Inside Living Cells

A set of cards detailing all you need to know for the Inside Living Cells topic for Edexcel Additional Science Biology.

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DNA and Chromosomes

  • In normal human cells there are 23 pairs of chromosomes.
  • Chromosomes and long coiled molecules of DNA.
  • A DNA molecules consists of two strands  which are linked to form a series of paired bases: adenine and thymine, cytosine and guanine. 
  • A gene is a section of DNA which codes for a particular characteristic.
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How DNA Works

  • DNA contains instructions for how to join amino acids.
  • These instructions are in a codes made up of the bases.
  • The sequence of bases tells the order that amino acids should be to make a protein.
  • There are 20 amino acids in total.
  • Proteins are molecules that the body needs.
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Making Proteins

To make proteins two things need to happen:

- the DNA code needs to be carried from the nucleus to the ribosomes 

- amino acids must enter the ribosomes to make up the proteins.

RNA (ribonucleic acid) has a similar structure to DNA. 

RNA is only a single strand and the base thymine is replaced with uracil.

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1. DNA unwinds and unzips and acts as a template for mRNA (memory).

2. mRNA forms next to the unzipped DNA.

3. The mRNA bases match up in the same way DNA does except T is replaced with U.

4. When it has been made, it rips away from the DNA.

5. The completed mRNA leaves the nucleus through the nuclear pore.

6. The DNA zips up again.

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This takes place in the ribosomes. A sequence of bases on the mRNA are translated.

1. mRNA attaches to a ribosome and the code is 'read' by the tRNA (transfer).

2. tRNA carries amino acids to the ribosome to link to the mRNA.

3. (There is a specific tRNA for each set of amino acids) tRNA pairs up with a triplet (3 bases) on the mRNA.

4. tRNA brings another amino acid which attaches to another triplet.

5. These two amino acids form a chemical bond.

6. A 3rd tRNA brings another amino acid which links as before forming a chemical bond with the second making a growing chain. 

7. Whilst this is happening the first tRNA goes for another amino acid.

8. The tRNA run a 'drop-off' service stringing amino acids together to make a protein.

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Genetic Engineering and Making Insulin

1. In DNA there is a gene for insulin.

2. A restriction enzyme cuts out the gene.

3. The same restriction enzyme is used to cut a space in the plasmid. 

4. Other enzymes then insert the section of human insulin into the space.

5. The plasmid is inserted into bacterium which multiplies rapidly.

6. Human insulin is made by these bacteria in the fermenter.

The insulin is used by insulin-dependent diabetics.

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Aerobic Respiration

  • Respiration is the process which releases the energy in glucose. 
  • Oxygen is needed for aerobic respiration, it produces carbon dioxide and water:


  • Special enzymes in the cells cause the glucose and oxygen to react and energy is released which can be used for work (movement). This is aerobic respiration.

The Equation

1. Glucose and oxygen are brought to the respiring cells by the bloodstream.

2. Carbon dioxide is taken away by the blood to the lungs where it's breathed out.

3. Water passes into the blood and is lost as sweat, breath and urine.

4. Energy is used for muscle contraction, metabolism and maintaining temperature.

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  • Diffusion is the spreading of particles from and area of high concentration to an area of low concentration.
  • When muscle cells are working hard their respiration rates increase because more energy is being used up.
  • This menas more oxygen needs to be absorbed and more CO2 needs to be removed.
  • This gas exchange takes place by diffusion in the lungs at an increased rate.
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Anaerobic Respiration

During vigorous exercise the lungs and bloodstream can't always deliver enough oxygen to the muscle cells. 

When this happens, the glucose can only partly be broken down, releasing a much smaller amount of energy.

Anaerobic respiration produces a little bit of energy very quickly, but most of the glucose is changed to lactic acid (waste product).

Anaerobic respiration can only be used by muscles for a short time because lactic acid would build up in the body causing muscle cramps.


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Oxygen Debt

  • The build up of lactic acid in anaerobic respiration causes acute fatigue in the muscles which results in 'oxygen debt'.
  • This causes the muscles to stop contracting efficiently.
  • After exercise the lactic acid must be broken down quickly to avoid cell damage: the oxygen debt must be repaid through deep breathing.
  • This provides enough oxygen to oxidise the lactic acid into carbon dioxide and water.
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Diet and Exercise

  • It's important to eat a balanced diet to provide the body with the energy and nutrients needed for it to work proper.
  • If you take in more energy than you use you will put on weight. 
  • Exercise is also important, you should exercise four or five times a week and aim to increase your heart rate by 75%.

Food Groups:

1. Fats/oils/sugar - consume sparingly

2. Dairy - 2/3 servings

3. Protein - 2/3 servings

4. Vegetable - 3/5 servings

5. Fruit - 2/4 servings

6. Carbohydrate - 6/11 servings

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A Fermenter

A fermenter is a large vessel used to grow micro-organisms. It requires:

  • aseptic (sterile) conditions - they mustn't be contaminated 
  • nutrients - the correct nutrients in order for them to grow
  • an optimum temp. - suitable for them to grow
  • the correct pH level
  • oxygenation - they need oxygen to respire
  • agitation (stirring) - to maintain an even temperature
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