Major activities at the cellular level that need energy in living organisms:
1. Active transport: as of ions and molecules
2. Movement: as of the cilia and muscles.
3. Anabolic reactions: such as DNA replication, protein synthesis, etc.
Adenosine triphosphate (ATP):
- ATP is the energy currency of the cell.
- Every cell produces or consumes ATP for its energy demands.
- Within each cell, energy-yielding reactions are linked with energy-consuming reactions via the universal intermediary molecule, ATP.
- Besides the ‘linking role’ played by ATP, its structural features help it qualify for the title of ‘universal energy currency’.
- ATP consists of three parts:
  - First, there is a ribose sugar.
  - Second, there’s a nitrogenous base called adenine.
  - And, the third part is the phosphate group.
- Adenine, together with the ribose sugar, is called adenosine. That’s what the A in ATP stands for. A phosphate group containing three phosphate molecules is attached to this adenosine.
- Moreover, adenosine can be combined with one, two, or three phosphate molecules to give, adenosine monophosphate (AMP), adenosine diphosphate (ADP), or adenosine triphosphate (ATP) respectively.
- ATP has all of its energy in bonds between the phosphate molecules. The two outer phosphate molecules contain high-energy bonds and more energy is released when they are broken.
- So when a phosphate molecule is removed from ATP by hydrolysis, adenosine diphosphate (ADP) is formed and 30.5 kJmol^-1 of energy is released.
- When the second phosphate molecule is hydrolysed, adenosine monophosphate (AMP) is produced, and again 30.5 kJmol^-1 of energy is released.
- But, the hydrolysis of the last phosphate molecule results in adenosine, releasing only 14.2 kJmol^-1 of energy.
- All these reactions are reversible. It is the interconversion of ATP to ADP that is all-important in providing energy for the cell.
- Features of ATP that makes it the universal energy currency:
  - ATP gets easily hydrolysed to provide energy to cells in need.
  - Not only is ATP readily hydrolysed, it is also small in size and water-soluble. This allows it to be easily transported within cells or the cytoplasm.
Synthesis of ATP in cells:
- ATP is mostly produced from ADP in the membranes of chloroplasts and mitochondria through the process of chemiosmosis.
  - During chemiosmosis, a phosphate molecule is added to ADP, resulting in the synthesis of ATP.
  - The process of adding a phosphate molecule to ADP is called phosphorylation.
  - The process of phosphorylation that occurs in the chloroplast membrane is called photophosphorylation.
  - In contrast, if it occurs in the mitochondrial membrane, it is called oxidative phosphorylation.
- ATP is also synthesised by another process called substrate-level phosphorylation.
  - In substrate-level phosphorylation, other donor molecule, known as substrate, provide the phosphate group to an enzyme to help it synthesise ATP.
  - This happens in the cytoplasm of the cell during the processes of glycolysis and the Kreb’s cycle.
Relative energy values of carbohydrates, lipids, and proteins as respiratory substrates:
- Carbohydrates, lipids, and proteins are calorigenic nutrients that provide energy once they are broken down during respiration.
- They are also called respiratory substrates because they act as donor molecules that provide the phosphate group, with the help of enzymes, for the synthesis of ATP, like in the case of substrate-level phosphorylation.
- Although carbohydrates are the first choice for instant energy, they do not have the highest energy value.
- The relative energy values per unit mass (energy density) of the respiratory substrates are as follows:
- Lipids have the highest energy value, whereas carbohydrates have the lowest.
- During aerobic respiration, most of the energy comes from the oxidation of hydrogen to water.
- The energy values of the respiratory substrates are determined by a principle:
  The greater the number of hydrogen atoms in the structure of the respiratory substrate molecule, the greater the energy value.
- The fatty acids in the lipids have more hydrogen atoms per molecule than carbohydrates and proteins.
- So, lipids have a greater energy value per unit mass than carbohydrates or proteins.
- Since lipids have a large number of hydrogen atoms for oxidation, they consume a lot of oxygen during aerobic respiration.
- Lipids take in more oxygen and release less carbon dioxide during aerobic respiration than simple carbohydrates like glucose.
Respiratory Quotient (RQ):
- The Respiratory Quotient is the ratio of the number of molecules of carbon dioxide produced to the number of molecules of oxygen taken in, as a result of respiration.
