Edexcel A (SNAB) A Level Biology:复习笔记4.2.1 Plant Cell Structure

• Plant cells contain most of the organelles found in animal cells, along with a few extra 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

Middle lamella

• This forms the outermost layer of the plant cell and acts like glue to stick adjacent plant cells together
• It provides stability to the plant

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 allows substances to be transported between plant cells and facilitates cell to cell communication

Pits

• These are very thin regions of the cell wall
• The pits in adjacent plant cells are lined up in pairs
• This facilitates transport of substances between cells

Detailed structure of plant cell wall

Chloroplasts

• Larger than mitochondria, also surrounded by a double-membrane
• 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 are the site of photosynthesis:
• Certain parts of photosynthesis occur in thylakoid membranes, while other parts happen in the stroma
• 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
• They are responsible for storing starch in plants and converting it back to glucose when the plant needs it

Structure of an amyloplast

Vacuole and tonoplast

• The vacuole is a sac in plant cells surrounded by the tonoplast (selectively permeable membrane)
• Contains cell sap, which is a mixture of different substances such as water, minerals, waste and enzymes
• Vacuoles have several functions in plant cell:
  • They keep cells turgid, which stops the plant from wilting
  • They can break down and isolate unwanted chemicals in plant cells
  • The tonoplast controls what can enter and leave

The structure of the vacuole

• It is important to be able to recognise the plant organelles mentioned earlier from electron microscope images

TEM electron micrograph of a plant cell showing key features