Photosynthesis—Part 1

Plant Cell Anatomy Review

Two Unique Structures

Cell wall

• Not found in animals
• Provides support for cell and limits its volume by remaining rigid
• Located outside the plasma membrane and is much thicker than the plasma membrane
• Consists of cellulose fibers embedded in other polysaccharides and proteins

Central vacuole

• Comprises 80 % or more of mature cells!
• A place to store all kinds of molecules and water

Figure 7.8, page 109, Campbell's Biology, 5th Edition

The Chloroplast

• The organelle in which photosynthesis occurs
• Origin: most likely a result of endosymbiosis of bacteria
• Contains chlorophyll, which gives it its green color
• Double-membrane structure:
  • Outer membrane
  • Inner membrane
• Thylakoid: a membrane-bound flattened sac that looks like a poker chip
• Granum: a stack of thylakoids (looks like a stack of poker chips)
• Stroma: fluid surrounding the thylakoids
• Thylakoid space: the space inside the thylakoid

Part of figure 7.18, page 118, Campbell's Biology, 5th Edition

Photosynthesis Introduction

• General equation: light + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Reactants: carbon dioxide and water
• Products: glucose and oxygen
• Energy conversion: light energy is converted into chemical energy (glucose)

Figure 8.1, Purves's Life: The Science of Biology, 7th Edition

Where Does the Oxygen Come From?

• Surprisingly, the oxygen that is produced during photosynthesis was discovered to come from the water, not the carbon dioxide!
• This research was conducted using radioactive isotopes to label and trace the atoms.
  • This is a good technique to remember for the AP Biology exam experimental question.

Figure 8.2, Purves's Life: The Science of Biology, 7th Edition

Photosynthesis Overview

The process of photosynthesis is composed of two sets of reactions:

Light reactions

• This is the “photo” part.
• Light energy is converted into chemical energy.
  • ATP
  • NADPH

Dark reactions (Calvin cycle)

• This is the “synthesis” part
• This can actually happen in the light or the dark; it is called the “dark reaction” because light is not needed.
• CO₂ is converted into glucose.

Figure 8.3, Purves's Life: The Science of Biology, 7th Edition

Light Reaction: Chloroplasts Capture All Light Energy from the Sun Except Green Light!

• Light from the sun is a form of energy.
• Plants absorb light (energy) of all colors coming from the sun except green.
  • We see plants as green because green is the only color reflected to our eyes.

Figure 8.5, Purves's Life: The Science of Biology, 7th Edition; figure 10.6, page 174, Campbell's Biology, 5th Edition

Absorption and Absorption Spectra

Absorption spectra

• Represent the frequencies of light that are absorbed by different molecules (pigments)
  • Chlorophyll a
  • Chlorophyll b
  • Carotenoids
• These pigments absorb light energy from the blue and red ends of the spectrum best.

Action spectra

• Represent the biological effectiveness of different frequencies of light (i.e., how well different frequencies of light drive photosynthesis)
• The green part of the spectrum is not as useless as it might first appear from the absorption spectra.

Figure 10.7(a) and (b), page 174, Campbell's Biology, 5th Edition

Chlorophyll Is Located in Chloroplasts

• Chlorophyll is a molecule (pigment) that absorbs the incoming light.
• It is embedded in the thylakoid membrane.
• Components:
  • Porphyrin ring head
    • Magnesium atom at its center
    • The actual light-absorbing structure
  • Hydrocarbon tail

Figure 10.8, page 175, Campbell's Biology, 5th Edition

Energy Absorption: A Molecular View

• Light energy causes an electron to move to a higher energy state (a higher orbital)
  • This is the point at which light energy has been changed to chemical energy!
• The natural tendency is for the electron to fall back to its ground state, giving off both heat and light.
• Light-absorbing pigments like chlorophyll prevent this; they capture and hold the electron at its high-energy state.

Figures 8.4 and 8.8, Purves's Life: The Science of Biology, 7th Edition

How a Photosystem Harvests Light

• Light-absorbing pigments like chlorophyll a and b are bound together in the thylakoid membrane into what is called a photosystem.
• The captured energy is transferred between molecules in the photosystem until it reaches the reaction-center chlorophyll—chlorophyll a.
• Two photosystems exist in the thylakoid membrane:
  • Photosystem I: reaction-center chlorophyll (P700) absorbs light best at 700 nm
  • Photosystem II: reaction-center chlorophyll (P680) absorbs light best at 680 nm

Figure 10.10, page 176, Campbell's Biology, 5th Edition

Noncyclic Electron Flow = Noncyclic Photophosphorylation

• Noncyclic: not a cycle—it is a one way flow of electrons and energy.
• Photophosphorylation
  • Photo: light
  • Phosphorylation: adding a phosphate
  • Thus, photophosphorylation means making ATP from ADP and inorganic phosphate using light energy (photons).
  • This is how light energy is initially captured and converted into chemical energy.
• Photosystem II
  • Absorbs light
  • Light energy causes an electron to be energized.
  • This energized electron makes its way to the primary electron acceptor.
  • The energized electrons are replaced by electrons that come from water.
  • This process produces oxygen (O₂).
• Electron transport chain
  • The energized electrons pass from photosystem II to photosystem I.
  • Similar to the electron transport chain in cellular respiration
    • Uses cytochromes, etc.
  • As the energized electrons move from a higher energy state to a lower energy state, their energy is converted into ATP.
  • This is called photophosphorylation because it uses light to make ATP.
    • In cellular respiration it is called oxidative phosphorylation, and the motion of protons (H⁺ ions) is used instead.
• Photosystem I to NADP⁺
  • When the electron reaches the bottom of the electron transport chain, it fills a hole left by an energized electron.
  • The electron that has been energized is captured by a different primary electron acceptor.
  • The energized electron is passed down a short electron transport chain to the final electron acceptor: NADP⁺, which becomes NADPH.

Figure 8.9, Purves's Life: The Science of Biology, 7th Edition

Light Reaction Review

• Two photosystems are embedded in the thylakoid membrane.
• Light (photons) of a particular wavelength is absorbed by pigment molecules like chlorophyll (which are located in the thylakoid membrane of the chloroplast).
• This absorption excites the electrons to a higher energy state.
• These electrons flow back down their energy gradient, moving through electron transport chains and producing chemical energy:
  • Between photosystem II and photosystem I: ATP
  • After photosystem I: NADPH
• The electrons that flow through this system are replaced by breaking up water molecules.
  • The product of the breakup is oxygen gas.
• Overall, the process transforms light energy into chemical energy in the form of ATP and NADPH.

Figure 10.12, page 178, Campbell's Biology, 5th Edition

Cyclic Electron Flow

• An alternative path for the electrons
• Electrons cycle back into the electron transport chain between photosystem II and photosystem I.
• No production of NADPH
• No release of oxygen (since it is reusing the electrons)
• Generates ATP

Figure 10.13, page 178, Campbell's Biology, 5th Edition