Photosynthesis Notes

Photosynthesis

Introduction

  • Photosynthesis is how green plants create food from inorganic substances, building up organic substances, a process called autotrophic nutrition.

  • Inorganic substances used include:

    • Carbon dioxide
    • Water
    • Minerals

  • Plants create:

    • Carbohydrates
    • Fats
    • Proteins
    • Vitamins

  • Photosynthesis is the process by which some organisms produce their own food from water and carbon dioxide using light energy.

  • Glucose is the main product, but other sugars, lipids, amino acids, and vitamins can be formed.

  • Chlorophyll, a green pigment found in chloroplasts, is needed to trap light energy.

Word Equation
  • Light + Carbon Dioxide + Water → Glucose + Oxygen (with Chlorophyll as a catalyst)
Symbol Equation
  • 6CO<em>2+6H</em>2O→Light, ChlorophyllC<em>6H</em>12O<em>6+6O</em>26CO<em>2 + 6H</em>2O \xrightarrow{\text{Light, Chlorophyll}} C<em>6H</em>{12}O<em>6 + 6O</em>2

Leaf Structure

External Structure

  • A typical leaf consists of:

    • Leaf base
    • Midrib
    • Axil
    • Margin
    • Stem
    • Tip
    • Vein
    • Petiole
    • Leaf blade or lamina
    • Branching veins
    • Venules

  • Leaf stalk (petiole) extending into the midrib.

  • Leaf blade or lamina.

  • Branching veins.

  • Different types of leaves:

    • Simple leaf: single leaf blade.
    • Compound leaf: made up of a number of leaflets.
Internal Structure

  • Cross-section of a leaf shows organized cells and tissue layers.

  • Key layers:

    • Upper Epidermis: includes a wax cuticle
    • Palisade Mesophyll
    • Spongy Mesophyll: contains air space
    • Lower Epidermis: includes a wax cuticle
    • Stoma
    • Guard Cell with Chloroplasts
    • Mesophyll layer

Leaf Layers: Structure and Functions

Upper Epidermis
  • Structure: Single layer of cells with no chloroplasts. Transparent, waxy waterproof cuticle covers it.
  • Functions:
    • Maintains leaf shape.
    • Prevents bacteria and fungi from entering.
    • Cuticle reduces water loss.
Palisade Mesophyll Layer
  • Structure: One or more layers of tightly packed, elongated cells with many chloroplasts.
  • Functions:
    • Responsible for making food by photosynthesis.
    • Chloroplasts contain chlorophyll, which absorbs light and converts it into chemical energy.
Spongy Mesophyll Layer
  • Structure: Layers of loosely packed cells of different shapes and sizes, with many air spaces.
  • Functions:
    • Helps in making food by photosynthesis.
    • Air spaces allow carbon dioxide and other gases to enter or leave by diffusion.
Lower Epidermis

  • Structure: Similar to the upper epidermis, with a thinner cuticle and stomata (special openings).

  • Functions:

    • Same as upper epidermis.
    • Stomata control the amount of carbon dioxide and other gases entering and exiting the leaf, which affects photosynthesis.

  • Stomata are guarded by special guard cells with thickened cell walls around the stoma.

Leaf Adaptations for Photosynthesis

  • Photosynthesis occurs inside chloroplasts, where enzymes and chlorophyll catalyze and supply energy to the reaction.

  • Leaves are adapted for efficient photosynthesis:

    • Supported by stem and petiole: Exposes leaf to sunlight and air; avoids shading.
    • Large surface area: Exposes leaf to sunlight and air.
    • Thin leaf blade: Allows sunlight to penetrate cells; allows CO₂ to diffuse in and O₂ to diffuse out quickly.
    • Transparent waxy cuticle & no chloroplasts in epidermis: Allows sunlight to penetrate the mesophyll layer.
    • Thin epidermis (usually one cell thick): Provides a short distance for quick diffusion of CO₂ and O₂.
    • Chloroplasts containing chlorophyll in mesophyll layer: Absorbs sunlight, providing energy to combine CO₂ and H₂O.
    • Palisade layer at top and rich in chloroplasts: Keeps as few cell walls as possible between sunlight and chloroplasts.
    • Palisade cells arranged end on: Exposes as much chlorophyll as possible to sunlight.
    • Spongy mesophyll with large air spaces: Allows CO₂ and O₂ to diffuse to and from all cells quickly.
    • Stomata in lower epidermis: Allows CO₂ and O₂ to diffuse in and out, and prevent water loss.
    • Xylem vessels within short distance of every mesophyll cell: Supplies water to chloroplasts for photosynthesis.
    • Phloem tubes within short distance of every mesophyll cell: Takes away organic products of photosynthesis.

