2020 Biology HSC Homeostasis-Notes PDF

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2020 Biology HSC Homeostasis-Notes...

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Homeostasis Homeostasis is the maintenance by an organism of a relatively constant internal state, regardless of external changes in the environment. The importance of homeostasis In order to bring about optimal metabolic efficiency, all chemical reactions within cells must occur efficiently and be effectively coordinated. These reactions are catalysed by enzymes, which are very sensitive to changes in their environment. It’s therefore essential that internal conditions be maintained at a level that allows the optimal functioning of enzymes to ensure that optimal metabolic efficiency is maintained. Enzymes are extremely sensitive to the temperature and pH of the internal environment, and changes to these as well as to the concentration of substrates in the reactions will affect their activity. Maintenance of homeostasis In mammals, both the nervous system and the e ndocrine (hormonal) system are involved in homeostasis. The phrase ‘relatively constant internal state’ used in the definition of homeostasis indicates that some change in the internal environment is allowed for. Variables in the internal environment, such as temperature or glucose concentration, are maintained within a narrow range, known as the tolerance limits. Each of these variables has an ideal or normal value, called the set point. The Negative Feedback System 1. Detecting change: receptors (sensory cells) within the body detect a change in a particular component of the internal environment (e.g. temperature, blood pH). This change is called a stimulus. 2. Counteracting the change: a response occurs that will reverse (or counteract) the change, which is brought about by the effector organs and will restore the body to its relatively constant internal state.

Link between these two changes is the control centre, which is responsible for maintaining fluctuations around the set point, receives information from the receptors about a change in a condition either too far above or below the set point. It then determines an appropriate response and sends a message to the effector to carry out activities to counteract the stimulus and return levels to the set point. This self-regulating system is known as a negative feedback mechanism. Message from receptors to control centre and response directed by control centre to change stimulus is the feedback. It’s called ‘negative’ because it counteracts stimulus. Coordination of the homeostatic mechanism The hypothalamus, a region in the lower central part of the brain, is an important control centre in maintaining homeostasis, in the negative feedback system. It contains receptors for certain factors and is a link between the nervous and endocrine systems. Negative feedback loops These can be used to show the way in which the body maintain specific internal conditions. Thermoreceptors detect changes in temperature (stimulus). Cooling the body - When receptors detect an increase in body temperature (stimulus), messages are sent to the anterior (Front) area of the hypothalamus, which is the heat-loss control centre. Messages are then sent via nervous system to effectors that initiate processes to lose heat and cool the body down. - Vasodilation - which involves blood vessels dilating, bringing blood closer to the skin and allowing heat to escape - Activation of sweat glands to secret liquid sweat, removing heat from the body when it evaporates - Activation of thyroid gland to lower metabolism rate in cells by reducing the amount of hormone thyroxine produced, thus generating less heat. Warming the body - When there is a decrease in temperature, messages are sent to the posterior (back) area of the hypothalamus. - Vasoconstriction - involves blood vessels constricting, removing blood from skin surface and conserving heat

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Contraction of hair erector cells, causing hair/fur to stand on end, trapping layer of warm air and reducing heat loss Release of thyroid-stimulating hormone (TSH) by pituitary gland under direction of hypothalamus, causing the thyroid gland to increase amount of thyroxine → increases a rate of metabolism, generating more heat. Rapid contraction of muscles (shivering), generating heat

Combined effect of these process increases body temperature, counteracts stimulus and returns to set point.

Mechanisms to maintain homeostasis Internal coordination systems Nervous and endocrine systems are internal systems that work together that homeostasis is maintained. They coordinate and provide pathways of communication for negative feedback systems that operate to maintain homeostasis. Receptors Receptors are responsible for detecting stimuli, any changes from set point, that are outside tolerance limits. They contain sensory cells and can take numerous forms depending on stimuli. They are concentrated in particular areas, forming sense organs such as eye, ear, tongue, etc. Interoceptors - receptors in the body that detect internal stimuli related to homeostasis. May be named according to the type of energy. - Thermoreceptors - detect changes in temperature. In the skin, these are nerve endings that are sensitive to heat or cold and send information to the brain about external temperature. - Chemoreceptors - detect concentration of certain chemicals inside the body. Located in blood vessels and detect pH and other chemicals (e.g. CO2, O2) - Osmoreceptors - detect changes in osmotic pressure and are located in the hypothalamus. Osmotic pressure in blood is determined by the concentration of substances dissolved in the blood plasma, and changes to it cause process to regulate amount of water. The nervous system Neural pathways by which messages travel in the body are provided by the nervous system. There are two main parts: central nervous system and peripheral nervous system. CNS is composed of brain and spinal cord; the PNS comprises all other nerves throughout the body that aren’t part of the CNS.

Peripheral nerves carry information to and from the CNS. the information carried by nerves consist of ‘messages’ transmitted in the form of electrochemical impulses. Neurons - These are nerve cells, and no two are the same, but contain three common structure. - Cell body contains a nucleus and many organelles found in other cells. These from the ‘grey matter’ of the CNS - Dendrites, branching extensions, that are extensions of the cytoplasm of the cell body. They receive messages in the form of impulses from other axons and conduct these nerve impulses towards the cell body. Singular: dendron - Axon, single, very long extension of the cytoplasm of cell body, carries messages away from cell body and forms ‘white matter’ of CNS. Functions and types - Sensory neurons - carry impulses from sensory cells in PNS to CNS. - Motor neurons - transfer messages from CNS to effectors such as muscles or glands. - Interneurons (association or connector neurons) - located within the CNS and are link between sensory and motor neurons. When a nervous impulse is transferred from the axon of one neuron to the dendrites of the adjacent neuron it must cross a small gap, or synapse, as the adjacent neurons do not actually touch. Transmission of nerve impulses - the action potential Electrochemical impulses involve a change in the electric potential of the cell membrane of the axon - this temporary change is known as an action potential.this is brought about by a change in concentration of ions, as movement of ions causes electrical impulse. At rest A neuron is said to be at rest i it’s not transmitting any electrochemical messages, and ions inside and outside the cell attempt to balance themselves out (not possible but they try).

