Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which solar light energy is transformed into chemical bond energy, stored in sugar and other organic molecules. This process occurs in chloroplasts and is fundamental to life on Earth.

• Photosynthesis synthesizes energy-rich organic molecules from energy-poor molecules such as CO2 and H2O.

• Uses CO2 as a carbon source and light energy as the energy source.

• Directly or indirectly supplies energy to most living organisms.

Modes of Nutrition

Autotrophic Nutrition

Autotrophic nutrition is the mode by which organisms synthesize organic molecules from inorganic raw materials. These organisms are called autotrophs.

• Photoautotrophs: Use light energy to synthesize organic molecules. Examples include plants, algae, and some prokaryotes.

• Chemolithoautotrophs: Use the oxidation of inorganic substances to obtain energy. Examples include some bacteria.

• Autotrophs are also known as producers because they produce organic molecules that enter an ecosystem’s food store.

Heterotrophic Nutrition

Heterotrophic nutrition is the mode by which organisms acquire organic molecules from other organisms, as they are unable to synthesize organic molecules from inorganic raw materials. These organisms are called heterotrophs or consumers.

• Examples include animals and decomposers (most fungi and some bacteria).

• Most plants are photoautotrophs, but some bacteria are heterotrophs.

Chloroplast Structure and Function

Chloroplast Anatomy

Chloroplasts are lens-shaped organelles found in the cells of leaves and other green tissues. They are the site of photosynthesis.

• Chloroplasts are primarily found in mesophyll, the green tissue in the leaf’s interior.

• CO2 enters the leaf through microscopic pores called stomata.

• Water is absorbed by roots and transported to leaves through vascular bundles.

• Chloroplasts are divided into functional compartments by a system of membranes:

  • Intermembrane space: The chloroplast is bounded by a double membrane, creating a narrow intermembrane space.

  • Thylakoid space: Thylakoids form another membrane system within the chloroplast. The thylakoid membrane segregates the interior into two compartments: thylakoid space and stroma.

  • Thylakoids: Flattened membranous sacs inside the chloroplast. Chlorophyll is found in the thylakoid membranes.

  • Grana: Stacks of thylakoids.

Summary Equation of Photosynthesis

The overall chemical equation for photosynthesis is:

Stages of Photosynthesis

Light Reactions

Light reactions convert light energy to chemical bond energy in ATP and NADPH. These reactions occur in the thylakoid membranes.

• Reduce NADP+ to NADPH.

• Light absorbed by chlorophyll provides the energy to reduce NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH, which temporarily stores the energized electrons transferred from water.

• O2 is given off as a by-product from the splitting of water.

Calvin Cycle (Dark Reactions)

The Calvin cycle consists of carbon-fixation reactions that assimilate atmospheric CO2 and reduce it to carbohydrate. These reactions occur in the stroma of the chloroplast.

• First, CO2 is incorporated into existing organic molecules by a process called carbon fixation.

• Then, the fixed carbon is reduced to carbohydrate.

• The Calvin cycle does not require light directly, but reduction of CO2 to sugar requires the products of the light reactions: NADPH and ATP.

• ATP provides the chemical energy.

Photosynthetic Pigments

Types and Functions of Pigments

Photosynthetic pigments are substances that absorb visible light. Light may be reflected, transmitted, or absorbed when it contacts matter.

• Different pigments absorb different wavelengths of light.

• Pigments that absorb all wavelengths appear black.

• Pigments that transmit or reflect some wavelengths appear colored.

• Each pigment has a characteristic absorption spectrum, which can be measured using a spectrophotometer.

• Chlorophyll a is the key light-absorbing pigment in photosynthesis.

• The action spectrum for photosynthesis does not exactly match the absorption spectrum for chlorophyll a, because other pigments (such as chlorophyll b and carotenoids) also participate.

Calvin Cycle Details

Key Steps and Molecules

• The Calvin cycle begins when each molecule of CO2 is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).

• This reaction is catalyzed by RuBP carboxylase (rubisco), one of the most abundant proteins on earth.

• The product of this reaction is an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate.

• The end-product of the Calvin cycle is a three-carbon sugar, glyceraldehyde 3-phosphate (G3P).

• For every three CO2 molecules that enter the Calvin cycle and three turns of the cycle, six G3P molecules are made; three are needed to make one glucose molecule.

• For the net synthesis of one G3P molecule, the Calvin cycle uses 9 ATP and 6 NADPH molecules (products of the light reactions). So, the Calvin cycle uses 18 ATP and 12 NADPH molecules to make one glucose molecule.

Why Shouldn't You Sleep Under a Tree?

Non-photosynthetic parts of plants depend on organic molecules exported from leaves in veins. They use the process of cellular respiration (by-product is CO2) to break down sugars for fuel.

• Most plants make more organic material than needed for respiratory fuel and for precursors of biosynthesis.

• Plants consume about 50% of the photosynthate as fuel for cellular respiration.

• Extra sugars are synthesized into starch and stored in storage cells of roots, tubers, seeds, and fruits.

• Heterotrophs also consume parts of plants as food.

Table: Comparison of Autotrophs and Heterotrophs

Table: Key Pigments in Photosynthesis

Additional info: The notes have been expanded to include definitions, examples, and tables for clarity and completeness. All equations are provided in LaTeX format as required.