Homeostasis & Response

  • Created by: AMcL1
  • Created on: 29-05-19 15:46


  • Homeostasis is the regulation of the internal conditions of a cell or organism in order to maintain optimum conditions for function in response to internal or external changes.
  • Enzymes denature by the following process: The pH goes towards the optimum, causing the kinetic energy in the enzymes to increase, meaning that there are more succesful collisions and therefore a higher rate of reaction. At the optimum pH (8), the enzyme activity is at its highest. Then, as the pH goes further away from the optimum, the enzymes start denaturing and so their activity decreases. Now, the active site has changed shape and can no longer break down the substrate. This is irreversible. 
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  • There are two different systems in the body that help to perform homeostasis. One is the nervous system and the other is the endocrine system.
  • The Nervous System - Responses from here are instant, rapid and precise, and are carried out in the form of electrical impulses. These are carried to a specific target organ.
  • The Endocrine System - Responses from here are slow and are chemical (hormones). These are carried everywhere, but only affect the specific target organ.
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Blood Temperature

  • Blood Temperature - This is monitored by the thermoregulatory centre of the brain. This part of the brain contains receptors that are sensitive to the temperature in the blood, and any changes in it. It responds to changes after receiving nervous impulses from temperature receptors in the skin.
  • If body temperature gets too high, the sweat glands in the skin begin to release more sweat. As this evaporates, it removes heat energy from the skin. Also, when body temperature is too high, vasodilation occurs. This process describes how the blood vessels underneath the surface of the skin (capillaries) dilate, allowing a larger bloodflow to move through them, meaning that heat from the blood can be lost as it flows closer to the skin.
  • If body temperature gets too low, the muscles in the body begin to contract and relax rapidly. This is called shivering. In order to do this, the muscles must respire quickly, which releases heat energy, warming the body up. In addition to this, the blood vessels beneath the skin (capillaries) become narrower and constrict, allowing less blood to flow through them, meaning that more heat is kept in the body as less heat energy is lost because the bloodflow is further away from the surface of the skin. 
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The Nervous System - Part 1

  • The nervous system is split into two parts: the central and the peripheral.
  • The Central Nervous System - This is composed of the coordinators of the nervous system: the brain and the spinal cord. 
  • The Peripheral Nervous System - This is the term used to describe all nerves that carry messages to and from the brain and spinal cord.
  • Types of Neurones - There are three main types of neurone: sensory, relay and motor. Firstly, sensory neurones carry signals from the receptor to the CNS. Relay neurones carry signals from any part in the CNS to another. And finally, motor neurones carry signals from the CNS to the effector, which triggers a response.
  • Signals move across neurones through the following process. Firstly, the electrical impulse travelling through the neurone travels across it until it reaches the end. Here, it triggers the release of a neurotransmitter substance. This diffuses across the synapse and binds with the receptor molecule on the next neurone. Then, the impulse continues.
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The Nervous System - Part 2

  • Reflex Reaction - This is a fast, automatic and protective response to a stimulus. It does not involve the conscious part of the brain and its process can be illustrated by a reflex arc. The reflex arc is described below:
  • Stimulus (external or internal) - Receptor (cell/organ) - Coordinator (brain/spinal cord) - Effector (muscles/glands) - Response (movement/behaviour)
  • The sensory neurones carry signals between the receptor and coordinator and motor neurones carry signals between the coordinator and effector.
  • An example of a reflex reaction shown by a reflex arc can be seen below:
  • Hot Plate (stimulus) - Skin (receptor) - Spinal Cord (coordinator) - Muscles in Arm (effector) - Move Hand/Drop Plate (response)
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The Brain

  • The brain controls the behaviour of the body and has interconnected neurones. It also has different regions for different functions.
  • Brain Mapping - Neuroscientists map brains by studying those with brain damage and electrically stimulating areas of the brain. They also use MRI (Magnetic Resonance Imaging).
  • Brain Treatment - There are many issues that surround treating the brain in order to cure nervous system disorders. In many cases, it is not possible, but where it is, surgery can be performed to remove excess fluid or a brain implant or tumour. The problems with this are that the brain is delicate and complex and so there are risks fo physical damage or increasing the issue.
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Hormones - Part 1

