Photosynthesis Notes

Photosynthesis

• Photosynthesis equation: $6CO<em>2 + 6H</em>2O + \text{light} + \text{chlorophyll} \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Carbon dioxide + water + light + chlorophyll yields Glucose + Oxygen.

Key Terminology

• Radiant energy: Energy from the sun.
• Chloroplast: Organelle in plants; site for photosynthesis.
• Chlorophyll: Green pigment needed for photosynthesis.
• Thylakoids (granal lamellae): Part of chloroplast that contains chlorophyll; stacks of thylakoids/lamellae; site for light-dependent phase.
• Stroma: Liquid part of chloroplast; site for light-independent phase.

Producers

• Green plants have chlorophyll in chloroplasts which traps radiant energy from the sun to produce carbohydrates via photosynthesis.
• Green plants are producers.
• Photosynthesis is an anabolic reaction (builds up).
• Has two stages: Light-dependent phase and Light-independent phase (Calvin cycle).

Role of ATP

• ATP (adenosine triphosphate) is a nucleotide that performs essential roles in the cell.
• Major energy carrier, providing energy for activities.
• Monomer used in the synthesis of RNA + DNA.
• Regulates many biochemical pathways.
• Synthesized in chloroplasts during photosynthesis.
• Used during the formation of polysaccharides (e.g., starch).

Importance of Photosynthesis

• Balances $CO_2$ and oxygen levels in the atmosphere.
• Uses carbon dioxide and releases oxygen.
• Uses radiant energy to produce chemical potential energy in the form of glucose, which serves as food.
• Proteins and lipids are made using stored starch.

Environmental Factors Affecting Photosynthesis

• Photosynthesis rate is affected by:
  • Intensity of light
  • Concentration of $CO_2$
  • Temperature

Intensity of Light:

• At low light intensity, the rate is low.
• As light intensity increases, the rate increases to a certain point.
• At optimum light intensity, photosynthesis occurs most rapidly.
• If light intensity increases past the optimum, the rate remains constant; other factors become limiting.

Comparison of Phases

• Light-Dependent Phase
  • Light is required.
  • Occurs in the grana/granum (thylakoids).
  • Light is absorbed by chlorophyll.
  • Water is absorbed and split (photolysis), releasing energy-rich $H^+$, electrons, and oxygen.
  • ATP and NADPH are created (phosphorylation); store energy used in the Calvin Cycle.
• Light-Independent Phase (Calvin Cycle)
  • Light is not required.
  • Occurs in the stroma.
  • $CO_2$ is absorbed from the atmosphere.
  • $CO_2$ and energy-rich Hydrogen atoms from the light-dependent phase are combined using ATP to form carbohydrates (glucose).
  • Excess glucose is stored as starch granules.

Temperature

• When the temperature is low, the rate of photosynthesis is low.
• Increasing temperature leads to a maximum rate.
• Optimum temperature yields the maximum rate.
• If temperature is higher than optimum, the rate decreases due to enzyme denaturation.

The concentration of carbon dioxide

• At a low carbon dioxide concentration in the rate of photosynthesis is low.
• As the carbon dioxide concentration level increases, the rate of photosynthesis also increases until it reaches a moximum

Greenhouses

• Glass or plastic structures trap heat and allow light to enter, used to grow plants.
• Maintain optimum factor levels.
• Artificial lights for extended photosynthesis.
• $CO_2$ pumped in or produced by burning gas lamps.
• Temperature kept at optimum using heating/cooling devices.

Greenhouse Effect

• Natural phenomenon where heat is trapped by $CO_2$ in the atmosphere.
• Keeps Earth at a temperature that allows for life.
• Enhanced greenhouse effect due to increased $CO_2$ is leading to global warming.

Structural Sustainability of Leaf for Photosynthesis

• Flat laminae (leaf blade) provide a large surface area for sunlight absorption.
• Thin leaves allow sunlight to penetrate and reach all cells.
• Cuticle and epidermal cells are transparent to allow light through to chloroplasts.
• Stomata enable $CO_2$ to diffuse in and oxygen to diffuse out.
• Palisade mesophyll layer:
  • Vertically arranged cells exposed to sunlight.
  • Located in the upper part of mesophyll (nearer to the sun).
  • Densely packed with chloroplasts.
  • Chlorophyll molecules arranged on flat membranes to maximize sunlight exposure.

Life Sciences Experiments

• Determine if a factor is required for photosynthesis.
• Experiment: Plant given all requirements except the tested factor.
• Control: Plant given all requirements.
• Starch test performed to prove photosynthesis took place.

Destarching a Plant

• Plant placed in a dark cupboard for 48 hours.
• Plant uses stored starch.
• Ensures starch present at the end is from the experiment.

Investigation 1: Starch Test

• Glucose is produced during photosynthesis and converted to starch.
• Method:
  • Place leaf in boiling water to soften cell walls and stop metabolism.
  • Place leaf in ethanol to extract chlorophyll (leaf will turn white).
  • Rinse leaf in water.
  • Spread leaf on petri dish and add iodine solution.
• Result:
  • Leaf turns blue-black if starch is present, proving photosynthesis.

Investigation 2: Is Light Necessary for Photosynthesis

• Method:
  • Destarch a potted plant.
  • Cover part of a leaf with aluminum foil.
  • Place plant in a sunny area.
  • Pick the leaf and remove the foil.
  • Do starch test.
• Results:
  • Experiment (leaf in foil): Iodine solution remains light brown.
  • Control (uncovered leaf): Iodine solution turns blue-black.
• Conclusion: Light is required for photosynthesis.

Investigation 3: Carbon Dioxide is Required

• $NaOH$, $KOH$, or sodolime: Remove $CO_2$
• Sodium bicarbonate or potassium bicarbonate: add $CO_2$
• Method:
  • Destarch 2 potted plants.
  • Place one in a sealed bell jar with sodium hydroxide (to absorb $CO_2$).
  • Place the other in bell jar with sodium bicarbonate (to release $CO_2$).
  • Place in a sunny area.
  • Pick a leaf from each plant and test for starch.
• Results:
  • Jar 1 (no $CO_2$): Iodine solution remains reddish-brown.
  • Jar 2 (with $CO_2$): Iodine solution turns from brown to blue-black.
• Conclusion:
  • Photosynthesis cannot take place in the absence of $CO_2$.
  • Photosynthesis takes place in the presence of $CO_2$.

Investigation 4: Chlorophyll is Required

• Variegated leaf (green and white parts) used.
• Method:
  • Place a potted plant with a variegated leaf in a sunny place.
  • Remove a leaf from the plant.
  • Test for starch.
• Results:
  • Experiment (white part): Iodine solution remains reddish-brown.
  • Control (green part): Iodine solution turns from brown to blue-black.
• Conclusion: Chlorophyll is essential for photosynthesis.

Investigation 5: Photosynthesis Produces Oxygen

• A glowing splint is used to test for oxygen.
• Method:
  • Set up the apparatus with pond weed in water.
  • Add sodium bicarbonate to the water to add $CO_2$
  • Place in a sunny area.
  • Collect gas bubbles in a test tube.
  • Insert a glowing wooden splint into the test tube.
• Results:
  • The glowing splint re-ignites or burns more brightly.
• Conclusion: Oxygen is produced during photosynthesis.