Untitled Notes

Question Q14-62:
- If light shines on chloroplasts and the rate of photosynthesis is measured as a function of light intensity, a curve is observed that reaches a plateau at a fixed rate of photosynthesis, denoted as x.
- Conditions that could increase the value of x:
- a. Increasing the number of chlorophyll molecules in the antenna complexes
- b. Increasing the number of reaction centers
- c. Adding a powerful oxidizing agent
- d. Decreasing the wavelength of light used

The rate of photosynthesis increases with light intensity until all reaction centers are hit by photons.
At saturating light levels, the limiting factor for photosynthesis becomes the number of reaction centers still available to be excited.
Possible ways to increase the rate of photosynthesis:
- Increase the number of reaction centers.
- Increase the recovery rate of reaction centers to their low-energy state.
Impact of changing chlorophyll molecules:
- Increasing chlorophyll does not affect response levels because it does not impact the number of reaction centers or their restoration rate.
Impact of a powerful oxidizing agent:
- It may hinder the reduction of the reaction center back to its resting state.

Question Q14-63:
- If a compound inhibits NADP+ reductase in illuminated chloroplasts, NADPH generation ceases.
- Ferredoxin does not accumulate in its reduced form because it can donate electrons to both NADP+ (via NADP+ reductase) and back to cytochrome b6-f complex.
- A cyclic flow of electrons occurs between ferredoxin, cytochrome b6-f complex, plastocyanin, and photosystem I.
- If photosystem II is also inhibited, outcomes are as follows:
- a. Less ATP generated per photon absorbed.
- b. ATP synthesis may cease.
- c. Plastoquinone accumulates in oxidized form (since it loses its electron source).
- d. Plastocyanin will accumulate in oxidized form.

Diagrammatic representation includes the components:
- Photosystem I and II
- Cytochrome b6-f complex
- Proton gradient used to generate ATP.
- NADPH is produced while water is split, facilitating electron flow.

Statement about organelles (Q15-2):
- a. Area of endoplasmic reticulum far exceeds plasma membrane area (about 20-30 times).
- b. Chloroplasts and mitochondria are surrounded by double membranes.
- c. Cytosol comprises about half the volume of eukaryotic cells, with organelles making up the other half.
- d. Chloroplasts and mitochondria have their own genomes, contrasting with the nucleus which encodes the organism's genome.
- Incorrect statements:
- The nucleus is not the only double-membrane organelle, nor is it the sole organelle containing DNA.

Characteristics of a newly discovered organelle:
- Small genome inside organelle.
- Surrounded by two membranes.
- Lack of vesicle pinching from the membrane.
- Contents include proteins similar to bacteria.
- Presence of ribosomes.
How might it have arisen?
- It likely evolved from an engulfed bacterium, based on the presence of these characteristics.

Protein Location:
- a. Synthesized in the cytosol.
- b. Synthesized in the chloroplast.
- c. On the endoplasmic reticulum.
- d. In both the cytosol and chloroplast.
Details:
- Proteins encoded by nuclear DNA are synthesized in the cytosol and directed to the chloroplast via sorting signals.
- Proteins encoded by chloroplast DNA are synthesized on ribosomes inside chloroplasts.

Proteins translated in the cytosol:
- Do not end up in:
- b. Mitochondria
- c. Interior of the nucleus
- d. Transport vesicles.
Proteins for transport vesicles are translated on ER-associated ribosomes.

Fully translated proteins in the cytosol without sorting signals
- End up in:
- a. The cytosol.
Proteins meant for mitochondria and the nucleus have specific sorting signals.

Protein A:
- Has a typical nuclear localization signal but is typically present in the cytosol.
- Moves to the nucleus upon hormone exposure to activate cell division genes.
Role of Protein B:
- Always complexed with protein A in unexposed cells.
- An experiment is conducted using cells lacking protein B to see its effect on protein A location and activity.
Experimental conclusions:
- Possible mechanisms:
- Protein B masks the nuclear localization signal.
- Hormones dissociate protein B, allowing protein A entry into the nucleus.
- Protein B might sequester protein A to a specific location, preventing nuclear entry in the absence of hormones.

True Statements:
- a. Signal sequences for mitochondrial proteins are at the N-terminus.
- b. Most mitochondrial proteins are encoded by nuclear genes and imported from the cytosol.
- c. Chaperone proteins assist in moving proteins across mitochondrial membranes.
Incorrect statements include:
- Mitochondrial proteins do not cross membranes in their native folded state.

Objective: Identify peroxisome-targeting sequence in the thiolase enzyme in yeast.
Fusion Protein Strategy:
- Hybrid genes encoding thiolase fused to histidinol dehydrogenase (HDH) create a system to test import functionality.
Targeting Sequence Results:
- Identified region: Between amino acids 100 and 125 is crucial for import into the peroxisome.
- Fusion proteins containing this sequence showed inability of yeast to grow on histidine-deficient media, confirming their need for targeting sequences.
- Further narrowing down determined the minimal targeting region necessary for import into the peroxisome.