Comparison of the Structure and Function of Chloroplasts and Mitochondria
Chloroplasts and Mitochondria are both complex cell organelles found
in eukaryote cells. These detailed organelles can be studied more
closely with the recent invention of the electron microscope, allowing
us to look specifically at the details of their structures in relation
to their function
A typical eukaryote cell contains an average of a thousand
. A mitochondrion is the main site of aerobic respiration
and the amount of mitochondria present in a cell represents its
metabolic activity. This is demonstrated in the high amount of
mitochondria found in the base tail of sperms, as a huge amount of
energy is needed in order for the sperm to swim up the fallopian tubes
to reach an ovum for fertilization to take place.
Oxygen + Glucose Carbon Dioxide Water Energy
6O C H O 6CO 6H O
Chloroplasts are most commonly found in the photosynthetic tissues of
plants and some protocists. Chloroplasts are found palisade cells and
play a crucial role in photosynthesis, the process by which plants
manufacture food. Similarly to mitochondria the internal organisation
of the chloroplasts is done in order to gain maximum light absorption.
Carbon Dioxide Water Oxygen Glucose Energy
6CO 6H O 6O C H O
A mitochondrion varies in shape from 1.0 to 2.5 a similar size to that
of chloroplasts, as they are two of the largest cell organelles
in eukaryote cells. This has been proven using the technique of
differential centrifugation; this devise involves spinning cell
organelles at different speeds gradually getting faster. In doing this
the denser organelles form a sediment at the bottom. Chloroplasts and
therefore mitochondria are both found in the sediment in the very
early stages of the process; they are evidently the densest of all the
Mitochondria and chloroplasts are similar in that they both have a
double phospholipid membrane with a fluid filled gap, controlling the
entry and exit of materials. The double membrane found around
chloroplasts is known as the "chloroplast envelope." These
similarities and differences in the structure of the organelles are
demonstrated in figure 1 and 2 shown below.
Figure 1 Figure 2
The mitochondria's inner membrane folds inward projecting into the
interior of the cell organelle forming "cristae." This structure
increases the surface area. The cristae are covered in energy rich ATP
(adenine triphosphate) one of the main products formed in the process
In contrast to mitochondria, chloroplasts contain thylakoids, each one
consists of a pair of membranes close to each other with a narrow
space between them containing chlorophyll, this is the substance that
traps light energy. The thylakoids form a network of flattened sacs,
which, stack up to form granum in order to produce a greater surface
area (like the cristae in the mitochondrion) for the maximum
absorption of light.
Inside the double membrane of chloroplasts is a substance called
Stroma, this is a substance similar to that of cytoplasm. In the
stroma enzymes are contained to aid the process of photosynthesis, and
stain grains can be found, which are used as a temporary store for the
products of photosynthesis. Similarly the inner membrane of the
mitochondrion contains a substance called the "matrix" and this also
contains many of the necessary enzymes in order to aid the process of
Detecting the exact site of photosynthesis was a complex task carried
out by the german botanist T.W.Engelmann. Engelmann used filamentous
algae; in each of these giant cells there is a ribbon like chloroplast
with a spiral shape. Engelmann used the fact that oxygen is given off
during photosynthesis to identify the site of the reactions. He used
the motile, oxygen sensitive bacterium, Pseudomonas, which tends to
cluster around areas, which have the highest concentration of oxygen.
When the algae was put on a slide and illuminated with light of
different wavelengths, it was found that the bacteria clustered mostly
near the chloroplasts when the wavelengths were 450 blue or 650 red.
These results are demonstrated in figure 3. These wavelengths resulted
in an increased level of photosynthesis, which produced more oxygen
and therefore attracted more of the pseudomonas bacteria. It has since
this discovery been proven that green plants photosynthesise more when
illuminated in red or blue light.
