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.