Organelles that capture and release energy.
Before we get to the organelles, we have to look at two
molecules that create the energy produced in those organelles
that follow.
The first molecule is ATP (Adenosine Triphosphate). ATP has the
following structure:
ATP is used to produce energy in the cell – think
of a flashlight powered by a fully charged
Lithium Ion battery. The energy is produced by
breaking the bond between the 2nd and 3rd
phosphate group.
This converts ATP to ADP (Adenosine
Diphosphate). ADP is then ready to be
“recharged” by adding another phosphate group
on to it.
We will see in a bit more detail in
Photosynthesis.
Chloroplasts – Chloroplasts are found in plants
and some animals (ex. Euglena)and can thought
of a solar power stations.
Chloroplasts convert sunlight into chemical
energy (stored in the bonds of sugars) through
the process called Photosynthesis.
Photosynthesis-the little engine
that could
Photosynthesis takes place in Chloroplasts.
Chloroplasts are specialized organelles that
contain pigment containing stacked membranes
called Thylakoids. The green pigment, contained
within the thylakoid membranes, is called
Chlorophyll.
The overall reaction of photosynthesis is:
6CO2 + 6H2O + light = C6H12O6 + 6O2
A simplified diagram of a chloroplast
NADPH NADP+
You will notice, on the previous diagram, the 2
molecules NADPH and NADP+ . These
molecules, along with the
ATP ADP cycle, are used to drive the Calvin
Cycle. (The Calvin Cycle is also called the “Dark
reaction” of photosynthesis because it is light
independent. We discuss this shortly).
NADPH NADP+ continued
NADP+ (nicotinamide adenine dinucleotide
phosphate)is an electron carrying protein that is
used to transfer high-energy electrons from the
“Light reaction” of photosynthesis to the “Dark
reaction”.
The NADP+ molecule reacts with an H2O
molecule and strips off the 2 hydrogen ions,
leaving an Oxygen ion behind. This Oxygen ion
combines with another
Oxygen ion to form O2. Oxygen gas is released
by the plant.
The NADPH molecule, acting as an energy
storage unit powers the Calvin Cycle (along with
ATP/ADP) by having the electrons stripped off,
returning to NADP+ and being reused.
We will now look at the Calvin Cycle.
The Calvin Cycle (aka the “Dark reaction”
The ATP and the NADPH produced by the “Light
reaction” contain an abundance of energy but neither
of these molecules are stable for more than a few
minutes. This is where the Calvin cycle comes in (the
Calvin cycle was named after American scientist Melvin
Calvin, the discoverer of this process).Plants use the
energy contained in ATP and NADPH to build stable,
high-energy carbohydrate molecules that can store
energy for long periods of time.
The end results
The two sets of photosynthetic reactions work
together – the light-dependent reactions trap
the energy from sunlight in chemical form, and
the light-independent reactions use that
chemical energy to produce stable high-energy
sugars from carbon dioxide and water. In the
process, heterotrophs like us, get plenty of food
and an atmosphere rich in oxygen.
Factoring affecting Photosynthesis
The most important factors that affect the rate
of photosynthesis are:
a) Temperature
b) Intensity of Light
c) Availability of water
Temperature: The enzymes that make Photosynthesis
possible have a preferred range of temperatures from
0°C to 35°C. Temperatures higher than that will slow
the process down. At low temperatures, the process
will cease entirely.
Light Intensity: As you would expect, the intensity
(brightness) of the light will have an impact. Bright light
will speed up the process but photosynthesis does have
a “top end limit”. The pigments in chlorophyll have
preferred parts of the spectrum – Violet/Blue and
Orange/Red (400-525nm and 650-700nm)
Availability of Water: Because water is one of
the raw materials of photosynthesis, a lack or
shortage of water will slow down or stop
photosynthesis. Water loss can also damage
plant tissues (shown by wilting or dying).
Plants living in arid or hot environments have
adapted by having leaves with waxy coatings to
reduce water loss.