Week 4 Last week when we learned about the organelles in a cell, we talked about the cell membrane or plasma membrane and how it is important in all cells in regulating what moves in and out of a cell. You might also remember (from week one) that one of the properties of life is that all things must maintain homeostasis. Homeostasis is a kind of balancing act and it means that, in order for something to stay alive, it must stay within certain boundaries. Since the outside world is changing all the time, living things must be able to control how they change (in other words, they cannot just change with every change in the environment). Once again, cell membranes help maintain homeostasis by allowing cells to control what is going on inside of a cell while the outside might be changing drastically. Recall from last week that we talked about solutions. Remember that a solution is made up of a solute (the stuff that gets dissolved) and the solvent (the thing that does the dissolving which is usually water). Cells are complex solutions which have a great deal of water in them (the solvent) and many different solutes (such as glucose, salts, proteins, etc.). You may also remember from last week that I told you that the cell membrane is Selectively Permeable. This means it is selective in terms of what types of molecules move in and out of the cell. We will learn about the structure of a cell membrane later on; but first, a little lesson about concentration differences. Figure 4.1a Let’s just say for a moment that the glucose (in brown) is not able to move back and forth across the cell membrane BUT that water (in blue) CAN (Figure 4.1a). If there is more water inside the cell than outside, which direction will water move? Will water move into the cell, out of the cell, or both? Well, your first response is probably that it will move

out of the cell. That is partially true, but it will actually move both directions. If one water molecule can move out of the cell, than another one can move in. BUT, the OVERALL movement (in other words, most of the water molecules or the net movement) will be moving out of the cell until an equilibrium is reached. At that point, the movement of water in the cell will equal the rate of movement out of the cell. This point is called the dynamic equilibrium of the reaction. Because the concentration of different chemicals is important in trying to figure out which way molecules will move, we have special names to describe the concentrations in and out of a cell. These terms are based on the concentration of the solute (not the solvent). So, if the inside of a cell has 10% salt and the outside of the cell has 2% salt, then we say the inside the cell is hyperosmotic (has more solute) to the outside. We could also say that, in this same case, the outside of the cell is hyposmotic (has less solute) to the inside. The other term, isosmotic, is used to refer to situations that have equal concentrations of solute. In figure 4.2a, there are three examples illustrating this. The large black circles represent the cell membrane of a cell. In these cases, the solute is glucose. Figure 4.2a

These concentration terms allow us to describe the overall direction that we expect molecules to travel. Now, let’s take a closer look at the structure of the cell membrane, and from there we can start describing what types of molecules can and cannot pass across the membrane. Figure 4.3a Figure 4.3a is commonly referred to as the “fluid-mosaic” model of the cell membrane. Most scientists believe this is what a cell membrane is like (however, recent studies suggest that this view might be changing). There are several parts to a cell membrane (and we will cover those shortly). The main part of the cell membrane, however, are the molecules called phospholipids. As their name implies, they are lipid molecules (made up mostly of carbon and hydrogen) with a phosphate attached to them. Specifically, we call the lipid part the “tails” and the phosphate part the “head” of the phospholipid. Figure 4.4a shows a chemical drawing of what a phospholipid looks like.

Figure 4.4a If you remember back to week two, when we discussed the properties of molecules like lipids, we said that carbon and hydrogen (the main components of lipids) form non-polar covalent bonds. This means that they share the electrons equally between the atoms which results in no slightly negative or slightly positive parts on a lipid. Also, you should remember that water can only bind to things with electrical charges. So, water does not stick to lipids (or does not “mix” with lipids) very well. Because of these properties, the phosphate head of a phospholipid (which does have an electrical charge) can mix with water and the carbon-hydrogen tails of a phospholipid cannot. We then apply the terms “hydrophilic” to the heads of a phospholipid (because they “love” water so to speak) and “hydrophobic” to the tails of a phospholipid (because they are “afraid” of water). Of course, they are molecules so they don’t really think about being afraid….these are just terms we use to describe how the molecules seem to react. Also, when you mix a bunch of phospholipids together in water, they will naturally arrange themselves so that water is only touching the phosphate heads. They thus form what is called the “phospoplipid bilayer”. In Figure 4.5a you can see the details of the phospholipid bilayer of a cell membrane. You should also note the hydrophilic and hydrophobic regions of the phospholipids.

