Biology Flashcards
What makes a good biological drawing?
- good use of space
- clear strong lines
- label lines are straight
- labels clearly written
- scale bar if needed
- lines touch the labelled structure
- no shading
What is ‘cell theory’?
it is a scientific theory which describes the properties of cells. These cells are the basic unit of structure in all organisms and also the basic unit of reproduction.
What is ‘classical cell theory’?
Classical cell theory included the following ideas:
· All organisms are made up of one or more cell(s).
· Cells are the fundamental and structural unit of life.
· All cells come from preexisting cells.
What is ‘modern cell theory’ and it’s 2 exceptions?
The key points of modern cell theory include:
· The cell is the fundamental unit of structure and function in living things.
· All cells come from preexisting cells by division.
· Cells contain hereditary information (DNA) which is passed from cell to cell during cell division.
· All cells are basically the same in chemical composition.
· All known living things are made up of cells.
· Some organisms are unicellular, made up of only one cell.
· Others organisms are multicellular, composed of countless number of cells.
Exceptions to the theory
· Viruses are considered by some to be alive, yet they are not made up of cells.
· The first cell did not originate from a pre-existing cell.
What is an electron microscope?
a microscope with high magnification and resolution, employing electron beams in place of light and using electron lenses.
How is the electron microscope different to a light microscope?
- light microscopes are smaller and lighter so are easier to set up
- light microscopes are less expensive
- electron microscopes use beams of electrons
- beams of electrons can be focused using electromagnets due to negative charge of electrons
- they have a much higher resolution
- higher magnification
- electron microscopes are in grey resolution
- electron microscopes use fluorescent screen
How are the images collected on electron microscope?
On photographic film or screens
Why must there be a vacuum inside the electron microscope?
The vacuum inside an electron microscope is important for its function. Without a vacuum, electrons being aimed at the sample would be deflected (knocked off course) when they hit air particles. But liquid water, which is abundant in biological samples, evaporates immediately in a vacuum. If this happened, a biological sample would vaporize in front of your eyes!
Define magnification
Magnification is how much bigger a sample appears to be under the microscope than it is in real life.
Define resolution
Resolution is the ability to distinguish between two points on an image i.e. the amount of detail
Facts about the resolution
The resolution of an image is limited by the wavelength of radiation (electrons or light) used to view the sample.
This is because when objects in the specimen are much smaller than the wavelength of the radiation being used, they do not interrupt the waves, and so are not detected.
The wavelength of light is much larger than the wavelength of electrons, so the resolution of the light microscope is a lot lower.
Using a microscope with a more powerful magnification will not increase this resolution any further.
It will increase the size of the image, but objects closer than 200nm will still only be seen as one point.
Light microscopy
Light Microscopy: This is the oldest, simplest and most widely-used form of microscopy. Specimens are illuminated with light, which is focussed using glass lenses and viewed using the eye or photographic film. Specimens can be living or dead, but often need to be stained with a coloured dye to make them visible. Many different stains are available that stain specific parts of the cell. All light microscopes today are compound microscopes, which means they use several lenses to obtain high magnification. Light microscopy has a resolution of about 200 nm, which is good enough to see cells, but not the details of cell organelles.
Electron microsope
Electron Microscopy. This uses a beam of electrons, rather than light , to “illuminate” the specimen. This may seem strange, but electrons behave like waves and can easily be produced (using a hot wire), focused (using electromagnets) and detected (using a screen or photographic film). A beam of electrons has an effective wavelength of less than 1 nm, so can be used to see small sub-cellular ultrastructure. The development of the electron microscope in the 1930s revolutionised biology, allowing organelles such as mitochondria, ER and membranes to be seen in detail for the first time.
The main problem with the electron microscope is that specimens must be fixed in plastic and viewed in a vacuum, and must therefore be dead. Other problems are that the specimens can be damaged by the electron beam and they must be stained with an electron-dense chemical (usually heavy metals like osmium, lead or gold). Initially there was a problem of artefacts (i.e. observed structures that were due to the preparation process and were not real), but improvements in technique have eliminated most of these.
There are two kinds of electron microscope. The transmission electron microscope (TEM) works much like a light microscope, transmitting a beam of electrons through a thin specimen and then focusing the electrons to form an image on a screen or on film. This is the most common form of electron microscope and has the best resolution. The scanning electron microscope (SEM) scans a fine beam of electron onto a specimen and collects the electrons scattered by the surface. This has poorer resolution, but gives excellent 3-dimentional images of surfaces
