1. MEMBRANES TRANSPORT In cellular biology the term membrane transport refers to the collection of mechanisms that regulate the passage of solutes such as ions and small molecules through biological membranes namely lipid bilayers that contain proteins embedded in them. The regulation of passage through the membrane is due to selective membrane permeability - a characteristic of biological membranes which allows them to separate substances of distinct chemical nature.
In other words, they can be permeable to certain substances but not to others.
The movements of most solutes through the membrane are mediated by membrane transport proteins which are specialized to varying degrees in the transport of specific molecules. As the diversity and physiology of the distinct cells is highly related to their capacities to attract different external elements, it is postulated that there is a group of specific transport proteins for each cell type and for every specific physiological stage[1]. This differential expression is regulated through the differential transcription of the genes coding for these proteins and its translation, for instance, through genetic-molecular mechanisms, but also at the cell biology level: the production of these proteins can be activated by cellular signaling pathways, at the biochemical level, or even by being situated in cytoplasmic vesicles.
a. Passive diffusion
Diffusion is a process in which the random motions of molecules or other particles result in a net movement from a region of high concentration to a region of lower concentration. A familiar example of diffusion is the dissemination of floral perfumes from a bouquet to all parts of the motionless air of a room. The rate of flow of the diffusing substance is proportional to the concentration gradient for a given direction of diffusion. Thus, if the concentration of the diffusing substance is very high at the source, and is diffusing in a direction where little or none is found, the diffusion rate will be maximized. Several substances may diffuse more or less independently and simultaneously within a space or volume of liquid. Because lightweight molecules have higher average speeds than heavy molecules at the same temperature, they also tend to diffuse more rapidly. Molecules of the same weight move more rapidly at higher temperatures, increasing the rate of diffusion as the temperature rises.
Driven by concentration gradients, diffusion in the cell usually takes place through channels or pores lined by proteins. Size and electrical charge may inhibit or prohibit the passage of certain molecules or electrolytes (e.g., sodium, potassium, etc.).
Osmosis describes diffusion of water across cell membranes. Although water is a polar molecule (i.e., has overall partially positive and negative charges separated by its molecular structure), transmembrane proteins form hydrophilic (water loving) channels to through which water molecules may move.
b. Facilitated diffusion
Facilitated diffusion involves the use of a protein to facilitate the movement of molecules across a membrane. In some cases, molecules pass through channels within the protein. In other cases, the protein changes shape, allowing molecules to pass through.
Facilitated diffusion works according to the same thermodynamic principle of transport along a gradient as passive diffusion. However, the transport is facilitated by the presence of channel proteins, which facilitate the transport of, in this instance, water or certain hydrophilic ions and molecules. These integral membrane proteins are present as pores immersed in the bilayer, that form a channel with a hydrophilic interior that allows the passage of highly lipophobic molecules such as those mentioned above. In unregulated channels the opening of the channel is continuous and unregulated. However, regulated channels require a signal to mediate their opening and closing.
c. Active transport and co-transport
Transmembrane proteins establish pores through which ions and some small hydrophilic molecules are able to pass by diffusion. The channels open and close according to the physiological needs and state of the cell. Because they open and close transmembrane proteins are termed "gated" proteins. Control of the opening and closing mechanism may be via mechanical, electrical, or other types of membrane changes that may occur as various molecules bind to cell receptor sites.
Active transport is movement of molecules across a cell membrane or membrane of a cell organelle, from a region of low concentration to a region of high concentration. Since these molecules are being moved against a concentration gradient, cellular energy is required for active transport. Active transport allows a cell to maintain conditions different from the surrounding environment. There are two main types of active transport; movement directly across the cell membrane with assistance from transport proteins, and endocytosis, the engulfing of materials into a cell using the processes of pinocytosis, phagocytosis, or receptor-mediated endocytosis.
Transport proteins found within the phospholipid bilayer of the cell membrane can move substances directly across the cell membrane, molecule by molecule. The sodium-potassium pump, which is found in many cells and helps nerve cells to pass their signals in the form of electrical impulses, is a well-studied example of active transport using transport proteins. The transport proteins that are an essential part of the sodium-potassium pump maintain a higher concentration of potassium ions inside the cells compared to outside, and a higher concentration of sodium ions outside of cells compared to inside. In order to carry the ions across the cell membrane and against the concentration gradient, the transport proteins have very specific shapes that only fit or bond well with sodium and potassium ions. Because the transport of these ions is against the concentration gradient, it requires a significant amount of energy.
Endocytosis is an infolding and then pinching in of the cell membrane so that materials are engulfed into a vacuole or vesicle within the cell. Pinocytosis is the process in which cells engulf liquids. The liquids may or may not contain dissolved materials. Phagocytosis is the process in which the materials that are taken into the cell are solid particles. With receptor-mediated endocytosis the substances that are to be transported into the cell first bind to specific sites or receptor proteins on the outside of the cell. The substances can then be engulfed into the cell. As the materials are being carried into the cell, the cell membrane pinches in forming a vacuole or other vesicle. The materials can then be used inside the cell. Because all types of endocytosis use energy, they are considered active transport.
