CHAPTER 6

Energy for living processes

Energy is required for an organism to carry out the basic living processes such as movements, digestion, reproduction, response, secretion, etc. The energy must be supplied constantly to the cells so that chemical reactions at the cellular level can be carried out. This in turn allows the basic living processes to be carried out by the organism.

Energy is available in organic food molecules in the form of carbohydrates. Respiration is required where food molecules are broken down and energy is produced. The energy is subsequently supplied to the cells so that the cells can carry out various processes at the cellular level. Processes at the cellular level include active transport, synthesis of protein, cell division, formation of gamete, nerve transmission and so on.

The main substrate for energy production

The main substrate for respiration is glucose. For humans and animals, glucose is derived from the digestion of carbohydrates. In plants, glucose is derived from photosynthesis.

Two types of respiration

Respiration can be defined as a metabolic process that involves the breaking down of organic nutrients, such as glucose, into simpler products to produce energy for the cells. There are two types of respiration, external respiration and internal respiration.

External respiration is the breaking down of organic nutrients through gaseous exchanges between body tissues and the environment. It involves breathing or inhaling of air containing oxygen from the atmosphere into the lungs followed by exhaling of air containing carbon dioxide back to the atmosphere.

Internal respiration, on the other hand, is the breaking down of nutrients in the cells through cellular respiration. It involves a series of chemical reactions in the cells. No oxygen is used in this respiration. As such, the respiration is categorised into two groups as follows

i.Aerobic respiration	– the respiration that uses oxygen
ii.Anaerobic respiration	– the respiration in the absence of oxygen

Aerobic respiration

Aerobic respiration is the breaking down of glucose in the presence of oxygen to release chemical energy. Oxygen is required to oxidise the glucose and the products of the oxidation are arbon dioxide, water and energy. Glucose is completely oxidised in order to release all its chemical energy.

Anaerobic respiration

Anaerobic respiration is a type of cell respiration which occurs in the absence of oxygen to release energy. Glucose is broken down in the absence of oxygen to release chemical energy. However, the glucose is not completely broken down and not all the energy in the glucose is released. Some of the energy in the glucose is stored as the by-product from the anaerobic respiration. Only small amounts of energy are released. Anaerobic respiration occurs in the cytoplasm.

Anaerobic respiration in human muscles

• Anaerobic respiration occurs in human muscles during vigorous exercise or vigorous activities, such as when a person is running. Oxygen is needed at a higher rate and is transported quickly to the muscles for rapid cell respiration. This enables the release of sufficient energy for the vigorous muscle activities to take place.

Anaerobic respiration in yeast

Yeast is a microorganism that undertakes anaerobic respiration. Such respiration is called fermentation. During fermentation, yeast secretes the enzyme called zymase. The zymase hydrolyses glucose in the absence of oxygen to form ethanol, carbon dioxide and energy.

The enzyme zymase secreted by the yeast speeds up the fermentation process. Only a small amount of energy is released in the fermentation process. A large amount of energy is still stored in the ethanol as chemical energy. This is because the glucose is not completely broken down in anaerobic respiration.

Comparison between aerobic respiration and anaerobic respiration

Different respiratory structures in organisms

Breathing is the mechanical process of gas exchanges. It involves the taking in of oxygen into the lungs (inhalation) and the removal of carbon dioxide from the lungs (exhalation). All organisms need respiratory structures for the gaseous exchanges. Their structures must be well adapted so as to maximise the rate of these exchanges. 

The gaseous exchanges occur via diffusion and take place on the surfaces of the respiratory structures. The respiratory surfaces must be adapted for maximum gaseous exchanges and this is done by increasing the total surface area. The larger the surface area of the respiratory surfaces, the higher will be the rate of diffusion for gaseous exchanges. 

Several characteristics should be present in respiratory surfaces for them to achieve the optimum rate for gaseous exchanges.

