Proteins

Question 1

Light microscopes use a pair of convex glass lenses that can resolve images that are 0.2um

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apart. The reason for this is that this is wavelength of light and therefore restricts the

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resolution that a light microscope resolve to. This is compared to electron microscopes which

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can distinguish between items 0.1nm apart.

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The magnification of an image as seen through a microscope can be calculated using the

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following equation:

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Magnification = size of image/size of real object

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Resolution is defined as the minimum distance apart that two objects can be distinguished as

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separate objects in an image. The greater the resolution the more clear the image will be.

Question 10

Electron Microscopes

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The limitation of light microscopes only resolving to a resolution of 0.2um means that electron

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microscopes can be used to look at objects that are closer than 0.2um apart. There are two

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main types of electron microscope

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these are transmission electron microscopes (TEM) and

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scanning electron microscopes (SEM). Electron microscopes work in a similar way to light

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microscopes

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but instead use a beam of electrons that are focused by electromagnets inside a

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vacuum environment. The vacuum environment is needed so that particles in the air do not

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deflect the electrons out of the beam alignment.

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The following details how each type of electron microscope works:

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1.Transmission Electron Microscope - a beam of electrons passes through a thin section of a

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specimen. Areas that absorb the electrons appear darker on the electron micrograph that is

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produced.

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2.Scanning Electron Microscope - in a scanning electron microscope a beam of electrons passes

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across the surface and scatter. The pattern of scattering builds up a 3D image depending on

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the contours of the specimen.

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There are some limitations though when using ellectron microscopes

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the limitations for SEM and

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TEM are:

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- The whole system must be in a vacuum so living specimens cannot be observed.

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- A complex staining process is required which may introduce artefacts into the image.

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- Specimens have to be very thin

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particularly for TEM so that the electrons can pass through.

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- SEM has a lower resolving power than TEM

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but both have greater resolving power than a light

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microscope.

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Cell Fractionation and Ultracentrifugation

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Cell fractionation is the process in which different parts and organelles of a cell a separated so

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that they can be studied in detail. The most common method of cell fractionation is differential

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centrifugation.

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The process of homogenation is detailed below:

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1.The cells are first blended in an homogeniser forming the resultant fluid called the

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homogenate. This tube of homogenate is then placed in a centrifuge and spun at a slow

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speed.

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2.The heaviest organelles

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the nuclei

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3.The fluid at the top

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called the supernatant

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the nuclei. The supernatant is then transferred to another tube and spun at a slightly faster

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speed. This time the pellet that forms contains the next heaviest organelle

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the

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mitochondria.

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4.This process continues so that each time the speed is increased the next heaviest organelle is

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sedimented and separated out.

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The homogenate at the beginning is placed in a cold

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buffered solution of the same water

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potential as the cells. This is to prevent the organelles from bursting under osmotic pressure

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to inactivate any enzymes from breaking down organelles and so that the pH does not

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fluctuate.Nucleus is a double membrane called the envelope containing ~3000 nuclear pores

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that enables molecules to enter and leave. It also contains chromatin and a nucleolus

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which is the site of ribosome production. A granular jelly like material called

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nucleoplasm makes up the bulk of the nucleus.

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• Rough endoplasmic reticulum is a series of flattened sacs enclosed by a membrane with

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ribosomes on the surface. RER folds and processes proteins made on the ribosomes.

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• Smooth endoplasmic reticulum is a system of membrane bound sacs. SER produces and

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processes lipids.

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• Golgi apparatus is a series of fluid filled

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flattened & curved sacs with vesicles

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surrounding the edges. Golgi apparatus processes and packages proteins and lipids. It

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also produces lysosomes.

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• Mitochondria are oval shaped

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bound by a double membrane called the envelope. The

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inner membrane is folded to form projections called cristae with a matrix on the inside

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containing all the enzymes needed for respiration. Centrioles are hollow cylinders containing a ring of microtubules arranged at right

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angles to each other. Centrioles are involved in producing spindle fibres for cell division.

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• Ribosomes are composed of two sub units and are the site of protein production.

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• Lysosomes are vesicles containing digestive enzymes bound by a single membrane.

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Prokaryotic cells such as bacteria contain:

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• Cell wall – Rigid outer covering

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made of peptidoglycan.

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• Capsule – Protective slimy layer

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which helps the cell to retain

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moisture and adhere to surfaces.

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• Plasmid – Circular piece of DNA.

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• Flagellum - a tail like structure

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which rotates to move the cell.

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• Pili - Hair-like structures which

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attach to other bacterial cells.

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• Ribosomes - Site of protein production.

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• Mesosomes - Infoldings of the inner membrane which contain enzymes required for

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respiration.

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Viruses are non-living structures which consist of nucleic acid (either DNA or RNA) enclosed in

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a protective protein coat called the capsid

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sometimes covered with a lipid layer called the

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envelope.

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Cells of multicellular organisms are organised into tissues

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tissues into organs and organs into systems.