Figure 4.5a The cell membrane contains other parts in addition to the phospholipids. For example, there are cholesterol molecules in the cell membrane that help keep it fluid like. Some animals have a lot of cholesterol in their cell membranes and it actually acts like a type of anti-freeze and keeps the cell membrane very fluid even in freezing cold weather. There are also several types of protein molecules that you can see embedded in the membrane. Some of these proteins act as channels or tunnels and they allow certain molecules to pass through. Others act as recognition proteins. These proteins are very important for your immune system because they can allow your body to recognize certain cells. Also, these recognition proteins are often very important in how different drugs work. Some cells have certain recognition proteins that allow certain drugs to bind to them that eventually affect the inside working of the cells.

How do things get in and out of a cell? Passive transport: Passive transport is the movement of molecules that goes along with the concentration gradients. So, if you have a high concentration of water on the outside of a cell and a low concentration of water on the inside of the cell, you would expect water to move BOTH directions: in and out of the cell. BUT, you would expect the OVERALL movement of water to move INSIDE the cell. This would be passive transport because the overall movement of water is going with the flow. It is kind of like the water is going

downhill. Think of a rock on the edge of a hill; the rock would naturally (or passively) roll down the hill. Active transport: Active transport is the movement of molecules that move AGAINST the concentration gradient. For example, let us say you have a high concentration of glucose on the inside of the cell and a low concentration of glucose on the outside of the cell BUT THE GOAL is to move even more glucose INSIDE the cell. This is like moving UPHILL. Just like a rock does not roll uphill on it’s own, energy will be required for active transport. Let’s take a closer look now at PASSIVE TRANSPORT: Take a look at this simple picture of a cell membrane (Figure 4.6a): Figure 4.6a Some molecules can go straight across the cell membrane. The only molecules that can do that are either non-polar molecules (such as cholesterol and carbon dioxide) and very small polar molecules (such as water). This is called simple diffusion. Simple diffusion is also used when describing any molecules moving from high to low concentration (not necessarily needing to cross a membrane). Sometimes (such as in your lab book), the term dialysis is used to specifically describe diffusion across a selectively permeable membrane. Now, if it is specifically water that is moving across the cell membrane, we call that specific type of simple diffusion, osmosis. Remember, osmosis only refers to the passive diffusion of water. Therefore, if you are studying by osmosis you are only

sucking water into your head, not facts and knowledge! These (simple diffusion, dialysis, and osmosis) are types of passive transport. Other molecules that are either big and polar (such as glucose) or are charged (such as ions) cannot go across the cell membrane. The reasons for this is that water molecules form hydrogen bonds with these types of molecules and these keep them from moving very much. Therefore, they usually don’t develop enough kinetic energy (energy in motion) to move across the cell membrane. So, molecules such as glucose or Na+ (a sodium ion) or K+ (a potassium ion) must enter or leave the cell through a special protein (See Figure 4.6a above). This is called facilitated diffusion. Facilitate diffusion is also a type of passive transport. So far, the type of movements we have described are for regularly sized molecules. But if you want to move something REALLY big inside or outside of a cell (such as a bacteria or a virus or large protein), then we have another system. Endocytosis and Exocytosis. In these, the membrane can fold in and surround the object. It eventually pinches off and encloses the item in a type of internal sac. Figure 4.7a depicts what EXOCYTOSIS would look like (the movement of something out of the cell): Figure 4.7a Endocytosis would look similar but in reverse. Instead, cells or large molecules or groups of molecules enter the cell as it folds inward. Often, there are receptors on the surface of the cell-membrane that help this happen. Endo and Exocytosis are both active types of transport. Often, Endo and exocytosis are further broken down based on what is actually being brought into the cell. For example, the term phagocytosis is often used to

describe “cell eating” or the process of bringing in solid materials (such as bacteria). The term Pinocytosis is used when a cell engulf liquids. A summary of the different types of transport are demonstrated in the table 4.1a: Table 4.1a

