You may be aware, that as the temperature decreases, things slow down. There is, in fact, a temperture so cold that nothing moves. This temperature is called absolute zero. Where can we find absolute zero?
The converse of this theory, is that as the temperature increases, so does motion, velocity. We can state this in a formula by saying the velocity is proportional to temperature. Where v is velocity and T is temperature. v ~ T .
All of the atoms have mass. Mass times velocity equals momentum, p. mv = p
When you play pool, the ball rolls along with a certain velocity. The ball has mass, mass time velocity is momentum. The pool ball can transfer this momentum. When your cue ball strikes another ball, it causes this ball to move. We have transferred momentum from one ball to another. Atoms and molecules do the same thing. Molecules at temperatures above absolute zero are moving and, when they strike another object, they transfer their momentum.
If you have a tire without any air in it, it is flat. You can fill a flat tire with air. As more of the molecules of air go into the tire, it starts to inflate. When enough air is inserted, the tire becomes hard. When you push your finger against the wall of an inflated tire, all of the molecules bouncing against the wall on the other side of the tire, push at your finger. As you push in against the tire, the molecules of air push the tire and your finger back from the other side.
When all of your molecules are the same, we can say that you have a concentrated substance, because all of the molecules are the same, the substance we have is very concentrated. If we have a beaker full of pure water, this is very concentrated water. If we have a dropper full of ink, this is very concentrated ink. We have to work hard to concentrate are purify anything. It is a natural force of nature to want to unconcentrate or unpurify anything -- to make it more dilute. Except if it is frozen, e.g., at absolute zero.
This power of dilution tends to drive the universe (Technically, this is know as increasing entropy or the chaos of the universe.) Things tend to become more dilute. So, if we put a drop of concentrated ink into a beaker of concentrated water, What will happen? (You can try this at home.) Both the ink and the water become less concentrated, more dilute.
The cause of dilution is diffusion, because all objects are in motion they tend to "push" to spread as far as they can. As they move apart, they are less concentrated, more dilute; they diffuse.
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Solution
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Diffusion
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Flux
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Concentration
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As you should recall, the cell membrane is a lipid bilayer. At body temperature, the lipids are liquid or fluid, hence the fluid mosaic nomenclature. Now this structure makes an effective barrier, because it is hydrophobic, it keeps out water and hydrophilic substances. Because the membrane is surrounded by water, this makes it difficult for lipid molecules to approach.
Since we need transport across the membrane, to allow the food and nutrients in and the excrement and waste materials out, we need to modify the structure of the membrane. One of the simplest ways is to allow molecules to "float" within the lipid bath. Some amino acids are lipophilic and some are lipophobic. If we construct a lipo-protein, it will float within the lipid layers. A hydrophilic, lipophobic protein, would be expelled from the lipid bath.
An interesting situation is when a protein has both properties. If we alternate regions of lipophilicity with lipidphobicity, parts of the protein will be contained within the membrane and parts will be expelled.
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Transmembrane 1
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Transmembrane 2
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In figure 1 we have a protein with alternating regions of lipophilicity and lipidphobicity. Parts are internalized within the membrane and parts are expelled. In figure two the parts of the protein have gathered together. The straight lipophobic transmembrane segments have clustered around each other and the lipids have been excluded from the central portion of the structure. What we have now is a "pore", a hole in the membrane, through which conforming substances may pass. Since it is possible to alter the size of the pore, and the structures guarding the "mouth" of the pore, we can be selective about what can pass through the pore.
Proteins can change shape. This may be due to a variety of inputs. One result of this capacity, is that we can have, e.g., a channel that can open and close. We therefore have a mechanism for controlling the flow of ions or molecules into and out of the cell. Since we can "open" or close these channels, we call them gates. Gates can be very specific. We have sodium gates, potassium gates, and calcium gates, to name a few.
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Ion Channels
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Charge
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It is possible to have more negative ions in the cell than positive ions. The inside of the cell then becomes negative with respects to the outside of the cell. Many cells will have a negative internal potential of 50 - 100 mV (1 mV = 1/1,000 of a volt). This establishes a potential across the cell membrane, which encourages the diffusion of positively charged ions into the cell and negatively charged ions out of the cell.
