Photosynthesis and Plant Nutrition

8.1 Photosynthesis

  • Photosynthesis is the process by which plants synthesize carbohydrates from raw materials using light energy.

  • The raw materials (reactants) of photosynthesis are:

    • Carbon dioxide (CO2) gas

    • Liquid water (H2O)

  • Oxygen (O2) is released as a by-product (waste product) of photosynthesis; a portion of this oxygen is utilized for respiration.

  • Chlorophyll is a green pigment found in the chloroplasts that captures light energy.

  • Word Equation for Photosynthesis:

    • Carbon Dioxide + Water → Glucose + Oxygen

8.2 The Role Of Chlorophyll

  • Chlorophyll absorbs the light energy essential for photosynthesis to occur.

  • Formula Equation of Photosynthesis:

    • Light energy is transformed into chemical energy stored in energy-storing molecules.

  • The stored chemical energy can then be converted to glucose, even in the absence of light.

8.3 Using Glucose

  • Glucose is the main product of photosynthesis.

  • Reasons why plants cannot store glucose directly:

    1. Solubility of Glucose:

    • Glucose is soluble, reducing the water potential within the cell, resulting in water entering the cell via osmosis.

    1. Size of Glucose Molecules:

    • Glucose molecules are relatively small and can easily escape from the cell.

  • Therefore, glucose must be:

    1. Respired to release energy or

    2. Converted into other forms:

    • Glucose can be converted into sucrose and transported elsewhere in the plant through the phloem (refer to page 28).

    • Glucose can also be stored as starch inside the cell, mainly in the cytoplasm and chloroplast.

    • Starch is insoluble and too large to escape from the cell.

8.4 Gas Exchange In Plants

  • Plants exchange gases mainly through stomata in the leaves.

  • Gases move in and out by diffusion.

Carbon Dioxide (CO₂)

  • Role in photosynthesis: Raw material

  • Role in respiration: Waste product

  • Daytime:

    • Photosynthesis rate is higher

    • CO₂ mostly diffuses into the plant

  • Night:

    • Respiration rate is higher

    • CO₂ mostly diffuses out of the plant

Oxygen (O₂)

  • Role in photosynthesis: Waste product

  • Role in respiration: Required

  • Daytime:

    • O₂ mostly diffuses out of the plant

  • Night:

    • O₂ mostly diffuses into the plant

Water Vapour

  • Water vapour diffuses out of the leaves all the time

  • This process is called transpiration

  • Transpiration helps to pull water from the roots to the leaves

Stomata

  • Carbon dioxide diffuses in through the stomata

  • Oxygen diffuses out through the stomata

Formula:

Word equation:

  • Carbon dioxide + Water → Glucose + Oxygen

  • (in the presence of light energy and chlorophyll)

Symbol (chemical) equation:

  • 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

  • (light energy and chlorophyll are required)



  • Gas exchange dynamics in relation to photosynthesis and respiration compared:

    Material

    Photosynthesis

    Respiration


    Carbon Dioxide

    Raw material

    Waste product


    Oxygen

    Waste product

    Required


    Water

    Raw material

    Waste product

    • Water vapor constantly diffuses out of leaves (transpiration, page 28), aiding in pulling water from roots to leaves.

    8.5 Leaf Structure

    • Leaf attachment to the stem is via a leafstalk (petiole).

    • Veins in leaves supply water and minerals and translocate nutrients such as sucrose.

    • Midrib: The principal vein providing support to the leaf and positioning the leaf blade (lamina) for light absorption.

    8.6 Leaf Adaptations for Photosynthesis


    • Anatomy of Leaf Tissue and Adaptations:

      Tissue

      Adaptations

      Functions


      Cuticle

      Waterproof layer; reduces water loss

      N/A


      Upper Epidermis

      Transparent (lack of chloroplasts); allows light passage

      Facilitates light absorption


      Palisade Mesophyll

      Numerous chloroplasts; vacuoles push chloroplasts outward

      Traps more light energy for photosynthesis


      Spongy Mesophyll

      Fewer chloroplasts and more air space

      Gaseous exchange and water evaporation


      Vascular Bundle

      Contains xylem and phloem

      Water, mineral transport, and nutrient translocation


      Lower Epidermis

      Contains guard cells and stomata

      Gaseous exchange and transpiration control

      • Guard Cells: Specialised cells that open and close the stoma in response to environmental conditions and regulate gas exchange and transpiration.