Respiratory structures and the breathing mechanisms in organisms

Protozoa (unicellular organisms)

Protozoa are unicellular microorganisms such as the Amoeba and paramecium. They are very small in size and have large total surface area to volume ratios. The gaseous exchanges are achieved by simple diffusion and they occur rapidly and efficiently across the thin plasma membrane

Oxygen from the atmosphere diffuses into the cells down the partial pressure gradient while carbon dioxide diffuses out of the cells through their permeable membranes via the same mechanism. The respiratory structure of the protozoa is thus a very simple one.

Respiratory structures and the breathing mechanism in humans

The human respiratory system consists of a complex respiratory structure. It is made up of the nasal cavity, pharynx, tracheae, bronchi, bronchioles and lungs. The respiratory process involvea air entering through the nostrils and subsequently into the pharynx, tracheae, bronchi, bronchioles, finally ending in air sacs called alveoli. There are as many as 700 million air sacs surrounded by a capillary network for the purpose of gaseous exchanges.

Human respiratory system

The trachea and bronchi are strengthened by cartilage rings (C-shaped). This is to prevent the respiratory tube from collapsing during breathing. At the same, the cartilage rings keep the tube open and allowed the passage of air.

The epithelium lining of the trachea and bronchi is moist. This is to trap dust and microorganisms present in the inhaled air. The trapped particles are moved by the cilia on the epithelial lining to the pharynx and then to the stomach for removal. The trachea branches into two bronchi each of which goes into a lung. Each of the bronchi branches into bronchioles and subsequently, branch and re-branch into finer tubes, finally ending in alveoli. Oxygen enters the alveoli and diffuses through the epithelium lining and the capillary walls into the blood.

The Breathing Mechanism in Humans during Inhalation and Exhalation

The breathing mechanism involves air entering the lungs during inhalation and air moving out of the lungs during exhalation. The lungs do not have muscles but breathing is made possible by the action of a set of antagonistic intercostals muscles and the action of the diaphragm muscles

The Exchange of Oxygen and Carbon Dioxide at the Alveoli

The exchange of oxygen and carbon dioxide at the alveoli is made possible because of the partial pressure of these two gases existing between the alveoli and the blood capillaries. This partial pressure exists because each component of gas in a mixture of gases exerts its own pressure. Partial pressure can defined as the fraction of the total pressure exerted by the gas.

In the atmosphere, there is 21% oxygen, so the partial pressure of oxygen Po in the atmosphere is 21% x 760 mmHg (total atmospheric pressure) = 159.6 mmHg. In the same way, there is 0.03% of CO₂ so that the partial pressure of CO₂ Pco in the atmosphere is 0.03% x 760 mmHg = 0.23 mmHg.

Inhalation

During inhalation, the partial pressure of oxygen P_o in the inhaled air in the alveoli is higher (105mmHg) compared to the partial pressure of oxygen P_o in the blood capillaries of the lungs (95mmHg). The higher partial pressure of oxygen in the alveoli forces the oxygen to be dissolved in the layer of moisture on the walls of the alveoli : it subsequently diffuses out into the blood capillaries.

 

Gaseous exchange across the alveolus surface and surrounding blood capillaries

Exhalation

Carbon dioxide, which is produced by the body cells, is brought by the blood capillaries to the alveoli. Hence, the partial pressure of carbon dioxide P_co in the blood capillaries entering the alveoli is higher (45 mmHg) than the partial pressure of carbon dioxide P_co in the alveoli (40mmHg). The difference in the partial pressure forces the carbon dioxide in the blood capillaries to diffuse into the alveoli and it is expelled during exhalation. 

Transport of Respiratory Gases

uring gaseous exchanges, oxygen diffuses into the blood capillaries. Once it is in the blood capillaries, the oxygen has to be transported to the body cells where it is needed. To be transported, oxygen dissolves in the plasma, diffuses in the red blood cells and combines with a respiratory pigment called haemoglobin to form oxyhaemaglobin.

The transport of oxygen is achieved via the blood circulatory system. Oxygenated blood flows from the lungs to the whole body in the form of oxyhaemoglobin. Oxyhaemoglobin is not stable. When cells lack oxygen, the oxyhaemoglobin breaks down and releases the oxygen to diffuse into the cells.

