Chapter Summary and Practice

Photosynthesis connects sunlight with food and atmospheric oxygen

Photosynthesis uses light energy to build carbon compounds, chiefly from CO₂ and water. Carbon assimilation provides material for plant growth and storage and supports terrestrial and aquatic food systems. In oxygenic photosynthesis, water supplies electrons and the oxygen gas released to the environment comes from water rather than from direct cleavage of CO₂. The net chemical summary is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ under illumination, while a fuller representation with 12 H₂O on the left and 6 H₂O on the right more explicitly tracks water's contribution.

Early observations developed into a stronger mechanistic explanation. Controlled starch tests on variegated or shaded leaves support the roles of chlorophyll-bearing tissue and light, while KOH absorption of CO₂ tests carbon supply. Priestley studied plant effects on enclosed air, Ingenhousz linked oxygen evolution to illuminated green tissue, Sachs linked photosynthetic sugar production to green chloroplast-containing tissue, and van Niel compared different hydrogen donors to infer oxygen's origin. Engelmann's oxygen-sensitive bacteria collected mainly in blue and red light, supporting the first action-spectrum interpretation.

Chloroplast structure predicts the two reaction domains

Photosynthesis takes place primarily in leaf mesophyll chloroplasts and other green tissues. The thylakoid membrane system, including grana and stroma lamellae, contains pigments, photosystems, the electron transport system and ATP synthase. The stromal compartment contains enzymes of carbon fixation. Water splitting and oxygen release are associated with photosystem II. The light reactions make ATP and NADPH, while the Calvin cycle uses these chemical resources to reduce fixed carbon to triose-phosphate compounds, which can feed sucrose, starch and other biosynthesis.

An absorption spectrum describes how strongly a pigment absorbs wavelengths, whereas an action spectrum describes effectiveness in photosynthesis. Chlorophyll a is the key reaction-centre pigment; chlorophyll b, xanthophylls and carotenes broaden light absorption and help protect against excess excitation. Action and absorption spectra have strong activity in blue and red regions but do not exactly coincide, because many pigments and other cellular factors contribute to the whole process.

The light reactions have two related outputs

The photosystems operate in order PS II then PS I for usual linear electron flow, despite their historical numbering. PS II has the P680 reaction centre and its oxygen-evolving complex uses water as an electron source. PS I has the P700 reaction centre and passes excited electrons to acceptors culminating in NADPH production. Each photosystem absorbs light, so electrons undergo two energy-raising photochemical events. The Z-scheme shows those energy changes, not the physical shape of thylakoid membranes.

Water oxidation follows 2H₂O → O₂ + 4H⁺ + 4e⁻. In linear flow, electrons proceed through PS II and intersystem carriers to PS I and then NADP⁺, while associated proton transfer establishes a gradient. In cyclic PS I flow electrons are returned to PS I through carriers, allowing additional ATP production without net NADPH or O₂ generation. Distinguish this cyclic electron return from the Calvin cycle's repeated regeneration of the five-carbon CO₂ acceptor.

The proton gradient powers chloroplast ATP synthesis

H⁺ accumulates towards the thylakoid lumen because PS II water oxidation releases protons there and electron transport transfers additional protons across the membrane. NADP⁺ reduction also uses protons on the stroma side. H⁺ flows down its electrochemical gradient from lumen to stroma through the CF₀ component of ATP synthase. The stromal CF₁ region catalyses ATP formation from ADP and Pi. This is chemiosmosis: ATP production depends on an intact membrane, a proton-motive force and ATP synthase.

The Calvin cycle regenerates its carbon acceptor

The Calvin cycle proceeds through carboxylation, reduction and regeneration. RuBisCO adds CO₂ to five-carbon RuBP, yielding two molecules of three-carbon 3-PGA after the intermediate splits. ATP and NADPH support reduction to triose phosphate. Further ATP is used to regenerate RuBP, allowing the cycle to continue. The net conventional cost is three ATP and two NADPH per CO₂ fixed into carbohydrate precursors. For three CO₂ this means nine ATP and six NADPH for one net triose phosphate; for six CO₂, eighteen ATP and twelve NADPH for one net hexose equivalent.

C₃ and C₄ pathways share a destination but differ in delivery

C₃ leaves usually carry out direct RuBisCO fixation and Calvin-cycle reactions in mesophyll chloroplasts. The first stable product is three-carbon 3-PGA. A typical C₄ leaf has two cooperating photosynthetic cell types: PEP carboxylase initially captures inorganic carbon with three-carbon PEP in mesophyll, forming four-carbon OAA, which is converted to transport forms such as malate or aspartate. These reach bundle-sheath cells, where CO₂ is released next to RuBisCO for the Calvin cycle. A three-carbon compound returns and is used to regenerate PEP at an energy cost.

Kranz anatomy, visible in many maize and sorghum leaves, shows a conspicuous ring of bundle-sheath cells and surrounding mesophyll. This helps establish the spatial separation central to the C₄ carbon-concentrating pathway. It does not mean only a small amount of carbon can be fixed: CO₂ is captured over the mesophyll area and delivered to the high-CO₂ bundle sheath. As a result, RuBisCO operates largely as a carboxylase, reducing losses due to photorespiration, particularly under high light and high temperature.

Photorespiration reveals competition at RuBisCO

RuBisCO can add O₂ rather than CO₂ to RuBP. Oxygenation produces one 3-PGA and one two-carbon 2-phosphoglycolate; subsequent recovery through chloroplast, peroxisome and mitochondrion consumes resources and releases some CO₂. Net carbohydrate synthesis is therefore less efficient when oxygenation is frequent. Low internal CO₂ relative to oxygen, heat and stomatal closure can increase the contribution of this reaction in C₃ leaves. Typical C₄ plants suppress the oxygenase route by concentrating CO₂, although 'absolutely no photorespiration under every possible condition' is too strong a universal claim.

