What name is given to the process by which water crosses a selectively permeable membrane?

a) Osmosis.

b) Diffusion.

c) Pinocytosis.

d) Phagocytosis.

e) Passive transport.

Diagram showing osmosis. Water moves in response to high solute (sugar) and low water
Diagram showing osmosis. Water moves in response to high solute (sugar) and low water

The correct answer is a) Osmosis.

Osmosis is the movement of water across a selectively permeable membrane from where there is a high to a low concentration of water.

Water balance has to be carefully regulated in the cells of living organisms in order for them to remain alive, thus osmoregulation is a very important process that occurs.

Where the concentration of water is high, the concentration of solutes is low, and vice versa. This means that water moves into areas where solute levels are high, in other words, water moves into hypertonic solutions.

Osmosis is a passive means of transport since it is following the natural concentration gradient of water molecules, and therefore does not need to obtain energy from ATP in order for it to occur.

Other passive transport methods include simple and facilitated diffusion, the latter process occurs when substances move through the membrane by passing through integral proteins that act as channels.

Active methods of transport all require an investment of energy, usually in the form of ATP. Such methods include pinocytosis and phagocytosis.

The pinocytosis involves the formation of small vesicles at the membrane and is used to bring in small particles and other liquids into the cell.

Phagocytosis also involves the formation of vesicles, but these are large in size compared with pinocytosis.


Osmosis is the process by which water moves into or out of a living cell across a selectively permeable membrane.

Osmoregulation is a process by which living organisms can control how much water is present in their cells. Osmosis is thus an important process for all living cells, which have to ensure that there is not too little or too much water present in the cell.

The process of osmosis is a type of passive transport since it does not require an input of energy. This is because molecules tend to move down their natural gradient from where there is a high concentration of water to where there is a low concentration of water.

Concentrations of water molecules on either side of a plasma membrane are relative and are also influenced by the concentrations of solute molecules.

In fact, if the solute concentration is very high, then the water concentration will be low, and vice versa. The result of this is that water moves to where there is a lot of solute present.

If there is a low solute concentration inside a cell then the contents are said to be hypotonic relative to the outside of the cell which is then said to be hypertonic. At the same time, the concentration of water molecules inside are higher than outside of the cell.

The net movement of water then is from inside of the cell to outside of the cell across the membrane. If the inside of the cell is hypertonic then the movement of water would be from outside to the inside of the cell.

Diffusion and facilitated diffusion

Other examples of passive transport include types of diffusion. Once again substances move down a concentration gradient from a high to low concentration.

Some small particles, such as gases, can simply diffuse through the phospholipids of the plasma membrane into the cell or out of the cell.

In other cases, substances cannot diffuse through the membrane in this way because of the lipid nature of the phospholipids.

In such situations, a special protein channel is used which facilitates the movement of these molecules through the lipid bilayer of the cell membrane. These channels are made of integral proteins that span the entire width of the membrane.

Pinocytosis and phagocytosis

These are two active transport methods, so called because they cannot function unless energy is available and can be used.

Energy is usually provided by the breaking of one of the phosphate bonds of the ATP molecule. This releases energy to enable these forms of transport to occur. The result of the reaction is that ADP and inorganic phosphate is formed in the process.


Pinocytosis involves the formation of small vesicles at the surface of the plasma membrane. These little vesicles then enable fluids and small sized particles to be taken into the cell, or out of the cell.

It is a method that is commonly used in many small unicellular protistan organisms. Environmental factors such as pH influence this process in Protista such as Amoeba.

The process is less specific than methods of endocytosis such as receptor-mediated processes in which particular receptor proteins are involved.

Pinocytosis is actually less efficient than receptor-mediated endocytosis since it uses a great deal more energy.

Dissolved organic nitrogen is commonly taken into algal species by the process of pinocytosis. This process even occurs in the intestines of animals to take up droplets of lipids from digested food.


This is a similar method to pinocytosis but it involves the formation of much larger vesicles in order for the cell to take in larger particles.

This is a method by which Protistans are able to effectively ingest food particles, including other small protists and bacterial cells.  Ciliates and other Protista have been seen to take in food particles into vacuoles, which then fuse with lysozymes inside the cell.

The result is that a phagolysosome is formed in which the food is digested by the enzymes. This same process occurs in animal macrophage cells.

The macrophages are cells of the immune system that are able to ingest foreign particles, such as bacteria, by forming phagosomes round them.

Antigen proteins on these foreign entities activate the process by which the macrophages ingest these particles. Once inside the macrophage, the particles are digested by enzymes.


  1. Editors of Encyclopedia Britannica (2019). Osmosis. Retrieved from Encyclopedia Britannica.
  2. C Chapman-Andresen, H Holtzer (1960). The uptake of fluorescent albumin by pinocytosis in Amoeba proteus. The Journal of Biophysical and Biochemical Cytology.
  3. A Aderem, DM Underhill (1999). Mechanisms of phagocytosis in macrophages. Annual Review of Immunology.
  4. Editors of Encyclopedia Britannica (2019). Pinocytosis. Retrieved from Encyclopedia Britannica.
  5. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.


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