Plant Physiology: Nutrition and Transport

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Plant Nutrition and Transport — Key Facts for Sri Lanka A/L Examination

Photosynthesis Equation: $$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$

Two Main Stages:

Stage	Location	Main Events
Light reactions	Thylakoid membranes	Photolysis of water, ATP and NADPH production
Dark reactions (Calvin)	Stroma	CO₂ fixation into glucose

Water Uptake in Plants:

• Root hairs: Increase surface area for absorption
• Osmosis: Water moves from high to low water potential
• Root pressure: Pressure from active ion uptake
• Transpiration pull: Cohesion-tension theory

Key Terms:

• Turgor pressure: Pressure of cell contents against cell wall
• Wilting: Loss of turgidity (no water = droopy leaves)
• Guttation: Exudation of water from leaf tips (not the same as transpiration!)

⚡ A/L Exam Tip: Transpiration is the driving force for water movement in the xylem. Remember the cohesion-tension theory!

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Plant Nutrition and Transport — Detailed Study Guide

Mineral Nutrition

Essential Mineral Elements:

Element	Symbol	Function	Deficiency Symptoms
Nitrogen	N	Protein synthesis, leaf growth	Yellowing (chlorosis) of older leaves
Phosphorus	P	Root growth, flowering	Purple leaves, stunted growth
Potassium	K	Enzyme activation, stomata	Scorched leaf edges
Magnesium	Mg	Chlorophyll component	Interveinal chlorosis
Iron	Fe	Chlorophyll synthesis	Young leaves turn yellow
Calcium	Ca	Cell wall formation	Deformed young leaves
Sulfur	S	Protein synthesis	Young leaves turn yellow

Macronutrients (needed in large amounts):

• N, P, K, Ca, Mg, S
• Remember: NPKCaMgS (NPK is most important!)

Micronutrients (needed in small amounts):

• Fe, Mn, Zn, Cu, B, Mo, Cl
• Only required in trace amounts

Nitrogen Cycle in Nature:

1. N₂ fixation: Atmospheric N₂ → NH₃ (by Rhizobium bacteria in legume root nodules)
2. Nitrification: NH₃ → NO₂⁻ → NO₃⁻ (by nitrifying bacteria)
3. Assimilation: Plants absorb NO₃⁻ and NH₃
4. Ammonification: Dead organisms → NH₃
5. Denitrification: NO₃⁻ → N₂ (returns to atmosphere)

⚡ A/L PYQ: “Explain the role of Rhizobium bacteria in nitrogen fixation.” Answer: Rhizobium converts atmospheric nitrogen (N₂) into ammonia (NH₃) which plants can use. They live in root nodules of legumes and provide nitrogen in exchange for carbohydrates.

Water Movement in Plants

Cohesion-Tension Theory:

1. Transpiration: Water evaporates from leaf stomata
2. Cohesion: Water molecules stick together (H-bonding)
3. Adhesion: Water molecules stick to xylem walls
4. Tension: Negative pressure pulls water up

Path of Water: $$\text{Root hairs} \rightarrow \text{Cortex} \rightarrow \text{Endodermis} \rightarrow \text{Xylem} \rightarrow \text{Stem} \rightarrow \text{Leaves}$$

Factors Affecting Transpiration:

Factor	Effect
Light	Increases (stomata open)
Temperature	Increases (more evaporation)
Humidity	Decreases (less gradient)
Wind	Increases (removes water vapor)

Significance of Transpiration:

• Creates pull for water absorption
• cools the plant
• Enables mineral transport
• Maintains turgor pressure

Phloem Transport

Pressure Flow Hypothesis (Münch, 1927):

1. Loading: Sugars actively transported into sieve tubes (source)
2. Water enters: Osmotic influx of water
3. High pressure: Water pushes sugars through phloem
4. Unloading: Sugars removed at sink (roots, fruits, growing points)

Source to Sink:

• Sources: Leaves (photosynthesis), storage organs
• Sinks: Roots, fruits, developing leaves, flowers

⚡ Common Mistake: Students confuse xylem and phloem function. Xylem transports water UP (unidirectional), phloem transports sugars BOTH directions (source to sink).

