Science Articles

Article / Updated 09-22-2022

Each quantum state of the hydrogen atom is specified with three quantum numbers: n (the principal quantum number), l (the angular momentum quantum number of the electron), and m (the z component of the electron’s angular momentum, How many of these states have the same energy? In other words, what’s the energy degeneracy of the hydrogen atom in terms of the quantum numbers n, l, and m? Well, the actual energy is just dependent on n, as you see in the following equation: That means the E is independent of l and m. So how many states, |n, l, m>, have the same energy for a particular value of n? Well, for a particular value of n, l can range from zero to n – 1. And each l can have different values of m, so the total degeneracy is The degeneracy in m is the number of states with different values of m that have the same value of l. For any particular value of l, you can have m values of –l, –l + 1, ..., 0, ..., l – 1, l. And that’s (2l + 1) possible m states for a particular value of l. So you can plug in (2l + 1) for the degeneracy in m: And this series works out to be just n2. So the degeneracy of the energy levels of the hydrogen atom is n2. For example, the ground state, n = 1, has degeneracy = n2 = 1 (which makes sense because l, and therefore m, can only equal zero for this state). For n = 2, you have a degeneracy of 4: Cool.

Article / Updated 09-21-2022

The mapping of skin receptors to a specific area of neocortex illustrates one of the most fundamental principles of brain organization, cortical maps. The projection from the thalamus is orderly in the sense that receptors on nearby parts of the skin project to nearby cortical neurons. The figure shows a representation of the skin map on the somatosensory cortex. The fundamental idea about a given area of cortex being devoted to receptors in a given skin area is that activity in this area of cortex is necessary for the perception of the skin sense (along with other parts of the brain). You perceive activity in that bit of cortex area as a skin sensation not because that area has some special skin perceiving neurons, but because it receives inputs from the skin and has outputs that connect to memories of previous skin sensation and other associated sensations. In other words, the perception produced by activity in this and other areas of cortex is a function of what neural input goes to it and where the outputs of that area of cortex go. This skin map on the cortex is called a homunculus, which means "little man." However, the surface area of the somatosensory cortex onto which the skin receptors project is not really a miniature picture of the body; it is more like a band or strip, as established in the studies by Canadian neurosurgeon Wilder Penfield. Because of the difficulty in mapping a three-dimensional surface onto a two-dimensional sheet (think of how two-dimensional maps of the earth compare to the three-dimensional globes), the image is distorted, depending on choices that the "map-maker" makes about what is relatively more important to represent accurately versus what is less. Also note that some areas of the body, such as the hands and fingers, are located in the cortex map close to areas such as the face that are actually quite distant, body-wise. Some researchers suggest that the phantom limb feelings (including pain) that sometime occur after an amputation may occur because some neural projections from the face invade the part of the cortex that was being stimulated by the limb and cause sensations to be perceived as being located there, even when the limb is gone.

Cheat Sheet / Updated 09-21-2022

Whether you're talking about evolution — or any other element of science — you should understand the process of scientific investigation, which proves or disproves a scientific theory. Take a look at a chart of our hominid ancestors as discovered through fossil records, and learn some key terms to grasp the course of evolution.

