Photosynthesis and Cell Respiration Worksheet with detailed questions on biological processes.

Certainly! Below is a complete solution to the Photosynthesis and Cell Respiration Worksheet, with detailed explanations for each question. Since I can't draw images directly, I’ll describe how to create the required drawings (chloroplast and mitochondrion) in text form.

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## Photosynthesis

1) Write the general reaction for photosynthesis. Are the chemical reactants and products balanced?

General Reaction:
\[
6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2
\]

- Reactants: Carbon dioxide (CO₂), water (H₂O), light energy
- Products: Glucose (C₆H₁₂O₆), oxygen (O₂)

✔ Yes, the equation is balanced.
- 6 carbon atoms on both sides
- 12 hydrogen atoms on both sides
- 18 oxygen atoms on both sides

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2) Prepare a rough drawing of a chloroplast labeling the following: chloroplast, grana, thylakoid membrane, stroma

Drawing Description:
- Draw an oval shape labeled "Chloroplast".
- Inside it, draw stacked disk-like structures called grana (plural of granum).
- Each disk in the stack is a thylakoid membrane.
- The space surrounding the grana (inside the chloroplast but outside the thylakoids) is the stroma.

Labeling:
- Chloroplast: outer boundary
- Grana: stacks of thylakoids
- Thylakoid membrane: individual disks in the stacks
- Stroma: fluid-filled space between grana

> 🖼️ *Visual Tip:* Think of a stack of coins (grana), each coin being a thylakoid; the entire structure is inside a larger bag (chloroplast); the space around the stacks is the stroma.

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3) What links photosystem I and photosystem II? What is the purpose of this link?

- The electron transport chain (ETC) links Photosystem II and Photosystem I.
- Electrons move from PSII → ETC → PSI.

Purpose:
- The ETC uses the energy from excited electrons to pump protons (H⁺) into the thylakoid lumen, creating a proton gradient.
- This gradient drives ATP synthesis via chemiosmosis (ATP synthase).

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4) Consider what happens when light is absorbed by the pigments of photosystem II. How are electrons affected by the light absorption? Where do the excited electrons go?

- When light hits chlorophyll in photosystem II, electrons are excited (gain energy).
- These high-energy electrons are passed to the primary electron acceptor.
- They then travel through the electron transport chain toward photosystem I.

> ⚡ The electrons are "boosted" to a higher energy level and begin moving down the ETC.

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5) In just 3–4 sentences summarize the light-dependent reactions. What are the two most important products?

- Light-dependent reactions occur in the thylakoid membranes of the chloroplast.
- Light energy splits water (photolysis), releasing O₂, H⁺, and electrons.
- Electrons move through the ETC, generating ATP and NADPH via chemiosmosis and reduction.
- The two most important products are ATP and NADPH, which are used in the Calvin cycle.

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6) What is the overall purpose of the Calvin Cycle?

- To fix carbon dioxide (CO₂) into organic molecules (like glucose) using ATP and NADPH from the light reactions.
- It builds sugars from inorganic carbon (CO₂) – essentially carbon fixation.

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7) Where does CO₂ enter the cycle?

- CO₂ enters the Calvin cycle at the beginning, where it combines with a 5-carbon sugar called ribulose bisphosphate (RuBP) in a reaction catalyzed by the enzyme RuBisCO.

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8) How are ATP and NADPH used?

- ATP provides energy to phosphorylate 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P).
- NADPH donates electrons (reducing power) to convert 3-PGA into G3P.
- Both are essential for carbon fixation and sugar synthesis.

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9) G3P (also called PGAL) can be used to form glucose, but most is recycled. Why?

- Most G3P is recycled to regenerate RuBP, so the cycle can continue fixing more CO₂.
- Only about 1 out of every 6 G3P molecules is used to make glucose or other carbohydrates.
- Recycling maintains the cycle’s ability to run continuously.

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## Cell Respiration

1) Write out the overall equation for cell respiration – compare it to the equation for photosynthesis

Cell Respiration:
\[
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}
\]

Photosynthesis:
\[
6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2
\]

🔹 Comparison:
They are reverse processes:
- Respiration breaks down glucose to release energy.
- Photosynthesis builds glucose using energy.

