Metabolism: Photoautotrophic Energy Processing
How do living systems process energy?
A Quick Recap
There is a reciprocal relationship between chemoheterotrophic
nutrition and photoautotrophic nutrition.
The inputs of one are the outputs of the other.
This explains this curious fact:
Plants and Such
Remember:
Plant Anatomy: In plants, photosynthesis happens at the leaves, organs which are specialized for the process.
At the leaf,
Light = Energy
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Light is a form of
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It is produced by
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Visible light is just one tiny slice of the larger electromagnetic spectrum.
Chlorophyll
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Chlorophyll is a
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Pigment: any
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Chlorophyll comes in two varieties
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While chlorophyll is the main photosynthetic pigment, it is not the only pigment found in chloroplasts.
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_________________________________: other pigments that allow the chloroplast to
Why are plants green?
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Sunlight contains almost all wavelengths of visible light.
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Chloroplasts do not
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When plants are exposed to light, chloroplasts preferentially
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Chlorophylls have an absorption spectrum that is highest in the blue and red portions of the visible light
spectrum.
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The accessory pigments expand the
Photosynthesis
An ______________________________ ,
_________________________________ process
Overview
Chloroplasts
Chloroplasts consist of a series of
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- Photosynthetic prokaryotes use specialized cell membrane
regions to accomplish photosynthesis.
Photosynthesis is a two part process:
1. The light reactions: Occur in the thylakoid
membranes. Light is used to drive the production of
ATP and NADPH (an electron shuttle). Water
provides the electrons needed and is converted to
oxygen gas (a waste product).
2. The Calvin cycle: Occurs in the stroma. The
ATP and NADPH produced in the light reactions are
used to incorporate carbon dioxide into a 3-carbon
sugar.
The Light Reactions (Thylakoid)
Light + Chlorophyll = Electrons!
When
This happens with ~1% of all the sunlight that strikes the
surface of the earth.
Isolated chlorophyll will fluoresce when exposed to light, as
excited electrons return to the ground state.
Photosystems
Complexes of protein and pigment molecules that are embedded in the
thylakoid membrane.
Direct incoming photons into the
Two types:
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Since chlorophyll is not going to have the electrons return to it, new electrons are needed.
_______________ provides the replacement electrons (________________________). This creates 4 protons
and 1 molecule of oxygen gas for every 2 water molecules consumed.
The oxygen gas is released as waste product, becoming a major input for aerobic cellular respiration.
Chemiosmosis
As the electrons move through the
ETC, they provide the energy for
chemiosmosis, in a fashion almost
identical to cellular respiration.
A few notable differences:
- In respiration, the energy comes
from the oxidation of glucose,
(oxidative phosphorylation). In
photosynthesis, the energy come from
protons (photophosphorylation).
- In respiration, protons were pumped from the
matrix into the intermembrane space by the ETC. In
photosynthesis, electrons are pumped from the stroma
into the thylakoid space.
Electron Flow
- Non-Cyclic Electron Flow
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Cyclic Electron Flow
Why?
Inputs
Calvin Cycle
Three Phases
1.
2.
3.
Outputs
Every step of the Calvin Cycle is controlled by an enzyme (not shown).
Where’s the Sugar?
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In order to get 1 G3P as a product of the Calvin Cycle, 3 molecules of carbon dioxide have to be joined
to three molecules of RuBP.
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Inputs
(per G3P)
Outputs
Versatility and Regulation
An evolutionary quirk – Rubisco evolved in conditions of low oxygen and gas concentration. As a result, its
active site has a high affinity for oxygen gas, which is a problem.
Photorespiration – The metabolic pathway that occurs when rubisco incorporates oxygen instead of carbon
dioxide into RuBP.
A metabolic dead end, uses ATP but produces no sugar.
As long as a plant
C3 Leaves: No adaptations for minimizing photorespiration
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Both stages of photosynthesis
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Oxygen and carbon dioxide
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Sugars are transported
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But there are environments where keeping stomates open will lead to _____________________
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Closed stomates =
=
=
2 Major Adaptations
C4 Leaves -
Carbon fixation occurs in mesophyll cells.
Carbon monoxide is incorporated into a
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The 4C acid is then transported to
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Since the bundle sheath cells are surrounded by
mesophyll,
CAM Plants –
Carbon fixation occurs during
The carbon dioxide is stored in an organic acid.
During the day,
Importance
Make sure you can …
… explain how photoautotrophic energy processing allows for the production of useful energy for organisms.
… explain why and how photoautotrophic energy processing is controlled.
… identify the oxidation and reduction reactions that occur in photosynthesis.
… explain the processes and identify all inputs and outputs of all steps of photosynthesis,
… relate the different steps of photosynthesis to their locations in the cell.
… compare the adaptations that have been made to reduce photorespiration.
… compare photosynthesis with cellular respiration.
… explain how photosynthesis provides the energetic foundation for the vast majority of life on Earth.