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O-level Biology: Co-odination and Response

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O-level Biology: Co-odination and Response
« on: September 08, 2014, 10:01:03 AM »
Co-ordination and Response  You have previously learned that one of the  7 characteristics of living organism is irritability or sensitivity. And this  is the ability to detect a change in the outer environment and respond to it. A  change in the environment is also called a stimulus (plural stimuli). Actions  taken by the body in order to co-operate with a stimuli are called responses.  The body detects a stimulus by parts in the body called receptors and is able  to respond to it through other parts called effectors. Two organ systems are  continuously working to detect and respond to stimuli, these organ system are  called the nervous system and the endocrine system.
 
The Nervous System:  The nervous system is a system of organs  working together to detect and respond to stimuli. The nervous system is made  up of two systems, the Central Nervous System (C.N.S) and the Peripheral Nervous  System (P.N.S) the peripheral nervous system connects the central nervous  system to the other parts of the body.
 Central Nervous System (CNS):  The  central nervous system is made up of the brain and the spinal cord. The spinal  cord is basically a big bundle of nerve cells running through a tunnel inside  the backbone which protects it while the brain is protected by the skull. The  central nervous system is what gives out orders to other parts of the body to  perform certain jobs.
 The Peripheral Nervous System PNS:  The peripheral nervous system is the other  part of the nervous system. The main job of the PNS is to detect stimuli and  send impulses to the CNS according to the stimuli. The PNS is made of receptors  and nerves that carry the impulses.
Receptor cells are ones whose function is  to detect something about its environment. There are many receptors in the body  that are able to detect many changes like temperature, touch, light, sound and chemicals. There are some organs in the body that are there to detect just one  stimulus, like the eye for example. These are called sensory organs and they  can be defined as a group of receptor cells responding to specific stimuli.
Effectors are the opposite of receptors.  Receptors are two detect the stimuli while effectors are two respond to it.  Effectors are usually muscles and glands.
 Neurons (Nerve Cells):  Neurones are one of the most important  structures of the nervous systems. Neurones act as a wire that transmits  electrical impulses all over the body. Like a cable that consists of many  wires, a bundle of neurones is called a nerve. There are 3 types of neurones, each type is to transmit electrical impulses from a specific place to another.
Motor Neurone: This  is a neurone that transmits electrical impulses from the Central nervous system  to the effectors.
This neurone is made up of three segments;  the cell body which is the start of the motor neurone and is in the CNS, axon  which stretches out from the cell body all the way to end of the neuron, and the motor plate which is the end of the neurone and is in the effector muscle.
 Neurones have features that are common  between most animal cells like a nucleus, cymiddlelasm and cell surface membrane,  but they also have some exclusive features like the axon. The axon is an  extended cymiddlelasm thread along which electrical impulses travel. Some motor  neurones have axons of length 1 metre. Axons are coated by a layer of myelin  called myelin sheath, this is an electrically insulating layer which is  essential for the proper functioning of the nervous system.
Another exclusive feature of neurones is dendrites, these are several short threads of cymiddlelasm coming out of the cell body. Their  function is to pick up electrical impulses from other cells.
The last exclusive feature of motor  neurones only is motor end plate. This is just the end of the axon which is in  the muscle. It passes the electrical impulses from the neurone to the muscle  fibres.
Sensory Neurones: like  other neurones, sensory neurones carry electrical impulses from one place to another. But sensory neurones carry electrical impulses in the direction different to that of motor neurones, from the receptors to the CNS.
The sensory neurone’s shape is unique. This  is because it is made of a cell body, with two arms extending out of it. The  first arm is the axon which’s other end is in the CNS. The second arm is  dendrite which’s other end is in the receptor. The dendrite is similar in structure to the axon except that it joins the receptor with the cell body. The  electrical impulses of the sensory neurone flow from the receptor, through the  dendrite to the cell body, then from the cell body to the CNS through the axon.
Relay Neurone: Relay  neurones are located in the CNS. Their job is to pass electrical impulses from  the sensory neurone onto the motor neurone, so it acts like a diversion.
Where neurones meet, they are not actually  touching each other. Instead there is a gap called synapse or junction box.  When the electrical impulses reach the end of a neurone, the neurone secretes a chemical transmitter which passes by diffusion to the other neurone causing the  impulses to be carried from the first neurone to the second.
 Reflex Arc (Nervous System in Action):  If your finger  touches a hot surface, receptor cells in the skin of your finger detect a  stimulus, which is a sudden rise in the temperature. The receptor uses the  energy of the stimulus to generate electrical impulses. These impulses are then  carried by the axons of the dendrites of the sensory neurone through cell body  to axon and from the axon to the CNS. At the CNS the electrical impulses travel  through the synapse to the relay neurone, which passes it onto the motor neurone. The nerve impulses are transmitted through the axon of the motor  neurone to the targeted muscle which contracts when electrical impulses reach  it, resulting in your finger being pulled away from the hot surface. This  pathway is called the reflex arc and happens in about a fraction of a second.
Reflex Arc: RECEPTOR → Sensory Neurone → CNS → Motor Neurone → EFFECTOR
   Voluntary and Involuntary Actions:  The reflex arc is a reflex action. Reflex  means it is automatically done without your choice. This is because when the electrical impulses reach the relay neurone in the CNS from the receptors, some  impulses are carried by other neurones to the brain, and some impulses are  passes onto the motor neurone to the effector muscle and the response takes  place. The electrical impulses going to your brain are much slower that the  ones going to the effector muscle directly. This is why the reflex action takes  place before you realise it, it is uncontrollable. Reflex actions are said to  be involuntary actions. Involuntary actions start at the sense organ heading to  the effector. They are extremely quick. Voluntary actions are the ones that you  make the choice to do. Like picking up a bag from the floor for example. Your  brain sends electrical impulses to the effector muscles ordering them to  contract so you could pick the bag up. Voluntary actions are slower than  involuntary actions and they start at the brain.
 
