Exchange Surfaces

• Why do multicellular organisms require specialised gas exchange surfaces?

  Their smaller surface area to volume ratio means the distance that needs to be crossed is larger and substances cannot easily enter the cells as in single-celled organisms
• How is surface area to volume ratio calculated?

  Ratio = Surface area / Volume
• Name 3 features of an efficient gas exchange surface.

  1. Large surface area, e.g. root hair cells
  2. Thin/short distance, e.g. alveoli
  3. Steep concentration gradient, maintained by blood supply or ventiliation, e.g. gills
• Describe the trachea and its function in the mammalian gaseous exchange system.

  • Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes
  • Lined by ciliated epithelium cells which move mucus, produced by goblet cells, towards the throat to be swallowed, preventing lung infections
  • Carries air to the bronchi
• Describe the bronchi and their function in the mammalian gaseous exchange system.

  • Like the trachea they are supported by rings of cartilage and are lined by ciliated epithelium cells and goblet cells
  • However they are narrower and there are two of them, one for each lung
  • Allow passage of air into the bronchioles
• Describe th bronchioles and their function in the mammalian gaseous exchange system.

  • Narrower than the bronchi
  • Do not need to be kept open by cartilage, therefore mostly have only smooth muscle and elastic fibres so that they can contract and relax during ventilation
  • Allow passage of air into the alveoli
• Describe the alveoli and their function in the mammalian gaseous exchange system.

  • Mini air sacs, lined with epithelium cells, site of gas exchange
  • Walls only one cell thick, covered with a network of capillaries, 300 million in each lung, all of which facilitates gas diffusion
• Explain the process of inspiration and the changes that occur throughout the thorax.

  • External intercostal muscles contract (while internal relax), pulling the ribs up and out
  • Diaphragm contracts and flattens
  • Volume of the thorax increases
  • Air pressure outside the lungs is therefore higher than the air pressure inside, so air moves in to rebalance
• Explain the process of expiration and the changes that occur throughout the thorax.

  • External intercostal muscles relax (while internal contract), bringing the ribs down and in
  • Diaphragm relaxes and domes upwards
  • Volume of thee thorax decreases
  • Air pressure inside the lungs is therefore higher than the air pressure outside, so air moves out to rebalance
• Explain how a spirometer works.

  Used to measure lung volume. A person breathes into an airtight chamber which leaves a trace on a graph which shows the volume of the breaths
• Define vital capacity.

  The maximum volume of air that can be taken in or expelled from the lungs in one breath. Can be calculated from the spirometer graph by finding the maximum amplitude
• Define tidal volume.

  The volume of air we breathe in and out during each breath at rest. Can be calculated from the spirometer graph by finding the amplitude at rest
• Define breathing rate.

  Th number of breaths we take per minute. Can be calculated from the spirometer graph by counting the number of peaks in one minute
• Name and describe the two main features of a fish's gas transport system.

  • Gills = located within the body, supported by arches, along which are multiple projections of gill filaments, which are stacked up in piles
  • Lamellae = at right angles to the gill filaments, give an increased surface area. Blood and water flow across them in opposite directions (countercurrent exchange system)
• Explain the process of gas exchange in fish.

  • Buccal cavity volume increased to enable water to flow in, reduced to increase pressure
  • Water is pumped over the lamellae by the operculum, oxygen diffuses into the bloodstream
  • Waste carbon dioxide diffuses into the water and flows back out of the gills
• How does the countercurrent exchange system maximise oxygen absorbed by the fish?

  Maintains steep concentration gradient, as water is always next to blood of a lower oxygen concentration. Keeps rate of diffusion constant and enables 80% of available oxygen to be absorbed.
• Name and describe the three main features of an insect's gas transport system.

  • Spiracles = holes on the body's surface which may be opened or closed by a valve for gas or water exchange
  • Tracheae = large tubes extending through all body tissues, supported by rings to prevent collapse
  • Tracheoles = smaller branches dividing off the tracheae
• Explain the process of gas exchange in insects.

