Chapter 8 Cell respiration and photosynthesis

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Chapter 8 Cell respiration and photosynthesis Powered By Docstoc
					         Chapter 8 Cell respiration and photosynthesis

                       Cellular respiration

This is a process that provides energy in the form of ATP from
the organic compounds we eat. It can be divided into 4 stages.
Glycolysis happens in the cytoplasm of the cell, the link reaction
and Krebs cycle occur in the matrix of the mitochondria and the
electron transfer chain takes place in the inner wall of the

The Mitochondria

   1) Glycolysis
This happens in the cytoplasm of cells and converts glucose
into 2 pyruvate molecules in this series of reactions.

   Glucose a 6 carbon sugar is the starting point
   Firstly ATP gives up a phosphate group becoming ADP
    this joins the glucose molecule. This step is then repeated.
   The molecule formed is now Hexose biphosphate.
   This then splits into 2 triose phosphate molecules each
    containing 3 carbon atoms and 1 of the phosphate groups
    from the ATP
   At this stage another phosphate group from the cytoplasm
    is added to each triose phosphate
   Each triose phosphate molecule is then oxidised by the
    removal of electrons and hydrogen by the hydrogen
    accepter molecule NAD+ which is reduced in the process
    to become NADH+H
   Each of the Triose phosphate molecules now forms a
    pyruvate molecule releasing phosphate groups to form 2
    molecules of ATP from 2 molecules of ADP (4 in total)
Glycolysis then has produced 4 ATP molecules from an initial
investment of 2 ATP molecules.

Input               Reactions             Output
1 Glucose molecule 2 oxidation            2 pyruvate
4 phosphate groups reactions by NAD+      molecules
(2 from ATP, 2                            4 ATP molecules
from the cytoplasm)

Glycolysis animation

  2) The link reaction

This starts in the cytoplasm and ends in the mitochondrial
matrix. It converts a pyruvate molecule into an acetyl coenzyme
A molecule.

   Each pyruvate molecule produced by glycolysis combines
    with Coenzyme A (CoA)
   This is then oxidised by NAD+ (which is reduced in the
   A molecule of CO2 is then lost (decarboxylation)
   This leaves a molecule of Acetyl CoA

     Input                   Reactions           Output
     1 pyruvate              1 oxidation         1 Acetyl CoA
     molecule                reaction by NAD+    molecule
     1 CoA molecule          1 decarboxylation   1 CO2 molecule

                    NAD+              NADH

 Pyruvate                                               Acetyl CoA

                       CoA               CO2

  3) The Krebs cycle

Is a cyclic process which occurs in the matrix of the
mitochondria and starts with an acetyl molecule and through a
series of oxidative decarboxylation and oxidation reactions
produces a small amount of ATP, hydrogen ions and electrons

   CoA gives up the acetyl molecule (a 2 carbon molecule)
    which combines with oxaloacetate ( a 4 carbon molecule)
    to producer citrate (a 6 carbon molecule)
   This is oxidised by NAD+ which accepts the hydrogen ions
    becoming (NADH +H+) and a CO2 molecule is removed
    (oxidative decarboxylation) producing a 5 carbon
   This process is then repeated with NAD+ again being the
    hydrogen accepter and CO2 being removed leaving a 4
    carbon molecule
   A phosphate molecule is then added to ADP producing 1
    molecule of ATP
   The 4 carbon molecule is then oxidised twice once by
    NAD+ and once by FAD (becoming FADH2)
   This leaves the 4 carbon oxaloacetate molecule that can
    now combine with another acetyl molecule

This process is repeated twice for each glucose molecule since
in glycolysis 1 glucose molecule produces 2 pyruvate molecules
each of which produces an acetyl molecule in the link reaction.
Each step is catalysed by specific enzymes or co-enzymes such
as NAD+, FAD and CoA

Input             Reactions                 Output
1 acetyl          3 NAD+ NADH + H+          3 NADH + H+
molecule          1 FAD       FADH2         1 FADH2
                  1 ADP + P = ATP           2 CO2
                                            1 ATP
Krebs cycle animation


  4) The electron transfer chain

This occurs in the christae (inner folds) of the inner membrane
of the mitochondria and uses the electrons and hydrogen ions
produced in earlier stages to form an electro chemical gradient

   Electrons from NADH and FADH2 are given up to a series
    of electron carriers (protein complexes) which are situated
    in the phospholipid bilayer of the inner membrane
   As they move along the electron carriers energy is given
    up and is used to pump hydrogen ions from the matrix of
    the mitochondria into the inter-membrane space
   This concentration of hydrogen ions can then be allowed
    to pass down the concentration gradient through the
    enzyme ATP synthase situated in the inner membrane to
    produce ATP
   The electrons and hydrogen ions then react with oxygen
    the last electron acceptor to form water

Inputs        Reactions                  Outputs
NADH +H+      NADH +H+     NAD           H2O
FADH2         FADH2      FAD             ATP
               +    -
              H + e + O2     H2O

Electron transfer chain animation

                               Krebs cycle



Light energy is converted into chemical energy
Carbon dioxide + water                   glucose + oxygen
There are different types of chlorophyll but generally they
reflect the green part of the spectrum and absorb the red/orange
and violet/blue part. Chlorophyll is contained in the chloroplast.