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The two main parts of the nervous system are the CNS and the PNS Nervous system contains millions of neurons (nerve cells), which transmit messages via electrochemical impulses. A typical neuron contains a cell body, dendrites, and an axon. The three types of neurons are sensory neurons, motor neurons, and interneurons (connector neurons) The synapse is the small gap between the axon terminals and dendritic terminals of adjacent neurons. Nerves are made up of bundles of neuronal fibres A stimulus will cause a change in ion concentrations across the cell membrane of the axon, which in turn alters the membrane potential. If the membrane potential reaches the threshold value, an action potential involving depolarisation, and then repolarisation, is instigated. Each potential causes another action potential in the next region of the neuron. This series of action potentials along the neuron is the nerve impulse. If the threshold value is not reached, there will be no action potential and therefore no nerve impulse generated. Neurotransmitters released at the axon terminal transfer the ‘message’ across the synapse to the adjacent neuron and stimulate an action potential in the dendrites of the receiving neuron.

The central nervous system The brain is the main control centre of the body and a very complex organ. It largely controls the maintenance of homeostasis, and consists of numerous parts that all work together to ensure optimal functioning.

The hypothalamus provides a link between the nervous and endocrine systems, to assist in the maintenance of homeostasis. The spinal cord extends from the medulla oblongata down through the vertebral column to the waist area. It contains the nerve fibres that provide the link for the pathway of nerve impulses between the PNS and the brain. The two main functions of the spinal cord are: - Acts as a conduction pathway for nerve impulses from the receptors around the body to the brain, and for nerve impulses from brain to effectors. - Coordinate reflex actions, such as removing your hand quickly when you touch something hot, before you feel the pain. The endocrine system The endocrine regulates the activity of the body. Its main components are hormones, which are chemical messenger molecules secreted by endocrine glands. Hormones are transported by the bloodstream to cells possessing receptors for the particular hormone. The hormones cause these

cells to change their activity to maintain concentration of these enzymes in the cells, known are target cells. Glands can be stimulated to secrete hormones by messages from the nervous system, by other hormones or by receptors located in the particular gland. The pituitary gland is referred to as the master gland and is just below (and in collaboration with) the hypothalamus. It releases hormones to regulate the activity of other glands. There are two distinct regions of the pituitary gland: anterior and posterior (back). Hormones released by the hypothalamus control the anterior area of pituitary gland, while posterior section is controlled by nerve impulses. The endocrine portion of the pancreas consists of structures called pancreatic islets, which contain alpha and beta cells. Chemoreceptors in beta cells detect high levels of glucose in the blood and stimulate insulin production, which causes it to be removed from blood by converting it to other substance. When glucose levels decrease, production of insulin decreases. Alpha cells respond to low levels of glucose, by producing glucagon.

Adaptations in endotherms Endotherms are organisms that are able to maintain their body temperature within a very narrow range of tolerance limits despite variations in ambient temperature, relying on metabolic activity and other internal sources. An adaptation is a characteristic that an organism possesses that will increase the survival and reproductive changes of that organism in its environment. Thermoregulation - regulation of body temperature. Behavioural adaptations If ambient temperature is too high, then they may change the position of their body to reduce the surface area exposed, seek shade shelter in burrows or move into water to cool down. Nocturnal activity is another common behavioural adaptation to assist in regulating body temperature.

Migration is another behavioural adaptation that assists in thermoregulation. Structural adaptations Structural adaptations that assist with temperature control include insulation, such as fur, hair, feathers, which trap a layer of air next to the skin, reducing heat loss. Blubber is another form of insulation. The surface area to volume ratio is also an important structural component of temperature regulation. Physiological adaptations These focus on functions within the body, and metabolic activity can be altered to assist the organism in maintaining its body temperature within the tolerance range. Photosynthesis and transpiration are examples.

Mechanisms to maintain water balance in plants Reducing internal temperature Some mechanisms for plants other than transpiration - Their leaves may be coated in a shiny waxy cuticle or a thick, leathery cuticle. This ensures that all the epidermal cells are waterproof, preventing loss of water by evaporation from these surface cells. - The leaf may have white hairs to reflect sunlight, which will reduce the temperature on the surface of the leaf and thus reduce evaporation. Reducing the exposure of transpiring plant structures to sunlight In some plants, the exposure of these organs and their stomata to light is reduced by: - The orientation of the leaves so that stomata are not exposed to direct light - Reduced surface area or organs that have the highest proportion of stomata - Complete loss of transpiring plant organs such as leaves or leaf-like parts Some examples of adaptations that reduce water loss are: - Reduced leaf size - Reduced size of flowers or no petals - Shedding leaves

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Orientation of leaves on the stem

Regulating the opening and closing of stomata Some plants minimise the loss of water by only opening the stomata during the cooler parts of the day. Water storage Some plants, called succulents, have adaptations such as fleshy stems or leaves that swell up and retain moisture when it is available’ they then survive by using this moisture during dry periods. Fruits Fruits are structures that are removed from plants so that the seeds they contain can be dispersed. Many plants produce woody fruits rather than fleshy fruits, which reduces the amount of water lost from the plant when the fruits fall off....