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Biology A-Level Exchange and Transport

May 4, 2025

AQA Biology A-Level Topic 3: Exchange of Substances

Overview

  • Specialised exchange surfaces are necessary in larger organisms due to their higher surface area to volume ratio.
  • Single-celled organisms have short diffusion distances, but multicellular organisms require specialised structures for efficient gas exchange.
  • Efficient exchange surfaces have:
    • Large surface area (e.g., root hair cells, folded membranes like mitochondria).
    • Thin barriers to shorten diffusion distance.
    • Good blood supply/ventilation to maintain a steep diffusion gradient (e.g., alveoli).

Gas Exchange in Different Organisms

Fish

  • Fish have gills with lamellae for gas exchange.
  • Counter-current flow of blood and water maintains a steep diffusion gradient.
  • Ventilation involves mouth opening, buccal cavity lowering, and operculum acting as a valve.

Insects

  • Oxygen transported directly to tissues via spiracles, trachea, and tracheoles.
  • Gas movement by diffusion, muscle contraction, and volume changes.

Plants

  • Leaves have stomata for gas entry and exit.
  • Air spaces allow gas movement and contact with mesophyll cells.

Mammalian Gaseous Exchange System

  • Lungs in chest cavity are protected by the rib cage and have a large surface area.
  • Air travels through nose, trachea, bronchi, and bronchioles to alveoli where gas exchange occurs.
  • Alveoli are thin-walled and surrounded by capillaries.
  • Ventilation involves inspiration and expiration:
    • Inspiration: Intercostal muscles and diaphragm contract, expanding thoracic volume, reducing pressure to draw air in.
    • Expiration: Muscles relax, decreasing thoracic volume, increasing pressure to expel air.

Spirometer

  • Device to measure lung volume.
  • Vital capacity: max air volume inhaled/exhaled
  • Tidal volume: air volume per breath at rest
  • Breathing rate: breaths per minute

Digestion and Absorption

  • Digestion involves breaking down large molecules into absorbable forms.
  • Carbohydrates: Digested by amylases, maltases, sucrases, and lactases.
  • Lipids: Emulsified by bile salts, digested by lipases.
  • Proteins: Broken down by endopeptidases, exopeptidases, dipeptidases.
  • Absorption involves amino acids (co-transport), monoglycerides, and fatty acids (diffusion).

Haemoglobin

  • Globular protein carrying oxygen via haem group.
  • Oxygen binding varies with partial pressure.
  • Dissociation Curves:
    • Illustrates haemoglobin saturation changes with O2 pressure.
    • Bohr effect: CO2 presence decreases oxygen affinity.

Circulatory System

  • Transport medium (blood) is pumped by the heart.
  • Closed double circulatory system with two pumps for lungs and body.

Heart Structure and Cardiac Cycle

  • Two pumps with atrium and ventricle each.
  • Cardiac Cycle:
    • Diastole: Relaxation, blood fills atria.
    • Atrial systole: Atria contract, blood to ventricles.
    • Ventricular systole: Ventricles contract, blood to arteries.

Blood Vessels

  • Arteries: Thick-walled, carry blood from heart.
  • Arterioles: Feed blood into capillaries.
  • Capillaries: Site of exchange, one cell thick.
  • Veins: Thin-walled, carry blood to heart, have valves.

Tissue Fluid

  • Supplies tissues with nutrients, removes waste.
  • Created by hydrostatic pressure in capillaries.
  • Tissue fluid moves back via lymphatic system.

Mass Transport in Plants

Xylem and Phloem

  • Xylem: Transports water/minerals, supports structure.
  • Phloem: Translocates sugars, consists of sieve tube elements and companion cells.

Transpiration

  • Water moves from roots, up stem, exits via stomata.
  • Transpiration stream aids photosynthesis, growth, temperature control.
  • Potometer measures transpiration rate.
  • Xerophytes have adaptations to reduce water loss.

Movement of Water

  • Water absorbed by root hair cells, moves via symplast and apoplast pathways.
  • Casparian strip forces water into symplast pathway.

Translocation

  • Active process moving sucrose from sources to sinks.
  • Evidence for mass transport includes pressure in sieve tubes, sucrose concentration gradients.

Experiments

  • Ringing: Demonstrates phloem's role in mass flow.
  • Tracer: Uses radioactive carbon to trace sucrose movement.