- Calculation of RQ values of different respiratory substrates from the equations for respiration:
- This is based on the determination of the volumes of carbon dioxide and oxygen that is known from the equation for the oxidation of specific respiratory substrates.
- RQ value of carbohydrates:
  - The equation for the oxidation of glucose through aerobic respiration can be denoted by:
    C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 6H₂O + Energy
  - Now the RQ value can be calculated as the molecules of carbon dioxide given out divided by the molecules of oxygen taken in, which gives the result of 1 when calculated as:
    RQ= $\frac{CO_2}{O_2}$
    =$\frac{6}{6}$
    = 1.0
- RQ value of lipids:
  - The equation for the oxidation of oleic acid is given as:
    C₁₈H₃₄O₂ + 25.5 O₂ → 18 CO₂ + 17H₂O
  - Since the number of carbon dioxide molecules given out is 18 and the number of oxygen molecules taken in is 25.5, the RQ for this data would be 0.7 when calculated as:
    RQ= $\frac{CO_2}{O_2}$
    =$\frac{18}{25.5}$
    =0.7
- RQ value of proteins:
  - In the case of proteins, no single RQ value can be assigned overall.
  - This is because of the complexity of ways by which different amino acids in the protein are metabolised. So, an RQ value of 0.8 to 0.9 is a general estimate.
- Respiratory Quotient (RQ) value of respiratory substrates:
  - Note:
    - RQ values are calculated only for aerobic respiration of the substrates. In the case of anaerobic respiration, there is no oxygen consumed. So, the denominator in the equation of the RQ becomes zero. And any number divided by zero is greater than one.
Investigation to determine the RQ of small invertebrates (e.g. blowfly larvae):
- This involves the use of a simple apparatus known as a respirometer.
- Apparatus set up for a simple respirometer:
  - On the left is an experimental tube, containing small invertebrates as respiring cells, supported by a gauze platform.
  - Below that is soda-lime, or a concentrated solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) that absorbs any carbon dioxide produced during respiration.
  - The experimental tube is connected to a graduated U-tube containing manometer fluid, for detecting changes in pressure inside the tube.
  - As changes in atmospheric pressure can alter the volume of oxygen produced, this effect is compensated by connecting it to a control tube on the right via the same capillary U-tube.
  - The control tube is exactly the same as the experimental tube, except that it contains inert non-respiring material like glass beads of the same volume as the invertebrates.
  - A syringe is fixed on top of the control tube to adjust the volume of manometer fluid to a certain level before the experiment starts.
- When the invertebrates start to respire, the carbon dioxide produced gets absorbed by the soda lime or KOH or NaOH.
- Any decrease in the volume of air surrounding the invertebrates, results from the consumption of oxygen by the invertebrates.
- The volume of oxygen consumed in unit time can be measured by reading the level of manometer fluid against the scale.
Measurement of RQ value of invertebrates:
- To measure the RQ of the invertebrates, the volume of oxygen consumed at a particular temperature is first calculated. Let’s suppose that it is x cm³ min^-1.
- Then the respirometer is set in the same way, but this time without soda-lime or KOH or NaOH.
- When the volume of oxygen consumed and carbon dioxide produced are the same, the level of manometer fluid will not change, and the RQ equals 1.
- But when more carbon dioxide is produced than the oxygen absorbed, the scale will show an increase in the volume of air in the respirometer. Let’s suppose this is equal to y cm³min^-1.
- The RQ can then be calculated as:
  RQ = $\frac{x+y}{x}$
- Conversely, when less carbon dioxide is produced than the oxygen absorbed, the volume of air in the experimental tube will decrease by, let’s suppose, z cm³min^-1.
- So, the calculation for RQ would be:
  RQ = $\frac{x-z}{x}$
- An important precaution to be considered is that the experiment has to be carried out at a constant temperature, say, in a water-bath, making sure there is no chance of air leakage.
- One can also do the experiment using germinating seeds and calculate the RQ value likewise.
- Note:
  - The calculation of the RQ value is not only useful to identify the type of substrate used for respiration, but also to identify the type of respiration, which can either be aerobic or anaerobic.

12.01 - Energy

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