Cell Structure

Cells
1. Organisms can be Prokaryotes or Eukaryotes
  1. All living things are made from cells
  2. Eukaryotic cells are complex and include all animal and plant cells
    1. Eukaryotes are organisms that are made up of eukaryotic cells
  3. Prokaryotic cells are smaller and simpler, e.g. bacteria
    1. A prokaryote is a prokaryotic cell (it's a single celled organism
      1. Bacteria are prokaryotes
        Bacterial cells are much smaller
        It has cytoplasm, a cell membrane and a cell wall
        Bacteria don't have chloroplasts or mitochondria
        Bacterial cells don't have a 'true' nucleus - instead they have a single circular strand of DNA that floats freely in the cytoplasm
        They may also contain one or more small rings of DNA called plasmids
2. Plant and animal cells have similarities and differences
  1. The different parts of a cell are called subcellular structures
    1. Animal cells
      1. Nucleus - contains genetic material that controls the activities of the cell
      2. Cytoplasm - gel-like substance where most of the chemical reactions happen
        It contains enzymes that control these reactions
      3. Cell membrane - holds the cell together and controls what goes in and out
      4. Mitochondria - these are where most of the reactions for aerobic respiration take place
        Respiration transfers energy that the cell needs to work
      5. Ribosomes - these are where proteins are made in the cell
    2. Plant cells
      1. They usually contains the same subcellular structures as an animal cell plus a few things animals cells don't have
        Rigid cell wall
        Made of cellulose
        It supports the cell and strengthens it
        Permanent vacuole - contains cell sap, a weak solution of sugar and sals
        Chloroplasts - these are where photosynthesis occurs, which makes food for the plant
        They contain a green substance called chlorophyll, which absorbs the light needed for photosyntheis
Microscopy
1. Cells are studied using microscopes
  1. Microscopes let us see hings that we can't see with the naked eye
    1. The microscopy techniques we can use have developed over the years as technology and knowledge have improved
      1. Light microscopes use light and lenses to form an image of a specimen and magnify it (make it look bigger)
        They let us see individual cells and large subcellular structures such as nuclei
        Electron microscopes use electrons instead of light to form an image
        They have a higher resolution
        Resolution is the ability to distinguish between two points, so a higher resolution gives a sharper image
        Electron microscopes let us see much smaller things in more detail, like the internal structure of mitochondria and chloroplasts. They even let us see tinier things like ribosomes and plasmids
2. You can calculate the magnification of an image using this formula:
  1. Magnification = image size/real size
    1. They need to have the same units
      1. The image size or real size can be calculated by rearranging the equation
        Image size = magnification x real size
        Real size = image size/magnification
    2. Example:
      1. A specimen is 50um. Calculate the width of the image of the specimen under a magnification of 100
        1) Rearrange the formula
        2) Fill in the values you know
        3) Remember the units in your answer
        4) Convert the units
        Image size = 100x50
        = 5000um
        = 5mm
        To convert from micrometres to milimetres you need to divide by 1000 e.g. 5000um / 1000 = 5mm
3. Standard form
  1. Because microscopes see such tiny objects, sometimes it's useful to write numbers in standard form
    1. This is where you change very big or small numbers with lots of zeros into something more manageable e.g. 0.017 can be written as 1.7x10-2
      1. To do this you just need to move the decimal point left or right
        The number of places the decimal point moves is then represented by a power of 10, this is positive if the decimal point's moved to the left, and the negative if it's moved to the right
        Example:
        A mitochondria is approximately 0.0025mm long. Write this figure in standard form
        1) The first number needs to be between 1 and 10 so the decimal point needs to move after the '2'
        2) Count how many places the decimal point has moved - this is the power of 10. Don't forget the minus sign because the decimal point has moved right
        2.5 x 10-3
Cell Differentiation and Specialisation
1. Cells don't all look the same. They have different structures to suit their different functions
  1. Cells differentiate to become specialised
    1. Differentiation is the process by which a cell changes to become specialised for its job
      1. As cells change, they develop different subcellular structures and turn into different types of cells
        This helps them carry out specific functions
        Most differentiation occurs as an organism develops
        In most animal cells, the ability to differentiate is then lost at an early stage, after they become specialised
        However lots of plant cells don't ever lose this ability
        The cells that differentiate in mature animals are mainly used for repairing and replacing cells, such as skin or blood cells
        Some cells are undifferentiated cells - they're called stem cells
  2. Examples of specialised cells:
    1. Sperms cells are specialised for reproduction
      1. The function of a sperm cell is basically to get the male DNA to the female DNA
        It has a long tail and a streamlined head to help it swim to the egg
        There are a lot of mitochondria in the cell to provide the energy needed
        It also carries enzymes in its head to digest through the egg cell membrane
    2. Nerve cells are specialised for rapid signalling
      1. The function of nerve cells is to carry electrical signals from one part of the body to another
        These cells are long (to cover more distance) and have branched connections at their ends to connect to other nerve cells and form a network throughout the body
    3. Muscle cells are specialised for contraction
      1. The function of a muscle cell is to contract quickly
        These cells are long so that they have space to contract and contain lots of mitochondria to generate the energy needed for contraction
    4. Root hair cells are specialised for absorbing water and minerals
      1. Root hair cells are cells on the surface of plant roots, which grow into long "hairs" that stick out into the soil
        This gives the plant a big surface area for absorbing water and mineral ions from the soil
    5. Phloem and Xylem cells are specialised for transporting substances
      1. Phloem and xylem cells form phloem and xylem tubes, which transport substances such as food and water around plants
        To form the tubes, the cells are long and joined end to end
        Xylem cells are hollow in the centre and phloem cells have very few subcelluar structures so that stuff can flow through them
Binary Fission
1. Prokaryotic cells can reproduce using a type of simple cell division called binary fission
  1. Prokaryote cells replicate by binary fission
    1. In binary fission, the cell splits into two
      1. 1) The circular DNA and plasmid(s) replicate
        2) The cell gets bigger and the circular DNA strands move to opposite 'poles' (ends) of the cell
        3) The cytoplasm begins to divide and new cell walls begin to form
        4) The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid(s)
  2. Bacteria can divide very quickly if given the right condition (e.g. a warm environment and lots of nutrients)
    1. Some bacteria, such as E.coli can take as little as 20 minutes to replicate in the right environment
      1. However if conditions become unfavourable, the cells will stop dividing and eventually begin to die

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	Created by Liffey Farrell over 7 years ago