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Metabolism 2: Photosynthesis

Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. Derived from content by David Knuffke.
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Thomas Hattori

on 12 October 2015

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Transcript of Metabolism 2: Photosynthesis

Photoautotrophic Energy Processing
Photosynthesis
Thylakoid
The Light Reactions
Stroma
Importance
Light!
Chloroplast
Calvin Cycle
CO
A Quick Recap
Light Makes Us Food!
There is a reciprocal relationship between chemoheterotrophic nutrition and photoautotrophic nutrition.


The inputs of one are the outputs of the other.
This accounts for this curious fact:
Aerobic Cellular Respiration:
Photosynthesis:
C H O + 6O
6CO + 6H O
2
2
2
6
6
12
6
6
2
C H O + 6O
12
2
2
6CO + 6H O
Plants and Such
Remember
Plants are NOT the only photoautotrophs
Most of the oxygen in the atmosphere is generated by cyanobacteria and aquatic protists
Purple Sulfur Bacteria
Cyanobacteria
Unicellular Algae
Seaweed
Plant Anatomy
In Plants, photosynthesis happens at the
leaves
, organs which are specialized for the process.
At the leaf,
mesophyll cells
are full of
chloroplasts
, the site of photosynthesis.
Light = Energy
Light is a form of
electromagnetic radiation
.

It is produced by the movement of electrons between orbitals.
Visible light is just one tiny slice of the larger
electromagnetic spectrum
.
Chlorophyll
Why are plants green?
Chlorophyll is a
pigment
.

Pigment: any molecule that interacts with light energy to produce a color.
Chlorophyll comes in two main varieties:
Chlorophyll A
, and
Chlorophyll B
.

While chlorophyll is the main photosynthetic pigment, it is NOT the only pigment found in chloroplasts.

Accessory Pigments
: other pigments that allow the chloroplast to absorb a wider range of light, and protect the chloroplast from light-related damage (example:
carotenoids
,
xanthophylls
)
Magnesium!
Sunlight contains almost all wavelengths of visible light.

Chloroplasts do not absorb all wavelengths of light equally.

When plants are exposed to light, chloroplasts preferentially absorb light in the blue and red parts of the spectrum.

Chlorophylls have an
absorption spectrum
that is highest in the blue and red portions of the visible light spectrum

The accessory pigments expand the useful range of light (the "
action spectrum
"), but green is still the least useful.
The unequal utility of different wavelenghts of light was first noticed by Theodore Engelmann, who obeserved higher rates of growth of aerobic bacteria on algae grown in blue and red wavelengths of light.
Chloroplasts consist of a series of membranous disks (
thylakoids
), arranged in stacks (
grana
).

The grana are inside of the inner membrane of the chloroplast.

The fluid/space surrounding the grana is called the
stroma
.

Photosynthetic prokaryotes use specialized cell membrane regions to accomplish photosynthesis.
Photosynthesis is a 2-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.
2
Light + Chlorophyll = Electrons!
When photons of light interact with chlorophyll, electrons in the Magnesium atom become excited.

This happens with ~1 % of all sunlight that strikes the surface of the earth.
Isolated chlorophyll will
flouresce
when exposed to light, as the 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 "
reaction center
" where chlorophyll a molecules produce excited electrons which are transferred to an
electron transport chain
.

Two types:
Photosystem II
: central chlorophyll works best at a light wavelength of 680 nm (
P680
). Found at the "start" of the ETC.
Photosystem I
: central chlorophyll works best at a light wavelength of 700 nm (
P700
). Found at the "end" of the ETC.

Since chlorophyll is not going to have the electrons return to it, new electrons are needed.

Water provides the replacement electrons ("
photolysis
"). This creates 4 protons and 1 molecule of oxygen gas for every 2 water molecules consumed.

The oxygen gas is released as a 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 in cellular respiration.

A few notable differences:
In respiration, the energy comes from oxidation of glucose ("
oxidative phosphorylation
"). In photosynthesis, the energy comes from photons ("
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 thlakoid space.
ATP
NADPH
O
Outputs
Inputs
Light
ADP+ Pi
NADP+
Water

Electron Flow
Non-Cyclic Electron Flow
Electrons move from photosystem II to photosystem I via the ETC. From photosystem II, they are transferred to the enzyme
NADP-Reductase
which uses them to reduce NADP+ into NADPH.

Produces both ATP and NADPH

Requires water.
Cyclic Electron Flow
Electrons move from photosystem I to the ETC before returning to photosystem I.

Only produces ATP.

Does not require water.
Why?
The Calvin cycle will require 9 ATP and 6 NADPH for every sugar produced.
Fun Fact:
Even if you are growing plants indoors, you are still using sunlight to do it, just a version that was stored in the chemical bonds of the fossil fuels that are being used to power the electric lights.
2
Fun Fact:
The Calvin cycle is named for Melvin Calvin, who discovered it by using radioactive C-14 to trace the path of carbon through the cycle.

He received a Nobel Prize for his efforts in 1961.

It is also commonly referred to as simply "
The Carbon Fixation Cycle
"

It is never, ever, called "The Dark Reactions"
Outputs
3 CO
9 ATP
6 NADPH
Inputs
1 G3P
9 ADP + Pi
6 NADP+

Enjoy this Analogy!
Three Phases:
Every step of the Calvin Cycle is controlled by an enzyme (not shown)
1. Carbon Fixation: Ribulose Bisphosphate Carboxylase (aka "
RuBisCo
") mediates the transfer of a molecule of Carbon Dioxide onto a molecule of
Ribulose Bisphosphate
(
RuBP
- 5 C)

2. Reduction: ATP and NADPH are used to rearrange RUBP into
Glyceraldehyde 3-phosphate
(
G3P
, aka
PGAL
) a three-carbon sugar.

3. Regeneration: ATP is used to reconstitute RuBP from G3P
Where's the Sugar?
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.

This makes 6 molecules of G3P, 1 of which is a net product.

The other 5 G3P are used to regenerate three molecules of RuBP.

G3P is a sugar building block. 2 G3P can make 1 6 carbon sugar. Many G3P can make a polysaccharide.
(per G3P)
Consider everyone you know, ever pet you have ever had, every ancestor in your lineage...they have all been able to exist for the simple fact that photoautotrophs make more food than they need and produce oxygen gas as a waste product.
Well...
Also...
Modern industry is more and more interested in using plants to do all sorts of things (like make
biofuels
, for instance).
C H O + 6O
2
6CO + 6H O
12
6
6
2
2
Oxidized
Reduced
Water will be
oxidized
(it is the "
reducing agent
").
Carbon will be
reduced
(it is the "
oxidizing agent
").
An
anabolic
,
endergonic
process
Without light, you wouldn't be able to eat anything. And you want to eat, don't you? DON'T YOU?!?!
G3P FTW!
Chloroplast
Mitochondria
Oxygen,
Organic Molecules (ex. glucose)
Carbon Dioxide,
Water
ATP
Heat
Light
Plants!
Coach says it's called the "
Z-Scheme
"
2
ATP, NADPH
ADP + Pi, NADP+
Full transcript