Article / Updated 09-01-2022

Celebrate everything that is beautiful about our planet by reconnecting with nature, learning more about the natural environment, or picking up a new eco-friendly habit or two. April 22 is Earth Day — a world-wide celebration commemorating everything that makes our blue-marble planet unique and beautiful, and all the things we can do to protect it. Starting all the way back during the environmental movement of the 1970s, Earth Day has grown to become a powerful motivator for individuals and companies alike to become more environmentally aware and responsible. From local volunteer cleanups to nation-wide conservation initiatives, this year is no different, with numerous events happening at both the grassroots and state levels. Want to join in on the celebration? Find Earth Day events in your area, by visiting https://www.earthday.org/ The First Earth Day Did you know... In response to public outcry to the Santa Barbara Oil Spill of 1969, U.S. Sen. Gaylord Nelson organized a nationwide "teach-in" about environmental issues to take place on April 22, 1970. More than 2,000 colleges and universities, 10,000 public schools, and 20 million citizens participated. Aside from volunteering or donating to an eco-friendly cause, there are plenty of other ways to celebrate Earth Day — here are just a few. Plant a healthy diet You’ve probably already heard about the massive environmental impact that animal farming has on the planet. It is second only to fossil fuels in terms of contributing to human-made greenhouse gas emissions, the cause of climate change. And, while the jump from a traditional omnivorous diet to a full-blown vegan one is not something most people can accomplish overnight, cutting down on red meat is a great first step. Something as simple as committing to meat-free Mondays can have a significant influence on your health and the environment. Get started by taking a crack at one or two of our favorite plant-based recipes found in Plant-Based Diet Cookbook For Dummies. Channel your inner green thumb You don’t need to become an expert horticulturist to help save our planet. Simply buying local or better yet, growing some of your own food can help reduce your carbon footprint — not to mention, teach you valuable transferable skills like diligence and patience. Plus, there’s just something so satisfying about working with your hands, especially if you’re used to working in front of a screen all day. You can start by growing some common herbs like rosemary or thyme. They are fairly resilient and, depending on where you live, can even be grown indoors. If you lack the space to garden, look for farmers markets in your area or, better yet, join a community garden, these gardens are becoming increasingly popular, even in the most urban of cities. Don't worry if you have no previous gardening knowledge — beginner-friendly resources, such as Gardening Basics For Dummies, will help you get there. You can also find a farmers market near you using the USDA National Farmers Market Directory. Stay informed It’s no secret that our planet is in grave danger as a result of climate change. But, contrary to what you may feel after reading all the increasingly worrisome headlines, you must remember — there’s still hope. And, while the biggest contributors of climate change, like animal farming and fossil fuel burning, may seem so far removed from your daily life, you’d be surprised how much of a difference you as an individual can make. Staying informed is perhaps the easiest way to help environmental causes. As an informed citizen, you have the power to choose more wisely what products to buy, what companies and practices to support, and even who you vote for in the next election. Don't know where to start? Check out Climate Change For Dummies to help you navigate this complex topic. More ways to greenify your life There’s always something more we can do to reduce our environmental impact on the planet, but that should not discourage us from taking action — after all, every little bit counts. One small change in your daily routine today will lead you to more and more lasting changes in the future. If you’re ready to explore even more ways to go green, check out Green Living For Dummies for a more comprehensive guide to sustainable living. From the team at Dummies, we wish all our fellow earthlings a happy and green Earth Day.

Article / Updated 08-17-2022

General relativity was Einstein’s theory of gravity, published in 1915, which extended special relativity to take into account non-inertial frames of reference — areas that are accelerating with respect to each other. General relativity takes the form of field equations, describing the curvature of space-time and the distribution of matter throughout space-time. The effects of matter and space-time on each other are what we perceive as gravity. Einstein immediately realized that his theory of special relativity worked only when an object moved in a straight line at a constant speed. What about when one of the spaceships accelerated or traveled in a curve? Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational field. As Einstein later related the discovery, he was sitting in a chair thinking about the problem when he realized that if someone fell from the roof of a house, he wouldn’t feel his own weight. This suddenly gave him an understanding of the equivalence principle. As with most of Einstein’s major insights, he introduced the idea as a thought experiment. If a group of scientists were in an accelerating spaceship and performed a series of experiments, they would get exactly the same results as if sitting still on a planet whose gravity provided that same acceleration, as shown in this figure. Einstein’s brilliance was that after he realized an idea applied to reality, he applied it uniformly to every physics situation he could think of. For example, if a beam of light entered an accelerating spaceship, then the beam would appear to curve slightly, as in the left picture of the following figure. The beam is trying to go straight, but the ship is accelerating, so the path, as viewed inside the ship, would be a curve. By the principle of equivalence, this meant that gravity should also bend light, as shown in the right picture of the figure above. When Einstein first realized this in 1907, he had no way to calculate the effect, other than to predict that it would probably be very small. Ultimately, though, this exact effect would be the one used to give general relativity its strongest support.

