1 Life in a Cell A little background, please. In 1665, an English scientist named Robert Hooke looked at thin slices of cork under a microscope. He saw something similar to the picture to the right. Hooke saw that cork had a lot of empty spaces. Hooke used the word cells to describe the empty spaces. He called them cells because they reminded him of the tiny rooms monks lived in called cells. He was the first person to do so. He used a new fangled instrument called a microscope. Today, biologists know that Hooke did not see living cells. He saw the walls of cells that were once alive. Anton Van Leeuwenhoek was the first person to see a living cell in the late 1600’s. A cell is the basic unit of living organisms. The type of microscope Hooke and Van Leeuwenhoek used is called a simple light microscope. It was not really anything more than a fancy magnifying glass that light was directed through. By the nineteenth century, microscopes had been improved. The science of producing lenses and optics had improved so much the compound light microscope was produced. Instead of a single lens like the simple microscope had, a compound microscope has many lenses through with the image is magnified in several steps. This gave the ability to see smaller objects in the cell. Scientists could see that the cell had parts. First Robert Brown discovered the central part of the cell, the nucleus. Then, two German biologists Matthias Schleiden and Theodor Schwann, did experiments to see what kinds of living things had cells. They formed hypotheses that all plants and animals were made of cells. The experiments of Schleiden, Schwann and other scientists lead to the development of the cell theory. The major ideas of the cell theory are listed below. 1. All living things are made of one or more cells. 2. Cells are the basic units of structure and functions in living things (they’re where the real action takes place) 3. All cells come from other cells. Today, compound light microscopes are not only common (almost every high school biology classroom has them) but they are really powerful. The really good ones can magnify an image 1500 times. In the 1940’s, a new kind of microscope was invented. It was called an electron microscope. The principle was similar to the light microscope but instead of shining a light through the extremely thin section, this microscope sent a beam of electrons through the section. This was amazing not only could you see the parts of the cell, but you could see the parts that make up the parts. Today, there are two widely used types of electron microscopes, transmission electron microscope (TEM) and scanning electron microscope (SEM). The TEM send a beam of electrons through the thin section. SEM bounces them off the surface of the section so intricate details of the surface can be seen. Both produce cool pictures 2 as you can see in the pictures to the left. With each advance in microscope technology came an equal advance in the knowledge of the cell. What was once information reserved for scientists, now is common knowledge in most high school biology courses. We have come a long way, baby. Cell parts and their jobs (or Parts are definitely not parts) Cells are microscopic units that make up all living things. Cells are alive. They do everything needed to stay alive. They carry on cellular respiration. They grow and reproduce. A cell has many different parts to do all of these jobs. As you study the parts of the cell look at the pictures on the next page so you can put a name with a face (or organelle in this case). The top picture is an animal cell and the one on the bottom is plant cell. 3 Cell membrane The cell membrane is sometimes called the plasma membrane. Both plant and animal cells have a cell membrane. It gives the cell some sort of shape but mostly it keeps things out that should be out and keeps the inside stuff inside. It also controls what can go in and out of the cell. The cell membrane is a double-layered membrane with lots of stuff sticking in it and through it. A close up diagram of the cell membrane can be seen in the picture below. Notice the two layers are called a lipid bilayer. That is because the two rows are made out of weird lipids. The head end is hydrophilic or water loving and the tail end is hydrophobic or water hating. This makes the hydrophilic heads want to be next to the water inside and outside the cell. When this happens, the hydrophobic tails get sandwiched between the water loving heads and get away from the water. Also notice the membrane has several big globby molecules that seem to go right through it. These are used to move large substances through the membrane and are called transporter proteins. In addition, cell membranes have little bumps and branches sticking out of them that act as markers that cells identify each other with. No two people have exactly the same cell markers with the exception of identical twins. When you receive an organ transplant, if the cell markers are not a close enough match to your own, your body will think it is some invading organism and will start attacking it. This is what doctors call rejection of a transplant. Cytoplasm (cell juice) Inside the cell membrane is a whole host of little cell structures called organelles. Organelle means little organ. These organelles have to have some way to move substances from one to another. The cell often needs to move substances that have been let into the cell to another area. The liquid inside the cell called the cytoplasm accomplishes all this. All cells have cytoplasm. It is very important for slow transport of substance and to keep everything nicely wet. The cell needs to be wet inside because the reactions that keep the cell alive are all are water based. Without the water, the right molecules may never bump into each other. 