Cell Membrane - Biology Q&A
1. What is a membrane?
Membrane is any delicate sheet that separates one region from another blocking or permitting (selectively or completely) the passage of substances. The skin, for example, can be considered a membrane that separates the exterior from the interior of the body; cellophane, used in chemical laboratories to separate solutions, acts as a membrane too.
2. Concerning their permeability how are membranes classified?
Membranes can be classified as impermeable, permeable, semipermeable or selectively permeable.
An impermeable membrane is that through which no substance can pass. Semipermeable membranes are those that let only solvents, like water, to pass through it. Permeable membranes are those that let solvent and solutes, like ions and molecules, to pass across it. There are also selectively permeable membranes, i.e., membranes that besides allowing the passage of solvent, let only some specific solutes to pass while blocking others.
3. What is diffusion?
Diffusion is the spreading of substance molecules from a region where the substance is more concentrated to another region where it is less concentrated. For example, during the boiling of water in a kitchen gaseous water particles tend to uniformly spread in the air by diffusion.
4. What is meant by concentration gradient? Is it correct to refer to “concentration gradient of water”?
Concentration gradient is the difference of 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 the solution. Since water in general is the solvent in this situation it is not correct to refer to “concentration of water” in a given solution.
5. What is the difference between osmosis and diffusion?
Osmosis is the phenomenon of 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.
One can consider osmosis as movement of water (solvent) and diffusion as movement of solutes, both concentration gradient-driven.
6. What is osmotic pressure?
Osmotic pressure is the pressure created in an aqueous solution by a region of lower solute concentration upon a region of higher solute concentration forcing the passage of water from that to this more concentrated region. The intensity of the osmotic pressure (in units of pressure) is equal to the pressure that is necessary to apply in the solution to prevent its dilution by the entering of water by osmosis.
It is possible to apply in the solution another pressure in the contrary way to the osmotic pressure, like the hydrostatic pressure of the liquid or the atmospheric pressure. In plant cells, for example, the rigid cell wall makes opposite pressure against the tendency of water to enter when the cell is put under a hypotonic environment. Microscopically, the pressure contrary to the osmotic pressure does not forbid water to pass through a semipermeable membrane but it creates a compensatory flux of water in the opposite way.
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 depends only on the quantity of molecules (particles) in relation to the total solution volume. Solutions with same concentration of particles even containing different solutes exert the same osmotic pressure.
Even when the solution contains a mixture of different solutes its osmotic pressure depends only on its total particle concentration regardless of the nature of the solutes.
8. How are solutions classified according to their comparative tonicity?
Comparative to another, a solution can be hypotonic (or hyposmotic), isotonic (or isosmotic) or hypertonic (or hyperosmotic).
When a solution is less concentrated than another the adjective hypotonic is given and the more concentrated is called hypertonic. When two compared solutions have the same concentration both receive the adjective isotonic. So this classification makes sense only for comparison of solutions.
9. Concerning permeability what type of membrane is the cell membrane?
The cell membrane is a selectively permeable membrane, i.e., it allows the passage of water and some selected solutes.
Cell Membrane Review - Image Diversity: cell membrane
10. What are the basic constituents of the cell membrane?
The cell membrane is formed of lipids, proteins and carbohydrates.
The membrane lipids are phospholipids, a special type of lipid to which one extremity a phosphate group is bound thus assigning electrical charge to this region of the molecule. Since phospholipids have one electrically charged extremity and a long neutral organic chain they can organize themselves in two layers of associated molecules: the hydrophilic portion (polar) of each layer faces outwards in contact with water (a polar molecule too) of the extracellular and the intracellular space and the hydrophobic chains (non polar) face inwards isolated from the water. Because this type of membrane is made of two phospolipid 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 associated to some of those proteins under the form of glycoproteins or bound to phospholipids forming glycolipids. The membrane carbohydrates form the glycocalix of the membrane.
This description (with further explanations) is known as the fluid mosaic model about the structure of the cell membrane.
