Question

Albert Szent-Gyorgi, a pioneer in early photosynthesis research, stated, "What drives life is a little electric current, kept up by the sunshine." What did he mean hy this? \(17 .\) BIOCHEMICAL CONNECTIONS What is implied about the energy requirements of photosystems I and II by the fact that there is a difference in the minimum wavelength of light needed for them to operate \((700 \mathrm{nm} \text { for photosystem } 1 \text { and } 680\) nm for photosystem 11 )?

Short answer

Photosystem II requires more energy (shorter wavelength) compared to photosystem I, reflecting their different energy roles.

Step-by-step solution

Understand the Context

Albert Szent-Gyorgi highlighted that life is powered by a small electric current.

Difference in Energy Requirements

To compare photosystem I and II, note that the minimum wavelength of light needed for photosystem I is 700 nm, and for photosystem II it is 680 nm.

Calculate Energy of Light

Use the energy formula for photons: \[ E = \frac{hc}{\text{wavelength}} \] where \( h \) is Planck's constant (6.626 × 10^(-34) J·s), and \( c \) is the speed of light (3.00 × 10^8 m/s).

Energy for Photosystem I

Substitute 700 nm (0.7 × 10^(-6) m) into the formula: \[ E_{PSI} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^8}{0.7 \times 10^{-6}} \approx 2.84 \times 10^{-19} \text{ J} \]

Energy for Photosystem II

Substitute 680 nm (0.68 × 10^(-6) m) into the formula: \[ E_{PSII} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^8}{0.68 \times 10^{-6}} \approx 2.93 \times 10^{-19} \text{ J} \]

Compare Energies

Notice that the energy required by photosystem II is slightly higher than that required by photosystem I, as the wavelength is shorter.

Conclusion

The difference in energy requirements reflects different roles in the electron transport chain and the efficiency of light utilization by photosystems.

Key concepts

electromagnetic spectrum

The electromagnetic spectrum refers to the range of all types of electromagnetic radiation. Radiation is the energy that travels and spreads out as it moves, visible in the form of light and other forms like X-rays and radio waves. The spectrum ranges from very short wavelengths such as gamma rays to very long wavelengths like radio waves.
In photosynthesis, plants use light from the visible part of the spectrum, which includes wavelengths from approximately 400 nm to 700 nm. Each color of light corresponds to a different wavelength. For example, blue light has shorter wavelengths around 450 nm, while red light has longer wavelengths close to 700 nm.
Understanding the electromagnetic spectrum is essential to grasp which parts of the light spectrum are used in photosynthesis. Particularly, photosystems I and II absorb light at specific wavelengths to initiate the process of converting light energy into chemical energy.

photon energy calculation

Calculating the energy of a photon is crucial in studying photosynthesis, as it helps in understanding the energy requirements of photosystems I and II. The energy of a photon can be calculated using the formula: \[ E = \frac{hc}{\text{wavelength}} \] Here, \( h \) represents Planck's constant (6.626 × 10^(-34) Joule·seconds), and \( c \) represents the speed of light (3.00 × 10^8 meters/second).
For example, to find the energy required by photosystem I, which absorbs light at 700 nm, you would substitute 700 nm (0.7 × 10^(-6) meters) into the formula:
\[ E_{PSI} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^8}{0.7 \times 10^{-6}} \]
This calculation yields approximately 2.84 × 10^(-19) Joules.
Similarly, for photosystem II, which absorbs light at 680 nm, the calculation would be:
\[ E_{PSII} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^8}{0.68 \times 10^{-6}} \]
This gives approximately 2.93 × 10^(-19) Joules. As seen, photosystem II requires slightly more energy than photosystem I.

photosystems I and II

Photosystems I and II are essential components in the light-dependent reactions of photosynthesis. These are large protein complexes embedded in the thylakoid membrane of chloroplasts.

Photosystem II (PSII) is the first step in the electron transport chain. It uses light energy to split water molecules into oxygen, protons, and electrons. The light absorbed by photosystem II has a wavelength of about 680 nm. After absorption, the energy helps in the excitation of electrons that move through the electron transport chain.

Photosystem I (PSI) follows photosystem II in the chain. It absorbs light at a slightly longer wavelength (700 nm). PSI uses the light energy to further excite the electrons received from PSII. This setup plays a crucial role in producing NADPH, an essential molecule for the Calvin cycle where carbon dioxide is converted into glucose.
Though the two photosystems work sequentially, their energy requirements differ due to their different roles and efficiencies. The difference in light absorption wavelengths between PSII and PSI reflects this.