Article / Updated 08-17-2022

General relativity was Einstein’s theory of gravity, published in 1915, which extended special relativity to take into account non-inertial frames of reference — areas that are accelerating with respect to each other. General relativity takes the form of field equations, describing the curvature of space-time and the distribution of matter throughout space-time. The effects of matter and space-time on each other are what we perceive as gravity. The theory of the space-time continuum already existed, but under general relativity Einstein was able to describe gravity as the bending of space-time geometry. Einstein defined a set of field equations, which represented the way that gravity behaved in response to matter in space-time. These field equations could be used to represent the geometry of space-time that was at the heart of the theory of general relativity. As Einstein developed his general theory of relativity, he had to refine the accepted notion of the space-time continuum into a more precise mathematical framework. He also introduced another principle, the principle of covariance. This principle states that the laws of physics must take the same form in all coordinate systems. In other words, all space-time coordinates are treated the same by the laws of physics — in the form of Einstein’s field equations. This is similar to the relativity principle, which states that the laws of physics are the same for all observers moving at constant speeds. In fact, after general relativity was developed, it was clear that the principles of special relativity were a special case. Einstein’s basic principle was that no matter where you are — Toledo, Mount Everest, Jupiter, or the Andromeda galaxy — the same laws apply. This time, though, the laws were the field equations, and your motion could very definitely impact what solutions came out of the field equations. Applying the principle of covariance meant that the space-time coordinates in a gravitational field had to work exactly the same way as the space-time coordinates on a spaceship that was accelerating. If you’re accelerating through empty space (where the space-time field is flat, as in the left picture of this figure), the geometry of space-time would appear to curve. This meant that if there’s an object with mass generating a gravitational field, it had to curve the space-time field as well (as shown in the right picture of the figure). Without matter, space-time is flat (left), but it curves when matter is present (right). In other words, Einstein had succeeded in explaining the Newtonian mystery of where gravity came from! Gravity resulted from massive objects bending space-time geometry itself. Because space-time curved, the objects moving through space would follow the “straightest” path along the curve, which explains the motion of the planets. They follow a curved path around the sun because the sun bends space-time around it. Again, you can think of this by analogy. If you’re flying by plane on Earth, you follow a path that curves around the Earth. In fact, if you take a flat map and draw a straight line between the start and end points of a trip, that would not be the shortest path to follow. The shortest path is actually the one formed by a “great circle” that you’d get if you cut the Earth directly in half, with both points along the outside of the cut. Traveling from New York City to northern Australia involves flying up along southern Canada and Alaska — nowhere close to a straight line on the flat maps we’re used to. Similarly, the planets in the solar system follow the shortest paths — those that require the least amount of energy — and that results in the motion we observe. In 1911, Einstein had done enough work on general relativity to predict how much the light should curve in this situation, which should be visible to astronomers during an eclipse. When he published his complete theory of general relativity in 1915, Einstein had corrected a couple of errors and in 1919, an expedition set out to observe the deflection of light by the sun during an eclipse, in to the west African island of Principe. The expedition leader was British astronomer Arthur Eddington, a strong supporter of Einstein. Eddington returned to England with the pictures he needed, and his calculations showed that the deflection of light precisely matched Einstein’s predictions. General relativity had made a prediction that matched observation. Albert Einstein had successfully created a theory that explained the gravitational forces of the universe and had done so by applying a handful of basic principles. To the degree possible, the work had been confirmed, and most of the physics world agreed with it. Almost overnight, Einstein’s name became world famous. In 1921, Einstein traveled through the United States to a media circus that probably wasn’t matched until the Beatlemania of the 1960s.

