Plant Physiology (Botany): water relations, photosynthesis, respiration, plant hormones, growth and movement

Key factors influencing normal seed germination

1. Water (imbibition): The seed must absorb water so the seed coat softens and the embryo cells become metabolically active. Water activates enzymes and hydrolyses stored food (starch, protein, fat) into soluble forms.
2. Temperature: An optimum temperature (usually 25–35 °C for most crop seeds) is needed for enzyme activity. Very low or very high temperatures slow or stop germination.
3. Oxygen (air): Germination is an aerobic process. Oxygen is needed for respiration to release energy for the growing embryo. Waterlogged or compact soil reduces oxygen and prevents germination.
4. Light: Most seeds germinate in dark or light, but some are light-sensitive. Light is essential only for certain seeds (e.g., lettuce); for most crop seeds it is not a limiting factor.
5. Viability and dormancy: Only viable (living) seeds with a healthy embryo can germinate. Some seeds have hard seed coats or internal dormancy that must be broken first.
6. Soil/medium conditions: Proper moisture, aeration and a suitable pH of the medium support normal germination.

Effect of exposing seeds to low temperature

Low temperature slows down enzyme activity and metabolic reactions, so germination is delayed or inhibited. However, in some seeds low temperature is beneficial: it provides stratification (chilling treatment), which breaks seed dormancy by overcoming the inhibitory effect of abscisic acid and promoting gibberellin activity. Thus, for dormant seeds of temperate plants (e.g., apple, peach), a period of low temperature is required for normal germination, while for non-dormant seeds it simply delays germination.

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to convert carbon dioxide and water into carbohydrates (glucose), releasing oxygen as a by-product. It occurs in the chloroplast and is summarised as:

$6CO_2 + 12H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2 + 6H_2O$

Photosynthesis has two phases: the light reaction (photochemical phase) in the thylakoid membranes, and the dark reaction (Calvin cycle) in the stroma. Non-cyclic electron flow (non-cyclic photophosphorylation) is the main light-driven electron transport of the light reaction.

Non-cyclic electron flow

It is called "non-cyclic" because the electron does not return to its original source; instead it is passed to NADP⁺, and the electron lost by chlorophyll is replaced by electrons obtained from the photolysis of water. It involves both photosystems (PS I and PS II) and produces ATP, NADPH and O₂.

Steps:

1. Excitation of PS II. Light of wavelength 680 nm is absorbed by the chlorophyll a molecules of PS II (P680). The electrons become excited and are raised to a higher energy level.

2. Electron transport from PS II to PS I. The excited electrons are picked up by the primary electron acceptor (pheophytin) and passed down an electron transport chain — pheophytin → plastoquinone (PQ) → cytochrome b₆f complex → plastocyanin (PC) — to PS I. As electrons move down this chain, energy is released and used to pump protons into the thylakoid lumen, creating a proton gradient.

3. Photolysis of water (Hill reaction). To replace the electrons lost by P680, water is split in the presence of light and Mn²⁺/Cl⁻ ions:

$2H_2O \xrightarrow{\text{light, PS II}} 4H^+ + 4e^- + O_2$

The electrons replace those lost by P680, the H⁺ ions accumulate in the lumen, and O₂ is released as a by-product. Thus water is the electron donor in non-cyclic flow.

4. Excitation of PS I. Light of wavelength 700 nm is absorbed by PS I (P700). Its electrons are excited and accepted by ferredoxin (Fd) via the primary acceptor.

5. Reduction of NADP⁺. The enzyme NADP reductase transfers electrons from ferredoxin to NADP⁺, which combines with H⁺ (from the lumen) to form NADPH:

$NADP^+ + 2e^- + H^+ \rightarrow NADPH$

6. Synthesis of ATP (chemiosmosis). The protons pumped into the lumen during electron transport create a proton gradient. These protons flow back to the stroma through the ATP synthase (CF₀–CF₁) channel, and the energy released drives the synthesis of ATP from ADP and inorganic phosphate. This is called non-cyclic photophosphorylation.

Net result: 2H₂O are split, giving 1 O₂, and the electron pair passing through the two photosystems yields about 2 ATP and 2 NADPH (per 2 electrons).

The ATP and NADPH formed are used in the stroma to reduce CO₂ to carbohydrate in the Calvin cycle.

