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Photosynthesis
Photosynthesis (‘photo’= light and ‘synthesis’ = manufacture) is the process by which
plants convert light energy into chemical energy (a.k.a. food) which may be used for the
plant’s metabolism and growth. Occurs in mesophyll cells and guard cells of the leaf.
Structure of a leaf:
Cross-section of a leaf:
light
Equation for reaction: 6CO2 + 6H2O -------> C6H12O6 + 6O2
chlorophyll
enzymes
- requires light to be trapped by chlorophyll and enzymes to catalyze the reactions
- occurs in the thylakoid membranes inside the chloroplast
Carbon dioxide (CO2): diffuses from the air into the leaf via stomata - reaches the
mesophyll cells due to air spaces between cells and diffuses through cell walls,
membranes and into the chloroplast
Water (H2O): absorbed by root hairs and carried up to the leaves via xylem tissue and to
the mesophyll cells by osmosis
Light: from the sun or an artificial light source- plants adapt to have as much light
hitting the leaf surfaces as possible… How? Spread of branches and leaves, and positive
phototactic response which makes plants grow towards the light and orient leaves to light
3 factors that affect light received:
1) Angle of sun’s rays
2) Length of growing season
3) Length of day
Chlorophyll: pigment that makes plants look green- 2 kinds in plants-:chl a and chl b
- found in the thylakoid membranes of the chloroplast
- absorbs light most strongly in the blue and red portions of the spectrum
- other chlorophylls (c1, c2, d, f) exist in algae and cyanobacteria
- other pigments (carotenoids and xanthophylls) assist in light capture
-in fall, cold temperatures (plant senescence) cause breakdown of chlorophyll to
colorless molecules, thus unmasking the other pigments and showing fall leaf colors
Minerals: needed for formation of enzymes to catalyze the reactions as well as the
formation of carbs, proteins, nucleid acids and lipids
-Nitrogen: need reactive nitrate ions from the soil to form proteins and chlorophyll
-Magnesium: obtained from the soil to form chlorophyll
Glucose (C6H12O6): end product of photosynthesis used by the plant for metabolism and
growth
- may be used as an energy source for respiration
- may be converted to starch and stored in the leaf. Why?
1) Soluble glucose might dissolve in water around the cell and be lost from the plant
2) Solubility increases concentration of sugar in the cell and can damage the cell
Starch is insoluble, so it can be stored as starch grains in chloroplast or in tubers…
Oxygen (O2): ‘waste’ product of photosynthesis that diffuses out of the leaf via stomata
Factors that affect the Rate of Photosynthesis
1) Light Intensity- as light intensity increases, the rate of photosynthesis increases until
the plant is photosynthesizing at its maximal rate.
2) Temperature- as temperature increases, the rate of photosynthesis increases. So a
plant will photosynthesize faster on a warm day than a cold day.
3) Concentration of reactants- if the reactants (CO2 and H2O) are limited in
concentration, the reaction itself will be limited by what’s available.
4) Weather- On hot days, stomata close to prevent water loss. If the stomata are closed
no CO2 is available and photosynthesis cannot take place.