Cell Membrane Review - Image Diversity: phospholipid bilayer membrane proteins glycocalyx
11. What are the respective functions of phospholipids, proteins and carbohydrates of the cell membrane?
Membrane phospholipids have a structural function, they form the bilipid membrane that constitutes the cell membrane itself.
Membrane proteins have several specialized functions. 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 labels) and there are still those that participate in the adhesion complexes between cells or between the internal surface of the membrane and the cytosketeleton.
Membrane carbohydrates, associated to proteins or to lipids, are found in the outer surface of the cell membrane and they have in general labeling functions for recognition of the cell by other cells and substances (for example, they differentiate red blood cells in relation to the ABO blood group system), immune modulation functions, pathogen sensitization functions, etc.
12. What are differentiations of the cell membrane?
In some types of cells, the cell membrane presents differentiations that are necessary for the specific functions of the cells. The main differentiations are the microvilli and the structures for reinforcement of adhesion or union between cells (cell junctions).
Microvilli are multiple external projections of the membrane resembling glove fingers. This differentiation is found in cells of tissues where it is advantageous to increase the size of the surface area in contact with the exterior, for example, in the enteric (intestinal) epithelium for absorption of nutrients.
Membrane differentiations for reinforcement of adhesion between cells occur mainly in epithelial tissues where the need for coverage and impermeability requires cells to be “glued” to neighboring cells. These differentiations can be interdigitations, desmosomes, tight junctions (zonula occludens), zonula adherens (adherens junctions) and gap junctions.
13. What is the relationship between concentration gradient and active and passive transport?
Passive transport is the movement of substances across membranes in favor of their concentration gradient, i.e., 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. In passive transport, because it is spontaneous, there is no energy spent; the active transport however requires energy (work) to occur.
Active transport works to maintain or increase the concentration gradient of a substance between two regions while passive transport acts in a manner 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 is the energy source 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. The active transportation uses chemical energy from ATP.
16. What is the difference between simple and facilitated diffusion? Facilitated by which type of molecule does the term “facilitated” mean?
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”, i.e., facilitators of their passage through the membrane.
Cell Membrane Review - Image Diversity: facilitated diffusion
17. How does the intensity of simple diffusion vary in relation to the concentration gradient of the moved 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 diminishes too.
18. How does the intensity of facilitated diffusion vary in relation to the concentration of the moved substance? What is the limiting factor?
Like simple diffusion facilitated diffusion is more intense when the concentration gradient of the substance increases and less intense when the gradient lessens. In facilitated diffusion however there is a limiting factor: the quantity of the permeases that facilitate the transport through the membrane. Even in a situation in which the concentration gradient of the diffusing substance increases, if there are not enough permeases to perform 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 achieved.
19. Without saturation of transport proteins and under the same concentration gradient 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 under equal concentration gradients of the moved substance.
20. How does facilitated diffusion present similarities with enzymatic chemical reactions?
One of the main examples of facilitated transport is the entrance of glucose from the blood into cells. Glucose from blood binds to specific permeases (hexose-transporting permeases) present in the cell membrane and by diffusion facilitated by these proteins it enters the cell to play its metabolic functions.
Facilitated diffusion resembles chemical catalysis because the transported substances bind to permeases like substrates bind to enzymes and in addition, after one transport job is concluded, the permease is not consumed and can perform other successive transports.
21. What are some examples of biological activities in which osmosis plays an important role?
Hemolysis (destruction of red blood cells) by entrance of water, the hydric regulation in plants and the entrance of water in the xylem of vascular plants are all examples of biological phenomena caused by osmosis.
Excessive dilution of the blood plasma causes, by osmosis, the entrance of too much water into red blood cells and then the destruction of these cells (hemolysis). Osmosis is also the main process for maintenance of the flaccid, turgid or plasmolytic states of plant cells. Osmosis is one of the forces responsible for the entrance of water into plant roots 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 in both processes there is participation of membrane proteins.
In active transport however the transported substance moves against its concentration gradient and with energy spent. Facilitated diffusion is a passive transport in favor of the concentration gradient and it does not require energy.
Cell Membrane Review - Image Diversity: active transport
23. Which are the molecules that make possible active transport through membranes?
Active transport is made by specific membrane proteins. These proteins are called “pumps” because they “pump” the moving substance through the membrane using energy from ATP molecules.
