Plant cell structureEdexcel International A Level Biology: Revision notes
Section 1
Cell wall and middle lamella
The cell wall surrounds the plasma membrane of every plant cell. It is made mainly of cellulose microfibrils, which are strong and give the wall tensile strength. It supports the cell and gives it shape, and it resists expansion when water enters by osmosis, so a turgid cell does not burst. The wall is freely permeable to water and solutes.
The middle lamella is a layer of pectin between the walls of neighbouring cells. It cements the cells together.
The cell wall is not the same as the plasma membrane. The wall is fully permeable; the membrane is selectively permeable.
Section 2
Plasmodesmata and pits
Plasmodesmata are narrow channels of cytoplasm, lined by plasma membrane, that pass through the cell walls and link the cytoplasm of adjacent cells. Substances and signals move between cells through them, by diffusion, so they form the symplast pathway.
Pits are thin regions of the cell wall where the wall is not thickened. Plasmodesmata are concentrated in pits, so transport between neighbouring cells is easier there.
Section 3
Chloroplasts
Chloroplasts are the site of photosynthesis. Each is surrounded by a double membrane (envelope). Inside are stacks of flattened membrane sacs, the thylakoids, which form grana. The thylakoid membranes contain chlorophyll and absorb light for the light-dependent reaction. The fluid stroma around them contains enzymes for the light-independent reaction, as well as small circular DNA, 70S ribosomes and sometimes starch grains.
Section 4
Amyloplasts
Amyloplasts are organelles that store starch. They have a double membrane but no thylakoids or chlorophyll. They are found in storage tissues such as potato tubers and seeds. They stain blue-black with iodine solution.
Starch is a good store because it is insoluble, so it does not affect the cell's water potential, and it is a large, compact molecule that cannot diffuse out of the cell.
Section 5
Vacuole and tonoplast
A mature plant cell has a large permanent vacuole filled with cell sap, a solution of sugars, ions, pigments and waste. The vacuole is surrounded by a single membrane, the tonoplast, which controls what enters and leaves.
The vacuole stores water and solutes, and the water in it gives turgor pressure, which keeps the cell firm and supports non-woody plants.
Section 6
Plant and animal cells; electron micrographs
Both are eukaryotic, with a nucleus, mitochondria, ribosomes, ER, Golgi apparatus and plasma membrane.
Plant cells only: a cellulose cell wall, a large permanent vacuole, chloroplasts (in some cells), plasmodesmata and pits.
Animal cells only: no cell wall, small temporary vacuoles, no chloroplasts, and energy stored as glycogen rather than starch.
In electron micrographs: the cell wall is a thick layer outside the plasma membrane; the vacuole is a very large sac with a single membrane; a chloroplast has a double membrane and stacks of thylakoids; an amyloplast has a double membrane and large pale starch grains but no thylakoids.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Plant cell structure
- A student examines an electron micrograph of a palisade mesophyll cell from a leaf. The cell has a thick layer outside its plasma membrane, a very large sac occupying most of the cell interior, and several oval organelles containing stacks of flattened membranes.The cell is placed in distilled water. Explain why it does not burst.2 marks
- A potato tuber stores energy over the winter. Cells from inside a tuber were stained with iodine solution and viewed under a light microscope. Many organelles in the cells contained dark blue-black granules.Explain why the tuber stores energy as starch in amyloplasts rather than as glucose in the cytoplasm.2 marks
- A researcher injects a fluorescent dye into one parenchyma cell in the stem of a plant. Within minutes the dye is detected in the neighbouring cells, even though the dye cannot cross the plasma membrane. Microscopy shows that where the dye passes between the cells, the cell wall is thin.Explain how the dye could move from the injected cell into its neighbours.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).