
20
Photosynthesis
20.1 Roles of photosynthesis in ecosystems
1
Photosynthesis is the process by which organic food (carbohydrates) are made from simple
inorganic substances (carbon dioxide and water) using light energy. Oxygen is released as a
by-product.
2
The word equation for the overall process of photosynthesis:
carbon dioxide + water
3
light energy
chlorophyll
carbohydrates +
oxygen
Significance of photosynthesis:
- Provides the basic food source for most organisms.
- Maintains energy flow in ecosystems.
- Maintains the balance of oxygen and carbon dioxide in the atmosphere.
20.2 Requirements for photosynthesis
1
To determine if a leaf has carried out photosynthesis, we can:
- detect the production of oxygen from the leaf using a glowing splint.
- detect the presence of starch in the leaf by iodine test.
2
If we plan to use the presence of starch in leaves as evidence that photosynthesis has occurred,
any starch that is present in the leaves before the investigation must be removed by
destarching (脫澱粉).
3
Destarching is the removal of starch from the leaves of a plant by putting it in the dark for at
least 24 hours. This is to make sure that the starch detected at the end of the investigation
was made during the investigation.
4
Light, carbon dioxide, chlorophyll and water are required for photosynthesis.
20.3 Site of photosynthesis
1
In plants, leaves are the main site of photosynthesis.
2
The morphology (形態) of the leaf, the structures of the leaf and the distribution of
chloroplasts in the cells are well adapted to maximize the absorption of light, facilitate gas
exchange, facilitate transport of materials or reduce water loss, such that photosynthesis
can occur at a higher rate.
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Adaptive features of a terrestrial dicotyledonous leaf for photosynthesis:
Feature of leaf
Whole leaf
Adaptation
- Leaf blade is broad and flat
- Provides a large surface area for
absorbing sunlight
- Leaf is thin
- Gases and light can reach the
photosynthetic cells easily
Internal
Palisade mesophyll
structure
- Consists of tightly packed cells that
of leaf
contain many chloroplasts
- Allows effective absorption of
sunlight
- Located on the upper side of the leaf
(exposed directly to sunlight)
Spongy mesophyll
- Consists of loosely packed cells with
- Allows gases to diffuse freely inside
the leaf
many air spaces
- The cells contain chloroplasts
- Allows absorption of sunlight
(fewer than palisade mesophyll)
Upper epidermis and lower epidermis
- Covered by a thin layer of waxy
- Reduce water loss from the leaf,
so that water is kept inside for
cuticle
photosynthesis and keeping the cells
turgid to support the leaf blade
Stoma (in epidermis)
- More on the lower epidermis
- Opens when the conditions are
- Surrounded by a pair of guard cells
(containing chloroplasts)
favourable for photosynthesis;
closes when the conditions are less
favourable. This regulates the
passage of gases and water vapour
into and out of the leaf
Midrib and veins
- Contain vascular bundles which are
made up of xylem and phloem
- Allow efficient transport of materials
(e.g. water and food) into and away
from the leaf
- Support and spread the leaf blade
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Structure of a chloroplast:
outer membrane
thylakoid
(類囊體)
stroma
(基質)
granum
(基粒)
inner membrane
chlorophyll molecules
thylakoid
membrane
5
Adaptive features of the chloroplast for photosynthesis:
Structure of
chloroplast
Stroma
Description
Adaptation
- Jelly-like fluid
- Contains enzymes that catalyse
photosynthetic reactions
- Holds starch grains which are temporary
stores of photosynthetic products
- Holds other photosynthetic products
(e.g. lipid droplets)
Thylakoid
- Membranous sacs
- Provides a large surface area to pack
- Large in number
more chlorophyll
- Arranged in stacks called grana
- Grana are interconnected
- Allows efficient transport of
photosynthetic products within the
chloroplast
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20.4 Process of photosynthesis
1
Photosynthesis occurs in two main stages: photochemical reactions (光化學反應) and carbon
fixation (碳固定).
a
Photochemical reactions (also called light reactions (光反應)):
- Occur in the thylakoids of chloroplasts
- Require light
- Involve three main processes:
excited electron
−
e
1b Electron transport
ADP + P
1a Light absorption
and electron
emission
Formation of ATP
2
ATP
NADP + H
NADPH
O2
−
electron
e
chlorophyll
(on thylakoid membrane)
1a
−
4e
2 H2O
3
+
4H
+
provides
hydrogen for
Photolysis of water
Light absorption and electron emission
Chlorophyll molecules in the thylakoid membrane capture light energy. Some
electrons of the chlorophyll molecule are raised to a higher energy level and are
emitted from the chlorophyll molecule.
1b
Electron transport
The excited electrons pass through an electron transport chain (電子傳遞鏈). Energy
is released in a stepwise manner.
2
Formation of ATP
The energy released in the electron transport chain is used for
photophosphorylation (光磷酸化) of ADP to form ATP.
3
Photolysis of water
The light energy captured by chlorophyll also drives the photolysis (光解) of water,
forming:
- hydrogen, which is accepted by NADP to form NADPH;
- oxygen gas, which is released into the atmosphere as a by-product.
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b
Carbon fixation (also called the Calvin cycle (卡爾文循環) or dark reactions (暗反應)):
- Occurs in the stroma of chloroplasts
- Does not require light
- Involves a cyclic series of reactions:
 Carbon dioxide fixation
carbon dioxide
5-C compound
Calvin cycle
3-C compound
ATP
from
photochemical
reactions
ADP + P
ATP
from
photochemical
reactions
NADPH
NADP
ADP + P
triose phosphate (3-C)
 Reduction of
 Regeneration of
3-C compound
carbon dioxide
acceptor
glucose (6-C)
 Carbon dioxide fixation and formation of a 3-C compound
Under the action of enzymes, one molecule of carbon dioxide is accepted by one
molecule of a 5-C compound (carbon dioxide acceptor) to form two molecules of a
3-C compound.
 Reduction of 3-C compound and formation of glucose
Using energy from ATP and hydrogen from NADPH formed in photochemical
reactions, the 3-C compound is reduced to triose phosphate (丙糖磷酸). Two
molecules of triose phosphate then combine to form one molecule of glucose. The
ADP and NADP regenerated are reused in photochemical reactions.
 Regeneration of the carbon dioxide acceptor
Most triose phosphate molecules are used to regenerate the 5-C carbon dioxide
acceptor, using energy from ATP, so that the Calvin cycle can repeat itself.
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2
Photochemical reactions and the Calvin cycle are linked by ATP and NADPH.
3
The balanced equation for the overall process of photosynthesis:
6 CO2 + 6 H2O
carbon dioxide water
4
light captured
by chlorophyll
C6H12O6 + 6 O2
glucose
oxygen
Differences between the photochemical reactions and the Calvin cycle:
Photochemical reactions
(Light reactions)
Occur in daytime
In daytime only
The Calvin cycle
(Dark reactions)
In daytime and at night
or at night?
Place of occurrence Thylakoid membranes of chloroplasts Stroma of chloroplasts
Energy change
Light energy is converted to chemical
Chemical energy in ATP and NADPH
energy in ATP and NADPH
is converted to chemical energy in
organic food
Input
- Light energy
- Carbon dioxide
- Water
- 5-C compound
- ADP and NADP
- ATP and NADPH
(from the Calvin cycle)
Output
(from photochemical reactions)
- ATP
- Glucose
- NADPH
- 5-C compound
- Oxygen
(regenerated for the Calvin cycle)
- ADP and NADP
(for photochemical reactions)
20.5 Fate of photosynthetic products
1
Triose phosphate is the primary photosynthetic product. It can be synthesized into glucose or
fructose.
2
Some glucose is used as an energy source, combine with fructose to form sucrose for
transport, or is built into cellulose for making cell walls.
3
Excess glucose is built into starch for storage. During non-photosynthetic periods, starch
can be broken down to form glucose or converted to sucrose.
4
Intermediates of glucose breakdown can be used to form glycerol and fatty acids for
making lipids; they can also combine with inorganic ions to form amino acids for making
proteins.
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20.6 Factors affecting the rate of photosynthesis
Effect of light intensity on the rate of photosynthesis:
 saturation point
rate of photosynthesis
1



