Photosynthesis supplement
Dr. Ross Clark
Where photosynthesis happens
Photosynthesis occurs in all green parts of plants.
Plants are green because they have
chloroplasts
Chloroplasts are green because they
contain chlorophyll
Chlorophyll
Chlorophyll a is the only pigment that is absolutely essential for
photosynthesis.
Chlorophyll b is another chlorophyll. But it is not essential for
photosynthesis
All of the other chloroplast pigments pass the energy they have absorbed
on to chlorophyll a molecules.
Chlorophyll is green because it reflects
green
Compare this graph with the one to the right.
Chlorophyll a has absorption peaks in the blue
and red parts of the visible spectrum. So does
chlorophyll b, but the peaks are not the same.
The “valley” in this absorption curve is in the
green-yellow part of the spectrum, which is the
color of leaves.
Other pigments, called accessory
pigments, absorb other wavelengths,
making chloroplasts more efficient.
That makes the overall absorption
curve smoother.
The accessory pigments carotene
(which is orange) and xanthophyll
(yellow) reflect the wavelengths of
light that they do not absorb.
Making a sugar molecule
is a two-step process
2
Some of the
captured
energy Is used
to make sugar
from carbon
dioxide.
*
1
First, light
energy is
captured by
the chloroplast pigments.
* The CO2 in this position is a mistake.
Ignore it.
Making a sugar molecule
is a two-step process
The sugarmaking step is
called the dark
reactions,
because it does
not require
light. It can
occur in the
dark or in light.
(This CO2 is a mistake;
ignore/erase it!)
The energy-capturing step is called
the light reactions, because light energy
is required: no light = no light reactions.
The whole photosynthetic process
occurs in chloroplasts
This is a diagram of a
chloroplast.
The thylakoids are
specialized membranes
that contain the
photosynthetic pigments.
They are where the light
energy is captured. The
stacks of thylakoids look
a little like stacks of
cookies. The stacks of
thylakoids are called
grana.
Here are a few more details
cutaway view of chloroplast
LIGHT REACTIONS
•
Light penetrates chloroplast
•
Light strikes pigment
molecules, and is absorbed
by electrons in the pigment
molecules
3
The energized electrons are used
to make ATP and energized
electron carrier molecules
ATP and energized electrons
are transferred to the chloroplast
stroma
More details, continued
DARK REACTIONS
The dark reactions are a cycle
sometimes called the Calvin
cycle. This cycle requires
several enzymes.
The most important enzyme
is rubisco. Rubisco picks up
carbon dioxide (CO2) and adds it
to 5-carbon sugars. Those
sugars now have 6 carbons.
As the cycle continues to turn,
it generates a 3-carbon sugar
that is not needed to keep the
cycle turning.
Two of those surplus 3-carbon
sugars are combined to form
glucose, a 6 carbon sugar.
There it is – the sugar, the key
to life!
Let’s revisit the photosynthesis picture
Overall process of photosynthesis
light
Water + carbon dioxide
chlorophyll a
minerals from soil
sugar
+ oxygen
Let’s revisit the photosynthesis picture
Overall process of photosynthesis
light
water + carbon dioxide
chlorophyll a
minerals from soil
sugar
+ oxygen
Let’s account for the things in this process –
Carbon dioxide – diffuses into plants from atmosphere
Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma
Where do water and oxygen fit into all this?
Let’s take a closer look at the diagram
we looked at earlier
Let’s take a closer look at the diagram
we looked at earlier
The very first thing that happens
is that light energy is absorbed by
electrons in the pigment
molecules.
Some of those electrons are
carried to the dark reactions and
never return to the pigment
molecules.
What happens when other rays
of light zap the pigment
molecules that lost their
electrons?
How can a pigment capture light
energy if it has lost its energycapturing electrons?
Let’s take a closer look at the diagram
we looked at earlier
The very first thing that happens
is that light is absorbed by
electrons in the pigment
molecules.
Some of those electrons are
carried to the dark reactions and
never return to the pigment
molecules.
What happens when other rays
of light zap the pigment
molecule that lost their electrons?
The answer is, those electrons
must be replaced. But HOW?
Let’s take a closer look at the diagram
we looked at earlier
The very first thing that happens
is that light energy is absorbed by
electrons in the pigment
molecules.
Some of those electrons are
carried to the dark reactions and
never return to the pigment
molecules.
What happens when other rays
of light zap the pigment
molecule that lost their electrons?
The answer is, those electrons
must be replaced. But HOW?
The answer to “how” is that
water (H2O) molecules are split
into hydrogen ions (H+), oxygen ions (O=), and electrons. The electrons replace the electrons
of the pigment molecules, and the oxygen ions combine to form O2.
The splitting of water during
photosynthesis is called
photolysis.
Photo means, “with light”
Lysis means, “splitting”
Manganese ions are necessary for the photolysis of water.
Let’s revisit the overall picture again
Overall process of photosynthesis
light
Water + carbon dioxide
chlorophyll a
minerals from soil
sugar
+ oxygen
Let’s account for the things in this process –
water – is used to replace electrons lost by pigment molecules
carbon dioxide – diffuses into plants from atmosphere
sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma
oxygen – is a left-over produced by splitting water to replace the
pigment electrons
= a left-over that changed the atmosphere
Let’s revisit the photosynthesis picture
Overall process of photosynthesis
light
Water + carbon dioxide
chlorophyll a
minerals from soil
sugar
+ oxygen
Let’s account for the things in this process –
water – is used to replace electrons lost by pigment molecules
Carbon dioxide – diffuses into plants from atmosphere
Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma
Oxygen – is a left-over produced by splitting water to replace the
pigment electrons
= a left-over that changed the atmosphere
a left-over that changed the planet
Let’s revisit the photosynthesis picture
Overall process of photosynthesis
light
Water + carbon dioxide
chlorophyll a
minerals from soil
sugar
+ oxygen
Let’s account for the things in this process –
water – is used to replace electrons lost by pigment molecules
Carbon dioxide – diffuses into plants from atmosphere
Sugar – produced by dark reactions (Calvin cycle) in chloroplast stroma
Oxygen – is a left-over produced by splitting water to replace the
pigment electrons
= a left-over that changed the atmosphere
a left-over that changed the planet
a left-over that made humans possible
Just think about that.
Take care of your plants.
They are taking care of you.