Transport in Plants
Plants need to transport water, minerals, and food between roots, stems, and leaves.
Means of Transport
- Diffusion: Passive, along concentration gradient. Slow over long distances.
- Facilitated diffusion: Via channel/carrier proteins. Passive.
- Active transport: Against gradient, requires ATP (pumps).
Water Potential
Psi_w = Psi_s + Psi_p
Psi_w: Water potential (pure water = 0, solutions have negative values).Psi_s: Solute potential (osmotic potential), always negative.Psi_p: Pressure potential (turgor pressure), usually positive.
Water moves from high water potential to low water potential.
Absorption of Water by Roots
- Root hairs in contact with soil water.
- Apoplast pathway: Through cell walls (fast).
- Symplast pathway: Through cytoplasm via plasmodesmata (slow).
- Transmembrane pathway: Across cell membranes.
Ascent of Sap (Water Transport)
Root pressure: Positive pressure in xylem. Responsible for guttation (water droplets from leaf tips). Not sufficient for tall trees.
Transpiration pull-cohesion-tension theory (Dixon and Joly):
- Transpiration from leaves creates tension.
- Cohesion between water molecules (H-bonds).
- Adhesion of water to xylem walls.
- Continuous water column pulled from roots.
Transpiration
Loss of water vapour from plant surfaces (mainly through stomata).
Types:
- Stomatal transpiration (major, 80-90%).
- Cuticular transpiration.
- Lenticular transpiration (through bark).
Factors affecting transpiration: Light, humidity, temperature, wind, soil water.
Significance: Cooling, mineral transport, water column maintenance.
Stomatal Mechanism: Guard cells regulate opening/closing. K+ ion movement and turgor changes control stomatal aperture.
Translocation of Food (Phloem Transport)
- Source: Site of production (leaves).
- Sink: Site of utilisation/storage (roots, fruits, seeds).
Pressure flow hypothesis (Munch):
- Sugars loaded into phloem at source (active transport).
- Water enters phloem by osmosis (higher pressure).
- Flow occurs from high pressure (source) to low pressure (sink).
- Sugars unloaded at sink.
Mineral Nutrition
Essential Elements
Criteria: Element must be required for life cycle, cannot be replaced by another, directly involved in plant metabolism.
Macronutrients (required in large amounts): C, H, O, N, P, K, Ca, Mg, S.
Micronutrients (trace amounts): Fe, Mn, Cu, Mo, Zn, B, Cl, Ni.
Functions of Key Minerals
| Element | Function | Deficiency Symptoms |
|---|---|---|
| N | Proteins, nucleic acids | Stunted growth, yellow leaves |
| P | ATP, nucleic acids, membranes | Poor root growth, dark leaves |
| K | Enzyme activation, stomatal opening | Marginal leaf burn |
| Mg | Chlorophyll component | Interveinal chlorosis |
| Fe | Electron transport, enzyme cofactor | Chlorosis (young leaves) |
| Ca | Cell wall formation | Stunted roots |
Nitrogen Metabolism
- Plants absorb N as NO3- (nitrate) or NH4+ (ammonium).
- Nitrogen fixation: Atmospheric N2 to NH3.
- Biological: Rhizobium (symbiotic), Azotobacter (free-living), Cyanobacteria.
- Industrial: Haber process.
- Nitrification: NH4+ -> NO2- (Nitrosomonas) -> NO3- (Nitrobacter).
- Assimilation: NO3- -> NO2- -> NH4+ -> amino acids.
Photosynthesis
Conversion of light energy to chemical energy (carbohydrates).
6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O
Site
Chloroplast (thylakoid membranes for light reactions, stroma for dark reactions).
Light Reaction (Photochemical Phase)
Occurs in thylakoid membranes:
- Photosystem II: Light absorbed, water split (photolysis):
2H2O -> 4H+ + 4e- + O2. - Electron transport chain: Electrons flow through PS II -> plastoquinone -> cytochrome b6f -> plastocyanin -> PS I.
- Photosystem I: Light re-energises electrons, transferred to NADP+ (forms NADPH).
- Chemiosmosis: Proton gradient across thylakoid membrane drives ATP synthesis (photophosphorylation).
Products: ATP, NADPH, O2.
