Organisation and the digestive system

Tissues and organs

  • A tissue: a group of cells with a similar structure and function

- Muscular tissue contracts to bring about movement,

- glandular tissue contains secretory cells producing and releasing substances,

- epithelial tissue covers the outside of the body and internal organs.

  • Organs: collections of tissues

- The stomach has muscular tissue to churn down the food and digestive juices together,

- glandular tissue to produce digestive juices breaking down food,

- epithelial tissue covers the inside and outside of the organ.

  • Pancreas: make hormones to control blood sugar and some enzymes digesting food.
  • Organ systems: groups of organs working together to perform specific functions, and form organisms, having adaptations to make them effective.
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The human digestive system

  • The food you eat is made up of large insoluble molecules which the body can't use, so they are digested to make up smaller, soluble molecules.
  • The digestive system  starts at the mouth and ends at the anus. Glands make and release digestive juices containing enzymes to break down food.

The small intestine: where soluble food molecules are absorbed into the body which are transported around the body.

  • a very large surface area due to villi,
  • a good blood supply
  • and short diffusion distance, increasing diffusion and active transport.

The muscular walls: squeeze the undigested food into the large intestine,

  • water is absorbed from the undigested food into the blood and forming faeces stored and passing out the body through the rectum and anus.

The liver: produces bile, digesting lipids.

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The chemistry of food


  • Provide us with fuel making reactions of life possible.
  • They contain carbon, hydrogen, and oxygen and made up of sugars. Some carbohydrates only have one sugar unit or two sugar units joined together. These are simple sugars.
  • Complex carbohydrates are made up of long chains of simple sugar units bonded together. Carbohydrates are broken down into glucose needed for cell respiration. Cellulose a carbohydrate supports plants.

Lipids are fats and oils.

  • They're all insoluble in water and the most efficient energy store of the body. They are made up of three molecules of fatty acids joined to a molecule of glycerol which is always the same but the fatty acids vary.
  • Proteins build up cells and tissues and enzymes. They are made of carbon, hydrogen, oxygen, and nitrogen.They are made up of long chains of small units called amino acids.They are folded, coilded, and twisted to make specific 3D shapes enabling other molecules to fit into the protein. The bonds holding the proteins are sensitive to temperature and pH and if it's broken the shape of the protein is lost and won't function; it is denatured. Proteins act as enzymes, antibodies, hormones, and structural components of tissues.
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Catalysts and enzymes

Catalysts speed up reactions but aren't used up in the reactions. 

  • The rate of chemical reactions in the body are controlled by enzymes, special biological enzymes speeding up reactions. Each enzyme interacts with a particular substrate (reactant). They are large protein molecule and its shape is vital for its function.
  • Long chains of amino acids are folded to produce a molecule with an active site having a unique shape so it can bind to a specific substrate molecule.

The substrate of the reaction to be catalysed fits into the active site of the enzyme:

  • once in place, the substrate and enzyme bind together and the products are released from the surface of the enzyme. Enzymes join small molecules together and break up large ones.

Enzymes control metabolism - sum of all the reactions in a cell or body.

  • Different enzymes catalyse specific types of metabolic reactions: building large molecules from small ones, changing one molecule into another, and breaking down large molecules into small ones.
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Factors affecting enzyme action

Factors affecting biological reactions:

  • concentration,
  • temperature, 
  • surface area.

As the temperature increases, so does the rate of reaction, until 40 degrees Celsius when the protein structure of the enzyme and the active site changes.

  • The substrate can't fit into the active site and the enzyme is denatured and the rate of reaction drops. Human enzymes work best at body temperature.

Without enzymes, reactions wouldn't happen fast enough to keep you alive. Not all enzymes work best at 40 degrees.

  • A change in pH affects the forces holding the chains so it changes shape and the active site is lost so it becomes denatured. Different enzymes work best at different pH levels. A change in pH levels stop them working completely.
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How the digestive system works

Digestive enzymes work outside the cells produced by specialised cells in glands and the enzymes pass out of the cells into the digestive system to come in contact with the food.

The mouth and small intestine - alkaline but the stomach - low acidic pH value.

Carbohydrases: enzymes breaking down carbohydrates .

Starch - sugars in the mouth and small intestine by  amylase made by the salivary glands and pancreas. Enzymes made in the pancreas go to the small intestine.

  • Proteins is catalysed by protease made by the stomach, pancreas, and small intestine. Break  into amino acids, which occurs in the stomach and small intestine.

Lipids - fatty acids and glycerol in the small intestine, catalysed by lipase made in the pancreas and small intestine.

Once food molecules leave the small intestine, they pass into the bloodstream to be carried around the body to cells needing them.

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Making digestion efficient

The stomach secretes pepsin to digest protein which works best in an acidic pH and  also produces a concentrated solution of hydrochloric acid allowing protease enzymes to work efficiently.

The stomach produces a thick layer of mucus coating the stomach and protecting it from being digested. 

Small intestine enzymes work best in alkaline conditions.

  • The liver- bile, stored in the gall bladder
  • As food comes into the small intestine from the stomach, bile is squirted on it through the bile duct; neutralising the acid added to it and providing alkaline conditions for enzymes to work freely.

Enzymes need to have a large area to work on. Bile emulsifies fats breaking up large drops of fat into smaller droplets, providing a large surface area of fats for lipase to act upon since it helps chemically break down the fats quicker into fatty acids and glycerol.

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Required practical: Identify the main food groups.

  • Carbohydrates: iodine test for starch - yellow-red iodine solution - blue-black if starch is present. Benedict's test for sugars - blue Benedict's solution - brick red on heating if sugar  is present.
  • Protein: Biuret test - blue Biuret reagant - purple if protein is present. Biuret solution is corrosive; wear protection.
  • Lipids: ethanol test - ethanol added to a solution - a cloudy white layer if a lipid is present. Ethanol is flammable and harmful.

Required practical:

Place different starch solutions of a known volume and concentration in a water bath not higher than 37 degrees, add a buffer solution at a different pH to each solution, set up spotting tiles for each test solution with a drop of iodine in each well, mix the same concentration and volume of amylase into each tube, start a stop clock as soon as the enzyme is added, take samples every 30 seconds using a pipette and adding each sample to an iodine-filled well, and observe and record results displayed graphically to compare the effect of pH on an amylase-catalysed reaction.

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