Species Interactions and Ecophysiology

Chemical defence toxins

Many plants and animals carry toxic secondary compounds that deter predators and herbivores. Cardiac glycosides, which have a steroid core linked to a sugar, are a classic example: they block the Na⁺/K⁺-ATPase pump in animal cell membranes, and both plants and toads produce them. Similarity between organisms is described with two terms that must not be confused. Homology is similarity inherited from a common ancestor, whereas analogy is similarity that evolved independently through convergent evolution. Structure and function must be judged separately, so two compounds can be structural analogs while acting on the same target.

• Homology — shared ancestry of a trait, e.g., the forelimbs of humans and bats.
• Analogy — similar traits arising from convergence, e.g., the wings of insects and birds.
• Phytoecdysone — a plant ecdysteroid that structurally mimics the insect moulting hormone, so it disrupts moulting and metamorphosis in herbivorous insects.
• Tannins and lignins — tannins bind proteins and reduce digestibility, while lignins are structural cell-wall polymers. Neither is a hormone mimic.

Thermoregulation strategies

Animals are classified by how they control body temperature. Homeotherms maintain a nearly constant internal temperature across a range of ambient temperatures by metabolic heat production, whereas poikilotherms let body temperature follow the environment and depend largely on external heat sources. Homeothermy and poikilothermy describe the pattern of regulation, which is distinct from the terms ectotherm and endotherm, which refer to the source of heat. Heat exchange depends on surface area relative to volume. Heat is lost across the body surface, while heat is produced in proportion to body volume, so a lower surface-to-volume ratio conserves heat. Larger bodies and compact shapes are therefore favoured in cold climates, which underlies Bergmann's rule for body size and Allen's rule for shorter appendages.

• Huddling — several animals share heat and reduce the total exposed surface, which lowers heat loss for each individual.
• Clutch number — reproductive timing depends on food supply and season length, not directly on surface area to volume.
• Long limbs and large ears — these increase surface area relative to volume and promote heat loss, which is the opposite of the cold-climate pattern.

Competitive coexistence mechanisms

Two closely related species can coexist at the same site when their niches differ enough to reduce competition. Niche partitioning divides a shared resource along one or more axes, such as food size, habitat height, timing of activity or breeding, and communication signals. A strong trait difference between sympatric species often reflects character displacement, in which competition selected for divergence from a common ancestor. This is divergent evolution, and the trait may also set which prey or resources each species can use, as when body size limits the prey a predator can kill and handle.

• Acoustic partitioning — species that call at different frequencies avoid signal interference and can coexist.
• Spatial partitioning — nesting at different heights reduces overlap in nest sites.
• Temporal partitioning — breeding at different times of the year reduces competition for mates and food.
• Same limited resource — species using an identical limited resource compete strongly and tend toward competitive exclusion rather than coexistence.

Anabolism and catabolism

Metabolism is the sum of biochemical reactions in an organism and has two complementary parts. Anabolism is the set of biosynthetic reactions that build complex molecules from simpler precursors, usually consuming energy in the form of ATP or reducing power. Catabolism breaks complex molecules down and releases energy that drives anabolic work.

• Growth — new cell material is synthesized from simple precursors, so it is anabolic.
• Storage — reserves such as glycogen or triglycerides are built up from smaller units, which is also anabolic.
• Respiration — oxidation of substrates to release energy is catabolic.
• Excretion — removal of waste products does not synthesize new compounds and is not an anabolic process.

To classify a process, ask whether it builds new molecules or breaks them down.

Pathogenic bacteria and human diseases

Each pathogenic bacterium is usually associated with characteristic infections, and exam questions test these pairings. Matching should be done by organism, because the same type of infection can have several bacterial causes, while each bacterium has a recognized clinical syndrome.

• Staphylococcus aureus — a common agent of skin, wound and postoperative (surgical-site) infections.
• Shigella species — cause bacillary dysentery, a form of diarrhoea that may contain blood and mucus.
• Neisseria species — N. meningitidis causes meningitis, whereas N. gonorrhoeae causes gonorrhoea.
• Yersinia pestis — the causative agent of plague, transmitted mainly by fleas that feed on infected rodents.

C3 and C4 photosynthesis

C3 and C4 are two photosynthetic pathways that differ in how carbon dioxide reaches the Calvin cycle. In C3 plants, Rubisco fixes CO₂ directly in mesophyll cells. In C4 plants, PEP carboxylase fixes CO₂ into a four-carbon acid in mesophyll cells, and the acid is decarboxylated in bundle-sheath cells, which concentrates CO₂ around Rubisco. The energy cost per CO₂ fixed differs. The C3 cycle needs 3 ATP and 2 NADPH. The C4 cycle needs the same Calvin-cycle energy plus the regeneration of PEP, which costs 2 more ATP, giving 5 ATP and 2 NADPH in total. The C4 pathway therefore has the higher energy requirement, and this cost is the price of the CO₂-concentrating mechanism.

• Rubisco — the carboxylating enzyme of the Calvin cycle; its oxygenation reaction leads to photorespiration.
• Kranz anatomy — the bundle-sheath arrangement typical of C4 leaves.
• Trade-off — the extra ATP cost of C4 is offset by reduced photorespiration in hot, high-light conditions.

Forms of energy in organisms

Organisms store and transfer energy in several forms, and classification depends on the physical state in which the energy is held. Potential energy is stored by virtue of position or configuration, while kinetic energy is the energy of motion. Within potential energy, the forms relevant to biology include chemical energy in bonds, electrochemical energy in ion gradients across membranes, and mechanical (elastic) energy stored in deformed structures.

• Elastic strain energy — stored when a solid is stretched or compressed away from its rest length, as in a stretched tendon.
• Electrochemical energy — a polarized membrane or an activated neuron depends on ion gradients, not on mechanical deformation.
• Relaxed muscle — at its rest length it stores little elastic energy.

To identify the form, ask what physically holds the energy: bond chemistry, ion separation, or deformation.

Mycorrhizal–plant mutualism

Mycorrhizal fungi colonize plant roots, and the outcome of the association depends on the nutrient status of the soil. The interaction is best described as lying on a continuum between mutualism and parasitism. Under nutrient-poor conditions the fungus supplies phosphorus and water in exchange for carbon from the host, which is mutualism. Under nutrient-rich conditions the benefit to the plant declines, and the fungus may draw carbon while returning little, which is parasitism.

• Mutualism — both partners gain, classically nutrients exchanged for photosynthate.
• Parasitism — one partner gains at the expense of the other.
• Commensalism — one partner gains while the other is unaffected, as when the fungus benefits and the plant is not measurably harmed.
• Predation — one organism consumes another; it does not describe a root–fungus association.