Plant cell structure and celluloseEdexcel A-Level Biology A: Revision notes
Section 1
Cell wall, middle lamella, pits and plasmodesmata
Plant cells are surrounded by a cell wall outside the cell surface membrane. It is made of cellulose microfibrils and gives the cell support and strength. It stops the cell from bursting when water enters by osmosis, so cells become turgid.
Between the walls of adjacent cells is the middle lamella, a thin layer containing pectin that cements the cells together.
Pits are thin areas of the wall where the wall is not thickened. Plasmodesmata are narrow channels lined with cell membrane that pass through the walls (often through pits) and link the cytoplasm of neighbouring cells. They let substances move and cells communicate, and they can also let viruses spread.
Section 2
Chloroplasts, amyloplasts, vacuole and tonoplast
A chloroplast has a double membrane. Inside are stacks of thylakoids (grana) containing chlorophyll, surrounded by the stroma, which may contain small starch grains. It is the site of photosynthesis.
An amyloplast is a membrane-bound organelle that stores starch in large grains. It has no thylakoids or chlorophyll, and is found in storage tissues such as potato tubers.
A plant cell usually has a large central vacuole that contains cell sap (sugars, ions, pigments and enzymes). It is surrounded by a selectively permeable membrane, the tonoplast. The vacuole keeps the cell turgid and stores substances.
Do not call the tonoplast the cell wall or the cell surface membrane. It is the membrane around the vacuole.
Section 3
Comparing plant and animal cells
Both have a nucleus, cytoplasm, a cell surface membrane, mitochondria, ribosomes, rough endoplasmic reticulum and a Golgi apparatus.
Plant cells also have:
- a cellulose cell wall and a middle lamella
- plasmodesmata and pits
- chloroplasts (in cells exposed to light) and amyloplasts
- a large permanent vacuole with a tonoplast
- starch as the carbohydrate store
Animal cells lack these, and usually store carbohydrate as glycogen. Their vacuoles, if present, are small and temporary.
Section 4
Recognising organelles in electron micrographs
In an electron micrograph (EM):
- cell wall: a thick band outside the cell surface membrane; between two cells there are two walls with a thin middle lamella between them
- plasmodesmata: narrow gaps or channels crossing the wall
- chloroplast: an oval organelle with a double membrane, stacks of thylakoids (grana) and sometimes starch grains
- amyloplast: an oval, membrane-bound organelle containing one or more large starch grains, without thylakoids
- vacuole: a large space occupying much of the cell, surrounded by the tonoplast
To tell a chloroplast from an amyloplast, look for stacked thylakoid membranes (chloroplast) or large starch grains and no thylakoids (amyloplast).
Section 5
Starch
Starch is the carbohydrate store of plants. It is a polysaccharide made of α-glucose joined by glycosidic bonds, and has two forms:
- amylose: an unbranched chain of 1,4 glycosidic bonds that coils into a helix
- amylopectin: a chain with 1,4 bonds and 1,6 branches
Starch is suited to storage because it is:
- insoluble, so it does not change the water potential of the cell or leave it
- compact, as the helix and branching store a lot of glucose in a small space
- quickly hydrolysed at the many ends of the branched amylopectin, releasing glucose for respiration
Section 6
Cellulose and hydrogen bonds
Cellulose is a structural polysaccharide made of β-glucose. In β-glucose the OH on carbon 1 is above the ring, so each glucose is inverted relative to its neighbours when the 1,4 glycosidic bond forms. The result is a long, straight, unbranched chain.
The OH groups of neighbouring chains form many hydrogen bonds. Each is weak, but together they are strong, and they bind the chains into microfibrils, which are arranged into fibres in the cell wall.
This gives high tensile strength, which supports the cell and resists the pressure of water entering the cell.
The strength of cellulose does not come from bonds within a chain. It comes from the many hydrogen bonds between parallel chains.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Plant cell structure and cellulose
- A student examines two plant cells with an electron microscope: a palisade mesophyll cell from a leaf and a cell from a potato tuber. Both cells have a large central vacuole surrounded by a membrane.Describe two differences between a chloroplast in the leaf cell and an amyloplast in the tuber cell.2 marks
- A virus infects a tobacco leaf and spreads through the leaf from cell to cell. Each plant cell is surrounded by a rigid wall, and neighbouring cells are held together by a layer between their walls. Microscopy shows thin channels crossing the walls between neighbouring cells, and thin areas of the wall called pits.Suggest how the virus could spread from cell to cell even though each cell is surrounded by a rigid cell wall.2 marks
- Cotton fibres, which are used to make textiles, are almost pure cellulose. Potato tubers store large amounts of starch. Both cellulose and starch are polymers of glucose.Explain how the structure of starch makes it suitable as a storage molecule in the potato tuber.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).