34 Fundamental Q&As on Cell Membrane

2. How are membranes classified according to their permeability?

Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable.

An impermeable membrane is one through which no substance can pass. Semipermeable membranes are those which only let solvents, such as water, pass through them. Permeable membranes are those which let solvents and solutes, such as ions and molecules, to pass through them. There are also selectively permeable membranes, which are membranes that, in addition to allowing the passage of solvents, let specific solutes pass through while blocking others.

3. What is diffusion?

Diffusion is the spreading of molecules of a substance from a region where the substance is more concentrated to another region where it is less concentrated. For example, when water is boiled, gaseous water particles tend to uniformly spread in the air via diffusion.

4. What does concentration gradient mean? Is it correct to refer to the “concentration gradient of water”?

The concentration gradient is the difference in the concentration of a substance between two regions.

Concentration is a term used to designate the quantity of a solute divided by the total quantity of a solution. Since water, in general, is the solvent in this situation, it is not correct to refer to the “concentration of water” in a given solution.

5. What is the difference between osmosis and diffusion?

Osmosis is the phenomenon of the movement of solvent particles (in general, water) from a region of lower solute concentration to a region of higher solute concentration. Diffusion, on the other hand, is the movement of solutes from a region of higher solute concentration to a region of lower solute concentration.

Osmosis can be considered the movement of water (solvent) whereas diffusion can be considered the movement of solutes, caused by a concentration gradient.

6. What is osmotic pressure?

In a aqueous solution, osmotic pressure is the pressure that a region of lower solute concentration puts on a region of higher solute concentration, forcing the passage of water from the area of lower solute concentration to the more concentrated region. The intensity of the osmotic pressure (in units of pressure) is equal to the pressure necessary to apply to the solution to prevent its dilution by osmosis.

It is possible to apply pressure to counteract the osmotic pressure on a solution, such as the hydrostatic pressure of the liquid or atmospheric pressure. In plant cells, for example, the rigid cell wall creates pressure that acts against the tendency of water to enter when the cell is in a hypotonic environment. Microscopically, the pressure that counteracts osmotic pressure does not prevent water from passing through a semipermeable membrane, but it does create a water flow in the opposite way as compensation.

7. Can solutions with the same concentration of different solutes have different osmotic pressures?

The osmotic pressure of a solution does not depend on the nature of the solute; it only depends on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles, despite containing different solutes, exert the same osmotic pressure.

Even when the solution contains a mixture of different solutes, its osmotic pressure only depends on its total particle concentration, regardless of the nature of the solutes.

8. How are solutions classified according to their comparative tonicity?

When compared to another solution, a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).

When a solution is less concentrated than another, it is considered hypotonic compared to that other solution. When it is more concentrated, it is considered hypertonic. When two solutions have the same concentration,  both are designated isotonic. Therefore, this classification makes sense only when comparing solutions.

9. What type of membrane is the cell membrane in terms of permeability?

The cell membrane is a selectively permeable membrane, meaning that it allows the passage of water and some select solutes.

• Cell Membrane Review - Image Diversity: cell membrane

10. What are the basic components of the cell membrane?

The cell membrane is formed of lipids, proteins and carbohydrates.

The lipids contained in the membrane are phospholipids, a special type of lipid, which is bound to a phosphate group on one end, thus giving an electrical charge to this region of the molecule. Since phospholipids have one electrically charged end and a long neutral organic chain, they can organize themselves into two layers of attached molecules: the hydrophilic portion (polar) of each layer faces outwards and is in contact with the water (also a polar molecule) located in the extracellular and intracellular space whereas the hydrophobic chains (non-polar) face inwards and are isolated from the water. Because this type of membrane is made of two phospholipid layers, it is also called a bilipid membrane.

Membrane proteins are embedded and dispersed in the compact bilipid structure. Carbohydrates appear in the outer surface of the membrane, attached to some of those proteins in the form of glycoproteins or bound to phospholipids, forming glycolipids. The carbohydrates in the membrane form the glycocalyx of the membrane.

This description of the structure of cell membranes is known as the fluid mosaic model.

• Cell Membrane Review - Image Diversity: phospholipid bilayer membrane proteins glycocalyx

11. What are the respective functions of phospholipids, proteins and carbohydrates in the cell membrane?

Phospholipids have a structural function in cell membranes. They form the bilipid membrane that the cell membrane is composed of.

Proteins have several specialized functions in cell membranes. Some of them are channels for substances to pass through the membrane; others are receptors and signalers of information; others are enzymes; others are cell identifiers (cellular markers); and they also participate in the adhesion complexes between cells or between the internal surface of the membrane and the cytoskeleton.

Membrane carbohydrates, attached to proteins or to lipids, are found in the outer surface of the cell membrane. In general, they are used to mark cells so that these cells and their functions are recognized by other cells and substances (for example, they differentiate red blood cells in the ABO blood group system). They also carry out immune modulation functions, pathogen sensitization functions, etc.

12. What is differentiation of a cell membrane?

In some types of cells, the cell membrane has different structures that are necessary for the specific functions of the cells. The main ones are the microvilli and the structures for the reinforcement of adhesion between cells (cell junctions).

