Investigating The Structure Of Cells

  • Created by: ernily
  • Created on: 26-12-14 16:04


Magnification: The number of times the image has been enlarged by the microscope.

Resolution: The ability of the microscope to distinguish clearly between 2 points that are extremely close together.

Light microscopes have a low resolution therefore less detail is seen than with an electron microscope.

They have a maximum resolution of 0.2 micrometres, and a maximum magnification of X1500.

They observe both living and dead material.

Electron microscopes have a high resolution therefore more detail is seen than with an electron microscope.

They have a maximum resolution of 0.0001 micrometres, and a maximum magnication of X500,000.

They observe dead material.

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Transmission Electron Microscope (TEM)

The transmission electron microscope consists of electron gun the produces an electron beam which is focused on to the specimen by a condensor electromagnet. The beam passes through the thin segment and is absorbed. 

They produce thin, 2D images and have a lower resolving power than the scanning electron microscope.

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Scanning Electron Microscope (SEM)

The scanning electron microscope direct an electron beam on to the surface from above. The beam is passed back and forth across a portion of the specimen in a regular pattern. The electrons are scattered.

They produce thick, 3D images of the specimen.

They have a higher resolving power than the transmission electron microscope.

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Ultrastructure of Epithelial Cell (Small Intestine

Lysosomes: Organelle containing digestive enzymes. They digest unwanted material in the cell.

Mitochondria: Organelles surrounded by two membrane layers. The inner layer is folded to form cristae. Mitochondria produce ATP from respiration.

Rough Endoplasmic Reticulum: Have ribosomes on the outside. Collects and transports proteins around the cell.

Ribosomes: Very small organelles with no membrane. They assemble protein molecules from amino acids.

Microvilli: Increases surface area to allow more efficient absorption of food molecules. 

Smooth Endoplasmic Reticulum: Similar to rough endoplasmic reticulum, but without the ribosomes. They synthesise lipids.

Cytoplam: Many important cell reactions take plae here (e.g. respiration). Food substances are made and stored.

Nucleus: The largest organelle, surrounded by the nuclear envelope. There are many holes called nuclear pores. The nucleus contains DNA which holds the genetic information necessary for controlling the cell.

Cell Surface Membrane: Surrounds the cell surface and is made of lipids and proteins. It controls the passage of substances in and out of the cell.

Golgi Apparatus: Flattened sacs surrounded by membrane. Vesicles are pinched off the end of the sacs. The golgi apparatus packages and processes molecules such as proteins that are made in the cell. It forms lysosomes.

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Magnification, Actual Size and Image Size

1 micrometer = 0.001mm

1mm = 1000 micrometers

To calculate the magnifications:

1) Measure the picture in mm

2) Make sure the actual size and image size are in the same units

3) Magnification = Image / Actual

To calculate actual size:

1) Measure the picture in mm

2) Convert into micrometers

3) Actual Size = Image Size / Magnification

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Cell Fractionation and Ultracentrifugation

1) Homogenisation

Homogenise means to break up the tissue and cells to release the organelles. This is done with a homogeniser. It produces a homogenate liquid of broken up cells.

Homogenisation is carried out in a solution which is

- Ice Cold: To prevent enzyme activity so organelles aren't damaged.

- Isotonic: So that there is the same water potential as organelles, which prevents osmosis.

- Buffered: To maintain the same pH.

2) Differential Centrifugation

Centrifugation means to spin fast in a machine called a centrifuge. Cell Fractionation involves spinning samples at progressively higher speeds to separate the different types of organelles.

When the homogenate is spun in a centrifuge, the heaviest organelle will sink to the bottom at lower speeds.

Pellet/Fraction: The mass of dense material at the bottom of the centrifuge tube after spinning, contains the densest organelles.

Supernatant: The fluid above the pellet, may contain less dense material.

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Prokaryotic & Eukaryotic Cells

Prokaryotic Cells:

  • 0.1 - 10 micrometers in size.
  • No nucleus, the DNA is in the cytoplasm.
  • DNA is arranged in a granular ring.
  • 70s Ribosomes.
  • Cell wall made of murein.
  • No membrane bound organelles.
  • No endoplasmic reticulum present.
  • Mesosome is the site of respiration.

Eukaryotic Cells:

  • 10 - 100 micrometers in size.
  • DNA inside the nucleus.
  • DNA arranged linearly in chromosomes.
  • 80s Ribosomes.
  • Cell wall in plant cells, not in animal cells.
  • Many membrane bound organelles.
  • Endoplasmic reticulum present.
  • Mitochondria is the site of respiration.
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