The difference in the concentrations of the molecules in the two areas is called the concentration gradient. The kinetic energy of the molecules results in random motion, causing diffusion. In simple diffusion, this process proceeds without the aid of a transport protein. It is the random motion of the molecules that causes them to move from an area of high concentration to an area with a lower concentration.
Diffusion will continue until the concentration gradient has been eliminated. Since diffusion moves materials from an area of higher concentration to the lower, it is described as moving solutes "down the concentration gradient". The end result is an equal concentration, or equilibrium , of molecules on both sides of the membrane. At equilibrium, movement of molecules does not stop.
At equilibrium, there is equal movement of materials in both directions. Not everything can make it into your cells. Your cells have a plasma membrane that helps to guard your cells from unwanted intruders. If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment.
What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells.
This barrier is called the plasma membrane , or cell membrane. The plasma membrane see figure below is made of a double layer of special lipids, known as phospholipids. The phospholipid is a lipid molecule with a hydrophilic "water-loving" head and two hydrophobic "water-hating" tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water.
Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer a bilayer to keep the cell separate from its environment.
Lipids do not mix with water recall that oil is a lipid , so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell. What exactly are hypertonic drinks? How do hypertonic sports drinks work?
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It acts as a mild central-nervous-system stimulant that can improve alertness and concentration and perk you up if you are feeling tired or lethargic. In practice, that means some well-timed caffeine can help you go harder and for longer. It also encourages our bodies to burn fat as fuel and it can reduce feelings of pain and fatigue. We're going to assume that the cellular membrane, this phospholipid bilayer, is semipermeable, that it will allow water molecules to pass in and out, so a water molecule could go from the inside to the outside, or from the outside to the inside, but we're gonna assume that it does not allow the passage of the solute particles, so that's why it's semipermeable.
It's permeable to certain things, or we could say, selectively permeable. Now, what do we think is going to happen? Well, the first thing that you might observe is we have a lower concentration of solute on the outside than we have on the inside, so at any given moment of time, you will have some water molecules moving in just the right direction to go from the outside to the inside, and you will also have some water molecules that might be in just the right place to go from the inside to the outside, but what's more likely to happen, and what's going to happen more over a certain period of time?
The water molecules that are on the outside, and we talk about this in the osmosis video, they're going to be less obstructed by solute particles. If this one happens to be moving in that direction, well, it's gonna make its way to the membrane, and then, maybe get through the membrane, while something, maybe, if this water molecule was moving in this direction, well, gee, it's gonna be obstructed now, maybe this is bouncing back, and it's gonna ricochet off of it, so the water molecules on the inside are more obstructed.
They're less likely to be able to fully interact with the membrane or move in the right direction. They're being obstructed by these solute particles. Even though you're going to have water molecules going back and forth, in a given period of time, you have a higher probability of more going in, than going out, so you're going to have a net inflow.
Net inflow of H2O, of water molecules. Now, a situation like this, where we're talking about a cell and it's in a solution that has a lower concentration of solute, it's important that we're talking about a solute that is not allowed to go to the membrane, the membrane is not permeable to that solute. We call this type of situation, this type of solution that the cell is immersed in, we call this a hypotonic solution.
Hypotonic solution. Anytime we're talking about hypotonic, or as we'll see, isotonic and hypertonic, we're talking about relative concentrations of solute that cannot get through some type of a membrane. Here is a simple example to illustrate these principles: Two containers of equal volume are separated by a membrane that allows free passage of water, but totally restricts passage of solute molecules. Solution A has 3 molecules of the protein albumin molecular weight 66, and Solution B contains 15 molecules of glucose molecular weight Into which compartment will water flow, or will there be no net movement of water?
When thinking about osmosis, we are always comparing solute concentrations between two solutions, and some standard terminology is commonly used to describe these differences:. Diffusion of water across a membrane generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called hydrostatic 'water-stopping' pressure.
The term osmolarity is used to describe the number of solute particles in a volume of fluid.
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