Specific Type of Transport Active or Passive Common Example of thing transported

Simple Diffusion/Dialysis Passive Carbon Dioxide, Oxygen

Osmosis Passive Water

Facilitated Diffusion Passive Sodium (Na+ ), Potassium (K+) ion channels

Active Active Sodium-Potassium ATP pump

Endocytosis/Exocytosis Active Bacteria, Large Proteins

Energy: In the first week, we talked about how one of the properties of life is that all living things require energy. This energy is required to maintain order. In turns out, just as when we described how diffusion causes molecules to move from higher to lower concentrations, all molecules move around from high concentration to low concentration. Living things require order but the natural state of things is to move towards disorder. We will revisit that idea in a minute. But first, let’s take a look at what energy is… (an example is how a steam engine works (Figure 4.8a)

Figure 4.8a Energy can be thought of in several different ways. It can be defined as the ability to do work. Another definition is that energy is the ability to bring about change against an opposing force. Energy can be stored (called potential energy) or it can be in motion (called kinetic energy). Imagine you have a giant boulder on the top of a hill. The boulder could roll down the hill if it was pushed. If it is just sitting at the top of the hill, the rock is said to have potential energy. If the rock is actually rolling down the hill, this would be kinetic energy. There are a couple of laws that deal with energy that you need to know. They are called the 1st and 2nd Laws of Thermodynamics. The 1st Law of Thermodynamics is that energy cannot be created or destroyed; only transferred or transformed. In Figure 4.8a, you can see that coal is being burned. This chemical energy is released and heat is given off. Some of the released energy boils the water and the steam from the water turns the wheel. Thus it is transforming the heat energy into mechanical energy. The 2nd Law of Thermodynamics is slightly more difficult to explain. First, you need to know the word entropy. Entropy is a measure of the disorder of things. The second law of thermodynamics states that with every energy transformation, entropy increases. Heat is the ultimate form of disorder. So, if you put all those ideas together, you get this: Every time energy is transferred or transformed, some of the energy is lost as heat. Heat cannot be used to do work (because it is too disordered). Now look at the above picture again. When the coal is burned, there is a certain amount of energy available. Some of

that energy is going into boiling the water. BUT NOT ALL OF IT because some of it escapes as heat. Lets now look at how energy is stored in chemical bonds. Here is a picture that will explain two types of chemical reactions (Figure 4.9a): Figure 4.9a Endergonic reactions are ones that store energy when they happen. Exergonic reactions are ones that release energy when they occur. In Figure 4.9a, you should note that when glucose molecules are hooked together to form glycogen, this is an endergonic reaction. This means you have to add energy to make the reaction occur. Now, if you break glycogen down into glucose, energy is released and thus is an exergonic reaction. Let me ask you this: if it takes 50 calories or so to take the glucose molecules and hook them together, how many calories do you think you would get out when you break the glycogen molecule? Initially, you might think you would get 50 calories out; but not quite. Remember that you always lose some of the energy as heat. So you would not be able to just go around and around in a big circle using the same 50 calories forever and ever. Here is another exergonic and endergonic reaction (Figure 4.10a). This one you will see a lot. It’s a molecule of ADP and ATP:

Figure 4.10a Don’t worry about memorizing all the pieces. Just look at the difference between the two. You should notice that ADP has two phosphate groups and if you add a third phosphate (which is an endergonic reaction) you have ATP. If you take ATP and remove one of the phosphates (which is an exergonic reaction) you have ADP. Notice that the phosphates are negative. Making ATP (from ADP) is endergonic because adding another negative phosphate requires energy because the other two negative phosphates repel it (opposite charges attract so like charges repel each other).