Ascent of sap is the upward movement of water and dissolved minerals from the roots through the xylem to the leaves and other aerial parts of the plant, against gravity.

Cohesion-tension (transpiration pull) theory (Dixon and Joly)

This is the most widely accepted explanation of how water reaches the top of tall trees. It rests on three properties:

1. Transpiration pull (tension): Water evaporates from the mesophyll cells through the stomata. The mesophyll cells then have a lower water potential and draw water from neighbouring cells and finally from the xylem vessels of the leaf veins. This creates a strong negative pressure (tension) in the xylem that pulls the water column upward.
2. Cohesion: Water molecules attract each other by hydrogen bonds. This holds them together as a continuous, unbroken column from leaf to root, and gives it high tensile strength.
3. Adhesion: Water molecules also stick to the hydrophilic walls of the narrow xylem vessels and tracheids. This, together with capillarity, supports the column and stops it from breaking or slipping down.

Steps in the process

• Transpiration from the leaf lowers the water potential of the leaf cells.
• Water is pulled out of the xylem of the leaf, and the tension is transmitted down the continuous water column to the root.
• Water enters the root from the soil by osmosis, passes across the cortex to the xylem, and replaces the water lost.

Maximum uptake of water

Because the pull is generated at the leaf and passes through the unbroken water column, the plant takes up water at a rate equal to the transpiration rate. Uptake is greatest when the continuity of the water column is maintained, the xylem is narrow, many stomata are open, and humidity is low and the air is warm and moving. The root works passively, acting as an absorbing surface. In this way the cohesion-tension mechanism can raise water to the top of trees that are over 100 m tall.

(d) Plasmolysis

When a fresh plant cell is kept in a hypertonic solution, water leaves the protoplasm by exosmosis and the protoplasm shrinks away from the cell wall — this process is called plasmolysis.

(a) Reduced auxins allows lateral buds to grow

The apical bud produces auxin, which travels downward and suppresses the growth of lateral (axillary) buds — a phenomenon called apical dominance. Removing the apical bud removes this source of auxin, so the lateral buds are released from inhibition and grow out into branches.

Flow chart of the EMP pathway (glycolysis)

Glycolysis occurs in the cytoplasm and is divided into two phases: the preparatory (investment) phase and the pay-off (oxidative) phase.

A. Preparatory phase (glucose → 2 glyceraldehyde-3-phosphate)

Glucose (6C)
   │  ATP → ADP   (hexokinase)
   ▼
Glucose-6-phosphate (6C)
   │            (phosphoglucose isomerase)
   ▼
Fructose-6-phosphate (6C)
   │  ATP → ADP   (phosphofructokinase)
   ▼
Fructose-1,6-bisphosphate (6C)
   │            (aldolase)
   ▼
Dihydroxyacetone phosphate ⇌ Glyceraldehyde-3-phosphate
        (3C)                    (3C)

B. Pay-off phase (2 glyceraldehyde-3-phosphate → 2 pyruvate)

2 Glyceraldehyde-3-phosphate (3C)
   │  + 2 NAD⁺ → 2 NADH   (glyceraldehyde-3-phosphate dehydrogenase)
   ▼
2 1,3-bisphosphoglycerate (3C)
   │  2 ADP → 2 ATP   (phosphoglycerate kinase)
   ▼
2 3-phosphoglycerate (3C)
   │            (phosphoglycerate mutase)
   ▼
2 2-phosphoglycerate (3C)
   │            (enolase)
   ▼
2 Phosphoenolpyruvate (3C)
   │  2 ADP → 2 ATP   (pyruvate kinase)
   ▼
2 Pyruvate (3C)

Net gain per molecule of glucose

So, one molecule of glucose yields a net gain of 2 ATP and 2 NADH (plus 2 molecules of pyruvate) through the EMP pathway.

Dark Reaction (Blackman's Reaction) — the Calvin Cycle

The dark reaction (also called the Calvin cycle or C₃ cycle) takes place in the stroma of the chloroplast. It does not need light directly, but it uses the ATP and NADPH produced during the light reaction to fix CO₂ into sugar. It has three major steps:

1. Carboxylation — CO₂ is accepted by ribulose-1,5-bisphosphate (RuBP) with the help of the enzyme RuBisCO, forming an unstable 6-carbon compound that splits into two molecules of 3-phosphoglyceric acid (3-PGA).
2. Reduction — Each 3-PGA is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde-3-phosphate (G3P).
3. Regeneration — Most of the G3P is used to regenerate RuBP (with more ATP) so the cycle can continue; the rest is used to build glucose.

