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Topic 7- Plant Cells
Plant Cell Structure
Plant cells have most of the same organelles as animal cells,
except for a few extras.
Cell walls are made from cellulose and are there to support and
protect the cell. They allow the cell to cope with changes in water
potential that would cause an animal cell to burst.
Chloroplasts contain chlorophyll, which gives them a green
pigmentation. They are the site of photosynthesis in plant cells.
The vacuole stores water and supports the cell.
Cell Walls and Cellulose
The functions of the cell wall are support and protection. This
means it needs to be tough, indigestible, flexible and strong.
Cellulose is made from -glucose arranged in chains.
The chains are joined together by hydrogen bonds.
The glycosidic bonds are protected by layers of glucose chains,
making it hard to digest. Only the surface layers can be digested.
This makes it better suited as a structural molecule rather than a
Lots of thin strands increase flexibility without compromising the strength of the polymer. Cellulose has good tensile
Cellulose is formed from microfibrils.
-glucose is an isomer of -glucose. -glucose has both its hydroxyl groups on one side, whereas -glucose has the hydroxyl
group on the first carbon molecule flipped, as below. This means that in cellulose, alternate glucose molecules are inverted.
-glucose molecules are joined together by -glycosidic bonds. This small difference causes a difference in structure of the
glucose chain. Whereas the -glucose bond is flexible,
meaning that starch molecules are coiled, the -glucose
bond is rigid so cellulose forms straight chains. These
chains are linked by hydrogen bonds.
The pigment chlorophyll gives chloroplasts their green colour.
There is a higher concentration of chlorophyll in the thylakoid
membrane. The thylakoid membrane is a phospholipid
bilayer with chlorophyll molecules embedded into it to absorb light
energy and transfer it to other protein molecules. ATP is generated
in this stage of photosynthesis.
The thylakoid membrane forms into stacks called grana (singular- granum).
The stroma is a fluid containing enzymes. They use ATP generated in photosynthesis to fix carbon dioxide into sugar.
The chloroplast has a double membrane that controls the movement of substances in and out of the chloroplast.
The grana increase the efficiency of reactions by capturing most of the light energy entering the cell.
Excess carbohydrate generated in photosynthesis is stored as starch grains. This is because it is insoluble so does not
affect the water potential of the organelle. Starch is also compact because it is coiled and has a small surface area for
bond breaking so it releases glucose molecules slowly. Starch is made from
Glycogen, like starch is a good energy store because it is compact, coiled
and branched molecule. Glycogen has more branches meaning that it can
be broken down faster and therefore is better for releasing glucose quickly.