Plant structure

Photosynthesis

Definition:
Plants use energy from the Sun to combine carbon dioxide (from the air) with water (from the soil) to produce glucose.

  • "Photo" means light.

  • "Synthesis" means to manufacture or make.

Photosynthesis Equation:

Equation:Β 2 𝐢𝑂2+𝐻2𝑂→𝐢6𝐻12𝑂6(π‘”π‘™π‘’π‘π‘œπ‘ π‘’)+𝑂2

Importance of Chlorophyll

  • Chlorophyll is a green pigment that traps the Sun’s energy.

  • It contains the mineral magnesium.

  • Chlorophyll molecules are packed inside special organelles called chloroplasts.

Leaf structure:

Layers of the Leaf:

Layer

Key Structures

Adaptations for Photosynthesis

Upper Epidermis

Waxy Cuticle

Waterproofs the leaf, thin layer of transparent cells lets light enter the middle layer

Palisade Mesophyll

Chloroplasts

Packed with chloroplasts to trap light, thin, upright cells maximize sunlight

Spongy Mesophyll

Air Spaces, Chloroplasts

Air spaces allow CO2 in and O2 out, packed with chloroplasts to trap light

Lower Epidermis

Guard Cells, Stomata

Flat layer of cells, small gaps (stomata) allow O2 out and CO2 in

Veins in Leaves

  • Xylem vessels carry water to the leaf.

  • Phloem tubes transport sucrose and amino acids away from the leaf.

Starch Testing in Leaves

Starch Formation:

  • Plants photosynthesize to produce glucose.

  • Glucose is converted into starch for storage.

Starch Testing Method:

  1. Remove the leaf to be tested.

  2. Kill the leaf by placing it in hot water.

  3. Remove chlorophyll by soaking the leaf in warm ethanol.

  4. Soften the leaf in hot water again.

  5. Lay the leaf flat on a white tile and add iodine solution.

Safety Tips:

  • Wear goggles to protect against hot liquids.

  • Use a water bath instead of a Bunsen burner to heat ethanol (it’s flammable).

Carbon Dioxide Requirement

  • Place the leaf in a bell jar with a COβ‚‚ absorber (e.g., sodium hydroxide, potassium hydroxide, soda lime).

  • No access to COβ‚‚ means no photosynthesis, no glucose production, and no starch formed.

Chlorophyll Requirement

  • Use a variegated plant (green and white leaves).

  • White areas (no chlorophyll): no photosynthesis β†’ stay orange with iodine.

  • Green areas (with chlorophyll): photosynthesize β†’ turn blue/black with iodine.

Light Requirement

  • Place the plant in the dark (e.g., a cupboard).

  • After enough time, leaves will test negative for starch (remain orange).

Raw Materials for Photosynthesis

Carbon Dioxide:

  • Comes from the air; diffuses in through stomata.

Water:

  • Absorbed from soil by root hair cells.

Glucose Uses:

  • Used in respiration to release energy.

  • Converted into starch for storage.

  • Used to make organic substances (e.g., sucrose, cellulose, oils).

  • Transported around the plant as sucrose in phloem tubes.

Mineral ions and plants:

Mineral use and deficiency symptoms

Mineral

Use within Plant

Deficiency Symptoms

Nitrates

Form proteins for growth

Poor growth

Magnesium

Forms part of the chlorophyll molecule

Yellow, pale leaves

Factors affecting the rate of photosynthesis

Limiting factors

  • Light intensity

  • Carbon dioxide concentration

Effect of light intensity:

  • As light intensity increases, the rate of photosynthesis also increases.

  • Eventually, the rate levels off when another factor (like COβ‚‚) becomes limiting.

Effect of carbon dioxide concentration:

  • Higher COβ‚‚ levels lead to a faster rate of photosynthesis.

  • This continues until the rate plateaus due to light or another factor limiting the process.