Flow chart of the Calvin cycle

                 CO₂
                  │
        ┌─────────┴──────────┐
        │  1. CARBOXYLATION  │
        │  RuBP + CO₂        │
        │  (RuBisCO enzyme)  │
        └─────────┬──────────┘
                  │
         2 × 3-PGA (3-phosphoglyceric acid)
                  │
        ┌─────────┴──────────┐
        │  2. REDUCTION      │
        │  ATP → ADP + Pi    │
        │  NADPH → NADP⁺     │
        └─────────┬──────────┘
                  │
         2 × G3P (glyceraldehyde-3-phosphate)
                  │
        ┌─────────┴──────────┐
        │  3. REGENERATION   │
        │  ATP → ADP + Pi    │
        │  G3P → RuBP        │
        └─────────┬──────────┘
                  │
              RuBP (reused)
                  │
        (6 turns → 1 glucose)

Explanation of the three steps

Step 1 — Carboxylation (fixation of CO₂): RuBP (a 5-carbon compound) combines with one molecule of CO₂. The enzyme RuBisCO catalyses this reaction. The product is an unstable 6-carbon compound which immediately breaks into two molecules of 3-PGA (3-carbon each). This is the actual fixation of carbon.

$\text{RuBP (C}_5\text{)} + \text{CO}_2 \rightarrow 2 \times \text{3-PGA (C}_3\text{)}$

Step 2 — Reduction: Each 3-PGA molecule is first phosphorylated by ATP to 1,3-bisphosphoglyceric acid, then reduced by NADPH (giving up H) to G3P. For every CO₂ fixed, 2 ATP and 2 NADPH are used here.

$\text{3-PGA} \xrightarrow{\text{ATP}} \text{1,3-bisphosphoglycerate} \xrightarrow{\text{NADPH}} \text{G3P}$

Step 3 — Regeneration of RuBP: Of the G3P formed, five out of every six molecules are used (using 1 ATP each) to rebuild RuBP, so the cycle keeps running. The remaining G3P molecules leave the cycle and are joined to make glucose.

Overall balance for one glucose (6 turns of the cycle):

$6\,\text{CO}_2 + 18\,\text{ATP} + 12\,\text{NADPH} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 18\,\text{ADP} + 18\,\text{P}_i + 12\,\text{NADP}^+$

Chemical equation of photosynthesis

$6\,\text{CO}_2 + 12\,\text{H}_2\text{O} \xrightarrow[\text{chlorophyll}]{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 + 6\,\text{H}_2\text{O}$

(For the dark reaction alone, the equation is simply: $6\,\text{CO}_2 + 12\,\text{NADPH} + 18\,\text{ATP} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 12\,\text{NADP}^+ + 18\,\text{ADP} + 18\,\text{P}_i$.)

(b) Stomata

Stomata are the small pores on the leaf epidermis whose opening and closing (by guard cells) controls the loss of water vapour, and hence regulates transpiration. Xylem conducts water, root hairs absorb it, and phloem translocates food — none of these regulate transpiration.

(b) The plant will rely on anaerobic respiration producing less ATP.

In a sealed jar the oxygen is used up and not replenished, so aerobic respiration (Krebs cycle and electron transport chain) cannot continue. The plant then carries out anaerobic respiration (fermentation), which yields only 2 ATP per glucose molecule instead of about 36–38 ATP in aerobic respiration, so energy production falls over time.

Diffusion vs osmosis (based on types of molecules)

• Diffusion involves the movement of any type of molecule or ion (e.g., O₂, CO₂, glucose, ions) from a region of higher concentration to a region of lower concentration, directly across the membrane or in the surrounding medium. It does not require a semipermeable membrane.
• Osmosis involves the movement of only solvent molecules, usually water, across a semipermeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration).

So the key difference is that diffusion can involve solute particles as well as gases, while osmosis is restricted to water (solvent) molecules moving through a selectively permeable membrane.

