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Biology: The Cell

We will start the biology section with the cell. Cells are the fundamental units of life, and you are required to know about them. There are two types of cells, eukaryotic cells and prokaryotic cells. 

Eukaryotic cells are the ones with the well-defined nucleus, a nuclear membrane and organelles like the mitochondria, Golgi bodies, endoplasmic reticulum, etc. Think of the eukaryotic cell in three regions. Region one: the outer cell wall and membrane. Region two: the cytoplasm, which contains the organelles and cytosol. Region three: the nucleus, which is encased in a nuclear membrane. 

Remember that all cells, except animal cells, usually have a cell wall and a cell membrane. Animal cells have a cell membrane only. They do not have a cell wall. Remember also that cell walls are made of carbohydrates. Cell membranes are made of proteins and phospholipids. Since the cell membrane has two layers of phospholipids, it is sometimes called a phospholipid bilayer. 

The cell membrane is made of proteins and phospholipids. You need to remember one theory about the arrangement of protein and phospholipid. This theory depicts the cell membrane as a bilayer of phospholipids with proteins kind of stuck into it on the inner and outer surfaces. The proteins stuck at the outer surface or the inner surface are called peripheral proteins. The proteins that are entirely inside the phospholipid bilayer are called integral proteins. Some of these proteins are large enough to penetrate the bilayer and protrude through both ends. 

A phospholipid molecule has a head and a tail. The head is water-soluble or hydrophilic, and the tail is water insoluble or hydrophobic. The hydrophilic heads point outward to the surface and inward to the cytoplasm. The hydrophobic tails meet in the interior of the bilayer. The cell membrane is important, therefore, because it regulates and maintains the cell’s internal environment, and it determines what gets into the cell and what stays out. 

To understand how the cell membrane does this, imagine a balloon filled with freshwater and imagine that the balloon is a cell. Imagine also that the balloon is freely permeable to salt. Now, suppose we put the balloon in a tank of salt water. The water inside the balloon does not have salt in it, but the water outside the balloon does. Now remember this rule, every solution tends to equalize the concentration of solutes throughout its volume and unless something stands in its way, it will do exactly that. Since we said the balloon is freely permeable to salt, salt will pass in the balloon until the salt concentration inside the balloon is the same as that outside the balloon. 

Now, suppose by some process we change the balloon’s properties in two ways. First, we make it impermeable to salt. Second, we make it possible for water to pass into the balloon, but impossible for water to pass out of the balloon. Now, imagine that we take the balloon out of the salt water tank and put it into a fresh water tank. Since the water inside the balloon has salt in it but the water outside the balloon does not, some salt would like to move out of the balloon in order to equalize the concentration. But that cannot happen, and because the system keeps trying to equalize the salt concentration inside and outside the balloon, it keeps moving water into the balloon and the balloon eventually pops. 

When in a solution or system of connected solutions, like the balloon in the tank, two different regions have different concentrations of the same solute. We call that difference a concentration gradient. Any movement of solvent or solute that tends to equalize concentrations will, of course, tend to eliminate the concentration gradient, and that movement is called movement with or along the concentration gradient. 

When you think about a membrane that is permeable to all of the solvents and solutes in its surroundings, then you call movement through that membrane simple diffusion. Think about a membrane that is not permeable to a solute, but is permeable to a solvent. When the solvent, which is usually water, moves across the membrane in order to equalize concentrations, we call that movement osmosis. 

Think about the expanding balloon in the example. Its solute concentration is higher than that of its surrounding medium. We say then that the fluid in the balloon is hypertonic. If the balloon in our example had a solute concentration that was lower than its surrounding medium, we say that the fluid in the balloon is hypotonic. 

Think again about the expanding balloon in our example. Because the fluid inside is hypertonic and because the balloon is impermeable to the outward flow of water, there is a tendency for water to move into the balloon. The balloon’s tendency to suck water in by osmosis is called osmotic pressure. Osmotic pressure is the tendency of water to move into a solution by osmosis. 

When a membrane is permeable to some substances but not others, it is called semi selectively permeable or semipermeable. The cell membrane is semipermeable because of its structure. Remember that the cell membrane is composed of a lipid bilayer and protein. Substances that are lipid-soluble tend to pass through the lipid bilayer pretty easily by simple diffusion. But for substances that are lipid insoluble such as proteins and charged ions, the lipid bilayer is a barrier to movement. Lipid insoluble substances cannot get through the lipid bilayer unless they have some special help. This special help comes from the membrane’s protein components. 

For these lipid insoluble substances, the cell membrane’s protein components somehow facilitate movement through the bilayer. In fact, when a lipid insoluble substance is able to move through the cell membrane along a concentration gradient or electrical gradient, we say that the substance is undergoing facilitated diffusion or facilitated transport. 

The way facilitated diffusion works is not clearly known. There is, however, a popular theory which states that some of the membrane’s protein molecules act as carrier molecules and because of their particular structure, they have the ability to combine with a particular solute, usually hydrophilic, to form a complex. This complex is soluble in the lipid bilayer of the membrane, and it passes through without coming in contact with the hydrophobic interior of the lipid bilayer. At the other end of the channel, the carrier molecule and the solute separate, and the solute is released. 

