Photosynthesis Overview

Photosynthesis

• Photosynthesis is the process of converting light energy (from sunlight) into chemical energy stored in glucose.
• The process requires carbon dioxide, light energy, and water.
• The product of photosynthesis is glucose and oxygen.
• Plants are autotrophs, meaning they produce their own food.

Chloroplasts

• Photosynthesis takes place within chloroplasts, which are found in plant leaves.
• Chloroplasts are a type of plastid.
• Thylakoid and stroma are types of chloroplasts.

Mesophyll Cell

• A typical mesophyll cell contains 30-40 chloroplasts.
• Chlorophyll is the primary pigment involved in photosynthesis.

Nature of Light and Photosynthesis

• Visible light drives photosynthesis.
• Shorter wavelengths have higher energy; longer wavelengths have lower energy.
• Plants absorb most light in the red and blue wavelengths, and least in green.

Pigments in Plants

• Chlorophyll a is the main pigment, absorbing violet-blue and red light.
• Chlorophyll b and carotenoids are accessory pigments.
• Carotenoids absorb different wavelengths and pass energy to chlorophyll a.
• Chlorophyll a is the most efficient pigment, absorbing violet-blue (approximately 430-450 nm) and orange-red light (approximately 640-680 nm).

Photosynthesis as a Redox Process

• Photosynthesis involves reduction (carbon dioxide gains electrons) and oxidation (water loses electrons).
• The overall chemical reaction for photosynthesis is: 6CO2 + 6H2O → C6H12O6 + 6O2.
• This reaction produces glucose (a sugar) and oxygen as a byproduct.

Stages of Photosynthesis

• Light-Dependent reactions occur in the thylakoid membrane.
• Dark reactions (Calvin Cycle) occur in the stroma.

Light Dependent Reactions

• Location: Thylakoid membranes of the chloroplasts.
• Process:
  • Water splitting (photolysis) releases oxygen.
  • Light energy absorbed by chlorophyll drives the transfer of electrons and H+ from water to NADP+, forming NADPH.
  • ATP is formed via chemiosmosis.
• Products: NADPH, ATP, and oxygen.

Calvin Cycle

• Location: Stroma of the chloroplasts.
• Energy Use: Consumes ATP and NADPH.
• Phases:
  • Carbon fixation: CO2 is incorporated into an organic molecule.
  • Reduction: The fixed carbon is reduced to carbohydrate using electrons from NADPH.
  • Regeneration: G3P (a three-carbon molecule) is created and then regenerated into RuBP(five-carbon compound). This cycle has to be repeated.

Photosystems

• Photosystem II (PSII) absorbs light and excites electrons.
• PSII splits water (H20), releasing H+, electrons, and O2.
• The electrons are moved to an electron transport chain, which generates ATP.
• Photosystem I (PSI) absorbs light and utilizes electrons from the chain, also generating NADPH.

Other Important Concepts in Photosynthesis

• Photolysis is the splitting of water to release electrons, hydrogen ions, and oxygen.
• Chemiosmosis is the process that utilizes the gradient of H+ ions (protons) to produce ATP.
• Different plants use varying photosynthetic pathways like C4 or CAM to adapt to their environment (e.g. high temperatures, or water stress).

Plant Organs

• Vegetative organs (stem, roots, leave) are involved in basic life processes like support, transport, growth and storage of plants.
• Roots : Roots anchor the plant in the ground, absorb water and nutrients, also store food.
• Stems : Stems support the plant, transport substances (water, nutrients, and sugars), and can provide food storage.
• Leaves : Leaves are the primary site of photosynthesis. They contain chlorophyll to trap sunlight and produce food.

Types of Cells in Plants

• Parenchyma cells are general purpose cells involved in photosynthesis, storage, and wound healing.
• Collenchyma cells provide flexibility and support.
• Sclerenchyma cells have thick cell walls and provide rigid support.

Reproductive Structures

• Flowers are the specialized reproductive structures of angiosperms.

• The parts of a flower include sepals, petals, stamen (male), pistil (female), receptacle, and peduncle.

• Complete flowers have all four basic parts, while incomplete flowers lack part of these parts.

• Perfect flowers have both male and female reproductive organs whereas imperferct flowers only have one.

• Fruits are mature ovaries that develop around a seed for protection of the plant. Simple fruits develop from one flower and one ovary, fruits from multiple ovaries develop into aggregate fruits and those that involve other flower parts into false fruits.

• Seeds are the reproductive structure of plants. They contain a plant embryo. Endosperms are food sources enclosed in the seed coat for a growing plant.