Cell Structure
(a) state the resolution and magnification that can be achieved by a light microscope, a transmission electron microscope and a scanning electron microscope
Light Microscope – uses a number of lenses to produce an image that can be viewed directly at the eyepiece. Light passes from a bulb under the stage, through a condenser lens and then through the specimen. This beam of light is passed through an objective lens (x4, x10, x40) and then the eyepiece lens (x10).
Overall Magnification = Objective Lens Magnification x Eyepiece Lens Magnification
Transmission Electron Microscope (TEM) – uses electromagnets to focus a beam of electrons, which is transmitted through the specimen. Denser parts of the specimen absorb more electrons, which makes them look darker on the image you end up with creating a contrast.
Scanning Electron Microscope (SEM) – an electron beam is scanned across the specimen. The electrons don’t pass through the specimen, they bounce off and are detected at multiple detectors.
(b) explain the difference between magnification and resolution
- Magnification – the number of times greater an image is than the object itself
- Resolution – the ability to distinguish two separate points as distinct from each other
(c) explain the need for staining sample for use in light microscopy and electron microscopy
Material needs to be stained to allow the specimen to be seen under the microscope. Images are produced because some parts of the object absorb more light/electrons than others to create contrast, but sometimes the object being viewed is completely transparent – this makes the whole thing look white because the light rays/electrons pass straight through. This problem is solved by staining the specimen. Different types of stains bind to different specific cell structures:
- Acetic Orcein stains DNA dark red
- Genetian Violet stains bacterial cell walls
Some specimens are embedded in wax, to prevent distortion of the structure when being cut.
Electron microscopes are stained with heavy metals (like lead) to scatter the electrons and create contrast.
(e) describe and interpret drawings and photographs or eukaryotic cells as seen under an electron microscope
(f) outline the functions of the structures listed in (e)
(g) outline the interrelationship between the organelles involved in the production and secretion of proteins
- In the nucleus is the DNA needed to make proteins. The DNA from the nucleus is copied into a molecule called mRNA
- The mRNA leaves the nucleus and attaches itself to a ribosome on the rough endoplasmic reticulum
- The ribosome reads the instructions and uses the code to make proteins and is folded and processed in the rough endoplasmic reticulum
- The proteins are transported to the Golgi apparatus in vesicles where they fuse with it to modify the protein
- The protein leaves the Golgi apparatus in vesicles and fuses with the plasma cell membrane to be secreted.
(h) explain the importance of the cytoskeleton in providing mechanical strength of cells, aiding transport within cells and enabling cell movement
The cytoskeleton is the network of protein fibres found within cells that gives structure and shape of the cell, aids transport and enables cell movement. Its functions are:
- to support the cell’s organelles by keeping them in position
- to strengthen the cell and maintain its shape
- to transport materials within the cell
- to make the cells move (flagella and cilia)
Microfilaments (Actin) are proteins which cause movement and move some organelles around inside cells by moving against each other.
Microtubules are made of a protein called tubulin and these proteins will move organelles and other cell contents along the fibres using ATP to drive movement.
Flagella (Undulopodia) and cilia are both hair-like extensions that stick out from the surface of the cell. There are two microtubules in a central bundle surrounded by nine microtubules arranged in a circle. Flagella is longer than cilia and usually occur in ones or twos in cells. Cilia is less than 10µm long and occur in large numbers. Flagella enables movement of the cell and cilia causes sweeping movements to move substances across the surface of the cells.
(i) compare and contrast, with the aid of diagrams and electron micrographs, the structure of prokaryotic cells and eukaryotic cells
Eukaryotic Cells – (means having a true nucleus) cells contain many organelles some of which are bound by a membrane e.g. animals, plants and fungi
Prokaryotic Cells – (means before nucleus) cells lack membrane organelles such as a nucleus e.g. bacteria. These cells are about to 10,000 times smaller than eukaryotic cells
(j) compare and contrast, with the aid of diagrams and electron micrographs, the structure and ultrastructure of plant cells and animal cells
