5.1

• Communication is essential for the survival of organisms as all living organisms must be able to detect and respond to changes in their internal and external environments.
• In multicellular organisms, the change necessary for survival is triggered by nervous and endocrine systems.
• Cell signalling involves the communication between cells, in the form of electrical signals which are carried by neurons or with the help of hormones.
• Neuronal cell signalling is faster and short term whereas chemical is slower and long term.
• Slow twitch fibres are specialised for slow contractions and are adapted to long periods of exercise such as marathon running, therefore they do not fatigue quickly.
• Slow twitch fibres contain many mitochondria and a lot of myoglobin, resulting in slow twitch fibres being dark in colour.
• Fast twitch fibres are adapted for rapid release of energy during intense exercise such as sprinting, with the contractions being intense and in short bursts.
• Fast twitch fibres have very few mitochondria and not a lot of mitochondria, therefore they are lighter in colour.
• Cell signalling in the form of endocrine signalling can be used for long distance signalling, where the signalling molecule is carried by the circulatory system.
• Other examples of signalling include paracrine signalling which occurs between cells which are in close proximity to each other and occurs directly or with the help of extracellular fluid.
• Autocrine signalling is a form of signalling where the cell releases signals to stimulate its own receptors thus triggering a response within itself.
• Homeostasis serves to ensure that a constant internal environment consisting of factors such as temperature, water potential, pH and blood glucose level is maintained, despite changes in the external environment of the organism.
• Homeostasis is achieved with the help of negative feedback which counteracts any change in internal conditions.
• In order for the negative feedback pathway to work, the following elements need to be present: sensory receptors such as temperature receptors to detect changes in internal conditions, effectors such as muscles and glands which can provide a response.
• If a change is detected, the receptors pass the message either via the nervous or hormonal system to the effectors such as liver or muscles which bring about a response to restore the optimum conditions.
• Another example of a control pathway is positive feedback which doesn’t occur as often as negative and has an opposing effect in that it increases the original change in the conditions.
• An ectotherm is an organism which regulates its body temperature with the help of external source.
• Ectotherms are unable to increase their respiration rate to increase the internal production of heat therefore they cannot rely on internal energy sources.
• Ectotherms control their body temperature by exchanging heat with their surroundings, for instance by exposing their body to sun, orientating it to either minimise or maximise sun exposure, hiding away from sun or increasing breathing for heat loss via evaporation of water.
• Endotherms are able to maintain a constant body temperature, independent of the external temperature.
• Endotherms contain thermoreceptors which monitor core body temperature changes and communicate them to the hypothalamus which in turn coordinates appropriate responses to restore the optimum temperature through either physiological or behavioural responses.
• Actions taken by endotherms to control body temperature through heat gain or heat loss include: Shivering – contractions of skeletal muscles stimulated by nerve impulses sent out by the hypothalamus, lead to increase in temperature as heat is released.
• Sweat glands – sweat production to decrease body temperature via evaporation.
• Hairs on skin – lie flat to minimise insulation and increase heat loss, raised to provide insulation and reduce heat loss.
• The cerebrum is the largest part of the brain composed of two halves known as the cerebral hemispheres.
• The membranes of the vesicles have aquaporins (water channel proteins) so when these are inserted into the plasma membrane it makes the cells more permeable to water.
• The mammalian nervous system consists of the central nervous system (brain and spinal cord) which is made up of grey and white matter, the peripheral nervous system made up of sensory and motor neurones which carry nerve impulses towards and away from the central nervous system, the autonomic nervous system made up of sympathetic and parasympathetic neurones which operate without conscious thought, and the somatic nervous system which is involved in voluntary control of skeletal muscles.
• Ethene speeds up ripening, to promote lateral growth, to promote fruit drop.
• The cerebrum is involved in controlling vision, thinking, learning as well as emotions as well as voluntary control of the body.
• Adrenaline cannot cross the plasma membrane, it must interact with the cell via the receptors on its surface.
• Gibberellins are used to delay senescence in citrus fruits, elongation of apples in combination with cytokinins, for elongation of grape stalks, in brewing of beer for production of malt, to increase yield of sugar cane, to speed up seed formation in young conifer trees, to prevent lodging.
• The autonomic nervous system controls homeostatic mechanisms as well as response to stress.
• Cytokinins are used to prevent yellowing of lettuce leaves, to promote shoot growth.
• In response to binding of adrenaline to the adrenergic receptor, a specific series of events is triggered.
• The autonomic nervous system consists of sympathetic and parasympathetic systems, which are antagonistic systems with opposing effects.
• The medulla oblongata, located at the base of the brain controls many vital body processes such as breathing, heart rate and blood pressure.
• Adrenaline is the first messenger whereas cyclic AMP is the secondary messenger.
• Different parts of the cerebrum have different functions, for instance the parietal lobe controls orientation, movement, some types of recognition and memory whereas the occipital lobe located at the back of the cerebrum is known as the visual cortex.
• The binding activates a membrane associated enzyme called adenyl cyclase which converts ATP to cyclic AMP which in turn interacts with a kinase enzyme.
• The kinase enzyme activates enzymes required to achieve a particular response.