ENZYMES AND CELL PROCESSES

ENZYMES AND CELL PROCESSES

1. ENZYMES

   • enzymes are proteins
   • act as biological catalysts
     • controlling speed of chemical reactions in all organisms
       • increasing rate of chemical reactions
         • without enzymes, metabolism would take place too slowly for life to exist
   • are specific
     • 1 enzyme catalyses only 1 type of reaction
   • each enzyme has a particular shape determined by sequence of amino acids
     • active site of an enzyme corresponds that of the substance it catalyses
       • the 'lock and key model'
         • in the way a key fits into a particular lock, the substrate fits into the active site of the enzyme
           • as the enzyme and substrate fit together,
             • chemical bonds are formed or broken
   • not consumed or broken down in reactions
     • i.e. 1 enzyme molecule can be reused at a fast rate
       • e.g. peroxidase in liver cells can catalyse breakdown of several million hydrogen peroxide molecules per min
   • shape maintained by weak H bonds.
     • susceptible to denaturing at high temp
       • as H bonds break, active site shape changes
         • enzyme unable to function

2. ENZYME ACTIVITY: FACTORS

   • Temperature
     • warmer the temp, faster the enzymes catalyse the reaction
       • at low temp, both enzyme and reactants move slowly
         • i.e. low kinetic Energy
         • less successful collisions between enzymes' active site and substrate
           • a slow ROR
             • as temp increases, both reactant and enzyme gain more kinetic energy
               • move faster and more successful collisions -> faster ROR
                 • at high temp, enzyme active site changes shape (denatures)
                   • substrate can no longer fit into active site
                     • enzyme is unable to function
                       • ROR decreases
             • (ROR= rate of reaction)
   • Substrate concentration
     • low substrate conc: less substrate
       • less chance of enzyme active site and substrate collisions
         • slow ROR
     • as substrate conc increases, more substrate is present
       • more chance of successful collisions with enzyme active site
         • faster ROR
     • at high substrate concentration
       • other factors will become limiting factors
         • ROR wont increase and levels off
         • e.g. enzyme conc, temperature
   • pH
     • enzymes work best at a specific pH
       • if pH is outside the range for an enzyme it denatures
         • substrate can then no longer fit and no longer acts as a catalyst
           • ROR decreases
       • e.g. amylase=7 pepsin=1~2
   • Co-factors
     • assist in catalysis
       • small inorganic ions or vitamins- coenzymes
     • necessary when only weak bonds form between enzyme and substrate
       • co-enzyme acts as a bridge, locking enzyme and substrate more tightly together
   • Inhibitors
     • substances that prevent enzymes catalysing reactions
       • poisons
     • can take over active site of enzyme
       • stop substrate from binding to active site
     • bonding to another part of enzyme
       • altering shape of active site
         • can no longer bind to substrate
           • usually temporary

3. PHOTOSYNTHESIS

   • occurs in chloroplasts of plant cells
   • process which plants produce GLUCOSE from CARBON DIOXIDE and WATER in presence of light Energy and chlorophyll
   • carbon dioxide + water ---------------> glucose + oxygen
     • light E and chlorophyll on the arrows
   • 6CO2 + 6H2O -------------> C6H12O6 + 6O2
   • Light Phase
     • occurs in grana
     • electrons in chlorophyll are excited by solar E striking them
       • passed along series of carrier molecules and synthesises ATP from ADP
         • electrons return to chlorophylll
           • water is split to Hydrogen has and oxygen gas
   • Dark phase
     • light independent reaction
     • occurs in stroma
     • CO2 and H enter a complex biochemical cycle
       • are rearranged to form C6H12O6 (glucose) as a final product
     • ATP used to run the cycle
   • Glucose Produced
     • stored as insoluble starch in cells of roots
     • used in respiration
     • used to make other needed organic chemicals
       • e.g. fats, amino acids
   • Importance of Photosynthesis
     • converts solar E into chemical E as organic molecules
       • plants= autotrophs, producers
         • i.e. self feeders
         • starting point of food chains.
     • photosynthesis by plants accounts for increase in level of O2 in atmosphere
       • O2 is essential for aerobic respiration
       • (as O2 is a waste product of Photosynthesis)

4. RATE OF PHOTOSYNTHESIS

   • Temperature
     • increasing temperature increases R.O.Photosynthesis up to an optimum temp
       • when temp increases too far above optimum temp,
         • enzymes controlling photosynthesis denature and can no longer catalyse reaction
           • therefore photosynthesis ceases
   • Light Intensity
     • increasing light intensity increases R.O.Photosynthesis up to a maximum
       • above this maximum/limit, further increase in light intensity will have no further effect on R.O.P
         • because either
           • (i) light absorbing pigments are saturated
           • (ii) other factors becoming limiting factors
             • e.g. conc of CO2 or temperature
   • CO2 concentration
     • increasing CO2 conc increases R.O.P up to a maximum
       • above this maximum/limit, further increase in CO2 conc has no further effect on photosynthetic rate
         • due to other limiting factors
           • e.g. temperature, light intensity
   • because 3 factors combine to determne R.O.P
     • R.O.P can alter if one of the other factors change

