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History of the cell

  • Robert Hooke:
    • “cell” from cork bark appearance under the microscope
    • “cell terminology”
  • Schwann’s Cell Theory
    • All organisms consist of ≥1 cell
    • The cell is the basic unit of structure for all cells (smallest thing we can isolate that is alive)
    • All cells arise only from pre-existing cells (mitosis/meiosis)

Prokaryotic and eukaryotic cells

  • Prokaryotic cells
    • No nucleus
    • DNA spread throughout the cell
  • Eukaryotic cells
    • Have a nucleus that contains the DNA
  • Note that plant cells are larger than animal cells, and they contain chloroplasts and a large central vacuole.

Unicellular and multicellular

  • Unicellular (single cell)
    • All prokaryotes and some eukaryotes
    • E.g. yeast, protozoa
  • Multicellular
    • Eukaryotes
    • Plants and animals
    • Allowed development of specialised cells
      • so that individual tissues etc could carry out specialised functions
      • organs, systems etc

Cell sizes

Relative sizes of various cells‎
  • large range of sizes
  • many different shapes: spinitum, spirochete, anabaens (a cyanobacterium), bacillus, E. coli, Staphylococcus, rickettsia, mycoplasma

Cell Organelles

An organelle is a specialised part of a cell that has its own particular function. They are membrane-bound (enclosed).

The organelles of a cell
Organelle Function
Rough endoplasmic reticulum Protein synthesis by ribosomes [not organelles as it’s not membrane bound]
Smooth endoplasmic reticulum Lipid metabolism
Golgi apparatus Refinement of synthesised proteins by glycosylation and modification of carbohydrate side chains
Lysosomes Use oxygen radicals to digest unwanted substances
Mitochondria Energy production by oxidative phosphorylation
Endosomes Absorb substances from the cell
Peroxisomes Use the enzyme catalase to digest unwanted substances via oxidative reactions
Secretory vesicles Formed by the incorporation of new plasma membrane. Used to export/import substances
Transport vesicles Move substances between cytoplasmic organelles
Plasma membrane Contain cell organelles and cytoplasm
Cytoskeleton [not an organelle as it’s not membrane bound]
Nucleus Store DNA
Nucleolus

NB: The plasma membrane, mitochondria and nucleus have a double membrane.

Nucleus

  • Double membrane
  • Nuclear pores inside the membrane
  • Genetic material lies within chromosomes
  • Chromosome is a structure made up of DNA and protein
  • Has its own cytoskeleton
  • Has no organelles (but has a nucleolus)
  • Has no protein synthesis
  • Stores DNA
  • Imports proteins
  • Exports mRNA

Endoplasmic Reticulum

  • Smooth:
    • No ribosomes attached
    • Lipid metabolism and synthesis
    • Detoxifies drugs and harmful compounds
    • Different amounts of sER are in different cells
  • Rough:
    • Has ribosomes attached, which are involved in protein synthesis
    • RNA is transcribed from DNA in the nucleus
    • It leaves the nucleus through nuclear pores
    • It is converted to protein in the RER
    • Protein is packaged into a transport vesicle in the SER
    • The transport vesicle fuses with the Golgi apparatus and its contents are released inside
    • At the Golgi, the protein is modified and “finished”
    • The Golgi apparatus packages the completed protein into a vesicle, which may:
      • Stay inside the cytoplasm or nearby the Golgi apparatus
      • Fuse with the plasma membrane and release its contents

Ribosomes

  • Protein synthesis
  • Can be either free in the cytoplasm or bound to the endoplasmic reticulum
  • Has a large and a small subunit. Growing polypeptide runs between the two subunits.

Golgi Apparatus

  • Glycosylation of secreted proteins
  • Sorting of cytosolic proteins
  • Modification of carbohydrates
  • Trims side chains of proteins
  • Trans vesicles fuse with plasma membrane
  • Different enzymes in different compartments
  • Located near the nucleus
  • Disc-shaped stack of membranes called the “Golgi stack”. There may be between 6 and 30 membranes per stack
  • Many sets of membrane-bound smooth surfaced cisternae.

