Cells structure

• Light microscope


  • Magnification: x1500
  • Resolution: 200 nm
• Electron microscope


  • Magnification: x250,000 - x500,000
  • Resolution: SEM - 3 nm, TEM - 0.5 nm

  
• Light microscope

  • Can observe living tissue
  • More portable
  • Easier to use - no technical training required
  • Possible to see real/natural colours and a live specimen
  • Can stain particular types of tissue for better visibility
• Magnification


  How much bigger a sample appears to be under a microscope than it is in real life
• Resolution


  The ability to distinguish between 2 separate points
• As resolution increases, image clarity and detail also increase
• Ribosomes (25nm) can't be seen with a light microscope as they don't interfere with the light waves
• Different stains are absorbed by different cell organelles so they can be observed more clearly
• Electron microscopes require a vacuum as electrons cannot be focused without a vacuum as they will collide with air molecules and scatter
• The sample must be dehydrated in an electron microscope as water boils at room temperature in a vacuum
• The cell is the basic unit of all living organisms
• Cell surface membrane


  • Selectively permeable membrane that allows for the exchange of certain substances
  • Barrier between cytoplasm and external environment
  • Cell recognition (surface antigens)
  • Selection of substances that enter/leave cells
• Nucleus


  • Controls cell's activities
  • Very dense, takes up colour the most when stained
  • Divides first during cell division
  • Surrounded by 2 membranes, known as the nuclear envelope which is continuous with the RER
  • Contains nuclear pores and nucleolus
• Ribosomes


  • Carry out protein synthesis
  • 80S - cytoplasm
  • 70S - chloroplasts & mitochondria
• Rough endoplasmic reticulum (RER)

  • Membranes that form an extended system of fluid-filled sacs (cisternae)
  • Ribosomes are attached to the RER and are the site of protein synthesis
  • Proteins made by the ribosomes enter the sacs and are often modified as they go through them
  • Small sacs (vesicles) break off from the ER and join to form the golgi body
• Golgi body / apparatus / complex

  • Stack of flattened sacs (cisternae) formed by the vesicles which bud off from the RER
  • Packages substances into vesicles for transport
  • Glycosylation
  • Phosphorylating proteins
  • Assembly of polypeptides into proteins (4º structure)
  • Folding proteins
  • Removing the 1st amino acid methionine to activate proteins
• Smooth endoplasmic reticulum (SER)

  Synthesizes lipids and steroids such as cholesterol and the reproductive hormones estrogen and testosterone
• Lysosomes


  • Spherical sacs surrounded by a single membrane
  • Contain hydrolytic enzymes
  • Responsible for digestion/breakdown of unwanted structures e.g., old organelles
  • Can even digest whole cells e.g., in mammary glands after the period of lactation
• Mitochondria


  • Carries out aerobic respiration
  • Synthesizes ATP (adenosine triphosphate)
  • More in cells that have a higher demand for energy e.g., muscle, liver, and root hair cells
  • Outer membrane contains a transport protein called porin
  • Energy released from energy-rich molecules e.g., sugars and fats during respiration is transferred to molecules of ATP
  • ATP is the energy-carrying molecule in all living cells
  • Once made, ATP leaves the mitochondrion and can spread rapidly to all parts of the cell where energy is needed
  • Its energy is released by breaking ATP down to ADP (adenosine diphosphate) in a hydrolysis reaction
• Microtubules


  • Long, rigid, hollow tubes found in the cytoplasm
  • Made of a protein called tubulin
  • Tubulin has 2 forms - ⍺ & β tubulin
  • ⍺ & β tubulin molecules combine to form dimers
  • Many dimers are joined end to end to form protofilaments
  • 13 protofilaments are in a ring to form a cylinder with a hollow center
  • Supports and gives shapes to the cell
  • The assembly of microtubules from tubulin molecules is controlled by special locations in cells called microtubule organizing centers (MTOCs)
• Centrioles (and centrosomes)


  • Outside the nucleus of animal cells, 2 centrioles are present close together at right angles in a region called the centrosome
  • Centrioles are hollow cylinders about 500 nm long
  • Produces spindle fibers
  • Organizes microtubules
• Chloroplasts


  • Diameter 3-10 um
  • Carries out photosynthesis
  • Contains starch grains, circular DNA, and 70S ribosomes
  • ATP is also produced here
• Cell wall


  • Gives cell definite shape
  • Rigid as made of cellulose
  • Freely permeable
  • Prevents cell from bursting
• Plasmodesmata


  • Plant cells are linked to neighboring cells by means of fine strands of cytoplasm called plasmodesmata which pass through pore-like structures in their walls
  • Allows the transport of water, sucrose, amino acids, ions, etc., between cells without crossing membranes
  • Allows communication/signaling between cells
• Vacuoles


  • Surrounded by a partially permeable tonoplast which controls exchange between the vacuole and cytoplasm
  • Helps regulate osmotic properties of cells
  • Fluid present in the vacuole consists of:
• Prokaryotic cells


  • Organisms that lack nuclei or proper nuclear membranes
  • Unicellular
  • 1-5um
  • Cell wall made of murein (peptidoglycan = protein + polysaccharides)
  • No membranes around organelles
  • 70S (smaller) ribosomes
  • Genetic material in the form of circular DNA
  • Have no ER
• Viruses are noncellular/acellular, have a protein coat and a nucleic acid core (DNA/RNA), replicate inside host cells only, show no characteristics of living organism, and have a symmetrical shape

  
• The virus DNA/RNA takes over the protein synthesizing machinery of the host cell which helps to make new virus particles

  
• Golgi body/apparatus/complex


  Membrane-bound organelle found in eukaryotic cells that modifies, sorts, and packages proteins and lipids for transport
• Packages substances into vesicles for transport


  Function of the Golgi apparatus to take in proteins and lipids from the RER and package them into vesicles for transport
• Glycosylation


  Process of adding sugar molecules to proteins and lipids, which is carried out by the Golgi apparatus
• Microtubules


  A long, rigid, hollow tube found in the cytoplasm, composed of tubulin protein
• α and β tubulin

  The two forms of tubulin protein that combine to form dimers
• Tubulin


  A protein that forms microtubules, existing in two forms: α and β
• Dimers


  The resulting molecule formed by combining α and β tubulin molecules
• Protofilaments


  Tubulin molecules joined end to end to form a microtubule
• 13 protofilaments


  The number of protofilaments arranged in a ring to form a microtubule cylinder
• Microtubule organizing centers (MTOCs)


  Special locations in cells that control the assembly of microtubules from tubulin molecules