Environmental factors influencing the rate of diffusion

1. Concentration gradient — The steeper the difference in concentration between the two regions, the faster the rate of diffusion. A greater gradient drives more net movement.
2. Temperature — Higher temperature increases the kinetic energy of molecules, so they move faster and diffusion occurs more rapidly. Lower temperature slows it down.
3. Surface area — A larger surface area available for exchange (e.g., alveolar surface, root hair surface) increases the rate of diffusion, as more molecules can cross at a time.
4. Distance (thickness of the medium/membrane) — The shorter the distance over which diffusion must occur, the faster the rate. Thicker barriers slow diffusion.
5. Size and nature of molecules — Smaller and lighter molecules (e.g., CO₂) diffuse faster than larger, heavier ones (e.g., proteins).
6. Medium/density — Diffusion is faster in gases than in liquids because gas molecules are farther apart and move more freely.

Photosynthetic pigments and capture of light energy (2 marks)

Photosynthetic pigments are coloured compounds in the thylakoid membranes. Each absorbs different wavelengths, so together they capture a wide part of the solar spectrum.

• Accessory pigments absorb wavelengths that chlorophyll a absorbs poorly and pass the energy by resonance to chlorophyll a at the reaction centre.
• Hence the action spectrum of photosynthesis follows the combined absorption spectrum of all pigments.

Non-cyclic photophosphorylation (3 marks)

It involves both PS II and PS I. Electron flow is one-way, water is the electron donor, and ATP, NADPH and O₂ are produced.

1. Light excites P680 of PS II; the electron goes to the primary acceptor (pheophytin).
2. Photolysis of water replaces the lost electron: $2H_2O \xrightarrow{\text{light}} 4H^+ + 4e^- + O_2$
3. The electron passes through plastoquinone, cytochrome b₆f and plastocyanin to PS I. Protons are pumped into the thylakoid lumen, forming a proton gradient.
4. Light excites P700 of PS I; the electron goes to ferredoxin.
5. NADP reductase transfers it to NADP⁺: $NADP^+ + 2e^- + H^+ \rightarrow NADPH$
6. Protons flow back through ATP synthase (CF₀-CF₁) and ADP + Pi becomes ATP (chemiosmosis).

Cyclic photophosphorylation (included in the 3 marks)

It involves PS I only. The electron from P700 goes to ferredoxin, then back through cytochrome b₆f and plastocyanin to P700. Protons pumped during this return drive ATP synthase, so only ATP is made (no NADPH, no O₂). It supplies extra ATP when the Calvin cycle needs more ATP than NADPH.

Charts (3 marks)

Non-cyclic (Z-scheme):

H2O --> PS II (P680) --> Pheophytin --> PQ --> Cyt b6f --> PC --> PS I (P700) --> Fd --> NADP reductase --> NADPH
 |  (photolysis: O2 + H+ + e-)  light       |                  light
 |                                          +--> H+ pumped into lumen --> ATP synthase --> ATP

Cyclic:

        light
          |
   PS I (P700) --> Fd --> Cyt b6f --> PC --+
        ^                |                 |
        +----------------|-----------------+
                         +--> H+ gradient --> ATP synthase --> ATP

(c) Pyruvic acid

Glycolysis breaks one molecule of glucose into two molecules of pyruvic acid (pyruvate), along with 2 ATP and 2 NADH. Ethanol and lactic acid are formed later by fermentation, and acetyl CoA is formed when pyruvate enters the mitochondrion.

(c) Gibberellin

Gibberellin induces parthenocarpy (development of fruit without fertilisation) and also promotes cell elongation, so it gives larger seedless fruits — as commonly done in grapes.

Process of photosynthesis (1 mark)

Photosynthesis is the process by which green plants use light energy, chlorophyll, CO₂ and water to make carbohydrate and release oxygen:

$6CO_2 + 12H_2O \xrightarrow[\text{chlorophyll}]{\text{light}} C_6H_{12}O_6 + 6O_2 + 6H_2O$

It occurs in the chloroplast in two phases: the light reaction in the thylakoid membranes (light is trapped; ATP and NADPH₂ are made and water is split) and the dark reaction (Calvin cycle) in the stroma (CO₂ is reduced to sugar using ATP and NADPH₂).

