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Flash cards for igcse biology

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All the different flash cards of definition etc... are gathered here for IGCSE

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Flash cards for igcse biology

  1. 1. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 1 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Home (/flashcards/) Create Flashcards (/flashcards/create-flashcards.php) Browse (/flashcards/browse/) Tour (/flashcards/tour/) Pricing (/flashcards/signup/business/) Text Size: S | M | L (855) 776-7763 IGCSE Biology All Topics Total Flash Cards » 38 Cells All animals and plants are made up of cells. Most cells have: A nucleus – controlling the activity of the cell. All cells have these at one point. Cytoplasm – where chemical reactions occur. Inside the cytoplasm are enzymes which speed up these reactions. Cytoplasm also contains mitochondria which is where energy is released. A cell membrane – to Login (/flashcards/login.php?return=/flashcards/) Sign Up Free (/flashcards/register.php) Search Flashcards
  2. 2. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 2 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics control the passage of substances in and out of the cell. Plants also have Cell walls – to strengthen the cell. Chloroplasts – to absorb sunlight energy to make food by photosynthesis. Vacuole –full of cell sap. Cells, tissues and organs Cells act together to form tissues. For instance, the cells on the surface of a leaf form "pallisade tissue". A group of cells with similar structures and a particular function are called a tissue. Tissues are grouped together to form an organ. A leaf is an organ. Organs are grouped together to form an organism such as a whole plant or animal. Cells ----> Tissue ----> Organ - ---> Organism Tissues are usually formed from specialised cells. The cells in the pallisade tissue are specialised to perform photosynthesis and contain many chloroplasts. Some other specialised cells are: Guard cells Egg cells Sperm cells Red blood cells
  3. 3. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 3 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Diffusion Diffusion is the net movement of particles from an area of high concentration to an area of a lower concentration. The steeper the concentration gradient, the more rapid the rate of diffusion. 2 examples of diffusion are: Oxygen (required for respiration) passes through cell membranes and gas exchange surfaces (e.g. alveoli in lungs) by diffusion Carbon dioxide enters leaves and leaf cells by diffusion Osmosis Osmosis is the movement of water from an area of high water concentration (more dilute) to an area of a low water concentration (stronger) through a partially permeable membrane. A partially permeable membrane allows water molecules to pass through (as they are smaller) but not solute molecules (they are too big). It acts like a sieve. Visking tubing is a partially permeable membrane. It is used in dialysis machines. Diffusion and osmosis can
  4. 4. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 4 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Active Transport only work if the concentration gradient is right. Sometimes an organism needs to transport something against a concentration gradient. The only way this can be done is through active transport, using energy produced by respiration. In Active transport, the particles move across a cell membrane from a lower to a higher concentration. Examples of active transport In plants: Plants need mineral salts (e.g. nitrates) for making proteins and growth. Nitrates are at a higher concentration inside the root cells than they are when dissolved in the water around the soil particles. If the plant relied on diffusion alone, the vital nitrate salts would drain out of the cells into the soil. So energy is deployed by the cells to actively transport nitrates across the cell membrane into the root cells, against the concentration gradient. In humans Active transport takes place during digestion of food in the small intestine. After food has been absorbed by the villi for some time, the concentration of food
  5. 5. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 5 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics molecules inside the villi increases, making it impossible for more food to diffuse into the villi. So simple sugars, amino acids, minerals and vitamins are actively absorbed into the villi, from an area of lower to an area of higher concentration Shareen Saqlain The Cell Cycle happens in cells Prophase The first and longest phase of mitosis, prophase, can take as much as 50-60 percent of the total time to complete mitosis. During prophase, the chromosomes become visible. The centrioles, two tiny structures located in the cytoplasm near the nuclear envelope, separate and take up positions on opposite sides of the nucleus. The centriols lie in a region called the centrosome that helps to organise the spindle, a fanliek microtubule structure that help separate the chromosomes. During prophase, the condensed chromosomes become attached to fibers in
  6. 6. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 6 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics the spindle at a point near the centromere of each chromatid. Metaphase The second phase of mitosis, metaphase, often only lasts a few minutes. During metaphase, the chromosomes line up across the center of the cell. Microtubules connect the centromere of each chromosome to the poles of the spindle. Anaphase Anaphase is the third phase of mitosis. During anaphase, the centromeres that join the sister chromatids separate, allowing the sister chromatids to separate and become individual chromosomes. The chromosomes continue to move until they have separated into two groups near the poles of the spindle. Anaphase ends when the chromosomes stop moving. Telophase Following anaphase is telophase, the fourth and final phase of mitosis. In telophase, the chromosomes, which were distinct and condensed, begin to disperse into a tangle of dense material. A nuclear envelope re-forms around each cluster of chromosomes. The spindle begins to break
  7. 7. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 7 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics apart, and a nucleolus becomes bisible in each daughter nucleus. Mitosis is complete Plant structure Plants are divided into flowers, stems, leaves and roots with root hairs. A generalised plant is shown in the illustration. The stem provides support for the leaves and flowers. It also allows water and food to travel both up and down the plant. The leaves make the food for the plant. Photosynthesis takes place in the leaves.