5. ADAPTATIONS OF THE PLANT

   • Adaptations of Chloroplasts
     • grana have large S.A to absorb light, maximising Photosynthesis
     • stoma is clear to allow light to pass through to grana, maximising Photosynthesis
     • oval in shape
       • increase S.A : Vol ratio so it can take in more CO2 and H2O faster
         • faster rate of diffusion
           • maximising Photosynthesis
   • Adpatations of the Leaf
     • waxy cuticle and epidermal layer is clear
       • so that maximum light can penetrate to palisade layer
         • maximising photosynthesis
     • thin and flat
       • reduce distance for gases and substances to diffuse through stomata to photosynthesising cells
     • thin
       • allows light to pass through
     • flat
       • maximises S.A exposed to light
     • air spaces in spongy mesophyll
       • allows gases to diffuse faster to photosynthesising cells
     • palisade cells
       • richly supplied with chlorophyll
       • on upper surface of leaf to capture light
       • packed tightly

6. RESPIRATION

   • process that occurs in mitochondria of all living cells, breaking down glucose to produce ATP to be utilised for cell processes
   • GLUCOSE + OXYGEN -----------------> CARBON DIOXIDE + WATER + ENERGY
   • C6H12O6 + O2 ---------------> CO2 + H20 + ENERGY
   • Aerobic Respiration
     • requires oxygen for complete breakdown of glucose to CO2 H2O and Engergy in the form of ATP and heat
     • Gylcolysis
       • occurs in cytoplasm (no oxygen required)
         • changes glucose -----> pyruvate
           • produces 2 molecules of ATP
       • Krebs cycle
         • occurs in matrix (requires oxygen)
           • pyruvate + O2 ----> CO2 + H+ ions
             • (CO2 is released as a waste product)
             • H+ ions enter the electron transfer chain and 2 ATP molecules are produced
         • Electron transfer
           • occurs in cristae (requires oxygen)
             • H+ ions passed along chain releasing Energy
               • 34~36 ATP molecules produced
   • carried out actively in cells with high E demands
     • e.g. muscle cells- contract and release - need ATP therefore have high no. of mitochondira
     • e.g. kidney tubules reabsorb nutrients (glucose) in filtrate using ATP through active transport therefore have high no. of mtochondria

7. CELL DIVISION

   • DNA (deoxyribonucleic acid) : large molecule in chromosomes that carries genetic info
     • double helix shape
     • each strand is made of many repeating units = nucleotides
     • bas pairs held together by weak H bonds
       • base pairing rule (A=T) (G=C)
     • 2 strands are anti parallel
       • 3' -> 5' and 5' -> 3'
   • DNA Replication
     • essential so that chromosomes are copied and give same genetic code to every new cell made
     • occurs prior to cell division
     • 1 identical DNA molecules are produced
     • semi-conservative
       • 1 side of final DNA strand = original
         • other side is new
     • Factors affecting DNA Replication
       • controlled by enzymes
         • any factor affecting enzymes affects rate of DNA replication
           • i.e. temp, amount of nucleotides, amount of enzymes
       • plants grow more in spring
         • warm and unlimited resources
       • animals grow more at certain stages of their life cycle
         • e.g. foetus or at young
     • during interphase
   • 1. DNA molecule unzipped by DNA helicase breaking H bonds b/w complementary base pairs
     • 2. nucleotides from within nucleus used to produce new strand of DNA
       • 3. as DNA unzipps, DNA polymerase adds new nucleotides to 3' end of original strand producing a continuous strand
         • 4. DNA polymerase only adds to nucleotides to 3' end of original strand
           • DNA polymerase must work from the replication down the strand producing Okazaki fragments
           • 5. Okazaki fragments bonded by enzyme ligase to form a continuous strand
             • 6. 2 identical complete DNA molecules twist into a helix
   • Mitosis
     • cell division process producing 2 genetically identical cells for growth and repair
       • so that same cell functions and processes continue
     • Mitosis is rapid when:
       • new cells are forming during periods of growth
         • e.g. zygote, embryo, infant and young organism or plants coming our of periods of dormacy
       • repairing after tissue damage
         • e.g. dermis of skin dividing rapidly to form replacement cells
       • in the bone marrow
         • constantly producing red blood cells