There are many ways for transport to occur across the cell membrane. If there is a pore (technically a semipermeable membrane) then we can have diffusion of molecules of correct properties, e.g., size, if it is small enough in can go through.
We have proteins designed to carry specific molecules across the membrane. These are known as mediated transport. Because of the way these little guys work, they can only carry so many molecules, only work so fast. An interesting perspective can be seen in figure 2, flux and solute. Here we see a graph of free diffusion; the greater the concentration imbalance, the greater the flux. The* case with mediated transport is quite different. Since the protein can only work so fast, there is a maximum flux, or velocity,at which flow occurs. The important point here is that no matter how large the concentration, the maximum flow does not change.
What is next in importance is the direction of flow. Free diffusion flows along the gradient. Assisted or facilitated diffusion also flows along the gradient. Active transport is used to "pump" substances against the gradient.
To "pump" against a gradient is work. It takes energy to do work. Our cells use a molecule called Adenosine TriPhosphate, ATP, to work. The breakdown and release of energy from ATP to ADP + Pi, is coupled to the pumping action.
A classic pump is the Sodium - Potassium ATPase pump. The pump pumps potassium into the cell, sodium out of the cell and utilizes the energy contained in a molecule of ATP to accomplish this. The pump is actually a protein, a functional protein called an enzyme. Enzymes which break down large molecules into smaller molecules is connotated by placing an "ase" on the end of the word. Since this pump/enzyme breaks down ATP, it is known as an ATPase.
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Mediated
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Flux and Solute
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Flux Direction
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Primary Active 1
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Primary Active 2
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Secondary Active
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Co and* Counter
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Calcium Transport
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Sodium Transport
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Organic Transport
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Since the pump creates on imbalance of, e.g., sodium. There is a concentration gradient for which sodium will diffuse. We can couple this desire for sodium to diffuse to regions of lower concentration. We can couple or tie other activities to this force - this is known as secondary transport. Depending whether the flow is the same direction as the concentration gradient, or against it, we have co- and counter- current secondary active transport.
Many substance move through pores in the membrane. We can figure out the dynamics of this process if we determine the concentrations of the different substances involved. We know already, from the power of dilution, that solutions like to become less concentrated. Ions and molecules, therefore, tend to migrate in search of lower concentrations.
If we separate solutions by a semipermeable membrane, depending upon the properties of the membrane, only some molecules will be able to diffuse across the membrane. If we have different concentrations on different sides of the membrane, we can allow water to flow from one compartment to another until equilibrium is reached, the concentrations of solutions on each side of the membranes is the same. However, the side that had the highest initial concentration of molecules will have the greatest volume of water. The cells of our bodies will expand and contract to balance equilibrium. If placed in a dilute, hypo tonic solution, water will be absorbed by the hyper tonic side, until equilibrium is reached of there is ptosis of the cell.
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Movement
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Water Solutions
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Equilibrium 1
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Equilibrium 2
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Cell Volume
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When a cell grabs a bunch or glob of substance by invagination of the surface, this process is called endocytosis, bringing it into the cell. When the converse occurs, and material is expelled from the cell, his is known as exocytosis.
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Endocytosis
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Vesicles
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Because our lipid membranes are intrinsically hydrophobic, passage of water across the external membrane is a major task of our skin, the integumentary system. A variety of mechanisms are created to decrease or eliminate the free flow of water into and out of our bodies. The epithelial cells has the duty to stand watch and prevent the unmitigated movement of water into and out of cells of our body.
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Water Epithelium
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Glands are created from epithelial tissue. They are divided into a wide variety of types, based upon function.
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Glands
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Gland can release their products directly into the regions "outside" of our bodies, e.g., the secretions of exocrine glands such as saliva or milk, or they can release their products "inside" directly into the blood stream, hormones from the endocrine glands such as adrenaline or growth hormone.
