Xylem and the transpiration stream

Xylem vessels are dead, hollow tubes with no end walls and lignified walls that are waterproof and prevent collapse under tension.

Cohesion–tension theory:

• Water evaporates from the surfaces of the mesophyll cells and diffuses out through the stomata: transpiration.
• This lowers the water potential of the mesophyll, so water is drawn from the xylem, putting the column under tension.
• Water molecules stick to each other by hydrogen bonding (cohesion) so the whole column is pulled up as one, and stick to the lignified walls (adhesion).
• Water enters the root by osmosis through the root hair cells, and mineral ions are taken up by active transport — which lowers the water potential of the root further.

Transpiration rate rises with light intensity (stomata open), temperature (more kinetic energy, steeper gradient), air movement (removes the humid boundary layer) and falls with high humidity.

Xerophytes (2.1.15): marram grass has a thick waxy cuticle, stomata sunk in pits, hairs, and rolled leaves — all of which trap humid air next to the stomata, reducing the water potential gradient, and lengthen the diffusion pathway. Hydrophytes go the other way: air spaces (aerenchyma) for buoyancy and oxygen supply, and stomata on the upper surface.

Your turn — potometer

Phloem and translocation

Phloem is living. Sieve tube elements have perforated sieve plates and lose most of their organelles; each is supported by a companion cell packed with mitochondria.

Mass flow hypothesis:

• At the source (a photosynthesising leaf), sucrose is actively loaded into the sieve tube — the companion cell uses ATP to pump H⁺ out, and sucrose then enters by co-transport with H⁺.
• This lowers the water potential in the sieve tube, so water enters by osmosis from the xylem, raising the hydrostatic pressure.
• At the sink (a root, fruit or growing bud), sucrose is unloaded and used or stored, so water leaves and the pressure falls.
• Sap therefore flows down a hydrostatic pressure gradient from source to sink — which is why phloem transport can go up or down.

Evidence: aphid stylet experiments show that sap flows fastest near the source and contains sucrose; metabolic poisons and low oxygen stop translocation (proving that an active step is involved) but do not stop transpiration.

Xylem or phloem?

Tap a feature, then tap the tissue it belongs to.

The heart and the cardiac cycle

Mammals have a closed double circulation. The left ventricle has the thickest wall because it must generate enough pressure to force blood around the entire systemic circuit; the right ventricle only serves the low-pressure pulmonary circuit.

The heart is myogenic. The sinoatrial node (SAN) in the right atrium sets the rhythm; the wave of excitation spreads over both atria (atrial systole), reaches the atrioventricular node (AVN), which delays it so the atria empty first, then passes down the bundle of His to the apex and up the Purkyne fibres, so the ventricles contract from the bottom up.

Your turn — cardiac output

Haemoglobin and the Bohr shift

Haemoglobin has four polypeptide chains, each with a haem group containing Fe²⁺, so it can carry four O₂ molecules. Binding is cooperative: the first oxygen changes the shape of the molecule, making the next easier to bind — which is why the oxygen dissociation curve is S-shaped (sigmoid).

• At the high partial pressure of oxygen in the lungs, haemoglobin is almost fully saturated — it loads oxygen.
• At the low partial pressure in respiring tissue, saturation falls steeply — it unloads oxygen exactly where it is needed.

The Bohr shift: respiring tissue produces CO₂, which dissolves to form carbonic acid, lowering the pH. This changes haemoglobin’s shape, reducing its affinity for oxygen, so the curve shifts to the right: at any given pO₂, more oxygen is released. The harder a tissue respires, the more oxygen it is given.

Comparisons that carry marks: fetal haemoglobin has a higher affinity (curve to the left) so it takes oxygen from the mother’s blood across the placenta. Myoglobin has a very high affinity, acting as an oxygen store in muscle. Small mammals with a high metabolic rate have haemoglobin with a lower affinity (curve to the right), unloading oxygen readily.

The Bohr shift

The breathing system and smoking

The respiratory tree: trachea → bronchi → bronchioles → alveolar ducts → alveoli. Cartilage rings keep the trachea and bronchi open; the airways are lined by ciliated epithelium with goblet cells, so mucus traps particles and the cilia sweep it upwards.