  • Hormones are chemicals released into the blood that impact target organs with long-lasting effects. Main examples in the human body are listed below.
  • Adrenaline - This is produced by the adrenal gland and targets the vital organs of the body, preparing it for action (fight or flight).
  • Thyroxine - This is produced by the thyroid gland and doesn't have a specific target organ. Its function is to stimulate the basal metabolic rate. 
  • Progesterone - This is produced by the ovaries and targets the uterus, helping to maintain the lining of the womb.
  • Luteinising Hormone (LH) - This is produced by the pituitary gland and targets the ovaries, triggering egg release (ovulation) and progesterone production.
  • Follicle Stimulating Hormone (FSH) - Once again, this is produced by the pituitary gland and targets the ovaries, but this hormone triggers egg ripening and oestrogen production instead. 
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Hormones - Part 2

  • Insulin - This is produced in the pancreas, targets the liver and controls blood sugar levels.
  • Testosterone - This is produced in the testes and targets the male reproductive organs. Its purpose is to control puberty and sperm production.
  • Anti-Diuretic Hormone (ADH) - ADH is produced in the pituitary gland and affects the kidneys. It is a hormone that helps to control blood water levels. 
  • Oestrogen - This is a female hormone that is produced in the ovaries. It has many target organs, including its site of production - the ovaries, as well as the pituitary gland and the uterus. Its functions are to control puberty and the menstrual cycle, trigger the production of LH and also suppress the function of FSH.
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ADH and the Kidneys

  • If there isn't enough water in the body, more ADH is released by the pituitary gland. This increase in ADH causes the kidney tubules to become more permeable, helping them to reabsorb more water back into the blood. This process is controlled by negative feedback. Negative feedback is the process where a change in a condition from a set level causes a series of actions that return the condition to the set level.
  • Kidney Failure - Patients with kidney failure can either have kidney transplants or kidney dialysis. Kidney dialysis is where a machine and a drip are used to take out the harmful waste and retain the healthy substances.
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Blood Sugar Levels & Diabetes

  • If blood sugar levels are too high, the pancreas produces insulin. This hormone triggers the conversion of glucose into glycogen so that it can be stored in muscles and the liver. This conversion lowers blood sugar levels.
  • If blood sugar is too low, the pancreas produces glucagon. This stimulates the conversion of glycogen (which is currently in the liver and muscles) back into glucose, so that it can be released into the bloodstream to increase blood sugar to a safe level again.
  • The process is constantly repeating. 
  • Diabetes is split into two types, and is the name given to an incurable disease that impacts blood sugar levels.
  • Type 1 Diabetes - This form of the disease is when the pancreas doesn't produce sufficient insulin, and so blood sugar levels are constantly high. It is treated with insulin injections and a healthy diet.
  • Type 2 Diabetes - This form of the illness is when the body does not respond to insulin as the blood sugar level is already too high because of an unhealthy diet and lifestyle. One risk factor that can cause type 2 diabetes is obesity. The only way it can be treated is with a healthy diet and lots of excersize.
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  • Osmosis is the movement of water molecules from an area of high concentration to an area of low concentraiton across a partially permeable membrane.
  • In osmosis, the term dilute is used to describe an area with more water molecules and the term concentrated is used to describe and area with less water molecules.
  • The balance of ions in the body is regulated by the kidneys. Ions are absorbed through the consumption of food, and if there is an upset in ion balance, it affects osmosis.
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  • The Production of Urine - Proetins are made up of amino acids, which are not stored in the blood. This means, if there are any excess amino acids, they will be converted into fats and carbohydrates, through a process called deamination. This occurs in the liver. One of the waste products of deamination is ammonia, which is toxic. This is converted to urea. The urea then combines with water to form urine, which is filtered out of the blood by the kidneys. This final filtration allows the kidneys to reabsorb any useful mineral ions that are still in the water in the urine.
  • Uses of Urine - Urine can be used for many different things, including pregnancy, STI, UTI and hydration tests. 
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The Eye