4 The Nucleus (the big cheese of the cell) The nucleus is the cell part that controls most of the cell’s activities. All cells have a nucleus at some time during their lives. It is really important. It determines how and when proteins will be made. Proteins, as you know, are complex substances that can act as enzymes to make reactions go. Proteins can also be structural like those that form some cell parts. The nucleus is also where all the information is stored that will be passed onto the offspring. The picture below shows a nucleus and some of its important parts. Just like a cell, the nucleus has a membrane around it. It is called the nuclear membrane or nuclear envelope. It does most of the same things a cell membrane does. It keeps the inside, in and the outside, out. It also controls what can go in and out. The nuclear membrane has little holes in it called pores that let stuff pass through without too much trouble. It also is not made of one lipid bilayer like the cell membrane; it is a double lipid bilayer, one on top of the other. The information to control all of things that have to go on in a cell is stored in the cell’s DNA. Big wads of this DNA are called chromosomes. They are found inside the nucleus and usually stain darker colored. Chromosomes are usually all tangled up and spread out during most of the cell’s life. When they are spread out, they are not really visible. When the cell is going to divide, the chromosomes condense and they look like big roundy X’s. A picture of a chromosome can be seen to the right. Sitting in the nucleus is a smaller organelle called the nucleolus. The nucleolus is the cell part that makes ribosomes. You will read about ribosomes later but just remember that the nucleolus makes them for now. Just like a cell, inside the nucleus is a water solution. Now you might expect this since this container has lots of holes in it and it’s sitting in a container of water. The cytoplasm would naturally go into the nucleus through the holes. But since the nucleus is considered the most important organelle in a cell, its water can’t be called cytoplasm. Nooooo! It must be more fancy, dancy than that. It is called nucleoplasm. Big deal. Nucleoplasm is nothing but cytoplasm with a better address. Ribosomes (Time to make the protein) As mentioned earlier, the nucleolus has the job of making ribosomes. Ribosomes are tiny (I mean really tiny) organelles that make protein. Ribosomes are made out of a special kind of ribonucleic acid called ribosomal RNA (rRNA). Wow, what rocket scientist named that one? Two big pieces of rRNA are put together to make the ribosome. The protein is made between the two pieces. You can see a 5 picture of a ribosome to the right. Some ribosomes are found floating freely in the cytoplasm but more are found attached to another organelle, the endoplasmic reticulum (ER). Endoplasmic reticulum (FedEx of the cell) The cytoplasm is riddled with a network of canals called the endoplasmic reticulum. This organelle is in both plant and animal cell. The canals serve as a highspeed delivery system for the cell. Since the cytoplasm takes its own sweet time moving stuff from one place to another, the cell must have a special speedy delivery system. This ensures that materials are where they need to be, when they need to be there. The endoplasmic reticulum has a couple of other jobs as well. As mentioned before, it is a place where ribosomes attach. Pretty handy to make a protein in the ribosome, then whisk it to the place its needed in the ER. ER also makes and stores lipids in the cell. A picture of ER can be seen to the left. Golgi Apparatus (Gift wrap department of the cell) When the ER makes something, it usually puts it in a temporary package then sends it to the Golgi apparatus. The Golgi apparatus is a stack of flattened sacks that kind of look like bumpy pancakes. The Golgi sorts the substances in the incoming vesicles (sometimes called vacuoles) and groups them together. It then repackages them to travel around the cell. You can see a drawing of a Golgi to the right. Vacuole (Tupperware for the cell) Sometimes the cell doesn’t need what the organelles have produced right away. If you have extra food for a meal, sometimes it gets put in a plastic container and waits in your fridge until someone wants a snack. The cell also has containers for things it isn’t going to use immediately. They are called vacuoles or vesicles. They are just globs of some substance with a membrane around them. Plant cells have really large vacuoles that they use for storing water to maintain shape. Animal cells also have vacuoles but they are small compared to plant vacuoles. You can see how large the vacuole is in the plant cell above and to the left. 6 Lysosomes (Deadly vacuoles) Now, organelles don’t last forever, or even as long as a cell lives. Sometimes the cell takes in stuff that is dangerous to it. All these used up organelles and nasty poisonous stuff need to be broken down. That is the job of the lysosomes. Lysosomes are vacuoles with attitude. Like all vacuoles, the Golgi apparatus packages them. They contain digestive enzymes (chemicals) that break stuff down. The enzymes have to be kept inside a membrane or the chemicals would start eating perfectly healthy organelles and things the cell needs. When the chemicals are needed, the membrane fuses with the thing it is trying to destroy. This lets the enzymes to do their duty without damaging any of the valuable cell parts. The resulting smaller pieces are either used by the cell or passed out through the membrane. Both plants and animal cells have lysosomes. You can see a drawing of a lysosome being formed by the Golgi apparatus to the left. Centrioles (what do they do? Not really sure.) Just outside the nucleus in animal cell are two strange looking cylinders that hang out at right angles to each