  8. 8. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 8 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics The roots anchor the plant in the soil and take up water and salts (mineral ions) from the soil. The root hairs provide a large surface area for water and salt uptake. The flowers are reproductive organs. They attract insects that carry pollen from one plant to another. This process of transferring pollen from plant to plant is known as pollination. The structure of the leaf Leaves produce the food for the plant. The structure of the leaf is shown in the illustration. The leaf has prominent veins that contain two types of tubes, the xylem tubes and the phloem tubes. The leaf has the following parts (from top to bottom): Waxy cuticle Upper Epidermis Pallisade layer Spongy layer Veins Lower epidermis
  9. 9. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 9 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Guard cells that form stomata Leaves are green because they contain the green pigment called chlorophyll. Chlorophyll is used in photosynthesis. The structure of flowers lowers are composed of: Sepals - these are arranged underneath the flower and are typically green. Petals - often brightly coloured to attract insects. Stamens - stalk-like filaments that have anthers at the top which produce pollen. Pollen contains the male gametes. Pistil - contains one or several carpels that contain the ovaries with ovules, the female gametes. Sometimes the carpels are merged. A stalk called the style leads upwards from each pistil and is topped by a sticky stigma that receives the pollen. The pistil is the bottle shaped
  10. 10. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 10 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics structure. A pistil can be composed of one or many carpels and a flower can have several pistils. Plant growth Plant growth requires glucose produced by photosynthesis and energy produced by respiration. It also requires minerals obtained from the soil. Plant growth is controlled by plant hormones called auxins. Auxins Minerals needed for plant growth There are three minerals that are essential for plant growth: phosphates, nitrates and potassium. Small quantities of iron and magnesium are also needed, especially for the production of chlorophyll. Phosphates: used in photosynthesis and respiration. Phosphate deficiency: purple leaves and small roots. Nitrates: used in the production of aminno acids. Amino acids are combined to make proteins. Nitrate deficiency: yellowing of leaves and poor, stunted growth.
