Diffusion and Osmosis
Diffusion and osmosis basically just involve the spreading out of the particles of a substance over a given area so that they become evenly distributed. My old post on the Kinetic Theory of Matter is kind of related to this. But by "kind of" I mean very slightly. As in the only two important lines are:
- Particles move in random straight-line motion until they collide with each other or another solid object (like the inside of a glass jar) [according to the Kinetic Theory].
- For example, you can explain why gases spread through the kinetic theory idea of random straight-line motion.
Now for a bit more explanation on how diffusion works!
In a nutshell, particles will tend to move from an area of high concentration to an area of low concentration. For example, if you open a bottle of perfume, the gas particles carrying the odour of the perfume will move from the bottle (where the particles are in high concentration) to the rest of the room (where the particles are in low concentration). The process will continue until the particles are evenly distributed- or, should we say diffused? (I'm actually not entirely sure if you can use the word this way in a scientific context, but I'm assuming you can)- throughout the room.
Now, why might this be the case? Is it because the particles have egalitarian principles and believe that every inch of space should have an equal amount of particles? Nope, I don't think so. Diffusion can be explained by the kinetic theory, as stated above. The particles inside gases and liquids are constantly moving, and they occasionally collide with each other. They continue moving in their straight-line motion until a collision occurs. Once a gas or liquid particle "breaks free" of the giant mob of concentrated particles down at the concentrated end, it can continue on, relatively uninhibited, while it passes through areas of lower concentrations. Eventually this happens to all of the molecules until they are evenly spread out over the available space, where the chances of collision are roughly equal no matter which direction the particle moves in next.
Now for some terminology: The difference in concentration that causes diffusion to occur is called the concentration gradient or diffusion gradient. The greater the difference, the steeper the gradient, and the faster diffusion will occur. The movement of particles along a diffusion gradient is called the net diffusion. "Net" or "overall" is used as a reference to those particles that are moving in the opposite direction (since the kinetic theory states that particles move in random straight line motion. I have no idea whether this is true or whether this is yet another drawback of the kinetic theory).
Also btw if you dissolve something in solution, making the particles of the solute (substance you're dissolving/ substance in smaller quantity) spread out amongst the particles of the solvent (substance you're dissolving things in/ substance in larger quantity), the solute and the solvent will BOTH have diffusion gradients, just in opposite directions. If you're dissolving sugar, for example, sugar will have a diffusion gradient in one direction, while water will have a diffusion gradient in the other direction.
I was about to wrap up the topic right there but then I realised that I haven't spoken about osmosis yet. Oops.
Osmosis is basically the same thing as diffusion. In fact my book says that it's "a special case of diffusion." It's special because while diffusion just spreads the particles throughout any gas or liquid or whatever, osmosis also involves a differentially permeable membrane. That sounds all technical unless you break it down: a membrane is basically like a dividing wall of some sort (okay, not very scientific, and probably the worst explanation ever, but works for now), "permeable" means that things can pass through it and "differentially" means that it discriminates between different substances (also not a very scientific explanation, and makes the membrane sound almost racist or sexist or something-ist). In short, a differentially permeable membrane is a divider of some sort that allows some substances to pass through, but not others. Let's see how this can impact diffusion.
Since differentially permeable membranes only allow certain substances to pass through, osmosis looks at not only of the diffusing substances but at how substances that can't pass through the membrane affect diffusion.
Here's a diagram of a container with a differentially permeable membrane dividing it in half. As you can see, the water molecules are evenly distributed on each side:
As the concentration of water molecules remains the same throughout the liquid, equal numbers of water molecules pass through the membrane in each direction, and the net movement of the molecules is zero (or at least I think it is).
If you take a substance that won't pass through the membrane and dissolve it into the water on one side of the container, however, something interesting happens:
Since the other substance can't pass through, each side has a different ratio of water to other substance molecules. To make up for this, more water diffuses from the left side to the right side, where water now has to compete with the other substance for supremacy. Or something. (My book says something about water molecules moving from a place of higher concentration to a place of lower concentration, but I have trouble writing that after being told in Chemistry that liquids like water generally aren't considered to have concentrations, unlike aqueous solutions, which do.)
Now for some fancy words! I've talked a lot about osmosis without really using the word much. Aside from my earlier description, osmosis is also considered to be the diffusion of water across differentially permeable membranes like the membranes of cells (water is considered most often since it's so darn common and so important to the human body). Osmotic pressure refers to the pressure caused by osmosis when solutes that can't pass through the membrane are added, like in the example above where the water level rises because of the added substance. (Or at least that's what I think the word means.) The higher the number of solute particles, the higher the osmotic pressure.
Enzymes
As you may well know, there are many ways of speeding up the rate of a chemical reaction, and using a catalyst is one of them. There's a bit more about speeding up reactions on my post titled Reaction Rates.