Role of chlorophyll pigments (2 marks)

• Chlorophyll a is the primary pigment and the reaction-centre pigment (P680 in PS II, P700 in PS I). Only it can donate the excited electron to the electron transport chain. Absorption peaks are near 430 nm (blue) and 662 nm (red).
• Chlorophyll b and the carotenoids (carotene, xanthophyll) are accessory pigments. They absorb other wavelengths and pass the energy to chlorophyll a, so more of the spectrum is used. This helps plants capture more light when it is limiting, for example in shade.
• The Mg²⁺ porphyrin head absorbs photons and the phytol tail anchors the molecule in the thylakoid membrane. An absorbed photon raises an electron to a higher energy level, which starts the light reaction.

Non-cyclic electron transport: ATP and NADPH₂ (5 marks)

It involves both PS II and PS I. Electron flow is one-way, with water as the electron donor, and it gives ATP, NADPH₂ and O₂.

1. Excitation of PS II: Light of about 680 nm excites P680; the electron is taken up by the primary acceptor (pheophytin).
2. Photolysis of water: The lost electrons are replaced from water, and O₂ is released: $2H_2O \xrightarrow{\text{light, PS II}} 4H^+ + 4e^- + O_2$
3. Transport to PS I: The electron passes along pheophytin → plastoquinone → cytochrome b₆f → plastocyanin. Protons are pumped into the lumen, creating a proton gradient.
4. Excitation of PS I: Light of about 700 nm excites P700; its electron goes to ferredoxin.
5. Formation of NADPH₂: NADP reductase passes the electrons to NADP⁺: $NADP^+ + 2e^- + H^+ \rightarrow NADPH_2$
6. Formation of ATP: Protons flow back through ATP synthase (CF₀-CF₁), making ATP from ADP and Pi (photophosphorylation).

Support to the dark reaction: In the stroma, NADPH₂ reduces 3-PGA to glyceraldehyde-3-phosphate, and ATP phosphorylates 3-PGA and regenerates RuBP, so CO₂ is fixed into carbohydrate.

The light-independent reactions (also called the Calvin cycle or C₃ cycle) occur in the stroma of the chloroplast. They do not require light directly, but they depend on the ATP and NADPH produced by the light-dependent reactions. The cycle fixes CO₂ into carbohydrate through three phases: carboxylation, reduction, and regeneration.

1. Carboxylation (CO₂ fixation) CO₂ from the atmosphere enters the stroma and combines with a 5-carbon acceptor molecule, ribulose-1,5-bisphosphate (RuBP). This reaction is catalysed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), the most abundant enzyme on Earth. The unstable 6-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA), each with 3 carbon atoms. This is the actual step where inorganic carbon becomes organic.

2. Reduction Each molecule of 3-PGA is phosphorylated by ATP to form 1,3-bisphosphoglycerate, and then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P). For every CO₂ fixed, 2 ATP and 2 NADPH are consumed in this phase. G3P is a 3-carbon sugar phosphate.

3. Regeneration of RuBP Most of the G3P (5 out of every 6 molecules) is used to regenerate RuBP through a series of reactions so that the cycle can continue. This regeneration step requires 1 ATP per CO₂ fixed. The remaining 1 molecule of G3P is the net product.

Overall balance To produce one molecule of glucose (6C), the cycle must turn 6 times: $6\,\text{CO}_2 + 18\,\text{ATP} + 12\,\text{NADPH} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 18\,\text{ADP} + 18\,\text{P}_i + 12\,\text{NADP}^+$

---

Utilization of ATP and NADPH (3 marks)

The ATP and NADPH formed in the light-dependent reactions are used in the Calvin cycle as follows:

• NADPH acts as the reducing agent. It donates hydrogen (electrons) to reduce 3-PGA into G3P during the reduction phase. Without NADPH, the fixed carbon cannot be reduced to sugar.
• ATP provides chemical energy and phosphate groups. It is used in two places:
  • In the reduction phase, ATP phosphorylates 3-PGA to 1,3-bisphosphoglycerate, making it reactive enough to accept hydrogen from NADPH.
  • In the regeneration phase, ATP is used to convert G3P back into RuBP, allowing the cycle to continue.
• Thus, ATP supplies energy while NADPH supplies reducing power. Together they drive the endergonic conversion of CO₂ into glucose. Once used, they form ADP, Pi, and NADP⁺, which return to the thylakoid membrane to be recharged by the light-dependent reactions.

