Cell – Structure and Function

Examination, with microscopes, of sections of animal and plant tissues, leads pupils to an awareness of the cellular structure of living organisms.

Cells are the basic units of life To observe cells under the microscope we often stain them to make the structures more visible.  Some common stains are:

• Methylene blue – a good general purpose stain
• Iodine – Stains starch grains.

Some organisms are good for observing cells:

• Elodea – Canadian pondweed is excellent for plant cells
• Cheek scrape cells – excellent for observing animal cells.

The parts of a cell are:

The Animal Cell

A – The nucleus controls the cell’s activities B – The cytoplasm the chemical reactions of life occur here C – The cell membrane controls the entry and exit of material from the cell. The cell membrane is selectively permeable – that is it allows some substances to pass through and not others

The Plant Cell

The plant cell also has A – nucleus, B Cytoplasm and C – cell membrane.

It also has: D – The cell wall rigid structure made of cellulose give shape to the cell E – The vacuole liquid filled centre of the cell, gives the cell rigidity F – Chloroplasts filled with the green pigment chlorophyll – photosynthesis occurs here.

• A plant cell always has a cell wall
• Animal cells never have cell walls, large vacuoles or chloroplasts.

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Investigating Diffusion

The aim of this sub-topic is to promote a knowledge and understanding of the process of diffusion and osmosis.

Diffusion is the movement of solute particles from a region of higher solute concentration to a region of lower solute concentration until the solute is evenly spread throughout the solvent.

• Solute molecules are molecules of a substance dissolved in a solvent (liquid).
• Solute molecules in a liquid are constantly moving
• That means that any dissolved molecules will be spread slowly through the liquid
• This process is known as diffusion.

Diffusion is the way many molecules:

• move across membranes:
  • between the air and the blood through the lungs.
  • Oxygen moves into the blood, carbon dioxide moves out.
• from the inside of the gut into the blood:
  • sugar and amino acids diffuse out of the gut into the blood.
• between the blood and the cytoplasm of cells through the cell membrane:
  • Oxygen and food diffuse from the blood into the cytoplasm and carbon dioxide and waste diffuse out of the cell into the blood.
• move within cells.

Osmosis is a special case of diffusion

• Osmosis is the diffusion of water
• We describe osmosis in terms of water concentration
  • Pure water has the highest possible water concentration
  • As molecules of solute are added the water concentration goes down.

• Osmosis can only occur across a selectively permeable membrane (see cell membrane above).
• Water moves from the higher water concentration to the lower water concentration.
• In animal cells the point where the water concentration within the cell balances the water concentration outside the cell is at 0.85% salt solution.

You should be able to say what happens in each of the events shown by arrows on the diagram.

In plant cells the situation is slightly different

• The possession of a tough cellulose cell wall makes it impossible for the cell to burst.
• In a high water concentration:
  • The vacuole expands as a result of osmosis – animal cells don’t have these large, central vacuoles.
  • Water flows into the vacuole and the cell wall bulges. 
  • The cell is described as turgid.
• In a low water concentration:
  • the vacuole shrinks,
  • the cell membrane pulls away from the cell wall,
  • the cell is described a plasmolysed.

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Investigating Cell Division

This sub-topic seeks to develop pupils’ knowledge and understanding of the process of mitotic cell division, providing an opportunity for all pupils to demonstrate their skills in obtaining and presenting information about chromosome behaviour during cell division.

• Cell division increases the number of cells in an organism.
• The control of cell division is within the nucleus.
• Within the nucleus there are thread like structures called chromosomes which:
  • Control the cell
  • Carry the hereditary information of the cell (see topic “Inheritance”).
• Therefore it is essential that the two cells produced during cell division have a complete set of chromosomes:

Mitosis is the process whereby the nucleus of the cell divides producing two identical copies of the chromosomes. One cell with 46 chromosomes divides to form two cells – each with 46 chromosomes.

The number of chromosomes in the cell of an organism is called the chromosome complement (note the spelling)

Each species has it’s own chromosome complement – ours is 46. Errors in mitosis can rely in the incorrect chromosome complement that results in a damaged organism. An error that occurs occasionally results in three copies of chromosome 21 instead of the normal two – this results in Down’s syndrome.

The process of mitosis is described below:

You should be able to:

• Arrange these events in order if they are shuffled up.
• Describe what happens in each case.

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Investigating Enzymes

Pupils should know that cells produce enzymes and that many cellular processes depend upon the activity of enzymes. This activity can be affected by changes in factors such as temperature and pH.  Pupils should understand the specific relationship between an enzyme and its substrate.

A catalyst:

• Is a chemical which speeds up a chemical reaction Without being changed itself.
• An enzyme is a biological catalyst.
• It is made of protein It shows an optimum (best) temperature
  • The optimum temperature for human enzymes is 37ºC
• It has an optimum pH
  • The optimum pH for catalase is 9 and the optimum pH for pepsin is 2.5
• It is destroyed above a certain temperature (usually over 55-60 ºC) It is specific.
  • Enzymes do just one job Each type of enzyme (and there are thousands of types) only carries out one type of reaction e.g. amylase will only change starch into maltose – nothing else.

• The chemical(s) an enzyme acts on is its substrate(s) The chemical(s) produced by the enzyme is the product(s)

• Enzymes are required in cells
  • to speed up chemical reactions
  • to control the cell – if an enzyme is present the reaction happens; if not it doesn’t

• Some enzyme reactions break down chemicals e.g.
  • Catalase acts on its substrate hydrogen peroxide to make the products oxygen and water.
  • Amylase acts on its substrate starch to make the product maltose.
  • Some enzymes synthesise (build up) more complex molecules from simpler molecules e.g.
  • Potato phosphorylase acts on its substrate glucose-1-phosphate to make the product starch.

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Investigating Aerobic Respiration

Respiration is a cellular process.  Pupils should understand the cell’s need for energy and should have a basic knowledge and understanding of how energy is released from food in the presence of oxygen.

• All living cells need energy for:
  • Movement – e.g. muscle cells, white blood cells Growth Mitosis and cell division To make chemical reactions happen To move materials across the membrane.
• Food is the source of the energy.
  • One gram of fat contains more than twice the energy of carbohydrates or proteins. This is the reason our bodies use fat as a storage chemical for energy.
• Oxygen is needed for aerobic respiration.

Aerobic respiration is:

• A chemical process Occurring in all living cells Which releases energy from food Oxygen is needed for respiration Carbon dioxide and water are produced as by products. The molecules of carbon dioxide were once part of the food before it was broken down by aerobic respiration.

  Food + oxygen energy + CO₂ + H₂O

  Food and oxygen produce energy and Carbon dioxide and water

• Respiration is not 100% efficient:
  • Not all the energy available from the food is changed into useful energy for the cell. Some energy is lost as heat. This heat is used to keep our bodies warm.

The word metabolism represents the sum total of chemical reactions occurring in the cell.

• The speed at which these reactions occur is called the metabolic rate.
• Most of the cell’s chemical reactions require energy
• Consequently, without food the cells of the body eventually run out of energy and stop working.

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