Biology F211 Cells Part 1

Living organisms consist of cells
1. Magnification - the degree to which the size of an image is larger than the object itself
2. Resolution - the degree to which it is possible to distinguish between two objects that are very close together
3. The Light microscope
  1. Use a number of lenses to produce an image that can be viewed directly at the eyepieces
  2. Light passes from a bulb under the stage, through a condenser lens, then through the specimen
  3. The beam of light is focused through the objective lens, then through the eyepiece lens
  4. Have a number of objective lenses that can be rotated into position
  5. Advantages & Disadvantages
    1. Magnification - Upto x1500
    2. Resolution - Maximum resolving power using light is 200nm (disadvantage)
    3. Specimens - A wiide range including living and dead plants and animals can be viewed. Also smear preparations of blood or cheek cells can also be viewed
Cell size and magnification
1. Limits of resolution
  1. Maximum for the human eye is 100 micrometres
  2. Maximum for light microscope is 200 nanometres
  3. Maximum resolution for an electron microscope is 020 nanometres
2. Calculations
  1. Actual size = image size/magnification
  2. Magnification = Image size/actual size
  3. Image size = magnification x actual size
Electron microscopes and cell details
1. Transmission Electron Microscope
  1. The electron passes through a very thin prepared sample
    1. Electrons pass through the denser parts of the sample less easily therefore giving contrast
  2. The final image produced is a 2D picture
  3. The magnification possible with a TEM is x500 000
2. Scanning electron microscopes
  1. The electron beam is directed onto a sample. The electrons don't pass through the specimen
  2. Electrons 'bounced off' sample
  3. The final image produced is a 3D view of the surface of the sample
  4. The magnification possible with an SEM is about x100 000
3. Advantages
  1. The resolution is 0.1nm (2000x more than in a light microscope)
  2. Electron microscope can be used to produce detailed images of the structures (organelles) inside cells
  3. The SEM produces 3D images that can reveal the deatail of contours and cellular and tissue arrangements - not possible with a light microscope
4. Disadvantages
  1. Electron beams are deflected by the molecules in the air, so samples have to placed in a vacuum
  2. Extremely expensive items
  3. Preparing samples and using an electron microscope both require a high degree of skill and training
Cells and living processes
1. Ultrastructure - what is seen under an electron microscope (the contents of a cell)
2. Most organelles are found in both plant and animal cells
  1. The different organelles that make up a cell, work together - each contributing to the survival of the cell
3. Cytoskeleton - fibres that keep the cells shape stable by providing internal framework
  1. Actin filaments - able to move against each other. Cause some of the movements seen in white blood cells and move organelles inside cells
  2. Microtubules - cylinders about 25nm in diameter.
    1. Made of a protein called tubulin
    2. May be used to move a microorganism through a liquid or to waft a liquid past the cell
    3. Other proteins present on the the microtubules move organelles and other cell contents along the fibres
      1. These proteins are known as microtubule motors - they use ATP to drive these movements
    4. How vesicles move from the ER to the Golgi apparatus
4. Flagella and Cilia
  1. Undulipodia (flagella) and cilia can move because the microtubules can use energy from ATP
  2. Cilia are shorter than undulipodia (about 10 micrometeres long)
5. Vesicles and vacuoles
  1. Vesicles are membrane bound sacs found in cells. Used to carry many different substances around cells
  2. Plant cells - a large cell vacuole maintains cell stability. It is filled with water and solutes so that it pushes the cytoplasm against the cell cytoplasm against the cell wall; making the cell turgid.
    1. Especially important in non-woody plants
6. Plant cell walls
  1. On the outside of plant cell plasma membranes
  2. Made of cellulose; a carbohydrate polymer made up of glucose sub-units
  3. Cellulose forms a sieve-like network that makes the wall strong.
  4. Because it is held rigid by the pressure of the fluid inside the cell it supports the cell and so helps support the whole plant
Organelles - Structure & function
1. Surrounded by membranes
  1. The Nucleus
    1. Largest organelle. Surrounded by a nuclear envelope
    2. Houses nearly all the cell's genetic information. Chromatin consists of DNA and proteins
    3. Has a dense, spherical structure inside it - called the nucleolus. It makes RNA and ribosomes
  2. Endoplasmic Reticulum (ER)
    1. Consists of a series of membrane bound flattened sacs called cisterne
    2. Rough ER - Transports proteins that were made on the attached ribosomes
    3. Smooth ER - involved in making the lipids the cell needs
  3. Golgi apparatus
    1. Stack of membrane-bound flattened sacs
    2. Receives proteins from the ER and modifies them
      1. May add sugar molecules to it
    3. Packages modified proteins into vesicles so that they can be transported
  4. Mitochondria
    1. May be spherical or sausage shaped
    2. The site where adenosine triphosphate is produced during respiration
    3. Inner membrane is highly folded to form cristae. Central part of the mitochondrion is called the matrix
  5. Chloroplasts
    1. Found only in plant cells and the cells of some protoctists
    2. Are the site of photosynthesis in plant cells
    3. Inner membrane is continuous, with an elaborate network of flattened membrane sacs called thylakoids
  6. Lysosomes
    1. Spherical sacs surrounded by a single membrane
    2. Contain powerful digestive enzymes
2. Organelles without membranes surrounding them
  1. Ribosomes
    1. Tiny organelles. Some are in cytoplasm and some are bound to ER
    2. Site of protein synthesis in a cell
      1. Act as an assembly line where coded information (mRNA) from the nucleus is used to assemble proteins from amino acids
    3. Consists of two subunits
  2. Centrioles
    1. Small tubes of protein fibres (microtubules).
      1. There is a pair of them next to the nucleus in animal cells and in the cells of some protoctists
    2. Known to take part in cell division
      1. Form fibres, known as spindle
Organelles at work
1. Division of labour
  1. 1.mRNA copy of the instructions (gene) for insulin is made in the nucleus
  2. 2.mRNA leaves the nucleus through a nuclear pore
  3. 3.mRNA attaches to a ribosome. Ribosome reads the instructions to assemble the protein
  4. 4. Insulin molecules are 'pinched off' in vesicles and travel towards Golgi Apparatus
  5. 5. Vesicle fuses with Golgi Apparatus
  6. 6. Golgi apparatus processes and packages insulin molecules, ready for release
  7. 7. Packaged insulin molecules are 'pinched off' in vesicles from Golgi apparatus and move towards the cell surface membrane
  8. 8. Vesicle fuses with cell surface membrane
  9. 9. Cell surface membrane opens to release insulin molecules outside
2. Prokaryotes vs Eukaryotes
  1. Eukaryotes
    1. Have a nucleus
    2. Contain organelles, some of which are bound by membranes
    3. Have a complicated internal structure
  2. Prokaryotes
    1. They are bacteria and are 1-5 micrometeres long
    2. Only have one membrane - cell surface membrane
      1. Contains no membrane bound organelles such as mitochondria and chloroplasts
    3. Surrounded by a cell wall made of peptidoglycan - not cellulose
    4. DNA in the form of single loop sometimes called the 'bacterial loop'
    5. ATP production takes place in specialised infolded regions of the cell surface membrane called mesosomes
    6. Some strains of bacteria are resistant to antibiotics - one such example is MRSA
    7. Skin is covered with a 'normal flora' of bacteria. Help to prevent harmful microorganisms getting into the body
    8. DNA is not surrounded by a membrane. The general area where the DNA lies is called the nucleoid

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Biology F211 Cells Part 1

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