Photophosphorylation and Anabolism - Exam 3

What is photophosphorylation?

The process by which light energy is converted into chemical energy (ATP). It occurs during the "light reactions" of photosynthesis when photons excite electrons, driving ATP synthesis via a proton gradient.

How does photophosphorylation relate to photosynthesis?

Photophosphorylation is the energy-capturing part of photosynthesis. Photosynthesis combines phototrophy (light energy capture) and carbon fixation (building organic molecules from CO2).

Where do the light reactions occur in eukaryotes?

In specialized thylakoid membranes of chloroplasts, which contain pigments and electron transport components

What is a photopigment?

A light-absorbing molecule that captures photons and transfers energy to electrons. They are not proteins but are often bound to them

What is the structure of a typical photopigment molecule?

Most have a porphyrin ring with a central Mg2+, allowing absorption in blue and red wavelengths—ideal for photosynthesis

Why are some wavelengths more effective for photosynthesis?

Light in blue and red ranges (400-750 nm) carries the right energy levels to excite electrons in pigments like chlorophyll, while UV or far-red light is less useful or damaging

What is the function of photopigments in photosystems?

Photopigments capture light and funnel energy to a central reaction center pigment pair that donates high-energy electrons to an electron transport chain

What are photosystems?

Organized clusters of pigments and proteins that work together to absorb light and generate energized electrons. There are two main types: Photosystem I and II

How are photopigments arranged within photosystems?

They are arranged in "antennae" that collect light energy and transfer it toward the reaction center, where electrons are excited and passed to an acceptor

What is the first type of photosystems and what do they do?

Photosystem I (PSI): Uses light energy to reduce NADP⁺ to NADPH; PSI drives NADPH formation.

What is the second type of photosystems and what do they do?

Photosystem II (PSII): Uses light to split water and provide electrons to the ETC, generating ATP; PSII starts the light reactions

In which domains of life are photosystems found?

Both are present in cyanobacteria and eukaryotic photosynthetic organisms. Archaea lack chlorophyll-based photosystems but may use other light-driven systems

What is oxygenic photosynthesis?

It's photosynthesis that produces O2 by splitting water to obtain electrons. Carried out by cyanobacteria, algae, and plants

What is anoxygenic photosynthesis?

Occurs in some bacteria (green sulfur, purple sulfur, heliobacteria). It does not produce O2 because it uses electron donors like H2S instead of H2O and typically uses only one photosystem

What distinguishes oxygenic from anoxygenic photosynthesis?

Oxygenic: uses water as an electron source → O2 released. Anoxygenic: uses alternative donors (H2S, Fe2+) → no O2 released, different pigments (bacteriochlorophylls).

What are bacteriochlorophylls?

Light-absorbing pigments used in anoxygenic photosynthesis. They absorb light at longer wavelengths than chlorophyll and are found in bacteria like Rhodobacter

What is bacteriorhodopsin-based phototrophy?

A light-driven system used by some archaea (like Halobacterium). Instead of chlorophyll, it uses bacteriorhodopsin, a membrane protein that pumps protons using light energy to generate ATP

What molecule does bacteriorhodopsin use to capture light?

It uses retinal, a light-sensitive pigment similar to that in human eyes, embedded within the protein

Why do halophilic archaea use bacteriorhodopsin-based phototrophy?

They live in extremely salty, oxygen-poor environments where chlorophyll-based photosynthesis isn't practical, so they rely on this simpler light-powered proton pump

What is the proton motive force in photosynthetic organisms?

As electrons move through the photosynthetic electron transport chain, protons are pumped across the membrane, forming a gradient that powers ATP synthase — just like in respiration

What is the role of ATP synthase in photophosphorylation?

ATP synthase uses the proton gradient created by light-driven electron flow to phosphorylate ADP → ATP

What are the products of the light reactions?

ATP and NADPH — both are used in the Calvin cycle (dark reactions) for carbon fixation

What is the Calvin cycle?

The series of reactions that use ATP and NADPH from the light reactions to convert CO2 into organic molecules like glucose

What is the role of NADPH in photosynthesis?

It provides the reducing power (electrons and hydrogen) needed to build carbohydrates from CO2 in the Calvin cycle

How does cyclic photophosphorylation differ from noncyclic?

Cyclic: Electrons from PSI return to the ETC instead of reducing NADP⁺ → produces ATP only, no NADPH; Noncyclic: Electrons flow from water → PSII → PSI → NADP⁺ → produces both ATP and NADPH and releases O₂.

Why do some organisms use cyclic photophosphorylation?

To balance their ATP/NADPH ratio when more ATP is needed than NADPH for cellular processes

What are amphibiolic pathways?

Pathways that serve dual roles in metabolism — both catabolic (breaking down) and anabolic (building up). For example, the Krebs cycle provides energy and biosynthetic precursors.

What is anabolism?

Anabolism (biosynthesis) is the process of building complex molecules (proteins, nucleic acids, lipids) from simpler precursors using energy from ATP and reducing power from NADPH

Where do anabolic precursors come from?

They are intermediates from catabolic pathways like glycolysis and the Krebs cycle (ex: pyruvate, acetyl-CoA, α-ketoglutarate)

Why are anabolic and catabolic pathways often interconnected?

Because energy and intermediates flow between them — catabolic reactions generate the ATP and NADPH that anabolism requires

What are the major macromolecules synthesized in bacteria like E. coli?

Amino acids (proteins), nucleotides (DNA/RNA), lipids (membranes), and peptidoglycan (cell wall)

How does E. coli regulate anabolic synthesis?

It uses feedback inhibition and gene regulation to avoid wasting energy — only synthesizing macromolecules when needed

What is the overall "big picture" of metabolism?

Energy from catabolic processes fuels anabolic processes. Metabolism is a network of interconnected reactions maintaining balance between energy use and energy production

Why are amphibolic pathways advantageous?

They save energy and resources by allowing cells to switch between breaking down and building up molecules depending on needs

What happens if light intensity or wavelength changes?

Different photopigments absorb different wavelengths, allowing organisms to adapt to various light environments (e.g., deep water or shaded conditions)

What does "carbon fixation" mean?

It refers to converting inorganic CO2 into organic carbon compounds, such as glucose — a key step in autotrophic growth

What organisms perform photophosphorylation but not full photosynthesis?

Some halophilic archaea — they capture light to make ATP but do not fix carbon or produce oxygen

What role does Mg2+ play in chlorophyll?

It stabilizes the porphyrin ring and facilitates light absorption in blue and red regions of the spectrum

Why is water important in oxygenic photosynthesis?

It's the source of electrons and protons for the light reactions; its oxidation releases O2 as a byproduct

What is the function of antenna complexes in photosystems?

They capture photons and pass excitation energy toward the reaction center — increasing light-absorption efficiency

What is the main difference between chlorophyll a and bacteriochlorophyll?

Chlorophyll a absorbs at shorter wavelengths (blue/red light), while bacteriochlorophyll absorbs at longer wavelengths, suitable for low-light environments