Difficulty: Intermediate | Prerequisites: Chapter 8 (metabolism, enzymes, ATP), Chapter 9 (ETC, chemiosmosis, redox).
Photosynthesis is the anabolic counterpart to cellular respiration. Where respiration breaks glucose down for energy, photosynthesis builds glucose from CO2 and water using sunlight. This chapter covers the two main stages (light reactions and the Calvin cycle), the structure of the chloroplast, the two photosystems, and alternative carbon-fixation pathways (C3, C4, CAM). The chemiosmosis mechanism here parallels the one in mitochondria, so understanding Ch. 9 makes this chapter far more accessible.
Photosynthesis converts light energy into chemical energy stored in glucose: 6 CO2 + 6 H2O + sunlight produces C6H12O6 + 6 O2. The light reactions (thylakoid membranes) split water, release O2, and produce ATP and NADPH. The Calvin cycle (stroma) uses that ATP and NADPH to fix carbon from CO2 into sugar via the enzyme rubisco.
Photosynthesis
The process that converts solar energy into chemical energy (glucose). "Photo" = light energy; "synthesis" = building sugar.
Autotroph
An organism that produces its own organic molecules from CO2 using sunlight (photoautotroph) or chemical energy. Almost all plants are photoautotrophs.
Heterotroph
An organism that obtains organic material by consuming other organisms. Depends on autotrophs for food and O2.
Chloroplast
The organelle where photosynthesis occurs. Contains thylakoid membranes (stacked into grana) and the stroma (fluid-filled space around them).
Thylakoid
Flattened membrane sacs inside the chloroplast containing the photosynthetic pigments. Stacked thylakoids form a granum. The light reactions occur here.
Stroma
The fluid-filled space surrounding the thylakoids inside the chloroplast. The Calvin cycle (light-independent reactions) occurs here.
Stomata
Pores in the lower epidermis of leaves, responsible for gas exchange (CO2 in, O2 out). Can open and close to prevent water loss in hot conditions.
Light reactions
The first stage of photosynthesis, occurring on the thylakoid membranes. Splits water, releases O2, reduces NADP+ to NADPH, and generates ATP via photophosphorylation.
Calvin cycle (C3 cycle)
The second stage, occurring in the stroma. Uses ATP and NADPH from the light reactions to fix CO2 into sugar using the enzyme rubisco. Also called the light-independent reactions or Blackman reactions.
Photosystem II (PSII)
Functions first in linear electron flow. Absorbs light at 680 nm. Contains P680 reaction centre, chlorophyll a molecules, and pheophytin. Splits water to replace lost electrons.
Photosystem I (PSI)
Functions second in linear electron flow. Absorbs light at 700 nm. Contains P700, 96 chlorophyll a molecules, carotenoids, and iron-sulfur clusters. Reduces NADP+ to NADPH via ferredoxin.
Linear electron flow
The primary pathway: electrons flow from PSII through the ETC to PSI and on to NADP+. Produces both ATP and NADPH. Uses both photosystems.
Cyclic electron flow
An alternative pathway using only PSI. Electrons cycle from PSI back to the ETC via ferredoxin. Produces only ATP (no NADPH, no O2 released).
Rubisco
The enzyme that catalyses carbon fixation in the Calvin cycle, combining CO2 with RuBP. The most abundant protein on Earth.
C4 pathway (Hatch-Slack)
An alternative carbon-fixation pathway using PEP carboxylase instead of rubisco for initial fixation. Separates fixation and the Calvin cycle into different cell types. Example: corn. Less efficient overall but avoids photorespiration.
CAM (Crassulacean Acid Metabolism)
A pathway in which stomata open at night to fix CO2 and close during the day to conserve water. Used by desert plants like cacti.
NADPH
The electron carrier produced by the light reactions and consumed by the Calvin cycle. The photosynthesis equivalent of NADH in respiration.
6 CO2 + 6 H2O + sunlight energy --> C6H12O6 + 6 O2
Oxygen is a byproduct of photosynthesis, not the primary product. It comes from the splitting of water during the light reactions.
Thylakoids: membrane sacs containing photosynthetic pigments, stacked into grana
Stroma: the fluid surrounding the grana, where the Calvin cycle runs
Upper and lower epidermis: protective layers of the leaf. Stomata are located in the lower epidermis for gas exchange
Ground tissue: parenchyma, collenchyma, and sclerenchyma cells that support the leaf
Vascular tissue: xylem (carries water up) and phloem (distributes sugars)
The light reactions accomplish four things: split H2O, release O2, reduce NADP+ to NADPH, and generate ATP from ADP.
Photosystem II (PSII) functions first despite its numbering:
Absorbs light at 680 nm (P680 reaction centre)
Contains about 20 different proteins, 50+ chlorophyll a molecules, 2 reaction-centre molecules, 2 accessory molecules, and 2 pheophytin molecules
When light excites P680, electrons are boosted to the primary electron acceptor; water is split to replace those electrons, releasing O2
Photosystem I (PSI):
Absorbs light at 700 nm (P700 reaction centre)
Contains 12 different proteins, 96 chlorophyll a molecules, 2 P700 molecules, 4 accessory molecules, 22 carotenoids, 4 lipids, and 3 Fe4S4 clusters
Electrons are passed to ferredoxin, then to NADP+ reductase, producing NADPH
Linear electron flow (the primary pathway):
Uses both photosystems
Produces ATP and NADPH
Electrons flow: H2O --> PSII --> electron transport chain --> PSI --> ferredoxin --> NADP+ reductase --> NADPH
Cyclic electron flow:
Uses only PSI
Produces only ATP (no NADPH, one O2 released)
Electrons cycle from ferredoxin back to the PSI reaction centre via the cytochrome complex
The Calvin cycle fixes carbon from CO2 into organic sugar. It requires ATP and NADPH from the light reactions. The enzyme rubisco catalyses the fixation step.
