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LAMC
BIOLOGY

LAMC Bio 3 Lecture Notes: Chapter 7 - Photosynthesis

Reading Assignment: Chapter 7

Learning Outcomes:
By the end of today’s lecture, you should be able to:
- explain the steps involved in photosynthesis.
- write the general equation for photosynthesis.
- explain the difference between C3, C4 and CAM photosynthesis

I. Photosynthesis:

Photosynthesis is a two-part process: the trapping of energy (the light reaction) and then using that energy to produce sugar molecules in the Calvin cycle.
Plants use sugars as building blocks for other organic compounds, including cellulose.
Plants, and other photosynthesizers, are the ultimate source of food for almost all other organisms.

Autotroph means “self-feeder,” and the term is applied to any organism that makes its own food without eating, decomposing, or absorbing other organisms or organic molecules.

The site of photosynthesis in eukaryotic cells is the chloroplast.  Photosynthesis is an endergonic reaction.
6CO₂ + 6H₂O + Light Energy à C₆H₁₂O₆ + 6O₂
[carbon dioxide + water + light energy + à glucose + oxygen]

1. Light reactions: steps that convert light energy to chemical energy and produce O2 gas as a waste product.
These reactions occur in the thylakoid membranes and produce high energy molecules (ATP and NADPH). During the light reactions, a leaf absorbs some light wavelengths (blue and red) but not others (green).
The kinetic energy of light is absorbed, which excites electrons.
The excited electrons are passed along an electron transport chain in a series of redox reactions.
The energy released by these redox reactions is used to generate ATP and NADPH.

2. Calvin cycle: a cyclical series of steps that assembles glucose from CO2 molecules.
These reactions occur in the stroma (the fluid outside the thylakoids but inside the inner chloroplast membrane) and use the energy and electrons from ATP and NADPH respectively in “carbon fixation.” Light is not directly required, but because production of ATP and NADPH requires light, the Calvin cycle steps usually occur during daytime.
 

II. C4 and CAM metabolism:

C4 & CAM metabolism are adaptations to dry climates.

In areas where water is in short supply, the stomata of plants remain closed in order to conserve water, so CO₂ concentration in the leaf becomes limiting and photosynthesis can shut down.

C4 metabolism – an intermediate C4 product is formed with the CO₂ and moved to a different part of the leaf so that the CO₂ concentration in that part of the leaf remains high enough for the Calvin cycle to proceed.

CAM metabolism – CO₂ diffuses into the plant at night, when water loss is reduced.  The CO₂ is made into an intermediate compound that is stable enough to be stored for several hours.  The CO₂ is then used during the day when light can power photosynthesis.

©2003-2004 Dilek Sanver-Wang, MS
Los Angeles, CA USA.  All Rights Reserved.