Coming Up….(but you need to know at least this part for this coming quiz!)… All livings things need energy. Other than plants (and a few other kinds of things), most organisms need to eat food. This food is then transformed into a high energy molecule called ATP. ATP is basically the main energy source of living things. ATP and other molecules are generated through the many chemical reactions that are going on in a living organism. The sum of all of these chemical reactions going on in an individual living thing are called metabolism.

The set of chemical reactions that generate ATP are collectively called cellular respiration. Cellular respiration involves four basic steps: glycolysis, an “in-between step” (Pyruvate Dehydrogenase Complex), the Krebs cycle, and the Electron Transport Chain. There are many important inputs and outputs to cellular respiration.

One important input is oxygen. Oxygen is needed to complete the final steps of cellular respiration. We will learn the other inputs and outputs later. For now, just realize that the main goal of cellular respiration is to make ATP.

When breaking down food and converting the energy into ATP, heat is released (this is due to the second law of thermodynamics from above). This heat is what makes many animals (such as humans) have high body temperatures. Animals that generate a lot of heat through cellular respiration are called endothermic animals. Although technically the word metabolism is the sum of all chemical reactions going on inside a living thing, we often measure metabolism by measuring the amount of cellular respiration taking place. Likewise, since there are so many steps in cellular respiration, we tend to focus on one part of cellular respiration to measure metabolism. To do this, scientist often measure how much oxygen is being consumed or how much carbon dioxide is being produced (or both). Therefore, animals that consume oxygen at a fast rate must be doing a great deal of cellular respiration and must have a high metabolic rate. Another way to put it is that animals that are endotherms consume lots of oxygen and have high metabolic rates and consistently high body temperatures.

Some animals do not generate a lot of heat through their metabolism. These animals are called ectothermic animals because they rely primarily on the environment to generate heat.

As an example of all of this, I studied the metabolic rates of lizards during sleep and wakefulness. For my particular research, I specifically measured how much oxygen was being consumed by my Desert Iguanas at different temperatures. Since Desert Iguanas are ectothermic, they consume more oxygen as their body temperature increases. Therefore, their metabolic rates increase with increased temperature.

ASSIGNMENT #4 – Print this sheet off and turn it in with your lab next week. This sheet of paper goes on top (then the lab).

1) In the picture, the percentage of solute in container “A” is: A) 10% B) 20% C) 90% D) 80% E) 100% 2) In the picture, the percentage of solvent in container “B” is:

A) 10% B) 20% C) 90% D) 80% E) 100% 3) In the picture, the percentage of solute in container “A” is__________ to container “B”? A) hyperosmotic B) hyposmotic C) isosmotic 4) Sodium ions generally enter the cell via: A) Dialysis B) Osmosis C) Facilitate Diffusion D) Simple Diffusion E) All of the above 5) Animals that consume a great deal of oxygen nearly all of the time will (Choose all that apply): A) have a high metabolic rate B) a low metabolic rate C) generally be described as being endothermic D) generally be described as being ectothermic

6) What is the chemical cause of cystic fibrosis (book or internet)?

7) What is the glycocalyx

Words that you may be asked to define or use in fill-in-the blank types of questions: dynamic equilibrium, osmosis, hyperosmotic, hyposmotic, isosmotic, solute, solvent, solution, diffusion, dialysis, active transport, passive transport, endocytosis, exocytosis, phagocytosis, pinocytosis, 1st law of thermodynamics, entropy, second law of thermodynamics, energy, exergonic, endergonic, cellular respiration, glycolysis, Krebs cycle, electron transport chain, ectotherm, endotherm, metabolism, oxygen, ATP, ADP, PDC