  • Gases move in and out of the tracheae through the spiracles
  • A diffusion gradient allows oxygen to diffuse into the body tissue while waste CO2 diffuses out
  • Contraction of muscles in the tracheae allows mass movement of air in and out
• Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion

  • The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
  • The small volume means the diffusion distance to all organelles is short
• As organisms increase in size their SA:V ratio decreases

  • There is less surface area for the absorption of nutrients and gases and secretion of waste products
  • The greater volume results in a longer diffusion distance to the cells and tissues of the organism
• • Large multicellular animals and plants have evolved adaptations to facilitate the exchange of substances between their environment
  • They have a large variety of specialised cells, tissues, organs and systems
  • Eg. gas exchange system, circulatory system, lymphatic system, urinary system, xylem and phloem
• The Need for a Specialised System for Gas Exchange

  • Supply of Oxygen:
  • Organisms require ATP in order to carry out the biochemical processes required for survival. The majority of ATP is produced through aerobic respiration which requires oxygen
• The Need for a Specialised System for Gas Exchange

  • Removal of Carbon Dioxide:
  • Carbon dioxide is a toxic waste product of aerobic respiration
  • If it accumulates in cells/tissues it alters the pH
• Diffusion for Single-celled Organisms vs Multicellular Organisms

  • Chlamydomonas, single-celled organism, has a diameter of 20μm. Oxygen can diffuse across cell wall and membrane
  • Maximum distance oxygen molecules have to diffuse is 10μm, would only take 100 milliseconds
  • If diffusion distance increased to 15cm ,diffusion time would increase to 7 hours
  • This demonstrates how diffusion is a viable transport mechanism for single-celled organisms but not for larger multicellular organisms
  • The time taken for oxygen to diffuse from the cell-surface membrane to the tissues would be too long
• • The metabolic rate of an organism is the amount of energy expended by that organism within a given period of time
  • The basal metabolic rate (BMR) is the metabolic rate of an organism when at rest. The BMR is significantly lower than when an organism is actively moving
• The metabolic rate of an organism can be measured/estimated using different methods and apparatus:

  • Oxygen consumption (respirometers)
  • Carbon dioxide production (carbon dioxide probe)
  • Heat production (calorimeter)
• • Experiments conducted by scientists have shown that the greater the mass of an organism, the higher the metabolic rate
  • Although metabolic rate increases with body mass the BMR per unit of body mass is higher in smaller animals than in larger animals
  • Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more energy to maintain their body temperature
• Plants have much lower metabolic rates than animals as they do not move around their habitat and don’t have to maintain a high body temperature.
• Effective exchange surfaces in organisms have a:

  • Large surface area
  • Short diffusion distance (thin)
  • Good blood supply
  • Ventilation mechanism
• • Many exchange surfaces within organisms have adaptations that increase their surface area
  • A larger surface area provides more space over which the exchange of substances with the environment can occur
• Increased surface area of root hair cells

  • Root hair cells are specialised cells found in the roots of plants. They play an important role in the absorption of water and mineral ions from the soil
  • Root hair cells have a root hair that increases the surface area (SA) so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)
• • The exchange of oxygen and carbon dioxide occurs between the alveoli and the capillaries in the lungs
  • Oxygen and carbon dioxide are exchanged in a process of simple diffusion; (passive movement from high to low concentration)
• • The air in the alveoli contains a high concentration of oxygen. The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration
  • The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide. The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled
• • 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
• 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
• 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
• • In order for the diffusion of a substance across an exchange site to continue for a prolonged period of time, the concentration gradient must be maintained
  • An adequate blood supply helps to maintain a concentration gradient as it is continuously flowing, bringing substances that have just entered the blood away from the exchange site
• Good blood supply in fish gills

  • Fish gills are adapted to directly extract oxygen from water as they have a large capillary network
  • The extensive capillary system that covers the gills 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
• Good blood supply in fish gills

  • Water with the highest oxygen content is found next to blood that is oxygenated
  • There is still a difference in concentration so diffusion of oxygen into the blood still occurs
  • Water continues to supply the blood with oxygen along the whole gill arch and ends with water with the lowest oxygen concentration adjacent to the most deoxygenated blood with continued diffusion occurring
• • A ventilation mechanism also helps to maintain a concentration gradient across an exchange surface
  • Ventilation (mass flow of gases) in the lungs 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. Breathing removes air with low amounts of oxygen and high amounts of carbon dioxide and replaces it with air that has high amounts of oxygen and low amounts of carbon dioxide