IGCSE Biology Section B/2 parts c and e

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Three factors affecting the movement of substances

1) Surface area to volume ratio

A larger surface area to volume ration would mean that diffusion, osmosis and active transport would be higher in these cells because there is a larger area to diffuse through.

2) Temperature

When the particles in a substance get hotter, they gain kinetic energy so they move faster in and out of cells.

3) Concentration gradient

If the concentration gradient is larger, the diffusion or osmosis (not active transport,) will happen more quickly because there would be many more particles on one side of the membrane. 

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Carbohydrates are made up of SIMPLE SUGARS.

Monosaccharide- simple sugar, a single unit such as glucose and fructose

Disaccharide- two monosaccharide groups linked together for example

  • Maltose = Glucose + Glucose 
  • Sucrose = Glucose + Fructose 
  • Lactose = Glucose + Galactose 

Polysaccharides - many simple (monosaccharides) sugars together in a long chain, for example:

  • starch 
  • cellulose


Found in pasta, rice, sugar. They provide energy. 


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Carbohydrate Structure


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Proteins and Lipids.

Proteins are long chains of amino acids. There are 8 essential amino acids.


Found in meat and fish and it is needed for growth and repair of tissues. Enzymes and hormones are example of proteins. Also protein can be used as energy in there is no other alternative.

A lack of protein can lead to kwashiorkor, a disease where fluid collects in the tissues.

Lipids have an E shaped structure of one glycerol to three fatty acids.


Found in butter and oils. Fats provide energy and are an energy store. They also provide insulation to the body to maintain its temperature. 

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Tests for glucose, starch and proteins.


You can add benedict's solution (reagent) which is blue to a sample. Put the test tube over heat (water bath,) making sure it doesn't boil and if glucose is present it will form a coloured precipitate. 

The colour changes from blue > green > yellow > orange > brick red

The higher the concentration of glucose, the more the colour will change, so for example if there is a high concentration of glucose, it will turn orange/red. This is useful to compare the concentration of glucose in varying samples.


Add iodine solution to the sample. If there is starch, it will turn from brown to dark black/blue.If there is no starch, it will remain brown. 


Add copper sulfate and sodium hydroxide to the sample and it will turn purple.

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The importance of vitamins and minerals.

  • Vitamin A is found in liver. It helps to improve vision, and maintains healthy hair.
  • Vitamin C is found in foods such as oranges. It prevents scurvy.
  • Vitamin D is found in eggs. It is needed to aid the absorption of calcium. Vitamin D can also be made when the skin is exposed to sunlight.
  • Calcium is found in milk and cheese and is needed to make bones and teeth.
  • Iron is found in red meat and is needed to make haemoglobin for healthy blood.
  • Water can be obtained through food and drink and is essential for most bodily functions. An intake of water is vital to replenish the water your body loses from sweating and breathing.
  • Fibre can be found in foods such as wholemeal bread and vegetables. It aids the movement of foods though the gut, in order to keep the gut regulating well.


There are 6 essential components> carbohydrates, proteins, lipids, vitamins, minerals and water.

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Energy in food

PAPER TWO- The amount of energy you require varies depending on:

  • Age- children and developing teenagers need more energy than older people. Also, the children in this age group are usually more active.
  • Activity level- Active people need more energy.
  • Pregancy- Pregant women need more energy than women who are not carrying a child, since they require energy for them and their babies.

calorimetry- burning food to investigate how much energy it contains

  • weigh a small amount of food and skewer it onto a mounted needle.
  • Add 25 cm cubed of water to a test tube, held by a clamp. Place a thermometer into the water and measure the temperature before-hand, because the change in temperature will indicate the amount of heat ENERGY released when the food has been burned. 
  • Set fire to the food and hold it under the tube (using a bunsen burner.) Keep re-igniting the food until it will no longer relight. 
  • Now measure the temperature of the water. 
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Energy in food


The accuracy of the experiment could be improved, as heat energy is lost to the surroundings, therefore the results will be lower, and showing that the food produces more energy that it really does. This can be improved by insulating the test tube so that some heat is retained in the heated water

How to calculate he amount of energy in food.

