Cells are ‘building blocks’ of life: all living things, whether plants, animals, people, or tiny microscopic organisms, are made up of cells. Although a cell is only about 10 micrometers across (one micrometer=one millionth of a meter!), there is still amazing complexity within it.
The plasma membrane around the cell is semi-permeable, meaning that some substances are able to enter the cell through it and some are not. Plant cells and some bacteria and algae cells have a protective cell wall in addition. Although animal cells don’t have a cell wall, they are protected by other cells, such as white blood cells that fight disease.
Inside the cell is a jelly-like fluid called cytoplasm that holds a cell’s organelles, special structures that perform specific cell functions. Some of the main organelles within the cell are the vacuoles, mitochondria, lysosomes, ribosomes, endoplasmic reticulum, Golgi apparatus, and cell nucleus. Think of organelles as being similar to the organs in your body: your heart, liver, and brain are all organs, performing specific functions to make your body work.
Most of these organelles are present in both animal and plant cells. The endoplasmic reticulum (ER) is important in the production or synthesis of cell components. Smooth ER produces lipids and membrane proteins, while rough ER (so called because it contains protein-producing ribosomes) produces all other proteins needed by the cell. These proteins are modified by the Golgi apparatus, which also stores and packages them for exportation from the cell. (You can think of the Golgi apparatus as a kind of shipping department in the cell.)
Vacuoles are the cell’s main storage units, holding food, water, or waste until it can be used or disposed of. Mitochondria are the ‘powerhouses’ of the cell, coverting nutrients to energy. Animal cells contain lysosomes that are responsible for the reactions that break down proteins, poly- and disaccharides, and some lipids. Your white blood cells use lysosomes to ‘eat’ disease with digestive enzymes.
The nucleus provides the ‘brains’ for this operation—the cell would be unable to do anything without it. The nucleus contains deoxyribonucleic acid, or DNA, which is the genetic material of life. Messenger ribonucleic acid, or RNA, is also important, as is makes a ‘negative’ copy (like the negative of a photograph) copy of the DNA and takes this information outside the nucleus to the ribosomes. At the ribosomes, transfer RNA ‘translates’ the code from the messenger RNA, allowing the ribosomes to form protein.
Eukaryotic cells, which include animal and plant cells, have a nucleus enclosed in membrane. Prokaryotic cells such as bacteria do not have a nuclear membrane; the genetic material is just clumped in the center of the cell instead.
Mitosisis asexual reproduction (without the union of male and female gametes) that takes place in cells. There are four stages to this process. In very simplified terms, the cell’s replicated DNA separates into two sets of identical chromosomes during prophase; the chromosomes are aligned down the center of the cell during metaphase; the duplicate chromosomes separate during anaphase; and in telephase, two identical copies—or clones—are formed out of what was one ‘mother’ cell, each with an identical set of chromosomes.
Sexual reproduction in cells, or meiosis, involves more stages and is much more complex, resulting in a new, unique combination of genetic material rather than making an identical copy. The two different kinds of reproduction serve two different purposes: mitosis for ‘easily’ reproducing large numbers of cells and then meiosis for creating unique individual creatures, seen most clearly in humans who are also uniquely made in his image.
Plant cells have the same parts as other cells do, but they also have a few special features. One of these features is a non-living protective shell called a cell wall. Only plant cells and some bacteria and algae have a cell wall. The cell walls are important to the overall structure of a plant: they provide rigidity in trees, for example.
Another difference between plant cells and animal cells is in plant cell vacuoles: the water that a plant takes in is held in the cell vacuoles and when the vacuoles are empty, the leaves go limp.
Leaf cells have another special feature: pigment-containing chloroplasts in certain cells enable them to produce energy and their own food through photosynthesis. What does that mean? Well, the chloroplasts within a cell contain different pigments, which are what gives a leaf its color. Green chlorophyll is the most common type of pigment, but there are also xanthophylls (yellow), cartenoids (yellow, orange), and anthocyanins (red). The chlorophylls usually hide the other pigments, except when autumn comes along and chlorophyll begins to break down. This is why leaves turn colors in the fall.
So then, what is photosynthesis? Simply put, it’s the capture of light energy to produce food. Light energy from the sun is transmitted through a leaf’s cells to chloroplasts, where chlorophyll and other absorbing pigments serve as receptors to collect the energy. In the process of photosynthesis, carbon dioxide from the air is converted into energy-rich carbon compounds called carbohydrates. As this happens, oxygen is given off into the air, providing the oxygen that we breathe.
