Photosynthesis: Light & Dark Reactions

• Photosynthesis is a fundamental process where plants, algae, and some bacteria convert sunlight, water, and carbon dioxide into sugars (glucose), releasing oxygen as a byproduct.

Leaf Structure: Photosynthesis Factory

• The broad, flat shape of a leaf maximizes sunlight absorption.
• Veins (vascular bundles) transport water and nutrients within the leaf.
• The mesophyll layer in the leaf is where most photosynthesis occurs.
• The upper and lower epidermis protect the leaf.
• Stomata (tiny pores) in the leaf allow for gas exchange (CO2 in, O2 out).

Two Phases of Photosynthesis

• Photosynthesis occurs in two main stages: light reactions and dark reactions.

Light Reactions (Light-Dependent Reactions)

• These reactions occur in the thylakoid membranes within the chloroplasts.
• Sunlight is absorbed by chlorophyll pigments.
• Chlorophyll pigments absorb different wavelengths of light (chlorophyll a and chlorophyll b).
• The absorption spectra shows how much each pigment absorbs
• Light energy splits water molecules, releasing oxygen, protons, and electrons.
• Energy is stored in ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
• Thylakoid membranes are stacked into structures called grana, where light reactions happen.
• The fluid-filled space surrounding the grana is the stroma, where dark reactions take place.
• Photophosphorylation is the process of using light energy to create ATP.
  • Cyclic photophosphorylation uses photosystem I and only produces ATP.
  • Non-cyclic photophosphorylation uses both photosystem I and II, producing ATP, NADPH, and oxygen.
  • Cyclic only uses photosystem I.
  • Non-cyclic uses photosystem I and II.

Dark Reactions (Light-Independent Reactions or Calvin Cycle)

• These reactions occur in the stroma.
• ATP and NADPH from the light reactions convert carbon dioxide into glucose.
• The Calvin cycle is a series of chemical reactions where CO2 is "fixed" into organic molecules.
• The end product is a 3-carbon sugar that can be used to build glucose and other carbohydrates.

Photosynthetic Pathways: C3, C4, and CAM Plants

• Plants have adapted to different environments, leading to variations in their photosynthetic pathways.
• C3 plants are the most common type where the first stable product of carbon fixation is a 3-carbon compound and are less efficient in hot, dry conditions because of photorespiration.
• C4 plants have adapted to minimize photorespiration, use a different enzyme to initially fix CO2, creating a 4-carbon compound.
• Because of the extra energy needed to transport the CO2 to the bundle sheath cells, the rate of photophosphorylation is less in C4 plants as compared to C3.
• CAM plants (Crassulacean Acid Metabolism) are adapted to extremely dry conditions and open their stomata at night to take in CO2, minimizing water loss.

Chemical Equation of Photosynthesis

• 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
  • (Carbon dioxide) + (Water) + (Light energy) → (Glucose) + (Oxygen)

Experiments to Demonstrate Photosynthesis

• Oxygen bubbles from an aquatic plant like Elodea under bright light demonstrate the release of energy.
• Boiling a leaf in alcohol to remove chlorophyll, then staining it with iodine demonstrates the presence of starch, with the iodine turning the starch blue-black.

Significance of Photosynthesis

• Photosynthesis is the primary source of energy for almost all life on Earth.
• Photosynthesis produces the oxygen we breathe.
• Photosynthesis removes carbon dioxide from the atmosphere, helping to regulate climate.
• Photosynthesis serves as the base of nearly all food chains.

Explore photosynthesis: plants, algae, & bacteria convert sunlight, water, & CO2 into sugars, releasing oxygen. Learn about leaf structure, light reactions in thylakoids, and dark reactions.