FYUG Even Semester Exam, 2025
BOTANY: Plant Physiology (BOTDSM-252)

Semester: 4th Semester | Paper Code: BOTDSM-252
Full Marks: 70 | Pass Marks: 28 | Time: 3 Hours

UNIT-I

Question 1(a): Differentiate between apoplastic and symplastic pathways. 2 Marks

Question 1(b): Write two factors that affect transpiration. 2 Marks

  1. Light: Increases transpiration by causing stomata to open and increasing leaf temperature.
  2. Humidity: High atmospheric humidity reduces the diffusion gradient, thereby decreasing the rate of transpiration.

Question 1(c): What do you mean by water potential? 2 Marks

Water potential (Ψw) is the measure of the free energy of water in a system. It determines the direction of water movement; water always moves from a region of higher water potential to a region of lower water potential.

Question 2(a): Discuss the mechanism of opening and closing of stomata with suitable model. 10 Marks

The most widely accepted model is the K+ Ion Transport Mechanism (Levitt's Model).

Stomatal Opening (Daytime):

  • Photosynthesis occurs in guard cells, leading to a decrease in CO2 concentration.
  • Malic acid is formed, which dissociates into malate ions and protons (H+).
  • Protons are actively pumped out of guard cells, and Potassium ions (K+) are actively taken in from subsidiary cells.
  • This increases the osmotic concentration of guard cells, causing water to enter (endosmosis).
  • Guard cells become turgid, their thin outer walls stretch, and the thick inner walls pull apart, opening the pore.

Stomatal Closing (Nighttime):

  • Photosynthesis stops, CO2 increases, and the plant hormone Abscisic Acid (ABA) may be produced.
  • K+ ions leak out of the guard cells back into subsidiary cells.
  • Osmotic potential decreases, water leaves the guard cells (exosmosis), and they become flaccid, closing the pore.

Question 2(b): Discuss the mechanism of transport of water by plants. 10 Marks

The transport of water from roots to leaves (ascent of sap) is explained by the Cohesion-Tension Theory.

  • Transpiration Pull: Evaporation of water from leaves creates a negative pressure (tension) in the xylem.
  • Cohesion: Water molecules stick to each other due to hydrogen bonding, forming a continuous water column.
  • Adhesion: Water molecules stick to the walls of xylem vessels.
  • Root Pressure: Active transport of minerals into roots creates an osmotic gradient that pushes water upward to a limited height.

UNIT-II

Question 3(a): Differentiate between active and passive transports. 2 Marks

Feature Passive Transport Active Transport
Energy (ATP) Not Required Required
Direction Along concentration gradient Against concentration gradient

Question 3(b): Write the composition of phloem sap. 2 Marks

Phloem sap is primarily composed of sucrose (the main transport sugar), water, amino acids, minerals, and various plant hormones.

Question 3(c): Write the role of Mg in plant. 2 Marks

  • It is the central atom of the chlorophyll molecule, essential for photosynthesis.
  • It acts as an activator for various enzymes, especially those involved in carbohydrate metabolism and ATP synthesis.

Question 4(a): With the help of suitable diagram, describe the mechanism of phloem loading and unloading in plants. 10 Marks

The movement of food in phloem occurs via the Pressure Flow Hypothesis (Munch's hypothesis).

  • Phloem Loading: At the source (leaf), sucrose is actively transported into the sieve tubes. This lowers the water potential, causing water to enter from the xylem, creating high turgor pressure.
  • Translocation: The mass flow of sap moves from high pressure (source) to low pressure (sink).
  • Phloem Unloading: At the sink (root/fruit), sucrose is actively or passively removed from the sieve tubes. Water then follows out back to the xylem, reducing turgor pressure.

Question 4(b): Write the physiological roles and deficiency symptoms of: (i) Mn (ii) Zn (iii) N (iv) P 10 Marks

  • Nitrogen (N): Role—Constituent of proteins and nucleic acids; Symptom—General chlorosis in older leaves.
  • Phosphorus (P): Role—Energy transfer (ATP) and cell membranes; Symptom—Purple or dark green coloration of leaves.
  • Manganese (Mn): Role—Water splitting in photosynthesis; Symptom—Interveinal chlorosis with grey spots.
  • Zinc (Zn): Role—Auxin biosynthesis; Symptom—Little leaf disease and malformed leaves.

UNIT-III

Question 5(a): What do you mean by oxidative phosphorylation? Where it occurs? 2 Marks

Oxidative phosphorylation is the process of ATP synthesis using energy derived from the Electron Transport Chain (ETC). It occurs in the inner mitochondrial membrane.

Question 5(b): What is CAM? Give one example. 2 Marks

Crassulacean Acid Metabolism (CAM) is a carbon fixation pathway where plants open stomata at night to fix CO2 into organic acids and keep them closed during the day to conserve water. Example: Pineapple or Cacti.