      8.7 Mineral Requirements In Plants

      • Key minerals essential for plant growth:

        1. Nitrates

        • Necessary for amino acid synthesis; building blocks of proteins and required for DNA synthesis in new cells.

        • A deficiency in nitrates results in stunted growth and lack of chlorophyll production due to insufficient protein synthesis.

        1. Magnesium Ions

        • Integral component of chlorophyll.

        • A shortage of magnesium leads to reduced chlorophyll levels and less photosynthesis, impacting overall plant growth.

      8.9 Limiting Factors Of Photosynthesis

      • Limiting Factor Definition: An environmental element that limits the rate of photosynthesis when in insufficient amounts.

      • Identified limiting factors:

        • Light intensity

        • Temperature

        • Carbon dioxide concentration

        • Water availability

      • Impact of limiting factors:

        • Low light intensity facilitates increased photosynthesis until a factor becomes limiting.

        • Optimum temperature promotes ideal enzyme activity for photosynthesis efficiency.

        • Water contributes as a transport medium, reactant in photosynthesis, and maintains turgidity.

      8.10 Destarching Plants

      • Destarching: The process of removing starch from plants prior to photosynthesis experiments to demonstrate that starch is only produced during the experiment.

      • Method: Putting the plant in a dark cupboard for two to three days ensures all stored starch is utilized before commencing with the experiment.

      8.11 Testing A Leaf For Starch

      • Procedure Steps:

        1. Boil for One Minute:

        • Breaks down cell walls and membranes ensuring efficient iodine absorption in later steps.

        1. Place in Warm Ethanol:

        • Ethanol extracts chlorophyll, preventing it from masking the color change when iodine is introduced.

        1. Rinse in Cold Water:

        • Softens the brittle leaf after alcohol treatment.

        1. Add Drops of Iodine:

        • Iodine reacts with starch, turning dark blue, the change in color is easier to observe on a white tile.

      8.12 Investigating the Limiting Factors

      • Use of variegated leaves for chlorophyll study (they possess both green and white patches).

      • Testing starch presence will show only green areas produce starch due to chlorophyll being required for light energy transformation into chemical energy.

      • Light-Proof Cover Experiment:

        • Demonstrates necessity of light; only exposed green part will test positive for starch when iodine is applied.

      • A potassium hydroxide treatment is used to absorb CO2 for one plant, controlling its carbon dioxide exposure, while another plant serves as a control.

      8.13 The Hydrogencarbonate Indicator

      • The hydrogencarbonate indicator measures CO2 levels, an acidic gas crucial for photosynthesis.

      • In the dark, high CO2 levels indicate reduced photosynthesis (higher respiration).

      • Bright light leads to lower CO2 levels due to photosynthesis.

      • Color Change Dynamics of the Hydrogencarbonate Indicator:

        1. Red: Normal (atmospheric CO2 levels) - Indicates equilibrium of respiration and photosynthesis.

        2. Yellow: High CO2 (acidic conditions) - CO2 levels high in darkness, indicating minimized photosynthesis.

        3. Purple: Low CO2 (alkaline conditions) - CO2 levels low in bright light, indicating a predominance of photosynthesis.

      8.14 Investigating Gaseous Exchange In Water Plants

      • During bright light exposure, CO2 levels decrease in pondweed (water plants) as the rate of uptake (photosynthesis) outpaces the release (respiration).

      • Changing color of the hydrogencarbonate indicator reflects the CO2 uptake.

      8.15 Investigating The Rate Of Photosynthesis

      • Rate measurement in water plants:

        • Often gauged by counting oxygen bubbles, or using inverted graduated tubes for more accuracy.

      • Limiting factors investigation involves manipulating one while controlling others to ensure accurate results.

      • Experimental designs include variations in CO2 concentration and light intensity to observe effects on photosynthesis rates.

      • The color of the hydrogencarbonate indicator is more likely red if a water plant and an animal share the same tube, as CO2 produced by the animal is absorbed by the plant during photosynthesis.

      • Syringe Usage: To adjust water levels when repeating the investigations.

      • Variables Managed in Investigations:
        | Variable | Manipulation Method | Control Method |
        |------------------------|---------------------------------------------------|------------------------------------------------|
        | Light Intensity | Change distance of the lamp | Use the same lamp at a fixed distance |
        | Temperature | Different water baths at varied temperatures | One water bath with a heat shield |
        | Concentration of CO2 | Sodium hydrogen carbonate as CO2 source | Fixed concentration of sodium hydrogen-carbonate |