In short, the Calvin cycle is the biosynthetic (dark) phase of photosynthesis, where ATP and NADPH from the light reactions drive the reduction of CO₂ into glucose.

(c) Leaves

The transpiration pull generated in the leaves (due to water loss from stomata) is the major force that pulls water up the xylem.

Type of germination: Epigeal germination (the cotyledons are lifted above the soil surface).

Mechanism:

1. Imbibition and swelling: The seed absorbs water, swells, and the seed coat ruptures. Enzymes become active and digest the stored food.

2. Emergence of radicle: The radicle emerges first and grows downward into the soil, anchoring the seed and absorbing water and minerals.

3. Rapid elongation of the hypocotyl: The hypocotyl (the region between the radicle and the cotyledons) elongates rapidly and grows upward. It is curved in the form of an arch (the hypocotyl arch), which protects the delicate plumule tip from injury as it pushes through the soil.

4. Lifting of cotyledons: As the hypocotyl elongates, it carries the cotyledons above the soil surface. The cotyledons thus become aerial and turn green (in many plants) to carry out photosynthesis for a short period.

5. Straightening and plumule growth: Once above the ground, the hypocotyl straightens, the seed coat is shed, and the plumule (embryonic shoot) develops into the shoot with the first foliage leaves.

6. Food supply: The cotyledons supply stored food to the growing seedling until the young plant becomes green and self-sufficient, after which they shrivel and fall off.

Examples: Bean (Phaseolus), castor, papaya, onion, tomato.

(b) Gibberellins

Gibberellins stimulate cell elongation and division in the subapical meristem of the stem, which lengthens the internodes (for example, they reverse dwarfism and cause bolting in rosette plants). IAA mainly promotes elongation in coleoptiles and apical dominance, ABA inhibits growth, and cytokinins promote cell division.

(a) Alters solute equilibrium, promoting water withdrawal from the cell

Plasmolysis occurs when a cell is placed in a hypertonic solution. The outside solute concentration is higher than inside, so water leaves the cell by exosmosis and the protoplasm shrinks away from the cell wall. Options (b) and (c) describe water entering or being trapped, which is the opposite of what happens.

Differences between aerobic and anaerobic respiration

Krebs cycle (citric acid cycle)

It takes place in the mitochondrial matrix. Pyruvate from glycolysis is converted to two-carbon acetyl CoA (oxidative decarboxylation), which enters the cycle.

1. Condensation: acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
2. Citrate is isomerised to isocitrate (6C).
3. Isocitrate is oxidised and decarboxylated to $\alpha$-ketoglutarate (5C), releasing $CO_2$ and forming NADH.
4. $\alpha$-Ketoglutarate is oxidised and decarboxylated to succinyl CoA (4C), releasing $CO_2$ and forming NADH.
5. Succinyl CoA is converted to succinate, forming one GTP (ATP) by substrate-level phosphorylation.
6. Succinate is oxidised to fumarate, forming $FADH_2$.
7. Fumarate is hydrated to malate.
8. Malate is oxidised to oxaloacetate, forming NADH, and the cycle is ready to start again.

Per acetyl CoA: 3 NADH, 1 $FADH_2$, 1 ATP (GTP) and 2 $CO_2$. For one glucose (2 turns): 6 NADH, 2 $FADH_2$, 2 ATP and 4 $CO_2$.

Diagram: draw a circle with oxaloacetate at the top, acetyl CoA entering, and the intermediates citrate, isocitrate, $\alpha$-ketoglutarate, succinyl CoA, succinate, fumarate and malate arranged clockwise, with arrows showing where $CO_2$, NADH, $FADH_2$ and ATP are produced.

(c) Movement of water from cell A to B

Water moves from the cell with the lower diffusion pressure deficit (DPD, higher water potential) to the cell with the higher DPD. Cell A has a DPD of 5 atm and cell B has 7 atm, so water moves from A to B until the two are in equilibrium.

(b) Hypertonic solution

The cell wall is freely permeable, so the external hypertonic solution passes through it. When the protoplast shrinks away from the wall by exosmosis, the gap is filled by this external hypertonic solution.

Plant growth hormones: organic chemicals produced in small amounts in one part of the plant that are transported to other parts, where they promote, inhibit or modify growth and development. The main ones are auxins, gibberellins, cytokinins, abscisic acid and ethylene.