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Plant Nutrition and Transport — Complete Notes for A/L Sri Lanka

Photosynthesis in Detail

Light-Dependent Reactions:

Location: Thylakoid membranes
Input: H₂O, light energy
Output: O₂, ATP, NADPH

Process:
1. Photolysis of water: 2H₂O → 4H⁺ + O₂ + 4e⁻
2. Electrons pass through PSII → ETC → PSI → NADP⁺ → NADPH
3. Chemiosmosis: H⁺ gradient drives ATP synthase → ATP

Cyclic vs Non-cyclic Photophosphorylation:

Feature	Non-cyclic	Cyclic
Electron source	Water	PSI only
Product	ATP + NADPH + O₂	ATP only
NADPH produced	Yes	No
Oxygen produced	Yes	No

C₃ Photosynthesis (Calvin Cycle):

Location: Stroma (chloroplast)
Input: CO₂, ATP, NADPH
Output: Glucose (G3P)

Steps:
1. Carbon fixation: CO₂ + RuBP → 2 × 3-PGA (Rubisco)
2. Reduction: 3-PGA → G3P (using ATP, NADPH)
3. Regeneration: G3P → RuBP (using ATP)
4. 2 G3P → 1 glucose (net gain)

Note: 6 CO₂ → 1 glucose requires 6 turns, 18 ATP, 12 NADPH

C₄ Photosynthesis (for hot climates):

• Plants: Maize, sugarcane, sorghum
• Kranz anatomy: Bundle sheath cells around vascular bundles
• First product: 4-carbon acids (oxaloacetate)
• More efficient in high light, high temperature

CAM Photosynthesis (for arid conditions):

• Plants: Pineapple, cactus, succulents
• Crassulacean Acid Metabolism
• Stomata open at night, close during day
• First product: Malic acid (stored)

⚡ A/L Important: Sri Lanka’s tropical climate makes C₄ plants like maize very efficient. C₃ plants (rice, most trees) are less efficient in high temperatures but dominate in cooler areas.

Mineral Absorption

Active Transport:

• Requires ATP
• Against concentration gradient
• Via carrier proteins
• Example: Ion pumps in root hairs

Passive Transport:

• No ATP needed
• Along concentration gradient
• Examples: Diffusion, facilitated diffusion

Mechanism of Ion Absorption:

1. H⁺ ions actively pumped out of root hair cell (ATP)
2. H⁺ gradient created
3. Anions (NO₃⁻, Cl⁻) move in via channel proteins
4. Cations (K⁺, Ca²⁺) may follow or be actively transported
5. Casparian strip in endodermis prevents backflow

Casparian Strip:

• Waxy band in endodermal cell walls
• Forces all water and minerals to go through cell membranes
• Prevents toxic substances from entering xylem
• Ensures selective absorption

GCE A/L Sri Lanka Past Paper Tips

Common Structured Questions:

1. “Describe the cohesion-tension theory of water movement in plants” (10 marks)
2. “Explain the process of photosynthesis, including the light and dark reactions” (15 marks)
3. “Describe the path of water from soil to leaf in a named plant” (8 marks)
4. “Compare the transport of water in xylem with transport of food in phloem” (10 marks)

Diagram Questions:

1. Draw and label the C₃ cycle (Calvin cycle) (8 marks)
2. Draw a labeled diagram of a transverse section of a dicotyledonous leaf, showing photosynthetic tissues (10 marks)
3. Draw a root hair cell and explain mineral absorption (6 marks)

Practical Questions:

• Test for starch in leaves (destarch plant, illuminate, test with iodine)
• Test for oxygen produced during photosynthesis (bubble count method)
• Effect of light intensity on transpiration rate

⚡ A/L Strategy: Always explain mechanisms with reference to specific plant examples from Sri Lanka (e.g., coconut, rice, mango) when asked!

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