A-Level Biology – Module 3: Exchange and Transport (Shuffled Revision)
Gas Exchange in Fish and Insects
Fish Gills:
Gills are composed of filaments and lamellae for large surface area.
Blood and water flow in opposite directions (countercurrent), maintaining a steep
concentration gradient.
Thin epithelium shortens diffusion distance.
Ventilation brings oxygen-rich water; circulation delivers oxygen-poor blood.
Insect Tracheal System:
Spiracles on body surface allow air in/out; valves minimise water loss.
Air moves through tracheae → tracheoles → directly to respiring cells.
At rest, diffusion is sufficient; during activity, insects ventilate to support gas
exchange.
Why Do Larger Organisms Need Specialised Systems?
Small organisms (like bacteria) manage with surface diffusion due to:
High surface area to volume ratio
Minimal diffusion distance
Low metabolic demands
Larger organisms (e.g. mammals, plants) require:
Specialised exchange surfaces (e.g. alveoli, villi)
Transport systems (e.g. circulatory system, xylem/phloem)
This is due to:
Multicellularity
Low SA:V ratio
Protective barriers
Structure and Function of the Lungs
Made of trachea → bronchi → bronchioles → alveoli
Trachea/Bronchi:
C-shaped cartilage for flexibility and support
Goblet cells secrete mucus; ciliated epithelium wafts mucus upwards
Bronchioles:
Lined with smooth muscle and epithelium
Can constrict or dilate to regulate airflow
Alveoli:
Millions of thin-walled sacs, increasing surface area
Surrounded by capillaries
Elastic fibres allow stretch and recoil during ventilation
Mechanics of Breathing
Inhalation:
Diaphragm contracts (flattens)
External intercostals contract (ribs lift)
Thoracic volume ↑, pressure ↓ → air in
Exhalation:
Muscles relax, alveoli recoil
Thoracic volume ↓, pressure ↑ → air out
Pulmonary Ventilation Equation:
PV = Tidal Volume × Ventilation Rate
Gas Exchange in Leaves
Leaf Features:
Palisade layer rich in chloroplasts for photosynthesis
Spongy mesophyll with air spaces for gas diffusion
Guard cells regulate stomatal opening
Day: CO₂ enters, O₂ exits
Night: O₂ enters, CO₂ exits (only respiration)
Water Transport in Plants
Xylem:
Dead hollow tubes
Lignified walls for strength and waterproofing
Movement by transpiration pull via cohesion-tension mechanism
Transpiration:
Evaporation from moist cell walls
Water vapour diffuses out through stomata
Factors Increasing Transpiration:
High light intensity
Warm temperatures
Low humidity
Windy conditions
Potometer: Measures rate of water uptake as proxy for transpiration
Phloem and Translocation
Phloem:
Made of sieve tubes and companion cells
Transports organic solutes (e.g. sucrose) via mass flow
Mass Flow Process:
Sucrose loaded into sieve tube (via active transport/cotransport)
Water follows by osmosis (lowers Ψ)
Hydrostatic pressure moves sap towards sink
At sink, sucrose is removed and water leaves
Circulatory System in Mammals
Double Circulation:
Blood passes through the heart twice per complete circuit
Maintains high pressure for efficient delivery
Components:
Heart = pump
Blood vessels = conduits
Blood = transport medium
Heart Chambers:
2 atria (receive blood), 2 ventricles (pump blood)
Left ventricle has thicker wall – pumps to whole body
Valves and Heart Function
Atrioventricular (AV) valves: Between atria and ventricles
Semilunar (SL) valves: Between ventricles and arteries
Valve Mechanics:
AV opens when atrial pressure > ventricular pressure
SL opens when ventricular pressure > arterial pressure
Cardiac Cycle:
Atria fill → Atria contract → Ventricles fill and contract → Valves open/close
appropriately
Cardiac Output Equation:
CO = Stroke Volume × Heart Rate
Coronary Heart Disease (CHD)
Causes:
Atheroma formation due to LDL cholesterol and high blood pressure
Plaque narrows arteries → reduced blood flow
Clot formation (thrombus) → myocardial infarction (heart attack)
Risk Factors:
Poor diet (saturated fats, salt)
Smoking
Age, genetics, lack of exercise
Structure and Role of Blood Vessels
Arteries: Thick walls, elastic fibres, narrow lumen – carry blood away at high
pressure
Capillaries: One-cell thick for exchange
Veins: Thin walls, valves, large lumen – carry blood under low pressure
Pressure Changes:
Highest in arteries → drops in arterioles/capillaries → lowest in veins
Capillaries and Tissue Fluid
Plasma: Main component of blood; carries cells and solutes
Tissue Fluid: Formed by hydrostatic pressure at arteriole end
Return: Osmosis and low Ψ draw fluid back at venous end
Lymph System: Collects excess fluid
Disorders:
High BP = more fluid forced out
Low protein diet = reduces osmotic return
Red Blood Cells and Oxygen Transport
Haemoglobin (Hb):
Quaternary structure with 4 haem groups (Fe²⁺)
Each Hb binds up to 4 O₂ molecules
O₂ Loading and Unloading:
High partial pressure = lungs → Hb has high affinity and loads O₂
Low partial pressure = tissues → Hb has low affinity and releases O₂
ODC Curve:
Sigmoid shape
Bohr Effect: CO₂ lowers affinity by shifting curve right
Special Cases:
Fetal Hb = higher affinity → left-shifted curve
Organisms in low O₂ = left-shifted ODC
Small/active organisms = right-shifted ODC for rapid O₂ release
Digestive System and Absorption
Digestive Enzymes:
Amylase & Maltase: Starch → Glucose
Protease (Endo, Exo, Di): Proteins → Amino acids
Lipase: Lipids → Monoglycerides + Fatty acids
Absorption in Small Intestine (SI):
Villi and microvilli for increased surface area
Glucose/AAs via sodium cotransport
Lipids via micelles → diffusion → chylomicrons → lymph
Lactose Intolerance
Cause: Lack of lactase enzyme
Effect: Undigested lactose in gut → draws water by osmosis (diarrhoea) and
fermented by microbes (flatulence)