Three phases per turn (the cycle turns three times to produce one G3P output):
Carbon fixation: rubisco attaches CO2 to RuBP (5C), producing two 3-phosphoglycerate (3C) molecules
Reduction: ATP and NADPH reduce 3-phosphoglycerate to G3P
Regeneration of RuBP: ATP is used to rearrange five G3P molecules back into three RuBP molecules
After three turns: 3 CO2 fixed, 1 net G3P produced (used to build glucose and other organic compounds).
C4 (Hatch-Slack) pathway: uses PEP carboxylase for initial CO2 fixation in mesophyll cells, then shuttles the 4-carbon product to bundle-sheath cells where the Calvin cycle runs. This spatial separation reduces photorespiration. Example: corn (higher photosynthesis efficiency).
CAM pathway: stomata open at night to fix CO2 using PEP carboxylase and close during the day to conserve water. The stored organic acids release CO2 for the Calvin cycle during the day. Example: cacti and succulents.
Feature | Chloroplast | Mitochondria |
|---|---|---|
Energy source | Light energy | Chemical energy from food |
Protons pumped into | Thylakoid space | Intermembrane space |
ATP synthase direction | Stroma (protons flow out to stroma) | Matrix (protons flow back to matrix) |
Product | ATP (used in Calvin cycle) | ATP (used by the cell) |
Students often think oxygen is the main product of photosynthesis. Oxygen is a byproduct, released when water is split during the light reactions. The primary product is glucose.
PSII functions before PSI despite the numbering. They were named in the order they were discovered, not the order they operate in.
The Calvin cycle is sometimes called the "dark reactions," but this is misleading. It does not require darkness; it simply does not require light directly. It runs in the stroma using ATP and NADPH produced by the light reactions.
C4 plants are not "better" than C3 plants in all conditions. C4 is advantageous in hot, dry climates because it avoids photorespiration, but C3 plants are more efficient under cooler, wetter conditions.
⚠️ Know the overall equation and be able to explain what each part represents.
⚠️ The essay asks you to explain the process of photosynthesis, including light and light-independent reactions, Calvin cycle, light-harvesting complexes, and chemiosmosis.
⚠️ Be able to compare chemiosmosis in the chloroplast with chemiosmosis in the mitochondria.
⚠️ Distinguish linear electron flow (produces ATP + NADPH, uses both photosystems) from cyclic electron flow (produces only ATP, uses only PSI).
⚠️ Know the difference between C3 (Calvin cycle with rubisco) and C4 (Hatch-Slack with PEP carboxylase).
True or False: The Calvin cycle occurs on the thylakoid membranes. (False, it occurs in the stroma.)
Fill in the blank: PSII absorbs light at ____ nm. (680)
True or False: Cyclic electron flow produces both ATP and NADPH. (False, it produces only ATP.)
Fill in the blank: The enzyme that fixes CO2 in the Calvin cycle is ______. (Rubisco)
True or False: C4 plants use PEP carboxylase for initial carbon fixation. (True)
Q: Explain the process of photosynthesis and how plants use energy to make sugars.
A: Photosynthesis occurs in two stages within the chloroplast. The light reactions take place on the thylakoid membranes: PSII absorbs light at 680 nm, splitting water to release O2 and boosting electrons through an electron transport chain to PSI (700 nm), which reduces NADP+ to NADPH. The electron transport chain also pumps protons into the thylakoid space, and their flow back through ATP synthase produces ATP (chemiosmosis). The Calvin cycle then runs in the stroma: rubisco fixes CO2 onto RuBP (carbon fixation), ATP and NADPH reduce the resulting molecules (reduction), and RuBP is regenerated. Three turns of the cycle fix 3 CO2 and produce 1 net G3P, which the plant uses to build glucose. The overall equation is 6 CO2 + 6 H2O + sunlight --> C6H12O6 + 6 O2.
Q: Compare linear and cyclic electron flow.
A: Linear electron flow is the primary pathway, using both PSII and PSI. Electrons flow from water through PSII, the ETC, PSI, and finally reduce NADP+ to NADPH. It produces both ATP and NADPH. Cyclic electron flow uses only PSI: electrons cycle from ferredoxin back to the cytochrome complex and then back to PSI, generating only ATP with no NADPH and no O2 released.
Q: How does chemiosmosis in the chloroplast compare to chemiosmosis in the mitochondria?
A: Both use an electron transport chain to pump protons across a membrane, creating a concentration gradient that drives ATP synthase. In chloroplasts, light energy drives the process, protons accumulate in the thylakoid space, and ATP synthase releases ATP into the stroma for the Calvin cycle. In mitochondria, chemical energy from NADH and FADH2 drives the process, protons accumulate in the intermembrane space, and ATP synthase releases ATP into the matrix for cellular use.
Photosynthesis is the anabolic partner of cellular respiration (Ch. 9). The glucose produced here is the substrate broken down in respiration. Both processes share the chemiosmosis mechanism.
Chloroplast structure connects to membrane transport (Ch. 7) and the thylakoid membrane functions like the inner mitochondrial membrane. Rubisco and the Calvin cycle enzymes connect to the enzyme regulation concepts in Ch. 8.
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