Organisms Responding to Changes - A-Level Biology PDF

3.6 Organisms respond to changes in their internal and external environments (A-level only) A stimulus is a change in the internal or external environment. A receptor detects a stimulus. A coordinator formulates a suitable response to a stimulus. An effector produces a response. Receptors are spe...

3.6 Organisms respond to changes in their internal and external environments (A-level only) A stimulus is a change in the internal or external environment. A receptor detects a stimulus. A coordinator formulates a suitable response to a stimulus. An effector produces a response. Receptors are specific to one type of stimulus. Nerve cells pass electrical impulses along their length. A nerve impulse is specific to a target cell only because it releases a chemical messenger directly onto it, producing a response that is usually rapid, short-lived and localised. In contrast, mammalian hormones stimulate their target cells via the blood system. They are specific to the tertiary structure of receptors on their target cells and produce responses that are usually slow, long-lasting and widespread. Plants control their response using hormone-like growth substances. 3.6.1 Stimuli, both internal and external, are detected and lead to a response (A-level only) **3.6.1.1 Survival and response (A-level only)** **Content Opportunities for skills** **development** Organisms increase their chance of survival by responding to changes in their environment. In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli. The effect of different concentrations of indoleacetic acid (IAA) on cell elongation in the roots and shoots of flowering plants as an explanation of gravitropism and phototropism in flowering plants. Taxes and kineses as simple responses that can maintain a mobile organism in a favourable environment. The protective effect of a simple reflex, exemplified by a three neurone simple reflex. Details of spinal cord and dorsal and ventral roots are not required. **3.6.1.2 Receptors (A-level only)** **Content Opportunities for skills** The Pacinian corpuscle should be used as an example of a receptor to illustrate that: receptors respond only to specific stimuli stimulation of a receptor leads to the establishment of a generator potential. The basic structure of a Pacinian corpuscle. Deformation of stretch-mediated sodium ion channels in a Pacinian corpuscle leads to the establishment of a generator potential. The human retina in sufficient detail to show how differences in sensitivity to light, sensitivity to colour and visual acuity are explained by differences in the optical pigments of rods and cones and the connections rods and cones make in the optic nerve. **3.6.1.3 Control of heart rate (A-level only)** **Contenunities for skills** Myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity. The roles of the sinoatrial node (SAN), atrioventricular node (AVN) and Purkyne tissue in the bundle of His. The roles and locations of chemoreceptors and pressure receptors and the roles of the autonomic nervous system and effectors in controlling heart rate. 3.6.2 Nervous coordination (A-level only) **3.6.2.1 Nerve impulses (A-level only)** **Content Opportunities for skills** **development** The structure of a myelinated motor neurone. The establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the movement of sodium ions and potassium ions. Changes in membrane permeability lead to depolarisation and the generation of an action potential. The all-or-nothing principle. The passage of an action potential along non-myelinated and myelinated axons, resulting in nerve impulses. The nature and importance of the refractory period in producing discrete impulses and in limiting the frequency of impulse transmission. Factors affecting the speed of conductance: myelination and saltatory conduction; axon diameter; temperature. **3.6.2.2 Synaptic transmission (A-level only)** **Content Opportunities for skillsdevelopment** The detailed structure of a synapse and of a neuromuscular junction. The sequence of events involved in transmission across a cholinergic synapse in sufficient detail to explain: unidirectionality temporal and spatial summation inhibition by inhibitory synapses. A comparison of transmission across a cholinergic synapse and across a neuromuscular junction. Students should be able to use information provided to predict and explain the effects of specific drugs on a synapse. 3.6.3 Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only) **Content Opportunities or skillsevelopment** Muscles act in antagonistic pairs against an incompressible skeleton. Gross and microscopic structure of skeletal muscle. The ultrastructure of a myofibril. The roles of actin, myosin, calcium ions and ATP in myofibril contraction. The roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation. (The role of troponin is not required.) The roles of ATP and phosphocreatine in muscle contraction. The structure, location and general properties of slow and fast skeletal muscle fibres.