Photosynthesis: Light Reactions

Photosynthesis Overview

• Chloroplasts have an outer membrane, inner membrane, granum, lumen, stroma, and thylakoids.
• Photosynthesis, uses light reactions.
• The Amoeba Sisters have an 8-minute video on photosynthesis.

Capturing Light

• The antenna complex contains chlorophylls and accessory pigments.
• The antenna complex resides in the thylakoid membrane.
• The antenna absorbs sunlight (photons).
• The antenna transfers energy from pigment to pigment until it reaches chlorophyll a in the reaction center.

Reaction Centre

• Ground state electrons in chlorophyll a gain energy.
• Excited electrons reach a higher potential energy level.
• Excited electrons have three possible fates.
• If the electron is gained by the primary acceptor in a redox reaction, the electron is transported through the light-dependent reactions.

Photoexcitation

• If photoexcitation happens but there is no primary electron acceptor available, the electron goes back to a lower potential energy state.
• This results in losing heat and light, known as fluorescence.
• Chlorophyll molecules usually do not fluoresce because a primary electron acceptor captures the excited electron.

Key Terms

• Stroma is the protein-rich semiliquid material inside a chloroplast.
• The thylakoid membrane is the photosynthetic membrane within a chloroplast that contains light-gathering pigment molecules and electron transport chain.

Thylakoid Membranes

• Thylakoid membranes contain Photosystem I (p700), which absorbs wavelengths of 700 nm (red light).
• Chlorophyll a is present for Photosystem I
• Thylakoid membranes contain Photosystem II (p680).
• Chlorophyll a is present for Photosystem II.
• Chlorophyll a is identical in both photosystems, but the photosystems absorb slightly different wavelengths because of their surrounding proteins.

Light-Dependent Reactions

• During chemiosmosis, ATP synthase facilitates proton movement down their concentration gradient from the lumen to the stroma.
• As protons move, the free energy created by the proton gradient (proton motive force) is used to make ATP from ADP and Pi (inorganic phosphate).
• This process is called photophosphorylation.

Light Reactions Results

• Light reactions produce ATP and NADPH, which are used in the next phase of photosynthesis: carbon fixation.
• Some energy is lost as heat, but most is captured in ATP molecules.

Non-Cyclic Electron Flow

Cyclic Electron Flow

• Cyclic electron flow creates ATP but not NADPH.
• Cyclic electron flow might be related to an evolutionary ancestor, such as a prokaryote or a precursor to an organelle.

Light Spectrum

• Light is the portion of the electromagnetic (EM) spectrum that is visible to the human eye.
• Only 5% of the sun's light is used for photosynthesis.
• Engelmann's experiment with spirogyra and aerobic bacteria showed that red and blue-violet light best support photosynthesis.

Chlorophyll Function

• Chlorophyll absorbs blue-violet and red light, thus reflect green.
• Chlorophyll transfers light energy to photosystems on the thylakoid membrane for light reactions.
• There are hundreds of chlorophyll molecules in a photosystem.
• Only chlorophyll "a" serves as the primary electron donor to the electron transport chain (ETC) during the light reactions of photosynthesis.
• Plants appear green because they reflect photons in the 500-600nm ranges (green-yellow).
• Reflected photons are absorbed by retina in our eyes, therefore they look green to us.

Chlorophyll A vs B

• Chlorophyll A is the most important.
• Chlorophyll A absorbs red wavelengths.
• Chlorophyll A receives more light energy than B.
• Chlorophyll A has a CH3 side group structurally.
• Chlorophyll B absorbs blue wavelengths.
• Chlorophyll B functions as an antenna pigment for chlorophyll a.
• Chlorophyll B has a COH side group structurally.

Accessory Pigments

• Accessory pigments help absorb and pass light energy on to chlorophyll a.
• Fall colors are due to:
  • Xanthophylls (yellow)
  • Anthocyanins (red/blue)
  • Carotenoids (orange/yellow)
• Anthocyanins stored in red onion cells are in the central vacuole. They cause plasmolysis in a hypertonic solution with salt/salt water.
• Beta-carotene (a carotenoid) is converted to 2 vitamin A molecules, which are oxidized to retinal, then to rhodopsin.
• Rhodopsin is a photopigment in rod cells of the eye that helps you see in low-light conditions.

Photosynthesis Stages

• Stage 1: Light Reactions.
  • Light reactions are the light-dependent reactions.
  • Require chlorophyll.
  • Occur on the thylakoid membrane.
  • Require light energy (photons) and H2O.
  • Produce ATP and NADPH (coenzyme).
• Stage 2: The Calvin Cycle.
  • The Calvin Cycle are the dark or light-independent reactions.
  • Occurs in the stroma of the chloroplast.
  • Requires ATP and NADPH.
  • Fixes carbon from CO2 into carbohydrates (e.g., glucose).