1.1 The world of Microscopes - Light and Electron
Light and Electron Microscopes
The first light microscopes were developed in the mid 17th century. They use a beam of light to form an image of an object. They have magnifications of up to x2000 and are relatively cheap. They can also examine live specimens and have easy portability.
The electron microscope was created in the 1930’s, allowing for scientists to learn more about subcellular structures. They use a beam of electrons to form an image and have magnifications of up to x2,000,000. SEM give dramatic 3D images with lower resolutions than than transmission microscopes which give higher quality 2D images. Electron microscopes are large, expensive and must be kept in rooms that meet specific conditions. The specimens must also be dead and dehydrated.
1.1 The world of Microscopes - Magnification
- 1 km = 1000m
- 1m = 100cm
- 1cm = 10mm
- 1mm = 1000 micrometres
- 1micrometre = 1000nm
- Therefore, 1 nm = 1 x 10 -9 metre
The magnification of a microscope is simply to product of the magnifications of the objective and eyepiece lens.
Magnification = size of image / size of real object
Magnifying and Resolving Power
Resolution is the ability to distinguish between two separate points and it is the resolving power of a microscope that affects how much detail it can show. A light microscope has a resolving power of 200 nm while and SEM has one of 10 nm and a TEM has one of 0.2nm
1.2 Animal and plant cells
The average animal cell is between 10-30 micrometres long.
- Nucleus - Controls all activities of the cell and is surrounded by nuclear membrane. It contains the genes on the chromosomes that carry the instructions for making and building new cells or organisms.
- Cytoplasm - It is where the organelles are suspended and where all the chemical reactions take place.
- Cell membrane - Controls the passage of substances such as glucose and mineral ions as well as urea or hormones
- Mitochondria - Structures in the cell where aerobic respirations takes place, providing energy for the cell. They are found in the cytoplasm and are only 1-2 micrometres in length and 0.2-0.7 in diameter.
- Ribosomes - Where protein synthesis takes place, making proteins for the cell.
1.2 Animal and plant cells cont.
Plant cells are normally bigger than animal cells , ranging from 10-100 micrometres. They contain all the features/organelles of a typical animal cell but also have features that are needed for specific functions.
Algae are simple aquatic organisms that used to be classified as plants as they have very similar features. They are now classified as protista. All plant and algal cells have a cellulose cell wall, that strengthens and gives support to the cell.
- Chloroplasts - They are found in all green parts of a cell, given their green colour due to the fact that they contain chlorophyll. This substance absorbs light to make food through methods of photosynthesis. Each chloroplast is around 3-5 micrometres long.
- Permanent vacuole - Filled with cell sap in order to keep the cells rigid.
1.3 Eukaryotic and Prokaryotic Cells
Eukaryotic cells all have a cell membrane, cytoplasm, and genetic material that is enclosed in a nucleus. The genetic material is a chemical called DNA, forming structures called chromosomes that are contained within a nucleus. All animals, plants, fungi and protista are eukaryotes.
Bacteria are single celled organisms that are examples of prokaryotic cells. At 0.2-2 micrometres, prokaryotes are much smaller than eukaryotes. Bacteria have a cytoplasm and cell membrane as well as a cell wall, but this is not made up of cellulose like most plant cells. In prokaryotic cells, the genetic material is not enclosed in a nucleus but rather the bacterial chromosome is a single loop of DNA found free in the cytoplasm. They may also contain small rings of DNA called plasmids. These code for specific features such as antibiotic resistance.
Some bacteria may have a protective slime capsule around the outside of the cell wall. Some types of bacteria may also have at least one flagellum which is a strand of protein that lashes around for movement.