Light, CO₂, temperature and water jointly determine rate

Blackman's law of limiting factors states that adding more of one resource may have little effect if another requirement is insufficient. With low light, a response curve usually rises as intensity increases; after saturation it reaches a plateau because another factor limits the rate. Severe light excess can cause photoinhibition. Increasing CO₂ often stimulates C₃ plants when light and other resources are suitable. Temperature controls many enzyme reactions and has a species-dependent optimum, while water stress often closes stomata and lowers internal CO₂.

An experiment comparing shade and sun leaves should record leaf age, size and pigment appearance rather than assuming a universal colour difference. When a leaf is kept dark for an extended period, chlorophyll may decline and the leaf may appear pale or yellow as other pigments become relatively more visible. Carotenoid pigments may persist longer under some such conditions, but both pigment quantity and leaf development determine the outcome. These are opportunities to infer from evidence rather than memorising a single unconditional answer.

A method for solving diagram and application problems

For a light-reaction diagram, begin by locating the thylakoid lumen and stroma, then identify the two reaction centres and the direction of electron flow. For a Calvin-cycle diagram, identify the five-carbon acceptor, three-carbon products and locations of ATP and NADPH use. For a C₃/C₄ comparison, specify the first acceptor, first stable product, fixing enzyme and cell location before describing photorespiration. For limiting-factor curves, interpret the slope and plateau rather than guessing values from the axis labels.

Mixed practice: diagrams, reasoning and quantitative accounting

Work through the questions using the full mechanism rather than relying on a one-word definition. Some tasks reconstruct the source experiments; others require balanced equations, carbon counting, identifying a limiting factor or distinguishing C₃ and C₄ leaf anatomy. Model answers beneath the practice section show the key causal points and clarify where observations do not establish a conclusion on their own.

Quick Quiz

Model Answers

1. Generally no. Both can have similar external growth forms. Stronger evidence comes from internal Kranz organisation and biochemical characters: initial acceptor, enzyme, first stable product, and locations of the Calvin cycle.

2. Typical Kranz anatomy: vascular tissue is surrounded by large chloroplast-rich bundle-sheath cells, themselves enclosed by mesophyll. This supports compartmentalised initial PEPcase fixation and Calvin-cycle activity.

3. A broad population of mesophyll cells first captures inorganic carbon and transports it as four-carbon compounds. CO₂ released in bundle sheath concentrates substrate around RuBisCO and suppresses photorespiratory losses.

4. RuBisCO's catalytic site can react RuBP with CO₂ or O₂. C₄ transport releases CO₂ near bundle-sheath RuBisCO, shifting competition toward carboxylation despite presence of oxygen.

5. No, the ordinary PS II and PS I reaction centres rely on specialised chlorophyll a. Chlorophyll b serves as an antenna accessory pigment that broadens absorption and transfers excitation to reaction-centre systems.

6. Chlorophyll abundance may decline in prolonged darkness and chloroplast development can change. More persistent yellow-orange carotenoid pigments may remain relatively visible, though the exact outcome depends on species and conditions.

7. Shade leaves often possess pigment and anatomical acclimations that improve weak-light capture, sometimes making them appear darker green. This is not universal; use matched leaves and account for age, chlorophyll content and species.

8. A lies in the rising, light-limited region. B marks near saturation and C the plateau, where another factor limits. D projects the approximate saturation intensity to the x-axis, while E marks the associated plateau rate on the y-axis.

9. C₃: RuBP (5C), 3-PGA (3C), RuBisCO, mesophyll Calvin cycle, potentially appreciable photorespiration. C₄: PEP (3C), OAA (4C), PEPcase in mesophyll, Kranz bundle-sheath Calvin cycle, greatly suppressed photorespiration.

10. Cyclic: primarily PS I, electrons return through carriers, yields ATP without net NADPH or O₂. Non-cyclic: PS II plus PS I, electrons from water to NADP⁺, synthesises NADPH and ATP and releases O₂.

11. C₃ mesophyll chloroplasts carry out the Calvin cycle without a distinct Kranz photosynthetic ring. Typical C₄ leaves have chloroplast-rich bundle-sheath cells surrounded by mesophyll; enzymes are separated between the two.

12. Show electrons from PS II through the carrier chain to PS I then NADP⁺. Light absorption at P680 and P700 creates two distinct energy rises, allowing water oxidation and later NADP⁺ reduction.

13. Water splitting and electron transport accumulate H⁺ in thylakoid lumen. H⁺ returns to stroma through CF₀, driving conformational/rotational changes in stroma-facing CF₁ that catalyse ATP formation from ADP and Pi.

14. Three CO₂ require 9 ATP and 6 NADPH, yielding one net triose-phosphate equivalent. Six CO₂ require 18 ATP and 12 NADPH, yielding two net triose equivalents that can form one hexose equivalent.

15. Its enzymes do not use photons directly, but they require ATP and NADPH from light reactions and commonly light-activated regulation. It normally operates while illuminated leaf chloroplasts supply those resources.

16. Destarch a plant, enclose part of a leaf in a sealed illuminated chamber with KOH, leave a matched region with CO₂ supply, then test for new starch. Compare the results while controlling light, time, tissue health and enclosure leaks.

17. The rate is constrained by the factor nearest insufficiency; extra light will not greatly increase photosynthesis while temperature keeps enzymatic reactions below the useful rate. Warming toward the optimum can then raise photosynthesis.

18. Photorespiration begins with RuBisCO oxygenation and involves recovery of 2-phosphoglycolate across organelles with CO₂ and energy costs. Mitochondrial respiration oxidises fuels to transfer energy to ATP and supports many cellular processes regardless of the RuBisCO branch.