You can test the importance of light energy in plant growth by doing a simple experiment. using 2-3 small plants. (Bean plants are a good choice, as they sprout quickly.) You’ll need one to be the control, with normal growing conditions, either outside in sunlight or inside by a bright window. See how light effects growth by covering the other test plants with a paper bag or small box during part of the day. Try covering one for four hours during the morning, and another for the whole day. Observe changes to the plants over the course of a week. Which grows the best? What is the result of light-deprivation?
To find out more about leaf pigments, do this next experiment. First, you’ll need to extract pigments from leaves. Collect several green leaves from different trees, a few from each one. Maples and others that have dramatic color changes in the fall will work best, but you can use any deciduous leaves (from trees that lose their leaves in the winter). Tear each set of leaves into several pieces and place them in a glass beaker or small drinking glass, then add just enough rubbing alcohol to cover them. (You can cover the containers with foil or plastic wrap to keep the alcohol from evaporating into the air.) Put the containers in a dish of hot tap water for about 30 minutes, until the alcohol turns green as the pigments from the leaves are absorbed into it.
Next, test to find out what colors are really present in a leaf. You’ll need coffee filters, filter paper, or chromatography paper for this part of the experiment. Cut a strip out of the middle of a coffee filter, about one inch wide, for each of the leaf sets that you want to test. Tape one end of the paper to a pencil or stick, and suspend it across the container, with the other end just touching the alcohol and pigment mixture. A bit of the mixture will travel slowly up the paper. After about 30-90 minutes you should be able to see the ‘green’ color break up into several different colors as the different pigments begin to separate. You’ll see different shades of green, and perhaps other colors as well. Which leaves had the most colorful pigments? Based on your experiment, which trees’ leaves do you think will turn the brightest and least brightest colors this fall? Try the experiment again with evergreen leaves or needles to compare the results.
Taking a Closer Look at Plant Cells
Learn even more about plants by studying different sections of real leaves. You can make your own microscope slide of a leaf section and view it under high power with a compound microscope to see cell detail. All you need is a fresh leaf specimen (use one without many holes or blemishes), a plain glass microscope slide, slide coverslip, sharp knife or razor blade, and water.
Before you begin, make sure the leaf is clean and dry. Lay it out flat on your working surface and slice about a 1” section crosswise out of the center using the knife. The cells surrounding the central vein of the leaf are what you will want to look at; so make sure you slice across a section of the vein. Then, starting at one of the short ends of the strip (the edges that you did not cut), tightly roll the leaf section. Carefully make several very thin slices off one end of the roll with a razor blade or knife. This is a ‘cross section’ of the leaf.
Make a wet mount on a plain slide with the inner part of the leaf section facing up (so the inner cells are visible). You can do this by adding a drop or two of water over the leaf section and then covering it with the coverslip. Look at the slide with your microscope’s 10x objective to see the general structure, and higher power to see cell detail. Record your observations on a copy of our free Microscope worksheet.
What are Bacteria?
You can run, but you can’t hide when it comes to bacteria. They can be found everywhere on earth, from Antarctica to the inside of your intestines! Some are good, aiding digestion and giving us tasty food like cheeses and yogurt. Many are harmful, causing serious diseases; these are called pathogenic bacteria. But what are bacteria?
Bacteria (or bacterium if you’re speaking of only one) are one-celled or unicellular microorganisms that don’t have chlorophyll and don’t have a distinct membrane-enclosed cell nucleus, like plant and animal cells do. Instead, the nuclear material—a single strand of DNA—is folded and clumped in the interior of the cell. Microorganisms that don’t have a distinct nuclear membrane are called prokaryotic organisms. Bacteria are classified in the kingdom Monera.
How are individual bacteria classified within the main kingdom? Scientists divide bacteria by shape: sphere, rod, and spiral. Spherical (round) bacteria include streptococcus, the cause of strep throat. Rod-shaped bacillus bacteria include anthrax and tetanus. Spiral bacteria have long bodies with a twist that form a spiral pattern when they are connected together; this group includes cholera.
Bacteria reproduce asexually, most commonly by binary fission, the division of a parent bacterium into two independent bacteria. This differs from the four stages of mitosis. In fission, a bacterium’s DNA forms a loop that attaches at a point on the cell wall. The DNA then replicates and the copy is attached to the cell wall near the first point. The wall then elongates till the two loops of DNA are far enough apart and then the two cells gradually separate. Fission occurs rapidly in as little as 20 minutes. Under perfect conditions a single bacterium could grow into over one billion bacteria in only 10 hours!
