4.1 Plant Cell Structure

• 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

A 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

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

Chloroplasts 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

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

The structure of the vacuole

• 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