DS

Photosynthesis and Energy Metabolism

Chapter 11: Phototrophic Energy Metabolism - Photosynthesis

1. Definitions

  • A. Photoheterotrophs

    • Definition: Organisms that use light for energy and organic compounds from their environment as their carbon source.

    • Example: Purple non-sulfur bacteria (e.g., Rhodospirillum).

  • B. Photoautotrophs

    • Definition: Organisms that use light for energy and CO2 as their carbon source.

    • Examples: Plants, cyanobacteria, algae, phytoplankton, some bacteria.

2. Chloroplasts

  • Definition: Membrane-bound organelles found in plant cells and algae.

  • Structure includes:

    • Outer membrane

    • Inner membrane

    • Thylakoid: Stacked into grana.

    • Stroma: Fluid-filled space around grana.

    • Other organelles: Nucleus, nucleolus, endoplasmic reticulum, mitochondria, lysosome, Golgi apparatus, ribosome, cell membrane, cell wall, vacuole.

3. Structure of Chloroplasts

  • Chloroplasts exhibit a double membrane structure consisting of:

    • Thylakoids: Site of the light-dependent reactions.

    • Stroma: Site of the Calvin Cycle (light-independent reactions).

4. Photosynthesis Reactions

  • Light-dependent Reactions: Takes place in thylakoid membranes.

    • Involves pigments that absorb light energy to convert it into chemical energy

  • Light-independent Reactions (Calvin Cycle) occur in the stroma.

    • Inputs: H2O, CO2, ADP + Pi, NADP+

    • Outputs: ATP, NADPH, O2, and sugar (C6H12O6).

5. Pigments in Photosynthesis

  • Pigment Definition: A molecule that absorbs light energy and converts it into chemical energy for photosynthesis.

  • Chlorophyll:

    • Most common pigment in photosynthesizers.

    • Structure: Long hydrophobic tail anchors molecule into the thylakoid membrane, with a porphyrin ring that contains carbon, nitrogen, and hydrogen atoms along with a magnesium ion (Mg2+) center.

    • Characteristics: Alternating single and double bonds in the porphyrin ring allow for free movement of electrons.

    • Photoexcitation: When electrons are excited by light, they may release energy (in fluorescence) or be transferred to an electron acceptor (in photosynthesis).

6. Types of Photosynthetic Pigments

  • Type A (Chlorophyll a):

    • Primary pigment that absorbs blue-violet and red light.

  • Type B (Chlorophyll b):

    • Accessory pigment that absorbs additional blue and orange-red light and transfers the absorbed energy to chlorophyll a.

  • Accessory Pigments:

    • Carotenoids: Absorb blue and green light; transfer energy to chlorophyll a.

    • Phycobilins: Absorb green or orange/red light; significantly found in cyanobacteria and red algae, especially in low-light environments.

7. Properties of Light

  • Light can act as a wave of electromagnetic radiation, traveling in waves with varying wavelengths and frequencies.

  • Energy correlates inversely with wavelength: the shorter the wavelength, the higher the energy.

    • Examples of wavelengths:

    • Gamma rays: 1 x 10^-12 m

    • Ultraviolet rays: 1 x 10^-8 m

    • Visible Light: 4 x 10^-7 m to 7 x 10^-7 m.

  • Photon: The smallest unit of electromagnetic energy, exhibiting both wave-like and particle-like properties, with each carrying a fixed amount of energy known as a quantum.

8. Photosystems

  • Photosystem: A complex of pigments and proteins embedded in thylakoid membranes, consists of chlorophyll a and accessory pigments, crucial for photosynthesis efficiency.

9. Steps of Photosynthesis

Light-Dependent Reactions

  1. Light hits chlorophyll a and accessory pigments within Photosystem II (PSII), exciting chlorophyll a's electrons.

  2. Excited electrons leave the P680 reaction center in PSII and are passed to an electron transport chain (ETC).

  3. Water is split to replace lost electrons, producing O2 and accumulating H+ (protons) inside the thylakoid space.

  4. Excited electrons move through electron carriers: plastoquinone → cytochrome b6f → plastocyanin.

  5. Energy from the electron transport chain pumps protons into the thylakoid space, building a proton gradient.

  6. Electrons reach Photosystem I (PSI), where they are re-excited and passed to ferredoxin (Fd), reducing NADP+ to NADPH.

  7. Proton motive force causes H+ to flow back into the stroma through ATP synthase, driving ATP synthesis.

Critical Products Produced

  • ATP and NADPH generated from the light reactions are critical for the Calvin Cycle.

10. Calvin Cycle (Light-Independent Reactions)

  1. CO2 enters the stroma; Rubisco enzyme attaches CO2 to RuBP (ribulose-1,5-bisphosphate).

  2. This forms an unstable 6-carbon intermediate that splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA).

  3. ATP and NADPH are utilized to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).

  4. Some G3P is used to build glucose and other carbohydrates, while the rest is recycled to regenerate Ribulose bisphosphate (RuBP).

11. Photosynthesis Summary

  • Overall reaction: 6CO2 + 6H2O + ext{light} ightarrow C6H{12}O6 + 6O2

    • Goal: Transform light energy to chemical energy stored in sugars.

12. Adaptations of Plants for Photosynthesis

  • Plants in hot or dry environments must adapt methods to protect carbon fixation from stress:

    • C3 Plants: Standard Calvin Cycle (3-carbon molecule 3-PGA), efficient in moderate temperatures; suffers in heat and drought (e.g., rice, wheat, soybeans).

    • C4 Plants: Modified pathway that separates carbon fixation spatially; stomata partially closed during the day to conserve water (e.g., corn, sugarcane). Fix CO2 into a 4- carbon molecule using the enzyme PEP carboxylase, which allows these plants to thrive in high temperatures and low CO2 concentrations.

    • CAM Plants: Operate by separating carbon fixation temporally; stomata open at night for CO2 fixation into 4C acids (malate) and closed during day (e.g., cacti, pineapples). Minimizes water loss by allowing photosynthesis to occur during cooler nighttime temperatures, thereby enhancing their ability to survive in arid environments.

13. Proton Motive Force Calculation

  • Conditions:

    • Membrane potential (Vm) = 0.30 V

    • pH outside thylakoid = 8

    • pH inside thylakoid = 6

    • Temperature (T) = 25ºC (298 K)

    • ext{∆pH} = 8 - 6 = 2

    • Formula:
      ext{pmf} = Vm + \frac{2.303 RT ext{∆pH}}{F}

    • R = 1.987 ext{ cal/mol•K}

    • T = 298 ext{ K}

    • F = 23,062 ext{ cal/mol} = 96485 ext{ C/mol}

    • Final Calculation:
      ext{pmf} = 0.30 + \frac{2.303 (1.987) (298) (2)}{23062} = 0.42 ext{ Volts}

    • Importance: This energy is used by ATP synthase under typical light conditions for ATP production.