Transport of carbon dioxide

The respiration at the body cells releases carbon dioxide. The carbon dioxide diffuses into the blood capillaries due to the differences in partial pressure. In the capillaries, the blood dissolves in the plasma and enters the red blood cells. A large part of the carbon dioxide is converted into bicarbonate ions. However, some carbon dioxide combines with haemoglobin to form carbaminohaemoglobin in the red blood cells.

The carbon dioxide is then transported to the alveoli in the form of carbaminohaemoglobin and bicarbonate ions in the blood plasma. At the alveoli, carbon dioxide is released from its compound, diffuses into the alveoli and is subsequently expelled during exhalation.

Gaseous Exchanges at the Body Cells

The gaseous exchanges at the body cells are also achieved by means of the gradient of the gas partial pressure. The partial pressure in the blood capillaries is higher than the partial pressure of oxygen in the body cells. As a result, oxygen is forced out of the blood capillaries into the body cells.

In addition, the low partial pressure in oxygen in the body cells results in the low affinity of haemoglobin to oxygen in the cells. This condition makes it more favourable for the oxygen to be released and diffuse into the body cells.

At the same time, the body cells contain a high concentration of carbon dioxide due to respiration at the cells. Hence, the partial pressure of carbon dioxide in the cells is higher than the partial pressure of carbon dioxide in the blood capillaries surrounding them. The carbon dioxide is forced to diffuse out of the body cells into the blood capillaries.

Rate of respiration and vigorous exercise

When a person is doing vigorous activities such as running, more energy is needed by the body to undertake the physical movements. This in turn increases the metabolic rate; the cells require more glucose and oxygen as more energy is given out in cellular respiration. This results in an increase in the breathing rate and heart beat. The increased breathing rate ensures that more oxygen is inhaled and the increase of the heart beats ensures that more blood is being circulated around the body with more oxygen being transported to the respiring body cells. At the same time, the excess carbon dioxide is expelled to the lungs.

The regulation of oxygen and carbon dioxide contents in the body

In humans, the control centre that regulates the basic rhythm of breathing is located in the medulla oblongata in the brain. The centre regulates the rhythm of breathing by controlling the strength and frequency of contraction and relaxation of the intercostal muscles and diaphragm muscles.

Breathing centre in the medulla oblongata

The control centre consists of a special group of cells called central chemoreceptors. The central chemoreceptors in the centre are triggered when there are changes in the oxygen and carbon dioxide concentrations in the body.

During vigorous activities, the rate of respiration in the body cells increases and this in turn increases the concentration of carbon dioxide in the blood. The high concentration of carbon dioxide in the blood lowers the pH value of the blood. The drop in the pH value is detected by the central and peripheral chemoreceptors. We shall look at how both chemoreceptors regulate the rhythm of breathing.

Central chemoreceptors

The drop in the pH value of the blood and tissue fluid bathing the brain stimulates the central chemoreceptors to emit nerve impulses to the respiratory centre. The respiratory centre sends nerve impulses to the intercostal muscles and the muscles of the diaphragm. The intercostal muscles and the diaphragm muscles contract rapidly, causing the rate of breathing and heart beats to increase.

The increases in the rate of breathing and the heart beats enable more oxygen to be supplied all over the body and more carbon dioxide to be produced. This will continue until the level of the pH returns to normal.

Peripheral chemoceptors

Besides the central chemoreceptors, periphery chemoreceptors also respond to changes of oxygen in the body. A decrease in the concentration of oxygen to a very low value, such as at high altitudes where the atmospheric oxygen value is low, will stimulate the periphery receptors to emit nerve impulses. The nerve impulses are then sent to the breathing centre at the medulla oblongata.

The breathing centre responds by sending nerve impulses to the respiratory muscles. Action of the muscles becomes more rapid and will increase the rate of breathing and ventilation. Nerve impulses are also sent to the heart causing the rate of the heart beats to increase. This causes more oxygen and glucose to be carried faster to the muscles for rapid cell respiration and releases energy for vigorous muscle activities. The increased rate of blood circulation also helps to remove carbon dioxide formed during cell respiration more rapidly.