C1.3 Photosynthesis

Light energy

Light energy

Plants + photosynthetic organisms (algae, plants, cyanobacteria) start food chains using photosynthesis to transform light energy from Sun into chemical energy in form of glucose

Photosynthesis equation

Water split for hydrogen and oxygen (released)

Released hydrogens reduce CO2 to form glucose

Photosynthetic pigments

Variety in chloroplasts, mostly chlorophyll

Also carotenoids including specific ones- carotene, xanthophyll

Type and concentration is unique to a plant species and can be seperated using chromatography

Chromatography

Allows determination of pigments in chloroplasts

Pigments with higher Rf values are smaller and more soluble

Electromagnetic spectrum

Plants use visible light section- has many colours but can be considered in three regions:

• Red end

• Green middle

• Blue end

Plant reactions to different wavelengths of light

Absorb it (energy absorbed and used)

Reflect it (energy not absorbed, we see the colour)

Ability of photoautotrophs to absorb light

Determined by pigments on chloroplast membranes

Absorption spectrum

Created by plotting amount of light absorbed against light wavelength

Varies depending on type of photosynthetic pigment present

Represents amount of light energy absorbed by pigment

Chlorophylls a+b have high light absorption in violet and red wave lengths

Pigments like carotenoids absorb light at different wavelengths compared to chlorophylls a+b

Other pigments less efficient at light absorption compared chlorophyll a+b

Action spectrum

Indicates rate of photosynthesis at different light wavelengths

Varies depending on type of photosynthetic pigment present

Represents rate of photosynthesis carried out by pigment

Chlorophylls a + b have relatively high rates of photosynthesis

Pigments like carotenoids allow photosynthesis at different wavelengths

Other pigments have less effective rate of photosynthesis compared to chlorophylls a + b

How light energy transforms into chemical energy

When a pigment absorbs light, energy is used to raise an electron to a higher energy level (excitation of electrons)

Requires a specific amount of energy (specific light photons)

Explains why different pigments absorb different wavelengths

Once electrons are excited to a higher level, energy is used for chemical bonds

Measuring rate of photosynthesis

Measure rate of oxygen production or carbon dioxide intake

Measure change in plant’s biomass (indirect reflection as many factors impact growth)

Law of limiting factors

Proposed by Frederick Frost Blackman in 1905

States a process with many limiting factors will have a rate limited by lowest value

Photosynthesis limiting factors

Amount of water, sunlight, temperature, carbon dioxide, chloroplasts, chlorophyll

Rising carbon dioxide levels and photosynthesis

Many experiments on this showed weed growth increases more than that of plants or trees

Peter Wayne’s study in controlled greenhouses showed doubling carbon dioxide led to 61% more ragweed growth

Experiments in natural settings, FACE

Air-free carbon dioxide enrichments experiments

Allows effect of increasing carbon dioxide to be seen in more natural + agricultural ecosystems→ more reliable