Special Cell Processes Cells need certain substances to stay alive. These substances include food and oxygen. How does a substance, such as oxygen, get through the cell membrane and into the cell? 1 Diffusion (Crowded or not, you choose) Imagine you really hungry and have to pass the library during period ¾. You may smell what is cooking for lunch. Sometimes this is a good thing, sometimes it is a bad thing. How did that smell, which is in the form of a gas, move from the cafeteria into the hallway by the library. The gas moved from where there was a large amount of it (in the café) to where there was only a small amount of it (the hallway). Why does the gas behave in this manner? When there is a large amount of a substance like the gas that is given off by the yeast rolls baking in the café, the molecules of that gas are close together and bump into each other a great deal. When they bump into each other, they go off in different directions from which they were traveling like the cue ball and a billiard ball. Because there are so many collisions, the molecules randomly spread out as much as they can. So where do they go? Where there are not so many molecules! Biologists like to call this an area of low concentration. Remember from IPS that concentration is the amount of solute (the thing dissolved) in a volume of solvent (the thing doing the dissolving). Eventually the molecules even out and the concentrations are equal. So, let us go back to our yeast roll smell. There is a high concentration of smell in the cafe. If the molecules are moving they will bump into each other and eventually by chance one will go through the crack around the door or through the ventilation duct. This will continue until the number of yeast roll smell molecules are equal inside the café and outside the cafe. This whole process that we just described is called diffusion. It is the movement of a substance from where there is a high concentration to where there is a small concentration. Diffusion will continue until the concentrations are equal. You can see this process in the picture below. Now that we are all expecting yeast rolls for lunch, what does all this food talk have to do with cells? Well, the membranes have cracks and holes in them that allow some substances to enter. They are technically called pores. Some molecules that are small enough, like oxygen, can pass through these pores. Others are too big and are left outside. Look at the picture below. It shows a cell with many holes or pores in its membrane. You can also see in picture A that the concentration of green oxygen molecules is greater outside the cell than inside the cell. In the
center picture, you can see some of the oxygen molecules have begun moving to the place with the lower oxygen concentration (inside the cell). In picture C, the number of green oxygen molecules inside and outside the cell is equal. When the concentrations inside and outside the cell are equal it is called equilibrium. 2 Osmosis (special diffusion) Water molecules also move across membranes. Like oxygen, they move from where there is a high concentration to where there is a lower concentration. The movement of water and other simple liquids across the cell membrane is called osmosis. Osmosis is important to cells because water molecules surround them. The number of water molecules inside and outside the cell must be about the same. When there are more water molecules outside the cell, the solution the cell is sitting in is said to be hypertonic. When a cell sits in a hypertonic solution ( as seen in the picture to the left, there is a higher water concentration inside the cell than outside. The water leaves the cell for where there is a lower concentration. This leaves the cell shriveled up and not working properly. When a cell is sitting in a solution that has more water outside the cell than inside, the solution is said to be hypotonic. Because there is more water outside the cell than inside, the water moves into the cell. You can see this in the picture to the right. If is cannot equalize the concentrations, water will keep moving in until the cell swells or bursts. This is the reason you get pruny if you sit is the bathtub too long. Your skin cells are sitting in a hypotonic solution, water is constantly enter them. They swell and can no longer fit into the normal space across your finger. A wrinkle develops in you skin to make room for the expanded cell. When a cell is sitting in an isotonic solution, it neither shrinks nor swells. An isotonic solution is one in the concentration of water is equal to the concentration of water inside the cell. You can see how a cell acts in an isotonic solution in the picture to the left. When you get an I.V. in the hospital, the solution is isotonic. Contact lens solution is also isotonic. After all your retina would not function very well if its cells were shrunken or swollen. And now for a little practical application. If you have some celery in your fridge for a while, it will get limp and floppy. It does not stay crisp like celery should be because the celery cells are constantly losing water to the air. They become very shriveled up and consequently aren t very good for eating. What kind of solution should you put the celery in to make it crisp once again? If you answered a hypotonic solution, you are right. A hypotonic solution will have more water outside the celery, causing it to move inside. What s a good cheap hypotonic solution? How about tapwater? If you put the celery in plain old tap water, the crispness returns to the celery and you can enjoy eating it once again.