light intensity
 optimum light intensity
 At low to moderate light intensity, the rate of photosynthesis increases proportionately
with light intensity as more energy is supplied to the photochemical reactions. Light
intensity is the limiting factor in this period.
 The saturation point (maximum rate of photosynthesis) is reached when the rate just starts
to level off. The light intensity at this point is called optimum light intensity.
 When light intensity increases further, the rate levels off as it is limited by other factors.
 At very high light intensities, chlorophyll and other photosynthetic components may be
damaged and the rate drops.
2
Effect of carbon dioxide concentration on the rate of photosynthesis:
rate of photosynthesis
high light intensity
curve B


low light intensity

curve A

optimum CO2
concentration at
low light intensity
CO2 concentration
optimum CO2 concentration
at high light intensity
 At low light intensity, the rate of photosynthesis increases proportionately with carbon
dioxide concentration as more substrates are supplied to the Calvin cycle. Carbon
dioxide concentration is the limiting factor.
 As the carbon dioxide concentration increases beyond the optimum value, the rate levels
off as it is limited by other factors.
 At high light intensity (curve B), the rate of photosynthesis increases, showing that light
intensity is the limiting factor at this range of carbon dioxide concentrations in curve A.
 As the carbon dioxide concentration increases further, the rate levels off again as it is
limited by another factor.
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3
Growing crops in greenhouses (温室) allows farmers to regulate the rate of photosynthesis of
their crops by monitoring environmental factors such as temperature, light intensity and carbon
dioxide concentration.
4
Growing crops in greenhouses can:
- improve productivity (生產力).
- reduce damages by pests, pollution and bad weather to improve quality of the crops.
- produce crops all year round to improve the crop yield.
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