Chapter 13 & 14 - Energy Generation in Chloroplasts
Chloroplasts and Photosynthesis
• Photosynthesis is process using the energy in sunlight and CO 2 to create the organic materials required of
present day cells
• The chloroplast is the special organelle in plants responsible for photosynthesis
Chloroplasts
• Similar to mitochondria
• Uses a proton pump to create ATP
• Stroma instead of matrix
• Has own RNA, DNA and ribosomes
• Difference is that the e- transport chain is in the thylakoid membrane – 3rd membrane that makes up the
thylakoids, a sac-like structure, so have a thylakoid space
• Granum – stack of thylakoids
Chloroplasts vs Mitochondria
Light and Dark Reactions
• Light or photosynthetic e- transfer reactions
– Sunlight energizes e- in chlorophyll which then moves down the e- transport chain in the thylakoid membrane
– e- gotten from H2O to make O2
– Electrochemical gradient is made in the stroma across the thylakoid membrane making ATP
– Generate NADPH from NADP+
• Dark or carbon-fixation reactions
– ATP and NADPH produced in light reaction used as energy and reducing power to take CO 2 and convert it to
a carbohydrate – sucrose
Photosynthesis Reactions
Chlorophyll
• Sunlight is composed of many different wavelengths ranging from violet to red
• Chlorophyll is green because it absorbs all the wavelengths but green
• The e- in chlorophyll gain a higher energy level when a wavelength is absorbed and then bounce around the ring –
porphyrin (blue)
Photosystem
• Chlorophylls are in a multiprotein complex called a photosystem
• Antenna is many molecules of chlorophyll that capture the sunlight’s energy that ultimately goes to the reaction
center
Reaction Center
• Set of proteins in the thylakoid membrane
• Special chlorophyll molecule that is an irreversible trap for an excited e • Transfers the e- to a more stable environment
ATP and NADPH
• The light reaction makes the ATP and NADPH (reducing power) to synthesize the sugar
• ATP made with the first photon of light absorbed and NADPH is made from the second photon of light
Summary of Light Reactions
• Electron from chlorophyll in photsystem II is donated to NADPH
• The replacement electron comes from the splitting of water
• When 4 electrons are removed (4 photons hit chlorophyll) O2 is released
Carbon Fixation
Carbon-Fixation or Calvin Cycle
• CO2 joins with a ribulose 1,5-bisphosphate (5 C) by a carboxylase called rubisco
– Rubisco is slow compared to other enzymes so therefore there is a large amount in the cell to compensate
for this
• 1 molecule of glyceraldehyde 3-phosphate (net product ) is generated and goes to make the sugar
• A large amount of energy goes to regenerating the ribulose 1,5-bisphosphate
• 3 ATP and 2 NADPH required for each CO2 molecule converted to carbohydrate
Glyceraldehyde 3-phosphate
• Converted into sucrose
• Can be shuttled into the glycolytic pathway in the mitochondria of plants to become pyruvate and eventually ATP
• Excess is converted into starch in the stroma which can be used at night as an energy source