Photosynthesis Quiz - Plant Biology

Photosynthesis Overview

• Photosynthesis is the process where plants convert light energy into biomass.
• Photosynthetically Active Radiation (PAR) is light energy between 400 and 700nm wavelengths.
• PAR is approximately 50% of incoming solar radiation.
• In full sunlight, PAR is about 2000 μmol photons/m²/s.
• Solar radiation is composed of 50% PAR, 47% infra-red radiation, and 3% ultraviolet radiation.

Chlorophyll

• The mesophyll of leaves is the main photosynthetic tissue in plants.
• Mesophyll cells contain chloroplasts, which hold chlorophyll pigments.
• Chlorophyll absorbs primarily red and blue light wavelengths, reflecting green light.

Leaf Cross-Section

• A leaf cross-section illustrates the key structures involved in photosynthesis.
• Key leaf structures include the epidermis, mesophyll, xylem, phloem, guard cells, and stomata.

Chloroplast Structure

• Chloroplasts contain several key organelles:
  • Inner and outer membranes
  • Grana (stacks of thylakoids)
  • Thylakoid lumen
  • Stroma (surrounds grana)
  • Stroma lamellae (connections between grana)

Organelles in a Chloroplast

• Stroma: the inner membrane that encloses the chloroplast, containing enzymes that convert CO2 into carbohydrates and the location of the Calvin cycle.
• Thylakoid: flat, sac-like membrane structures within the stroma, often stacked into grana; the site of light reactions.
• Stroma lamellae: thylakoid extensions that connect the individual grana.

Photosynthetic Pigments

• Chlorophyll a and chlorophyll b are the most abundant pigments.
• Carotenoids and phycocyanin are accessory pigments.

Process of Photosynthesis

• Photosynthesis involves a complex series of chemical reactions that convert CO2 and H2O into carbohydrates (C6H12O6).
• The chemical equation is: 6CO2 + 6H2O → C6H12O6 + 6O2
• Light reactions convert light energy to chemical energy, splitting water into oxygen and hydrogen ions.
• The light reactions produce ATP, NADPH, and oxygen.
• The Calvin cycle uses ATP and NADPH (from light reactions) to fix CO2 into a carbohydrate.
• The product of the Calvin cycle is glyceraldehyde-3-phosphate (G3P).

Output of Calvin Cycle

• Glyceraldehyde-3-phosphate (G3P) combines to form fructose.
• Fructose is then rearranged to form glucose or combines with fructose to form sucrose for transport throughout the plant.
• Glucose will undergo respiration to release energy (ATP).

Types of Photosynthesis

• C3 plants fix CO2 only via the Calvin cycle. Examples include rice, soybeans, and sunflowers.
  • C3 plants are shade-loving and thrive in cooler temperatures.
• C4 plants use two pathways: the Hatch-Slack pathway in the mesophyll cells and the Calvin cycle in bundle sheath cells. Examples include maize, sugarcane, and tropical grasses.
  • C4 plants are adapted to hotter, drier climates.
• CAM plants have specialized stomatal activity patterns, opening stomata at night to reduce water loss. Examples include cacti, orchids, and pineapples.

Photorespiration

• Photorespiration occurs under low CO2 conditions, where rubisco binds to oxygen instead of CO2.
• This is considered a wasteful process as it releases CO2 and less carbon is returned to the chloroplast.

Radiation Use Efficiency

• The maximum crop yield under optimum growth conditions depends on the amount of PAR intercepted by the leaf canopy.
• Crop biomass accumulation is proportional to the accumulation of intercepted PAR.
• Radiation use efficiency (RUE) represents how much biomass is produced from each absorbed PAR.

Factors Affecting Photosynthesis Rate

• Nitrogen, moisture, and temperature are major factors that influence the rate of photosynthesis.

Nitrogen

• Nitrogen is essential for chlorophyll formation, thus impacting photosynthesis.
• If nitrogen is deficient, plants can exhibit chlorosis, stunting plant growth.
• Reduced leaf size due to nitrogen deficiency reduces light energy capture.

Moisture

• Water is necessary for plant cell expansion, division, and differentiation.
• Drought reduces leaf canopy expansion, leading to decreased photosynthesis.

Temperature

• Photosynthesis generally increases with temperature up to an optimum.
• Exceeding the optimum temperature for a plant decreases photosynthetic rates.
• This occurs because membrane-bound electron transport processes become unstable, affecting the supply of reducing power, thus causing decreased photosynthesis.
• Plants have different optimal temperatures for photosynthesis based on adaptation to different environments.