Untitled Quiz

Photosynthesis: An Overview

• Photosynthesis is the process by which plants create sugar using light energy.
• The general equation for photosynthesis is: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂.
• Photosynthesis takes place in chloroplasts.

Photosynthesis in Chloroplasts

• Chloroplasts contain an internal membrane system of connected disc-shaped structures called thylakoids.
• Thylakoids stack to form grana.
• The region between the thylakoid membranes is the thylakoid space.
• The major pigment in chloroplasts is chlorophyll a.
• Chlorophyll a, along with other pigments, captures light energy from visible light and uses this energy to convert CO₂ and H₂O into glucose.

Leaf Structure

• Leaves are the photosynthetic organs of plants.
• Mesophyll cells perform photosynthesis.
• Mesophyll cells contain the largest number of chloroplasts.
• Stomata are pores in leaves that allow CO₂ in and O₂ out.

Stomata

• Stomata perform three functions:
  • Allowing CO₂ in
  • Allowing O₂ out
  • Evaporative cooling: When water exits through stomata, leaves cool.

Stomata Regulation

• Stomata regulate their size to maximize CO₂ uptake and minimize water loss.
• Stomata typically reduce in size under sunny, warm, dry, and windy conditions.
• Guard cells control the opening and closing of stomata.
• When water is scarce, guard cells lose water and close stomata.
• When water is abundant, guard cells swell and open stomata.

Photosynthesis: Why are Plants Green?

• Visible light is electromagnetic radiation with wavelengths between 400 and 700 nm.
• Plants appear green because green wavelengths of light are reflected.
• Other wavelengths are absorbed.
• Chlorophyll absorbs light most strongly in the blue and red portions of the visible spectrum.

Pigments

• Pigments are substances that absorb visible light.
• The reflected and transmitted light include the colors we see.
• Absorbed light is important in driving photosynthesis.
• Chlorophyll a is the main photosynthetic pigment
• Chlorophyll b is an accessory pigment.

Two Types of Reactions in Photosynthesis

• Photosynthesis involves two main processes:
  • Light-dependent reactions (light reactions)
    • Require light
    • Excite electrons and split H₂O
  • Light-independent reactions (dark reactions)
    • Do not directly require sunlight
    • Including Carbon fixation, the Calvin cycle

Light Reactions

• Occur in the grana.
• Split water, release oxygen, produce ATP, and form NADPH.

Dark Reactions (Calvin Cycle)

• Occur in the stroma.
• Forms sugar from carbon dioxide, using ATP and NADPH.
• A cyclic process that fixes carbon, utilizes energy molecules, and regenerates molecules.

Stages of Photosynthesis

• Light reactions produce NADPH and ATP.
• Dark reaction uses NADPH and ATP to form glucose from carbon dioxide.

The Light Reactions (Light-Dependent Reactions)

• The light reactions begin when light energy is trapped by chloroplasts.
• This light energy is absorbed by a complex cluster of chlorophyll molecules.
• This complex is called a photosystem.
• The first photosystem involved in the light reactions is photosystem II (or P680).
• Electron transport chain components transport lost electrons from photosystem II.
• Electrons are passed down a chain of electron acceptors to photosystem I (or P700)
• Photosystem II replenishes lost electrons from water; oxygen is produced as a by-product.
• This energy is used to produce ATP through chemiosmosis.
• The electrons reach photosystem I , which excites the electrons and passes them to NADP+, creating NADPH.

Photosystems

• Photosystems I and II are complexes containing chlorophyll a and other pigments

Electron Transport

• Electron transport occurs in the thylakoid membrane.
• Two mechanisms of electron transport:
  • Non-cyclic electron flow.
  • Cyclic electron flow.

Non-Cyclic Electron Flow

• The primary pathway.
• Process starts with Photosystem II (or P680) which is energized by light and releases electrons to an electron transfer chain
• Electrons pass through Cytochrome Complex
• The energy from the electron transport allows the active transport of hydrogen ions into the thylakoid space
• The final electron acceptor NADP+ is reduced to NADPH as it picks up a hydrogen ion giving NADPH.

Cyclic Electron Flow

• Cyclic electron flow generates more ATP using only Photosystem I.

ATP Synthase

• Protons pass through ATP synthase by facilitated diffusion.
• ATP is produced in the stroma.

Factors Affecting Photosynthesis

• Light intensity
• Carbon dioxide concentration
• Temperature

C3 Plants

• Stomata open during day and close at night.
• When stomata close in hot, arid conditions, O2 builds up within the cells and Rubisco binds to O2 rather than CO2, resulting in photorespiration.

C4 Plants

• Adaptation to hot, arid environments
• Mesophyll cells create 4-carbon molecules.
• Bundle sheath cells perform Calvin cycle.
• Allows Rubisco to have sufficient CO₂ to prevent binding with O₂

CAM Plants

• Adaptation to hot, arid environments.
• Stomata are closed during the day, and open at night.
• Carbon fixation happens during the night, and the Calvin cycle happens during the day.

Homework

• Read Chapter 3.1
• Do page 145 problems 4, 5, 6, and 7.
• Read Section 3.2 questions (page 154-155) Questions 1-3, 11.
• Read Section 3.3 Questions (page 166-167) Questions 1-4, 6, 8a(i-iii), 8b.