Enzymes, Transport and Exchange

• Collision theory states that chemical reactions occur only when the reactant particles collide with sufficient energy to react.
• The following factors influence the rate of reaction​

  • Increasing concentration / pressure​
  • Increasing temperature​
  • Increase the surface area of a solid reactant​
  • Use of a catalyst
• • Enzymes are a type of protein
  • Remember long chains of amino acids form proteins and the structure and folding of these proteins will dictate their function – The tertiary structure 
  • In the case of enzymes the unique folding structure will dictate the function of the enzyme including the shape of the enzymes ‘active site’
• Biological catalysts (biological because they are made in living cells, catalysts because they speed up the rate of chemical reactions without being changed)
• Enzymes speed up metabolic reactions by acting as a biological catalyst. ​

  Enzymes are proteins, they have an active site that has a specific shape. ​

  Active Site-part of the enzyme where substrate molecule binds to make an enzyme-substrate complex.​

  Enzymes are very specific due to their tertiary structure*. 
• • How Enzymes speed up reactions 
  • A certain amount of energy is required by the chemicals before a reaction will start- this is known as the activation energy. ​
  • Enzymes lower the activation energy required for a reaction to occur. This speeds up the rate of reaction.​
  • The substrate fits into the active site to form an enzyme-substrate complex.  This lowers the activation energy because: ​
  • If the molecules need to be joined, this holds them together​
  • If a molecule is broken down, fitting in the active site puts strain on the bonds 
• The effect of Temperature

  • Enzymes work fastest at their ‘optimum temperature’​
  • In the human body, this optimum temperature is about 37⁰C​
  • Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and the active site will lose its shape​
  • The enzyme has been denatured
• • increase in temperature results in increased rate of reaction- more heat = more kinetic energy so the molecules move faster. ​
  • substrate molecules are more likely to collide with the enzymes’ active sites. ​
  • energy of the collision also increases, therefore it is more likely to result in a reaction. ​
  • If temperature increases too much rate of reaction starts to decline. ​
  • increased temperature causes the enzymes’s molecules to vibrate, if temperature increases too much vibrations can cause bonds in tertiary structure to break​
• The effect of pH on Enzyme Activity

  • Increased H+ (acidic environment) or OH- (Alkali environment) will result in bonds on the enzyme being disrupted.​
  • Free H+ and OH- ions can bind to the protein and disrupt the structure of the enzyme.
  • Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2)​
  • Some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
• The effect of pH on Enzyme Activity

  • If the pH is too far above or too far below the optimum, the bonds that hold the amino acid chain together to make up the protein can be disrupted or broken​
  • This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity​
  • Moving too far away from the optimum pH will cause the enzyme to denature and the reaction it is catalysing will stop
• The effect of pH

  • All enzymes have an optimum pH value​
  • Above and below the optimum pH the H+ and OH- ions found in acids and alkalis can disrupt the ionic bonds (bonds between oppositely-charged atoms) and hydrogen bonds that hold the enzymes tertiary structure. The active site changes shape and the enzyme becomes denatured. 
• The effect of enzyme concentration:

  • Increasing the concentration of an enzyme increases the rate of reaction. ​
  • The more enzyme molecules there are present in a solution the more likely an enzyme-substrate complex will form. ​
  • However, if the amount of substrate is limited, there comes a point when there is more than enough enzyme to deal with the available substrate, so adding more enzyme has no further effect. ​This is known as a rate-limiting step
• Enzymes:

  • The properties and functions of enzymes that are determined by their tertiary structure: ​​

  • properties: ​

  • o The shape of the active site ​
  • o The role of bonding ​
  • o The effect of temperature on enzyme function ​​
  • • Role of enzymes: ​
  • o proteases including trypsin ​
  • o carbohydrases including amylase o lipase 
• Models of enzyme action: The Induced Fit Model  

  • The shape of an enzyme is changed slightly as a substrate binds to its active site​
  • Substrate causes or induces a slight change in the active site shape so that it can fit perfectly​
  • The enzyme changes shape, the substrate is activated so that it can react and the resulting product(s) are released​
  • Enzyme is left to return to its normal shape ready to receive another substrate molecule 
• Exchange of Substances:​

  In order for any organism to function properly, it needs to exchange substances between itself and the environment such as:​

  • Oxygen​
  • Carbon dioxide​
  • Water​
  • Dissolved food molecules​
  • Mineral ions​
  • Urea
• Substance Exchange

  • This exchange of substances occurs across the cell membrane​
  • There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport
• • The surface area to volume ratio is important in biology because it determines the efficiency of exchange surfaces in the organism. ​
  • A larger surface area to volume ratio means that there is more surface area available for the exchange of materials, making it easier for the organism to absorb necessary nutrients and eliminate waste products.
• • Unicellular (single-celled) organisms like amoeba have very large surface areas (SA) in comparison to their volumes​
  • This means that the distance between the surface of the organism to its centre is very small
• • For larger, multicellular organisms the distance between the surface of the organism to its centre is relatively long​
  • This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise
• The Need for Exchange Surfaces:

  • Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes​
  • This is why larger organisms need exchange surfaces within their transport systems to carry out diffusion, osmosis and active transport at a sufficient rate​
  • Exchange surfaces in animals include (covered in A&P):​
  • The lungs and alveoli for gas exchange​
  • The small intestines and villi for absorption of digested food
• • Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:​
  • Having a large surface area to increase the rate of transport​
  • A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across
• • animals have: A large network of blood vessels throughout the body:​
  • To reduce the distance of exchange of materials between cells and the bloodstream​
  • To move substances towards or away from exchange surfaces to maintain concentration gradients