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Notes on Biological Molecules
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- Biological Molecules
- Institution
- OCR
Notes on Biological Molecules
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Plant UltrastructurePlant Cells
Plant cells share all the common features of animal cells, but
also contain some additional organelles.
Plants gain all their energy from sunlight; cells in their leaves
contain many chloroplasts to convert this into a useful form.
Every plant cell is surrounded by a cell wall, and contains one or
more permanent vacuoles.
Chloroplasts
Chloroplasts use carbon dioxide, water and light energy to build
sugars. They are present in all green plants.
The chloroplast is surrounded by a double membrane. It is filled
with a liquid called the stroma, and contains stacks of thylakoid
membranes called grana.
The thylakoid membranes are the site of photosynthesis.
Chloroplasts use the light energy absorbed by chlorophyll to
build sugars. These are converted into complex carbohydrates
to store energy for all the plant’s needs.
Vacuoles
Permanent vacuoles only exist in plant cells. Animal cells can
contain temporary vacuoles but they are not common features.
A vacuole consists of a membrane called the tonoplast, filled
with cell sap – a watery solution of different substances,
including sugars, enzymes and pigments.
The vacuole is important in keeping the cell firm. When the
vacuole is full of sap the cell is said to be turgid.
The Cell Wall
The cell wall of a plant cell gives it support and structure. It is made of
the polysaccharide cellulose, and can function as a carbohydrate
store by varying the amount of cellulose it holds.
The cell wall does not seal off a cell completely from its neighbors.
There are pores within the walls called plasmodesmata. These
connect two cells together by their cytoplasm, enabling the exchange
and transport of substances.
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