Photosynthesis can be split into 2 stages

  • Light dependant
Light is used to split water (photolysis) into hydrogen ions,
oxygen, electrons and some ATP. The oxygen is a waste product
and the hydrogen ions and electrons are used to make ATP and

   • Light independent
Hydrogen ions from NADPH and energy from ATP are used to
fix 3 CO2 molecules into 2 molecules of triose phosphate (3C).
Two of these can then be combined to form 1 molecule of
The light dependant stage
This can be divided into 2 stages

  1. Non-cyclic photophosphorylation
  2. Cyclic photophosphorylation

Both these stages occur in the thylakoid membrane of the

  Non-cyclic photophosphorylation
Photosystems are groups of chlorophyll molecules with a
special molecule (the reaction centre) in the middle.

• Light is absorbed by photosystem 11 exciting electrons
  • These electrons are passed from chlorophyll molecule to
    chlorophyll molecule until they reach the reaction centre
  • From here they are then taken up by electron accepter X
    leaving a +ve chlorophyl ‘a’ molecule
  • The electrons are passed through a series of electron
    carriers in the membrane of the grana via a series of redox
    reactions ending up at photosystem 1. Energy is produced
    during this process
  • This energy is used to pump H+ ions across the thylakoid
    membrane into the lumen within creating a concentration
  • As in respiration the H+ ions can then be allowed to pass
    down the concentration gradient through ATP synthase
    producing ATP
  • Chlorophyl ‘a’ induces the lysis (photolysis) of water
    producing oxygen, (waste) hydrogen ions (pumped to the
    lumen of the grana until the concentration gradient drives
    them through ATP synthase converting ADP to ATP) and
    electrons which reduce chl ‘a+’ to chl ‘a’
  • Photoactivation happens in photosystem 1, the electrons
    being taken up by electron accepter Y. Unlike those of
    photosystem 11 these are accepted by NADP+ which
    combines with an H+ ions and is reduced to NADPH
  • The chlorophyl ‘a’ molecule of photosystem 1 receives the
    electrons from photosystem 11 and becomes uncharged

Input           Reactions                  Output
Light           H2O       H+ + e- + O2     NADPH
                ADP + P = ATP              ATP

Cyclic photophosphorylation
  • Again electrons are excited but this time they are passed
    along a series of electron carriers producing ATP and
    ending up back at PS 1.
  • It is a cyclic process and does not produce NADPH for the
    Calvin cycle (so does not produce complex carbohydrates)
    but does produce ATP
  • Is used when a build up of NADPH occurs causing a
    shortage of NADP+ (final electron acceptor)
  • Chlorophyl ‘a’ induces the lysis (photolysis) of water
    producing oxygen, (waste) hydrogen ions (pumped to the
    lumen of the grana until the concentration gradient drives
    them through ATP synthase converting ADP to ATP) and
    electrons which reduce chl ‘a+’ to chl ‘a’

Input                 Reaction              Output
Light                 ADP + P = ATP         ATP

Animation ( The enzyme names in this animation are not needed
just the basic description)

 The light independent reaction

                              Ribulose biphosphate

                                                                   2 x Glycerate 3 phosphate


                              Triose phosphate

              2 triose phosphate molecules join to form a glucose molecule.
              Glucose molecules can be used to make starch. 1 out of every 6 is
              used for starch and 5 for replenishing ribulose biphosphate.
Light independent reaction

  • This takes place in the stroma of the chloroplast
  • CO2 enters by diffusion and combines with ribulose
    biphosphate (RuBP) (5C) (carboxylation)
  • The catalyst is ribulose biphosphate carboxylase
  • This 6 carbon molecule splits into 2 glycerate 3-phosphate
    molecules (3C)
  • Glycerate 3-phosphate is reduced to triose phosphate by
    NADPH and ATP supplies the energy (from light
  • 2 triose phosphate molecules join to form a glucose
    phosphate molecule which can be joined to form starch
  • For every 6 triose phosphate molecules 1 is used for starch
    production and 5 for replenishing RuBP

Input          Reaction                 Output
                                  +   +
CO2            NADPH        NAD + H Triose phosphate
Ribulose       ATP      ADP
biphosphate    2 Triose phosphate = 1
               Glucose molecule

Light independent stage (The Calvin cycle)


Limiting Factors
  • Low light levels – products of the light dependant
    reactions will be in short supply ie NADPH and ATP
  • Low CO2 concentration – CO2 fixation slows and there is
    a build up of NADPH and ATP
  • Low temperature – enzymes of Calvin cycle slow and
    NADPH accumulates
  • High temperatures – RuBisCo does not work effectively
    NADPH builds up

Chloroplast structure and function

       Large surface of thylakoid membranes allows many of
        the light dependant reactions to happen simultaneously
       Small grana lumen allows a quick build up of H+ ions
       The stroma contains the correct ion and pH levels for
        RuBisCo to function


  • The development of new equipment for research requires
    great creativity can be said to parallel works of art
  • Correlation does not necessarily mean causation. To prove
    this all factors except 1 would have to be controlled and
    this is virtually impossible in living organisms.
  • Epidemiologists show links between factors and diseases
    but not necessarily proof.

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