It is important to know that the membrane’s properties of permeability are not constant and can change from moment to moment. The membrane’s permeability is adjusted according its protein components. At one moment, the membrane might be permeable to outward movement of sodium but not to its inward movement. At another moment, the situation might be reversed and sodium might move freely into the cell but be unable to move outward. 

The cell membrane is also thought to have tiny channels or pores, with each channel somehow selective in its permeability to lipid insoluble substances. There are thought to be special potassium channels, special calcium channels and special glucose channels. Just as selective permeability is related to specific carrier molecules, selective permeability is related to the presence of membrane channels. It is hypothesized that the channels have gates on them that can be opened or closed. 

Remember this, any movement of solute or solvent that tends to produce or increase a concentration gradient requires the expenditure of energy, and that is called active transport. If a system wants to move charged particles against an electrical gradient, it must expend energy and that is also called active transport. The process of active transport is also called a pump. An example of active transport mechanism is the sodium-potassium pump. Simple diffusion and osmosis do not require the cell to expend energy. These two processes, therefore, represent passive transport. 

Facilitated transport does not require energy because movement occurs along a concentration gradient or electrical gradient. For that reason, facilitated transport is not considered active transport. But even though the process does not require energy, the cell membrane has some active involvement in it, and therefore the process is not considered to be a form of passive transport either. 

So remember that substances may cross the cell membrane by (1) passive transport, which includes osmosis and simple diffusion; (2) facilitated transport; and (3) active transport; and fourth kind called endocytosis. 

Passive transport, facilitated transport and active transport apply to the movement of molecules and ions in and out of the cell. In order to take in larger particles, cell membranes use endocytosis in which a portion of membrane engulfs the object and pinches off to contain it within a vesicle inside the cytoplasm. Phagocytosis, or “cell eating,” is the endocytosis of relatively large particles. Pinocytosis, or “cell drinking,” is the endocytosis of dissolved materials. Receptor-mediated endocytosis is the endocytosis of specific particles using protein receptors found in clathrin-coated pit regions of the membrane. 

The cell membrane is actually one section of a larger membrane system. This system begins at the outer border of the cell, branches into the cytoplasm, winds its way through the cytoplasm and then surrounds the cell’s nucleus. Where this membrane is located at the periphery of the cell, it is called the cell membrane. Where it is located in the cytoplasm, it is called the endoplasmic reticulum. Where it surrounds the nucleus, it is called the nuclear membrane. 

The endoplasmic reticulum looks like a network of channels. Endoplasmic reticulum can be smooth or rough. Some portions of the endoplasmic reticulum have bumps on them, so these portions are called rough endoplasmic reticulum. The portions without the bumps are called smooth endoplasmic reticulum. These bumps sitting on the rough endoplasmic reticulum are ribosomes. Ribosomes are the sites at which the cell synthesizes protein. Smooth endoplasmic reticulum is not attached to ribosomes, and it is not involved in protein synthesis. The smooth endoplasmic reticulum is involved in lipid synthesis. 

At one or more points along its course, the smooth endoplasmic reticulum changes shape and looks like a network of flattened sacs instead of a network of channels. This region of endoplasmic reticulum is called the Golgi apparatus. The Golgi apparatus pinches off little pieces of itself and these are called vesicles. Make sure you do not confuse these vesicles with the vesicles that result from pinocytosis and phagocytosis. Golgi apparatus is involved in the packaging of proteins to be sent out of the cell. It packages these proteins in vesicles. These packaged proteins are modified along the way and usually, carbohydrates are added to them. Remember that sending materials out of the cell is called exocytosis. 
The cells must stick together to form body tissues. They do this by forming three types of cellular adhesions with each other. They are tight junctions, desmosomes and gap junctions. Tight junctions prohibit the passage of most substances between cells, while gap junctions allow the passage of small ions and molecules from cell to cell. Desmosomes hold adjacent cells together but allow substances to pass through. 

The endoplasmic reticulum, the ribosomes and the Golgi apparatus are three cellular organelles, and they happen to be closely associated with one another. There are eleven other structures associated with the cell, and you are required to know a little about them. 

Let us review these organelles and their functions. Chromosomes are composed of DNA and contain genetic information. Lysosomes contain hydrolytic enzymes and digest foreign substances and worn organelles. Centrioles are hollow rods composed of microtubules and form the spindle fibers during mitosis. Peroxisomes contain enzyme catalase and metabolize oxygen or hydrogen peroxide. Cilia and flagella function in cell movement by acting in a whip-like motion. Microfilaments are thin fibers made of actin associated with movement of cytoplasm. Microtubules form part of the cell’s skeleton and are involved in movement of chromosomes and also the structure from which cilia and flagella are formed. Nucleolus is the site at which ribosomal RNA is formed and is located in the nucleus. Vacuoles are fluid-filled spaces that function in expelling wastes. Plastids contain pigment and are found only in plants. Mitochondrion is a double membrane organelle with an inner matrix, and it produces ATP.