3 Active vs. Passive Transport (to use energy or not to use energy that is the question) The processes of diffusion and osmosis are natural processes that happen without the cell providing anything more than a membrane for substances to pass. Because the cell does not have to expend any energy, diffusion and osmosis are called collectively, passive transport. In addition to diffusion and osmosis there is another process that is considered passive transport. It is called facilitated diffusion. Facilitated diffusion is when protein molecules act as revolving doors in the cell membrane. You can see a picture of this to the right. Again, because facilitated diffusion is passive transport, the cell does not use any energy for this to happen. Active transport, as you may have guessed, is a process of moving substances across the cell membrane that requires the cell to use a little energy. Many of the substances that are moved by active transport are just too large to get through the small pores of the cell membrane or the transporter proteins. Endocytosis (All aboard!!!) Endocytosis is an active transport process that brings large molecules inside the cell. The way this happens is kind of cool. When a cell senses there is something too large to bring inside the membrane through passive transport, the cell membrane wraps around the substance. This process can be seen in the picture to the left. Once the cell membrane has completely surrounded the substance it pinches off a little vacuole containing the substance. In this way the substance ends up not only inside the cell but in a nice Tupperware container as well. When large solid molecules are being brought in through endocytosis it is called phagocytosis. When large liquid molecules are being brought in, the process is called pinocytosis. Pictures of phagocytosis and pinocytosis can be seen below.
4 Exocytosis (Last stop, Everyone out!) When a cell has to move some large particle to the outside, it also has to use energy. This process is called exocytosis. It happens just the opposite of endocytosis. You can see the steps in the process in the picture to the left. The Golgi apparatus packages the wastes in a vacuole. They travel near the cell membrane. The vacuole membrane fuses with the cell membrane and the vacuole breaks open. The once the vacuole is open, the wastes are on the outside of the cell and the cell are rid of them.
Cell Reproduction Every part of your body is made up of cells. You and all other living things started out as single cells. In some plants and animals, that cell came from part of one parent. In most plants and animals and in humans, the cell came from the joining of a cell from a male parent and a cell from a female parent (that s what sex really is you know). You have seen the difference in size between an adult dog and a puppy. The larger animal has millions more cells than its offspring. If the offspring started out as one cell, how does it become an adult with millions of cells? Where do all these new cells come from? Mitosis (your tosis) You started life as a single cell. You now have millions of cells in your body. Somehow, you have grown. The new cells in our body came from cell reproduction. Body growth and repair Living things grow. You were growing even before you were born. Living things also repair themselves when they are injured. As a child, you probably cut or scraped you skin often. You must have seen new skin grow back on your hands and knees many times. Your body must make new cells to grow and to repair itself. New cells are by the process of cell reproduction. One kind of cell reproduction is called mitosis. Mitosis is the division of the nucleus of a cell. This is usually followed by another process called cytokinesis in which the rest of the cell is split. These processes together result in two identical cells are made from one cell. Each new cell grows in size until it too is ready to reproduce by mitosis. All body cells in humans are formed by mitosis. Body cells are cells that make up most of the body, such as the skin, blood, bones, and stomach. You can see from the chart below that all body cells don t live for the same length of time. Body Cell Type Lifespan Brain cells 30-50 years Red Blood Cells 120 days Platelets (for clotting) 10 days Stomach lining cells 2 days Liver cells 200 days Intestine lining cells 3 days Skin cells 20 days Cells carry on mitosis at different rates in different organs to replace cells that are worn out. In which body organs do cells carry on mitosis most often? In which organs do cells carry on mitosis least often? In some body cells, such as muscle cells, mitosis never occurs after birth. You are born with all the muscle cells you will ever have. Big bulky weight lifters do not have more cells than you, their individual muscle cells are just much bigger than yours. Mitosis goes on during a person s entire life. You are constantly forming new cells to replace those worn out or lost. Mitosis begins before you are born and continues up 5
