Photosynthesis

Photosynthesis definition

The process by which plants make their own food (glucose) by capturing light energy and changing it into chemical energy.

Adaptations of the plant for photosynthesis

Many chloroplasts, absence of a cuticle for light absorption, many aerial parts to capture sunlight, a thin leaf blade (lamina).

Role of photosynthesis

Glucose for metabolism, animals feed on plants and gain energy, Oxygen produced used by most living things for respiration, fossil fuels formed as an indirect result of photosynthesis.

Equation of photosynthesis

6C02 +6H20 sunlight/chlorophyll = C6H12O6 + 602

Cell organelle where photosynthesis takes place

Chloroplasts (chlorophyll acts as a catalyst)

Initial energy source

Provided by the breakdown of ATP into ADP and phosphate.

ADP

Adenosine Diphosphate

ATP

Adenosine triphosphate (made by photophosphorylation)

Energy carriers

Sugars that carry electrons, used in photosynthesis and respiration

Light absorption

Chlorophyll pigment absorbs the light energy (forming ATP). Trapped energy eventually makes glucose or splits water into H+ protons, e- electrons, and O2. Process of hydrolysis

Equation of hydrolysis

2H20 /light/ 4H+ + 4e- + O2

Fate of hydrolysis products

Protons enter a proton storage pool, electrons taken by an electron acceptor and a series of energy carriers, O2 stays in leaf for respiration or released into atmosphere.

Light energises electrons

Some sunlight energy trapped by chlorophyll is passed onto electrons in chlorophyll to form high-energy electrons (excited electrons)

Formation of glucose

High energy electrons, protons, and CO2 are combined to form glucose. This is used in plant cells for respiration, converted to starch for storage, or changed to lipid/protein. Used to make cellulose for new walls.

Source of light

Sun, artificial light bulbs. An increase in light intensity increases the rate of photosynthesis (until light saturation)

Source of CO2

External atmosphere (respiration, combustion), internal respiration.

Source of water

Absorbed by roots of a plant and travels to cells in xylem vessels. Travels through stems into leaves.

Light stage of photosynthesis

Driven by light energy. Takes place in granum stacks of chloroplasts. Excited electrons are picked up by an electron acceptor, near a chlorophyll pigment, that then sends them through electron carriers in two pathways.

Pathway 1

The electrons flow through the carriers and back to the chlorophyll pigment. As the electrons move along, they give up their energy, which is then used to create ATP and water.

NADP

Nicotinamide Adeine Dinucleotide Phosphate

Pathway 2

Two electrons pass from chlorophyll to electron acceptor along to another series of electron acceptors. Electrons lose energy as they pass from electron acceptor to the next. This energy is used to make ATP. These negative electrons combine with NADP+ to form NADP-. H ions combine with this to form NADPH.

The dark stage

“Light independent stage”. Takes place in the stroma. This stage is controlled by enzymes (can be affected by temperature)

Events in the dark stage

CO2 combines with protons (H+ ions) and electrons to form glucose using energy from the breakdown of ATP. Hydrogen and electrons come from the breakdown of NADPH (high energy) into NADP (low energy), 2 electrons and H+ (reduction reaction).

Main event summary

Light energy absorbed by chlorophyll. Water is split (hydrolysis). Electrons are passed into the electron carriers, protons are stored in proton pools, O2 is released/ used in respiration. Sunlight transfers energy to electrons. High energy electrons, stored protons, and CO2 are used to make glucose.