  • Different light levels affect the muscles and their movements in the eye. Responses to light levels are described below.
  • In dim light, the circular muscles in the eye will relax, making the pupil bigger. The radial muscles will contract as well, which pulls the pupil further apart.
  • In bright light, the circular muscles in the eye will contract, causing the pupil to be smaller. Also, the radial muscles will relax, stopping the pupil from being pulled futher apart.
  • Accomodation - This is the process of changing the shape of the lens in the human eye in order to focus on near or distant objects. For distant objects, the ciliary muscles relax and the suspensory ligaments are pulled tight, causing the lens to become thin and only slightly refract light. For near objects the ciliary muscles contract and the suspensory ligaments are loosened, causing the lens to become thick and refract light strongly.
  • Hyperopia - This is also called long-sightedness, and is treated with a convex lens. It is when the eye is too short and the cornea is too flat, and so the image focuses behind the retina. 
  • Myopia - This is also named short-sightedness, and is treated with a concave lens. It is when the eye is too long and the cornea is too steep and so the image focuses infront of the retina.
  • With both hyperopia and myopia, the image seen by the eye appears blurred.
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Menstruation & Infertility

  • In menstruation, the pituitary gland release FSH. This triggers egg ripening and oestrogen production in the ovaries. The oestrogen then triggers LH production and suppresses FSH production. The LH then triggers egg release (ovulation) and progesterone production in the ovaries. This progesterone then maintains the womb lining. The process repeats.
  • Infertility - This is caused by either a blockage in the sperm ducts, a blockage in the fallopian tubes, the eggs not developing, the eggs not being released or insufficent fertile sperm being produced. One way in which infertile couple can still conceive is through IVF.
  • IVF - This stands for 'In Vitro Fertilisation', meaning 'in glass', which refers to the use of a test tube as it takes place in a laboratory. In IVF, the woman is injected with FSH and LH, which help the maturation of the eggs. These eggs are then collected from the mother, and are fertilised by the sperm from the father. The fertilised eggs (zygotes) then develop into embryos. Once they are small balls of cells, one or two are implanted into the mother's uterus. They use more than one in the hope that at least one of them will develop into a healthy foetus.
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Plants - Tropisms

  • A tropism is a growth in movement in response to a stimulus. A positive tropism is where the plant grows towards the stimulus and a negative tropism is where the plant grows away from the stimulus.
  • A phototropism is a tropism responding to the stimulus of light. An example of a positive phototropism is a plant shoot growing further towards the light.
  • A geotropism is a tropism responding to gravity as the stimulus. An example of a positive geotropism is roots growing down into the soil to stabilise the plant.
  • When a seed germinates in the soil, light is not a stimulus yet. The plant has to undergo a negative geotropism (where the shoot grows up and further away from the force of gravity) in order to find light.
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Plant Hormones & Auxins

  • Plant hormones coordinate and control the growth and responses to light and gravity.
  • An example of a plant hormone is an auxin. Auxins are made in the tip of a growing plant shoot and are unequally distributed due to light. There are more auxins where there is less light, meaning that the shaded side of the shoot has more auxins. These hormones help a plant to grow so that all areas of its shoot can photosynthesise. On the shaded side of the shoot, there are more auxins, and these auxins cause cell elongation, meaning that the shaded side grows faster than the well-lit side. This causes the brighter side to bend towards the light so that the end of the shaded side can reach it in order to make food through photosynthesis.
  • Selective Weed Killers - These are made up of synthetic (man-made) auxins. They cause weeds to grow so fast that they cannot supply enough energy to keep up, and so they die. They mainly affect broad leaved weeds.
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Ethene & Gibrellins

  • Ethene is an alkene that can control cell division and fruit ripening. For this reason, it is used in the fruit industry to cover different goods. Doing this means that the fruit won't ripen to early whilst being stored and transported.
  • Gibrellins help to initiate seed germination. They end seed dormancy and promote flowering.  They also increase fruit size. Because of this, they are used in the industry to help speed up the process of plants producing fruits and make it more efficient.
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Required Practical

  • This required practical investigates the effect of light on the growth of germinated seedlings.
  • Variables - The control variable (kept the same) is the amount of time the seedlings are under light for, the independent variable (changed) is the amount of light, and the dependent variable (measured) is the height of the plant. 
  • Method - Place cotton wool in each petri dish. Then, dampen the cotton wool. After that, sprinkle the seedlings in, and once they are germinated, place each dish in different light levels periodically, allowing them to be in indirect light every other day. Water the seedlings and measure the height of one per petri dish each day to find a daily average for each dish. This process should be continued for a week before ending the experiment. 
  • Expected Results - The seedlings in the petri dish placed in no light will have grown the most and will be the tallest as they are shaded and so they have more auxins. The seedlings that were in partial light will be quite tall as they have some auxins. And the seedlings in full light will be the shortest as there are less auxins because they aren't needed. 
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