6 to the time you die. Some cells like hair follicle cells continue to divide for a short time after you die. An introduction to Mitosis How does mitosis work? We could compare it to how a photocopy machine works. You teacher has given you this reading and being the diligent student you are, you immediately put it in your notebook. Your friend, however, was not so diligent as you. Your friend wants to borrow your copy of this reading. You are worried he will lose your copy as well. What do you do? You make a photocopy. You now have two identical copies of this reading. The process of mitosis is similar. You start with one cell. The end result is tow identical cells. The photocopy machine makes an exact copy in about 5 seconds. Mitosis may take several hours. What cell parts are involved in mitosis? The picture to the right shows what a typical animal cell would look like. Remember that the entire cell is surrounded by a cell membrane. Most of the material inside the cell is called cytoplasm. A cell nucleus is present in the cytoplasm. A nuclear membrane surrounds the nucleus. You have also read that inside the nucleus there are chromosomes. They may not be seen during most of the life of the cell because they are spread out. There are also centrioles stationed by the nucleus in the cytoplasm. An important step takes place in the nucleus of every cell before mitosis begins. It occurs during the longest stage in a cell s life cycle, called interphase. In fact, interphase is so long that if you imagine the entire cell cycle taking one hour, the cell would be in interphase for 50 minutes of the hour. That is why when you are looking at cells in an onion root tip, most of the cells you see are in interphase. Interphase is so long, scientists break it up into 3 parts. The first part is called G 1 for first growth stage. During this stage the cell is, you guessed it, growing. The second part of interphase is called the S stage for synthesis stage. During this stage the chromosomes double. If a cell needs all of its chromosomes to work properly, it makes sense that you need a complete copy to give to each offspring. Making 2 offspring, you will need 2 copies of the chromosomes. The two strands of each doubled chromosomes are held together at one point, the centromere. The two strands of chromosomes after is has become doubled are called sister chromatids. You can see a picture of sister chromatids with their centromere to the right. Each chromatid is an exact copy of the original chromosome. The last part of interphase is G 2 or growth stage 2. What is the cell doing during this part? Growing, of course? One important event happens during G 2, the centrioles double. The steps of mitosis
Follow along with the pictures as we describe the steps of mitosis for you. A complete diagram of the cell cycle will follow the complete description. Prophase After the cell has gone through interphase it is ready to divide. Prophase is the first step in the division of the nucleus. In prophase, the chromosomes that doubled in interphase shorten and condense. This is the first stage in the cell cycle that you can actually see individual chromosomes if you are looking at a cell with a microscope. The nuclear membrane breaks down during prophase. The centrioles begin to move to opposites sides of the cell. Little fibers begin to form called spindle fibers that act like guides to get the chromosomes to the opposite sides of the cell. 7 Metaphase The next stage in mitosis is called metaphase. This one is often the easiest to recognize when looking a cells in the microscope. In this phase, the sister chromatids are lined up in the middle of the cell, almost in a straight line. The centrioles are at opposite sides of the cell. The spindle fibers attach to the chromatids at the centromere. The spindle fibers extend from the centromere on the chromatids to one of the sets of centrioles. This is to make sure that each member of the pair goes to a different side of the cell. Anaphase After the chromosomes are lined up in the center of the cell, it is time for them to go their separate ways. This happens in anaphase. The sister chromatids are pulled apart by the spindle fibers and just begin traveling to opposite sides of the cell. Often when you see anaphase under the microscope the chromosome ends are still touching but the centers are heading in opposite directions. Telophase The last stage of mitosis is telophase. During this stage the chromosomes are at opposite sides of the cell. The spindle fibers disappear and the nuclear membrane begins to reform. Cytokinesis (splitting the rest of the cell) begins at the end of telophase.
After telophase and cytokinesis After telophase and cytokinesis have finished, the cell you originally began with is now two cells. These cells are about half the size of the parent cell and are usually called daughter cells. (I don t know why they are not son cells. Don t ask). These cells have the same number of chromosomes as the parent cell. They immediately start G1 of interphase where they grow for a considerable time. What is the benefit of mitosis? First mitosis helps us grow by producing new cells. Second, mitosis replaces cells lost through cell death and injury, such as when you cut your finger. Plant mitosis is very similar to animal cell mitosis but there are two differences you need to remember. The first difference is plant cells do not have centrioles. What the spindle fibers attach to is a mystery. The second difference is at the end of cytokinesis, after the cell membrane forms between the two daughter cells, a cell wall forms. 8
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