ATP, water and inorganic ions

the structure of ATP

• All organisms require a constant supply of energy to maintain their cells and stay alive

• This energy is required:

  • for anabolic reactions, building larger molecules from smaller molecules

  • to move substances across the cell membrane (active transport) or to move substances within the cell

• In all known forms of life, ATP (adenosine triphosphate) from respiration is used to transfer energy in all energy-requiring processes in cells

• This is why ATP is known as the universal energy currency

• ATP is a phosphorylated nucleotide

  • Adenosine (a nucleoside) can be combined with one, two or three a phosphorylated groups

    • One phosphate group = adenosine monophosphate (AMP)

    • Two phosphate groups = adenosine diphosphate (ADP)

    • Three phosphate groups = adenosine triphosphate (ATP)

hydrolysis of ATP

• Energy released during the reactions of respiration is transferred to the molecule ATP

• The use of ATP as an ‘energy-currency’ is beneficial for many reasons:

  • The hydrolysis of ATP can be carried out quickly and easily wherever energy is required within the cell by the action of just one enzyme, ATPase

  • A useful (not too small, not too large) quantity of energy is released from the hydrolysis of one ATP molecule - this is beneficial as it reduces waste but also gives the cell control over what processes occur

  • ATP is relatively stable at cellular pH levels

• Hydrolysis of ATP to adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) is catalysed by the enzyme ATP hydrolase, sometimes called 'ATPase'

• The hydrolysis of ATP can be coupled to energy-requiring reactions within cells such as:

  • the active transport of ions up a concentration gradient

  • enzyme-controlled reactions that require energy

  • muscle contraction and muscle fibre movement

• As ADP forms, free energy is released that can be used for processes within a cell, e.g. DNA synthesis

• The inorganic phosphate released during the hydrolysis of ATP can be used to phosphorylate other compounds, often making them more reactive

ATP synthesis

• On average, humans use more than 50 kg of ATP in a day, but only have a maximum of ~ 200g of ATP in their body at any given time

  • Organisms cannot build up large stores of ATP, and it rarely passes through the cell surface membrane

• This means the cells must make ATP as and when they need it

• ATP is formed when ADP is combined with an inorganic phosphate (Pi) group by the enzyme ATP synthase

  • This is an energy-requiring reaction

  • Water is released as a waste product, therefore, ATP synthesis is a condensation reaction

• ATP is made during the reactions of respiration and photosynthesis

  • All of an animal's ATP comes from respiration

types of ATP synthesis

• ATP can be made in two different ways:

  • Substrate-linked phosphorylation (occurs in the glycolysis stage of respiration)

  • Chemiosmosis (occurs in the electron transport chain stage of respiration)

water in cells

• Water is of great biological importance. It is the medium in which all metabolic reactions take place in cells

• Water is composed of atoms of hydrogen and oxygen

  • One atom of oxygen combines with two atoms of hydrogen by sharing electrons (covalent bonding)

• Water is a polar molecule

• Hydrogen bonds form between water molecules

  • As a result of the polarity of water, hydrogen bonds form between the positively and negatively charged regions of adjacent water molecules

properties of water

• Hydrogen bonds contribute to the many properties water molecules have that make them so important to living organisms:

  • An excellent solvent – many substances can dissolve in water

  • A relatively high specific heat capacity

  • A relatively high latent heat of vaporisation

  • Water is less dense when a solid

  • Water has high surface tension and cohesion

  • It acts as a reagent

water as a metabolite

• Water is a metabolite in many metabolic reactions, including condensation and hydrolysis reactions

  • In condensation reactions, smaller molecules combine to form a larger molecule, with the removal of a water molecule

    Examples include:

    • formation of peptide bonds between amino acids to make proteins

    • formation of glycosidic bonds in carbohydrates

    • formation of ester bonds in lipids

  • In hydrolysis reactions, water is added to break a bond within a larger molecule, splitting it into smaller units

    Examples include:

    • breaking proteins into amino acids

    • breaking starch into glucose

    • breaking triglycerides into fatty acids and glycerol

water as a solvent

• As water is a polar molecule, many ions (e.g. sodium chloride) and covalently bonded polar substances (e.g. glucose) will dissolve in it

  • This allows chemical reactions to occur within cells (as the dissolved solutes are more chemically reactive when they are free to move about)

  • Metabolites can be transported efficiently (except non-polar molecules, which are hydrophobic)

high specific heat capacity

• Specific heat capacity is a measure of the energy required to raise the temperature of 1 kg of a substance by 1 ^°C

  • Water has a high specific heat capacity of 4200 J / Kg ^°C, meaning a relatively large amount of energy is required to raise its temperature

• The high specific heat capacity is due to the many hydrogen bonds present in water. It takes a lot of thermal energy to break these bonds and a lot of energy to build them; thus, the temperature of water does not fluctuate greatly, meaning:

  • stable habitats can be provided, especially for aquatic organisms

  • water absorbs lots of heat with minimal temperature change

    • This helps maintain stable internal and external temperatures, essential for enzyme function

  • water in blood plasma transfers heat around the body, aiding temperature regulation

    • As blood flows through warmer tissues, it absorbs heat without large temperature shifts

latent heat of vaporisation

• To change state (from liquid to gas), a large amount of thermal energy must be absorbed by water to break the hydrogen bonds and evaporate

• This is an advantage for living organisms, as only a little water is required to evaporate for the organism to lose a great amount of heat

• This provides a cooling effect for living organisms, for example, the transpiration from leaves or the evaporation of water in sweat on the skin

cohesion and adhesion

• Hydrogen bonds between water molecules allow for strong cohesion between water molecules

  • This allows columns of water to move through the xylem of plants and the blood vessels in animals

  • This also enables surface tension where a body of water meets the air; these hydrogen bonds occur between the top layer of water molecules to create a sort of film on the body of water

• Water is also able to hydrogen bond to other molecules, such as cellulose, which is known as adhesion

  • This also enables water to move up the xylem due to transpiration

inorganic ions

• An ion is an atom (or sometimes a group of atoms) that has an electrical charge

  • An ion that has a positive charge is known as a cation

  • An ion that has a negative charge is known as an anion

• An inorganic ion is an ion that does not contain carbon

• Inorganic ions occur in solution in the cytoplasm and body fluids of organisms

  • The concentration of certain ions can fluctuate and can be used in cell signalling and neuronal transmission

hydrogen ions

• Hydrogen ions (H⁺) 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

iron ions

• There are 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

sodium ions

• Sodium ions (Na⁺) are 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⁺ in a process known as co-transport

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

phosphate ions

• Phosphate ions (PO₄^3-) attach 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 tobond 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