Chapter 12: Energy and Respiration

Living organisms are composed of cells, and within each cell, many activities and processes are constantly being carried out to maintain life. Work in a living organism requires energy and usable carbon compounds

An organic molecule is a compound including carbon and hydrogen. The term originally meant a molecule derived from an organism, but now includes all compounds of carbon and hydrogen even if they do not occur naturally.

  • Respiration in all living cells releases energy from the breakdown of organic molecules
  • Respiration involves the transfer of chemical potential energy from nutrient molecules (such as carbohydrates, fats and proteins) into a usable energy form (through the synthesis of ATP) that can be used for work within an organism

Glucose equations

Glucose + oxygen → carbon dioxide + water + energy

C6H1206 + 6 O2 → 6 CO2 + 6 H20 + 2870kJ

ATP

Adenosine 5′-triphosphate, or ATP, is the principal molecule for storing and transferring energy in cells. When one phosphate group is removed by breaking a phosphoanhydride bond in a process called hydrolysis, energy is released, and ATP is converted to adenosine diphosphate (ADP).

  • It is made up of:
    • Ribose sugar
    • Adenine base
    • Three phosphate groups

It is the creation of ATP from ADP using energy from sunlight, and occurs during photosynthesis. ATP is also formed from the process of cellular respiration in the mitochondria of a cell. This can be through aerobic respiration, which requires oxygen, or anaerobic respiration, which does not.

 

  • 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

ATP Synthesis

Energy for ATP synthesis can become available in two ways. In respiration, energy releases by reorganizing chemical bonds during glycolysis and the Krebs cycle is used to make some ATP. However, most ATP in cells is generated using electrical potential energy. ATP is formed when ADP is combined with an inorganic phosphate (Pi) group. This is an energy-requiring reaction. Water is released as a waste product (therefore ATP synthesis is a condensation reaction).

ATP synthase has three binding site and a part of the molecule that rotated as hydrogen ions (H+) pass. This produces structural changes in the blinding sites and allows them to pass sequentially through three phases:

  • blinding ADP and Pi
  • forming tightly bound ATP
  • releasing ATP

Types of ATP synthesis

  • ATP is made during the reactions of respiration and photosynthesis
    • All of an animal’s ATP comes from respiration
  • ATP can be made in two different ways:
    • Substrate-linked phosphorylation
    • Chemiosmosis

Substrate-linked phosphorylation

  • ATP is formed by transferring a phosphate directly from a substrate molecule to ADP

ADP + Pi —> ATP

Chemiosmosis

Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An example of this would be the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membrane during cellular respiration or photosynthesis.

Respiration

Respiration is the process in which organic molecules act as a fuel. The organic molecules are broken down in a series of stages to release chemical potential energies which is used to synthesize ATP.

Glucose breakdown can be divided into four stages.

  • glycolysis
  • link reaction
  • Krebs Cycle
  • Oxidative phosphorylation

Glycolysis

Glycolysis is a cytoplasmic pathway which breaks down glucose into two three-carbon compounds and generates energy. Glycolysis is used by all cells in the body for energy generation. The final product of glycolysis is pyruvate in aerobic settings and lactate in anaerobic conditions.

The Link Reaction

The Link reaction, also known as pyruvate decarboxylation forms an important link between the metabolic pathways of glycolysis and the citric acid or Krebs cycle. Pyruvate is decarboxylated: CO2 is removed. It is added to CoA to form Acetyl CoA.

Pyruvate + NAD + CoA → acetyl CoA + carbon dioxide + reduced NAD

Krebs cycle

Krebs Cycle is a sequence of reactions in the living organism in which oxidation of acetic acid or acetyl equivalent provides energy for storage in phosphate bonds (as in ATP) — called also citric acid cycle, tricarboxylic acid cycle. It includes following:

  • Decarboxylation of citrate
    • Releasing 2 CO2 as waste gas
  • Dehydrogenation of citrate
    • Releasing H atoms that reduce coenzymes NAD and FAD
    • 8H + 3NAD + FAD → 3NADH + 3H+ + FADH2
  • Substrate-level phosphorylation
    • A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP

Oxidative Phosphorylation

In the process of oxidative phosphorylation, the electrons are passed along a series of carrier. Some of the energy released in oxidative phosphorylation is used to move protons from the mitochondrial matrix to the inter membrane space. The movement of electrons sets up a gradient of protons across the inner membrane of the mitochondrial envelope. Protons pass back into the matrix, moving down their concentration gradient through protein channels in the inner membrane. An enzyme, ATP synthase, is associated with each of the proton channels. ATP synthase uses the electrical potential energy to the proton gradient to phosphorylate ADP to ATP. At the end of the carrier chain, electrons and protons are recombined and reduce oxygen to water.

Structure and Function of Mitochondria

Mitochondria are membrane-bound cell organelles (mitochondrion, singular) that generate most of the chemical energy needed to power the cell’s biochemical reactions. Chemical energy produced by the mitochondria is stored in a small molecule called adenosine triphosphate (ATP). Mitochondria have two phospholipid membranes.

  • The outer membrane is:
    • Smooth
    • Permeable to several small molecules
  • The inner membrane is:
    • Folded (cristae)
    • Less permeable
    • The site of the electron transport chain (used in oxidative phosphorylation)
    • Location of ATP synthase (used in oxidative phosphorylation)
  • The inter membrane space:
    • Has a low pH due to the high concentration of protons
    • The concentration gradient across the inner membrane is formed during oxidative phosphorylation and is essential for ATP synthesis
  • The matrix:
    • Is an aqueous solution within the inner membranes of the mitochondrion
    • Contains ribosomes, enzymes and circular mitochondrial DNA necessary for mitochondria to function

Synthesis of ATP in the mitochondria occurs during the last stage of respiration called oxidative phosphorylation.

Functions of mitochondria are given below:

  • The main function of mitochondria, which are organelles also known as the powerhouse of the cell, is to produce energy.
  • Perhaps the most well-known role of mitochondria is the production of ATP, the energy currency of cells
  • Mitochondrial calcium exchange is the flow of calcium in and out of a cell’s mitochondria, a process important in metabolic regulation and cell death.
  • Mitochondria control the intrinsic pathway, releasing proteins such as cytochrome c from their intermembrance space in response to cell stresses such as heat, infection, hypoxia, increased calcium and nutrient deprivation.

Mitochondria are thought to play crucial roles in the maintenance of pluripotency, differentiation, and reprogramming of induced pluripotent stem cells.