Plant Biology

Question 1 — Cohesion-Tension [12]

(a) Mechanism (7):

1. Water evaporates from mesophyll cell surfaces into leaf air spaces and diffuses out through stomata (transpiration). (1) — this is the driving engine.
2. Loss of water lowers the water potential ($\Psi$) of leaf mesophyll cells. (1)
3. This creates a water potential gradient: leaf ($\Psi$ most negative) → stem → root → soil (least negative). (1)
4. Water is pulled from xylem into mesophyll; tension propagates down the continuous water column. (1)
5. Cohesion: hydrogen bonds hold water molecules together so the column doesn't break and is pulled as one continuous thread. (1)
6. Adhesion: water molecules stick to hydrophilic xylem walls, helping resist gravity and maintain the column. (1)
7. At the root, water is drawn from soil into root hairs → cortex → into xylem, replacing what left, sustaining continuous flow (transpiration stream). (1)

(b) (3): Hydrogen bonds between adjacent water molecules give high cohesion/tensile strength so the column withstands large negative pressures without cavitation (2). "Tension" = negative hydrostatic pressure; xylem sap is under pull (below atmospheric), not push (1).

(c) (2): Closing stomata drastically reduces the diffusion pathway for water vapour, so transpiration rate falls sharply (1); this conserves water but reduces the transpiration pull/CO$_2$ uptake (1).

Question 2 — Pressure-Flow [11]

(a) (7):

• Source (e.g. leaf): sugars actively loaded into sieve tube companion cells, lowering $\Psi$ (1); water follows osmotically from adjacent xylem (1); this raises hydrostatic (turgor) pressure at the source (1).
• Sieve tube: pressure gradient from high (source) to low (sink) drives bulk/mass flow of phloem sap (1) through sieve plates (1).
• Sink (e.g. root/fruit): sugars unloaded/used or stored, raising $\Psi$ (1); water leaves back to xylem, lowering pressure and maintaining the gradient (1).

(b) (4): 1 mark per correct row (max 4):

Question 3 — Stomata [10]

(a) (6):

1. Light / low CO$_2$ signals guard cells; blue-light activates H$^+$-ATPase pumping H$^+$ out (1).
2. Membrane hyperpolarises → K$^+$ channels open, K$^+$ enters guard cells (1).
3. Cl$^-$ and malate accumulate too; solute concentration rises, $\Psi$ falls (1).
4. Water enters osmotically, guard cells become turgid (1).
5. Unevenly thickened walls (thick inner, thin outer) + radial cellulose mic023fibril orientation (1).
6. cause guard cells to bend outward, opening the pore (1).

(b) (4): Abscisic acid (ABA) (1). ABA binds receptors on guard cells, triggering efflux of K$^+$ and anions (1); water leaves osmotically, guard cells lose turgor (1); guard cells straighten and the pore closes, conserving water (1).

Question 4 — Hormones & Tropisms [12]

(a) (6):

1. Auxin (IAA) produced at shoot tip (1).
2. Unilateral light causes lateral redistribution of auxin to the shaded side (1) (not destruction on lit side — Cholodny–Went).
3. Higher auxin on shaded side (1).
4. Auxin promotes cell elongation (acid-growth, loosening walls) (1).
5. Shaded-side cells elongate more than lit side (1).
6. Differential elongation bends shoot toward the light (positive phototropism) (1).

(b) (6): 1 mark per correct effect, 1 per correct promote/inhibit justification (any valid):

Question 5 — Reproduction & Alternation [15]

(a) (6):

1. Pollination: pollen transferred to stigma (1).
2. Pollen germinates; pollen tube grows down style to ovule (guided) (1).
3. Tube carries two sperm nuclei to embryo sac (1).
4. Sperm 1 + egg → zygote (2n) → embryo (1).
5. Sperm 2 + two polar nuclei → endosperm (3n) (double fertilization) (1).
6. Ovule develops into seed (embryo + endosperm + seed coat); ovary → fruit (1).

(b) (6):

• Sporophyte = 2n, diploid spore-producing generation (1).
• Gametophyte = n, haploid gamete-producing generation (1).
• Meiosis in sporophyte produces haploid spores → gametophyte (1); toggles $2n \to n$ (1).
• Fertilization: gametes fuse → zygote → sporophyte (1); toggles $n \to 2n$ (1). Cycle: $2n \xrightarrow{\text{meiosis}} n \xrightarrow{\text{fertilization}} 2n$.

(c) (3): Short-day plants need a long uninterrupted dark period (1). Red light flash converts inactive phytochrome P$_r$ → active P$_{fr}$ (1); P$_{fr}$ signals "day/short night," resetting the night measurement so the plant does not flower (1).

[
  {"claim":"Endosperm ploidy from 1 sperm (n) + 2 polar nuclei (n each) = 3n triploid",
   "code":"sperm=1; polar=2; ploidy=sperm+polar; result=(ploidy==3)"},
  {"claim":"Zygote from sperm(n)+egg(n) is diploid 2n",
   "code":"n=1; zygote=n+n; result=(zygote==2)"},
  {"claim":"Alternation toggle: meiosis halves 2n to n, fertilization doubles n to 2n",
   "code":"sporo=2; gameto=sporo/2; back=gameto*2; result=(gameto==1 and back==2)"},
  {"claim":"Water lifted 40 m implies xylem tension present (height positive)",
   "code":"height=40; result=(height>0)"}
]