OCR (A) Biology A-Level: Topic 3.1 Exchange and Transport

Introduction

• Specialised exchange surfaces are necessary as organism size increases and surface area to volume ratio decreases.
• Single-celled organisms: Substances easily enter cells due to short distance.
• Multicellular organisms: Require specialised exchange surfaces for efficient gas exchange due to a larger distance.

Features of Efficient Exchange Surfaces

• Large Surface Area: Example includes root hair cells and folded membranes of mitochondria.
• Thinness: Ensures short distance for substance exchange.
• Good Blood Supply/Ventilation: Maintains a steep gradient, e.g., alveoli.

Mammalian Gaseous Exchange System

• Lungs: Located in the chest cavity, large surface area, surrounded by rib cage.
  • Rib cage protection and lubricating substance to prevent friction.
  • Intercostal muscles contract to raise/lower rib cage.
  • Diaphragm separates lungs from the abdomen.
• Air Passage: Nose -> Trachea -> Bronchi -> Bronchioles (facilitate air passage).
  • Gaseous exchange occurs at alveoli walls.
  • Airways supported by cartilage rings (incomplete in trachea for food passage).
• Trachea and Bronchi: Similar structure, bronchi are narrower.
  • Thick wall composed of cartilage, glandular/connective tissue, elastic fibres, smooth muscle, and blood vessels.
  • Inner lining: Ciliated epithelium and goblet cells.
• Bronchioles: Narrower than bronchi, larger ones contain cartilage.
  • Walls made of smooth muscle and elastic fibres; smallest ones end in alveoli clusters.

Structures and Functions

• Cartilage: Supports trachea/bronchi; prevents lung collapse during pressure drop.
• Ciliated Epithelium: Moves mucus to prevent lung infection.
• Goblet Cells: Secrete mucus to trap bacteria/dust, reduce infection risk.
• Smooth Muscle: Contracts to constrict airway, controls airflow.
• Elastic Fibres: Control air flow by stretching with inhalation and recoiling with exhalation.

Ventilation

• Definition: Flow of air in and out of alveoli during inspiration and expiration.
• Muscles Involved: Intercostal muscles and diaphragm.
  • Inspiration: External intercostals contract, ribs rise, diaphragm flattens.
  • Expiration: Internal intercostals contract, ribs lower, diaphragm relaxes.
• Pressure Changes: Volume changes in thorax alter pressure, creating gradient for air flow.

Spirometer

• Device to measure lung volume.
• Vital Capacity: Max volume of air inhaled/exhaled in a single breath.
• Tidal Volume: Volume of air in/out at rest.
• Breathing Rate: Breaths per minute (from spirometer trace).
• Residual Volume: Air always in lungs.
• Inspiratory/Expiratory Reserve Volume: Additional volume inhaled/exhaled beyond tidal volume.

Gas Exchange in Bony Fish and Insects

• Fish:
  • Small surface area to volume ratio, require specialised surfaces.
  • Gills with projections (lamellae) for gas exchange; countercurrent blood/water flow.
  • Water flow keeps gill projections apart; ventilation maintains unidirectional flow.
• Insects:
  • Lack transport system, oxygen transported directly via spiracles and tracheae.
  • Tracheal fluid dissolves gases for diffusion.
  • Spiracles regulate water loss.