Chapter 1 - Photosynthesis

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Questions and Answers

What drives the synthesis of carbohydrates during the dark reactions of photosynthesis?

Oxygen and carbon dioxide

Water and chlorophyll

ATP and NADPH (correct)

Light and photons

Which organelle is responsible for carrying out photosynthesis in plants and algae?

Nucleus

Chloroplast (correct)

Mitochondria

Ribosome

What is the fluid-filled region that lies between the thylakoid membrane and the inner membrane of the chloroplast called?

Intermembrane space

Thylakoid lumen

Stroma (correct)

Granum

How do leaves appear green to the human eye?

They absorb red and violet light and reflect green light Signup and view all the answers

What is the primary role of chlorophyll in photosynthesis?

To absorb and transfer light energy Signup and view all the answers

What occurs when an electron in a pigment absorbs energy during photosynthesis?

It becomes an excited state Signup and view all the answers

Which spectrum plots the rate of photosynthesis against the wavelength of light?

Action spectrum Signup and view all the answers

Which pigments primarily absorb blue and blue-green wavelengths of light?

Carotenoids Signup and view all the answers

What is the primary function of carotenoids in plants?

To protect against excessive light Signup and view all the answers

What does the absorption spectrum depict?

A pigment's light absorption as a function of wavelength Signup and view all the answers

What is the main purpose of the light reactions in photosynthesis?

To generate ATP, NADPH, and O2 Signup and view all the answers

Where do the dark reactions, specifically the Calvin cycle, occur?

In the stroma Signup and view all the answers

Which photosystem is responsible for the initial step of photosynthesis?

Photosystem II Signup and view all the answers

How is ATP synthesized in chloroplasts?

By chemiosmotic photophosphorylation Signup and view all the answers

What is the role of the electron transport chain (ETC) in photosystem II?

To pump H+ ions into the thylakoid lumen Signup and view all the answers

Which statement correctly describes the Z scheme of photosynthesis?

It depicts the zigzag energy level changes of electrons Signup and view all the answers

What is produced at the end of the light reactions apart from ATP and NADPH?

O2 Signup and view all the answers

What pigment molecule does Photosystem I primarily use?

P700 Signup and view all the answers

Which molecule is produced as a result of the oxidation of water by PSII?

Oxygen Signup and view all the answers

What is the primary role of ATP synthase in photosynthesis?

To produce ATP using a proton gradient Signup and view all the answers

In the Calvin Cycle, what is the first product formed when CO2 is incorporated into RuBP?

3PG Signup and view all the answers

What characteristic of C4 plants helps minimize photorespiration?

Production of a four-carbon molecule in carbon fixation Signup and view all the answers

Which part of the photosynthetic process is favored when NADP+ levels are low?

Cyclic photophosphorylation Signup and view all the answers

What is the primary waste product of photorespiration?

Carbon dioxide Signup and view all the answers

How many ATP and NADPH molecules are required to incorporate six CO2 into carbohydrates through the Calvin Cycle?

18 ATP and 12 NADPH Signup and view all the answers

Which enzyme is responsible for the carbon fixation step in the Calvin Cycle?

Rubisco Signup and view all the answers

What is the final product of the Calvin Cycle used for glucose synthesis?

G3P Signup and view all the answers

Which factor does NOT influence the efficiency of the Calvin Cycle?

Soil pH levels Signup and view all the answers

What role do autotrophs play in the ecosystem?

They produce energy-rich food compounds using solar energy. Signup and view all the answers

Which group of organisms specifically uses sunlight to convert water and carbon dioxide into sugar?

Phototrophs Signup and view all the answers

What event is believed to have contributed to the extinction of dinosaurs by disrupting photosynthesis?

An asteroid impact blocking sunlight Signup and view all the answers

During which part of photosynthesis is light energy converted into chemical energy?

Light reactions Signup and view all the answers

What is the main function of chlorophyll in photosynthesis?

To absorb light energy Signup and view all the answers

What distinguishes consumers from autotrophs in an ecosystem?

