Biology 110

Photosynthesis 1 and 2

What happens in photosynthesis? Reagents and products.

Why is photosynthesis important for life on earth?

Highlights of the history of photosynthesis research:

Photosynthesis consists of two groups of reactions, one that requires light (photochemical "light reaction") and the other that doesn't (biochemical "dark reaction").

Light reaction: Light is absorbed by a pigment molecule and the light energy is converted into useable chemical energy.

Dark reaction: The useable chemical energy from the light reaction is used to reduce carbon dioxide to sugar (photosynthetic carbon reduction cycle - PCR cycle).

Photosynthesis takes place in chloroplasts.

Light is electromagnetic radiation in the visible region of the spectrum (l 400 - 700 nm).

Pigments are molecules that absorb energy of photons at particular wavelengths - absorption spectra. Electrons in the molecules become excited to higher energy levels when photons are absorbed.

Photosynthetic pigment molecules are clustered in groups of about 200 molecules in "antennae" in thylakoid membranes.

Photochemistry of the light reaction: Know the reagents and products of the light reaction and the three main results of the electron transport chain.

Biochemistry of the dark reaction: Know the reagents, products and the three main steps of the PCR cycle.

Photorespiration is a loss of fixed carbon resulting from the oxygenation of RUBP by Rubisco.

Oxygen competes with carbon dioxide for active site of Rubisco enzyme molecule particularly at:

C4 photosynthesis and CAM are alternative photosynthetic pathways that eliminate photorespiration by concentrating CO2 at the site of sugar synthesis by the PCR cycle.

Sample question from past exams:

Explain the following:

a. The light and dark reactions of photosynthesis are mutually interdependent.

b. The sac-like structure of chloroplast thylakoids is essential for ATP synthesis.

c. CAM plants are capable of living in extremely dry environments.

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