Cells at Work

3.1

Enzymes make life possible Most reactions that take place in the cell are carried out with the help of enzymes. (Organic catalysts) Enzymes are substances that can increase the rate at which chemical reactions happen. This enables products to be produced more quickly. These reactions might still occur but at an extremely low rate. Substrates: These are substances that enzymes bind to. Active site: The part of the enzyme that binds to the substrate.

Figure 3.1a Enzyme and Substrate: Lock and key Enzymes can bring molecules together or split them apart.

Enzymes make life possible We know cells: Manufacture organic molecules Transform energy Breaking down and recycling unwanted substances. The chemical reactions involved in these activities are regulated by hundreds of enzymes working in chains. The product of one reaction is the substrate for the next reaction.

Enzymes make life possible Many different reactions occur at the same time in the cell because the cytoplasm has many different compartments. Enzymes are located at particular sites within cells and are used to increase the efficiency by which cells restore and release energy. Enzymes are also important in digestion.

Enzymes are organic catalysts ENZYMES SPEED UP REACTIONS Speed up reactions that otherwise occur more slowly. Most chemical reactions are reversible, the same enzyme can catalyse a reaction in either direction. Enzymes do not change the final amount of the product. Enzymes are not used up in a reaction, so cells can use them over and over again. Enzymes become ‘worn out’ and must be replaced. Vitamins in our diet do this.

Enzymes are organic catalysts ENZYMES ARE SPECIFIC Each enzyme catalyses only one type of reaction. They are specific because of their 3D structure. Enzymes action involved binding between the enzyme and the substrate. (Active site) The active sites on an enzyme and its substrate is structurally complimentary. (Jigsaw puzzle) This is why specific conditions have to be present in order for the enzyme can bind to its substrate. Eg: A change in pH will change the shape of the enzyme.

Enzymes are organic catalysts ENZYMES AFFECTED BY TEMPERATURE Enzymes are affected by temp. Warming increases the rate of the reaction. Molecules move faster and the enzyme and substrate are in contact more often, and the faster the product is produced. Too much heat can damage the structure of the enzyme and it begins to denature. All proteins are denatured (structure is changed) by heat when a certain ‘critical temperature’ is reached. At this point the enzyme can no longer bind with the substrate.

Denaturing is an irreversible change in protein structure. Boiling in water denatures most enzymes. LOVE THIS INTERESTING FACT! Some vegetables are blanched in boiling water before being frozen. The boiling water denatures enzymes that would otherwise cause the vegetable to ‘rot’ during storage. Most enzymes have an optimum temp. This is just below the ‘critical temp’. – Greatest effect on the speed of the reaction. Optimum temp. depend on the organisms body temp.

Enzymes are organic catalysts ENZYMES ARE AFFECTED BY OTHER FACTORS The rate at which the reaction takes place is affected by the amount of enzyme or substrate or whether there is an accumulation of product. More s or e – speeds up Accumulation of s or e – slows down Because the enzyme speeds up the reaction or slows it down, it does NOT affect the AMOUNT produced. There are other molecules that can also fit the enzyme’s active site and slow the reaction – competitive inhibitors.

Enzymes are organic catalysts SOME ENZYMES NEED HELP Coenzymes are very small molecules – less complex than proteins. Associated with particular enzymes and essential for activity. Many coenzymes cannot be synthesised by animals and need to be obtained from plants or microorganisms. Many vitamins provide this and are considered vital for our diets.

3.2

Cells need energy to work All types of living cells must perform the same basic processes. Cell replication places extra demands on an already very busy cell. The whole process of cell replication occurs in under an hour.

Chemical energy Cells require energy for their activities. All cells use chemical energy to carry out their energy-requiring activities. Chemical energy is stored in bonds or connections that join atoms together in molecules. If a molecule is broken, energy is released. Organic molecules such as glucose, fats and proteins have many energy containing bonds that can be broken apart in cells to release energy.

ATP – chemical energy for cells Cells store ‘ready to use’ energy in the form of ATP. (adenosine triphosphate) ATP is a molecule with a high energy phosphate bond that can be broken to release a small amount of energy. More than one bond can be broken depending on the needs of the cell. ie: energy dependent activities. ATP becomes ADP (adenosine diphosphate) once its given up its energy. When cells convert energy from one form to another, some energy is lost to it’s surroundings, usually as heat.

For example: During exercise we use the chemical energy of ATP to make our muscles contract. The more work we do, the more energy we use and the more energy is lost as heat, which warms our muscles. It is this continual generation of heat energy that allows mammals and birds to maintain stable body temp. that are usually higher than their surroundings.

3.3

Providing energy in cells Cells get their energy to make ATP by breaking apart glucose molecules. The chemical energy in the gluc. is released in a series of small steps that involve many enzymes, so that the energy is released in many small ‘packets’. The amount of energy that can be harvested from glucose depends on whether oxygen is present or not. When oxygen is available – aerobic – 36 to 38 molecules. Aerobic pathway – oxygen present Anaerobic pathway – no oxygen present

Glycolysis The first stage in the breakdown of glucose is called glycolysis. Lysis – splitting of glucose into two pyruvate molecules. Occurs in cytosol – anaerobic – does not require oxygen. For each molecule of glucose, glycolysis produces two ATP molecules very rapidly. This stage is common to both aerobic and anaerobic pathways. What happens next depends on whether or not there is adequate oxygen.

Aerobic respiration in the Mitochondria If oxygen is available - aerobic – 36 to 38 molecules of ATP are produced. This stage occurs in the Mitochondria. Aerobic respiration takes place in a series of steps that require oxygen, and involved two linked pathways – the Krebs cycle and the electron transport chain. The number of mitochondria in a cell in relation to the cell’s energy requirements. Very active cells, such as heart muscle cells, have many thousands of mitochondria.

Fermentation Anaerobic and occurs in the cytosol. If there is insufficient oxygen available, pyruvate passes into an anaerobic pathway, known as fermentation and there is no further formation of ATP. The pyruvate produced during glycolysis undergoes fermentation into either lactic acid (animals) or carbon dioxide and alcohol (plants and microorganisms) Fermentation prevents the build up of pyruvate which can slow a chemical reaction.

Comparing pathways For each molecule of glucose, aerobic respiration produces 20 times the number of ATP molecules produced by glycolysis. Most cells of plants and animals carry out aerobic respiration. They rely on the glycolysis on for a few seconds when there is not enough oxygen avail.

Making and handling organic molecules Page 46 - HOMEWORK What is an organic molecule? Role and function of: Nucleus Ribosomes Endoplasmic reticulum Golgi Apparatus Lysosome Vacuoles