Dark Reaction (Calvin Cycle / C3 Cycle)
Occurs in stroma:
- Carbon fixation: CO2 + RuBP -> 2 x 3-PGA (catalysed by RuBisCO).
- Reduction: 3-PGA -> G3P (using ATP and NADPH).
- Regeneration: RuBP regenerated.
6 CO2 molecules produce 1 glucose molecule (requires 18 ATP + 12 NADPH).
Photorespiration
- RuBisCO can bind O2 instead of CO2.
- Wastes energy, reduces efficiency.
- No ATP/G3P produced.
- Occurs when CO2 is low / O2 is high.
C4 Pathway (Hatch-Slack Pathway)
- Spatial separation of carbon fixation and Calvin cycle.
- Mesophyll cells: CO2 + PEP -> OAA (catalysed by PEP carboxylase).
- Bundle sheath cells: OAA releases CO2 for Calvin cycle.
- More efficient (no photorespiration).
- Examples: Maize, Sugarcane, Sorghum.
CAM Pathway
- Temporal separation (fix CO2 at night, Calvin cycle in day).
- Example: Cacti, Succulents.
Factors Affecting Photosynthesis
Light intensity, CO2 concentration, temperature, water availability.
Blackman's law of limiting factors: Rate is limited by the slowest factor.
Respiration in Plants
Aerobic Respiration
C6H12O6 + 6O2 -> 6CO2 + 6H2O + 36-38 ATP
Glycolysis (cytoplasm): Glucose -> 2 pyruvate + 2 ATP + 2 NADH.
Link reaction (mitochondrial matrix): Pyruvate -> Acetyl CoA + CO2.
Krebs cycle (mitochondrial matrix): Acetyl CoA fully oxidised to CO2 + NADH + FADH2 + 2 ATP.
Electron Transport Chain (inner mitochondrial membrane): NADH/FADH2 donate electrons -> proton gradient -> ATP synthase produces ~34 ATP.
Anaerobic Respiration
Without O2: Pyruvate -> Ethanol + CO2 (yeast) or Lactic acid (muscles). Net ATP: Only 2 (from glycolysis).
Respiratory Quotient (RQ)
RQ = CO2 released / O2 consumed
- Carbohydrates: RQ = 1.
- Fats: RQ < 1 (about 0.7).
- Proteins: RQ about 0.9.
Worked Examples
Example 1: Why do C4 plants have an advantage over C3 plants in hot, dry climates? Solution: C4 plants have no/low photorespiration. PEP carboxylase fixes CO2 efficiently even at low CO2 concentrations.
Example 2: How many ATP molecules are produced from one glucose molecule in aerobic respiration? Solution: Net gain = 36-38 ATP (2 from glycolysis, 2 from Krebs, 32-34 from ETC).
Common Mistakes
- Photosynthesis vs respiration: Photosynthesis produces glucose; respiration breaks it down.
- C3 vs C4: First stable product in C3 is 3-PGA (3C); in C4 is OAA (4C).
- Photorespiration vs dark respiration: Photorespiration occurs in chloroplasts and peroxisomes, no ATP produced.
- Transpiration vs guttation: Transpiration is water vapour loss; guttation is liquid water droplets.
ISC Exam Focus
- Theory (70%): Transport mechanisms, photosynthesis, respiration, mineral nutrition.
- Application (30%): Equations, RQ calculation, comparing C3/C4/CAM.
- ISC frequently asks: "Explain the Calvin cycle" and "Compare C3 and C4 pathways".
Self-Test Questions
Q1: Define transpiration. What is its significance? Answer: Water vapour loss from plants. Significance: cooling, mineral transport, water column.
Q2: Write the equation for photosynthesis.
Answer: 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O (light + chlorophyll).
Q3: Differentiate between C3 and C4 plants. Answer: C3: first product 3-PGA, normal leaf anatomy, photorespiration present. C4: first product OAA, Kranz anatomy, no photorespiration.
Q4: What is the RQ of carbohydrates and fats? Answer: Carbohydrates: RQ = 1. Fats: RQ about 0.7.
Q5: Name the end products of glycolysis. Answer: 2 pyruvate, 2 ATP, 2 NADH.
Q6: How many ATP are produced from one glucose in aerobic respiration? Answer: Net ATP = 36-38 molecules.