Microvilli are multiple external projections of the membrane resembling glove fingers. They are found in the cells of tissues in which it is advantageous to increase the size of the surface area in contact with the exterior, for example, in the enteric (intestinal) epithelium for the absorption of nutrients.

Structures that promote the strengthening of the adhesion between cells occur mainly in epithelial tissues where the need for coverage and impermeability requires cells to be “glued” to neighboring cells. These structures can be interdigitations, desmosomes, tight junctions (zonula occludens), zonula adherens (adherens junctions) and gap junctions.

13. What is the relationship between the concentration gradient and active and passive transport?

Passive transport is the movement of substances across membranes in favor of their concentration gradient, rather, from a more concentrated region to a less concentrated region. Active transport, on the other hand, is the transport of substances across membranes against their concentration gradient, from a less concentrated to a more concentrated region. No energy is used in passive transport because it is spontaneous. Active transport, on the other hand, requires energy (work) to occur.

Active transport works to maintain or increase the concentration gradient of a substance between two regions while passive transport works to reduce the concentration gradient.

14. What are the three main types of passive transport?

The three main types of passive transport are simple diffusion, osmosis and facilitated diffusion.

Cell Membrane Review - Image Diversity: passive transport

15. What energy source is used in active transport through biological membranes?

The energy necessary for active transport (against the concentration gradient of the transported substance) to occur comes from ATP molecules. Active transport uses chemical energy from ATP.

16. What is the difference between simple and facilitated diffusion? What does the term “facilitated” refer to? 

Simple diffusion is the direct passage of substances across the membrane in favor of their concentration gradient. In facilitated diffusion, the movement of substances is also in favor of their concentration gradient but the substances move bound to specific molecules that act as “permeabilizers”, that is, facilitators of their passage through the membrane.

• Cell Membrane Review - Image Diversity: facilitated diffusion

17. How does the intensity of simple diffusion vary depending on the relation to the concentration gradient of the transported substance?

The higher the concentration gradient of a substance, the more intense its simple diffusion will be. If the concentration gradient diminishes, the intensity of simple diffusion also diminishes.

18. How does the intensity of facilitated diffusion vary depending on the concentration of the transported substance? What is the limiting factor?

Like simple diffusion, facilitated diffusion is more intense when the concentration gradient of the substance is higher and less intense when the gradient is lower. However, in facilitated diffusion, there is a limiting factor: the quantity of the permeases that facilitate transport through the membrane. Even in a situation in which the concentration gradient of the diffusing substance is high, if there are not enough permeases to carry out the transport there will be no increase in the intensity of the diffusion. This situation is called saturation of the transport proteins and it represents the point at which the maximum transport capacity of the substance across the membrane is reached.

19. In a situation in which the transport proteins are not saturated, how can the speed of simple diffusion be compared to the speed of facilitated diffusion?

The action of facilitator proteins in facilitated diffusion makes this type of diffusion faster than simple diffusion (for the same concentration gradient of the transported substance).

20. What does facilitated diffusion have in common with enzymatic chemical reactions?

One of the main examples of facilitated transport is the entrance of glucose from blood into cells. Glucose from blood binds to specific permeases (hexose-transporting permeases) present in the cell membrane and, via diffusion facilitated by these proteins, it enters the cell to carry out its metabolic functions.

Facilitated diffusion resembles chemical catalysis because the transported substances bind to permeases like substrates bind to enzymes and, after one transport job is finished, the permease is not consumed and can transport other molecules.

21. What are some examples of biological activities in which osmosis plays an important role?

Hemolysis (the destruction of red blood cells) by the entrance of water, hydric regulation in plants and the entrance of water into the xylem of vascular plants are all examples of biological phenomena caused by osmosis.

Excessive dilution of blood plasma causes, via osmosis, the entrance of too much water into red blood cells and the subsequent destruction of these cells (hemolysis). Osmosis is also the main process in the maintenance of the flaccid, turgid or plasmolytic states of plant cells. Osmosis is one of the forces responsible for the entrance of water into the roots of plants, since root cells are hypertonic in comparison to the soil.

22. What do facilitated diffusion and active transport have in common? What are the differences between them?

Facilitated diffusion can be confused with active transport because membrane proteins participate in both processes.

However, in active transport the transported substance moves against its concentration gradient, consuming energy. Facilitated diffusion is passive transport in favor of the concentration gradient and does not require energy.

• Cell Membrane Review - Image Diversity: active transport

23. Which molecules make active transport through membranes possible?

Active transport is made possible by specific membrane proteins. These proteins are called “pumps” because they “pump” the moving substance through the membrane by using energy from ATP molecules.

24. How is the sodium-potassium pump involved in the functions of cell membranes? What is the importance of this protein for cells?

The sodium-potassium pump is the transport protein that maintains the concentration gradient of these ions between the intra and the extracellular spaces. This protein is phosphorylated in each pumping cycle and then pumps three sodium ions outside the cell and two potassium ions inwards. The phosphorylation is caused by the binding of a phosphate donated by one ATP molecule that is then converted into ADP (adenosine diphosphate).