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2) List four phases of cell respiration

1. Glycolysis
2. Pyruvate Oxidation (Link Reaction)
3. Krebs Cycle (Citric Acid Cycle)
4. Electron Transport Chain (ETC) and Oxidative Phosphorylation

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3) What are the starting and ending products of glycolysis?

- Starting: 1 molecule of glucose (6 carbons)
- Ending: 2 molecules of pyruvic acid (3 carbons each), 2 ATP (net), 2 NADH

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4) In what situation is pyruvic acid reduced to lactic acid?

- When oxygen is limited (anaerobic conditions), such as during intense exercise.
- Pyruvate is converted to lactic acid to regenerate NAD⁺ so glycolysis can continue.

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5) What happens to animal cells when too much lactic acid accumulates in muscles? How do muscle cells solve the problem?

- Lactic acid buildup causes muscle fatigue, soreness, and cramps due to low pH (acidosis).
- After exercise, oxygen becomes available again.
- Muscle cells convert lactic acid back to pyruvate, which enters aerobic respiration to produce more ATP and clear the lactic acid.

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6) In aerobic respiration, pyruvic acid continues on to cell respiration. Where does this occur? How is pyruvic acid changed prior to entry in the Krebs cycle?

- Occurs in the mitochondria.
- Before entering the Krebs cycle, pyruvate is oxidized and decarboxylated:
- Pyruvate → Acetyl-CoA + CO₂ + NADH
- This occurs in the mitochondrial matrix.

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7) In what part of the mitochondrion do the events of the Krebs cycle occur?

- The mitochondrial matrix.

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8) What molecule actually enters the cycle?

- Acetyl-CoA enters the Krebs cycle by combining with oxaloacetate to form citrate.

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9) What is the role of enzymes in the Krebs cycle?

- Enzymes catalyze each step of the cycle.
- They ensure the reactions happen quickly and efficiently under cellular conditions.
- Examples: citrate synthase, isocitrate dehydrogenase, etc.

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10) Where is CO₂ removed? What becomes of it?

- CO₂ is released during the oxidation of isocitrate and α-ketoglutarate in the Krebs cycle.
- It diffuses out of the cell and is exhaled via the lungs.

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11) What molecule accepts the electrons after they pass through the electron transport chain?

- Oxygen (O₂) is the final electron acceptor.
- It combines with electrons and H⁺ to form water (H₂O).

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12) How much ATP is produced in cell respiration overall, and in each of the stages?

| Stage | ATP Produced |
|-------|--------------|
| Glycolysis | 2 ATP (net), 2 NADH |
| Pyruvate Oxidation | 2 NADH (from 2 pyruvates) |
| Krebs Cycle | 2 ATP, 6 NADH, 2 FADH₂ |
| Electron Transport Chain & Oxidative Phosphorylation | ~28–34 ATP (depends on NADH/FADH₂ yield) |

Total ATP ≈ 30–32 ATP per glucose molecule

> 🔍 Note: NADH produces ~2.5 ATP, FADH₂ produces ~1.5 ATP.

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13) Prepare a drawing of a mitochondrion, labeling the following: outer membrane, inner membrane, and where the different stages occur

Drawing Description:

- Draw an oval with a double membrane:
- Outer membrane: smooth, outer layer
- Inner membrane: folded into cristae (infoldings)
- Space between membranes: intermembrane space
- Innermost space: matrix

Labeling:
- Outer membrane: surrounds the organelle
- Inner membrane: highly folded; site of electron transport chain and ATP synthase
- Matrix: site of Krebs cycle and pyruvate oxidation
- Intermembrane space: where H⁺ accumulates during ETC

Where each stage occurs:
- Glycolysis: Cytoplasm (not in mitochondrion)
- Pyruvate Oxidation: Matrix
- Krebs Cycle: Matrix
- Electron Transport Chain: Inner membrane
- Oxidative Phosphorylation: Inner membrane (via ATP synthase)

> 🖼️ *Visual Tip:* Imagine a bag within a bag — the inner one has folds (cristae). Label the folds as ETC site, the center as Krebs cycle site.

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✔ Summary Table: Key Comparisons

| Process | Location | Inputs | Outputs |
|------------------|----------------------|-------------------------|-----------------------------|
| Photosynthesis | Chloroplast | CO₂, H₂O, light | Glucose, O₂ |
| Cell Respiration | Mitochondria | Glucose, O₂ | CO₂, H₂O, ATP |

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