Photosynthesis. Light Dependent Reaction vs. Light Independent Reaction.
Photosynthesis Plants and other photoautotrophs use the following reaction to make food:
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Transcript of Photosynthesis Plants and other photoautotrophs use the following reaction to make food:
Photosynthesis
Plants and other photoautotrophs use the following reaction to make food:
6 CO2 + 6 H2O + Energy C6H12O6 + 6 O2
In plants, the reactions take place in the chloroplast:
cross-section of a leaf
chloroplast
• The energy in the equation is provided by light from the sun.
• light strikes the leaf of a plant and is absorbed by pigments (called chlorophylls) in the chloroplast.
• the chlorophylls absorb certain wavelengths of light, and as a result, photosynthesis occurs best at these wavelengths.
The chlorophyll molecules are embedded in the thylakoid membrane, arranged into a network called a photosystem.
Chlorophyll molecules are called antenna pigments because they anchor themselves into the membrane here…
and stick out to absorb light energy using electrons in a porphyrin ring (up top).
Photosynthesis takes place in 2 stages:
1. The light dependent reactions - where energy from the sun is converted into chemical energy (in the form of ATP and NADPH - a mobile electron carrier).
2. The light independent reactions - where the energy made in stage 1 is used
to make organic compounds (glucose) from CO2
photosynthesis movie
The Light Dependent Reactions
• 3 parts:
• a photon is absorbed by a chlorophyll electron
• electron transport - the excited electron is moved through membrane carriers, pumping H+ to create a gradient and eventually reduces NADP+ to NADPH.
• H+ moves through ATP synthase to drive the formation of ATP from ADP.
Photosystems
• the primary light harvesting units of chloroplasts.
• in the chloroplast, chlorophyll molecules are found in clusters called photosystems.
• a photosystem consists of an antenna complex and a reaction centre - embedded in the thylakoid membrane.
• an antenna pigment absorbs a photon and transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule in the reaction centre.
• the chlorophyll a molecule transfers this electron to a primary electron acceptor.
Electron Flow and Chemiosmosis
• Two photosystems (1 and 2) are used to make ATP and NADPH.
• photosynthesis starts when a photon strikes PS II.
• Z-protein splits water into 2 H+ , O2 and electrons.
• protons are pumped into the lumen (middle) of the thylakoid and eventually make ATP.
• the electrons leave the ETC by reducing NADP+ to form NADPH.
Light Independent Reactions (the Calvin Cycle)
• ATP and NADPH generated in light reactions used to fuel the reactions which take CO2 and break it apart, then reassemble the carbons into glucose.
• called carbon fixation: taking carbon from an inorganic molecule (atmospheric CO2) and making an organic molecule out of it (glucose)
• occurs in the stroma• don’t require light• also known as C3 Cycle
Steps in Light Independent Phase:
1. Carbon Fixation • An enzyme, RuBisCo, combines a CO2 molecule
with a 5C molecule called RuBP (ribulose biphosphate) to produce 2, 3C molecules called 3-PGA (3-phosphoglycerate).
2. Reduction Reactions • 3-PGA phorphorylated by ATP then reduced by
NADPH to produce G3P (glycerate 3-phosphate).3. RuBP Regeneration • G3P phosphorylated by ATP to re-create RuBP to
restart the cycle. For every 2 G3P, 1 molecule glucose is removed from cycle, so need 6 CO2 to create 1 glucose.