1. 10.3 and 10.4 Saturday, 21 November 2009
Starch, glycogen and cellulose (3.2.4 continued)
Organisms make a large range of different molecules with different
properties related to their function from a limited range of smaller
molecules.
So what makes them all different?
It is the way they are joined together.
Starch
This is a polysaccharide that is found in plants in the form of small
grains. Larger amounts are found in the seeds and other storage organs
such as potato tubers. Starch forms an important component of food
and is a major energy source in most diets.
Starch is made of a long chain of α-glucose monosaccharides linked my
glycosidic bonds. These bonds are formed by condensation reactions.
Glucose molecules are joined together in
a chain with water produced after each
on.
Once the chain has been made
(unbranched) to coils up tightly making
a small compact molecule. (next key diagram)
Starch is an energy store at it is suited to its purpose.
 It is insoluble and so does not tend to draw water into the cells by
osmosis (has not impact on cell water conc),
 This also means it is not easily diffused out of cells, so stays up
 Because it is compact a lot of starch can be stored in a small
space,
 Once hydrolysed again (broken back down into monomers) it form
alpha glucose which is easily to transport and used in respiration.
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2. 10.3 and 10.4 Saturday, 21 November 2009
Animals do not make starch, however, a similar polysaccharide called
glycogen is found and serves the same role.
Glycogen
It has a similar structure but is shorter and much more branched. As it
is the major carbohydrate storage product in animals it is sometimes
referred to as ‘animal starch’. Again it is stored as small grains this time
in muscles and the liver. It is suited for storage for much the same
reasons. The highly branches structure makes it even easier to
hydrolyse and release the glucose. This is never in plants.
Cellulose
This is important for structure and support in plants. It is different from
both starch and glycogen in one major respect: it is made of monomers
of β-glucose rather than α-glucose. Whilst this might not seem a great
difference it produces a fundamental difference in the structure and
function of the polysaccharide.
These two forms of glucose are (stereo) isomers, because they contain
the same atoms, but they differ in the arrangement of their atoms in
space. Alpha and beta glucose differ only in the direction that -H and
-OH groups point on carbon 1. Alpha glucose has an -OH group that
points "downwards", away from the ring, whereas the -OH on carbon 1
of beta glucose is in line with the ring. This means that to form the
glycosidic links, each β-glucose molecule must be rotated by 180o
compared to its neighbour. So the –CH2OH groups also alternates.
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3. 10.3 and 10.4 Saturday, 21 November 2009
How does this alter the structure?
Rather than forming a coiled chain like starch, cellulose has straight,
unbranched chains. These run parallel to one another forming many
hydrogen bonds to form cross-linkage between adjacent chains. This
gives massive strength to the structure because whilst each bond is
weak the sheer number of them makes a massive difference.
The cellulose molecules are
grouped together to form
microfibrils which, in turn, are
arranged in parallel groups called
fibres. (see page 157).
Cellulose is a major component
of plant cell walls and gives them
the rigidity they need. It is also
important because it prevents
cells from bursting as water
enters it by osmosis (unlike animal cells). This happens because it gives
and inward pressure that stops the cell expanding. Plant cells become
turgid and push against each other. Plants need this for increased
surface area for photosynthesis.
10.4
Plant cell structure
Plant cells are eukaryotic cells so have a distinct nucleus and membrane-
bound organelles, such as mitochondria and chloroplasts.
Leaf palisade cells
These are typical plant cells that carry out photosynthesis. Looking at
the diagram you can see how it is adapted to suit its function.
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4. 10.3 and 10.4 Saturday, 21 November 2009
 They are long thin cells to
help absorb sunlight.
 Numerous chloroplasts that
arrange themselves in good
positions to collect the
maximum amount of light
depending on the
surroundings.
 Have a large vacuole that
pushes the cytoplasm and
chloroplasts to the edge of
the cell.
Chloroplasts are the organelles that carry out photosynthesis.
Whilst they can and do vary in size and shape they are typically disc-
shaped, between 2-10 µm long and 1µm in diameter.
 The chloroplast envelop is a
double plasma membrane that
surrounds the organelle. It is
highly selective in what it allows
to enter and leave the
chloroplast.
 The grana are stacks of up to
100 disc-like structures called
thylakoids. Within the
thylakoids is the photosynthetic
pigment called chlorophyll. Some thylakoids have tubular
extensions that join up with thylakoids in adjacent grane. The
grana are where the first stage of photosynthesis takes place.
 The stroma is a fluid-filled matrix where the second stage of
photosynthesis takes place. Within the stroma are a number of
other structures, such as starch grains.
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