Unit 2

• The human body is composed of trillions of cells, of many different types
• 4 essential facts in cell theory


  • Animals and plants are made up of many eukaryotic cells
  • All cells are derived from the division of existing cells, although certain cells can change (differentiate) into other forms later on
  • The cell represents the smallest unit that can perform all essential physiological processes (i.e. those processes essential for life)
  • Every cell maintains its own homeostasis
• Homeostasis


  The stability of the internal environment (i.e. in terms of solute potential, temperature, pH etc.) even when the external environment changes
• 2 distinct taxa of cells


  • Eukaryotic cells
  • Prokaryotic cells
• All multicellular organisms are made up of eukaryotic cells
• Cell metabolism


  The chemical process involved in converting carbohydrates, fats, and proteins into the energy needed for cellular functions
• Adenosine triphosphate (ATP)


  A special "unit of currency" for transferring energy in living cells
• Cell energy production


  1. Anaerobic (without oxygen) glycolytic pathway, occurring in the cytoplasm

  2. Aerobic (with oxygen) pathway, occurring in the mitochondria
• Eukaryotic cells


  Cells defined by the presence of a membrane-bound nucleus, and the presence of other membrane-bound organelles, such as the mitochondria

  
• All multicellular organisms are eukaryotes, although monocellular eukaryotes also exist, such as the protozoa
• The human body is approximately 60% (by weight) water
• Extracellular fluid


  Comprises blood, lymph and the interstitial fluid, the latter being the extracellular fluid that surrounds cells in most tissue
• Cytoplasm


  The complete contents of the cell, including the cytosol and organelles
• Cell membrane


  Also known as the plasma membrane, it is a physical barrier between the outside and inside of the cell
• Primary functions of the cell membrane


  • To act as a barrier to prevent general ingress / egress of material, thus preserving the cell's status as an individual unit
  • To control movement of desirable material into the cell (e.g. glucose, certain ions), and the expulsion of waste material and secretions out of the cell, without otherwise breaching the barrier
  • To communicate with the world outside the cell
  • To make physical connections
• Phospholipid bilayer


  The predominant composition of the plasma membrane, with the polar phosphate heads orientating towards the polar cytosol inside the cell and the watery medium outside, and the hydrophobic tails lining up on the inside of the bilayer

  
• Membrane transport proteins


  Selective and specific for the molecules they transport, often using energy to mediate passage
• Membrane receptors


  Bind signals, such as hormones and immune mediators, to their extracellular portions, causing a conformational change that transmits a signal to intracellular messenger molecules
• Types of membrane transport


  • Passive transport (diffusion, facilitated diffusion)
  • Active transport
• Passive transport


  Requires no energy input as transport follows a concentration gradient
• Diffusion


  Spontaneous movement of a membrane permeable substance across the membrane, down the concentration gradient for the substance
• Membrane proteins that mediate communication


  Bind signals, such as hormones and immune mediators, to their extracellular portions. Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules.
• Receptor proteins


  Specific and selective for the molecules they bind (similar to the lock and key mechanism)
• Transporters (membrane transport/carrier proteins)


  Specialized membrane-spanning proteins that assist in the movement of ions, peptides, small molecules, lipids and macromolecules across a biological membrane
• Types of transport


  • Passive
  • Active
• Types of passive transport


  • Diffusion
  • Facilitated transport
• Diffusion


  Spontaneous movement of a membrane permeable substance across the membrane, down the concentration gradient for the substance (from an area of higher concentration, to an area of lower concentration)
• Facilitated transport


  The movement of a membrane impermeable substance across the membrane via transporters, down the concentration gradient
• Active transport


  Requires energy (usually from ATP hydrolysis) to transport substances into a cell, sometimes against the concentration gradient
• Types of active transport


  • Primary active transport
  • Secondary active transport
• Primary active transport


  The transport protein contains an ATPase, which hydrolyses ATP to generate the energy required for transport (e.g. ion pumps)
• Secondary active transport


  There is no direct coupling of ATP with the transporter. Instead, the transporter makes use of an existing difference in electrochemical potential between cell and fluid. Pumping ions out of the cell via primary active transport creates this electrochemical potential, and hence ATP is still (indirectly) consumed by this mode of transport.
• Factors affecting diffusion across a membrane


  • Concentration gradient
  • Surface area of the membrane/membrane thickness
  • Molecular size
  • State of ionisation
  • Lipid solubility
• Concentration gradient


  If a molecule can pass through a membrane, there is a natural flow from areas of high concentration to areas of low concentration
• Surface area of the membrane

  As the surface area of the membrane increases, the rate of diffusion also increases, as there is more space for molecules to diffuse across the membrane
• Membrane thickness


  The shorter the distance the substances have to move, the faster the rate of diffusion
• Molecular size


  In general terms, the smaller the molecule is, the easier it can move through a membrane by passive diffusion
• It is generally believed that the molecular weight (MW) of a compound must be under 500 Dalton to allow skin absorption and larger molecules cannot pass the corneal layer
• State of ionisation


  Most drugs used in clinical practice are weak acids or bases that exist in ionised and unionised forms depending on the pH. It is the lipid-soluble, non-ionised form of the drug that readily diffuses across the membrane.
• Acid-base reaction


  1. HA (acid) + H2O ⇌ H3O+ + A- (conjugate base)

  2. H3O+ (acid) + A- ⇌ HA + H2O (conjugate base)