UNIT THREE- AQA A LEVEL BIOLOGY

• Terrestrial insects


  • Have an exoskeleton made of chitin, a hard fibrous material
  • Contain a lipid layer to prevent water loss
  • Do not have lungs, instead have a tracheal system for gas exchange
• Tracheal system


  The gas exchange system in insects, involving trachea, tracheoles, and spiracles
• Adaptations to reduce water loss

  1. Small surface area to volume ratio

  2. Lipid layer on exoskeleton to be waterproof

  3. Spiracles that can open and close to control water loss
• Spiracles


  • Round, valve-like openings along the insect's abdomen
  • Can open and close to control water loss and gas exchange
• Trachea


  • Network of internal tubes within the insect
  • Have rings to keep them open and prevent collapse
  • Deliver oxygen to every tissue
• Gas exchange mechanisms


  1. Diffusion - down concentration gradients

  2. Mass transport - by muscle contractions pumping gases

  3. Fluid pressure changes - due to anaerobic respiration and water movement
• Adaptations for gas exchange

  • Large surface area via tracheal system
  • Short diffusion distance between spiracles and trachea
  • Maintenance of concentration gradients by cellular respiration
• Gas exchange surface


  • Large surface area to volume ratio
  • Short diffusion distance
  • Mechanism to maintain concentration gradients
• Gas exchange in fish gills

  1. Water enters mouth and flows over gill filaments

  2. Gill filaments have many thin gill lamellae at right angles

  3. Capillary network in lamellae provides short diffusion distance
• Counter-current flow


  • Water and blood flow in opposite directions over the gills
  • Maintains diffusion gradient across entire length of lamellae
• Counter-current flow


  Ensures equilibrium is not reached, allowing diffusion of oxygen along entire length of lamellae
• Fick's law: Diffusion rate is proportional to surface area, concentration difference, and inversely proportional to diffusion distance
• Fick's law


  The rate of diffusion is proportional to the surface area of the exchange surface, multiplied by the concentration gradient, divided by the diffusion distance
• Leaf adaptation for diffusion

  • Large number of stomata
  • Air spaces
  • Large surface area of mesophyll cells
• Fick's law describes proportionality, not equality
• Adaptation for gas exchange in single-celled organisms


  • High surface area to volume ratio
  • Thin cell membrane as exchange surface
  • Permeable cell wall
• Earthworms do not have special gas exchange organs, but rely on diffusion across their body surface
• Earthworms have a closed circulatory system with blood pigments to transport gases
• Flatworms have more efficient diffusion systems than earthworms due to their flat shape
• Insect tracheal system


  • Tubes directly connecting external environment to body cells
  • Spiracles as openings
  • Tracheal tubes getting smaller (tracheoles) to reach cells
  • Ability to regulate spiracles to control water loss
• Insect gas exchange regulation

  1. Concentration gradient drives CO2 out, O2 in

  2. Abdominal twitching increases gas flow

  3. Water potential changes during respiration draw water into cells, exposing more tracheal surface
• Tracheal system limits the maximum size of insects due to diffusion constraints
• Breathing


  The movement of air into and out of the lungs
• Respiration


  The chemical reaction that happens in every cell to release energy in the form of ATP
• Ventilation


  The scientific term for breathing
• Gaseous exchange/Gas exchange


  The diffusion of oxygen from the air in the alveoli into the blood and carbon dioxide in the other direction
• Key organs in the gas exchange system

  • Lungs
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• How humans breathe in/out (Ventilation)

  1. External intercostal muscles contract, pulling ribcage up and out

  2. (Internal intercostal muscles relax)

  3. Diaphragm contracts, pulling down

  4. Decreases pressure in lungs, air flows in (inspiration/inhalation)

  5. (External intercostal muscles relax) internal intercostal muscles contract, pulling ribcage down and in

  6. Diaphragm relaxes, moving up

  7. Increases pressure in lungs, air flows out (expiration/exhalation)
• Larger volume in thoracic cavity


  Decreases pressure in lungs, air flows in
• Smaller volume in thoracic cavity


  Increases pressure in lungs, air flows out
• Alveoli


  • Tiny air sacs at the end of bronchioles
  • About 300 million in each lung
  • Surrounded by capillaries
  • Single layer of epithelial cells for short diffusion distance
  • Provide large surface area for gas exchange
  • Maintain concentration gradients for diffusion
• The Wealth of Nations was written in 1776
• When analyzing markets, a range of assumptions are made about the rationality of economic agents involved in the transactions
• Rational (in classical economic theory)

  Economic agents are able to consider the outcome of their choices and recognize the net benefits of each one
• Consumers act rationally by


  Maximizing their utility
• Producers act rationally by


  Selling goods/services in a way that maximizes their profits
• Workers act rationally by


  Balancing welfare at work with consideration of both pay and benefits
• Governments act rationally by


  Placing the interests of the people they serve first in order to maximize their welfare
• Rationality in classical economic theory is a flawed assumption as people usually don't act rationally
• A firm increases advertising

  Demand curve shifts right