IB Intro to Cells

• Cell differentiation


  The process where cells become specialized for specific functions
• Cell compartmentalization


  • Enabled unicellular organisms to develop specialized functions through specific areas of their cell
  • One example is the nucleus region which contains DNA molecules
  • Another example is the compartmentalisation of energy-producing areas formed by the endosymbiosis of mitochondria
• Even with specialized compartments, unicellular organisms have their limitations, and so multicellular organisms evolved
• Specialization


  Enables the cells in a tissue to function more efficiently as they develop specific adaptations for that role
• Differentiation


  The development of distinct specialized cells
• Specialized eukaryotic cells


  • Have specific adaptations to help them carry out their functions
  • The shape of the cell
  • The organelles the cell contains (or doesn't contain)
  • Cells that make large amounts of proteins will be adapted for this function by containing many ribosomes (the organelle responsible for protein production)
• Prokaryotic cells


  Cell structure of organisms that are the simplest, being the first to evolve on Earth
• Domains of prokaryotes


  • Bacteria or Eubacteria
  • Archaebacteria or Archaea
• Archaebacteria or Archaea


  Typically found in extreme environments such as high temperatures and salt concentrations, include methanogens (organisms that exist in anaerobic conditions and produce methane gas)
• Prokaryotic cells


  • Small, ranging from 0.1µm to 5.0µm
  • Lack a nucleus
• Structure of prokaryotic cells


  • Cytoplasm not divided into compartments, lacks membrane-bound organelles
  • 70S ribosomes
  • DNA in a loop
  • Cytoplasm
  • Plasma membrane
  • Cell wall
• Eukaryotic cells


  Cells with a more complex ultrastructure than prokaryotic cells
• Eukaryotic cells


  • Cytoplasm is divided up into membrane-bound compartments called organelles
  • Organelles are bound by a single or double membrane
• Compartmentalization of the cell

  Dividing the cell into separate compartments
• Advantages of compartmentalization

  • Enzymes and substrates can be localized and available at higher concentrations
  • Damaging substances can be kept separated, e.g. digestive enzymes are stored in lysosomes
  • Optimal conditions can be maintained for certain processes, e.g. optimal pH for digestive enzymes
• The numbers and location of organelles are to be altered depending on the requirements of the cell
• Gene expression


  The process by which the information encoded in a gene is used to direct the synthesis of a functional gene product, such as a protein or RNA molecule
• Every nucleus within the cells of a multicellular organism contains the same genes, that is, all cells of an organism have an identical genome
• Despite cells having the same genome, they have a diverse range of functions because during differentiation certain genes are expressed ('switched' on)
• Whether a gene is expressed or not

  Is triggered by changes in the environment
• Controlling gene expression


  The key to development as the cells differentiate due to the different genes being expressed
• Once certain genes are expressed the specialization of the cell is usually fixed so the cell cannot adapt to a new function
• Lipid Bilayers: Basis of Cell Membranes

  • Phospholipids form the basic structure of cell membranes, which are formed from phospholipid bilayers
  • They are formed by a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails
  • As phospholipids have a hydrophobic and hydrophilic part they are known as amphipathic
  • The phosphate head of a phospholipid is polar and therefore soluble in water (hydrophilic)
  • The fatty acid tail of a phospholipid is nonpolar and therefore insoluble in water (hydrophobic) 
• Membrane proteins

  Proteins in the plasma membrane that carry out additional functions beyond the main barrier function of the phospholipid bilayer
• Types of membrane proteins

  • Integral
  • Peripheral
• Integral proteins

  • Partially hydrophobic, i.e. amphipathic
  • Embedded in the phospholipid bilayer
  • Can be embedded across both layers or just one layer
• Peripheral proteins

  • Hydrophilic
  • Attached to either the surface of integral proteins, or to the plasma membrane via a hydrocarbon chain
  • Can be inside or outside the cell
• The protein content of membranes can vary depending on the function of the cell
• Membranes with high protein content

  • Mitochondria
  • Chloroplasts
• Membrane protein functions


  • Transport
  • Receptors
  • Cell adhesion
  • Cell-to-cell recognition
  • Immobilized enzymes
• Transport proteins


  Allow ions and polar molecules to travel across the membrane
• Types of transport proteins

  • Channel proteins
  • Carrier proteins
• Channel proteins


  Form holes, or pores, through which molecules can travel
• Carrier proteins


  Change shape to transport a substance across the membrane, e.g. protein pumps and electron carriers
• Each transport protein is specific to a particular ion or molecule
• Transport proteins allow the cell to control which substances enter or leave
• Receptors

  • For the binding of peptide hormones, e.g. insulin, neurotransmitters or antibodies
  • The binding generates a signal that triggers a series of reactions inside the cell
• Immobilized enzymes

  • Integral proteins with the active site exposed on the surface of the membrane
  • Can be inside or outside the cell
• Cell adhesion

  Allows cells to attach to neighbouring cells within a tissue
• Cell-to-cell recognition

  Glycoproteins act as cell markers, or antigens, for cell-to-cell recognition