3.2 Plants, Photosynthesis & Cellular Respiration

Living systems need a continuous supply of energy. On the NEX Science exam, Biology questions about plants, photosynthesis, and cellular respiration test whether you understand where energy comes from, how it is captured and released, and how plant cells differ from animal cells. No physics equations are required—focus on organelles, pathways, products, and the relationship between the two processes.

Why Energy Pathways Matter

Photosynthesis captures light energy and stores it in organic molecules (mainly sugars). Cellular respiration breaks down those molecules to form ATP (adenosine triphosphate), the molecule cells use to power transport, synthesis, movement, and signaling. Autotrophs such as plants and algae perform both processes; animals and most fungi rely on organic fuel produced by autotrophs and then respire it.

Think of the relationship as a cycle of matter and a flow of energy:

• Matter (C, H, O) cycles between CO₂, H₂O, and organic compounds
• Energy enters ecosystems as sunlight and leaves mainly as heat after work is done

Plant Cell Features (Exam Checklist)

Plant cells are eukaryotic and share many organelles with animal cells (nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus). Distinctive or emphasized plant features include:

Chlorophyll absorbs mainly violet-blue and red wavelengths and reflects green—hence the green appearance of most leaves. Chloroplasts have inner and outer membranes and stacks of thylakoid membranes (grana) where light reactions occur; the fluid stroma hosts the Calvin cycle.

Photosynthesis Overview

Overall equation (simplified, balanced form used in most intro texts):

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

Carbon dioxide and water are reactants; glucose (or other carbohydrates) and oxygen are products. Light energy is required; it is not a chemical "ingredient" on the left in the same sense as molecules, but without it the process does not run.

Photosynthesis is commonly divided into two stages:

1. Light-Dependent Reactions (Thylakoid Membrane)

• Pigments capture photons; energy boosts electrons
• Water is split (photolysis): electrons replace those lost by chlorophyll; O₂ is released as a byproduct; protons contribute to a gradient
• Electron transport helps form ATP and NADPH (energy carriers)

2. Light-Independent Reactions / Calvin Cycle (Stroma)

• Carbon fixation: enzyme rubisco incorporates CO₂ into organic intermediates
• ATP and NADPH from the light reactions drive reduction and sugar formation
• Product carbohydrates can be stored as starch, used in respiration, or converted to other biomolecules

"Light-independent" does not mean the Calvin cycle thrives in permanent darkness—it depends on ATP and NADPH generated by light reactions, so it slows when light reactions stop.

Cellular Respiration Overview

Cellular respiration releases energy from organic molecules. With oxygen present (aerobic respiration), the overall simplified equation is essentially the reverse of photosynthesis in terms of matter:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP + heat)

Main stages:

Glycolysis (Cytoplasm)

• Glucose (6-carbon) is split into two molecules of pyruvate (3-carbon)
• Small net yield of ATP and reduced electron carriers (NADH)
• Does not require oxygen (anaerobic pathway segment), but aerobic cells continue to mitochondria

Krebs Cycle (Citric Acid Cycle) — Mitochondrial Matrix

• Pyruvate is processed; carbon is released as CO₂
• More NADH and FADH₂ are produced; a small amount of ATP (or GTP) forms directly

Electron Transport Chain (ETC) — Inner Mitochondrial Membrane

• High-energy electrons from NADH and FADH₂ pass through protein complexes
• Energy pumps protons, creating a gradient; ATP synthase makes most of the cell’s ATP (oxidative phosphorylation)
• Oxygen is the final electron acceptor, combining with electrons and protons to form water

Without oxygen, the ETC backs up. Many cells then rely on fermentation (e.g., lactic acid fermentation in human muscle under oxygen debt) to regenerate NAD⁺ so glycolysis can continue—yielding far less ATP than full aerobic respiration.

Mitochondria are therefore essential for efficient aerobic ATP production in eukaryotic cells—including plant cells, which respire day and night even though they photosynthesize in the light.

ATP: Immediate Energy Currency

ATP stores energy in its phosphate bonds. Hydrolysis to ADP + Pᵢ releases free energy for endergonic cell work; respiration (and photophosphorylation in chloroplasts) recharges ADP to ATP. Cells do not "store" large ATP reserves for long periods; they regenerate it continuously. For the exam, link ATP to both photosynthesis (made in light reactions) and respiration (made mainly at the ETC).

Photosynthesis vs Cellular Respiration

A useful memory hook: photosynthesis builds glucose and frees oxygen; respiration burns glucose with oxygen and frees carbon dioxide and water, while recycling energy into ATP.

Connecting Pathways in the Plant

During daylight, a leaf cell may run photosynthesis rapidly while still respiring. Sugar produced in chloroplasts can be:

• Exported as sucrose to non-photosynthetic tissues (roots, fruits)
• Stored as starch
• Oxidized in mitochondria for ATP

Oxygen produced by photosynthesis can support respiration; CO₂ from respiration can be refixed in the Calvin cycle. The net gas exchange of a plant (whether it takes in CO₂ or O₂ overall) depends on light intensity, temperature, and metabolic demand—concepts that appear in graph-interpretation items (rate of photosynthesis vs light intensity is a classic curve that plateaus when another factor becomes limiting).

NEX Application Tips

• Match organelle → process: chloroplast ↔ photosynthesis; mitochondrion ↔ aerobic respiration
• Identify reactants vs products from the balanced equations
• Do not confuse where O₂ is produced (water splitting in light reactions) with where CO₂ is used (Calvin cycle)
• Remember plants have both chloroplasts and mitochondria
• If a question describes oxygen debt or anaerobic conditions, think glycolysis + fermentation, not a full ETC yield

This section pairs naturally with cell structure (organelles) and later health topics (oxygen delivery, metabolism). Keep the equations, the three respiration stages, and the comparison table automatic before exam day.