OCR module 3 entire topic biology

• surface area to volume ratio


  the importance of this for organisms
• small organisms like amoeba have a very large surface area compared to their volume
• this means they have a big surface area for the transport substances and a short distance between the outside and the Very middle of the organism
• simple diffusion is sufficient to meet their metabolic needs
• large organisms have a smaller surface area compared to their volume and a larger distance from the outside to the middle of the organism
• they also have higher metabolic rates meaning they require more oxygen for respiration
• large organisms require adaptations to increase the efficiency of exchange across their surface
• the adaptations we'll focus on

  • Gills in fish
  • Alveoli in humans
  • Tracheal system in insects
• the running theme is looking at what provides the large surface area, what helps to maintain a concentration gradient, and what reduces the diffusion pathway
• Trachea

  Also known as the windpipe, has c-shaped rings of cartilage to support it and keep it permanently open
• Tracheal epithelial cells

  Ciliated cells that sweep away mucus, and goblet cells that produce the mucus
• Tracheal smooth muscle

  Can contract to constrict the tracheal lumen and reduce airflow, or relax to dilate the lumen
• The trachea splits into bronchi, which further split into bronchioles, leading to the alveoli</b>
• The bronchi and bronchioles also have cartilage to provide structural support and keep them open
• Alveoli


  The site of gas exchange, where oxygen diffuses into the blood and carbon dioxide diffuses out
• Adaptations for efficient gas exchange in the lungs

  • Large surface area provided by millions of alveoli
  • Short diffusion distance due to single cell layer alveolar walls
  • Concentration gradient maintained by ventilation and blood flow
• Ventilation

  The mechanism of breathing, involving the diaphragm and intercostal muscles to change thoracic volume and pressure
• Ventilation maintains the concentration gradient in the alveoli for efficient gas exchange
• Inspiration

  Increase in thoracic volume and decrease in pressure causes air to flow into the lungs
• Expiration


  Decrease in thoracic volume and increase in pressure causes air to flow out of the lungs
• Spirometry can measure lung volumes and breathing rates
• Fish face the challenge of lower oxygen concentration in water compared to air
• Fish ventilation

  1. Open mouth to increase buccal cavity volume and decrease pressure, causing water to flow in

  2. Operculum valve closes, expanding operculum cavity and decreasing pressure

  3. Raise buccal floor to force water over gills and out operculum
• Adaptations for efficient gas exchange in fish gills

  • Large surface area provided by many gill filaments and lamellae
  • Short diffusion distance across thin gill lamellae
  • Concentration gradient maintained by counter-current flow of water and blood
• Insects have a tracheal system for gas exchange, with spiracles, tracheae, and tracheoles
• Adaptations for efficient gas exchange in insect tracheae

  • Large surface area provided by many branching tracheoles
  • Short diffusion distance across thin tracheal walls
  • Concentration gradient maintained by abdominal muscle contractions pumping air in and out
• Insects can also use anaerobic respiration during flight, producing lactic acid and causing water to move into cells by osmosis
• Types of circulatory systems in animals

  • Open circulatory system (invertebrates)
  • Closed circulatory system (vertebrates and some invertebrates)
• Open circulatory system


  Transport medium (hemolymph) pumped directly into body cavity, few transport vessels, low pressure
• Closed circulatory system


  Transport medium (blood) remains within blood vessels, allows efficient gas and small molecule transport
• Types of closed circulatory systems

  • Single closed (fish)
  • Double closed (mammals, birds)
• Single closed circulatory system

  Blood passes through heart once per cycle, flows through gills then body
• Double closed circulatory system

  Blood passes through heart twice per cycle, has separate pulmonary and systemic circuits
• Pigmented protein


  For example, hemoglobin
• Single closed circulatory system

  • Blood passes through the heart once per cycle
  • Only one circuit that the blood takes
• Organisms with single closed circulatory systems

  • Fish
• Single closed circulatory system in fish

  1. Blood passes through two sets of capillaries immediately after being pumped out of the heart

  2. Blood flows through capillaries in the gills to become oxygenated

  3. Blood flows through capillaries delivering it to the body before returning it back to the heart
• Single closed circulatory system would not enable efficient gas exchange for mammals but it does work for fish because they have that counter current flow mechanism
• Double closed circulatory system

  • Blood passes through the heart twice per cycle
  • Two separate circuits the blood would take
• Organisms with double closed circulatory systems

  • Birds
  • Most mammals