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Here is a diagram of a plant
palisade mesophyll cell:
I have underlined the labels you need to know
about in green.
You might get asked how this cell is specialised
for its function. Well, this kind of cell is found in a leaf and it has
many chloroplasts for photosynthesis.
Chloroplasts
Chloroplasts contain photosynthetic pigments. You
have probably heard of "chlorophyll" which is needed for
photosynthesis. If there was only one kind of photosynthetic pigment you
would expect all green plants to be exactly the same shade of green, but
they are not. This is because there are several different photosynthetic
pigments and different plants have different amounts of each one and they
are not all the same colour.
Photosynthetic Pigments
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Chlorophyll a |
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Chlorophyll b |
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Chlorophyll c |
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Chlorophyllides |
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Xanthophyll |
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Carotene |
They are all slightly different colours. You do
not need to worry about this for your GCSE exam, but you will need to know
about this when you start your "A" Level biology course. It is
possible to separate these pigments from leaves using a technique called
"chromatography". Thin Layer Chromatography (TLC) is an
excellent way of proving that there is more than one kind of chlorophyll.
The colours you will see are blue-green, green, yellow-green, yellow,
orange and even grey. The grey pigment is actually a breakdown product
from chlorophyll.
Photosynthesis
Plants can use light energy to convert carbon
dioxide and water into glucose and oxygen. To do this they must have
chlorophyll. Photosynthesis takes place in the green parts of plant; we
call these the leaves !!!!! You know that there is no chlorophyll in the
cells in the roots because you know that roots are NOT green. Anyway,
roots are usually underground and therefore in the dark so photosynthesis
would not be possible even if the root cells did contain chlorophyll.
All of the last paragraph was
totally obvious: so it should be obvious that the following diagram shows
a plant cell even though it does not contain chloroplasts. Here is a
"root hair" cell:
Osmosis and turgidity
Osmosis is very important in root hair cells.
Providing that the soil is moist, it is possible for water to enter the
root hair by osmosis. (See my Osmosis
Page if you have forgotten about this.)
Water passes from a region of high water concentration (wet soil) through
a semi-permeable membrane (the cell membrane) to a region of lower water
concentration (the cytoplasm). This makes the cell turgid. As these cells
develop they absorb water by osmosis and the hair can be pushed between
soil particles.
The most important thing to remember about root
hairs is that they increase the surface area between the root and the
soil: this is necessary for the absorption of water and mineral salts.
Examiners like to ask you what is special about
this cell. They put a diagram like this in the exam paper (perhaps without
the labels) and then ask "How is this cell specialized or adapted for
its function?" You will write down "It has a large surface area
to speed up the absorption of water and mineral salts from the soil."
Poor old examiner will have to give you a mark. Actually examiners
like to give you marks, but only if you know the right answer.
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