Bio - Topic 2 - Cells

Definition of cell

The smallest unit of life

The theory of ‘spontaneous generation’

People used to believe that living organisms could spontaneously appear from non-living matter

Cell theory

All living things are made of individual units, cells, which are the basic units of life, and that all cells arise from other cells.

Features of Prokaryotic Cells

Nucleus: None. DNA floats in an open area called the nucleoid.

Structure: Simple and very small. No membrane-bound parts.

Organisms: Always single-celled (unicellular), like bacteria and archaea.

DNA: A single loop or strand

Features of Eukaryotic Cells

Nucleus: Yes. DNA is safely stored inside a membrane.

Structure: Complex and larger. Has tiny working parts (organelles) like mitochondria.

Organisms: Can be single-celled or multi-celled, like plants, animals, fungi, and protists.

DNA: Multiple straight lines

Features of both Prokaryotic Cells and Eukaryotic Cells

Both have DNA as genetic material.

Both have ribosomes to make proteins.

Both are filled with cytoplasm (jelly fluid).

Both have an outer cell membrane

Animalia cells

Multicellular eukaryotes without a cell wall. Animals are holozoic (eat other organisms with internal digestion of nutrients). The animal kingdom is the largest known kingdom with 1 million known species.

Fungus

Fungi are eukaryotes with a cell wall made with a molecule called chitin. Can be unicellular (eg. yeast) or multicellular (mushrooms)

Light microscopes

microscopes that pass light through a specimen and then use lenses to magnify the image produced

Electron microscope

Electron microscopes pass a beam of electrons through a specimen. Electrons will be absorbed by the denser parts of the sample, and scattered or able to pass through less dense areas, after which they are picked up by an electron detector and used to form an image.

How much can electron microscopes magnify very small objects

electron microscopes can magnify very small objects by about 500 000 times

Freeze fracture microscopy

Freezing a sample and then using a specialised tool to break the sample into small pieces which are then observed using an electron microscope to see the internal structure. This is a particularly useful technique for being able to visualise structures that are not normally visible, such as the internal plasma membrane.

Cryogenic electron microscopy

involves freezing a sample to cryogenic temperatures to fix the molecules, making them more firm or stable. The specimen is then viewed using electron microscopy. Freezing the sample improves the resolution of the image formed and reduces damage that may occur from the electron beam.

Immunofluorescence

A fluorescent tag, called a fluorophore, is attached to antibodies specific for antigens on a structure or cell being viewed. When the antibody binds to the antigen, the structure is then ‘tagged’ with immunofluorescence. When a certain wavelength of light is shone onto the fluorescence tag, the tag will emit light of a different wavelength that can then appear as brightly coloured spots, allowing the visualisation of the location of these target molecules.

Fluorescent dyes

Fluorescent dyes can be used in light microscopy. When the dye is added to the sample it will preferentially attach to certain structures. As in immunofluorescence, the labelled areas will appear as brightly coloured spots, allowing visualisation of the target molecule throughout the specimen.

Resolution of a microscope

Resolution is the shortest distance between two separate points in a microscope’s field of view that can still be distinguished as separate objects. The higher the resolution, the smaller this distance is and the clearer the image produced. Remember that a resolution of 0.1 nm is higher than a resolution of 200 nm. The lower the value, the higher the resolution.

True or False: Viruses can be viewed using a light microscope.

False