Photosynthesis: Light-Dependent Reactions

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Questions and Answers

During photosynthesis, what is the primary role of water?

  • To directly absorb light energy from the sun.
  • To regulate the opening and closing of stomata for gas exchange.
  • To transport carbon dioxide into the leaves through xylem.
  • To provide electrons for the electron transport chain and hydrogen ions that end up in glucose. (correct)

How does the structure of the thylakoid membrane contribute to its function in photosynthesis?

  • Its single layer accelerates the flow of electrons through transport chains.
  • Its phospholipid bilayer maintains concentration gradients crucial for ATP synthesis. (correct)
  • Its location in the stroma allows for easy access to carbon dioxide.
  • Its rigid protein composition facilitates the direct absorption of photons.

In the light-dependent reactions of photosynthesis, what is the immediate fate of electrons after they are excited in Photosystem I?

  • They are re-excited and used to reduce NADP+ to NADPH. (correct)
  • They are transferred to ATP synthase to generate ATP.
  • They are passed along proteins releasing energy to pump H+ ions
  • They are used directly to split water molecules.

What happens to the 3 PGA (3-phosphoglycerate) molecules formed during the Calvin cycle?

<p>They are converted into glucose and regenerate RuBP. (C)</p> Signup and view all the answers

How does wavelength relate to the energy of electromagnetic radiation, and what is the significance of this relationship in photosynthesis?

<p>Shorter wavelengths have higher energy, which can be harnessed by photosynthetic pigments. (A)</p> Signup and view all the answers

During the electron transport chain in photosynthesis, H+ ions are pumped across the thylakoid membrane. What is the direct result of this pumping?

<p>Establishment of a proton gradient to drive ATP synthase. (A)</p> Signup and view all the answers

Consider a plant exposed to green light. How would this affect the rate of photosynthesis, and why?

<p>Photosynthesis would decrease because chlorophyll absorbs green light poorly. (C)</p> Signup and view all the answers

How is the Calvin cycle directly dependent on the light-dependent reactions of photosynthesis?

<p>It uses ATP and NADPH generated during the light-dependent reactions. (D)</p> Signup and view all the answers

If a plant cell is placed in an environment with a high concentration of oxygen, how might this affect the rate of the Calvin cycle, and why?

<p>The Calvin cycle would decrease due to the increased rate of photorespiration. (B)</p> Signup and view all the answers

Which of the following electromagnetic radiations has the shortest wavelength?

<p>X-rays (B)</p> Signup and view all the answers

Flashcards

Photosynthesis

The biochemical process where plants convert light energy, water, and carbon dioxide into glucose and oxygen.

Light-dependent Reactions

Reactions that need light to occur and convert light energy into chemical energy in the form of ATP and NADPH.

Light-independent Reactions

Reactions in photosynthesis that use the energy (ATP and NADPH) from the light-dependent reactions to convert carbon dioxide into glucose.

Chlorophyll

The main pigment in plants that absorbs sunlight during photosynthesis.

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Stomata

Tiny pores on plant leaves that allow carbon dioxide to enter and oxygen to exit.

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Xylem

The tissue in plants that transports water from the roots to the leaves.

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Stroma

The fluid-filled space surrounding the grana inside a chloroplast where the Calvin cycle takes place.

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Thylakoid

Internal compartments of the chloroplast where the light-dependent reactions of photosynthesis occur.

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Grana

Stacks of thylakoids within the chloroplast.

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Electron Transport Chain (ETC)

A series of protein complexes that transfer electrons, releasing energy to pump H+ ions and create an electrochemical gradient during photosynthesis.

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Study Notes

Photosynthesis

  • Consists of light-dependent and light-independent reactions.
  • Requires water, carbon dioxide, and sunlight.
  • Chlorophyll absorbs photons from the sun.
  • Carbon dioxide enters and oxygen exits through stomata.
  • Water is transported from the roots to the leaves through xylem tissue.
  • Chloroplasts are suspended in stroma, a fluid.

Thylakoids

  • Store chlorophyll and are stacked into grana.
  • Possess a lumen and phospholipid bilayers.
  • Crucial for maintaining concentration gradients.
  • Location of light-dependent reactions.

Light-Dependent Reactions & Photosynthesis II

  • Involve photon absorption, water splitting, electron transport, NADH, and ATP formation.
  • Light energy is absorbed in the chloroplast.
  • Electrons are excited, increasing their energy state.
  • Water is split to release oxygen.
  • Electrons are transferred along an electron transport chain.

Electron Transport Chain

  • Electrons travel along proteins, releasing energy to pump H+ ions, creating a gradient.
  • In Photosystem I, electrons are re-excited to reduce NADP+ to NADPH.
  • H+ ions drive ATP synthase to form ATP from ADP.
  • Also known as the Calvin Cycle.
  • During the Calvin cycle, CO2 is converted into 3 PGA (carbon compounds).
  • The goal is to produce glucose, using ATP and NADPH.
  • Other products include starch, cellulose, and lipids.
  • The remaining carbon molecules are used to regenerate RuBP and produce other organic compounds.

Wavelengths and UV Rays

  • Electromagnetic spectrum in order of most to least powerful: Gamma rays, X-rays, UV rays, Visible light, Infrared, Microwaves, Radio waves.
  • Shorter wavelengths have higher energy; longer wavelengths have lower energy.
  • Wavelength distinguishes different forms of electromagnetic radiation.
  • Gamma rays have the most energy, and X-rays have a shorter wavelength than UV rays.
  • Infrared radiation is red while UV light ranges from blue/indigo/violet.

Miscellaneous

  • In light reactions, water is oxidized into oxygen.
  • In the Calvin cycle, carbon dioxide is reduced into glucose.
  • Water is the source of hydrogen and electrons in glucose.
  • Oxygen is the source of oxygen atoms in glucose.
  • NADPH is oxidized in the Calvin Cycle while NADP+ is reduced.
  • Oxygen is produced by reactions in the thylakoid.

Electron Transport Chain & ATP Production

  • Chlorophyll releases electrons and water splitting produces hydrogen and oxygen ions.
  • Electrons move down an electron transport chain; energy is used to move NADPH by combining 2 electrons and 1 hydrogen atom to make NADPH+.
  • The energy is used to add an inorganic phosphate onto ADP, forming ATP.

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