  11. 11. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 11 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Potassium: maintains electrical potentials and helps enzyme action. Potassium deficiency: leaves become yellow with spotty, brown, dead areas. Photosynthesis Like all living things, plants need food to live. This food is used for energy and to make new materials when plants grow. Plants are able to take two inorganic chemicals, carbon dioxide gas and water, to make an organic chemical, glucose. This simple food can be used as an energy source or converted into other useful organic molecules. The process requires an input of energy. Plants have found a way to capture the energy from sunlight using a pigment called chlorophyll. Once this light energy has been captured it can be used to create glucose, converting the light energy into chemical energy. Oxygen gas is released as a waste chemical. As light energy is used to create organic materials the
  12. 12. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 12 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics process is named Photosynthesis. The formula for photosynthesis is: Respiration in plants Respiration is the production of energy from glucose and oxygen with the release of carbon dioxide and water as waste products. This is the opposite of photosynthesis which is the production of glucose and oxygen from the energy in sunlight, carbon dioxide and water. Respiration: C H O + 6O → 6CO + 6H O + Energy released Glucose + Oxygen → Carbon dioxide + water + Energy released Photosynthesis: 6CO + 6H O + Energy → C H O + 6O Carbon dioxide + water + Energy → Glucose + Oxygen Plants use the energy from respiration to power the processes involved in growth. The energy obtained from respiration is used to turn glucose into many other substances. Typical uses of glucose are: Storage products 6 12 6 2 2 2 2 2 6 12 6 2
  13. 13. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 13 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Uses of glucose - glucose is used to make starch which can be converted back to glucose as required. Potatoes and rice are examples of parts of plants that contain starch.- glucose is converted into lipids, especially in seeds. Sunflower oil and rapeseed oil, which are used in cooking, come from sunflower and oilseed rape seeds. Structural products - glucose is converted to cellulose to make cell walls. Other products - glucose and nitrates are used to make amino acids which are used to make proteins.- glucose is also a basic raw material for making chlorophyll. The Human Digestive System Yummy in my TUMMY The Mouth Digestion begins inside the mouth, where chewing does 2 things - makes the food easier to swallow and increases the surface area (this helps to speed up digestion). Also inside the mouth, amylase (for digesting starch) can be
  14. 14. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 14 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics found, produced by the salivary glands. The gullet then carries food from the mouth to the stomach by its muscular squeezing action called peristalsis. Stomach The stomach does a number of things, including: pummels and churns the food produces protease enzymes to digest protein produces hydrochloric acid which kills bacteria and gives the ideal pH for protease to work. Liver The liver produces bile which emulsifies fats i.e. breaks them down into small droplets for a larger surface area. This will increase the rate at which the fat is digested by lipase. Bile also neutralizes the acid produced by the stomach to provide ideal alkaline conditions for enzymes in the small intestine. The gall bladder stores the bile made by the liver until it is needed. The small intestine produces amylase, lipase and protease. The pancreas, a pistol shaped organ, produces the enzymes amylase, lipase and protease
  15. 15. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 15 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Small Intestine and releases them into the small intestine when needed. The enzymes are used to fully digest the food molecules, so that they are small enough to diffuse into the bloodstream. Food molecules diffuse in the small intestine, which is ideal for this purpose. The small intestine walls are very thin so that there is a short diffusion pathway. It is long and folded with villi to increase surface area. There is a rich blood supply to maintain a steep concentration gradient for diffusion. Large Intestine & Anus When all the useful products have diffused into the blood, the remaining waste reaches the large intestine where the majority of the remaining water is absorbed into the blood stream. Finally the waste products leave the body in the form of faeces through the anus. Breathing and Respiration The Basics
  16. 16. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 16 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Breathing The zone of the body between the neck and the bottom of the ribs is known as the thorax. The major organs in the thorax are the heart and lungs. The lungs and associated airways allow us to breathe. In the head the airways consist of the mouth and nasal passages. Air and food has a common passage in the throat. Larynx or voicebox. This is where there is speech and sound generation. Trachea or windpipe. Two tubes that are each
  17. 17. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 17 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics known as a bronchus, plural bronchi. Bronchioles which are subdivisions of each bronchus. Alveoli which are sacks at the end of the airways that allow oxygenation of the blood. Pleural cavity" pleural membrane. The key features of breathing are that when we breathe in the intercostal muscles between the ribs and the diaphragm both contract; when we breathe out both of these muscles relax. When we breathe in the contraction of the intercostals pulls the sternum up and away from the body and the descent of the diaphragm increases the volume of the thoracic cavity. Notice that in the resting state (breathing out) the diaphragm bulges up under the lungs, the lungs themselves are slightly elastic and pull the diaphragm back to this position
