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We say cell wall is dead but nobody says cell membrane is dead.
Is cell membrane living or dead ? If it is not dead then why is it not included in protoplasm :
Protoplasm is the living contents of a cell that is surrounded by a plasma membrane.
(Source : Wikipedia)
It is neither living nor dead. It is simply not alive the way that any other collection of chemicals is not alive. Is salt alive or dead?
A cell's membrane consists of a collection of chemicals (mainly lipids and proteins). Individual chemicals can never be considered living or dead, any more than rocks or water can be living or dead. While the actual definition of life is tricky, everyone agrees that in order to be alive you need to (at the very least)
- Somehow interact with your environment
- Be able to make more copies of yourself
Therefore, the cells themselves are alive but the individual parts of the cells are not since they cannot make copies of themselves (this gets complicated when considering some organelles but never mind that here). Similarly, you are alive but your teeth are not. While you can make more of you (you can have children) your teeth cannot make more teeth.
Cell Membrane was discovered by Swiss botanist Carl Naegeli and C. Crammer in 1855.
Cell membrane also called plasma membrane, cytoplasmic or protoplasmic membrane. Cell
Membrane is the second layer in plant cell present below the cell wall while in animal cell it
is the first layer. Cell Membrane surrounds the cytoplasm and other organelles in it.
In 1972 two scientists S.J Singer and CL Nicolson proposed fluid mosaic model explaining the
structure of the cell membrane.
FLUID MOSAIC MODEL
According to this model “Cell membrane is made up of phospholipid by layer and proteins.
Some Cholesterol and Carbohydrates are also present in cell membrane. The phospholipid
bilayer forms a Fluid Sea in which proteins are floating.
Phospholipids have two Ends.
Polar spherical heads are located over the cell surface. They have phosphate group. They
are call hydrophilic (Water Loving)
They are also called Non polar ends. They face each other in middle of bilayer. Tails of both
layers attract reach other and repel water. So they are Hydrophobic. (Water Hating).
Cholesterol is also present in the cell membrane. They make the membrane less permeable
for water soluble substances. It makes the membrane rigid structure.
Carbohydrates are present in two forms i.e Glycolipids and Glycoproteins.
Cell membrane contains two types of protein.
1) Peripheral Proteins:
These Proteins also called Extrinsic Proteins. These are attached to inner outer membrane
2) Integral Proteins:
These Proteins also called Intrinsic Proteins. These are embedded in the lipid bilayer. These
Proteins perform the following functions.
i) Some link to sugar-Protein markers on the cell surface.
ii) Some move ions or molecules across the membrane.
iii) Some attach the membrane to cell inner Cytoskeleton.
Fig . Cell Membrane Structure Fluid Mosaic model.
FUNCTIONS OF THE CELL MEMBRANE
1) It gives shape and protection to cell.
2) Transport material into and out of the cell.
3) Act as a receptor site and recognize chemicals, hormones and neurotransmitter, and help in signaling.
4) It is the boundary which separates the part of the cell from outer environment.
5) It helps in exocytosis and endocytosis.
6) Regulates material moving into and out of the cell and from one part to another.
FLUIDITY OF CELL MEMBRANE
Cell membranes are fluid, meaning they are not fixed in position and can adopt amorphous
shapes. Membrane fluidity is enhanced at higher temperatures i.e Increase in temperature
decreases the Fluidity of Cell membrane and it is also affected by the composition of the
As the amount of cholesterol increases, the fluidity of cell membrane decreases.
Saturated fatty acids are straight while unsaturated fatty acids bends down.
Saturated fatty acids decrease the fluidity while unsaturated fatty acids increase the fluidity
of the membrane.
Polar and Nonpolar Substances:
Polar substances increase the fluidity of the membrane while Nonpolar Substances
decreases the fluidity.
ASYMMETRICAL NATURE OF CELL MEMBRANE
Asymmetrical means two sides of the membrane are not the same.
The cell membrane tends to have different composition on one side of the membrane than on the other side of the membrane. The differences can be caused by the different ratios or types of amphipathic lipid-based molecules, the different positioning of the proteins (facing in or facing out), or the fixed orientations of proteins spanning the membrane. Additionally, there are different enzymatic activities in the outer and inner membrane surfaces.