Energy in food (J)= Mass of water (g) x Temperature change (celsius) x 4.2

(1 cm cubed is the same as 1g of water)

Energy is Joules per gram of food (J/g) = Energy in food (J) / Mass of food (g)

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Enzymes and Digestion

The digestive enzymes break down larger molecules such as proteins, starch and fats because these are insoluble and cannot travel through the cell membranes.

  • Starch can be broken down from the polysaccharide to the disaccharide of maltose by amylase.
  • Then the maltose can be broken down into glucose (monosaccharide) by maltase.
  • Protease breaks down proteins into amino acids.
  • Lipases convert lipids into glycerol and fatty acids.


Bile is produced by the liver and stored in the gall bladder before it is released to neutralise the acidic conditions produced in the stomach. This is because the enzymes in the stomach work at their optimum when the conditions are acidic. Bile also emulsifies fats, (breaking it down into droplets) so that there is a larger surface area of the lipid for the enzyme to work on. Therefore the digestion of the fat is faster.

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The Five Main Stages Of Digestion

  • Ingestion
  • Putting food or the substance into your mouth.
  • Digestion
  • Digestion is the break down of large, insoluble molecules. There are mechanical (teeth) and chemical (enzymes) ways to digest the food.
  • Absorption
  • Absorption is the process of moving the soluble molecules through the walls of the intestines into the blood. Water is mainly absorbed in the large intestine.
  • Assimilation
  • Is the process of moving the absorbed molecules into body cells. The molecules then become part of the cell, for example amino acids makinds cellular proteins.
  • Egestion
  • Not everything can be digested, so this forms faeces which are disposed of through the ****, since they have no use to the body. This process is egestion.
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The Alimentarey Canal


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The Alimentary Canal

1) Salivary glands in the mouth produce amylase to break down the food. Mechanical work by the teeth also breaks up large pieces of food.

2) The food travels down to oesophagus to the stomach through peristalsis.

3) In the stomach, pepsin is produced, a protease which converts proteins into peptides. The stomach walls also secrete hydrochloric acid to kill any bacteria on the food, but also because pepsin has it's optimum pH value around 2. After the stomach, the substances are held back by a ring of muscle called a sphincter, which releases it into the first part of the illeum, (the duodenum.)

4)The pancreas produces lipase, amylase and trypsin (a protease,) which are released into the duodenum, along with bile from the gall bladder which emulsifies the fats, and neutalises the pH.

5) The illeum aborbs the digested molecules, through the villi, and most of the water. This is diffused into the blood stream. Then the colon absorbs the remaining water that the illeum has left, and the remaining waste forms faeces, stored in the rectum, and expelled from the ****.

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Absorption In The Small Intestine


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Abosrption in the small intestine

There are adaptations in the small intestine to make it suitable for the absorption of food.

1) It is very long in length, so there is time to abosrb all the molecules before it reaches the end

2) The walls of the small intestine are covered in tiny projections called villi which increase the surface area, also because there are millions of them.

3) Each cell on the surface of the villus also has MICRO VILLI, which increases the surface area further, allowing molecules to be absorbed into the blood easily.

4) They have a good supply of blood which increases (steepens) the concentration gradient, since there is a low concentration of the molecules in the blood compared to the gut.

5) They have a single layer of surface cells to help absorb the molecules more quickly, as it makes the diffusion distance smaller.

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There are circular muscle fibres in rings around the gut, and longitudinal muscle layers, running down the length of the gut. When the circular muscles contract, the longitudinal muscles relax, making the gut narrower. When the longitudinal contracts and the circular muscles relax, the gut becomes wider. This pushes the food along and means that it does not rely on gravity to be digested. 

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