3.3.2 Gas Exchange SPECIFICATION

Transcription

1 alevelbiology.co.uk Gas Exchange SPECIFICATION Adaptations of gas exchange surfaces, shown by gas exchange: Across the body surface of a single-celled organism In the tracheal system of an insect (tracheae, tracheoles, and spiracles) Across the gills of fish (gill lamellae and filaments including the counter-current principle) By the leaves of dicotyledonous plants (mesophyll and stomata) Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants. The gross structure of the human gas exchange system limited to the alveoli, bronchioles, bronchi, trachea and lungs. The essential features of the alveolar epithelium as a surface over which gas exchange takes place. Ventilation and the exchange of gases in the lungs. The mechanism of breathing to include the role of the diaphragm and the antagonistic interaction between the external and internal intercostal muscles in bringing about pressure changes in the thoracic cavity. Students should be able to interpret the information relating to the effects of lung disease on gas exchange and/or ventilation, interpret data relating the effects of pollution and smoking on the incidence of lung disease, analyse and interpret data associated with specific risk factors and the incidence of lung disease, evaluate the way in which experimental data led to statutory restrictions on the source of risk factors, and recognize correlations and causal relationships. Source: AQA Spec

2 3.3.2 Gas Exchange One of the major material exchanges happening in organisms is the exchange of gases. In particular, gases that are important to be exchanged are oxygen and carbon dioxide. Different organisms have developed different adaptations related to gas exchange. Single-celled Organisms 1. Diffusion is the main mode of exchange of material for single-celled organisms. They use diffusion to absorb and release gases. 2. Due to their small size, single-celled organisms have a high surface area where gas exchange can occur. They also developed a thin surface which make the diffusion pathway shorter. Insects 1. Insects developed a tracheal system for gas exchange. 2. Trachea contains pores in its surface called spiracles. It is through these spiracles that air moves. 3. Oxygen moves into the cell, down a concentration gradient (concentration of oxygen outside the cell is higher than inside the cell). 4. To reach individual cells of the insect s body, the trachea branch off into smaller tracheoles. 5. Like oxygen, carbon dioxide also moves down a concentration gradient, from the inside of the cell, to the spiracles. Eventually, carbon dioxide will be released into the atmosphere. 6. Rhythmic abdominal movements facilitate the moving in and out of air in the spiracles. Tracheal System of Insects

3 3.3.1 Surface Area to Volume Ratio Fish 1. Since fish live underwater, it is much harder to obtain oxygen from the environment because of the lower oxygen concentration in water than in air. 2. Fish developed gills which serve as the exchange organ. The gills contain thin plates, called gill filaments. These thin plates increase the surface area for gas exchange. 3. To increase the surface area further, tiny structures called lamellae are present in the surface of the gill filaments. 4. The respiratory system of a fish is well connected to the circulatory system to facilitate transport of material. A lot of blood capillaries are contained in the lamellae. 5. Movement of gases is from high concentration to low concentration. 6. A counter-current system happens in the lamellae to facilitate blood flow in one direction, and the water flow in the opposite direction. In this way, transfer of oxygen across a concentration gradient (from high oxygen-containing water to the low oxygen-containing blood) becomes more efficient. Fish Gills

4 3.3.2 Gas Exchange Mammalian Systems Lungs are the primary gas exchange organs in mammals and most vertebrate. Commonly this organ is connected to a mass transport system in the form of blood vessels of the circulatory system. Pulmonary ventilation happens by inhaling or breathing in air through a nasal cavity. As the air passes through the cavity it is warmed to body temperature. The lungs contain a number of parts that facilitate the gas exchange. Trachea - It functions to funnel the inhaled air into the lungs, while also facilitating the removal of inhaled air out of the lungs. Bronchi - These are smaller passages where air enters the lungs from the trachea. Bronchioles - The bronchi further divide into smaller, and smaller passages called bronchioles. Anatomy of Human Respiratory System

5 3.3.1 Surface Area to Volume Ratio Alveolar ducts - At the end of each bronchiole, grape-like structures are present called alveolar ducts. Each of these ducts contain about a hundred alveolar sacs. Each of these sac contains about 30 alveoli. Alveoli - These structures contain thin-walled cells that are directly connected to capillaries. Oxygen are transferred down a concentration gradient from the alveoli, into the blood cells. At the same time, carbon dioxide is transferred from the blood cells to the alveoli. Note: There are a large number of alveoli in the lungs to increase the total surface area where gas exchange can occur. Mesophyll Terminal bronchioles Dicotyledonous Plants 1. Diffusion is the main mode of exchange of material for single-celled organism. They use this means to absorb and release gases. 2. Due to their small size, single-celled organisms have a high surface area where gas exchange can occur. They also developed a thin surface which make the diffusion pathway shorter. 3. There are mesophyll cells (middle leaf) which serves for gas exchange. These cells contain pores called stomata that open to allow gas exchange, and close to control loss of water. The closing and opening of these pores is controlled by guard cells.

6 3.3.1 Surface Area to Volume Ratio Control of Water Loss Most of the time, gas exchange involves loss of water. Because of this, the plants and animals developed some adaptations to minimize loss of water even with gas exchange occurring. Since plants need to open their stomata to allow gas exchange, there should be a mechanism by which the plants can control the loss of water. This is solved by the presence of guard cells. Guard cells become swollen when water enters them. This swelling causes the stomata to open. When the plant gets dehydrated, the guard cells lose the water and as it becomes flaccid, the stomatal pore closes. Insects can close their spiracles to avoid losing much water. They also have tiny hairs and waxy cuticle to prevent evaporation. Opening and closing of Stomata Image Source: Wikimedia Commons Cross-section of Xerophyte Leaf Image Source: Wikimedia Commons Xerophytes are a type of succulent plant that can survive harsh conditions such as very high temperature environments with very low supply of water. To prevent further loss of water, they have hairs in their epidermis to trap moist air around the stomata. The stomata are sunken in pits capable of trapping moist air. This configuration also lowers the concentration gradient of water between the air and the leaf, lessening the tendency for water to evaporate away. Relatively, these xerophytes have a lower number of stomata to lessen sites where water can escape. The leaves have a waxy cuticle to reduce evaporation rate.