Biology 3 – Exchange & transport

Why large organisms need exchange surfaces and mass transport?

Smaller SA:V; diffusion distance too long to supply all cells

SA:V scaling?

Surface area ∝ length², volume ∝ length³, so SA:V falls as size increases

SA:V and metabolic rate?

Small animals have a larger SA:V, so lose more heat per gram; faster respiration releases heat to replace it, so a higher metabolic rate

Single-celled organisms?

Large SA:V, short diffusion distance; gases diffuse across the cell-surface membrane

Insect tracheal system?

Spiracles → tracheae → tracheoles; gases diffuse directly to and from cells

Tracheole adaptations?

Highly branched, so large surface area; thin walls and reach the tissues, so short diffusion pathway; abdominal pumping ventilates

Insects limiting water loss?

Waxy waterproof cuticle; spiracles close; hairs around spiracles; small SA:V

Fish gills?

Gill filaments covered in lamellae: large surface area, thin epithelium, many capillaries

Counter-current flow?

Blood and water flow in opposite directions, so a concentration gradient is kept along the whole lamella

Leaf gas exchange?

Many stomata; air spaces in spongy mesophyll; large surface area of mesophyll cells; thin leaf, short diffusion pathway

Xerophyte adaptations?

Sunken stomata, hairs, rolled leaves (trap moist air, reduce water potential gradient); thick waxy cuticle; fewer stomata

Human airways?

Trachea → bronchi → bronchioles → alveoli

Alveolar epithelium?

One thin, flat cell thick; many alveoli give a large surface area; close to capillaries, short pathway; ventilation and blood flow keep the gradient

Oxygen's route into blood?

Across the alveolar epithelium, then the capillary endothelium

Inspiration?

Diaphragm contracts and flattens; external intercostals contract, internal relax; ribcage up and out; volume up, pressure below atmospheric; air moves in

Expiration?

Diaphragm relaxes and domes; external intercostals relax (internal contract when forced); ribcage down and in; volume down, pressure up; air moves out

Pulmonary ventilation?

Tidal volume × breathing rate

Lung disease effects?

Reduced surface area, thicker epithelium or lost elasticity → slower diffusion, lower FEV₁

Correlation vs causation?

Correlation shows an association; it doesn't prove cause, as other factors may be involved

Carbohydrate digestion?

Amylase hydrolyses starch to maltose; membrane-bound disaccharidases (maltase, sucrase, lactase) hydrolyse disaccharides to monosaccharides

Lipid digestion?

Bile salts emulsify lipids into droplets (larger surface area); lipase hydrolyses ester bonds → monoglycerides and fatty acids

Micelles?

Bile salts + monoglycerides + fatty acids; carry them to the epithelium, keeping their concentration high; they diffuse in

Lipids after absorption?

Triglycerides re-formed in the ER; combined with proteins in the Golgi as chylomicrons; leave by exocytosis into lacteals

Protein digestion?

Endopeptidases: internal peptide bonds. Exopeptidases: terminal bonds. Membrane-bound dipeptidases: dipeptides → amino acids

Amino acid absorption?

Co-transport with Na⁺, as for glucose

Haemoglobin structure?

Quaternary: four polypeptides (two α, two β), each with a haem group containing Fe²⁺; carries four O₂

Loading and unloading?

High pO₂ (lungs): high affinity, loads. Low pO₂ (respiring tissues): lower affinity, unloads

Why is the curve S-shaped?

Cooperative binding: the first O₂ changes haemoglobin's shape, making the next ones bind more easily

Bohr effect?

Higher CO₂ lowers pH, reducing haemoglobin's affinity; curve shifts right; more O₂ unloaded in respiring tissues

Curve shifted left?

Higher affinity; loads at low pO₂ (e.g. fetal haemoglobin, animals in low-oxygen environments)

Curve shifted right?

Lower affinity; unloads more readily (e.g. small or active mammals with a high respiration rate)

Mammalian circulation?

Closed double circulation: pulmonary and systemic

Named vessels?

Coronary arteries; vena cava and aorta; pulmonary artery and vein; renal artery and vein

Route through the heart?

Vena cava → right atrium → AV valve → right ventricle → SL valve → pulmonary artery → lungs → pulmonary vein → left atrium → AV valve → left ventricle → SL valve → aorta

Why is the left ventricle wall thicker?

More muscle to generate the higher pressure needed for the systemic circulation

Atrial systole?

Atria contract; atrial pressure > ventricular; AV valves open; blood into ventricles

Ventricular systole?

Ventricles contract; pressure > atria, so AV valves close; pressure > arteries, so SL valves open; blood ejected

Diastole?

Muscle relaxes; ventricular pressure < arteries, so SL valves close; atria and ventricles fill

Valve rule?

Open when pressure behind is higher; close when pressure in front is higher; prevent backflow

Cardiac output?

Stroke volume × heart rate (heart rate = 60 ÷ time for one cycle)

Arteries?

Thick muscle and elastic tissue (stretch and recoil, smooth flow, maintain pressure); thick wall resists high pressure; narrow lumen

Arterioles?

More muscle; constrict or dilate to control blood flow to tissues

Veins?

Wide lumen; thin wall, little muscle and elastic tissue; valves prevent backflow; low pressure

Capillaries?

Wall one cell thick (short diffusion pathway); narrow lumen; many and branched (large surface area)

Tissue fluid formation?

High hydrostatic pressure at the arteriole end forces water and small molecules out; proteins and cells stay in the blood

Tissue fluid return?

At the venule end, plasma proteins give the blood a lower water potential; water returns by osmosis; excess drains into the lymph

Xylem?

Long, hollow dead cells with no end walls, strengthened with lignin; a continuous water column

Cohesion-tension?

Water evaporates from leaves, creating tension; hydrogen bonds give cohesion, so a continuous column is pulled up; adhesion to xylem walls

Evidence for cohesion-tension?

Trunk diameter narrows in the day (tension); a broken xylem vessel draws air in rather than leaking water

Factors affecting transpiration?

Light (stomata open), temperature (more evaporation), humidity (lower = steeper gradient), wind (removes moist air)

Phloem?

Sieve tube elements (no nucleus, sieve plates) and companion cells (many mitochondria)

Mass flow?

Source: sucrose actively loaded (co-transport with H⁺ via companion cells); water potential lowered; water enters by osmosis; high hydrostatic pressure. Sink: sucrose unloaded and used; water leaves; lower pressure. Flow source → sink

Ringing experiment?

Remove a ring of bark and phloem; sugars build up above the ring, tissue below dies, so phloem carries sugars

Tracer experiment?

Supply ¹⁴CO₂; autoradiography shows labelled sugars in the phloem moving to sinks

Evidence against mass flow?

Sugars travel to different sinks at different rates; sieve plates would slow flow

Oxygen dissociation curve axes?

x: partial pressure of oxygen (pO₂). y: % saturation of haemoglobin with oxygen

Insect tracheoles during exercise?

Muscle cells respire anaerobically, making lactate, which lowers their water potential; water moves out of the tracheoles into cells by osmosis, drawing air further in

Heart valves?

Atrioventricular valves (tricuspid right, bicuspid left) between atria and ventricles; semilunar valves at the base of the aorta and pulmonary artery

Bile?

Made in the liver, stored in the gall bladder; bile salts emulsify lipids. It contains no enzymes