Rate of Photosynthesis

  • Four factors affect the rate of photosynthesis: light, carbon dioxide, water, and temperature.
Light Intensity
  • In the dark, a plant cannot photosynthesize.
  • In dim light, it photosynthesizes slowly.
  • As light increases, the rate increases until the plant reaches its maximum rate; other factors then become limiting.
Carbon Dioxide Concentration
  • Carbon dioxide is a reactant in photosynthesis.
  • More carbon dioxide leads to faster photosynthesis until a limiting factor is reached.
Water
  • Lack of water slows down photosynthesis.
  • Stomata close to avoid water loss, reducing carbon dioxide intake.
Temperature
  • Photosynthesis involves enzymes.
  • Higher temperatures increase the rate until the optimum temperature is reached.
  • A rise of 10°C10°C can double the rate, but temperatures above 40°C40°C denature the enzymes.

The Compensation Point

  • All living things, including plants, respire. Respiration requires oxygen intake and carbon dioxide output all the time.

  • Photosynthesis requires carbon dioxide and gives off oxygen, but only in the light.

  • Equation for respiration:

  • Glucose+Oxygen→CarbonDioxideGlucose + Oxygen \rightarrow Carbon Dioxide

  • Equation for photosynthesis:

  • CarbonDioxide+Water→Glucose+OxygenCarbon Dioxide + Water \rightarrow Glucose + Oxygen

  • During daylight, plants photosynthesize and respire, using all carbon dioxide produced by respiration for photosynthesis and all oxygen needed by respiration is provided by photosynthesis.

  • Only when the rate of photosynthesis is faster than the rate of respiration will carbon dioxide be taken in and the excess oxygen given out.

  • The compensation point is when the rate of photosynthesis exactly matches the rate of respiration.

  • There is no intake or output of carbon dioxide or oxygen.

  • The sugar produced by photosynthesis exactly compensates for the sugar broken down by respiration.

  • The compensation point is the light intensity at which the rate of photosynthesis is equal to the rate of respiration.

Fate of Glucose

  • Glucose, the first carbohydrate made, can:

    • Release energy by respiration in the leaf.
    • Turn into starch and be stored in the leaf.
    • Be used to make other organic substances.

  • Glucose is a monosaccharide and is not a great storage molecule because:

    • It's soluble causing osmotic unbalance.
    • It's reactive.

  • Many glucose molecules join to form starch, which is a polysaccharide, for storage because:

    • It is not very reactive.
    • It is not very soluble.

  • The plant uses glucose as a starting point for making all the other organic substances it needs like sucrose, cellulose, oils and amino acids.

  • Glucose is converted into sucrose and transported to other parts of the plant.

  • Since the sucrose molecule is small and soluble, it can be transported easily to other parts of the plant like roots, stems and fruits. There it is converted into starch and stored. Sucrose can be then broken down back into glucose for energy.

Importance of Photosynthesis to Animals

  • Plants convert solar energy into chemical energy, making it available to animals.
  • Plants convert inorganic compounds (carbon dioxide and water) to organic compounds (carbohydrates), making them available to animals.
  • Plants convert organic compounds (carbohydrates) into other important organic compounds such as proteins, oils and vitamins, making them available to animals.
  • Plants help in the recycling of atmospheric carbon dioxide and in the production of oxygen, which is needed by animals in aerobic respiration.

