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OCR Biology - Microscopes, Organelles and Cell Signalling.

Updated on January 7, 2013

Exam Tips!

This hub will briefly go over some of the topics in OCR's January 'Cells, Exchange and Transport' exam and outline key facts and information that you must know!
To study topics in more detail I have made a number of other hubs listed below:

Electron Microscope.
Electron Microscope.
Light Microscope.
Light Microscope.

Microscopes

Definitions:

  • Magnification is the degree to which the size of the image of an object is larger than the object itself.
  • Resolution is the degree to which you can distinguish between two objects that are close together.


Scanning Electron Microscopes:

  • An electron beam is directed onto a sample but does not pass through it.
  • The image produced is 3D image of the surface of the sample.
  • Scanning electron microscopes have a maximum resolving power of 0.1nm and a maximum magnification of x100 000.


Transmission Electron Microscopes:

  • An electron beam is directed onto and passes through a very thin prepared sample.
  • Electrons pass through denser parts of the sample less easily.
  • The image produced is 2D.
  • Transmission electron microscopes have a maximum resolving power of 0.1nm and a maximum magnification of x500 000.


Light Microscopes:

  • They use a number of lenses that produce an image to be directly viewed at the eyepieces.
  • Light microscopes have 4 objective lenses of different magnifications (usually x4, x10, x40 and x100)
  • Light microscopes have a maximum resolving power of 200nm and a maximum magnification of x1500.
  • Staining allows the specimen to be seen in more detail.
  • Some stains bind to specific cell structures/organelles.

Organelle
Structure
Function
Golgi Apparatus
Membrane-bound, flattened sacs.
The Golgi apparatus recieves, packages and modifies proteins sent from the ribosomes. The proteins are packaged into vesicles which are then transported (some go to the cell surface membrane and are secreted out of the cell).
Mitochondria
Either sausage shaped or spherical. They have two membranes with fluid between. The inner membrane is highly folded.
Mitochondria are the site of ATP (Adenosine Triphosphate) production during respiration. Almost all of the cells activities that require energy are driven by ATP.
Nucleus
The largest organelle in the cell. It contains chromatin and is surrounded by a nuclear envelope (two membranes with a fluid-space between them). Pores in the nuclear envelope allow substances to enter and leave the nucleus. Inside the nucleus is the nucleolus.
The nucleus houses most of the cells genetic material. The nucleolus creates RNA and ribosomes that then pass into the cytoplasm.
Endoplasmic Reticulum (ER)
Lots of flattened, membrane-bound sacs. Rough ER is studded with ribosomes, smooth ER is not.
The rough ER transports the proteins that the ribosomes produced and smooth ER is involved in the production of lipids that the cell needs.
Ribosomes
Tiny organelles consisting of 2 subunits. Some ribosomes are found int he cytoplasm and some are bound to rough ER.
They are the site of protein synthesis.
Chloroplasts
Only found in plant cells and the cells of protoctists. They have two membranes with luid between them.
They are the site of photosynthesis.
Lysosomes
Spherical, membrane-bound sacs.
They contain powerful enzymes which break down and digest materials.
Centrioles
Small tubes consisting of protein fibres or microtubules.
Form spindle fibres and take part in cell division.

Cell signalling

  • In multicellular organisms the survival of the whole organism requires each cell to be able to play it's own role.
  • Cells must be able to detect internal or external signals used to carry out and coordinate the processes needed to live.
  • In order to detect these signals cells have receptors on their surface which are often protein molecules.
  • Hormones often mediate cell communication.
  • Cells with receptors for hormone molecules are called 'target cells'.
  • Hormones are chemical messengers that are released into the organism after being made in specific tissues.
  • A hormone molecule will bind to a receptor on the cell surface if the shape is complimentary, like two pieces of a jigsaw puzzle!
  • This induces a response within the cell.

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