Article / Updated 08-11-2022

Environmental science is all about finding ways to live more sustainably, which means using resources today in a way that maintains their supplies for the future. Environmental sustainability doesn’t mean living without luxuries but rather being aware of your resource consumption and reducing unnecessary waste. Reduce household energy use. Energy conservation is itself a source of energy. Here are several simple ways to reduce your household energy use: Turn off appliances and lights that you’re not using. Install energy-efficient appliances. Use a programmable thermostat that lowers or raises the temperature when you’re not home. Set your thermostat lower than usual in the winter and bundle up. Open windows to allow a breeze instead of turning on the air conditioning. Hang clothes to dry instead of using the dryer. Use an electric teakettle rather than a stovetop kettle to boil water. Replace incandescent light bulbs with compact fluorescent bulbs (CFLs). Eat locally. A powerful way to live more sustainably is to eat locally. The convenience of supermarkets has changed how people think about food. You can stroll through aisles stocked with fruits, vegetables, and other products from all over the world any time of year. But these products consume huge amounts of fossil fuel energy to get from those global locations to your corner supermarket. Dispose with disposables. Previous generations didn’t dream of single-use razors, forks, cups, bags, and food storage containers, but these days, you can find a plastic version of almost any object and then throw that object away after you use it. Many of the environmental health issues today stem from toxins released into the environment by trash. Even trash that’s properly disposed of, such as that in a landfill, requires careful monitoring to ensure that dangerous chemicals don’t enter the surrounding environment. When you make a purchase, consider the item’s life expectancy: How long can the item be used? Will it have more than one use? When you’re done with it, will it end up in the trash? Start investing in reusable products for the items you most often throw away. Plant seeds. Try growing your own food. Simply plant a few seeds in a corner of your yard or in a container on your porch or windowsill. You don’t need acres; a few square feet on a patio, along the driveway, or in a window box can provide enough space to grow edible herbs, fruits, and vegetables. Recycle. Recycle as much as possible! If your neighborhood or apartment complex doesn’t offer recycling pickup, either find a drop-off location or request the curbside service. Buying products labeled post-consumer lets companies know that recycling is the way to go! For other items, such as CFLs, batteries, cellphones, and electronics, find an appropriate recycler. Be sure to ask electronics recyclers where these materials go for recycling and avoid companies that ship electronic waste overseas for unregulated “recycling” and salvage operations. Goodwill Industries International is one place that accepts electronics for responsible recycling. Resell and donate items. Items that you no longer need can get an extended life through resale and donation. By extending the life of any product, you help reduce dependence on disposable or cheaply made single-use products that end up in landfills. Try reselling clothing and children’s things through a secondhand or consignment retailer or consider donating them to a nonprofit resale organization (such as Goodwill) or charity organization (such as the Salvation Army or American Cancer Society) that will redistribute them to those in need. Drink from the tap. Dependence on bottled water has added more than a million tons of plastic to the waste stream every year. One reason people rely on bottled water is because they believe it’s safer and better tasting than tap water. But most municipal water supplies in the U.S. provide safe, clean, fresh water (and many bottled waters are just bottled from city water supplies anyway). If you don’t like the flavor of your tap water, consider the one-time investment in a filtration system. If you like the convenience of bottled water, purchase refillable bottles and keep one in your fridge, one in your car, and one at the office. Encourage your employer to install filters and offer glasses or reusable bottles at work, too. Save water. An easy way to live more sustainably is to conserve household water use. Consider installing water-efficient toilets or dual-flush toilets that let you choose whether to use a full flush (for solid waste) or half-flush (for liquid waste). Newer clothes washers can automatically sense the smallest level of water needed for each load. Smaller changes, such as switching to water-saving shower heads and adding aerators to your sink faucets, are also effective ways to significantly reduce household water use. To conserve water outdoors, use landscaping adapted to your local environment. When buying plants, look for drought-tolerant species and varieties and be sure to plant them in proper soil and sun conditions to reduce their need for excess watering. Set up sprinkler systems so they don’t water the sidewalk, the driveway, and other paved, impermeable surfaces. Rely less on your car. Using fossil fuels to support one person in each car on the road is clearly no longer sustainable. Investigate mass transit options in your town or city, such as a bus system, a light rail train system, or carpool and vanpool services for commuters. When traveling close to home, walk or ride your bike. Purchase fair-trade products. When you purchase items that are imported from all over the world — particularly coffee, cocoa, sugar, tea, chocolate, and fruit — look for the fair-trade certification. This designation tells you that these items were grown using sustainable methods of agriculture and that local people are receiving fair prices for the goods they produce. Items that don’t have the fair-trade certification may have been produced unsustainably and may be the product of exploitative labor practices that don’t benefit the local people.