Question 5(c): What is red drop? 2 Marks

Red drop is the sharp decline in the quantum yield of photosynthesis when plants are exposed to monochromatic light of wavelength greater than 680 nm (far-red light).

Question 6(a): What do you mean by photorespiration? Discuss the mechanism of photorespiration. 10 Marks

Photorespiration (C2 Cycle) is a process where the enzyme RuBisCO binds with oxygen instead of CO2, leading to a loss of fixed carbon.

Mechanism (The Triple Organelle Pathway):

  1. Chloroplast: RuBP reacts with O2 to form one molecule of 3-PGA and one molecule of Phosphoglycolate.
  2. Peroxisome: Glycolate is converted to Glyoxylate and then to the amino acid Glycine.
  3. Mitochondrion: Two molecules of Glycine react to form one molecule of Serine, releasing CO2 and NH3.

Question 6(b): Write notes on: (i) Pentose phosphate pathway (ii) Glycolysis 5+5=10 Marks

  • Glycolysis: The metabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate in the cytosol, yielding a net 2 ATP and 2 NADH.
  • Pentose Phosphate Pathway (PPP): An alternative to glycolysis that generates NADPH for biosynthesis and pentose sugars (ribose-5-phosphate) for nucleic acid synthesis.

UNIT-IV

Question 7(a): What are the main groups of plant hormones? 2 Marks

The five major groups are: Auxins, Gibberellins, Cytokinins, Abscisic Acid (ABA), and Ethylene.

Question 7(b): Write two functions of Abscisic acid. 2 Marks

  1. Induces stomatol closure during water stress (Stress Hormone).
  2. Promotes seed dormancy and inhibits precocious germination.

Question 7(c): Name one hormone helps in cell division. 2 Marks

Cytokinin.

Question 8(a): Discuss physiological roles of auxin and Gibberellin in plants. 10 Marks

  • Auxin: Promotes cell elongation, apical dominance, root initiation in cuttings, and prevents abscission of young leaves.
  • Gibberellin: Promotes internodal elongation (bolting), breaks seed dormancy, induces parthenocarpy, and promotes malting in the brewing industry.

Question 8(b): Write short notes on: (i) Role of ethylene in plants (ii) Cytokinin 5+5=10 Marks

  • Ethylene: A gaseous hormone primarily involved in fruit ripening and promoting senescene and abscission of leaves/flowers.
  • Cytokinin: Promotes cytokinesis (cell division), delays leaf senescence (Richmond-Lang effect), and promotes lateral bud growth by overcoming apical dominance.

UNIT-V

Question 9(a): What are the different groups of photoperiodism? 2 Marks

Plants are grouped into: Short-Day Plants (SDP), Long-Day Plants (LDP), and Day-Neutral Plants (DNP).

Question 9(b): Differentiate between SDP and LDP. 2 Marks

  • SDP: Flower when the day length is shorter than a critical duration (require a long uninterrupted dark period).
  • LDP: Flower when the day length is longer than a critical duration.

Question 9(c): What do you mean by photomorphogenesis? 2 Marks

It is the development of form and structure in plants that is controlled by light, independent of photosynthesis.

Question 10(a): Write short notes on: (i) Vernalization (ii) Role of phytochrome in flowering 5+5=10 Marks

  • Vernalization: The induction of flowering by a period of low temperature (chilling) treatment.
  • Phytochrome: A pigment that exists in two interconvertible forms, Pr (Red light absorbing) and Pfr (Far-red light absorbing). Pfr is generally the physiologically active form that triggers flowering responses.

Question 10(b): Discuss about the role of red light and far-red light on photomorphogenesis with examples. 10 Marks

The red/far-red light response is mediated by the phytochrome system.

  • Red Light (660 nm): Converts Pr to Pfr, promoting responses like seed germination (in Lettuce) and inhibiting flowering in Short-Day Plants.
  • Far-Red Light (730 nm): Converts Pfr back to Pr, reversing the red-light effect and promoting flowering in Short-Day Plants if given during the dark period.

Exam Focus Enhancements

Important Formulas/Principles

  • Water Potential: Ψw = Ψs + Ψp (Solute potential + Pressure potential)
  • Photosynthesis: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O

Common Mistakes

  • Confusing Diffusion (passive) with Active Transport (requires energy).
  • Forgetting that RuBisCO has an affinity for both CO2 and O2 in photorespiration.

Presentation Strategy

  • Use flowcharts for the metabolic cycles (Glycolysis, C2 Cycle).
  • Always label the guard cell walls (thick vs thin) in stomatal diagrams.

Generated for Knowlet | 2026 Academic Series