Photosynthesis: Light Energy to Sugar
Photosynthesis
Plants are autotrophs, meaning they are self-nourishing.
Electromagnetic Spectrum
- Shortest wavelength = Gamma rays
- Then X-rays
- Then UV radiation
- Then visible light (ROYGBIV)
- Then infrared radiation
- Then microwaves
- Longest wavelength = Radio waves
Photons are packets of light energy. They have differing amounts of energy depending on their wavelengths.
Pigments
The color you see is the wavelengths not absorbed.
The light-catching part of a molecule contains electrons that move to higher energy levels when light is absorbed.
- Major pigment: Chlorophyll a
- Accessory Pigments: Chlorophyll b, Carotenoids, Anthocyanins, Phycobilins
Bacteria: Pigments are in plasma membranes.
Plants: Photosystems (I and II) are in the thylakoid membrane system.
Photosynthesis vs. Aerobic Respiration
Photosynthesis: An energy-storing pathway that releases oxygen and requires carbon dioxide.
Aerobic Respiration: An energy-releasing pathway that requires oxygen and releases carbon dioxide.
12H2O + 6CO2 = 6O2 + C6H12O6 + 6H2O
Water + Carbon dioxide = Oxygen + Glucose + Water
Light-Dependent Reactions
Pigments absorb light energy and give up electrons, which enter electron transfer chains.
Water molecules split (photolysis) to replace electrons in photosystem II. ATP and NADH form, and oxygen is released.
Pigments that gave up electrons get replacements.
Non-cyclic Electron Flow: Photosynthesis
Light pigments absorb photon energy from the sun. This forces electrons out of photosystem II.
Replacement electrons come from splitting water (photolysis), producing O2 and H+. O2 diffuses out, but H+ are stuck inside.
Electrons released from photosystem II go into the electron transfer chain.
H+ are attracted to the negative charge and get shuttled into the inside of the thylakoid.
Electrons continue to photosystem I.
Non-cyclic Photosynthesis
The H+ gradient forces H+ ions through ATP synthase, and ATP is formed. (This type of ATP formation is called chemiosmosis).
Electrons continue through photosystem I and are used to reduce NADP+ to make NADPH (gain an electron).
ATP and NADPH are used later in the light-independent reaction that makes glucose.
Cyclic Electron Flow: Ancient Photosynthesis
Photosystem I gets photon energy from surrounding pigments. It loses an electron.
The electron passes into the electron transfer chain and loses a little energy with each transfer.
H+ are attracted to the negative charge and get shuttled inside the thylakoid, creating a higher concentration of H+.
The H+ gradient forces H+ ions through ATP synthase, and ATP is formed.
The electron ends up being shuttled back into photosystem I and is reused (cyclic).
No NADPH is produced. Oxygen is not a product.
Light-Independent Reactions: Calvin-Benson Cycle
These reactions can proceed in the dark and take place in the stroma. It is a sugar factory.
Carbon dioxide + ATP + NADPH = Glucose + ADP + NADP+
The reaction is cyclic, and RuBP (ribulose bisphosphate) is regenerated.
Calvin-Benson Cycle
These reactions do not depend directly upon the presence of light.
This cycle occurs in the stroma and requires 18 ATP, 12 NADPH, and CO2.
The Calvin-Benson cycle is the opposite of the citric acid cycle in that the citric acid cycle produces ATP and NADP, and glucose is broken down to make CO2. The Calvin-Benson cycle uses ATP, NADPH, and CO2 is used to make glucose.
The C3 Pathway (Most Plants, Including Beans and Peas)
The first intermediate in the Calvin-Benson Cycle has three carbons = C3 pathway. These plants don’t grow well in hot, dry areas.
Photorespiration
On hot, dry days, stomata close.
Inside the leaf, oxygen levels rise, and carbon dioxide levels drop.
Difference: Rubisco (enzyme) attaches RuBP to oxygen instead of carbon dioxide = photorespiration (not good).
It takes twice as many turns of the Calvin-Benson Cycle (produces less sugar, and the plant doesn’t grow as well).
C4 Plants (Adaptation to Hot, Dry Climates: Corn)
Carbon dioxide is fixed twice; once in mesophyll cells to form malate, then pyruvate releases CO2 that is fixed into the Calvin-Benson cycle.
CAM Plants (Desert: Cacti)
Carbon is fixed twice (in the same cells).
Night: Stomata are open. Carbon dioxide is fixed to form malate.
Day: Stomata are closed. Malate releases carbon dioxide, which is fixed in the Calvin-Benson cycle.