Cytoplasmic streaming in onion cells

All cells are confined by a cell membrane, and in the case of plant cells this membrane is accompanied by a cell wall. Inside this envelope, each cell contains a lot of different organelles, molecules like proteins, carbohydrates and nutrients along with a cytoskeleton for structural support.

The organelles and macromolecules need to move around to get where they are needed. One way of achieving this is through passive diffusion. However, this process is very slow in the crowded cellular cytoplasm where up to 30% of the volume is taken up by macromolecules. To speed things up the cell utilizes a phenomenon called cytoplasmic streaming, also known as protoplasmic streaming and cyclosis. This kind of molecular movement, in contrast to diffusion, is not a passive but an active process which uses energy in the form of ATP (adenosine triphosphate). The energy is spent on moving specialized transport molecules called myosin, kinesin and dynein, along the cellular cytoskeleton while carrying the macromolecules and organelles.

In some cells the cytoplasmic streaming is especially visible. One example of these cells are the epidermal cells from a common onion (allium cepa). The epidermal layer in an onion is the thin membrane-like layer between all of the “real” onion layers. This is a fantastic specimen for observing cytoplasmic streaming for a number of reasons.

Firstly, the bulb of an onion is not used for photosynthesis so there are no chloroplasts blocking the view. Secondly, the epidermis is only a single cell layer in thickness so there is nothing above or beneath the cells to distort the view. And lastly, the epidermis is almost completely transparent which allows us to observe the motion in high detail and contrast.

Onion epidermis, 20x objective magnification, darkfield illumination.

Onion epidermis, 60x objective magnification, brightfield illumination.

In the clip you are able to see the cytoplasmic streaming in the epidermal cells in action. Note how the streaming transports molecules to and from the nucleus (pale blue condensation roughly in the middle of each cell) and around the boundary of each cell. The transport happens in more or less the same direction along the cytoskeleton which the carrier molecules uses as microscopic highways.

Scientific source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590424/