Upper phospholipids are phosphatidyl choline and sphingomyelin.
Lower Phospholipids are phosphatidyl serine and phosphatidyl ethanolamine.
Proteins are also different on the both sides.
TRANSPORT ACROSS PLASMA MEMBRANE
ACTIVE TRANSPORT VS PASSIVE TRANSPORT
1) Active transport is the Movement of molecules or ions against concentration gradient
i.e from an area of lower concentration to an area of higher concentration with expenditure
of energy while ,
Passive transport is the movement of molecules or ions toward the
concentration gradient i.e from an area of higher concentration to an area of lower
concentration without the expenditure of energy.
2) Active transport also called uphill movement of molecules while Passive transport
also called downhill movement of molecules.
3) Equilibrium maintenance is not necessary in active transport while Equilibrium
maintenance is required in passive transport.
4) Active transport is faster while passive transport is slower.
5) Active transport is affected by O2 and cynoid concentration while passive transport
is not affected by these.
6) Active transport is unidirectional process while passive transport is bidirectional process.
7) Macromolecules like proteins, carbohydrate (sugars), lipids, large cell are few of the materials which are transported by active transport
while Oxygen, monosaccharaides, water, carbon dioxide, lipids are the few soluble materials which are being transported through passive transport.
8) Examples of Active transport are Endocytosis, Exocytosis, Proton pumps and Sodium potassium pumps while examples of passive transport are osmosis, diffusion and facilitated diffusion.
Diffusion is said to be the movement of molecules from the more concentrated solution to
the less concentrated solution through the permeable membrane. Cell membrane does not
spend energy when molecule through it.
The movement of molecules which involves the proteins as their helpers is called as facilitated diffusion.
Many molecules cannot diffuse through cell membrane due to their size or charge, such molecules are taken into or out of cell with the help of transport protein present in the cell membrane. When transport protein help to move molecules from high to low concentration without expenditure of energy, the process is called facilitated diffusion.
FIG. Facilitated Diffusion
It is the Diffusion of solvent (water) through a semi-permeable membrane. It is controlled by the relative concentration of solutes in the water on both sides of membrane. Water always moves from a hypotonic solution (with lower concentration of solutes) to a hypertonic solution (with higher concentration of solutes).
Few examples of Osmosis are following:
(i) When a cell is placed in a hypotonic solution (which has lower solute concentration than the cell) the rate of movement of water inside the cell is more. In such conditions, animal cells swells and may rupture due to the absence of cell wall while the plant cells becomes turgid (due to their hard cell wall).
(ii) When a cell is placed in an isotonic solution (solution in which the concentration of solutes is equal to that of the cell), the rate of osmosis outward is equal to the rate of osmosis to inward. In such a condition, animal cells retain their volume constant while plant cells become flaccid (loose), because the net uptake of water is not enough.
(iii) When a cell is placed in a hypertonic solution (which has higher salt concentration than the cell), water moves out. In such conditions, animal cells shrink in size. In plant cells, the cytoplasm shrinks within the cell wall.
ENDOCYTOSIS AND EXOCYTOSIS
Endocytosis is a general term for the process that brings macromolecules, large particles and even small cells into the cell.
In endocytosis, the cell membrane invaginates (folds inward) and takes in the materials from the environment, forming a small pocket. The pocket deepens, forming a vesicle. This vesicle separates from the plasma membrane and migrates with its contents to the cell’s interior.
The initial event in this process is the binding of vesicle membrane with the cell membrane. Then, the contents of the vesicle are released to the environment and the vesicle membrane is incorporated into cell membrane.
When a plant cell is surrounded by water or hypotonic solution, the water moves into the cell vacuole by osmosis. The vacuole increases in size and pushes the cell contents against the cell wall. This pressure which is exerted by the cytoplasm against the cell wall is known as turgor pressure and the phenomenon is called turgor.
In turgid condition, the plant cell does not burst because the cell wall is strong and relatively inelastic.
