Hello to all my readers. I’ll be starting today’s discussion from where I stopped in my last article on WHAT’S IN A CELL? I will be talking about the different types of microscopes: the light and the electron microscopes and other interesting stuffs about the features of eukaryotic cells.
THE LIGHT MICROSCOPE
This instrument is also known as a compound microscope because two lenses, the eyepiece lens and the objective lens, are combined to produce a much greater magnification than is possible with a single lens. The total magnification is calculated by multiplying the magnification of the two lenses together. For example, if the eyepiece lens has a magnification of ×10 and the objective lens is ×50, the total magnification is ×500.
The light microscope has powers of magnification of up to ×1500, good enough to see cells, larger organelles and individual bacteria, but not powerful enough to reveal smaller structures such as plasma membranes, viruses or individual molecules.
An important feature of a microscope is its resolving power, which should not be confused with the magnification. Two objects close together may appear as one single image when viewed under the light microscope. Increasing the magnification does not allow you to resolve the two objects into separate images; the objects just appear to be a larger single image. Resolving power is as important as magnification when investigating structural details.
The limitation of the light microscope is due to the nature of light itself. The wavelength of light determines the maximum effective magnification and the resolving power. The wavelength of visible light is around 500-650 nm and the resolving power – the resolution – of the light microscope is 200 nm (0.2 μm), so two objects separated by less than 200 nm appear as one object.source
THE ELECTRON MICROSCOPE
The electron microscope (EM) was invented in the 1930s. Today’s modern transmission EMs can magnify up to 50 million times and can resolve two objects that are less than 78 picometres apart (one picometre is one trillionth of a metre). They can visualize individual atoms. The development of the EM has had a huge impact on biology. Organelles inside cells can be seen in great detail and new ones have been discovered.
While the light microscope uses lenses to focus a beam of light, the EM uses electromagnets to focus a beam of electrons. The wavelength of the electrons is much smaller than the wavelength of light, so the resolving power of the EM is much greater than the light microscope.
The main disadvantage of the EM is that the electron beam must travel in a vacuum because, being so small, electrons are scattered when they hit air molecules. Specimens for the EM must therefore be prepared (killed, dehydrated and fixed) so that they retain their structure inside a vacuum. Such harsh preparation methods can damage cells, and cause artefacts – features that do not exist in the living cell-to appear. For example, microsomes, tiny vesicles surrounded by ribosomes, were seen when animal cells were examined using the electron microscope. At first, cell biologists thought that these were organelles they had not noticed before, but they later realised that microsomes were fragments of endoplasmic reticulum produced by the fixing process.source
SCANNING AND TRANSMISSION ELECTRON MICROSCOPES
There are now several types of EMs, including the transmission EM, the scanning EM, the reflective EM, the scanning transmission EM and the tunnelling EM.
In a transmission electron microscope (TEM), a beam of electrons is transmitted through the specimen. The specimen must be thin and it is stained using electron-dense substances such as heavy metal salts. These substances deflect electrons in the beam and the pattern that the remaining electrons produce as they pass through the specimen is converted into an image.source
The scanning EM and the tunnelling EM are the most useful in biology. Scanning EMs record the electrons that bounce off the surface of an object, so they produce stunning three-dimensional images that can provide a wealth of information.
The scanning tunnelling microscope (STM) is completely different to a scanning electron microscope. It scans an electrical probe over a surface and picks up the weak electric current that flows between the probe and the surface. The STM was invented in 1981 by Gerd Binnig and Heinrich Rohrer who worked for IBM at the time. They won the Nobel Prize for their development of the STM in 1986.
The STM is special because it allows scientists to ‘see’ individual molecules and even atoms. The three-dimensional images produced can show the individual atoms within a crystal lattice, for example. This kind of EM can resolve to a distance of 0.2 nanometres and it is interactive. It can be used to manipulate atoms, control or trigger chemical reactions or add or remove electrons. It is mainly used in the physical sciences but has been used to visualize the surface of cell membranes.source
Now, let’s talk about each of the features of an animal cell as viewed under both the light and electron microscopes. Starting with the plasma membrane…..
THE PLASMA MEMBRANE
The plasma or cell surface membrane is the boundary between the cell and its environment. It has little mechanical strength but plays a vital role in controlling which materials pass in and out of the cell. Although basically a double layer of phospholipid molecules arranged tail to tail, the plasma membrane is a complex structure, studded with proteins. These can be embedded in the membrane or they can penetrate the bilayer forming pores (holes or channels) through which molecules can pass.source
The nucleus is the largest and most prominent organelle in the cell. Almost all eukaryote cells have a nucleus – red blood cells in mammals and phloem cells in plants are exceptions. Every nucleus is surrounded by a nuclear envelope. This consists of two membranes that are separated by a gap of 20 to 40 nm.
The nucleus is usually spherical and about 10 μm in diameter. It contains the cell’s DNA, which carries information that allows the cell to divide and carry out all its cellular processes. When the cell is not actively dividing, the DNA is spread throughout the nucleus as chromatin. Close examination of the chromatin reveals two different levels of density. Dark-staining chromatin, consisting of tightly packed DNA, is known as heterochromatin; the lighter, more loosely packed material is called euchromatin. Euchromatin contains the DNA that is being actively read to produce proteins; in heterochromatin, the DNA is packed together, and is not being read.
