GCSE OCR Gateway B3 (first Half)

Molecules of Life

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Cell Structure

Molecules of life

Cell Structure

 -  The number of mitochondria in a cell depends on the activity of the cell.

 -  Respiration occurs in the mitochondria

Cells such as liver or muscle cells have large numbers of mitochondria. This is because the liver carries out many functions, and muscle cells need to contract.

Ribosomes are smaller than mitochondria and are also found in the cytoplasm. They are too small to be seen with a normal microscope.

 Ribosomes are the site of protein synthesis.


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DNA and the genetic code

Molecules of life

DNA and the genetic code

- The nucleus contains genes Each gene:

is a section of a chromosome made of DNA

codes for a particular protein

- DNA is made of 2 strands coiled to form a double helix, each strand containing chemicals called bases. There are 4 different types of bases: A-T and G-C. This is called complementary base pairing. Each gene contains a different sequence of bases.

- Proteins are made in the cytoplasm but DNA cannot leave the nucleus. A copy must be made that can leave the nucleus and carry the code to the cytoplasm.

- The DNA base code controls which protein is made. This is because the base sequence in the DNA codes for the amino acid sequence in the protein, each acid being coded for by a sequence of 3 bases

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Molecules of life

mRNA and Discovering the structure of DNA

- The code needed to produce a protein, is carried from the DNA to the ribosomes by a molecule called mRNA

- Many of the proteins that are made are enzymes, which can control the activity of the cell.

Watson and Crick

Using data from other scientists, Watson and Crick built a model of DNA. Two of the importantpieces of data that they used were:

- Photographs taken using x-rays which showed that DNA had 2 chains wound in a helix

- Data indicating that bases occured in pairs e.g G-C

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Grouping Organisms

Proteins and Mutations

Grouping organisms

- All Proteins are made up of long chains of amino acids joined together.

- Proteins have different functions. Some examples are:

Structural proteins used to build tissues, e.g Collagen

Hormones, which carry messages to control a reaction, e.g Insulin controls blood sugar levels

Carrier proteins, e.g Haemoglobin, which carries oxygen


- Each protein has it's own number and order of amino acids. This makes each type of protein molecule a different shape and gives it a different function.

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Proteins and Mutations

- Enzymes speed up reactions in the body so are called biological catalysts. Reactions such as: respiration, photosynthesis and protein synthesis in living cells


- Each enzyme can only work on one particular substrate. This is called specificity. All enzymes work best at a particular temperature and pH. This is called the optimum. Any change away from this will slow down the reaction.

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Enzymes Continued and Q10s

Proteins and Mutations

At low temperatures molecules are moving slowly and so the enzyme and substrate are less likley to collide.

At very high or low pH values and at high temperatures the enzyme active site changes chape. This is called denaturing. The substrate cannot fit so cannot react so quickly.

- It is possible to work out how temperatures alter the rate of reaction by calculating the temperature coefficient, called Q10. This is done for a 10 degree celcius change in temperature using:

Q10 = Rate at higher temperature / Rate at lower temperature

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Proteins and Mutations

- Mutations may occur spontaneously but can be made to occur more often by radiation or chemicals

- When they occur, mutations:

- may lead to the production of different proteins

- are often harmful but may have no effect

- occasionally, may give an individual an advantage

- Although every cell in the body has the same genes, it does not mean that all the same proteins are made. This is because different genes are switched off  in different cells. This allows different cells to perform different functions.

- Gene mutations alter or prevent the production of the protein that is normally made, because they change the base code of DNA, and so change the oder of amino acids in the protein.

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- Respiration occurs in every cell in our body (mitochondria)

- It's the process of releasing energy from within glucose.

- This energy cannot be used directly, so it is used to make a substance called ATP.

- ATP acts as the energy source for many cell processes and transports energy to where it's needed.

- Respiration is controlled by enzymes, this means the rate of respiration is affected by pH and temperature.

- two types of respiration are anaerobic and aerobic.

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Aerobic respiration

- Aerobic = with oxygen

- This occurs when there's plenty of oxygen.

- It's the most efficient way of releasing energy from glucose.

- This respiration is used most of the time.

- Equations

- Glucose + oxygen -> cardon dioxide + water + energy

- C(6)H(12)O(6) + 6O(2) -> 6CO(2) + 6H(2)O + energy.

- Respiration rate increase = increase in oxygen comsumption and carbon dioxide      production

- Rate of oxygen consumption can allow for an estimate of the metabolic rate (amount of energy being used)

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Anaerobic Respiration

- Anaerobic = without oxygen

- When you do vigorous exercise, your body cannot supply the muscles with enough oxygen for aerobic respiration.

- Your heart rate and breathing rate increase, but you still have to begin anaerobic respiration.

- It releases less energy per glucose molecule. In anaerobic respiration, the glucose is only partially broken down. Lactic acid is also produced, this then builds up in your muscles and causes muscle fatigue and can become painful. 

- The incomplete breakdown of glucose resulting in the build up of lactic acid is called the oxygen debt. During recovery, the breathing rate and heart stay high so that: rapid blood flow can carry lactic to the liver, extra oxygen can be supplied, enabling the liver to break down the lactic acid.

glucose -> lactic acid + energy

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Measuring Respiration Rate

- The respiratory quotient tells you whether someone is respiring aerobically or anaerobically.

- RQ = Amount of carbon dioxide produced / amount of oxygen used

- The RQ Is usually between 0.7 and 1 ( aerobic)

- If RQ is above 1.0 then the person is short of breath and is repairing anaerobically as well.

- Changes in the pH and the temperature can also change the respiration rate because they affect enxymes, and respiration is controlled by enzymes.

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Becoming Multicellular

Cell Division

- There are a large number of advantages of being multicellular, as humans. It allows an organism to become larger and more complex.

- It also allows cells to take on different jobs. This is called cell differentiation.

- However, when an organism becomes multi cellular, it needs to have systems that can:

- allow communication between all the cells in the body.

- supply all the cells with enough nutrients.

- control exchanges with the environment such as heat and gases.

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- Before mitotis happens, DNA must be replicated. This involves: The two strands of the DNA molecule 'unzipping' to form single strands. Then new double strands are formed by DNA bases lining up in complementary pairings.

- In Mitosis, the chromosomes line up along the contre of of the cell and divide. The copies the move to opposite poles of the cell.

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- The type of cell production that involves gametes is called meiosis.

- Gametes are haploid because they contain only one chromosome from each pair. This means that the zygote gets one copy of a gene from one parent and another copy from the other parent. This produces genetic variation

- In meiosis, there are 2 divisions. First the single strands are copied to make X-shaped chromosomes and chromosomes with the same  genes pair up. Then:

     - In the first division, one chromosome from each pair moves to opposite poles of the cell.

     - In the second division, the copies of each chromosome come apart and move to opposite poiles of the cell.


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