Photosynthesis includes light reactions and dark reactions

1. light reactions: The light reactions require light. Energy of light is converted to chemical energy and conserved as “high energy” compound ATP reducing power of NADPH

2.dark reactions : The light-independent reactions occur either in the light or in the dark. – NADPH and ATP produced by the light reactions are used in

the reductive synthesis of carbohydrate from CO2 and water

The “goal” of photosynthesis is to synthesize the carbohydrate “glucose”.– Carbon dioxide is reduced to glucose (see equation

below). – The electrons needed for this reduction come from

water.– The energy needed for this reduction comes from

light (ATP, NADPH).– The equation is:

Energy + 6CO2 + 6H2O C6H12O6 + 6O2

Synthesis of Glucose Sun light

photosystem

• Two kinds of photosystems are involved in photosynthesis in plants.

• Photosystem I (PSI) is defined as containing reaction center chlorophylls with maximal light absorption at 700 nm.

• Photosystem II (PSII) absorbs light about 680 nm.

• both of the two photosystems are pigment/protein complexes that are located in thylakoids

The roles of photosystem I (PS I) and photosystem II (PS II)

• PS I provides reducing power NADPH • PS II uses light energy to drive two

chemical reactions - the split of water producing O2 and releasing electrons into an electron transport chain (Photosynthetic Electron Transport).

Types of photosynthesis

• C3– The majority of plants

• C4– CO2 temporarily stored as 4-C organic acids resulting in more

efficient C exchange rate– Advantage in high light, high temperature, low CO2

– Many grasses and crops (e.g., corn, sorghum, millet, sugar cane)

• CAM– Stomata open during night– Advantage in arid climates– Many succulents (e.g., cacti, euphorbs, bromeliades, agaves)

Cellular Respiration

• Breakdown of glucose begins in the cytoplasm: the liquid matrix inside the cell

• At this point life diverges into two forms and two pathways– Anaerobic cellular respiration (aka fermentation)– Aerobic cellular respiration

Cellular Respiration Reactions

• Glycolysis– Series of reactions which break the 6-carbon

glucose molecule down into two 3-carbon molecules called pyruvate

– Process is an ancient one-all organisms from simple bacteria to humans perform it the same way

– Yields 2 ATP molecules for every one glucose molecule broken down

– Yields 2 NADH per glucose molecule

Anaerobic Cellular Respiration

• Some organisms thrive in environments with little or no oxygen– Marshes, bogs, gut of animals, sewage treatment ponds

• No oxygen used= ‘an’aerobic• Results in no more ATP, final steps in these pathways serve

ONLY to regenerate NAD+ so it can return to pick up more electrons and hydrogens in glycolysis.

• End products such as ethanol and CO2 (single cell fungi (yeast) in beer/bread) or lactic acid (muscle cells)

Aerobic Cellular Respiration

• Oxygen required=aerobic• 2 more sets of reactions which occur in a

specialized structure within the cell called the mitochondria– 1. Kreb’s Cycle– 2. Electron Transport Chain

Kreb’s Cycle

• Completes the breakdown of glucose– Takes the pyruvate (3-carbons) and breaks it

down, the carbon and oxygen atoms end up in CO2 and H2O

– Hydrogens and electrons are stripped and loaded onto NAD+ and FAD to produce NADH and FADH2

• Production of only 2 more ATP but loads up the coenzymes with H+ and electrons which move to the 3rd stage

*At high CO2, C3 more efficient than C4 at all temps. (photosynthesis only, not other processes)

CO2 effects on photosynthesis

• C4 > C3 at low CO2 • But, C3 > C4 at high CO2

Photosynthetic N-use efficiency

• C4 plants need (have) less leaf N than C3• Photosynthesis higher per unit N in C4• Humans are increasing global N, which

benefits C3 more than C4• Increasing CO2 decreases leaf N content,

more in C3 than C4

Photosynthetic water-use efficiency

• C4 plants use less water than C3• (cause stomates open less)• Water availability may increase or decrease

in the future.