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Plant Nutrition

Photosynthesis & leaf structure

The Photosynthesis Equation

6CO₂ + 6H₂O + light + chlorophyll → C₆H₁₂O₆ + 6O₂

Photosynthesis converts light energy into chemical energy (glucose)

Happens in chloroplasts (contain chlorophyll)

Glucose: raw material for growth, energy for respiration

Light-Dependent & Light-Independent Reactions

Light-dependent (in thylakoids): Uses light energy. Splits water. Releases O₂. Produces ATP + NADPH.

Light-independent (in stroma, "dark reactions"): Uses ATP + NADPH from light reactions. Produces glucose from CO₂. Happens in light AND dark.

Both stages needed to convert CO₂ + H₂O to glucose

Limiting Factors of Photosynthesis

Light intensity: Increase light → rate increases (until saturation). Used in greenhouses.

CO₂ concentration: Increase CO₂ (to ~1%) → rate increases. Too much beyond 1% is wasteful.

Temperature: Increase temp (to ~25-30°C) → enzyme activity increases. Above ~40°C → enzymes denature.

To maximise photosynthesis: provide bright light, elevated CO₂, warm temperature

Leaf Structure for Photosynthesis

Upper epidermis: Clear, transparent. Allows light through.

Palisade mesophyll: Packed with chloroplasts. Many mitochondria. Light-absorbing layer. Main photosynthesis site.

Spongy mesophyll: Loosely packed cells. Large air spaces for gas exchange. Fewer chloroplasts.

Lower epidermis: Contains stomata (pores) for gas exchange & water loss.

Stomata & Guard Cells

Stomata: Pores in leaf (mostly on lower side). Allow CO₂ in, O₂ out, water vapour out.

Guard cells: Surround each stoma. Control opening & closing.

Guard cells have starch (low osmotic potential). Light → starch converts to glucose → osmotic potential drops → water enters by osmosis → cells swell → stoma opens

Opening: daytime (light available) Closing: night (save water)

Leaf Veins — Xylem & Phloem

Xylem: Dead, lignified cells. Transports water & mineral ions UP from roots (via transpiration pull).

Phloem: Living sieve tubes + companion cells. Transports glucose & amino acids UP and DOWN to all plant parts (via active transport).

Veins have both: xylem brings in water, phloem carries away glucose

Photosynthetic Adaptations

Palisade cells: Elongated, packed with chloroplasts → max light absorption

Large leaf area: Captures more light

Air spaces in mesophyll: Gas diffusion pathway (CO₂ in, O₂ out)

Veins: Supply water, carry away glucose

Thin epidermis: Minimal light absorption loss

Exam Tips

✓ Know photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

✓ Three limiting factors: light, CO₂, temperature

✓ Xylem = water UP; phloem = glucose UP and DOWN

✓ Stomata mostly on lower epidermis (saves water)

✓ Know leaf tissue layers: epidermis, palisade, spongy, epidermis

Key Takeaways

✅ Photosynthesis: CO₂ + H₂O → glucose + O₂ (light needed)

✅ Limiting factors: light, CO₂, temperature

✅ Palisade mesophyll: main photosynthesis site (chloroplasts)

✅ Stomata: gas exchange; guard cells: control opening

✅ Xylem (water UP) & phloem (glucose UP/DOWN)

Photosynthesis = life on Earth! 🌿☀️