Photosynthesis: Using Light to Make Food (Chapter 7) – Study Notes

Photosynthesis: Using Light to Make Food

Key Terms and Definitions

This section defines essential terms related to photosynthesis, plant physiology, and the biochemistry of energy conversion in plants.

• Biofuel: Fuel derived from living organisms, typically plants or algae, used as a renewable energy source.

• Fossil fuel: Nonrenewable energy sources formed from ancient organic matter, such as coal, oil, and natural gas.

• Photosynthesis: The process by which green plants, algae, and some bacteria convert light energy, carbon dioxide, and water into glucose and oxygen.

• Chloroplast: The organelle in plant and algal cells where photosynthesis occurs.

• Pigment: A molecule that absorbs specific wavelengths of light; in plants, pigments are crucial for capturing light energy.

• Chlorophyll a: The primary pigment in chloroplasts that absorbs light mainly in the blue-violet and red regions; essential for the light reactions of photosynthesis.

• Chlorophyll b: An accessory pigment that broadens the spectrum of light a plant can use by absorbing blue and orange light.

• Carotenoids: Accessory pigments that absorb blue and green light, protecting chlorophyll from damage by excess light energy.

• Stomata: Small pores on the underside of leaves that allow gas exchange (CO2 in, O2 out).

• Stroma: The fluid-filled space inside the chloroplast where the Calvin cycle occurs.

• Thylakoid: Flattened membrane sacs inside the chloroplast where the light reactions take place.

• Grana: Stacks of thylakoids within the chloroplast.

• Light reactions: The first stage of photosynthesis, occurring in the thylakoid membranes, where light energy is converted to chemical energy (ATP and NADPH).

• Calvin Cycle (light-independent reactions): The second stage of photosynthesis, occurring in the stroma, where ATP and NADPH are used to fix carbon dioxide into glucose.

• NADPH: A reduced electron carrier molecule produced in the light reactions and used in the Calvin cycle.

• Carbon fixation: The process of incorporating inorganic CO2 into organic molecules during the Calvin cycle.

• Electromagnetic spectrum: The range of all types of electromagnetic radiation, including visible light.

• Wavelength: The distance between two consecutive peaks of a wave; determines the color of light.

• Visible light spectrum: The portion of the electromagnetic spectrum visible to the human eye (approximately 380–750 nm).

• Absorption: The process by which pigments take in light energy.

• Reflection: The process by which light bounces off a surface; the color seen is the wavelength reflected.

• Transmission: The passage of light through a substance.

• Photon: A particle of light energy.

• Ground state: The lowest energy state of an electron in an atom or molecule.

• Excited state: A higher energy state of an electron after absorbing a photon.

• Decay: The process by which an excited electron returns to its ground state, releasing energy.

• Photosystem: A complex of proteins and pigments in the thylakoid membrane that captures light energy for photosynthesis.

• Reaction center: The part of a photosystem where energy is transferred to an electron, initiating the light reactions.

• Primary electron acceptor: The molecule in the reaction center that receives excited electrons from chlorophyll a.

• C-3 plants: Plants that use the Calvin cycle for the initial steps of carbon fixation, producing a three-carbon compound (3-PGA).

• C-4 plants: Plants that fix CO2 into a four-carbon compound as the first step, helping to minimize photorespiration.

• CAM plants: Plants that open their stomata at night to minimize water loss and fix CO2 in a two-step process.

Major Site of Photosynthesis in Plants

The majority of photosynthesis occurs in the leaves of plants. Leaves are adapted for this function because they have a large surface area to capture sunlight and contain many chloroplasts in their mesophyll cells. The presence of stomata allows for efficient gas exchange, which is essential for photosynthesis.

Structure and Function of a Chloroplast

Chloroplasts are double-membraned organelles found in plant and algal cells. Their main function is to carry out photosynthesis.

• Outer and inner membranes: Enclose the organelle and regulate movement of materials.

• Stroma: The fluid interior where the Calvin cycle occurs.

• Thylakoids: Membranous sacs where light reactions take place.

• Grana: Stacks of thylakoids that increase the surface area for light absorption.

Reactants and Products of Photosynthesis

The overall chemical equation for photosynthesis is:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

• Reactants: Carbon dioxide (CO2), water (H2O), and light energy

• Products: Glucose (C6H12O6) and oxygen (O2)

Relationship Between Photosynthesis and Cellular Respiration

Photosynthesis and cellular respiration are complementary processes. Photosynthesis converts light energy into chemical energy stored in glucose, while cellular respiration breaks down glucose to release energy for cellular activities.

• Organisms performing both: Plants, algae, and some protists (they photosynthesize and respire)

• Organisms performing only respiration: Animals, fungi, and most bacteria

The products of photosynthesis (glucose and O2) are the reactants for cellular respiration, and vice versa.

Oxidized and Reduced Forms of NADPH

• Oxidized form: NADP+

• Reduced form: NADPH

NADP+ accepts electrons during the light reactions to become NADPH, which then donates electrons in the Calvin cycle.

Location of Light Reactions and Calvin Cycle

• Light reactions: Occur in the thylakoid membranes of the chloroplast

• Calvin cycle: Occurs in the stroma of the chloroplast

Implications of Global Deforestation

Deforestation reduces the number of trees and plants available to absorb CO2 through photosynthesis, contributing to increased atmospheric CO2 and climate change. It also leads to loss of biodiversity and disruption of ecosystems.

Absorption of Visible Light by Chloroplasts

• Poorly absorbed wavelengths: Green light (around 500–570 nm) is poorly absorbed and mostly reflected, which is why plants appear green.

• Mainly responsible for photosynthesis: Blue (about 430–450 nm) and red (about 640–680 nm) wavelengths are most efficiently absorbed and drive photosynthesis.

Pigments in Chloroplasts and Their Roles

Chloroplasts contain several types of pigments, each with a specific role in capturing light energy:

Pigment	Role	Absorption Spectrum
Chlorophyll a	Main pigment; directly involved in light reactions	Blue-violet and red
Chlorophyll b	Accessory pigment; broadens usable light spectrum	Blue and orange
Carotenoids	Accessory pigments; protect against excess light	Blue and green

These pigments work together to maximize the absorption of light energy for photosynthesis.