AS Biology: Unit F211: Cells, Exchange and Transport

The Light Microscope

Magnification is the degree to which the size of an image is larger than the object itself.

Resolution is the degree to which it is possible to distinguish between two objects that are very close together.

How it works

• Use a number of lenses to produce an image that can be viewed directly at the eyepieces. 

• Light passes from a bulb under the stage, through a condenser lens, through the specimen & is focused through the objective lens before going to the eyepiece. 

• Four objective lenses are present and can be rotated to change the magnification. Lenses are usually: x4, x10, x40 and x100.

• Eyepiece lens magnifies this image again, usually x10.

Staining and Sectioning in Light Microscopes

A lot of biological material is not coloured, some may distort when you try to cut it. Overcome by:

• Chemicals called "coloured stains" bind to chemicals in the specimen, allowing it to be seen. 
• Some stains bind to specific cell structures. 
• E.g. staining DNA Dark Red with Acetic Orcein; & staining bacterial cell walls violet using Gentian. 
• Staining refers to any process that helps to reveal or distinguish different features. 

• Sectioning: specimens are embedded in wax, followed by thin sections being cut without distorting the structure of the specimen. 
• This is useful for making sections of soft tissue: E.g. Brain Tissue. 

Cell Size and Linear Magnification

Measuring specimens under a microscope can be limited by resolution. 

The resolution of the:

• Human Eye: 100μm.
• Light Microscope: 200nm.
• Electron Microscope: 0.2nm.

The Graticule and The Stage Micrometer 

• The graticule scale is arbitrary - it represents different lengths and different magnifications, hence needs to be calibrated. 
  
• The stage micrometre is a slide containing 1mm (1000μm) with 100 divisions.
• This is placed on the microscope stage
• To calculate the length of 1 eyepiece unit (epu):

The Electron Microscope

• The light microscope has a low resolution. 
• If magnification is above x1500, the image isn't clear.
• The wavelength of light ranges from 400-750nm, structures closer to 200nm would appear as on object. 
• To achieve a greater resolution, an electron microscope has to be used.

• Electron Microscopes: Generate a beam of electrons. The beam wavelength is 0.0044nm.
• The microscopes use magnets to focus the beam onto a specimen.
• The image produced is projected onto a screen to make a grayscale image.

The need for staining in EMs 

• Staining samples with lead salts = scatter the electrons differently to give contrast.
• Since: the final image produced is in grayscale, the image can be coloured using specialised computer software.
• These images are labeled as 'false-colour' electron micrographs. 

The Cytoskeleton

• Refers to the network of protein fibres found within cells that give structure and shape to the cell.
  • As well as move organelles around inside cells. 
  • It provides mechanical strength to them.,
  • As well as aids transport within cells & enabling cell movement.

Some of the fibres - called Actin Filaments. Able to move against each other. These muscle fibres can move some organelles around inside cells.

-Microtubules -  used to move microorganism through a liquid or to waft a liquid past a cell.

-Microtubules are made of the tubulin (protein). 

-Other proteins present (microtubule motors) move organelles and other cell contents along the fibres. 

-This is how chromosomes are moved during mitosis and how vesicles move from the endoplasmic reticulum to the Golgi apparatus. They use ATP to drive these movements.

Cilia and Flagella

• Eukaryotes (organisms that have cells with a nucleus) have hair-like extensions that stick out from the surface of the cells.
• Cilia often occur in large numbers in a cell.
  
• The sweeping movement of these move substances such as mucus across the surfaces of cells in the ciliated epithelial tissue.

• Flagella are longer and usually occur in ones or twos in a cell. 
• Ones that from the tail of sperm or bacteria. E.g. able to move the whole cell, using energy from ATP. 
  

The Vesicles, Vacuoles and Cell Wall 

Vesicles

• Are membrane-bound sacs found in cells. They carry many different substances around cells.

Vacuole

• The vacuole is filled with water and solutes so that it pushes the cytoplasm against the cell wall, making the cell turgid.
• This is important as it helps support the plant. 

Cell Wall

• The cell wall is made out of cellulose. 
• It forms a sieve-like network of strands that makes the cell wall strong. 
• The cell wall is held rigid by the turgor pressure, so it supports the cell & helps support the whole plant.

Refer to the printout of table, stating function & structure of other organelles.

Protein Synthesis

• Specific instructions to make the hormone are found in a gene.
• Genes are sections of DNA in the chromosomes in the nucleus.
• The nucleus copies the instructions in the DNA into a molecule called mRNA.
• The mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome. 
• The ribosome reads the instructions and uses the codes to assemble the hormone.
• The assembled protein is pinched off in a vesicle and transported to the Golgi Apparatus.
• The Golgi Apparatus packages the protein and may also modify it so that it is ready to release,
• The protein now packages into a vesicle, fuses with the cell surface membrane. 
• The membrane opens to release the hormone outside. 

Prokaryotes and Eukaryotes 

• Prokaryotes don't have a nucleus. They are bacteria and are much smaller than Eukaryotic cells (20-40μm). Other differences:
  • They only have one membrane.
  • They are surrounded by a cell wall made of peptidoglycan.
  • They contain much smaller ribosomes.
  • They have a single, circular loop of DNA, opposed to the linear, sperate strands of eukaryotic chromosomes.
  • Many also contain very small loops of DNA called plasmids.
  • The DNA is not surrounded by a membrane. 
  • ATP production takes place in specialised regions of the membrane called mesomes.
  • Some have Flagella. Made up of a spiral protein called flagellin.