the biomass at each trophic level.