  18. 18. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 18 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Oxygenation of the blood The two main functions of the lungs are to oxygenate the blood and to remove waste carbon dioxide. The blood is oxygenated in the alveoli. The alveoli are thin walled and surrounded by capillaries. The blood enters the capillary network around the alveoli from the pulmonary artery and leaves the capillary network via the pulmonary vein. Oxygen diffuses into the blood through the alveolar and capillary walls and carbon dioxide diffuses out of the blood. The alveoli have a surface area of about 70 square metres to make this gas exchange as fast as possible. Carbon dioxide dissolves in water and can easily and
  19. 19. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 19 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics reversibly form compounds such as carbonic acid and bicarbonates. Oxygen does not dissolve much in water, to overcome this problem the oxygen in the blood is stored in red blood cells. These contain haemoglobin which can combine with oxygen to form oxyhaemoglobin. The red blood cells contain the oxygen in the blood. The blood transports oxygen from the lungs to the rest of the body. Oxy-haemoglobin is bright red and haemoglobin is dark red, this is why veins look dark and why all the diagrams show veins in blue and arteries in red. The exception is the pulmonary artery which carries dark red, de-oxygenated blood to the lungs and the pulmonary vein which carries bright red oxygenated blood away from the lungs.The special adaptions of the alveoli for gas exchange are: Thin walls Huge surface area Covered in capillaries to provide blood A wet lining to dissolve gases
  20. 20. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 20 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Aerobic respiration Respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. Aerobic respiration uses oxygen to oxidise glucose and produce energy. The equation for the oxidation of glucose is: C H O + 6O → 6CO + 6H O + Energy released Glucose + Oxygen → Carbon dioxide + water + Energy released In a fire there is a massive uncontrolled release of energy as light and heat. Respiration is a similar process but it occurs in gradual steps. Most animals and plants use aerobic respiration as a primary source of energy. glucose + oxygen = carbon dioxide + water = energy When a person is doing very heavy exercise and the blood cannot supply enough oxygen another sort of respiration occurs. This converts glucose into energy without the need for oxygen and is known as anaerobic respiration. The reaction is: Glucose → Energy 6 12 6 2 2 2
  21. 21. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 21 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Anaerobic respiration released + lactic acid Anaerobic respiration releases less energy than aerobic respiration. Unfortunately the insufficient blood supply that leads to anaerobic respiration also means that the lactic acid builds up in the muscles. High lactic acid concentrations are painful and felt as cramp. When exercise stops, the blood supply is able to provide enough oxygen to convert the lactic acid to carbon dioxide and water but this takes time and the muscle pain may continue after exercise until the lactic acid has been converted. The delay in the removal of lactic acid is known as the oxygen debt. Carbon dioxide and lactic acid both cause increases in breathing rate and heart rate to allow the body to repay the oxygen debt. The oxygen debt is the reason why we continue to be out of breath even after exercise. If athletes are very fit their circulation can provide extra oxygen more rapidly and their recovery time, the time required to restore normal breathing and pulse, will be
  22. 22. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 22 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics shorter than in people who are not fit. Yeasts and anaerobic respiration The direct conversion of glucose to energy without the use of oxygen occurs in many yeasts and fungi. The ethanol that is used in alcoholic drinks is a result of anaerobic respiration in yeast, the reaction is: Glucose → Energy released + ethanol + carbon dioxide Brewers use various types of brewers yeast to produce alcohol. In fizzy alcoholic drinks such as champagne the bottles are tightly stoppered to prevent the carbon dioxide from escaping. The Heart a Muscle The blood transports food, proteins, blood cells, gases, water, minerals and waste products around the body. Blood contains: Plasma - a straw coloured liquid in which the blood cells are suspended and the other components of the blood are dissolved.
  23. 23. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 23 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Blood Red blood cells - these are red and doughnut shaped for a large surface area. They carry oxygen from the lungs to the tissues as oxy- haemoglobin (the oxygenated form of haemoglobin). They do not have a cell nucleus. White blood cells - these are colourless cells with a large cell nucleus. They defend the body against disease by engulfing micro- organisms (bacteria and viruses) and producing antitoxins and antibodies. Platelets - these are fragments of cells and very small. They do not have a cell nucleus. They form blood clots at the site of injuries. Clots protect the body from further infection and blood loss.
