topic 2

• Digestive enzymes work outside of cells; they digest large, insoluble food molecules into smaller, soluble molecules which can be absorbed into the bloodstream.
• Metabolism is the sum of all the reactions happening in a cell or organism, in which molecules are synthesised (made) or broken down.
• Enzymes are biological catalysts made from protein.
• Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature).
• Substrates temporarily bind to the active site of an enzyme, which leads to a chemical reaction and the formation of a product(s) which are released.
• Enzymes: How They Work

  • Enzymes catalyse specific chemical reactions in living organisms – usually one enzyme catalyses one particular reaction:

   

  • The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
  • Enzymes have specific three-dimensional shapes because they are formed from protein molecules
  • Proteins are formed from chains of amino acids held together by peptide bonds
  • The order of amino acids determines the shape of an enzyme
  • If the order is altered, the resulting three-dimensional shape changes
• The specific shape of an enzyme is determined by the amino acids that make the enzyme.
• The three-dimensional shape of an enzyme is especially important around the active site area; this ensures that the enzyme’s substrate will fit into the active site enabling the reaction to proceed.
• Enzymes work fastest at their ‘optimum temperature’ in the human body, the optimum temperature is around 37⁰C.
• Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together, causing the enzyme to distort and lose its shape, reducing the effectiveness of substrate binding to the active site, reducing the activity of the enzyme.
• Eventually, the shape of the active site is lost completely and the enzyme is described as being ‘denatured’.
• Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost.
• The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9).
• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed.
• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop.
• Lock & Key Model - A model used to explain how enzymes work. It suggests that there is a perfect fit between the active site of an enzyme and its substrate. This means that only one type of substrate can fit onto the active site of an enzyme.
• Chemical Digestion

  • The purpose of digestion is to break down large, insoluble molecules into smaller, soluble molecules that can be absorbed into the bloodstream
  • Large insoluble molecules, such as starch and proteins, are made from chains of smaller molecules which are held together by chemical bonds. These bonds need to be broken
  • Enzymes are biological catalysts – they speed up chemical reactions without themselves being used up or changed in the reaction
  • There are three main types of digestive enzymes – carbohydrases, proteases and lipases
• Carbohydrases

  • Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch into maltose, which is then broken down into glucose by the enzyme maltase
  • Amylase is made in the salivary glands, the pancreas and the small intestine
• Proteases

  • Proteases are a group of enzymes that break down proteins into amino acids in the stomach and small intestine
  • Protein digestion takes place in the stomach and small intestine, with proteases made in the stomach (pepsin), pancreas and small intestine
• Lipases

  • Lipases break down lipids (fats) to glycerol and fatty acids.
  • Lipase enzymes are produced in the pancreas and secreted into the duodenum
• The Role of Bile

  • Cells in the liver produce bile which is then stored in the gallbladder
  • Bile has two main roles:
  • It is alkaline to neutralise hydrochloric acid from the stomach. The enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach
  • It breaks down large drops of fat into smaller ones, increasing surface area. This is known as emulsification.
  • The alkaline conditions and larger surface area allows lipase to chemically break down fat (lipids) into glycerol and fatty acids faster (the rate of fat breakdown by lipase is increased)
• • The products of digestion are used to build new carbohydrates, lipids and proteins required by all cells to function properly and grow
  • Some glucose released from carbohydrate breakdown is used in respiration to release energy to fuel all the activities of the cell
  • Amino acids are used to build proteins like enzymes and antibodies
  • The products of lipid digestion can be used to build new cell membranes and hormones
• All gas exchange surfaces have features to increase the efficiency of gas exchange including a large surface area to allow faster diffusion of gases across the surface, thin walls to ensure diffusion distances remain short, good ventilation with air so that diffusion gradients can be maintained, and a good blood supply (dense capillary network) to maintain a high concentration gradient so diffusion occurs faster.
• Gas exchange occurs by the process of diffusion; breathing is essential in maintaining high concentration gradients between the air in the alveoli and the gases dissolved in the blood.
• Breathing keeps the oxygen level in the alveoli high and the carbon dioxide level low.
• Air passes through the following structures when we breathe in: Trachea, Bronchus, Bronchiole, Alveoli, and the Diaphragm.
• The Diaphragm is a thin sheet of muscle that separates the chest cavity from the abdomen and is ultimately responsible for controlling ventilation in the lungs.
• When the Diaphragm contracts, it flattens, increasing the volume of the chest cavity (thorax), which causes a decrease in air pressure inside the lungs relative to outside the body, drawing air in.
• When the diaphragm relaxes, it moves upwards back into its domed shape, decreasing the volume of the chest cavity (thorax), which causes an increase in air pressure inside the lungs relative to outside the body, forcing air out.
• The external and internal intercostal muscles work as antagonistic pairs, meaning they work in different directions to each other.
• During inhalation, the external set of intercostal muscles contract to pull the ribs up and out, increasing the volume of the chest cavity (thorax), decreasing air pressure, drawing air in.
• During exhalation, the external set of intercostal muscles relax so the ribs drop down and in, decreasing the volume of the chest cavity (thorax) increasing air pressure, forcing air out.
• When we need to increase the rate of gas exchange, for example during strenuous activity, the internal intercostal muscles will also work to pull the ribs down and in to decrease the volume of the thorax more, forcing air out more forcefully and quickly, this is called forced exhalation.
• There is a greater need to rid the body of increased levels of carbon dioxide produced during strenuous activity.
• This allows a greater volume of gases to be exchanged.
• The human heart is part of a double circulatory system.
• The circulatory system is a system of blood vessels with a pump (the heart) and valves that maintain a one-way flow of blood around the body.
• The heart has four chambers separated into two halves: the right side of the heart pumps blood to the lungs for gas exchange, this is the pulmonary circuit, and the left side of the heart pumps blood under high pressure to the body, this is systemic circulation.
• The benefits of a double circulatory system include: blood travelling through the small capillaries in the lungs loses a lot of pressure which reduces the speed at which it can flow, and by returning oxygenated blood to the heart from the lungs, the pressure can be raised before sending it to the body, meaning cells can be supplied with oxygenated blood more quickly.
• The heart is labelled as if it was in the chest, so what is your left on a diagram is actually the right-hand side (and vice versa).