Animal cells are surrounded by a cell plasma membrane which apart from protecting the cell contents also moderates what enters and leaves its internal environment.
Describe and explain clearly how the cell transports the following substances across the membrane :
ii) sodium and potassium ions
iv) enzymes to be used by other cells in the body
i) Osmosis involves the movement of water molecules through a selectively permeable membrane. When a cell plasma membrane separates two fluids that are not isotonic, water will move across the membrane from the hypotonic fluid into the hypertonic one. The diffusion will continue until the two fluids are isotonic, or until pressure against the hypertonic fluid counters it. Water movement occurs through the membrane lipid bilayer and through channels called aquaporins.
ii) Sodium and potassium ions cross membrane through special channels that are cometimes open and sometimes closed. When the membrane is at rest, the sodium channels are closed, preventing almost all sodium flow. Some kinds of stimulation can open the sodium channels. When the membrane is at rest, potassium channels are nearly but not entirely closed, so potassium flows slowly. Stimulation opens them more widely also, as it does for sodium channels. The sodium-potassium pump, a protein complex, repeatedly transports three sodium ions out of the cell while drawing two potassium ions into it. The sodium-potassium pump is am active transport that requires energy. As a result of the sodium-potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are similarly more concentrated inside than outside.
iii) Facilitated diffusion moves large molecules that need some help in passing the membrane, for example, glucose molecules. A glucose transporter is an example of a transport protein that works in facilitated diffusion. This protein changes shape when it binds to a molecule of glucose. The shape change moves the glucose to the opposite side of the membrane, where it detaches from the transport protein. Then, the glucose transporter reverts to its original shape.
iv) Exocytosis provides a means by which enzymes are released from cells. A vesicle containing enzymes to be expelled moves towards the surface of the cell and fuses with the plasma membrane. The vesicle then opens to the exterior and its contents leave the cell. The vesicle membrane then becomes part of the plasma membrane. The vesicles are often derived from the Golgi apparatus.