Sodium and potassium
Sodium is the major positive ion (cation) in fluid outside of cells. The chemical notation for sodium is Na+. When combined with chloride, the resulting substance is table salt. Excess sodium (such as that obtained from dietary sources) is excreted in the urine. Sodium regulates the total amount of water in the body and the transmission of sodium into and out of individual cells also plays a role in critical body functions. Many processes in the body, especially in the brain, nervous system, and muscles, require electrical signals for communication. The movement of sodium is critical in generation of these electrical signals. Too much or too little sodium therefore can cause cells to malfunction, and extremes in the blood sodium levels (too much or too little) can be fatal.
Potassium is the major positive ion (cation) found inside of cells. The chemical notation for potassium is K+. The proper level of potassium is essential for normal cell function. Among the many functions of potassium in the body are regulation of the heartbeat and the function of the muscles. A seriously abnormal increase in potassium (hyperkalemia) or decrease in potassium (hypokalemia) can profoundly affect the nervous system and increases the chance of irregular heartbeats (arrhythmias), which, when extreme, can be fatal.
2. TONICITY
Robert R. Wise (Plant Physiologist -University of Wisconsin Oshkosh ) say that Tonicity refers to the response of *cells or tissues* to the solutions in which they are immersed. If cells are placed in a hypertonic solution, net movement of water will be out of the cell, causing the cell to shrivel. If cells are placed in a hypotonic solution, net movement of water will be into the cell, causing the cell to swell or burst. Tonicity is useful only in reference to a particular cell or tissue.
In web, Tonicity is a measure of the osmotic pressure (as defined by the water potential of the two solutions) of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in an external solution. Like osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane. Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane. There are three classifications of tonicness that one solution can have relative to another. The three are hypertonic, hypotonic, and isotonic.
Hypertonicity
A hypertonic solution is a solution having a greater solute concentration than the cytosol. It contains a greater concentration of impermeable solutes on the external side of the membrane. When a cell’s cytoplasm is bathed in a hypertonic solution the water will be drawn into the solution and out of the cell by osmosis. If water molecules continue to diffuse out of the cell, it will cause the cell to shrink, or crenate. A hypertonic solution is used in osmotherapy to treat cerebral hemorrhage.
Hypotonicity
A hypotonic solution is a solution having a lesser solute concentration than the cytosol. It contains a lesser concentration of impermeable solutes on the external side of the membrane. When a cell’s cytoplasm is bathed in a hypotonic solution the water will be drawn out of the solution and into the cell by osmosis. If water molecules continue to diffuse into the cell, it will cause the cell to swell, up to the point that cytolysis (rupture) may occur. In plant cells, the cell will not always rupture. When placed in a hypotonic solution, the cell will have Turgor Pressure and proceed with its normal functions.
Isotonicity
A condition or property of a solution in which its solute concentration is the same as the solute concentration of another solution with which it is compared.
In Cells
In eukaryotic animal cells, a hypertonic environment forces water to leave the cell so that the shape of the cell becomes distorted and wrinkled, a state known as crenation. In plant cells, the effect is more dramatic. The flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called plasmodesmata. The cell takes on the appearance of a pincushion, and the plasmodesmata almost cease to function because they become constricted — a condition known as plasmolysis. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point.
Some organisms have evolved intricate methods of circumventing hypertonicity. For example, saltwater is hypertonic to the fish that live in it. They need a large surface area in their gills in contact with seawater for gas exchange, thus they lose water osmotically to the sea from gill cells. They respond to the loss by drinking large amounts of saltwater, and actively excreting the excess salt. This process is called osmoregulation.
In a hypotonic environment, animal cells will swell until they burst, a process known as cytolysis. Fresh water fish urinate constantly to prevent cytolysis. Plant cells tend to resist bursting, due to the reinforcement of their cell wall, which provides effective osmolarity or osmolality. In some cases of suspensions intended for intramuscular injection, a slightly hypotonic solution is preferred in order to increase the dissolution and absorption of the drug by absorbing water from the surrounding tissues.
3. THE APPLICATION TONICITY
Actually, simple practicum can be done in several ways including:
a. With Blood Test
Blood dripped into a solution of hypertonic, isotonic and hypotonic. Then take a sample of blood cells from each solution. After that, viewed and observe it under microscope, then we can see that bias changes in cell shape in each solutions, it is caused by tonicity.
b. With plant cells or organ
the way to practice with the method is to take 3 pieces of potato that has been cut to be cube with same size. After that weigh or scale each potato that has been cut. Then put it into a hypertonic, isotonic and hypotonic solution. Let stand for 1 hour. After that lift and drain briefly, then re-weigh the potatoes. The addition of potatoes or mass reduction is an indicator of changes in cell mass due to mass and volume changes in cells caused by fluid tonicity