The first stage of photosynthesis involves the absorption of light and
occurs in the thylakoid membranes. This produces the product ATP and
reduces the coenzyme NADP. This stage is known as the light dependent
stage. The initial stage of respiration takes place outside the
mitochondria in the cytoplasm and this stage is called "glycolysis,"
which is the breaking down of glucose (a six carbon sugar) into two
molecules of pyruvate (a 3 carbon sugar) this is demonstrated in
figure 4. Similarly to the light dependent stage ATP is produced
however in contrast NADH is produced as opposed to NADP as hydrogen is
removed and transferred to the hydrogen acceptor NAD. The light
dependent stage needs light, as it is this that the electrons are
dependent on. The light energy excites the electrons in the area of
the thylakoid known as pigment system two, giving them lots of energy
until eventually one escape from the molecule. Eventually the electron
reaches an electron carrier and is passed back along a series of
carriers until it reaches chlorophyll in the pigment system area one
of the thylakoid. The electron carriers are at different energy
levels, so as the electron is passed down, energy is released. It is
this energy that is used to synthesis ATP. The process of photolysis
is the breaking of water with light energy and this occurs in the
light dependent stage. The hydrogen ions combine with the electron and
NADP to form NADPH. This process is shown more clearly in figure 5.
The second stage of photosynthesis takes place in the stroma of the
chloroplast. This reaction involves the products from the light
dependent stage of photosynthesis using NADPH, ATP and CO in order to
form sugars. This stage of the reaction does not require light so it
is called light independent. The next stage of respiration occurs
similarly in the matrix of the mitochondrion where the link reaction
and the krebs cycle takes place.
In the light independent stage CO is accepted by a five carbon sugar
(ribulose biphosphate, RuBP), which is then catalysed by an enzyme
(Ribulose biphosphate carboxylase, Rubico), thus forming a six carbon
compound. Similar to the process of glycolysis this six-carbon sugar
is then broken down into two molecules of three carbon sugars known as
glycerate-3-phosphate. The glycerate-3-phosphate is reduced to a
triose sugar, glyceraldehyde phosphate (GALP), using the NADPH and
ATP. GALP is then used to build more six carbon sugars such as
glucose, amino acids, fatty acids and glycerol. However 5/6 of GALP is
used to regenerate RuBP using ATP. In reality very little glucose is
produced for the plants living needs, the majority of the GALP
produced is used to ensure that this process keeps taking place. This
process in its entirety is called the "Calvin Cycle," and the cycle is
shown in its entirety in figure 6.
The link reaction takes place in order for the production of the
acetyl co-enzyme A to be produced this is necessary for the Krebs
cycle to take place and is often considered part of the Krebs cycle.
The most important role of the Krebs cycle is to provide hydrogen for
the electron transport chain to provide energy for the formation of
ATP. The cycle is shown clearly in figure 7. The Krebs cycle produces
the necessary hydrogen this is then used in the electron transport
chain in order to provide energy for the formation of ATP. The Krebs
cycle produces; 3 molecules of NADPH, 1 molecule of FADH , 1 molecule
of ATP, 2 molecules of CO from one molecule of acetyl co-enzyme A.
However one molecule of glucose is worth 2 molcules of acetyl
co-enzyme A and therefore all the products are doubled.
However respiration involves a third stage occurring in the inner
membrane of the mitochondria. Oxygen is required for this final stage
of aerobic respiration. The electron transport chain involves a chain
of carrier molecules like that in the initial stages of
photosynthesis. Hydrogen atoms and electrons are passed along the
electron transport chain. The hydrogen atoms are passed on to other
carrier molecules from the hydrogen carriers reduced NADH and FADH .
NADH is the first carrier in the chain and it passes its hydrogen on
to FAD. The hydrogen atoms split into hydrogen ions and electrons.
The electrons are transferred along a series of electron carriers.
This is demonstrated clearly in figure 8. The hydrogen ions stay in
solution in the space between the inner and outer membranes of the
mitochondria. Finally the electrons recombine with the hydrogen ions
from hydrogen atoms and are passed on to oxygen to form water. Oxygen
is therefore the final electron acceptor, the reaction being catalysed
by the enzyme Cytochrome oxidase. The transfer of electrons along the
chain releases sufficient energy to make ATP from ADP + Pi. This
process produces a huge amount of ATP, a total of 34 molecules.
There are many similarities and differences demonstrated clearly
through the content of this essay. Both organelles provide an
irreplaceable role in the production of food and energy. Without these
organelles organisms would not be able to live thus evidently showing
how important these organelles are.