The importance of turgor in plants are as follows:
i) It plays an important role in maintaining the shape of the plant.
ii) It provides supports to plants especially in young tissues.
iii) It also helps in closing and opening of the stomata.
iv) Some flowers open during the day time and close at night. This is also due to change in turgor in the cells of sepals is of flowers.
34 Fundamental Q&As on Cell Membrane
A 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 inside and the outside of the body cellophane, used in chemical laboratories to separate solutions, also acts as a membrane.
More Bite-Sized Q&As Below
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.
Diffusion and Osmosis
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, oth 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.
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The Phospholipid Bilayer
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.
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.
Microvilli and Cell Junctions
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.
Active and Passive Transport, Simple and Facilitated Diffusion
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.ꂬtive 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.
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.
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.
The Sodium-Potassium Pump
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 thenonverted 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.
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.
Plant Cell Wall
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,ਊnd 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.
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).
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Cell Membrane Functions
Now let’s look at what functions the cell membrane performs:
- The barrier function of the cell membrane – the membrane as a real border guard, stands guard over the borders of the cell, stopping harmful or unsuitable molecules. This is the main function of the cell membrane.
- The transport function of the cell membrane – the membrane is not only a border guard at the cell gate but also a kind of customs checkpoint. Nutrients are constantly exchanged with other cells and the environment thru the cell membrane.
- Matrix function – it is the cell membrane that determines the location of cell organelles, regulates the interaction between them.
- The mechanical function is responsible for restricting one cell from another and for correctly connecting the cells with each other and for forming them into a homogeneous tissue.
- The protective function of the cell membrane is the basis for building a protective shield of the cell. Solid wood, dense skin, the protective shell of a tortoise can be examples of the protective function of the cell membrane.
- Energy function – photosynthesis and cellular respiration would not be possible without the participation of the protein contained in the cell membrane. An important cellular energy exchange takes place through protein channels.
- Receptor function – and again we return to the membrane proteins. In addition to the actual energy exchange, they have another very important function – they serve as receptors of the cell membrane, thanks to which the cell receives a signal from hormones and neurotransmitters. All this is necessary for the normal course of hormonal processes and the conduct of a nerve impulse.
- The enzymatic function is another important function performed by some proteins in the cell. For example, thanks to this function, digestive enzymes are synthesized in the intestinal epithelium.
Researchers took a nanoscale snap of a living cell membrane for the first time in history
Researchers have snapped the most detailed ever image of a cell membrane, all the way down to the nanoscale. The image could finally settle a long-standing debate in biology on how it functions, and introduce a powerful new tool to biologists’ toolkit.
Image credits John Katsaras et al, PLOS ONE (2017).
Every living cell’s membrane is put together from a thin sandwich of lipids, which are fat molecules, interspersed with other bits of organic materials such as proteins and carbohydrates. It’s a pretty nifty system — the fatty bilayer, for example, keeps all the watery stuff inside the cell from mixing with the watery stuff outside the cell. Proteins act as pumps and decide what goes through the membrane and when, or serve as landing areas for signaling molecules so the cell can talk with its pals. Some carbohydrates act as ID tags. Then there’s one other bit whose function — as far as can be summarized if you keep tabs on cytological debates, which I’m sure most of you do — seems to be solely to sow discord and disagreement into the ranks of biologists.
These tiny bits are known as lipid rafts and, although there’s a pretty solid body of documentation as to what they are and what they do, haven’t really caught with all cellular biologists. The short of it is that they act as independent, more compact domains than the rest of the membrane, making it behave a little wobbly, and their movements allow the cell to activate or inactivate proteins along its membrane.
So, a team led by John Katsaras, Senior Biological Systems Scientist at Oak Ridge National Laboratory’s Neutron Sciences Directorate, decided to take a picture and find out.
“It became a debate,” Katsaras said. “Some people believed they exist, while others believed they didn’t. There was a lot of circumstantial evidence that could support either side.”
The way they went about it could fundamentally change how living nanoscale structures are studied in the future.
Looking at the really small
When biologists want to take a peek at the going-ons inside a cell, they normally use fluorescent compounds designed to attach to a particular molecule and tag it, making it visible under the optical microscope. But since we don’t really know what lipid rafts do (so we don’t know where to add the fluorescent tags), and because they’re probably too tiny to spot under the microscope, this doesn’t really work in their case.