Physiological functions of auxin (e.g. IAA)

• Promotes cell elongation, particularly in stems and coleoptiles.
• Causes apical dominance (the apical bud suppresses lateral buds).
• Stimulates root formation in cuttings and fruit setting and growth (parthenocarpy).
• Causes phototropism and geotropism responses.
• Delays leaf and fruit abscission, and promotes cambial activity.
• Used as selective weed killer (2,4-D).

Shortcomings of excessive use in crops

• High concentration inhibits growth instead of promoting it and can kill the plant (2,4-D is a herbicide).
• Causes abnormal growth, deformed leaves and fruits, and may harm non-target broad-leaved plants.
• Residues may contaminate soil and water and affect the environment.

Link between glycolysis and the Krebs cycle (4 marks)

Glycolysis occurs in the cytoplasm and breaks one glucose (6C) into two molecules of pyruvate (3C), with a net gain of 2 ATP and 2 NADH. Under aerobic conditions pyruvate enters the mitochondrial matrix through its membrane. There the link reaction (oxidative decarboxylation), catalysed by the pyruvate dehydrogenase complex, converts each pyruvate to acetyl CoA (2C), releasing one $CO_2$ and forming one NADH:

$\text{Pyruvate} + \text{CoA} + NAD^+ \rightarrow \text{Acetyl CoA} + CO_2 + NADH$

Acetyl CoA then enters the Krebs cycle by combining with oxaloacetate. Thus pyruvate is the connecting product, and acetyl CoA is the entry point.

Flow chart of the Krebs cycle (4 marks)

Acetyl CoA (2C) + oxaloacetate (4C), then:

1. Citrate (6C)
2. Isocitrate (6C)
3. $\alpha$-Ketoglutarate (5C): $CO_2$ released, NADH formed
4. Succinyl CoA (4C): $CO_2$ released, NADH formed
5. Succinate (4C): ATP (GTP) formed
6. Fumarate (4C): $FADH_2$ formed
7. Malate (4C)
8. Oxaloacetate (4C): NADH formed, and the cycle repeats.

Draw these as a circle with arrows, showing acetyl CoA entering at the top.

Per acetyl CoA: 3 NADH, 1 $FADH_2$, 1 ATP and 2 $CO_2$ (double for one glucose).

(a) Root

Transpiration is the loss of water vapour from the aerial parts of a plant: through stomata of leaves, lenticels of stems and cuticle. Roots are underground and absorb water, so they do not transpire.

(b) Maize

Maize is a $C_4$ plant. Its leaf shows Kranz anatomy, in which bundle sheath cells containing large chloroplasts form a wreath around the vascular bundles and mesophyll cells surround them. Wheat, barley and oats are $C_3$ plants.

Pathway: Glycolysis (EMP pathway), the conversion of one glucose molecule into two molecules of pyruvic acid.

Site: The cytoplasm (cytosol) of the cell. It does not need oxygen.

Steps of the pathway

Preparatory (energy investment) phase

1. Glucose (6C) is phosphorylated by hexokinase, using 1 ATP, to give glucose-6-phosphate.
2. Glucose-6-phosphate is isomerized by phosphohexose isomerase to fructose-6-phosphate.
3. Phosphofructokinase adds a second phosphate using 1 ATP, forming fructose-1,6-bisphosphate.
4. Aldolase splits it into two 3C sugars, dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). DHAP is converted to G3P by triose phosphate isomerase, so two G3P are available.

Pay-off (energy generation) phase, occurring twice per glucose

5. G3P is oxidized and phosphorylated by G3P dehydrogenase with inorganic phosphate, forming 1,3-bisphosphoglycerate and reducing $NAD^+$ to NADH (2 NADH in total).
6. 1,3-bisphosphoglycerate gives its phosphate to ADP by phosphoglycerate kinase, forming 3-phosphoglycerate and ATP (substrate-level phosphorylation).
7. 3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglyceromutase.
8. Enolase removes water, forming phosphoenolpyruvate (PEP).
9. PEP gives its phosphate to ADP by pyruvate kinase, forming pyruvic acid and ATP.

Net gain of ATP

ATP used $= 2$; ATP produced $= 2\times 2 = 4$.

$\text{Net ATP} = 4 - 2 = 2 \text{ ATP}$

along with 2 NADH and 2 pyruvate per glucose.