Consumers cannot photosynthesize and depend on other organisms for energy. Signup and view all the answers

Which of the following molecules is produced during the light reactions of photosynthesis?

Oxygen Signup and view all the answers

In which part of the cycle do plants fix carbon dioxide into organic molecules?

Calvin Cycle Signup and view all the answers

Study Notes

Photosynthesis Overview

Photosynthesis is the process where some organisms capture solar energy and transform it into chemical energy for living things.
Producers make their own energy-rich food.
On land, producers use chlorophyll.

Key Concepts

Overview of Photosynthesis
Reactions Harnessing Light Energy
Synthesizing Carbohydrates via the Calvin Cycle
Variations in Photosynthesis

Autotrophs

Autotrophs are organisms that make their own food.
Phototrophs use solar energy for photosynthesis.
Chemotrophs use different chemical processes to get energy.
Autotrophs convert water (H₂O) and carbon dioxide (CO₂) into sugar and oxygen (O₂).

Photosynthesis or not?

Many organisms cannot photosynthesize; these are consumers.
Consumers obtain energy from consuming other organisms.
Consumers use cellular respiration to obtain energy from food.

Heterotrophs

Heterotrophs are organisms that must take in food to get energy.
Heterotrophs include herbivores (consume autotrophs), carnivores (consume other heterotrophs), and omnivores (consume both).

Dinosaur Extinction

A huge asteroid hit Earth near Mexico, sending up dust that blocked the sun.
The lack of sunshine caused a lack of food, ultimately causing dinosaurs to die off.

Photosynthesis Powers the Biosphere

The biosphere is the region on Earth's surface where living organisms exist.
Photosynthesis in plants, algae, and cyanobacteria drives the biosphere.
Cells use organic molecules for energy, with plants replenishing these molecules through photosynthesis.
Photosynthesis produces oxygen.

Photosynthesis Stages

Two stages:
- Light reactions: Light energy absorbed by chlorophyll is converted to ATP and NADPH.
- Dark reactions (Calvin Cycle): ATP and NADPH drive carbohydrate synthesis.

Photosynthesis and Cellular Respiration

Photosynthesis: CO₂ + H₂O + light energy → sugar + O₂
Cellular Respiration: sugar + O₂ → CO₂ + H₂O + energy (ATP)
The products of one reaction are the reactants of the other reaction.

Chloroplast

Chloroplast is an organelle where photosynthesis occurs in plants and algae.
Chlorophyll is the green pigment involved in photosynthesis in leaves.
Mesophyll cells in leaves receive light, water, and carbon dioxide for photosynthesis.
Carbon dioxide enters and oxygen exits through pores called stomata.

Chloroplast Anatomy

Outer and inner membranes have an intermembrane space.
A third membrane (thylakoid membrane) contains pigment molecules.
Thylakoids are membrane-bound compartments.
Granum a stack of thylakoid membranes.
Stroma is the fluid-filled region outside the thylakoid membranes.

Reactions Harnessing Light Energy

During photosynthesis, light energy is transferred from the sun to a pigment molecule in a plant.
Light is a type of electromagnetic radiation that travels as waves.
Electromagnetic spectrum: encompasses all possible wavelengths of electromagnetic radiation.
Light also behaves as particles called photons.
Shorter wavelengths carry more energy.

Photosynthetic Pigments

Pigments absorb light energy and reflect others.
Leaves appear green because they absorb red and violet light and reflect green light.
Absorption of light boosts electrons to higher energy levels.
Plants have different pigments to absorb light at many wavelengths.
Excited electrons release energy as heat or light.

Structure of Pigment Molecules

Plants have different pigment molecules that absorb light energy for photosynthesis.
Chlorophylls (a and b) contain a porphyrin ring and a magnesium ion for this photosynthesis process.
Carotenoids are other pigments in plants that are found mostly in flowers and fruit.