An electron microscope could probably make them out with ease, but the thing is that to find out how these rafts behave you need to observe a living cell. Since cells are made so tiny, atoms are basically brick-sized compared to them. Electrons, then, are bullet- or pellet-sized. To a living cell, an electron microscope is basically a death-spewing chaingun. So that won’t work either.
In the end, the team decided to use a mix of genetic and chemical labeling techniques to add a hydrogen isotope to the membranes of living Bacillus subtilis cells. Then, they used a method called neutron scattering to chart the arrangement of different molecules in the bacterium’s cell membrane. Neutron scattering was picked because it’s less energetic than electron microscopy, meaning the particles aren’t (necessarily) deadly to the bacteria.
So why are the isotopes there? Well, although less energetic, neutrons are way heavier than electrons. So it’s not exactly deadly, but the particles are powerful enough to affect the cell and interfere with its membrane’s internal processes. Furthermore, while it could spot the rafts, neutron scattering couldn’t tell it apart from the rest of the membrane, so the team needed to tag them with something that stands out.
Bag and tag
Since 99.98% of all hydrogen atoms currently in existence only have a single proton for a nucleus, the isotopes the team used, which have an extra neutron attached to the nucleus and are known as deuterium, is pretty conspicuous. And while they chemically function the same (since neutrons don’t affect the atom’s valence/electrical balance), physically they do differ enough to scatter neutrons in a different way — so they were both easy to spot and unlikely to occur naturally.
The team genetically edited a new strain of B. subtilis with a slightly different ratio of hydrogen to deuterium in its membrane compared to wild strains. If there were no rafts, they should see a uniform distribution of these altered fat molecules throughout the membrane.
Instead, their imaging showed areas with pronounced differences in lipid arrangement, which matched the proposed size of the lipid rafts — very strong evidence for their existence. Even better, the technique they developed for the study could fundamentally change how biologists peer into the workings of living cells.
“The people who study these things tend to use particular types of probes,” says Katsaras.
“They didn’t use neutron scattering because it wasn’t in the biologist’s wheelhouse. Our novel experimental approach opens up new areas of research.”
These differentiated areas aren’t visible in the team’s model, but it does an exemplary job of showing how a cell’s outer layers are structured — watery cytoplasm covered with the lipid layer the team was investigating in the middle, and the outer cell wall at the top.
The full paper “The in vivo structure of biological membranes and evidence for lipid domains” has been published in the journal PLOS ONE.
Most cells reproduce through the process of mitosis, also known as cell division. Mitosis occurs in both unicellular and multicellular organisms. Cells duplicate themselves for procreation in the case of unicellular creatures, while mitosis in multicellular organisms replaces old cells and is responsible for tissue growth.
Mitosis results in two daughter cells that have the exact genetic material of the original cell. In mitosis, the genetic material — which dictates structure and function in each cell — duplicates and the cell divides down the middle, with each new cell possessing structures identical to the original cell.
Special cell membrane structures in special cell types
To perform certain cellular functions, some cells possess unique cell membrane structures. Here are some examples:
[In this figure] TEM image of the small intestine epithelium surface. Microvilli are microscopic cellular membrane protrusions that increase the surface area for maximizing nutrient absorption.
Photo source: Atlas of plant and animal histology.
[In this figure] T-tubules (transverse tubules) are extensions of the cell membrane that penetrate into the center of skeletal and cardiac muscle cells. T-tubules permit the rapid transmission of the action potential into the cell, allowing heart muscle cells to contract more forcefully.
Photo source: wiki
[In this figure] Electron micrograph showing the surface of endothelial cells’ membrane coated with a thick layer of carbohydrate components, called glycocalyx. Endothelial cells are the cell types that line the inner lumens of blood vessels.