Some bacteria can also reproduce by forming endospores within the cell body. An endospore will only produce one new bacterium, but it’s very resistant to extended conditions of heat, cold, or dryness, and it is difficult to kill except with chemicals or high heat. All the species in the Bacillus genus of bacteria produce endospores.
Gram-positive and Gram-negative Bacteria
In 1884 Hans Christian Gram discovered that all types of bacteria could be divided into two different groups — ones that retained a stain (this kind is ‘gram-positive’) and ones that didn’t (‘gram-negative’).
Gram’s unique method for identifying these two groups became the first step in any bacterial identification process. Even the simple determination that a bacteria specimen is gram-positive or gram-negative can direct a doctor in diagnosis, as different bacteria cause different diseases. For example, the bacteria that causes scarlet fever is gram-positive, while that which causes typhoid or cholera is gram-negative. In addition, this classification process can help a doctor determine proper treatment, as some gram-negative bacteria are able to resist many common antibiotics.
So, how does it work? The stain will wash from a gram-negative cell because its cell wall contains more lipids (fatty substances) than a gram-positive cell. The washing solvent dissolves the lipid layer in gram-negative bacteria, allowing the color to be drawn from the cell. In contrast, the solvent causes the gram-positive cell wall to dehydrate, closing the pores and trapping the stain inside the cell.
Kingdom Protista: the Protozoans
They’re not aliens from another planet, in spite of the name! Protists are unicellular eukaryotic organisms: their cell nuclei are enclosed in membranes. They live in water (or watery tissues within the body, in the case of some diseases) and are classified in their own kingdom. You might have heard of some of these protists before: amoeba, euglena, paramecium, dinoflagellates, slime mold, and even most algae. Kingdom Protista seems to be the catch-all category of the cell world!
Protozoa use different kinds of movement: an amoeba uses amoeboid movement, flowing along with pseudopods, or temporary foot-like extensions. (This is also the way the white blood cells in our bodies move.) A euglena moves with a whip-like tail called a flagellum, and a paramecium uses tiny hairlike-threads called cilia on its body to propel it along.
Eating habits amongst protozoans vary, too. Some protists, such as euglena or volvox (a type of algae), use chloroplasts to generate energy through photosynthesis similar to the way plants do. Euglenas also serve as decomposers, by feeding off dead organisms. The amoeba, on the other hand, engulfs its prey with its pseudopodia and brings the food into its food vacuole (a sac where the microorganism’s food is stored until digested). A paramecium sweeps its food down an oral groove lined with cilia, into a gullet that closes off when full and becomes a food vacuole.
Many diseases are caused by protozoa, often transmitted through drinking bad water or through an insect bite. Sleeping sickness, malaria, dysentery, and Giardia (an intestinal disease) are all caused by protozoa.
You can observe protozoa by taking a sample of pond water and viewing it under magnification. A compound microscope is necessary to see any individual protozoa, although you can see the largest colonies of protozoa, such as volvox, with just a 30x stereo microscope. Scoop a cup or so of pond water (or water from a puddle or river) into a jar. You should view the protozoa specimens within 24 hours, as the composition of the sample changes over time. Some pond water specimens, such as daphnia, hydra, and planaria, are visible without magnification, since they are multicellular. However, all protists are too tiny to see without magnification of at least 100x. (If you don’t live by a pond, you can use our protozoa hatchery kit to grow your own specimens.)
What kind of detail do you see? Can you identify different kinds of protists? What physical characteristics (like flagella or pseudopods) can you see?
Fighting Disease: Antibiotics and Vaccines
Our bodies are specially designed with an immune system to fight disease, but sometimes there’s too much infection for us to fight alone. Antibiotics destroy bacteria cells within a person or animal’s body, without harming normal cells. They are often able to cure once-fatal diseases, such as the bacterial infection scarlet fever. Amoxicillin, penicillin, and erythromycin are common antibiotics that inhibit bacterial cell functions. Antibiotics are derived primarily from bacteria or fungi (mold), such as Penicillium.
Antibiotics don’t work exclusively against bacteria: some ‘broad-spectrum’ ones are also effective against protists. Malaria is a disease caused by protozoa that are carried by certain mosquitoes. Antibiotics, such as doxycycline, can be used in both treatment and prevention of malaria.
