2.4 cells

• The structure of an enzyme is composed of a protein and possible nonprotein called a co-factor. It has an active site, which joins or breaks substrate / a 3D protein folded and joined to other proteins by hydrogen and sulphide bonds to form an active site which the substrate binds to.
• Enzymes’ function is to catalyse or speed up biological reactions by lowering activation energy. They are specific to a reaction and are not used up in the reaction.
• Enzymes are involved in each step of DNA replication. During DNA replication, a specific enzyme unwinds the DNA helix exposing nucleotides. Another enzyme synthesises the new DNA strands by adding complementary nucleotides to the template strands.
• During DNA replication, enzymes function using the induced fit model. The active site and substrate are not initially perfect matches for each other. The active site continues to change until the substrate is completely bound to it, at which point the final shape of the enzyme is determined.
• Factors that affect enzymes during DNA replication include temperature, pH, substrate concentration, co-factors, enzyme concentration, and poisons.
• All enzymes have an optimum temperature. At very low temperatures, enzyme action slows, and at high temperatures the 3D shape may change and denature. Denaturing is irreversible and means the enzyme is inactive.
• An increase in substrate concentration means more can combine with enzymes’ active site and therefore increase the rate of reaction until the available enzyme molecules become ‘saturated’ and the rate of reaction levels off.
• Enzymes have an optimum pH range. Outside their optimum range can cause an enzyme’s active site to denature, preventing substrates from binding.
• High enzyme concentration allows for increased rate of reaction.
• Co-enzyme alter the shape of the active site in such a way that it can effectively combine with substrates. A decrease in co-factor would reduce the rate of reaction or even stop the enzyme.
• Inhibitors are poisons that alter or block the active site, preventing substrates from binding.
• Enzymes are involved in each step of DNA replication, and therefore the rate of DNA replication is dependent on factors affecting enzyme action. Therefore these factors would also affect growth, repair and meiosis / mitosis / cell division.
• Mitotic cell division is the process in which a parent cell divides into two daughter cells. It occurs in somatic / body cells as part of the cell cycle. The daughter cells are identical to the parent cell. Cells divide for growth and repair as well as to maintain large surface area to volume ratio.
• The stages of mitosis include interphase (G1 phase), prophase, metaphase, anaphase, telophase and cytokinesis.
• The efficiency of a cell is reliant upon gaining essential materials quickly, so processes such as cellular respiration and DNA replication can occur at an optimal rate. Similarly the cell needs to be able to exclude wastes such as carbon dioxide and water quickly to avoid toxicity or lysis. The efficiency of diffusion determines the overall efficiency of cell function and this is all dependent on an optimal SA:V
• DNA replication


  The cell makes an exact copy of its DNA before cell division so that there is a full set of genetic information available in each cell after cell division has occurred
• DNA replication


  1. DNA helicase unwinds and unzips the double helix structure of the DNA molecule

  2. DNA polymerase synthesises the new strands by joining free nucleotides one at a time in the 5'-3' direction based on the complementary base pairing rule where A-T and C-G on the template strand

  3. The new nucleotides are only added at the 3' side of the polynucleotides

  4. The leading strand is synthesised continuously but the lagging strand is synthesised discontinuously, creating Okazaki fragments

  5. Okazaki fragments are joined together by the enzyme DNA ligase by re-establishing bonds between the sugar and phosphate molecules (phosphodiester bonds)
• DNA replication is a series of enzyme controlled reactions where a DNA molecule is copied to produce two identical double stranded DNA molecules
• Replication fork


  Created when DNA helicase unwinds and unzips the double helix structure of the DNA molecule, breaking the hydrogen bonds that hold the bases together and exposing the nucleotides
• As a result, two identical DNA molecules are created, with one strand being the old original strand and the other being new
• Semi-conservative DNA replication


  One strand is 'conserved' in the new molecule
• Rough endoplasmic reticulum


  Network of folded membranes with ribosomes attached to its surface, giving it a rough appearance
• Smooth endoplasmic reticulum


  Network of folded membranes without ribosomes attached to its surface, giving it a smooth appearance
• White blood cells


  Have extensive system of rough endoplasmic reticulum
• Cells in the liver


  Contain large amount of smooth endoplasmic reticulum
• White blood cells


  • Involved in the production of antibodies, which are made up of proteins
  • Rough endoplasmic reticulum is the manufacture and transport of proteins
  • Ribosomes are attached on the surface of RER where proteins are made
• Liver cells


  • Produce steroid hormones that aid in the detoxification of drugs and alcohol
  • Smooth endoplasmic reticulum is the production of steroids hormones, lipids and storage for carbohydrates and calcium ions
• Aerobic respiration


  Glucose is broken down by a series of enzyme controlled reactions into carbon dioxide and water in the presence of oxygen to produce energy in the form of ATP
• Aerobic respiration


  Energy is produced in the form of ATP from food
• Aerobic respiration occurs in both plants and animals
• Mitochondria


  The organelle where the process of aerobic respiration occurs
• Mitochondria


  • Rod shaped
  • Increases the efficiency of energy production in respiration
  • Increases the surface area for the diffusion of oxygen and food in and carbon dioxide and water out
• Mitochondria


  • Have double membranes to allow for compartmentalisation to enable the regulation of pH and temperature to optimise conditions for respiration
• Cristae


  The inner membrane of the mitochondria which is highly folded to increase the surface area to enable reactions in the Electron Transport Chain to occur quickly
• Matrix


  The liquid medium in the mitochondria which provides a medium for chemical reactions to occur such as the Krebs Cycle
• Number of mitochondria in a cell


  Depends on their energy requirement
• Organisms that have a high energy requirement have a higher number of mitochondria than those who have a low energy requirement
• Animals tend to have a higher energy requirement as they carry out complex processes such as movement, breathing, digestion and excretion, than plants
• Muscles are involved in most of these processes and therefore have a high energy requirement and thus a lot of mitochondria to supply the energy they need for the contraction of muscles in movement
• Sperm cells also have a high energy requirement as they have a flagellum that allows movement towards the ova for fertilisation