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Photosynthesis

Unit 4 - Populations and the environment
3 Photosynthesis
• Photosynthesis occurs in the chloroplasts within the leaf of a plant
• Leaf structure
• large surface area
• leaves arrangement such that overlapping/
shadowing is minimised
• thin, short diffusion pathway, most light absorbed in
the first few millimetres
• transparent cuticle and epidermis lets light through
to photosynthetic mesophyll cells beneath
• long, narrow mesophyll cells packed with
chloroplasts
• numerous stomata for gaseous exchange
• stomata open and close in response to light intensity
• many air spaces in lower mesophyll layer to allow
diffusion of CO2 and O2
• network of xylem to bring water to leaf cells, and
phloem to carry away sugars produced in
photosynthesis
Structure and role of chloroplasts
• disc-shaped, 2-10µm long and 1µm wide, double membrane
• The thylakoids are where the light-dependent stage
occurs
...
They are joined together by intergranal lamellae
...
Photolysis of water
H2O → 2H+ + 2e- + 1/2O2
• The electrons replace the electrons lost when light hits photosystem II
Light energy is absorbed by photosystem II which excites a pair of electrons to a
higher energy level and leave the chlorophyll molecule
The electrons are absorbed by an electron carrier and is passed down successive
lower energy level electron carriers (electron transfer chain on thylakoid membrane)
The electrons lose energy as they are passed down and this energy is used to
combine inorganic phosphate and ADP to form ATP
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3
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5
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7
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8
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2

Photosynthesis
Light-independent reaction

Rubisco enzyme

⅚

⅙

1
...
It diffuses through plasma membrane, cytoplasm,
chloroplast membranes into the stroma
2
...
The unstable 6C compound breaks down into 2 x glycerate-3-phosphate (GP)
4
...
NADP is re-formed and returns to the light-dependent stage to be reduced again by
accepting more protons
6
...
g
...
5/6 of triose phosphates are used to regenerate RuBP using ATP from the
light-dependent stage
...

• The law of limiting factors: At any given moment, the rate of a physiological process is
limited by the factor that is at its least favourable value
...

• The volume of oxygen produced and
carbon dioxide absorbed will increase
to a point at which it is exactly
balanced by the oxygen absorbed and
carbon dioxide produced by
respiration; no NET exchange of gases
...

• Increases in light intensity will cause a
proportional increase in rate of photosynthesis and increasing volume of oxygen will
be released and carbon dioxide absorbed
• A point will be reached when further increases in light intensity no longer has an
effect on photosynthesis
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• Effect of carbon dioxide concentration
• 0
...
1%
• At higher CO2 concentrations, more CO2 is available to combine with RuBP in the
light-independent stage so the rate of photosynthesis increases
• Effect of temperature
• The rate of photosynthesis increases in direct proportion with temperature
• Between 0-25℃, the rate of photosynthesis is doubled for each 10℃ increase
• In many plants, the optimum is 25℃
• Increasing the temperature increases the kinetic energy of enzymes involved so
more successful collisions occur and rate of photosynthesis increases
• Above the optimum, rate of photosynthesis decreases as the temperature causes
the enzymes involved to become denatured

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