Photo source: derangedphysiology
Movement across Cell Membranes [back to top]
Cell membranes are a barrier to most substances, and this property allows materials to be concentrated inside cells, excluded from cells, or simply separated from the outside environment. This is compartmentalisation is essential for life, as it enables reactions to take place that would otherwise be impossible. Eukaryotic cells can also compartmentalise materials inside organelles. Obviously materials need to be able to enter and leave cells, and there are five main methods by which substances can move across a cell membrane:
1. Lipid Diffusion (or Simple Diffusion) [back to top]
A few substances can diffuse directly through the lipid bilayer part of the membrane. The only substances that can do this are lipid-soluble molecules such as steroids, or very small molecules, such as H2O, O2 and CO2. For these molecules the membrane is no barrier at all. Since lipid diffusion is (obviously) a passive diffusion process, no energy is involved and substances can only move down their concentration gradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.
2. Osmosis [back to top]
Osmosis is the diffusion of water across a membrane. It is in fact just normal lipid diffusion, but since water is so important and so abundant in cells (its concentration is about 50 M), the diffusion of water has its own name - osmosis. The contents of cells are essentially solutions of numerous different solutes, and the more concentrated the solution, the more solute molecules there are in a given volume, so the fewer water molecules there are. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.
Water Potential. Osmosis can be quantified using water potential, so we can calculate which way water will move, and how fast. Water potential ( Y , the Greek letter psi, pronounced "sy") is simply the effective concentration of water. It is measured in units of pressure (Pa, or usually kPa), and the rule is that water always "falls" from a high to a low water potential (in other words it's a bit like gravity potential or electrical potential). 100% pure water has Y = 0, which is the highest possible water potential, so all solutions have Y < 0, and you cannot get Y > 0.
Osmotic Pressure (OP). This is an older term used to describe osmosis. The more concentrated a solution, the higher the osmotic pressure. It therefore means the opposite to water potential, and so water move from a low to a high OP. Always use Y rather than OP.
Cells and Osmosis . The concentration (or OP) of the solution that surrounds a cell will affect the state of the cell, due to osmosis. There are three possible concentrations of solution to consider:
The effects of these solutions on cells are shown in this diagram:
These are problems that living cells face all the time. For example:
3. Passive Transport (or Facilitated Diffusion). [back to top]
Passive transport is the transport of substances across a membrane by a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule (a bit like enzymes), so substances can only cross a membrane if it contains the appropriate protein. As the name suggests, this is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient. There are two kinds of transport protein:
4. Active Transport (or Pumping). [back to top]
Active transport is the pumping of substances across a membrane by a trans-membrane protein pump molecule. The protein binds a molecule of the substance to be transported on one side of the membrane, changes shape, and releases it on the other side. The proteins are highly specific, so there is a different protein pump for each molecule to be transported. The protein pumps are also ATPase enzymes, since they catalyse the splitting of ATP g ADP + phosphate (Pi), and use the energy released to change shape and pump the molecule. Pumping is therefore an active process, and is the only transport mechanism that can transport substances up their concentration gradient.
The Na + K + Pump. This transport protein is present in the cell membranes of all animal cells and is the most abundant and important of all membrane pumps.
The Na + K + pump is a complex pump, simultaneously pumping three sodium ions out of the cell and two potassium ions into the cell for each molecule of ATP split. This means that, apart from moving ions around, it also generates a potential difference across the cell membrane. This is called the membrane potential, and all animal cells have it. It varies from 20 to 200 mV, but and is always negative inside the cell. In most cells the Na + K + pump runs continuously and uses 30% of all the cell's energy (70% in nerve cells).
The rate of diffusion of a substance across a membrane increases as its concentration gradient increases, but whereas lipid diffusion shows a linear relationship, facilitated diffusion has a curved relationship with a maximum rate. This is due to the rate being limited by the number of transport proteins. The rate of active transport also increases with concentration gradient, but most importantly it has a high rate even when there is no concentration difference across the membrane. Active transport stops if cellular respiration stops, since there is no energy.
5. Vesicles [back to top]
The processes described so far only apply to small molecules. Large molecules (such as proteins, polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membrane vesicles.
Sometimes materials can pass straight through cells without ever making contact with the cytoplasm by being taken in by endocytosis at one end of a cell and passing out by exocytosis at the other end.
Cell Membrane Structure
Encyclopaedia Britannica / UIG / Getty Images
The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell's chemical climate and assist in the transfer of molecules across the membrane.
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