Metabolism 1

What is Cellular Respiration?

Cellular respiration is the process by which energy is harvested. It is the complete oxidation of glucose. Food energy is the amount of energy in food that is available through digestion. Energy is used to do work. In order for cellular respiration to occur, we must obtain oxygen, otherwise anaerobic respiration occurs.

To show the amount of energy released from different substances, we can say:

• Carbohydrates release 4.28 cal/g
• Lipids release 9.46 cal/g
• Proteins release 4.32 cal/g

However, 60% of the energy from glucose metabolism is given off as heat. This means only 40% can be used in the cell.

Cellular respiration is different in plants than animals. In plants, photosynthesis occurs in the chloroplasts, which are stimulated by light. Chloroplasts produce glucose, which the mitochondria then converts to carbon dioxide and water as by-products.

What is Metabolism?

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. Metabolism is divided into two categories:

1. Catabolism is a set of metabolic pathways in which molecules are broken down into smaller units and energy is released. Metabolism involves only catabolic reactions.
2. Anabolism is a set of metabolic pathways in which smaller units are combined to construct larger molecules.

Metabolism is undergone to produce adenosine triphosphate (ATP), a nucleotide consisting of adenine, ribose sugar and three phosphate groups. The energy is stored between the 2nd and 3rd phosphates, which is then released when the bond is broken. In order to release energy, ATP is hydrolysed to ADP and Pi (inorganic phosphate).

Mostly, the mitochondria is the site of energy production. Most energy is produced in the mitochondrial matrix, which is the inner most part of the mitochondria. The efficiency of the mitochondrial function is increased by folds in the membrane, termed cristae.

Organisms are classified into two categories, which are based on how they obtain energy:

• Autotrophs produce their own organic molecules through photosynthesis
• Heterotrophs consume organic molecules made by autotrophs or consumed by heterotrophs

These organic molecules are the fuel molecules from which cellular respiration draws energy. The oxidation of these fuel molecules releases electrons, which are stored by high energy electron carriers; NAH+ and FADH. These electrons are released when NAD+ and FADH enter the electron transport chain.

Glucose catabolism: a brief introduction

The general equation representing glucose catabolism can be written:

glucose + oxygen → water + carbon dioxide + 36 ATP

Glucose catabolism involves redox reactions, where electrons are lost from one atom (oxidation) and gained by another atom (reduction). In summary, oxidation is the loss of electrons, loss of hydrogen but gain of oxygen. Reduction is the gain of electrons, gain of hydrogen and loss of oxygen.

Firstly, glucose is converted into pyruvate so it can enter the mitochondria (glycolysis). The citric acid cycle is then undergone in the mitochondria, and electrons are released which are trapped by high energy electron carriers, as mentioned above. These molecules move through the electron transport system, losing energy. Oxygen attracts the high energy carriers and as the electrons move, they lose energy and hydrogen ions are gained. ATP results.

Phosphorylation

Phosphorylation involves the addition or coupling of a phosphate group to a molecule, particularly sugars, so the cell can accumulate the sugars. There are two types of phosphorylation:

• Substrate level phosphorylation is the transferring of a phosphate directly to ADP from another molecule. It is usually carried out by the action of enzymes called kinases. In this phosphorylation, PEP transfers a phosphate to ADP, converting it to ATP and pyruvate, which is the end of the first step in metabolism.
• Oxidative phosphorylation occurs during the electron transport chain, where ATP synthase and energy from a proton (H ion) gradient are used to make ATP.

Mitochondrial Anatomy

Mitochondria are sausage-shaped organelles with a double membrane. The outer membrane contains pores with large diameters that are permeable to ions and small organic molecules, e.g. pyruvic acid. The inner membrane contains carrier proteins which move pyruvic acid into the mitochondrial matrix. The inter membrane space separates the outer and inner membranes.