biology chapter 2

• Eukaryotic cells evolved from prokaryotic cells.
• Before the invention of microscopes, we knew nothing of bacteria, cells, sperm, pollen grains, chromosomes - the list is endless.
• Microscopes have given us the power to understand disease, see how a new life is formed, watch the dance of the chromosomes as cells divide, and manipulate the processes of life itself.
• A microscope is an instrument which enables you to magnify an object hundreds, thousands and even hundreds of thousands of times.
• We can see many large organisms with the naked eye, but microscopes open up a whole world of unicellular organisms.
• By making visible the individual cells which make up multicellular organisms, microscopes allow us to discover how details of their structures relate to their functions.
• The first types of microscopes to be developed were light microscopes in the 16th to 17th century.
• Since then they have continued to be developed and improved.
• By the mid-19th century, scientists, for the first time, had access to microscopes with a high enough level of magnification to allow them to see individual cells.
• Cell theory states that: both plant and animal tissue is composed of cells, cells are the basic unit of all life, and cells only develop from existing cells.
• Light microscopy continues to be important as it is easily available, relatively cheap and can be used out in the field, and it can be used to observe living organisms as well as dead, prepared specimens.
• The development of cell theory is an example of how scientific theories change over time as new evidence arises.
• Robert Hoke, an English scientist, observed the structure of thinly sliced cork using an early light microscope and described the compartments he saw as 'cells' - coining the term we still use today.
• Anton van Leeuwenhoek, a Dutch biologist, developed a technique for creating powerful glass lenses and used his handcrafted microscopes to examine samples of pond water, observing bacteria and protoctista and describing them as 'little animals' or 'animalcules' - today we call them microorganisms.
• Robert Brown, an English botanist, was the first to describe the nucleus of a plant cell.
• Matthias Schleiden, a German botanist, proposed that all plant tissues are composed of cells.
• Jan Purkyne, a Czech scientist, was the first to use a microtome to make ultra-thin slices of tissue for microscopic examination, and based on his observations, he proposed that not only are animals composed of cells but also that the "basic cellular tissue is clearly analogous to that of plants".
• Theodor Schwann, a German physiologist, made a similar observation and declared that "all living things are composed of cells and cell products".
• Opaque specimens can be illuminated from above with some microscopes.
• The objective lens produces a magnified image, which is magnified again by the eyepiece lens.
• Smear slides involve using the edge of a slide to smear the sample, creating a thin, even coating on another slide.
• Rudolf Virchow, a German biologist, published Robert Remak's findings as his own a decade later in 1855.
• Gram stain technique is used to separate bacteria into two groups, Gram-positive bacteria and Gram-negative bacteria.
• In basic light microscopy, the sample is illuminated from below with white light and observed from above (brightfield microscopy).
• Illumination is usually provided by a light underneath the sample.
• Robert Remak, a Polish/German biologist, was the first to observe cell division in animal cells, disproving the existing theory that new cells originate from within old cells.
• There are a number of different ways in which samples and specimens can be prepared for examination by light microscopy.
• Dry mount involves viewing solid specimens whole or cut into very thin slices with a sharp blade, this is called sectioning.
• To prepare a sample for staining, it is first placed on a slide and allowed to air dry, then heat-fixed by passing through a flame.
• Squash slides involve preparing a wet mount, then gently pressing down the cover slip with a lens tissue.
• The cytosol (aqueous interior) of cells and other cell structures are often transparent.
• This objective/eyepiece lens configuration allows for much higher magnification and reduced chromatic aberration than that in a simple light microscope.
• A compound light microscope has two lenses - the objective lens, which is placed near to the specimen, and an eyepiece lens, through which the specimen is viewed.
• Wet mount involves suspending specimens in a liquid such as water or an immersion oil, then placing a cover slip on from an angle.
• The whole sample is illuminated at once in wide-field microscopy.
• Louis Pasteur disproved the theory of spontaneous generation of cells by demonstrating that bacteria would only grow in asterile nutrient broth after it had been exposed to the air.
• Stains increase contrast as different components within a cell take up stains to different degrees.
• The large surface area of the folded membrane was considered to be an important site for the process of oxidative phosphorylation.
• Some species of bacteria may have mesosomes as part of their normal structure, but this is not the general consensus.
• In a Scanning Electron Microscope (SEM), a beam of electrons is sent across the surface of a specimen and the reflected electrons are collected, with a resolving power from 3-10nm, resulting in lower resolution but stunning three-dimensional images of surfaces.