Individual segments of DNA called genes contain the information necessary to make individual proteins, including the enzymes that control most of the cell’s activities. In fact, a central concept in biology that is true for all cells, prokaryotes and eukaryotes, is that: many genes code for making enzymes that, in turn, control the activities of the cell.
When a cell is dividing, its chromosomes become visible. Nuclei also have one or more nucleoli. These dark-staining, spherical structures are ribosome-producing centres: they synthesise ribosomal RNA and package it with ribosomal proteins to make ribosomes.source
The nuclear envelope joins with the membrane of the endoplasmic reticulum (ER), a system of complex tunnels that are spread throughout the cell.
On much of the outside surface of the ER in a eukaryotic cell are the sites of attachment for ribosomes. This gives it a grainy appearance and its name, rough ER.
The main function of rough ER is to keep together and transport the proteins made on the ribosomes. Instead of simply diffusing away into the cytoplasm, newly made proteins are threaded through pores in the membrane and accumulate in the space called the ER lumen. Here, they are free to fold into their normal three-dimensional shape. Not surprisingly, a mature cell that makes and secretes large amounts of protein – such as one that makes digestive enzymes – has rough ER that occupies as much as 90 per cent of the total volume of the cytoplasm. The rough ER is also a storage unit for enzymes and other proteins.source
Small vesicles containing newly synthesised proteins pinch off from the ends of the rough ER and either fuse with the Golgi body or pass opposite directly to the plasma membrane.
ER with no ribosomes attached is known as smooth ER. Smooth ER tends to occur in small areas that are not continuous with the nuclear membrane. Smooth ER is not involved in protein synthesis but is the site of steroid (lipid hormone) production. It also contains enzymes that detoxify, or make harmless, a wide variety of organic molecules, and it acts as a storage site for calcium in skeletal muscle cells.
Liver cells contain large amounts of smooth ER. The enzymes on the inner surface of the smooth ER help the body cope with the sudden influx of large amounts of alcohol – as in binge drinking. The smooth ER can double its surface area and its enzymes within a few days to cope with the extra demand. It returns to normal when the alcohol has been dealt with. Drinking too much and too often can overwhelm this process and can cause permanent liver damage.source
Ribosomes are small, dense organelles, about 20 nm in diameter, present in great numbers in the cell. Most are attached to the surface of rough ER but they can occur free in the cytoplasm. The impression of protein synthesis shows the ribosome’s distinctive shape. Ribosomes are made from a combination of ribosomal RNA and protein (65 per cent RNA : 35 per cent protein).source
Ribosomes are Involved in protein synthesis. They assemble amino acids in the right order to produce new proteins. The ribosome uses the code on messenger RNA (mRNA) to put amino acids together in chains to form specific proteins. This is another central concept in Biology that you will learn more about later on.
A gene is a Piece of DNA that codes for a particular Protein. A copy of the gene in the form of messenger RNA passes out of the nucleus and travels to the ribosome where it controls protein synthesis.
Generally, proteins that are to be used inside the cell are made on free ribosomes, while those that are to be secreted out of the cell are made on ribosomes that are bound to ER membranes.source
THE GOLGI BODY
The Golgi body is a tightly-packed group of flattened cavities or vesicles. The whole organelle is a shifting, flexible structure; vesicles are constantly being added at one side and lost from the other. Generally, vesicles fuse with the forming face (the one nearest to the nucleus) and leave from the maturing face (the one nearest to the plasma membrane).source
The Golgi body appears to be involved with the synthesis and modification of proteins, lipids and carbohydrates. Studies have shown that proteins made on the ribosomes attached to ER are packaged into vesicles by the ER. Some of the vesicles join with the Golgi body, and the proteins they contain are modified before they are secreted out of the cell.source
Lysosomes are small vesicles 0.2- 0.5 μm in diameter that contain a mixture of digestive enzymes called lytic enzymes. It is important that the membrane of lysosomes remains intact because if the enzymes leak out, they could digest vital molecules in the cell.source
Why do cells need such potentially lethal structures? They have several uses:
- to supply the enzymes which destroy old or surplus organelles;
- to digest material taken into the cell. After a white cell has engulfed bacterium, for example, lysosomes discharge enzymes into the vacuole and digest the organism. This process is called phagocytosis;
- to destroy whole cells and tissues. Parts of tissues and organs often need to be removed after they have performed their function. The muscle of the uterus is reduced after giving birth and milk-producing tissue is destroyed after breast-feeding finishes. Bone is also constantly made and reabsorbed throughout life.
Lysosomes are similar in structure to many other vesicles in the cell and are thought to be made in the same way: inactive digestive enzymes from the rough ER pass through the Golgi body where they are activated and packaged into vesicles.source
Mitochondria are relatively big, individual organelles that occur in large numbers in most cells. They are usually spherical or elongated (sausage-shaped) and are 0.5-1.5 μm wide and 3-10 μm long. Their function is to make ATP (adenosine triphosphate) by aerobic respiration. ATP is a molecule that diffuses around the cell and provides instant chemical energy to the processes that require it.source
Mitochondria have a double membrane; the outer membrane is smooth while the inner one is folded. This arrangement gives a large internal surface area on which the complex reactions of aerobic respiration can take place. Mitochondria are particularly abundant in metabolically active cells, tissues such as muscle and tissues involved in active transport.source