Photosynthesis involves two main sets of reactions: light-dependent reactions where light energy is absorbed and electron transport is used to make ATP and NADPH; and light-independent reactions where CO2 is fixed into sugars using the energy from the light reactions. The light reactions take place in the chloroplasts and involve the absorption of light by pigments and the splitting of water to provide electrons and protons. The Calvin cycle fixes CO2 into carbohydrates using the ATP and NADPH produced in the light reactions. Factors like light intensity, temperature, and CO2 concentration can limit the rate of photosynthesis.

1. Photosynthesis
Photosynthesis as an energy
transfer process
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2. • Photosynthesis transfers light energy into
chemical potential energy of organic
molecules
• This energy can then be released for work
in respiration
• Photoautotrophs – green plants, the
photosynthetic prokaryotes and both
single-celled and many-celled protoctists
(including the green, red and brown algae)
• Chemoautotrophs – nitrifying bacteria
(obtain their energy from oxidising
ammonia to nitrite, or nitrite to nitrate)
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4. Outline of the process
• Photosynthesis is the trapping (fixation) of CO2 and its
subsequent reduction to carbohydrate, using H from H2O
• Overall equation for photosynthesis in green plants is:
light energy
nCO2 + nH2O (CH2O)n + nO2
chlorophyll
• Hexose sugars and starch are commonly formed:
light energy
6CO2 + 6H2O C6H12O6 + 6O2
chlorophyll
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5. • 2 sets of reactions involved:
– Light-dependent reactions (light energy
necessary)
• Only takes place in the presence of suitable
pigments which absorb certain wavelengths of light
• Light energy is necessary:
– for the splitting of water into hydrogen and oxygen
– to provide chemical energy (ATP) for the reduction of
CO2 to carbohydrate in the light-independent reactions
– Light-independent reactions (light energy
not needed)
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6. The light-dependent reactions
• Include the synthesis of ATP in
photophosphorylation and the splitting of
water by photolysis to give H+
• H+ + NADP NADPH
• ATP and NADPH - passed from the light-
dependent to the light-independent
reactions
• Photophosphorylation of ADP to ATP:
– Cyclic
– Non-cyclic
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7. • Cyclic photophosphorylation
– Only photosystem I
– Light absorbed by photosystem I and
passed to chlorophyll a (P700)
– An e- in the chlorophyll a molecule is
excited and emitted
– Captured by an e- acceptor and passed
back to a chlorophyll a (P700) molecule
via a chain of electron carriers
– Synthesis of ATP
– ATP passes to light-independent
reactions
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8. • Non-cyclic photophosphorylation
– ‘Z scheme’
– Light absorbed by both photosystem and
excited e- emitted from the primary pigments
of both reaction centres (P680 and P700)
– e- absorbed by e- acceptors and pass along
chains of e- carriers leaving the photosystems
positively charged
– The P700 of photosystem I absorbs electrons
from photosystem II
– P680 receives replacement e- from the
splitting (photolysis) of water
– ATP synthesised
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9. • Photolysis of water:
– Photosystem II includes a water-splitting
enzymes which catalyses the breakdown of
water:
H2O → 2H+ + 2e- + ½O2
– H+ combine with e- from photosystem I and the
carrier molecule NADP to give reduced NADP
2H+ + 2e- + NADP → reduced NADP
– This passes to the light-independent reactions
and is used in the synthesis of carbohydrate
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11. Light-independent reactions
• The fixation of CO2
• CO2 combines with a 5C sugar {ribulose
biphosphate (RuBP)} 2 molecules of a 3C
compound {glycerate-3-phosphate (GP/PGA)}
• GP is reduced to triose phosphate (3C sugar) in
the presence of ATP and NADPH
• Some condense to form hexose phosphates,
sucrose, starch and cellulose or are converted to
acetyl CoA to make amino acids and lipids
• Others regenerate RuBP
• The enzyme ribulose biphosphate carboxylase
(rubisco), catalyses the combination of CO 2 and
RuBP
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12. Calvin cycle
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13. Leaf structure and function
• Has a broad, thin lamina, a midrib and a network of
veins, leaf stalk (petiole)
• To perform its function the leaf must:
– Contain chlorophyll and other photosynthetic pigments arranged
in such a way that they can absorb light
– Absorb CO2 and dispose of the waste product O2
– Have a water supply and be able to export manufactured
carbohydrate to the rest of the plant
• Large surface area of lamina makes it easier to absorb
light and thinness minimises diffusion pathway for
gaseous exchange
• Upper epidermis is made of thin, flat, transparent cells
which allow light through to the cells of the mesophyll ,
where photosynthesis takes place
• A waxy transparent cuticle provides a watertight layer
• Cuticle and epidermis together form a protective layer
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14. • Stomata are pores in the epidermis
through which diffusion of gases occurs
• Each stoma is bounded by 2 sausage-
shaped guard cells
• Changes in turgidity cause them to
change shape so that they open and close
the pore
• Guard cells gain and loss water by
osmosis
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16. • The palisade mesophyll is the main site of photosynthesis (many
chloroplasts per cell than in the spongy mesophyll)
• Adaptations for light absorption:
– Long cylinders arranged at right-angles to the upper epidermis (reduces
number of light-absorbing cross walls in the upper part of the leaf so that
as much light as possible can reach the chloroplasts)
– Large vacuole with a thin layer of cytoplasm (restricts chloroplasts to a
layer near the outside of the cell where light can reach them most
easily)
– Chloroplasts can be moved within cells (to absorb the most light or to
protect it from excessive light intensities)
• Adaptations for gaseous exchange:
– Cylindrical cells pack together with long, narrow air spaces between
them (large surface area of contact between cell and air)
– Cell walls are thin (gases can diffuse through them easily)
• Spongy mesophyll is adapted as a surface for the exchange of
CO2 and O2
– Smaller number of chloroplasts
– Photosynthesis only at high light intensities
– Irregular packing and large air spaces produced provide a large surface
area of moist cell wall for gaseous exchange
• Veins in leaf help to support large surface area of leaf
– contains xylem and phloem
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18. Investigation of limiting factors
• External factors:
– Light intensity
– Temperature
– CO2 concentration
• Light intensity
– Rate initially increases as the light intensity increases
– Rate reaches a plateau at higher light intensities
• Temperature
– At high light intensities the rate of photosynthesis increases
as the temperature is increased over a limited range
– At low light intensities, increasing the temperature has little
effect on the rate of photosynthesis
• Photochemical reactions are not generally affected by
temperature
• Since temperature affects rate, there must be 2 sets of reactions
– Light-dependent photochemical stage
– Light-independent, temperature-dependent stage
• Limiting factor
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