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MODULE 5
ENERGY TRANSFER
IN AND BETWEEN
ORGANISMS
BIOLOGY
NOTES
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INDEX
TOPIC 1: PHOTOSYNTHESIS . . . . . . . . . . . . . . . . . . . . . . . 5
TOPIC 2: RESPIRATION. . . . . . . . . . . . . . . . . . . . . . . . . 10
TOPIC 3: ENERGY AND ECOSYSTEMS. . . . . . . . . . . . . . . . . 17
TOPIC 4: NUTRIENT CYCLES . . . . . . . . . . . . . . . . . . . . . . 21
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TOPIC 1
Photosynthesis
Photosynthesis
Photosynthesis is the process by which plants and some other organisms convert light energy
into chemical energy. The chemical equation for photosynthesis is:
6CO2 + 6H2O + energy from photons — *chlorophyll*
C6H12O6 + 6O2
Photosynthesis is a 2 stage process which takes place in chloroplasts.
Chloroplasts
•• Surrounded by an envelope
•• Outer membrane is permeable to small ions
•• Inner membrane is less permeable
○○ Folded into lamellae
○○ One granum consists of several stacked thylakoids
○○ Between grana is the intergranal lamellae
•• The stroma is the fluid filled matrix, which contains the necessary enzyme to carry out the
light dependent reactions
Chloroplast Adaptations
•• Many grana provide a large surface area for photosynthetic pigments
•• Photosynthetic pigments are arranged into photosystems
•• Proteins embedded in grana to hold the photosystems in place
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Light Dependent Stage
•• Photophosphorylation
○○ When a photon hits a chlorophyll molecule, the energy is transferred to 2 electrons
exciting them
–– “Photoionisation”
○○ Electrons captured by electron carriers and passed along a series of electron carriers
○○ Energy is used to pump protons across the thylakoid membranes
○○ Proton gradient is formed and protons flow down the concentration gradients through
ATP channels
○○ This is chemiosmosis
○○ This produces a force which joins ADP and Pi to form ATP
–– ATP is used in the light independent stage of the reaction
–– The making of ATP using light energy is called photophosphorylation
•• Cyclic Photophosphorylation
○○ Only uses PS1
○○ Electrons pass to an electron acceptor, then return to the chlorophyll molecule from
which they were lost
○○ No photolysis of water
○○ No generation of reduced NADP
○○ Small amounts of ATP made
○○ This may be used in the light independent reaction to actively transport potassium into
the cell, lowering the water potential and causing ions to flow in by diffusion
○○ This causes the guard cells to swell and the stomata to open
•• Non-cyclic Photophosphorylation
○○ Involves both PS1 and PS2
○○ Light strikes PS2, exciting a pair of electrons
○○ Electrons leave the chlorophyll molecule from the primary pigment reaction centre
○○ Electrons pass along the chain of electron carriers
○○ Energy is released, which is used to synthesize ATP
○○ Light strikes tPS1, causing the release of 2 electrons
○○ Electrons from oxidized PS2 replace those lost from PS1
○○ Electrons from photolysed water replace those lost from PS2
○○ Protons fro photolysed water take part in chemiosmosis, to make ATP
○○ They are captured in NADP in the stoma and used in the light-independent stage
Non light dependent stage
○○ This takes place in the stroma
○○ It is called the Calvin Cycle
○○ Although light is not used, the products of the light-dependent stage are used, so the
light independent stage will soon cease if light supply ceases
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•• The Calvin Cycle
○○ Carbon dioxide from the air diffuses into the leaf
○○ Diffuses through air spaces and reaches the spongy palisade layer
○○ Diffuses through the cellulose cell walls and into the chloroplast envelope
○○ In the stroma, carbon dioxide combines with the 5C ribulose bisphosphate
○○ Reaction is catalysed by the rubisco enzyme
○○ Two 3C compounds, called glycerate phosphate (GP) are produced
–– Some GP is used to make amino acids and fatty acids
○○ GP is reduced and photophosphorylated to another 3C compound, triose phosphate (TP)
–– ATP and reduced NADP are used in this reaction
○○ 5/6 TPs are recycled to ribulose bisphosphate
–– Pairs of TP combine to form hexose sugars
•• Conversion of Energy
○○ Light converted through cyclic phosphorylation and non-cyclic phosphorylation
○○ Both of which are light –dependent
○○ In Cyclic phosphorylation:
–– Light energy absorbed by PS1
–– Energy excites electrons, raising their energy level
–– This causes electrons to be transferred to carriers
–– As electrons drop down energy levels, they release energy
–– Energy is used to pump H+ ions through ATP synthase from thylakoid space to the
stroma
–– This is the process of chemiosmosis
–– In this process, ATP is produced
–– This process is used to make glucose
○○ In non-cyclic phosphorylation:
–– Light energy causes hydrolysis of water
–– This is photolysis
–– 2 electrons are produced, which pass into the PS2
–– They are excited and transferred to the electron carriers
–– As they pass along the carriers, they release energy,
–– This is used to pump H+ ions into the stroma at the same time
Role of Water in the Light Dependent Stage
•• Photosystem 2 contain an enzyme that can split water into protons, electrons and oxygen
•• This is photolysis
•• Some of the oxygen is used in aerobic respiration
•• Water is a source of hydrogen ions, which are used in chemiosmosis
•• Protons are accepted by a coenzyme NADP, which is reduced to reduced NADP and used in
the light independent stage
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