BIO TEST 2

🌿 Photosynthesis: An Overview

Photosynthesis is the process by which plants use light to synthesize carbohydrates. It is the process of using light energy to build something, in this case, sugars.

⚛ The Net Equation of Photosynthesis

The net equation for photosynthesis is:

6H2O+6CO2+Light⟶C6H12O6+6O26H2​O+6CO2​+Light⟶C6​H12​O6​+6O2​

• Reactants: Water and carbon dioxide

• Products: Glucose and oxygen

Water enters the plant through the roots, while carbon dioxide enters the leaves through tiny openings called stomata. Oxygen exits through the same stomata.

📍 Chloroplasts: The Site of Photosynthesis

The chloroplast is the organelle responsible for carrying out photosynthesis, analogous to how the mitochondria is the organelle responsible for cellular respiration. These processes are opposites of each other:

• Photosynthesis: Carbon dioxide + Water + Light Energy → Glucose + Oxygen

• Cellular Respiration: Glucose + Oxygen → Carbon Dioxide + Water + Energy

Chlorophyll, the pigment responsible for absorbing light energy, is found in the thylakoids. One stack of thylakoids is called a granum, and multiple stacks are called grana. The fluid inside the thylakoid is the lumen, while the fluid inside the chloroplast is the stroma. The chloroplast has an inner and outer membrane, with the intermembrane space in between.

Chlorophyll absorbs blue and red light but reflects green light, which is why plants appear green.

☀ Stages of Photosynthesis

Photosynthesis is divided into two stages:

• Light-dependent reactions: Occur in the thylakoids.

• Light-independent reactions (Calvin Cycle): Occur in the stroma. They're called light independent reactions or dark reactions because they can proceed without the assistance of light energy.

In the light-dependent reactions, water is oxidized into oxygen gas. NADP++ picks up electrons and is reduced into NADPH. Some energy transferred by light is used to make ATP from ADP and phosphate, a process called chemiosmosis, using ATP synthase.

Products of the light-dependent reactions:

• Oxygen gas

• ATP

• NADPH

Reactants of the light-dependent reactions:

• Water

• NADP++

• ADP

• Phosphate

The Calvin cycle takes in carbon dioxide and reduces it into sugars like glucose. NADPH is oxidized back into NADP++, and ATP is used to power the process as it converts back into ADP and P.

Reactants of the Calvin Cycle:

• Carbon dioxide

• ATP

• NADPH

Products of the Calvin Cycle:

• Sugars (e.g., glucose)

• NADP++

• ADP

• P

⚡ Light-Dependent Reactions: The Electron Transport Chain

The electron transport chain is located in the thylakoid membrane.

1. A light particle strikes photosystem II (P680), exciting electrons in chlorophyll.

2. Chlorophyll loses electrons, which flow into plastoquinone.

3. To replenish lost electrons, chlorophyll takes electrons from water, oxidizing it into oxygen gas (one water molecule produces one oxygen atom, two hydrogen ions, and two electrons).

4. Plastoquinone carries electrons to the cytochrome b6f complex, which pumps protons from the stroma into the lumen, creating a concentration gradient.

5. Electrons move from cytochrome b6f to plastocyanin, a copper-containing protein.

6. Plastocyanin transfers electrons to photosystem I (P700).

7. Electrons gain more energy by being struck by another photon of light in photosystem I.

8. Electrons move to ferredoxin, an iron-sulfur protein.

9. Ferredoxin carries electrons to NADP reductase, an enzyme that reduces NADP++.

10. NADP++ accepts electrons and a hydrogen ion, reducing into NADPH.

11. Hydrogen ions flow through ATP synthase due to the concentration gradient (chemiosmosis).

12. ATP synthase rotates, combining ADP and phosphate to make ATP.

In summary:

• Water is oxidized into oxygen gas.

• NADP reductase produces NADPH.

• ATP synthase produces ATP.

Key Point: Photosystem II converts water into oxygen gas.

🔄 Light-Independent Reactions: The Calvin Cycle

The Calvin Cycle has three main parts:

1. Fixation of carbon dioxide.

2. Reduction.

3. Regeneration of RuBP (ribulose 1,5-biphosphate).

⚙ Steps of the Calvin Cycle

1. Carbon dioxide enters the cycle and reacts with ribulose biphosphate (RuBP), catalyzed by the enzyme rubisco.

   • RuBP is a five-carbon molecule with phosphate groups on carbons 1 and 5.

   • This reaction turns into 3-phosphoglycerate (PGA).

   Rubisco: An enzyme that catalyzes the carboxylation of ribulose-1,5-bisphosphate (RuBP), the first major step of carbon fixation in the Calvin cycle.

2. When one molecule of CO2 reacts with one molecule of RuBP, it initially creates a six-carbon molecule, which breaks down into two three-carbon molecules. PGA is a three-carbon molecule with one phosphate group.

🔢 Carbon Math

• Three RuBP molecules (5 carbons each) have 15 carbons.

• Adding three CO2 molecules adds 3 carbons.

• Total: 18 carbons, resulting in six 3-phosphoglycerate molecules.

• Enzyme kinase transfers a phosphate.

Calvin Cycle Reactions 🔄

Phosphorylation of 3-Phosphoglycerate

• Six ATP molecules are used to phosphorylate 3-phosphoglycerate.

  • This converts 3-phosphoglycerate into 1,3-bisphosphoglycerate.

• PGA kinase catalyzes the conversion.

  Kinase enzymes catalyze the transfer of a phosphate group from one molecule to another.

• ATP gives up a phosphate to become ADP, and that phosphate is transferred to 3-phosphoglycerate.

Reduction of 1,3-Bisphosphoglycerate

• Six molecules of NADPH are used to reduce 1,3-bisphosphoglycerate into G3P (glyceraldehyde 3-phosphate).

• NADPH converts into NADP+.

• The reaction is catalyzed by G3P dehydrogenase.

  Dehydrogenase enzymes remove hydrogen from NADPH.

• This step results in six G3P molecules.

  • One of the six G3P molecules is used to produce sugars like glucose and fructose.

  • The other five G3P molecules are used to regenerate the three ribulose bisphosphate molecules.

    • The total number of carbons is 15 (5 G3P molecules x 3 carbon atoms each).

Net Result of the Calvin Cycle 🎯

• The Calvin Cycle converts three molecules of CO2 into one molecule of G3P.

Stoichiometry of Glucose Production 🧪

• To make one molecule of glucose, six molecules of CO2 are needed.

  • This produces two molecules of G3P.

• The process requires:

  • 18 ATP molecules.

  • 12 NADPH molecules.

Summary of Photosynthesis Processes ☀

• Light-dependent reactions: Occur in the thylakoid membrane.

• Light-independent reactions (Calvin Cycle): Occur in the stroma of the chloroplast.