The purpose of this lab is to introduce you to scientific measurement and give you a sense of scale regarding the organisms, cells, and structures that we will be examining this semester.
We will be considering the diversity of life, from unicellular prokaryotes and acellular viruses to gigantic multicellular eukaryotes. The largest animal can reach sizes of about 30 meters in length (blue whale, Balaenoptera musculus). Plants, algae, and fungi can grow even larger. The largest known fungus, an Armillaria soldipes colony in Oregon, is 3.8km in diameter. A marine plant in the Mediterranean, Posidonia oceanica, is around 8km in length.
On the other end of the scale, many organisms we will study this semester can be observed only under a microscope. Cellular organisms can be as small as 200nm in diameter (Mycoplasma genitalium). Viruses can be even smaller, beyond the resolution of a light microscope. The smallest viruses, such as porcine circovirus, are less than 20nm in diameter.
You may wish to visit the following interactive resource from the Genetic Science Learning Center at the University of Utah to get a sense of the scale of cells and their components: http://learn.genetics.utah.edu/content/cells/scale/
We will also consider cellular ultrastructure this semester, and will therefore review some of the major organelles and their sizes. If you have forgotten the names and functions of the organelles of the cell, it would be useful to review them early in the course.
In biology, understanding three-dimensional structures is extremely important. This is a difficult thing to grasp from a textbook or lecture, since cells and other structures are necessarily represented as flat. This lab is designed to help you to visualize cells in three dimensions.
Metric System (a review)
Scientists use the metric system and you should be familiar with the units most relevant to biological systems. You should also be comfortable converting between multipliers of the metric system. You should be familiar with metric units of length, area, volume, mass, temperature, and energy.
|area/surface area||square meter||m2|
|temperature||degree Celsiusor kelvin||°CK|
These units (apart from temperature) are modified with prefixes that indicate multiples of the unit. These are shown on the table below. The prefixes you will see most often in biology are in bold.
For this lab, we will be measuring cells using the microscope. Biological structures range in length from kilometers to nanometers. In Lab 1, you should have calculated that the smallest possible resolution of a light microscope is about 200 nm. Scientists use an electron microscope to study structures smaller than 200 nm. The electron microscope uses a beam of electrons instead of light, and resolution can be as good as 0.1 nm.
Transmission electron microscopy(TEM) directs a beam of electrons through a specimen that has been sliced into thin sections. The sections are often treated with a heavy metal such as osmium to improve contrast. An electron beam is transmitted through the specimen. Some electrons that travel through the specimen and make contact with a screen on the other side, resulting in an image.
Scanning electron microscopy (SEM) instead scans the surface of a cell or structure with a beam of electrons that results in an image showing the surface topography of a specimen. Electrons are either backscattered, or excite electrons in the sample, which are emitted and then captured to produce an image. The figure above shows the difference between images produced by TEM and SEM
A central concept in biology is that of variation. Populations of cells or organisms vary in their traits. Variation can be apparent to naked eye – for example, coloration or size – or it might be more difficult to detect, as in the case of genetic, biochemical, or behavioral variation. We can study variation at any level of biological organization, from molecules to ecosystems. This lab is concerned with variation at the cellular level. Cell structure correlates with cell function and variations in structure can inform our understanding of function.
Within a multicellular organism, cells vary widely according to function. Most of them do not look very much like the flat, generalized cartoon of a cell found in every biology textbook. They vary in size, shape, and number and type of organelles. We will examine several cell types from both unicellular and multicellular organisms in this course, and today’s lab will familiarize you with some of the variability between cell types, as well as variation within a single cell type.
Populations and measurements of central tendency and variability
When we measure variation, it is useful to have a way to describe the central tendency of the population as well as the variability. Central tendency is most often expressed as the mean, or average. Median is a useful description of central tendency for populations that do not follow a normal distribution (if the population is normally distributed, the mean and median will be the same).
However, two populations with the same mean can be very different in their variance. A simple way to express variance is the range. This is simply the difference between the highest and lowest number in a dataset. Although simple, the range is often of limited utility, particularly when the number of observations is small and/or there are outliers.
A more common way to express variance is as standard deviation. A biologist would most likely calculate standard deviation using a spreadsheet. It is not difficult to calculate by hand or a simple calculator, but it is time-consuming.