Lysosomes

  • Site of cellular digestion
  • Contain enzymes for digestion
  • Acid hydrolases (active at pH 5)
  • Primarry (enzyme)
  • Secondary (enzyme + substrate)

Mitochondria

  • Located throughout cytoplasmic compartment
  • Used to be aerobic organisms in symbiosis
  • Function:
    • Energy production
    • Electron transport chain (= respiratory chain)
  • Raw materials are 1) oxygen 2) pyruvate and fatty acids
  • Products are 1) carbon dioxide 2) ATP
  • Contain some DNA

Membrane structure

  • Phospholipid bilayer and proteins (the proteins may reach through the inside layer, the outside layer, or both)
  • 1972 Singer and Nicholson model: proteins ”floating” within lipid bilayer like a “liquid” surface
  • 1975 Unwin and Henderson: Integral membrane proteins. Both hydrophobic and hydrophilic. Glycoproteins – carbohydrate groups on the outer surface.
  • 1997 “Membrane rafts”: Motile patches of surface specialisation. Cholesterol is embedded in the membrane as well.
  • In prokaryotes (bacteria), there is just the one membrane, and just one compartment.
  • In eukaryotic cells, there are many different membrane compartments.
  • The plasma membrane is very flexible, exhibits fluidity, and adapts to physical/mechanical disturbances. Hence cells without a cytoskeleton would tend to a spherical shape (liposome)
  • However, the cytoskeleton is anchored to the cell membrane and gives the cell its shape (e.g. the red blood cell is a biconcave disc)
  • Cytoskeleton is made up of 3 filament types:
    • Microfilaments (actin filaments)
    • Intermediate filaments
    • Microtubules
  • The cell membrane can also have proteins that specifically localise only in one area of the membrane e.g. sperm have a tail (on one side) and an acrosomal cap (on the other site).
  • Furthermore, microtubules in the cytoskeleton are responsible for moving vesicles.
  • Membrane proteins line the cell membrane, allowign for specific functions. This includes:
    • Transport channels
    • Proteins that initiate enzyme reactions (e.g. G protein-coupled receptors)
    • Receptors
    • Cell identity
    • Cytoskeleton link
    • Cell adhesion

Plant Cell Organelles

  • Cell wall
    • Rigid structure outside cell membrane
    • No ability to move, osmotic stresses
  • Cell organelles
    • Central vacuole
    • Chloroplasts
  • The plasma cell membrane
    • Membranes form compartments:
      • Metabolic and biochemical specialisation
      • Localisation of function
      • Import and export (vesicles)
      • Regulation of transport
      • Detection of signals
      • Cell-cell communication (by proteins in membrane)
      • Cell Identity (lets immune system sort out what belongs to you. This is constantly being checked)
    • They allow you to localise enzymes etc that might otherwise destroy the cell (e.g. in mitochondria).
    • The cell itself has a plasma membrane.
    • The organelles have their own membranes, whose basic structure is similar.

Cellular Import and Export

The process of endocytosis and exocytosis
  • Endocytosis = import.
  • Exocytosis = export.
  • Lysosomes digest incoming vesicles, and so endocytosis is different to exocytosis in this way.
  • By fusing vesicles with the plasma membrane, new receptors can be incorporated into it (just place the receptors in the phospholipid layer of the vesicle)
  • Conversely, in endocytosis, membrane is “pinched off” and brought into the cell.
  • This is the process of membrane turnover.





Bacterial Membranes

There are 2 types of bacterial membranes. These have several distinct physical properties which can be used to differentiate them in the lab and hence aid in diagnosis.


Comparing the membrane structure of gram positive and negative bacteria

Gram Negative Bacteria

  • Have a double cell membrane (an inner plama membrane and an additional outer lipid bilayer surrounding the peptidoglycan layer)
  • The cell wall is composed of only a thin layer of peptidoglycans
  • Hence they do not retain the crystal violet dye (it is easily washed back out)

Gram Positive Bacteria

  • Have a single cell membrane, which is comparable to the inner (plasma) membrane of gram-negative bacteria
  • The cell wall is a very thick layer of peptidoglycans
  • Hence the bacterium retains the dye

Gram Staining

  • The gram-staining characteristics of bacteria are denoted as positive or negative, depending upon whether the bacteria take up and retain the crystal violet stain or not.
  • Gram-positive bacteria retain the colour of the crystal violet stain in the Gram stain. This is characteristic of bacteria that have a cell wall composed of a thick layer of a particular substance (specifically, peptidologlycan containing teichoic and lipoteichoic acid complexed to the peptidoglycan).
    • The Gram-positive bacteria include staphylococci ("staph"), streptococci ("strep"), pneumococci, and the bacterium responsible for diphtheria (Cornynebacterium diphtheriae) and anthrax (Bacillus anthracis).
  • Gram-negative bacteria lose the crystal violet stain (and take the colour of the red counterstain) in Gram's method of staining. This is characteristic of bacteria that have a cell wall composed of a thin layer of a particular substance (specifically, peptidoglycan covered by an outer membrane of lipoprotein and lipopolysaccharide containing endotoxin).
    • The Gram-negative bacteria include most of the bacteria normally found in the gastrointestinal tract that can be responsible for disease as well as gonococci (venereal disease) and meningococci (bacterial meningitis). The organisms responsible for cholera and bubonic plague are Gram-negative.


Process of Gram Staining