Ventilation: in inspiration the external intercostal muscles contract, moving the ribcage up and out, and the diaphragm contracts and flattens — the thoracic volume increases, so pressure falls below atmospheric and air flows in. Quiet expiration is largely passive: the muscles relax and the elastic tissue recoils.

The alveolus is a superb exchange surface: about 300 million of them give a vast surface area; the wall is a single layer of squamous epithelium, giving a very short diffusion distance; and the dense capillary network plus continuous ventilation maintains the concentration gradient.

Smoking (2.1.8): tar paralyses and destroys the cilia, so mucus accumulates — the “smoker’s cough” — and bacteria are not removed: chronic bronchitis. Enzymes released by the resulting inflammation destroy the alveolar walls, so the alveoli merge into large air spaces: emphysema drastically reduces the surface area for gas exchange. Carcinogens in tar cause mutations leading to lung cancer, and nicotine and carbon monoxide together raise the risk of CVD.

Adaptation, ecological factors and niche

Adaptation is any behavioural, physiological or morphological feature that helps an organism meet an environmental challenge.

Ecological factors that determine distribution:

• Climatic — temperature range, water availability, light intensity, light quality, day length.
• Edaphic (soil) — pH, availability of macronutrients and micronutrients, and soil aeration.
• Biotic — competitors, predators, disease, and the accumulation of waste.

An ecological niche is the role and position of a species in its habitat — everything it does and everything it needs. Two species cannot occupy the same niche indefinitely: one will out-compete the other.

Sampling a habitat

• Random sampling — lay out two tape measures as axes and use a random number generator for the coordinates. This avoids bias, so the sample is representative.
• Line transect — record every species touching a line at set intervals: used to show zonation along an environmental gradient.
• Belt transect — place quadrats along the line, giving quantitative abundance data along the gradient.
• Quadrats and pin frames — estimate density (individuals per m²), frequency (the % of quadrats containing the species) and percentage cover (for plants that are hard to count individually).
• Pitfall traps, sweep nets and pooters — for invertebrates.

Always record the abiotic factors at the same time (light meter, pH probe, thermometer, soil moisture) — otherwise you have a distribution with nothing to correlate it against. And take many samples: a larger sample size makes the mean more reliable and reduces the effect of chance.

Your turn — population density

Your turn — frequency

Which technique?

Tap the question on the left, then the technique that answers it.

Biodiversity and classification

All organisms share the same biochemical basis of life — the same four groups of biological molecules and, essentially, the same genetic code — which is powerful evidence of common ancestry.

Classification is hierarchical: domain, kingdom, phylum, class, order, family, genus, species — each group nested inside the one above, with no overlap. The binomial system gives every species a unique two-part Latin name. Modern classification is phylogenetic: it aims to reflect evolutionary relationships, and it is increasingly based on molecular evidence — DNA and rRNA sequences and protein comparisons — rather than appearance alone.

Biodiversity can be considered as the number of species, the genetic diversity within a species, and the range of habitats. It matters ecologically (a diverse ecosystem is more resilient), economically (food, medicines, tourism) and ethically.

Human impact on biodiversity

Threats:

• Habitat destruction and fragmentation — deforestation, drainage of wetlands, hedgerow removal. Small fragments hold small populations, which lose genetic diversity through genetic drift and inbreeding.
• Over-exploitation — fishing and hunting faster than a population can reproduce.
• Pollution — including eutrophication: fertiliser runs off into water; algae bloom; the bloom blocks the light so the plants below die; saprobiotic bacteria decompose them, multiply, and use up the dissolved oxygen; fish and invertebrates suffocate.
• Introduced species — with no natural predators they out-compete natives.

Conservation: in situ (nature reserves, SSSIs, legal protection, restoring habitats) keeps species in their natural habitat and preserves their ecological interactions. Ex situ (zoos, botanic gardens, seed banks) is a safety net for species too threatened to survive in the wild, but is expensive, holds a small gene pool, and does not conserve the habitat.

Eutrophication

The big ideas to know

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