In some cases where an antibiotic is used to treat a disease, the disease-causing bacteria or protozoa will develop resistance to the drug, meaning that that particular antibiotic will no longer be effective at destroying the resistant organisms. Antibiotic research is a continuous process, since the need often develops for bigger and better antibiotics to wipe out resistant diseases.
Antibiotics won’t work on viruses. You need vaccines to prevent viral diseases such as hepatitis or polio. A vaccine is a weakened form of a disease, which produces antibodies when injected in a person or animal. These antibodies allow the immune system to recognize and attack a stronger form of the disease.
Viruses: Dead or Alive?
We’ve all had a virus at one time or another, whether it was the flu, chicken pox, or something worse. But what is a virus? It has two main characteristics: its genetic material (DNA or RNA) is encased in a protective protein coat, and it is unable to reproduce itself the way cells can. Because of this inability, viruses are considered to be non-living.
So how does a virus work? When it infects a body or plant, the virus will inject its genetic material into a cell, taking over the cell’s protein production. It uses its genetic ‘code’ to direct the cell to replicate the virus, until the cell has so many copies of the virus that it ruptures, sending the infection to other cells.
Thankfully, our bodies have been equipped with ‘fighter’ cells, like white blood cells that can devour some viruses and other diseases!
Developing the penicillin antibiotic was a group effort by some British scientists. In 1928 Alexander Fleming discovered that a certain type of mold, Penicillium notatum, prevented the growth of staphylococci bacteria. However, it wasn’t until a decade later that research on penicillin as an antibiotic began, headed up by Howard Florey and Ernst Chain. One of their biggest challenges was producing a sufficient amount of pure penicillin to test effectively.
Then Norman Heatley joined the research. He developed ways to grow penicillin on a larger scale (it grew well in hospital bedpans!) as well as to produce purer penicillin. In 1940 the group tested the antibiotic properties of penicillin on eight laboratory mice. The mice were injected with disease-causing bacteria and then four of the mice were given penicillin. Only the four untreated mice died from the disease. Florey hailed the results as a miracle.
Production was still a problem, though, especially since England was at war and funding was tight. Florey and Chain took it to the U.S., where the government (after the bombing of Pearl Harbor) encouraged medical companies to mass produce penicillin. By the end of WWII, there was enough to treat all of the wounded Allied soldiers.
In 1945, Fleming, Chain, and Florey received the Nobel Prize for their contribution to science.
Pre-1600: In the 11th century, the Arab Alhazan described the use and characteristics of glass lenses. Two hundred years later, the English natural philosopher Roger Bacon was familiar with lenses. Eyeglasses, however, were not invented until the late 1200s.
1600s: In 1608 the telescope was invented, with Galileo improving upon it with his own models. Around 1600, the microscope was invented, possibly by Hans and Zacharias Jansen. Due to poor lens quality, the early compound microscopes (ones that used two lenses) could only magnify an object up to 20 or 30 times its normal size.
The first big microscopy advances came in 1665, when Robert Hooke published the Micrographia, a collection of copper-plate illustrations of objects he had observed with his own compound microscope. He coined the term ‘cell’ when looking at a piece of cork under 30x.
In the late 1660s, Antony van Leeuwenhoek began to grind his own lenses and make simple microscopes. Each microscope was really a powerful magnifying glass rather than a compound microscope. Leeuwenhoek’s hand-ground lenses could magnify an object up to 200 times! He observed animal and plant tissue, sperm cells and blood cells, minerals, fossils, and much more. He also discovered nematodes and rotifers (microscopic animals), and he discovered bacteria while looking at different samples of plaque from his own and others’ teeth.
1700-1800s: Not much change in the basic microscope design occurred, but better lenses were crafted (using purer glass and different shapes) to solve problems like color distortion and poor image resolution. In the late 1800s, Ernst Abbe discovered that oil-immersion lenses prevented light distortion at highest magnification power.
- 1900s till now: In 1931, a pair of German scientists invented the electron microscope. This kind of microscope directs a beam of speeded-up electrons at a cell sample; as the electrons are absorbed or scattered by different parts of the cell, they form an image that can be captured by an electron-sensitive photo plate. This model enables scientists to view extremely small parts, magnified as much as one million times. The only drawback is that living cells can’t be observed with electron microscopes. However, compound microscopes are being improved with digital and other new technology, making microscopy better for everyone from homeschooled kids to microbiologists.
See cool black & white images plus learn more about how an electron microscope works at the Scanning Electron Microscopy page.
For a look at the incredible variety of bacteria, visit the Cyanobacterial Image Gallery.