AQA A Level Biology复习笔记1.6.4 Inorganic Ions

Inorganic Ions

 

  • An ion is an atom (or sometimes a group of atoms) that has an electrical charge
    • An ion that has a +ve charge is known as a cation
    • An ion that has a -ve charge is known as an anion

     

  • An inorganic ion is an ion that does not contain carbon
  • Inorganic ions play an important role in many essential cellular processes
  • Inorganic ions occur in solution in the cytoplasm and body fluids of organisms
    • Some occur in high concentrations and others in very low concentrations
    • The concentration of certain ions can fluctuate and can be used in cell signalling and neuronal transmission
    • Each type of inorganic ion has a specific role, depending on its properties

     

Properties & Roles of Inorganic Ions

  • You should know the following inorganic ions, as well as their properties and roles in the body:
    • Hydrogen ions (H+)
    • Iron ions (Fe2+/Fe3+)
    • Sodium ions (Na+)
    • Phosphate ions (PO43-)
    • Calcium ions (Ca2+)

     

Hydrogen ions

  • Hydrogen ions are protons
  • The concentration of H+ in a solution determines the pH
  • There is an inverse relationship between the pH value and the hydrogen ion concentration
    • The more H+ ions present, the lower the pH (the more acidic the solution)
    • The fewer H+ ions present, the higher the pH (the more alkaline the solution)

     

  • The concentration of H+ is therefore very important for enzyme-controlled reactions, which are all affected by pH
    • The fluids in the body normally have a pH value of approximately 7.4
    • The maintenance of this normal pH is essential for many of the metabolic processes that take place within cells
    • Changes in pH can affect enzyme structure
    • For example, abnormal levels of hydrogen ions can interact with the side-chains of amino acids and change the secondary and tertiary structures of the proteins that make up enzymes
    • This can cause denaturation of enzymes

     

Iron ions

  • There are actually two versions of iron ions (known as oxidation states)
    • Iron (II) ions, also known as ferrous ions (Fe2+)
    • Iron (III) ions, also known as ferric ions (Fe3+)

     

  • Iron ions are essential as they can bind oxygen
    • Haemoglobin is the large protein in red blood cells that is responsible for transporting oxygen around the body
    • Haemoglobin is made up of four polypeptide chains that each contain one Fe2+
    • This Fe2+ is a key component in haemoglobin as it binds to oxygen
    • Myoglobin in muscles functions in a similar way (it is an oxygen-binding protein) but is only made up of one polypeptide chain (containing one Fe2+)

     

  • Iron ions are also essential as they are involved in the transfer of electrons during respiration and photosynthesis, so they are key to the biological generation of energy
    • Iron ions are an essential component of cytochromes (that are themselves a component of electron transport chains)
    • Cytochrome c contains an iron ion that is essential to its function
    • During the electron transport process, this iron ion switches between the Fe3+ and Fe2+ oxidation states, which allows for electrons to be accepted and donated

     

Sodium Ions

  • Na+ is required for the transport of glucose and amino acids across cell-surface membranes (e.g. in the small intestine)
    • Glucose and amino acid molecules can only enter cells (through carrier proteins) alongside Na+
    • This process is known as co-transport
    • First, Na+ is actively transported out of the epithelial cells that line the villi
    • The Na+ concentration inside the epithelial cells is now lower than the Na+ concentration in the lumen of the small intestine
    • Na+ now re-enters the cells (moving down the concentration gradient) through co-transport proteins on the surface membrane of the epithelial cells, allowing glucose and amino acids to enter at the same time

     

  • Na+ is also required for the transmission of nerve impulses

Phosphate Ions

  • PO43- attaches to other molecules to form phosphate groups, which are an essential component of DNA, RNA and ATP
  • In DNA and RNA, the phosphate groups allow individual nucleotides to join up (to form polynucleotides)
  • In ATP, the bonds between phosphate groups store energy
    • These phosphate groups can be easily attached or detached
    • When the bonds between phosphate groups are broken, they release a large amount of energy, which can be used for cellular processes

     

  • Phosphates are also found in phospholipids, which are key components of the phospholipid bilayer of cell membranes

Calcium Ions

  • Ca2+ is essential in the movement of organisms:
    • In synapses, calcium ions regulate the transmission of impulses from neurone to neurone

     

  • Ca2+ also stimulates muscle contraction
    • When an impulse reaches a muscle fibre, Ca2+ is released from the sarcoplasmic reticulum
    • This Ca2+ binds to troponin C, removing the tropomyosin from myosin-binding sites on actin
    • This allows actin-myosin cross-bridges to form when the muscle fibre contracts

     

  • Ca2+ can also help to regulate protein channels, which affects the permeability of cell membranes
  • Many enzymes are activated by Ca2+, making these ions key regulators in many biological reactions
  • The presence of Ca2+ is also necessary for the formation of blood clots (it is known as a clotting factor)

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