specialised exchange surfaces & features

• What SA:V ratio do 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
• What SA:V ratio do larger organisms?

  smaller SA:V ratio

  • 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
• Why do we Need 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
  • Removal of Carbon Dioxide:
  • Carbon dioxide is a toxic waste product of aerobic respiration
  • If it accumulates in cells/tissues it alters the pH
• What are features of efficient gas exchange?

  large surface area: provides a large area over which the exchange of materials can occur

  thin layers: this creates a short diffusion pass for gases, increasing the speed of exchange

  blood supply: good, constant blood supply maintains a large diffusion gradient and ensure the exchanged substances are constantly moving to the area needed

  ventilation: maintains the diffusion gradient, making the process faster and more efficient
• Why do multicellular organisms require specialised 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 a single-celled organism.
• How is surface area to volume ratio calculated?

  ratio = surface area / volume
• What are 3 features of an efficient gas exchange surface?

  large surface area e.g root hair cells

  thin/short diffusion distance e.g alveoli

  steep concentration gradient, maintained by blood supply or ventilation e.g gills
• How can the metabolic rate of an organism be measured?

  using different methods and apparatus:

  • Oxygen consumption (respirometers)
  • Carbon dioxide production (carbon dioxide probe)
  • Heat production (calorimeter)
• What is the relationship between mass and metabolic rate?

  • Experiments conducted by scientists have shown that the greater the mass of an organism, the higher the metabolic rate
  • Therefore, a single rhino consumes more oxygen within a given period of time compared to a single mouse
  • 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
• • 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
• As organisms increase in size, what happens to their SA:V ratio?

  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
• How are root hair cells an efficient exchange surface?

  • Root hair cells have a root hair that increases the surface area so the rate of water uptake by osmosis is greater (can absorb more water and ions than if SA were lower)
  • 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
• How are the alveoli an efficient exchange surface?

  • O2 and CO2 are exchanged in a process of simple diffusion
  • The air in the alveoli contains a high concentration of O2 . The oxygen diffuses from the alveoli and into the blood capillaries
  • The blood in the capillaries has a low concentration of O2 and a high concentration of CO2. The CO2 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, thus the gases have a very short diffusion distance so gas exchange is quick & efficient
• What are some others features of the alveoli?

  • The large number of alveoli increases the SA available for oxygen and carbon dioxide to diffuse across
  • Extensive capillary network
  • The walls of the capillaries are only 1 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
• How are fish gills an efficient exchange surface?

  • 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
  • The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood
• What is the ventilation mechanism in mammalian lungs?

  • A ventilation mechanism also helps to maintain a concentration gradient across an exchange surface
  • Ventilation 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
• What is ventilation?

  the mass flow of gases