Chloroplasts and Photosynthesis

• Chloroplasts are oval-shaped organelles within plant cells, essential for photosynthesis.
• Chloroplasts possess a double membrane structure, similar to mitochondria.
• The outer membrane in chloroplasts is permeable, allowing small molecules to pass through, while the inner membrane is relatively impermeable.
• The stroma is a fluid-filled space inside the double membrane of chloroplasts, similar to the mitochondrial matrix.
• Chloroplasts have a third internal membrane system composed of thylakoids, which are disk-like sacs stacked into grana.
• Thylakoids are essential for capturing light energy during photosynthesis.
• Photosynthesis uses sunlight to convert carbon dioxide and water into energy-rich organic molecules like sugars.
• Light energy drives reactions in chloroplasts, producing ATP (adenosine triphosphate) and NADPH, essential molecules.
• ATP is the main energy currency of the cell, and NADPH is an electron carrier.
• ATP and NADPH are subsequently used to convert CO2 into sugars.

Two Main Sets of Reactions in Photosynthesis

• Light reactions occur in the thylakoid membranes.
• Chlorophyll pigments absorb sunlight during light reactions.
• Energy from sunlight transports electrons, creating a proton gradient.
• ATP and NADPH are produced during light reactions.
• Oxygen gas (O2) is produced as a byproduct of light reactions.
• Dark reactions, or carbon fixation reactions, occur in the stroma.
• Dark reactions use ATP and NADPH from light reactions to convert carbon dioxide into sugars like glucose.

ATP Production in Plants

• During the day, chloroplasts make ATP, but mitochondria produce most of the ATP for plant growth and other processes.
• Root cells, which do not have chloroplasts, rely on mitochondria for ATP production.
• Sugars produced in the leaves are exported to root cells, where mitochondria break them down to make ATP.
• Both leaf and root cells depend on mitochondria to supply ATP to the entire plant.

Light Reactions

• The process begins when chlorophyll molecules absorb sunlight.
• Each photosynthetic unit contains hundreds of chlorophyll molecules.
• Only one chlorophyll molecule, called the reaction-center chlorophyll, transfers electrons.
• Antenna pigments gather light and pass the energy to the reaction center quickly.
• Photosystem II (PSII) absorbs a photon of light, becomes energized, and splits water molecules.
• Water molecules are split into oxygen (O2), protons (H+), and electrons (e-).
• Splitting water molecules is the source of the oxygen humans breathe.
• Electrons are transferred through an electron transport chain, a series of protein complexes in the thylakoid membrane.
• Plastoquinone (Q) carries electrons from PSII to the cytochrome b6-f complex.
• As electrons flow, more protons are pumped into the thylakoid lumen, increasing proton concentration.
• The increase in proton concentration creates a proton gradient.
• Electrons are passed to plastocyanin, which carries them to Photosystem I (PSI).
• In PSI, electrons gain another energy boost from sunlight.
• Electrons are passed to ferredoxin, an iron-sulfur protein.
• Ferredoxin interacts with ferredoxin-NADP+ reductase (FNR), which reduces NADP+ to NADPH.
• A proton gradient is created across the thylakoid membrane and is used by ATP synthase.
• Protons move back into the stroma through ATP synthase.
• The energy released is used to convert ADP and inorganic phosphate (Pi) into ATP.
• Some herbicides like diuron, atrazine, and terbutryn block electron flow through PSII.
• Blocking electron flow through PSII prevents ATP and NADPH production, killing plants.
• The herbicide paraquat interferes with PSI.
• Paraquat steals electrons that would normally go to ferredoxin and transfers them to oxygen, creating reactive oxygen species.
• The reactive oxygen species molecules damage chloroplasts and can affect mitochondria in humans by disturbing electron flow in complex I.
• Affecting mitochondria in humans makes some herbicides dangerous to non-plant organisms.

Comparison of Chloroplasts and Mitochondria

• Chloroplasts and mitochondria have similarities and differences.
• Both organelles have their own DNA and ribosomes.
• Both organelles are surrounded by a double membrane.
• Both organelles generate ATP using a proton gradient and large protein complexes in their inner membranes.
• Chloroplasts have an extra internal membrane, the thylakoid membrane.
• Mitochondria have cristae, which are folds of their inner membrane.
• Mitochondria use both a proton gradient and a charge gradient across the inner membrane to drive ATP production.
• Chloroplasts mainly rely on a proton gradient.
• The thylakoid membrane allows ions like chloride (Cl-) and magnesium (Mg2+) to move across, balancing out the charge.
• Final electron acceptors differ such that, in mitochondria (during oxidative phosphorylation), the final electron acceptor is oxygen (O2), and the process releases carbon dioxide (CO2).
• In chloroplasts, the final electron acceptor is NADP+, and the process produces oxygen and uses carbon dioxide in the Calvin cycle.

Calvin Cycle

• The Calvin cycle is also known as the dark reactions or carbon fixation reactions.
• The Calvin cycle relies on ATP and NADPH produced in the light reactions.
• The enzyme Rubisco captures CO2 from the atmosphere and attaches it to a sugar called RuBP (ribulose bisphosphate).
• A series of chemical reactions then create glucose (C6H12O6), the main sugar product of photosynthesis.
• The chemical equation for one round of the Calvin cycle to produce one glucose molecule is: 6 RuBP + 6 CO2 + 18 ATP + 12 NADPH → 6 RuBP + C6H12O6 + 18 ADP + 18 Pi + 12 NADP+.
• Rubisco is inefficient and can only fix around 3 CO2 molecules per second.
• Plant cells produce a large amount of Rubisco to compensate for its inefficiency.
• Rubisco is the most abundant protein on Earth, making up around 50% of the protein content in a leaf.
• There is an estimated 5 to 10 kilograms of Rubisco for every human on the planet.
• Producing glucose through the Calvin cycle is energy-intensive, using 18 ATP for each molecule of glucose.
• Oxidative phosphorylation in the mitochondria breaks down glucose to produce about 30 ATP molecules, resulting in a net energy gain for the plant.