Nutrition Modes

Introduction to Energy Flow and Recycling in Ecosystems

In ecosystems, energy flows in one direction, while nutrients are recycled. Organisms obtain energy and carbon through different nutritional modes, which are classified based on their energy source and carbon source.

• Autotrophs: Organisms that produce their own food from inorganic sources.

• Heterotrophs: Organisms that obtain food by consuming other organisms.

Classification of Nutrition Modes

• Example: Cyanobacteria are photoautotrophs; humans are chemoheterotrophs.

Anatomy of the Chloroplast

Structure and Function

The chloroplast is the site of photosynthesis in plant cells. It contains its own DNA and specialized structures for capturing light energy and converting it into chemical energy.

• Thylakoid Membranes: Flattened sacs where light-dependent reactions occur.

• Stroma: Fluid-filled space where the Calvin cycle (light-independent reactions) takes place.

• Grana: Stacks of thylakoids.

Additional info: Like mitochondria, chloroplasts have their own DNA and ribosomes, supporting the endosymbiotic theory.

General Formula of Photosynthesis

Overall Reaction

Photosynthesis is the process by which plants convert light energy, water, and carbon dioxide into glucose and oxygen.

• General Equation:

• Oxygen is produced from the oxidation of water.

• Glucose is produced from the reduction of carbon dioxide.

• Water is the oxidizing agent in photosynthesis because it loses electrons.

Stages of Photosynthesis

Light Reactions and Calvin Cycle

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle. Each stage occurs in specific locations within the chloroplast and involves distinct inputs and outputs.

• NADP+ is reduced to NADPH during light reactions.

Light and Pigments

Absorption of Light by Pigments

Plant pigments, such as chlorophyll a, chlorophyll b, and carotenoids, absorb light at specific wavelengths to drive photosynthesis.

• Chlorophyll a: Main pigment, absorbs best at blue-violet and red wavelengths.

• Chlorophyll b: Accessory pigment, broadens the range of light absorption.

• Carotenoids: Accessory pigments, absorb additional wavelengths and protect against photo-damage.

• Green part of the visible spectrum is reflected, making plants appear green.

Engelmann's Experiment

Engelmann demonstrated that photosynthesis is most efficient at wavelengths absorbed by chlorophyll, using algae and aerobic bacteria.

Light Reactions

Photosystems and Electron Transport

Light reactions occur in the thylakoid membranes and involve two photosystems (PS II and PS I) that capture light energy and transfer electrons through an electron transport chain.

• Photosystem II (PS II): Contains P680 chlorophyll, oxidizes water to replace lost electrons.

• Photosystem I (PS I): Contains P700 chlorophyll, receives electrons from PS II via the electron transport chain.

• Primary electron acceptor is reduced when it gains electrons.

• NADP+ is reduced to NADPH at the end of the electron transport chain.

Chemiosmosis

During light reactions, a proton gradient is established across the thylakoid membrane, driving ATP synthesis via ATP synthase.

• High H+ concentration in the thylakoid space creates a proton-motive force.

• ATP and NADPH produced are used in the Calvin cycle.

Calvin Cycle

Steps and Enzymes

The Calvin cycle is the set of light-independent reactions that fix carbon dioxide into organic molecules. It occurs in the stroma of the chloroplast.

• Step 1: RuBisCO enzyme catalyzes the reaction between CO2 and RuBP.

• Step 2: Reduction of a 3-carbon intermediate to form G3P.

• Step 3: ATP is used to regenerate RuBP from G3P.

• To generate 1 molecule of G3P: 3 CO2, 9 ATP, and 6 NADPH are required.

• The Calvin cycle is anabolic and does not directly depend on light energy.

• O2 released during photosynthesis comes from water, not CO2.

Alternative Ways of Carbon Fixation

C4 and CAM Pathways

Plants in hot or dry climates have evolved alternative methods of carbon fixation to minimize water loss and photorespiration.

• Rubisco has high affinity for CO2, but photorespiration increases in hot climates.

• C4 and CAM plants reduce water loss and photorespiration by separating carbon fixation spatially or temporally.

Additional info: Photorespiration is a wasteful process that occurs when Rubisco binds O2 instead of CO2, leading to loss of fixed carbon.