Enzymes are catalysts, or organic catalysts, to be exact- they speed up the reactions of living organisms. They are proteins that decrease the amount of energy needed to start a reaction so that reactions can begin at normal body temperatures. (See the aforementioned Reaction Rates for more details on activation energy.)
Molecules involved in a reaction are called substrates. Enzymes combine with these to make an enzyme-substrate complex. But not just any enzyme will do- each enzyme has a specific function and can only combine with a specific substrate in a specific reaction. This is because each enzyme and each substrate have different shapes and structures, and only complimentary ones will work together [insert lock-and-key analogy here]. After the enzyme-substrate complex reacts with another molecule, the enzyme will combine with a different substrate to start the process all over again. It's a lot quicker than it sounds, though- an enzyme can interact with millions of substrate molecules every minute.
Enzymes are kinda complex in that they are pretty sensitive and are affected by temperature and pH. For an enzyme to function at its best, temperature and pH need to be optimal. Normally, the higher the temperature, the faster the reaction, but if enzymes are present, you shouldn't raise the temperature past the optimal range. Also, an increased concentration of enzymes will do the job faster. More hands mean light work after all. Finally many enzymes are lazy buggers who won't do anything by themselves require the presence of certain ions or non-protein molecules (a.k.a. co-factors) before they will get off their asses and work catalyse a reaction.
Finally it's time for the last topic!
Organic Compounds: Carbohydrates, Proteins, Lipids and Nucleic Acids
(This is where I wish that last year I had hardened up and written more Chem posts because then I could link to them rather than have to write more about them now. The stuff we did about soaps and detergents in 3AB is actually quite relevant to the bit about lipids in that they both involve glycerol and long chains of organic molecules. Oh well.)
I'm tired so I'm just going to super summarise things now. Or, at least, I'm going to do my version of "super summarising" which is basically not really summarising anything at all. I'll just type faster. Meh. In any case, don't expect pictures. Google 'em yerself.
Carbohydrates (what most people prefer to refer to as "carbs" because "carbohydrates" is too long)- contain C, H and O (carbon, hydrogen and oxygen to the uninitiated). There are always twice as many hydrogen atoms as oxygen atoms. Simple sugars (e.g. glucose, fructose, galactose) are also known as monosaccharides. Not sure how these three were classified as "simple sugars," though the fact that they do seem to be relatively small molecules with a 1-ring structure probably has something to do with it. Two monosaccharides can combine via condensation reactions (basically a reaction that combines two small molecules by releasing water- it's in the 3AB course but haven't gotten around to speaking about them, sorry) to form a disaccharide (since di- means "two"). For example, glucose and fructose can combine to make sucrose. Finally, if loads of monosaccharides combine in this way, you get a polysaccharide, like glycogen (the form in which carbohydrates are stored in the body), starch or cellulose. Carbohydrates are important as they provide energy to the body cells.
Proteins- contain C, H, O and N (N being nitrogen). Many also contain S or P (sulphur or phosphorus). To understand proteins we need to first look at their building blocks- amino acids. There are about 20 different amino acids, and they can be combined in various ways through condensation reactions (just like how monosaccharides can be combined in various ways to make di- or polysaccharides). There's a fancy name for the bond between two amino acids: it's called a peptide bond. Because of this, two amino acids joined together are called a dipeptide, while ten or more joined together are called polypeptides. (I suppose those in between are just given prefixes like tri-, quad-, quin- etc.) If there are over 100 amino acids joined together, you have a protein. Proteins make make up much of the structure of body cells as well as take part in many chemical reactions in the body. They're necessary for building big muscles too :)
Lipids- really big molecules that are stored in the body as energy reserves and are insoluble in water. Fats, the best-known lipids, contain C, H and O (but not much of the latter), and are formed from condensation reactions between glycerol and fatty acids. Glycerol is an organic molecule with 3 carbon atoms and 3 OH groups. Each OH group can undergo a condensation reaction with a fatty acid (essentially a long chain of carbon atoms with the carboxylic acid group -COOH at the end). Fats are sorted into mono-, di- or triglycerides depending on how many fatty acids react with the glycerol (those with one fatty acid are monoglycerides etc.). 98% of fats in foods are triglycerides (product of glycerol and three fatty acids). Aside from fats, other lipids include phospholipids, a part of cell membranes, and steroids, which include cholesterol and the sex hormones.
Nucleic Acids- contain C, H, O, N and P. First discovered in the nuclei of cells, giving rise to their name. Just like the other types of molecules described previously, they are made up of many smaller molecules joined together in many different ways. This time, the basic unit is called a nucleotide, and consists of a nitrogen base, a sugar and a phosphate group. The two main types are RNA (ribonucleic acid) and DNA (deoxyribonucleic acid). They differ in the number of chains of nucleotides (RNA has one, DNA has two) and the type of sugar (RNA has ribose, DNA has deoxyribose).
All right, enough rambling from me. Here's the REAL summary:
Looks like the next couple of chapters of this book are going to cover the digestive system. Stay tuned...
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