24. How does the sodium-potassium pump present in the cell membrane work? What is the importance of this protein for the cell?
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 it pumps three sodium ions outside the cell and puts two potassium ions inwards. The phosphorylation is made by the binding of a phosphate donated by one ATP molecule that then is converted into ADP (adenosine diphosphate).
The job of the sodium-potassium pump, also known as sodium-potassium ATPase, is fundamental to keep the characteristic negative electrical charge in the intracellular side of the membrane of the resting cell and to create adequate conditions of sodium and potassium concentrations inside and outside the cell to maintain the cellular metabolism.
Cell Membrane Review - Image Diversity: sodium-potassium pump
25. What is mass transportation across the cell membrane?
Mass transportation is the entrance or the exiting of substances in or from the cell 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 in the cell 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 that 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 enclosing the particles; the vesicle then detaches from the membrane and enters the cytoplasm receiving the name phagosome.
Cell Membrane Review - Image Diversity: pynocitosis phagocytosis
27. How does the plant cell wall react when it is placed under hypotonic medium?
The plant cell wall (the covering of the cell external to the cell membrane) is made of cellulose, a polymer of glucose.
When the cell is put under hypotonic medium it absorbs too much water through osmosis. In that situation the cell wall pressure acts to compensate the osmotic pressure thus forbidding excessive increase of the cellular volume and the cell lysis.
28. What is meant by suction force of the plant cell? Does the suction force facilitate or make difficult the entrance of water into the cell?
The suction force (SF) is the osmotic pressure of the plant cell vacuole, i.e., of the vacuolar internal solution.
Since the vacuolar solution is hypertonic in comparison to cytosol it attracts water thus increasing the cytosol concentration. With 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 wall resistance of plant cells? Does this resistance facilitate or make difficult the entrance of water into the cell?
Wall resistance, or turgor pressure (TP), is the pressure made by the distension of the plant cell wall in opposition to the increase of the cell volume. The wall resistance works against the entrance of water in the cell, i.e., it acts forcing the exiting of water and compensating the entrance of the solvent by osmosis.
30. What does the formula DPD = SF – TP mean?
DPD is the abbreviation of diffusion pressure deficit, SF (suction force) is the vacuolar osmotic pressure and TP is the turgor pressure.
The difference between SF and TP determines whether water tends or not to enter the cell. If SF > TP, DPD > 0 and water tends to enter the cell by osmosis. If TP > SF, DPD < 0 and water cannot enter the cell by osmosis.
31. What are the values of DPD for plant cells under hypertonic, isotonic and hypotonic media?
In plant cells under hypertonic medium there is loss of water for 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, situation characterized by the retraction of the cell membrane that detach from the cell wall.
In plant cells under isotonic medium there is no increase of the internal water volume, SF > 0 and TP = 0 (since the cell wall is not distended). The cell membrane slightly touches the cell wall and in this situation the cell is called a flaccid cell.
In plant cells under hypotonic medium there is tendency of water to enter, SF = TP (since the osmotic pressure is totally compensated by the distension of the cell wall) and DPD = 0. The cell that has expanded itself to this point is called a turgid cell.
Cell Membrane Review - Image Diversity: plasmolysed cell flaccid cell turgid cell
32. What is the formula of the DPD for withered (shrunken) plant cells? How is that situation possible?
Withered plant cells are those that have shrunk due to loss of water by evaporation without enough replacement. In this situation the cell membrane retracts and detaches from the cell wall. The cell wall moreover 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 deplasmolysis of plant cells?
The plant cell when placed under hypertonic medium loses a great amount of water and its cell membrane detaches from the cell wall. In that situation the cell is called a plasmolysed cell. When the plasmolysed cell is placed under 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 meat and dried fruits?
Substances that maintain a highly hypertonic environment, like sugar and salt, are used in the production of dried meat, fruits or fish (for example, cod) because the material to be conserved is then dehydrated and the resulting dryness prevents the growth of populations of decomposer beings (since these beings also lose water and die).
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