Article / Updated 08-10-2022

All eukaryotic cells have organelles, a nucleus, and many internal membranes. These components divide the eukaryotic cell into sections, with each specializing in different functions. Each function is vital to the cell's life. The plasma membrane is made of phospholipids and protein and serves as the selective boundary of the cell. The nucleus is surrounded by a nuclear envelope with nuclear pores. The nucleus stores and protects the DNA of the cell. The endomembrane system consists of the endoplasmic reticulum, the Golgi apparatus, and vesicles. It makes lipids, membrane proteins, and exported proteins and then “addresses” them and ships them where they need to go. Mitochondria are surrounded by two membranes and have their own DNA and ribosomes. They transfer energy from food molecules to ATP. Chloroplasts are surrounded by two membranes, contain thylakoids, and have their own DNA and protein. They transform energy from the sun and CO2 from atmosphere into food molecules (sugars). The cytoskeleton is a network of proteins: actin microfilaments, microtubules, and intermediate filaments. Cytoskeletal proteins support the structure of the cell, help with cell division, and control cellular movements.

Article / Updated 08-10-2022

The eukaryotic cells of animals, plants, fungi, and microscopic creatures called protists have many similarities in structure and function. They have the structures common to all cells: a plasma membrane, cytoplasm, and ribosomes. All eukaryotic organisms contain cells that have a nucleus, organelles, and many internal membranes. With all the wonderful diversity of life on Earth, however, you’re probably not surprised to discover that eukaryotic cells have many differences. By comparing the structure of a typical animal cell with that of a typical plant cell, you can see some of the differences among eukaryotic cells. Cell walls, additional reinforcing layers outside the plasma membrane, are present in the cells of plants, fungi, and some protists, but not in animal cells. Chloroplasts, which are needed for photosynthesis, are found in the cells of plants and algae, but not animals. Large, central vacuoles, which contain fluid and are separated from the cytoplasm with a membrane, are found in the cells of plants and algae, but not animals. Centrioles, small protein structures that appear during cell division, are found in the cells of animals, but not plants. Home office: The nucleus The nucleus houses and protects the cell’s DNA, which contains all of the instructions necessary for the cell to function. The DNA is like a set of blueprints for the cell, so you can think of the nucleus as the office where the blueprints are kept. If information from the blueprints is required, the information is copied into RNA molecules and moved out of the nucleus. The DNA plans stay safely locked away. The boundary of the nucleus is the nuclear envelope, which is made of two phospholipid bilayers similar to those that make up the plasma membrane. The phospholipids bilayers of the nuclear envelope are supported by a scaffold of protein cables, called the nuclear lamina, on the inner surface of the nucleus. The nuclear envelope separates the contents of the nucleus from the cytoplasm. The structures within the nucleus are DNA in the form of chromosomes or chromatin: When a cell is about to divide to make a copy of itself, it copies its DNA and bundles the DNA up tightly so that the cell can move the DNA around more easily. The tightly bundled DNA molecules are visible through a microscope as little structures in the nucleus called Most of the time, however, when a cell is just functioning and not about to divide, the DNA is very loose within the nucleus, like a bunch of long, very thin spaghetti noodles. When the DNA is in this form, it is called chromatin. Nucleoli where ribosomal subunits are made: Information in the DNA needs to be read in order to make the small and large subunits needed to build ribosomes. The cell builds the ribosomal subunits in areas of the nucleus called nucleoli. Then, the cell ships the subunits out of the nucleus to the cytoplasm, where they join together for protein synthesis. When you stain cells and look at them under the microscope, nucleoli look like large spots within the nucleus. The DNA plans for the cell are kept in the nucleus, but most of the activity of the cell occurs in the cytoplasm. Because the DNA is separate from the rest of the cell, a lot of traffic crosses back and forth between the nucleus and the cytoplasm. Molecules enter and exit the nucleus through small holes, called nuclear pores, that pass through the nuclear membrane. Groups of proteins organize into little rings that penetrate through the nuclear