 The Human Eye:  The human eye is a sensory organ. This  means it is an organ of tissues working together to detect and respond to a  specific stimulus, which is light.
Features of the Human Eye:
  • Lens: changes shape to focus light on  retina
  • Ciliary muscles: contracts and relaxes to  adjust thickness of the lens
  • Suspensory ligaments: loosens and tightens  to adjust thickness of lens
  • Iris: widens and narrows to control amount  of light entering the eye depending on light intensity
  • Choroid: middle layer surrounding the eye.  It contains many blood vessels
  • Sclera: outer most tough, protective layer  of the eye.
  • Retina: inner most layer. It is sensitive  to light and it is where the fovea is and it has rods and cons
  • Fovea: very light sensitive spot
  • Blind spot: Where the optic nerve touches  the eye. No light sensitive cells in this area.
  How We See:  When the light hits an object, it is  reflected in all directions. When a light ray reflected from the object hits  your eye you see that object. At the back of your eye, there is a spot on the retina called the fovea (blind spot). This spot is full of light sensitive  cells. When the light ray falls on the fovea, the light sensitive cells  generate electrical impulses that travel through the optic nerve to brain. When  the electrical impulses reach the brain, the brain generates the image you see.  This all happens in less than a fraction of a second.
 
But  this is the general idea only. Light rays enter the eye from every direction.  If they are not focused on the fovea, they will most probably not hit it and we  won’t see. Here comes the role of the front part of the eye. When the light ray  hits the eye at an angle, it first has to penetrate the cornea which refracts (bends) the light ray inwards. The cornea acts as a converging (convex) lens.  Then the light penetrates the lens which refracts the ray a little more inwards  focusing the light ray on the fovea. And thus the light ray is focused on the  retina. When the ray hits the retina, the closer to the fovea the sharper the  image is.
Accommodation:  The angle at which the light ray hits the  hits the eye depends on the distance of the object. Every light ray that hits  the eye needs a certain amount of refraction in order to be directed to the  fovea. This is why the lens has the ability to widen and narrow according to the distant of the object you’re looking at in order to make the light ray hit  the retina at the right spot. This is called accommodation. Light rays  refracted from close objects are diverging (spreading out), they need to be  refracted inwards to be focused on the fovea. When you look at a close object,  it takes some time till the vision becomes clear. This is because at first, the  light ray is not correctly refracted, so it hits the retina away from the  fovea. The electrical impulses are generated and sent to the brain which  realises that the image is not clear. The brain then sends electrical impulses  to the ciliary muscles making them contract. When the ciliary muscles contract the suspensory ligaments become loose, this makes the lens become thicker and  rounder for more refraction of the light rays. Now the light rays are correctly  refracted and hit the retina at the fovea, the image becomes clear.
For far visions it  is the exact opposite. The rays reflected from far objects are almost parallel.  Very little refraction should be done. The brain sends electrical impulses to  ciliary muscles making them relax, the suspensory ligaments now tighten up and pull  the lens which become narrow.
DistanceCiliary musclesSuspensory
 ligaments
Lens
Near[/t][/t][/t][/t]
[/t]
Contract
Loosen
Widens
