What exactly IS photosynthesis?
By Megan Whitley
Formal Definition
• Photosynthesis can be
defined as the
physico-chemical
process by which
photosynthetic
organisms use light
energy to drive the
synthesis of organic
compounds.
What photosynthesis really is:
Plants use sunlight to make
food. In scientific terms, converting
light energy into chemical bond
energy. The plant is trying to make
glucose, its food source.
Summary Reaction:
6 CO2 + 6 H2O Æ C6H12O6 + 6 O2
The Leaf
• The leaf is the primary photosynthetic
organ of typical land plants.
– The leaf has an extensive surface area exposed to the
sun in order to absorb the maximum amount of light
(there are, of course, adaptive differences among
plants)
- There are several important features of leaves:
- epidermis:
epidermis found on upper and lower surface,
contains guard cells that control the opening and
closing of stomata,
stomata where diffusion of CO2 and O2
occurs
The Leaf, cont’d
- The major portion of the leaf is the mesophyll,
mesophyll
which actually contains the photosynthetic cells,
which are the parenchyma cells.
- Each parenchyma cell has lots of chloroplasts.
• The chloroplast is the cellular organelle where photosynthesis
takes place.
- chloroplasts have double membranes
- there are many thylakoids (membranous sacs) inside
- a stack of thylakoids is a granum (sing. grana)
- granums may be attached by a lamella
- the grana are in liquid-filled compartments called the stroma
Let’s clear that up a bit…
Chlorophyll molecules embedded in the thylakoid membranes give the
leaf its green color
Pigments
• Photosynthetic pigments are embedded in the
thylakoid membranes.
chlorophyll a: primary photo pigment
-absorbs blue and red wavelengths of light
-must be present to carry out photosynthesis
light Æ chl a Æ electrons boosted to high energy levels
Æ release of electrons starts photosynthesis
accessory pigments: chlorophyll b, carotenes,
xanthophylls
-absorb different wavelengths of light
So how do these Pigments work?
• There are electrons in these pigments that absorb
light energy.
• When light is absorbed, the electrons get boosted
from their ground state to their excited state (e-!)
through inductive resonance.
resonance
• When the electrons of chlorophyll a get excited (e-!),
they actually get released from the molecule. This
starts photosynthesis.
• This is why chl a is called the primary photo pigment
• The accessory pigments help to absorb more light
energy and pass it along to chlorophyll a (through
inductive resonance).
Now it starts to get tricky…
Let’s remember:
Summary Reaction:
6 CO2 + 6 H2O Æ C6H12O6 + 6 O2
The total photosynthetic process is made up of
two series of reactions:
-light reactions (require light energy
-dark reactions (require chemical energy)
Scary, huh? Let’s break it down…
We’ll start with the Light Reactions
Light Reactions
• These reactions take place in the thylakoid membrane
and require light energy to take place.
• There are two photosystems, photosystem I (PSI) and
photosystem II (PSII).
• In each of these photosystems, there is a reaction
center made up of two chlorophyll a molecules. These
reaction centers are called P680 (PSII) and P700 (PSI).
• There is an electron acceptor (EA) embedded in the
thylakoid membrane. When the two chl a molecules in
the P680 reaction center absorb light energy, each
transfers a high energy electron (e-!) to an EA.
Light Reactions, cont’d
• Meanwhile, in P680 (PSII), an enzyme is actively
splitting water molecules (H2O). Splitting in the
presence of light is called photolysis.
• The bond is broken between the O atom and the H
atoms. <H2/O> The electrons go to the chl a
molecules and the two H+ atoms are deposited in
the thylakoid interior (lumen).
lumen The oxygen atoms
combine to form O2.
• That first EA that receives the (e-!) from chl a
transfers these electrons to two more electron
acceptors. There is a loss of energy with each
transfer.
Light Reactions, cont’d
• Some of the lost energy has been spent pumping 2
H+ from the stroma into the lumen.
lumen
• Now, let’s focus on PSI,
PSI moving down the chain.
• The chl a molecules in the P700 (PSI) reaction center
absorb light energy and release (e-!) to another type
of EA.
• This provides vacancies that are filled by the two
electrons flowing out of PSII.
• The electrons that are released from P700 (PSI)
reaction center go on to reduce a molecule of NADP,
NADP
using 2 H+ from the lumen:
NADP Æ NADPH + (H+)
Light Reactions, cont’d
Let’s visualize this:
First,
-two chl a molecules absorb
light energy and transfer an
(e-!) to an EA
-water is split, producing
oxygen molecules and H+ in
the interior of the thylakoid
-electrons are transported
through a series of EAs (while
protons are being pumped
into the lumen)
Light Reactions, cont’d
Next,
-these electrons (from PSII)
enter PSI and are absorbed
by the chl a molecules
-the chl a molecules release
(e-!) to another type of EA
-the electrons released from
PSI cause the reduction of
NADP to NADPH and H+,
consuming 2H+ from the
lumen
Light Reactions, cont’d
• The orderly flow of electrons is the key to the
photosynthetic process. These are the results of this
electron flow:
-NADPH, which is used in glycolysis in dark reactions, is
produced
-a powerful electrochemical gradient is created by the build-up
of H+ ions in the lumen ~ at the same time the stroma
becomes negatively charged
• The system wishes to achieve equilibrium and
therefore the H+ ions in the stroma flow through an
enzyme complex, ATP Synthase. This transfer
provides the energy for the reaction ADP+Pi Æ ATP
to occur.
• The light reaction produces NADPH and ATP and O2.
Light Reactions, cont’d
A few more visuals…
ATP Synthase and how it uses the
electron pump to create ATP
Summary of the light reactions:
Dark Reactions
• These reactions take place in the stroma and on the
thylakoid surfaces facing the stroma.
• It is essential these reactions follow the light
reactions because they require NADPH and ATP.
• Dark reactions don’t have to take place in the dark,
they can take place without the presence of light.
They convert the chemical bond energy of NADPH
and ATP into the chemical bond energy of glucose.
• The most common pattern of the dark reactions is
the Calvin Cycle.
Cycle
Dark Reactions, cont’d
• The Calvin Cycle is just a cycle in which the same
reactions are repeated over and over. There must be
some starting material already present and
continually regenerated in order for this to occur.
Our starting material is a five carbon (5-C) sugar.
• The Calvin Cycle can be divided into three phases.
• In phase I,
I carbon fixation occurs. Carbon dioxide
molecules enter the leaf mesophyll through the
stoma and go into the parenchyma cells.
• The CO2 molecule binds to the 5-C sugar aided by an
enzyme. This makes a six carbon sugar that is highly
unstable so it immediately splits into two three
carbon molecules.
Dark Reactions, cont’d
• In phase 2,
2 which is the reduction portion of the
cycle, the 3-C molecules are phosphorylated by ATP
and receive electrons from NADPH.
• This chemical reduction makes two glyceraldehyde 3phosphate(G3P) molecules (3-C each). Some of
these G3P molecules are the products of the cycle,
but others stay in the cycle.
• In phase 3,
3 which is regeneration of the starting
product, G3P molecules react to form molecules of
the starting product.
• The cycle is ready to run again.
• Two molecules of G3P are bonded in an enzymatic
reaction to make glucose, the end molecule (of
photosynthesis) and food supply for the plant.
Dark Reactions, cont’d
A visual of the Calvin Cycle:
And there you have it…