Gas exchange Flashcards
Gas exchange:
Across the body surface of a single-celled organism
Chlamydomonas is a single-celled organism that is found in fresh-water ponds
It is spherical in shape and has a diameter of 20μm
Oxygen can diffuse across the cell wall and cell surface membrane of Chlamydomonas
The maximum distance that oxygen molecules would have to diffuse to reach the centre of a Chlamydomonas is 10μm, this takes 100 milliseconds
Diffusion is an efficient exchange mechanism for Chlamydomona
Gas exchange:
In the tracheal system of an insect (tracheae, tracheoles and spiracles)
All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
A spiracle is an opening in the exoskeleton of an insect which has valves
It allows air to enter the insect and flow into the system of tracheae
Most of the time, the spiracle is closed to reduce water loss
Tracheae are tubes within the insect breathing system which lead to tracheoles (narrower tubes)
The tracheae walls have reinforcement that keeps them open as the air pressure inside them fluctuates
A large number of tracheoles run between cells and into the muscle fibres - the site of gas exchange
For smaller insects, this system provides sufficient oxygen via diffusion
A concentration gradient is created as oxygen is used by respiring tissues allowing more to move in through the spiracles by diffusion
Carbon dioxide produced by the respiring tissues moves out through the spiracles down a concentration gradient
Very active, flying insects need a more rapid supply/intake of oxygen. They create a mass flow of air into the tracheal system by:
Closing the spiracles
Using muscles to create a pumping movement for ventilation
Also, during flight the production of lactate in the respiring muscles, lowers the water potential of muscle cells
Water found at the narrow ends of the tracheoles is then drawn into the respiring muscle by osmosis
This allows gases to diffuse across more quickly
Gas exchange:
Across the gills of fish (gill lamellae and filaments including the counter-current principle)
Oxygen dissolves less readily in water
A given volume of air contains 30 times more oxygen than the same volume of water
Fish are adapted to directly extract oxygen from water
Structure of fish gills in bony fish:
Series of gills on each side of the head
Each gill arch is attached to two stacks of filaments
On the surface of each filament, there are rows of lamellae
The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
Mechanism:
The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water - it is a counter-current system
The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
Gas exchange:
By the leaves of dicotyledonous plants (mesophyll and stomata)
In order to carry out photosynthesis, plants must have an adequate supply of carbon dioxide
There is only roughly 0.036% CO2 in the atmosphere, so efficient gas exchange is necessary
Leaves have evolved adaptations to aid the uptake of carbon dioxide
Structure of a leaf:
Waterproof cuticle
Upper epidermis - layer of tightly packed cells
Palisade mesophyll layer - layer of elongated cells containing chloroplasts
Spongy mesophyll layer - layer of cells that contains an extensive network of air spaces
Stomata - pores (usually) on the underside of the leaf which allow air to enter
Guard cells - pairs of cells that control the opening and closing of the stomata
Lower epidermis - layer of tightly packed cells
Mechanism:
When the guard cells are turgid (full of water) the stoma remains open allowing air to enter the leaf
The air spaces within the spongy mesophyll layer allows carbon dioxide to rapidly diffuse into cells
The carbon dioxide is quickly used up in photosynthesis by cells containing chloroplasts - maintaining the concentration gradient
No active ventilation is required as the thinness of the plant tissues and the presence of stomata helps to create a short diffusion pathway
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
Terrestrial Insects:
Small insects living on the ground are surrounded by air and prone to drying out
Insects possess a waterproof exoskeleton that prevents water loss
The waterproof waxy coating of the exoskeleton makes gas exchange by diffusion very difficult
As a result, insects have evolved a breathing system (the tracheal system)which consists of many tubes that carry oxygen directly to all tissues and cells of the body
Spiracle are openings in the exoskeleton of insects that are connected to the tracheal system
Xerophytic Plants:
Plants that live in conditions with a plentiful supply of freshwater have leaves with a short diffusion distance through the stomata and a large surface area provided by the air spaces in the spongy mesophyll
These factors make them vulnerable to water loss
Plants that live in conditions where freshwater is limited have evolved very effective adaptations to conserve water:
Very few stomata
Sunken stomata
Hairs surrounding stomata
Needle-shaped or small leaves
Waxy cuticle
Plants with these adaptations are described as xerophytic
The gross structure of the human gas exchange system limited to the alveoli, bronchioles, bronchi, trachea and lungs
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
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
The essential features of the alveolar epithelium as a surface over which gas exchange takes place
Large number of alveoli:
The average human adult has around 480 - 500 million alveoli in their lungs
This equals a surface area of 40 - 75 m2
The large number of alveoli increases the surface area available for oxygen and carbon dioxide to diffuse across
Thin walls:
The walls of the alveoli are only one cell thick and these cells are flattened
This means that gases have a very short diffusion distance so gas exchange is quick and efficient
Extensive capillary network:
The walls of the capillaries are only one cell thick and these cells are flattened, keeping the diffusion distance for gases short
The constant flow of blood through the capillaries means that oxygenated blood is brought away from the alveoli and deoxygenated blood is brought to them
This maintains the concentration gradient necessary for gas exchange to occur
Ventilation and the exchange of gases in the lungs
Ventilation (mass flow of gases) in the lungs and the continuous flow of blood in the capillaries helps to ensure that there is always a higher concentration of oxygen in the alveoli than in the blood
The movements involved in breathing causes the air in the alveoli to change, which supplies fresh oxygen and takes away carbon dioxide
The oxygen in the alveoli diffuses into the red blood cells which are rapidly carried away in the blood and replaced by oxygen-depleted red blood cells
Exercise causes oxygen demands to increase which can be facilitated by an increased rate of ventilation
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
In inspiration:
1) external intercostal muscles - contract
2) internal intercostal muscles - relax
3) ribcage - up and out
4) diaphragm - contracts and flattens
5) volume of thorax/thoracic cavity - increases
6) pulmonary pressure (in lungs) - decreases
7) air - moves down a pressure gradient into the lungs
In expiration:
1) external intercostal muscles - relax
2) internal intercostal muscles - contract
3) ribcage - down and in
4) diaphragm - relax and rises to dome shape
5) volume of thorax/thoracic cavity - decreases
6) pulmonary pressure (in lungs) - increases
7) air - air forced out down a pressure gradient
