Edexcel International A Level Biology

Revision Notes

4.1 Plant Cell Structure

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Plant Cell Structure & Ultrastructure

  • The structure of plant cells is made up of a complex system of organelles and ultrastructures
  • Plant cells contain many of the organelles found in animal cells, along with a few other organelles that are only found in plant cells

Cell wall

  • Cell walls are formed outside of the cell membrane and offer structural support to the cell
    • This structural support is provided by the polysaccharide cellulose
  • Cell walls are freely permeable, and will allow most substances to enter the plant cell

Middle lamella

  • This forms the outermost layer of the plant cell and acts like glue to stick adjacent plant cells together
  • It is mainly composed of a polysaccharide called pectin

Cell components_Cell wallA diagram to show the cell wall and middle lamella of one plant cell

Plasmodesmata

  • Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells
  • This interconnected system of cytoplasm between plant cells is known as the symplast

Pits

  • These are very thin regions of the cell wall
  • The pits in adjacent plant cells are lined up in pairs
  • They are formed due to the absence of secondary thickening in the cell walls in the areas where plasmodesmata are present

detailed-structure-of-plant-cell-wall

Detailed structure of plant cell wall

Chloroplasts

  • Chloroplasts are larger than mitochondria
  • Surrounded by a double-membrane
  • Within the chloroplast there are membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana (singular = granum)
  • Grana are joined together by lamellae (thin and flat thylakoid membranes)
  • Chloroplasts also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Cell components_ChloroplastChloroplasts are found in the green parts of plants - the green colour is due to the presence of the pigment chlorophyll

Amyloplasts

  • Small, membrane bound organelle containing starch granules
  • Large numbers are found in plant storage organs, such as potato tubers

structure-of-an-amyloplast

Structure of an amyloplast

Vacuole and tonoplast

  • The vacuole is a sac in plant cells surrounded by the tonoplast (selectively permeable membrane)
  • They are large, permanent structures in a plant cell
  • Contains cell sap, which is a mixture of different substances such as water, minerals, waste and enzymes
  • The concentration of the cell sap enables water to enter the vacuole by osmosis

Cell components_VacuoleThe structure of the vacuole

Plant Cell Structure & Ultrastructure: Function

  • The ultrastructures and organelles listed above each perform a specific function in a plant cell

Cell wall

  • The cellulose component of cell walls provides structural support to the cell
  • Due to its rigid nature, it is responsible for the regular shape of a plant cell

Middle lamella

  • It provides stability to the plant by ensuring that adjacent plant cells are adhered together

Plasmodesmata

  • The cytoplasmic strands connect the contents of adjacent plant cells
  • This allows substances to be transported between cells and facilitates cell to cell communication

Pits

  • Since the cell wall is very thin in these regions, it allows for the transport of substances between cells
  • This is particularly useful in xylem vessels, where it allows for the lateral flow of water and mineral ions between adjacent vessels

Chloroplasts

  • Due to the presence of chlorophyll and thylakoids, chloroplasts are the site of photosynthesis
    • Certain parts of the process occur in thylakoid membranes, while other parts happen in the stroma

Amyloplasts

  • They are responsible for storing starch in plants and converting it back to glucose when the plant needs it

Vacuole and tonoplast

  • Vacuoles have several functions in plant cells:
    • They keep cells turgid, which stops the plant from wilting
    • Store various substances, such as pigments and waste products
    • Break down and isolate unwanted chemicals in plant cells
    • The tonoplast controls what can enter and leave

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