  24. 24. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 24 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics How the heart works Due the continuous cardiac cycle which takes an approximate second, the heart is the fastest and the strongest muscle in the body. The strongest external muscle is the tongue, however the heart is much stronger. The heart works in three stages: 1. Blood flows into the atria. 2. Both atria contract at the same time, forcing blood into the ventricles. 3. Both ventricles contract at the same time, forcing blood through the pulmonary artery and aorta. The heart valves ensure that the blood goes in the correct direction, they stop backflow. When the atria contract the valves between the atria and the ventricles open passively and the high pressure in the arteries keeps the valves between the ventricles and the arteries closed. When the ventricles contract the valves between the ventricles and the atria shut and the valves between the ventricles and the arteries open.
  25. 25. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 25 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics The net effect of the action of the heart is as follows. Deoxygenated blood from the body enters the right atrium and flows into the right ventricle where it is pumped through the lungs. In the lungs the blood is oxygenated. It then flows into the left atrium and on into the left ventricle where it is pumped out at sufficient pressure to reach every part of the body. The atria have thin walls and act as reservoirs, the ventricles have thick, muscular walls and act as pumps. The valves in the heart prevent backflow. You need to know the following names of arteries and veins: Aorta - the big artery that comes out of the left ventricle. Pulmonary artery - the big artery that comes out of the right ventricle. Vena cava - this refers to the big vein that connects to the right atrium. It has two sections the "superior" vena cava that drains blood from the top part of the body and the "inferior" vena cava that drains blood from the body below the heart. To make the sections clear it is often said that there are two
  26. 26. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 26 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics "vena cavae", the superior vena cava and the inferior vena cava. Either learn the names of the two sections or, when you draw the heart put the arrow to the vena cava as close to the heart as possible. Pulmonary vein - the vein that is directly attached to the left atrium. The menstrual cycle is a cycle of events that occurs in the womb (uterus) and ovaries of female mammals. It is associated with the production of eggs and preparing the uterus for the implantation of fertilised eggs. The menstrual cycle occurs over a period of about 28 days. The changes during the cycle are due to four hormones, progesterone, oestrogen, FSH (follicle stimulating hormone) and LH (luteinising hormone). Progesterone and oestrogen have wide ranging effects on the body but in the context of the menstrual cycle progesterone is mainly involved in maintaining the lining of the uterus and oestrogen is mainly concerned with building up the lining of
  27. 27. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 27 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics The menstrual cycle the uterus. FSH stimulates the production of eggs and LH stimulates the release of the egg. FSH and LH are produced by the pituitary gland in the brain. There are four stages in the menstrual cycle. Stage 1. Days 1-4. Menstruation (bleeding) occurs. The lining of the uterus disintegrates and is shed. This is due to low levels of progesterone. Stage 2. Days 4-14. The uterine lining grows back. This is due to high levels of oestrogen. Stage 3. Day 14. The egg (called an ovum) is released. This is due to LH. Stage 4. Days 14-28. The lining of the uterus is maintained in case the egg becomes fertilised and implanted in the uterus. Maintenance of the lining is due to high levels of progesterone. The four hormones interact with each other. FSH causes Oestrogen release and oestrogen inhibits FSH. LH stimulates both oestrogen and progesterone production.