Experiments on Photosynthesis

De-starching
  • During photosynthesis, plants produce soluble glucose, which is quickly changed to insoluble starch. So, the only examinable product of photosynthesis is starch.
  • Before any photosynthesis-related experiment, a plant must first be de-starched, meaning all the plants' starch reserves are used up, proving that any starch present at the end of a photosynthesis-related experiment was produced during the actual experiment, not before.
  • Method:
    • Water well a potted plant and place it in a dark place for at least 48 hours.
    • Carry out the test for starch.
  • Result and Conclusion:
    • If a blue-black colour is obtained, then the plant has not de-starched completely and needs to be put back in the dark.
    • If no blue-black colour is observed, then de-starching is complete.
Testing A Leaf For Starch
  • Method:
    • Dip your leaf into a beaker of boiling water for about ten seconds. This will kill it and make it soft (denature enzymes). reactions stop.
    • TURN OFF BUNSEN BURNER. because ethanol is highly flammable.
    • Put the leaf into a test tube of ethanol. Stand the test tube in the beaker of hot water for about ten minutes. The ethanol will boil and this will decolorize the leaf.
    • Wash the leaf by waving it to and fro in the beaker of water. Why? to rinse away ethanol.
    • Put the leaf in a Petri dish and cover it with dilute iodine solution.
    • A blue-black colour shows that starch is present.
To Find Out If a Plant Needs Light In Order to Make Starch
  • Method:
    • You will need a potted plant, which has been de-starched.
    • Attach a strip of black paper or foil to the upper and lower sides of a leaf, as shown in the illustration.
    • Put the plant in a well-lit place.
    • After several days detach the leaf and test it for starch.
  • Interpretation:
    • The part of the leaf that was covered and received no sunlight remained yellow brown showing that photosynths is did not take place and starch was not produced.
    • The area that received sunlight produced starch.
    • Therefore light is needed for photosynthsis to take place.
To Find Out If A Plant Needs Chlorophyll to Make Starch
  • Method:
    • You will need a potted plant with variegated leaves e.g. geranium. The plant should have been put in the light for several days.
    • Detach one of the leaves and draw its upper side, making a clear distinction between the green and non-green areas.
    • Now carry out a starch test on the whole leaf.
  • Interpretation
    • The green parts of the leaf turned blue black showing that starch had been produced by photosynthesis.
    • The white parts of the leaf did not have any starch present.
    • This shows us that chlorophyll is needed for photosynthsis
To Find Out If A Plant Needs Carbon Dioxide In Order To Make Starch
  • Method:
    • You will need two potted plants, which have been de-starched. Geranium, balsam or coleus will do.
    • Put a dish of dampened soda lime on the soil beside one of the plants.
    • Cover the plant with a polythene bag as shown in the upper illustration. The soda lime will absorb carbon dioxide from the air inside the bag, so this plant will be deprived of carbon dioxide.
    • Put a dish of saturated sodium hydrogen carbonate solution on the soil beside the other plant.
    • Cover the plant with a polythene bag as shown in the lower illustration. The sodium hydrogen carbonate will slowly give out carbon dioxide into the bag, so this plant will have plenty of carbon dioxide.
    • Place both plants side by side in a well-lit place for about 48 hours.
    • After about 48 hours take a leaf, or part of a leaf, from each plant and test them for starch.
  • Interpretation
    • The leaf from the plant that had soda lime to absorb carbon dioxide remained yellow brown showing us that carbon dioxide is needed for photosynths is to take place.
    • In the control plant where carbon dioxide was released by the hydrogen carbonate the leaf turned blue black in colour.
To Find Out If a Water Plant Gives Off Oxygen

  • Put some Canadian pondweed into two separate beakers of water.

  • Cover the weed with an upturned funnel and test tube.

  • Place one of the beakers in the light and the other one in the dark.

  • After a few days compare what has happened in the two cases.

  • Test the gas for oxygen with a glowing splint.

  • Which pondweed produced oxygen gas?

    • The pondweed placed in sunlight.

  • Interpretation:

  • The pondweed placed in the light released oxygen gas showing us that light is needed for photosynths is to take place and produce oxygen.

To investigate the change in the rate of photosynthesis with varying light intensity
  • List the readings that you would take.
    • The boiling tube is moved further away from the light source at different distances on the meter ruler. At each distance point moved the number of bubbles released by the pondweed in two minutes are counted.
  • State the use of the tank or beaker of water between the pondweed and lamp.
    • The tank or beaker of water is present to absorb heat energy produced by the lamp so that only the variable of light intensity is investigated.