The job of the sodium-potassium pump, also known as sodium-potassium ATPase, is fundamental in the maintaining of the characteristic negative electrical charge on the intracellular side of the membrane of the resting cell and in creating adequate conditions of sodium and potassium concentrations inside and outside the cell to maintain cellular metabolism.

• Cell Membrane Review - Image Diversity: sodium-potassium pump

25. What is mass transport across the cell membrane?

Mass transport is the entrance or exit of substances through the process of being engulfed by portions of membrane. The fusion of internal substance-containing membranous vesicles with the cell membrane is called exocytosis. The entrance of substances into the cell after they have been engulfed by projections of the membrane is called endocytosis.

26. What are the two main types of endocytosis?

Endocytosis is the entrance of material into the cell through being engulfed by portions of the cell membrane.

Endocytosis can be classified as pinocytosis or phagocytosis. In pinocytosis, small particles on the external surface of the membrane stimulate the invagination of the membrane inwards and vesicles full of those particles then detach from the membrane and enter the cytoplasm. In phagocytosis, bigger particles on the external surface of the membrane induce the projection of pseudopods outwards to enclose the particles. The vesicle then detaches from the membrane and enters the cytoplasm, receiving the name phagosome.

• Cell Membrane Review - Image Diversity: pinocytosis phagocytosis

27. How do plant cell walls react when placed in a hypotonic medium?

Plant cell walls (the cover of the cell external to the cell membrane) are made of cellulose, a polymer of glucose.

When a plant cell is placed in a hypotonic medium, it absorbs too much water through osmosis. In that situation, the cell wall pressure acts to counteract the osmotic pressure, thus preventing excessive increases in cellular volume and cell lysis.

28. What is meant by the suction force of a plant cell? Does suction force facilitate or hinder the entrance of water into the cell?

Suction force (SF) is the osmotic pressure of the plant cell vacuole, or rather, the cell sap found inside the vacuole.

Since cell sap is hypertonic in comparison to cytosol, it attracts water, thus increasing the cytosol concentration. Through the osmotic action of the vacuole, the cytosol becomes hypertonic in relation to the exterior and more water enters the cell.

29. What is the turgor pressure of plant cells? Does it make it easier or harder for water to enter plant cells?

Turgor pressure (TP) is the pressure caused by the distension of the plant cell wall against the increase of the cell volume. Turgor pressure works against the entrance of water into the cell, as it forces the exit of water and counteracts the entrance of the solvent via osmosis. 

30. What does the formula DPD = SF – TP mean?

DPD is the abbreviation for diffusion pressure deficit; SF (suction force) is vacuolar osmotic pressure; and TP is turgor pressure.

The difference between SF and TP determines whether water tends to enter the cell or not. If SF > TP, DPD > 0, water tends to enter the cell by osmosis. If TP > SF, DPD < 0, water cannot enter the cell by osmosis.

31. What are the values of DPD for plant cells in hypertonic, isotonic and hypotonic media?

When plant cells are placed in a hypertonic medium, they will lose water to the exterior, SF > 0 (the vacuolar pressure is high because it is concentrated) and TP = 0 (there is no distension of the cell wall since the cellular volume is reduced), so DPD = SF. These cells are called plasmolysed cells, and they are characterized by the retraction of the cell membrane, which detaches from the cell wall.

When plant cells are placed in a isotonic medium, there is no increase in the internal water volume, SF > 0 and TP = 0 (since the cell wall is not distended). The cell membrane touches the cell wall just slightly, and the cell is called a flaccid cell.

When plant cells are placed in hypotonic medium, water tends to enter them, SF = TP (since the osmotic pressure is fully compensated by the distension of the cell wall) and DPD = 0. A cell that has expanded to this point is called a turgid cell.

• Cell Membrane Review - Image Diversity: plasmolysed cell flaccid cell turgid cell

32. What is the formula for the DPD of wilted (shrunken) plant cells? How is this situation possible?

Wilted plant cells are those that have shrunk due to the loss of water by evaporation without enough replacement. In this situation, the cell membrane retracts and detaches from the cell wall.  Moreover, the cell wall expands in length to stimulate the entrance of water, making TP < 0. Since DPD = SF – TP and TP is negative (< 0), its formula becomes DPD = SF + |TP|.

33. What is the deplasmolysis of plant cells?

When placed in a hypertonic medium, plant cells lose a large amount of water and their cell membranes detach from their cell walls. In that situation, the cell is called a plasmolysed cell. When a plasmolysed cell is placed in a hypertonic medium it absorbs water and becomes a turgid cell. This phenomenon is called deplasmolysis.

34. Why are salt and sugar used in the production of dried meats and dried fruits?

Substances that maintain a highly hypertonic environment, such as sugar and salt, are used in the production of dried meats, fruits or fish (for example, cod) because the material to be conserved is dehydrated and the resulting dryness prevents the growth of populations of decomposer organisms (since these organisms also lose water and die).