envelope to form the nuclear pores. The traffic in and out of the nuclear pores include the following: RNA molecules and ribosomal subunits made in the nucleus must exit to the cytoplasm. Proteins made in the cytoplasm but needed for certain processes, such as copying the DNA, must cross into the nucleus. Nucleotides, building blocks for DNA and RNA, must cross into the nucleus so that the cell can make new DNA and RNA molecules. ATP molecules that provide energy for processes inside the nucleus like assembly of DNA molecules. Traffic through the nuclear pores is controlled by proteins called importins and exportins. Proteins that are to be moved into or out of the nucleus have specific chemical tags on them that act like zip codes, telling the importins and exportins which way to move the protein with the tag. The movement of molecules into and out of the cell requires the input of energy from the cell in the form of adenosine triphosphate (ATP). Post office: The endomembrane system The endomembrane system, shown in the following figure, of the eukaryotic cell constructs proteins and lipids and then ships them where they need to go. Because this system is like a large package-shipping company, you can think of it as the post office of the cell. The endomembrane system has several components: The endoplasmic reticulum is a set of folded membranes that begins at the nucleus and extends into the cytoplasm. It begins with the outer membrane of the nuclear envelope and then twists back and forth like switchbacks on a steep mountain trail. The endoplasmic reticulum comes in two types: Rough endoplasmic reticulum (RER) is called rough because it’s studded with ribosomes. Ribosomes that begin to make a protein that has a special destination, such as a particular organelle or membrane, will attach themselves to the rough endoplasmic reticulum while they make the protein. As the protein is made, it’s pushed into the middle of the rough ER, which is called the Once inside the lumen, the protein is folded and tagged with carbohydrates. It will then get pushed into a little membrane bubble, called a transport vesicle, to travel to the Golgi apparatus for further processing. Smooth endoplasmic reticulum (SER) doesn’t have attached ribosomes. It makes lipids — for example, phospholipids for cell membranes. Lipids from the SER may also travel to the Golgi apparatus. The Golgi apparatus looks a little bit like a stack of pancakes because it’s made of a stack of flattened membrane sacs, called cisternae. The side of the stack closest to the nucleus is called the cis face of the Golgi, whereas the side farthest from the nucleus is called the trans Molecules arrive at the cis face of the Golgi and incorporate into the nearest cisterna. Lipids become part of the membrane itself, while proteins get pushed into the middle, or lumen, of the cisterna. The Golgi apparatus constantly changes as new cisternae form at the cis face, and old cisternae are removed from the trans face. As molecules make their journey through this flowing system, they’re modified and marked with chemical tags, so that they’ll get shipped to their proper destination. Vesicles are little bubbles of membrane in the cell and come in several types: Transport vesicles carry molecules around the cell. They’re like the large envelopes that you put your letters in. Transport vesicles travel from the ER to the Golgi and then to the plasma membrane to bring molecules where they need to go. They travel by gliding along protein cables that are part of the cytoskeleton. Lysosomes are the garbage disposals of the cell. They contain digestive enzymes that can break down large molecules, organelles, and even bacterial cells. Secretory vesicles bring materials to the plasma membrane so that the cell can release, or secrete, the materials. Peroxisomes are small organelles encircled by a single membrane. Often, they help break down lipids, such as fatty acids. Also, depending on the type of cell they are in, peroxisomes may be specialists in breaking down particular molecules. For example, peroxisomes in liver cells break down toxins, such as the ethanol from alcoholic beverages. In plants cells, glyoxisomes, a special kind of peroxisome, help convert stored oils into molecules that plants can easily use for energy. Altogether, the endomembrane system works as a sophisticated manufacturing, processing, and shipping plant. This system is particularly important in specialized cells that make lots of a particular protein and then ship them out to other cells. These types of cells actually have more endoplasmic reticulum than other cells so that they can efficiently produce and export large amounts of protein. As an example of how the endomembrane system