other. These little cylinders are called the centrioles or centrosomes. Just before the cell is ready to divide, the centrioles move to opposite sides of the nucleus. That’s what scientists do know for sure. Here’s where they aren’t quite so sure. When the chromosomes in the nucleus condense, little fibers called spindle fibers grow out of the centrioles and attach themselves to the chromosomes. The centrioles, they think, act as little anchors for the chromosomes to pull against to get to the opposite sides of the cell. When they get to the opposite sides the cell, the cell can divide. Why do animal cells have them and plant cells don’t? They don’t know. Centrioles are so tiny they were not even seen before the electron microscope was invented. They have only known about them for a short time so there is bound to be lots to learn. You can see a drawing of centrioles in the picture to the right. Cell wall (plants only, please) Plant cells, fungi cells, and some bacteria have a special outer coating that covers the cell membrane. This overcoat is called a cell wall seen to the left. It is thicker than a cell membrane and gives the cell lots of shape and protection. In plants its made out of cellulose, a carbohydrate that is indigestible for humans. When people eat lots of plants (fruits and veggies) the cellulose usually goes right through their digestive tract, pretty much cleaning out any thing in its path. People who don’t eat lots of fruits and veggies run the risk of being constipated. Yuck. In fungi, it is made out of another kind of carbohydrate called chitin. Strangely enough, lobsters and crabs use chitin to make their shells. Bacteria have cell walls 7 made out of other carbohydrates. Although, the cell wall is the outermost boundary, it does not decide what gets in or out of the cell. That is still the job of the cell membrane. Mitochondria (nuclear power plants for the cell) All of the activities of the cell take lots and lots of energy. Simple sugars can come in but they need to be broken down to have the energy released. The releasing energy job is done by an organelle called a mitochondrion (pl. mitochondria). You can see a mitochondrion in the picture to the right. Mitochondria look like little cocktail wieners. They have an outer membrane and an inner membrane that is folded. It is on the inner membrane where most of the energy is released. The mitochondria takes the released energy and packages it in small, portable, molecules called ATP. ATP can be used as an energy source in any reaction in the cell. What is more ATP from plant mitochondria can be used in animal cell reactions. In other words, ATP is a universal compound. If a cell is really active like a heart muscle cell, it will have more mitochondria than a cell that is less active like a skin cell. This makes sense because more activity requires more energy; to get more energy, you need more mitochondria to supply it. Chloroplast (energy transformers) Chloroplasts are only found in animal cells and other cells that carry on photosynthesis. You may remember photosynthesis changes light energy from the sun into chemical energy like sugar (glucose). There isn’t a cell that exists that can use light energy directly without changing it to chemical energy first. Chloroplasts look a great deal like green jellybeans. The have an outer membrane and an inner membrane that holds stacks of disks that contain chlorophyll. The chlorophyll is the thing that captures the light so it can be changed into chemical energy. For this to happen, the cell needs carbon dioxide and water to build the glucose molecule. Building the molecule requires energy that has to be put in. When the molecules are broken down in the mitochondria, the energy is released. You can see a picture of a chloroplast to the left. Cytoskeleton (dem bones, dem bones?) The cell membrane and cell wall in plants are not the only organelles that contribute to a cell shape. The cytoskeleton gives both plant and animal cell their shape. The cytoskeleton is a network of little protein tubes found in the cytoplasm. Using these tubules, the cell is able to change maintain its shape or change it if the need arises. Cells that move a great deal have more 8 substantial cytoskeletons as seen in the picture to the right. Cells that do not have to move much will have less substantial cytoskeletons. Flagella (whip it, whip it good) Some cells are not happy just floating along in the current of liquid they are sitting in. Some cells like to take charge of their destiny and are responsible for their own movement. One of the ways cells can move is by using an organelle called a flagellum (pl. flagella). It is a long taillike appendage that pokes out of the cell membrane and/or cell wall. Cells can have one or more flagella. Flagella are found in both plant and animal cells but not all plant and animal cells have them. Flagella help the cell to move by whipping back and forth. This usually does not make for a very smooth movement, but it gets them to go where they want. You can see a picture of flagella to the right. Cilia (row, row, row your boat) Other cells that move around do not have long flagella. Instead they have lots of little short hair-like appendages called cilia. The cilia usually cover the cell. Cilia are found mainly on animal cells but can occasionally be found on plant cells. Cilia move the cell along by beating in unison like men rowing a Viking ship. The cilia also move substances along the cell surface in stationary cells. Cells in your respiratory tract have cilia on them to move dirt and mucus to where it can be swallowed or coughed up. This is a very important function. If the dirt stays in your lungs it could cause infection and illness. So, coughing up a little phlegm is not such a bad thing. You can see a picture of cilia to the left.