  28. 28. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 28 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics Before ovulation LH release is stimulated by oestrogen but after ovulation it is inhibited by both oestrogen and progesterone. In summary, the hormones have the following effects: Oestrogen: causes growth of the uterine lining. Inhibits FSH. Stimulates release of LH and hence release of the egg. Inhibits LH after ovulation. Progesterone: maintains the uterine lining. Inhibits LH after ovulation. LH: Stimulates the release of the egg (called ovulation). Stimulates oestrogen and progesterone production. FSH: Stimulates egg development and the release of oestrogen. Birth control tablets contain high levels of progesterone and oestrogen. The oestrogen inhibits FSH production so that eggs cease to develop. FSH is used to treat infertility because it stimulates the production of eggs. The nervous system consists of the brain, spinal cord and relay neurones (Central nervous system) and peripheral nervous system
  29. 29. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 29 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics The Nervous System (sensory neurones and motor neurones). The nervous system lets the organism react to the environment and surroundings and coordinate their behaviour. The senses Sense organs contain receptors that are sensitive to stimuli. Typical stimuli are due to changes in chemical composition, mechanical effects, heat, sound and light. Stimuli are the changes that are detected. Receptors detect the change. There are five sense organs that are studied at this stage, the nose, tongue, eyes, ears and skin. The nose has smell receptors that are sensitive to a wide range of chemical stimuli. Much of what is called 'taste' is actually smell; the range of flavours in food is very limited if the nose is blocked. The tongue has taste receptors that are sensitive to chemical stimuli. There are four basic tastes: bitter, salt, sweet and sour. The eyes have receptors that are sensitive to light, the ears have receptors that are
  30. 30. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 30 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics sensitive to sound and the skin has receptors that are sensitive to temperature, touch, pressure and stretch. Light from the surroundings enters the eye through the pupil. It is focussed by the cornea and lens so that it forms an image on the retina. The cornea performs crude focussing and the lens performs active, fine focussing. The eye is filled with transparent liquid (the humours). The iris controls the size of the pupil and so controls how much light gets into the eye. In bright light the circular muscles in the iris contract, this makes the pupil smaller and allows less light into the eye. In dim light the radial
  31. 31. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 31 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics The eye muscles contract, this makes the pupil larger and allows more light into the eye. The lens changes shape to allow the eye to focus on things at various distances. The shape of the lens is changed by the action of the ciliary muscles. The ciliary muscles form a circle that is attached to the lens by the suspensory ligaments. To focus on distant objects the ciliary muscles relax which pulls the suspensory ligaments tight and makes the lens thinner. To focus on nearby objects the ciliary muscles contract which allows the suspensory ligaments to relax so that the lens forms a more spherical shape. The retina contains cells that are sensitive to light. These cells are called rods and cones. The rods are more sensitive than the cones but do not provide any colour information. In dim light our view of the world is largely provided by rods and appears as a black and white image. There are three types of cone and these are each sensitive
  32. 32. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 32 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics to different colours of light. The cones are especially densely packed in the part of the retina called the fovea; this dense packing means that the fovea is most sensitive to fine detail in the image. The part of the eye where the optic nerve enters is called the blind spot. The blind spot does not contain any light receptors (it has no rods or cones). Neurones Neurones are cells that are specialised to transmit electrical impulses around the body. They consist of three principle components: the dendrites, the cell body and the axon. Each neurone has its own nucleus. (The US spelling of neurone is "neuron"). There are three types of neurone: sensory neurones, relay neurones and motor neurones. Neurones connect to other neurones by means of synapses. Nerve impulses travel down the axon to one side of the synapse where the
  33. 33. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 33 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics electrical signal causes chemicals to be released. These chemicals diffuse across the gap and generate an electrical impulse in the neurone on the other side of the synapse. Reflex arcs The nervous system has many reflexes. These are automatic responses to stimuli. Very quick responses are managed by Reflex arcs. A reflex arc consists of the following components: Stimulus Receptor Sensory neurone Relay neurone Motor neurone Effector Response Pain reflex: a painful stimulus causes a muscle to contract automatically
  34. 34. 02/05/15 10:28 amFlashcards Table on IGCSE Biology All Topics Page 34 of 34http://www.proprofs.com/flashcards/cardshowall.php?title=igcse-biology-all-topics A typical reflex arc called the "pain reflex" is illustrated. In the pain reflex the stimulus is mechanical damage, the receptor is a "pain receptor", the pain receptor generates an electrical impulse in a sensory neurone which creates an electrical impulse in a relay neurone, which in turn creates an electrical impulse in a motor neurone. The impulse in the motor neurone stimulates a muscle, which is an effector, and this creates movement away from the painful stimulus as a response. Another reflex is the "patellar" or "knee jerk reflex" where a blow beneath the knee-cap makes the leg straighten. The contraction of the pupil in response to bright light is yet another reflex.

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