Absorption versus Action Spectrum

Absorption spectrum plots a pigment's light absorption as a function of wavelength.
Chlorophylls strongly absorb red and violet light, and reflect green light.
Action spectrum plots the rate of photosynthesis as a function of wavelength.
Different pigments allow plants to absorb light at many different wavelengths.

Algal Photobioreactors

Algal photobioreactors are used to grow algae for biofuel or other purposes.

Two Stages of Photosynthesis

Light reactions occur in the thylakoid membrane.
Light reactions utilize light energy to produce ATP, NADPH, and O₂.
Dark reactions (Calvin Cycle) happen in the stroma.
Dark reactions use CO₂, ATP, and NADPH to form sugars.

Light Reactions

Captured light energy is transferred to other molecules, producing energy.
Thylakoid membranes of chloroplasts have distinct complexes of molecules—photosystems I & II
Photosystem II is discovered first.
Light excites electrons which enter a transport chain.

Photosystem II & Electron Transport Chain

Light excites electrons in the pigment molecules within the light-harvesting complex of Photosystem II.
Energy moves to the P680 pigment molecule.
Oxidizes water, releasing O₂ and H⁺.
Electrons exit Photosystem II and go to electron transport chain

Photosystem I

Light hits the light-harvesting complex of Photosystem I.
High energy electrons are removed from the P700 pigment.
Electrons are transferred to NADP+ to form NADPH.
Electrons flow linearly from Photosystem II to Photosystem I, reducing NADP+ to NADPH.

Formation of ATP in Chloroplasts

ATP is formed via chemiosmosis called photophosphorylation.
H⁺ flow from thylakoid lumen into stroma via ATP synthase.
Increased H⁺ in the thylakoid lumen is due to water splitting, ETC pumping H⁺, and NADPH formation.

Z Scheme

Photosynthesis involves increase and decrease in the energy per electron as it moves through Photosystem II and Photosystem I.
Electrons start with least energy in Pigment molecule in Photosystem II.
Light excites electrons in Photosystem II

Three Chemical Products Generated

Oxygen: Produced in the thylakoid lumen following water oxidation by Photosystem II.
NADPH: Produced in stroma from high-energy electrons from PS I; high energy-electrons are boosted to form NADPH.
ATP: Produced in stroma using the electrochemical gradient/proton gradient of H⁺ to drive ATP synthase, and create ATP.

Noncyclic and Cyclic Electron Flow

Noncyclic: Electrons initiate at PS II, flowing to NADPH (occurs in equal proportions to ATP).
Cyclic: Electrons flow through PSI which results only in ATP production

Dark Reactions: Calvin Cycle

Carbon fixation, reduction, and regeneration steps.
CO₂ incorporated into carbohydrates via RuBisCO enzyme
RuBisCO incorporates CO₂ into RuBP in a 6-carbon intermediate stage that cleaves into 2 3PG.
Requires ATP and NADPH to reduce 3PG to G3P (glyceraldehyde-3-phosphate), the initial carbohydrate product formed.
10 G3P molecules are used to regenerate RuBP for further Carbon fixation steps.

Photorespiration

Rubisco can function as an oxygenase leading to photorespiration.
A process in which oxygen is added to RuBP instead of carbon dioxide.
More likely to occur in hot and dry environments when CO₂ levels are low.

Variations in Photosynthesis

Environmental conditions influence the efficiency of the Calvin cycle.
Light intensity, temperature, and availability of water affect photosynthesis.
Stomata are tiny openings in plant tissue that regulate gas exchange (CO₂ and water vapor).

C₄ Plants

Evolved a mechanism to minimize photorespiration.
Mesophyll cells initially incorporate CO₂ into a 4-carbon compound (oxaloacetate).
Bundle sheath cells release CO₂ to drive the Calvin cycle—a way of increasing CO₂ levels around Rubisco, thus preventing photorespiration, and improving the efficiency of photosynthesis.

CAM Plants

Separate processes over time, opening stomata at night, capturing CO₂, converting it to malate, and releasing CO₂ during the day to drive the Calvin cycle, and conserving water.

Review Video Summary

Photosynthesis overview from a video.

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