functions, follow the pathway of synthesis and transport for an exported protein: A ribosome begins to build a protein, such as insulin, that will be exported from the cell. At the beginning of the protein is a recognizable marker that causes the ribosome to dock at the surface of the rough endoplasmic reticulum. The ribosome continues to make the protein, and the protein is pushed into the lumen of the RER. Inside the lumen, the protein folds up, and carbohydrates are attached to it. The protein is pushed into the membrane of the RER, which pinches around and seals to form a vesicle, and the vesicle carries the protein from the RER to the Golgi. The vesicle fuses with the cis face of the Golgi apparatus, and the protein is delivered to the lumen of the Golgi, where the protein is modified. The protein eventually leaves in a vesicle formed at the trans face, which travels to the plasma membrane, fuses with the membrane, and releases the protein to the outside of the cell. The fireplace: Mitochondria The mitochondrion (see the following figure) is the organelle where eukaryotes extract energy from their food by cellular respiration. Mitochondria are like the power plants of the cell because they transfer energy from food to ATP. ATP is an easy form of energy for cells to use, so mitochondria help cells get usable energy. Part of the process that extracts the energy from food requires a membrane, so mitochondria have lots of internal folded membrane to give them more area to run this process. Mitochondria actually have two membranes, the outer membrane and the inner membrane. The inner membrane is the one that is folded back and forth to create more area for energy extraction; the folds of this membrane are called cristae. The outer membrane separates the interior of the mitochondrion from the cytoplasm of the cell. The two membranes of the mitochondrion create different compartments within the mitochondrion: The space between the two membranes of the mitochondrion is the intermembrane space. The inside of the mitochondrion is the Mitochondria also contain ribosomes for protein synthesis and a small, circular piece of DNA that contains the code for some mitochondrial proteins. The ribosomes and DNA of mitochondria resemble those found in bacterial cells. In the kitchen: Chloroplasts Chloroplasts, shown in the following figure, are the place where eukaryotes make food molecules by the process of photosynthesis. Chloroplasts are found in the cells of plants and algae. Like mitochondria, chloroplasts have two membranes, an inner membrane and an outer membrane. In addition, they have little sacs of membranes called thylakoids stacked up in towers called grana. The multiple membranes of the chloroplast divide it into several different spaces: The intermembrane space is between the inner and outer membranes. The central, fluid-filled part of the chloroplast is called the The interior of the thylakoid is another fluid-filled space. Like mitochondria, chloroplasts contain their own ribosomes for protein synthesis and a small, circular piece of DNA that contains the code for some chloroplast proteins. Scaffolding and railroad tracks: The cytoskeleton The structure and function of cells are supported by a network of protein cables called the cytoskeleton, shown in the following figure. These proteins underlie membranes, giving them shape and support, much like scaffolding can support a building. Cytoskeletal proteins run like tracks through cells, enabling the movement of vesicles and organelles like trains on a railroad track. When cells swim by flicking whip-like extensions called cilia and eukaryotic flagella, they’re using cytoskeletal proteins. In fact, you use cytoskeletal proteins literally every time you move a muscle. Cytoskeletal proteins come in three main types, with each one playing a different role in cells: Microfilaments are made of the protein Microfilaments are the proteins that make muscle cells contract, help pinch animal cells in two during cell division, allow cells like amoebae to crawl, and act as railroad tracks for organelles in some types of cells. Microtubules are made of the protein tubulin. Microtubules are the proteins inside of cilia and flagella. They move chromosomes during cell division and act as railroad tracks for the movement of vesicles and some organelles. Intermediate filaments are made of various proteins. They often act as reinforcing proteins. For example, the protein lamin that strengthens the nuclear membrane is an intermediate filament. Likewise, the keratin that strengthens your skin cells and makes them resistant to damage is an intermediate filament. You can easily mix up the words “microtubules” and “microfilaments.” Remember that “microtubules” are made of “tubul-in,” and they’re found in the “tube-shaped” cilia and flagella. (Okay, I’m stretching it on that last bit, but if it helps to remember it. . . .) Motor proteins Actin microfilaments and microtubules are long, cable-like proteins. They partner with motor proteins, proteins that use ATP to “walk” along the cables by repeatedly binding, changing shape, and releasing. Thus, the motor proteins use chemical energy to do cellular work in the form of movement. Several motor proteins work with microfilaments and microtubules: Myosin often acts as a partner to actin. For example, when myosin walks along actin microfilaments in muscle cells, it causes the actin microfilament to slide. The sliding of actin microfilaments is what causes muscle contraction. Myosin also attaches to cellular components, such as chloroplasts in plant cells, and then walks along microfilaments. The movement of the motor proteins causes the cellular components to flow around the cell in a process called cytoplasmic streaming. Dynein partners with microtubules inside of cilia and eukaryotic flagella. When dynein walks along microtubules on one side of a cilium or flagellum, it causes the microtubules to bend. The bending of different parts of cilia and flagella makes them flick back and forth like little whips. Kinesin is another partner with microtubules. One end of the kinesin molecule attaches to vesicles, while the other end walks along the microtubules. The movement of kinesin causes the vesicles to slide along the microtubules like freight cars on a railroad track. Cilia and flagella Cilia and flagella are essentially the same structure, but cilia are typically shorter and more numerous on the surface of the cell whereas flagella are typically longer in length and fewer in number. Cilia are found on cells that make up the surfaces of tissues, such as cells in the respiratory and genital tracts of humans, where the cilia beat to move fluid and materials along the surface. For example, in the human respiratory tract, the beating of cilia moves mucus upward where you can cough it out of the body. Some cells, such as microscopic protists and sperm cells, swim using cilia and flagella. The internal structure of cilia and flagella is distinctive. If you cut a cilium or a flagellum crosswise and look at the circular end with an electron microscope, you’ll see the same pattern of microtubules in in both cilia and flagella, shown in the following figure. The microtubules are grouped in pairs, called doublets, that are similar to two drinking straws laid tightly together side by side. The microtubules appear in a 9+2 arrangement, where nine pairs of microtubules (nine doublets) are arranged around the outside of the circle, while one pair of microtubules is in the center of the circle. Rebar and concrete: Cell walls and extracellular matrices The plasma membrane is the selective boundary for all cells that chooses what enters and exits the cell. However, most cells have additional layers outside of the plasma membrane. These extracellular layers provide additional strength to cells and may attach cells to neighboring cells in multicellular organisms. Typically, these layers are composed of long cables of carbohydrates or proteins embedded in a sticky matrix. The long, cable-like molecules work like rebar in concrete to create a strong substance. Two main types of extracellular layers support eukaryotic cells: Cell walls are extra reinforcing layers that help protect the cell from bursting. Among eukaryotes, cell walls appear around the cells of plants, fungi, and many protists. The primary cell walls of plants and algae are made of cellulose. If the plant is a woody plant, lignin is also present. (Lignin is a complex molecule that hardens the cell walls of plants.) Fungal cell walls are made of chitin. The layer around animal cells is the extracellular matrix (ECM), shown in the following figure. This layer is made of long proteins, such as collagen, embedded in a polysaccharide gel. The ECM supports animal cells and helps bind them together. Animal cells actually attach themselves to the ECM via proteins, called integrins, that are embedded in the plasma membrane. The integrins bind to the actin microfilaments inside the cell and to ECM proteins called fibronectins that are outside the cell.

Cheat Sheet / Updated 08-04-2022

Biology is the study of the living world. All living things share certain common properties: They are made of cells that contain DNA; they maintain order inside their cells and bodies; they regulate their systems; they respond to signals in the environment; they transfer energy between themselves and their environment; they grow and develop; they reproduce; they have traits that have evolved over time.