Far
Relaxes
tighten
narrows
 
 Rods and Cones:  The retina is full of light sensitive cells  called photoreceptors. There are two types photoreceptors, they are rods and cones. Rods and cones are specialised types of neurons. They look alike but  they are a little different in function.
Rods are sensitive to dim light. At night  or in dark places, most light detection electrical impulses transmission is  done by rods. Vitamin A is essential for proper functioning of rods, if Vitamin  A lacks it can lead to night blindness. Rods are spread all over the retina.
Cones are sensitive to bright and coloured  light. All cones are packed in one area, the fovea.
 The  Pupil:
The pupil of the eye is the dark round area  in the centre of it. It is surrounded by a coloured ring structure called the  iris. The pupil and the together play a big role in protecting the eye from  damage by limiting the amount of light entering the eye. If too much light fall  on the retina, the rods and cones get damaged. The iris and pupil change their  size to smiddle that happening. The iris contains two sets of muscles; Circular and Radial muscles. Circular muscles run around the iris and radial muscles run from the centre to the outside. When circular muscles contract they make the  pupil smaller. When the radial muscles contract the stretch the pupil outwards  making it wider.
In bright light, too much light starts entering  the eye, which is dangerous for the rods and cones, which detect the high light intensity. The rods and cones start a reflex arc by sending electrical impulses  to the brain via sensory neurone. The brain responds by sending electrical  impulses to the muscles of the iris via motor neurone. These impulses make the  circular muscles contract and the radial muscles relax limiting the amount of  light entering the eye, thus protecting the rods and cones from damage.
If you walk into a  dark room, the rods and cones sense the little amount of light. They start  another reflex arc and send electrical impulses to the brain which responds by  sending electrical impulses the muscles of the iris. The radial muscles  contract and the circular muscles relax widening the pupil to let more light  in.
Antagonistic Muscles:  You have just learned that in order for the  pupil to get narrower or wider, two muscles work simultaneously, when one  contracts the other relaxes. Pairs of muscles like that are called antagonistic muscles.
The most known antagonistic muscle pair is  the biceps and triceps of the arm. The bi and the tri for short, they are what  causes the movement of the arm. They work simultaneously to bend or straighten the arm. The biceps is located in front of the humerus bone of the upper arm.  The biceps is joined to the radius bone of the lower arm and the triceps is  joined to the ulna bone of the lower arm. Muscles are attached to bones by  strong fibres called tendons.
 
When you want to bend your arm the brain send two electrical impulses, one to the bi making it contract and one to the  tri telling it to relax. When the bi contracts, it becomes shorter pulling the  bones to which it is attached close and bending the arm. This causes the fibres  of the tri to stretch while they are relaxed.
To straighten your  arm, the brain send electrical impulses to both muscles making the bi relax in  order to leave the muscle it is attached to free. The tri contracts and becomes  shorter pulling the muscle it is attached to into place and straightening the  arm.
The biceps can be called a flexor because  it flexes (bends) the arm. The triceps can be called an extensor because it  extends (straightens) the arm.
 Drugs:  A drug is a chemical substance that  modifies and affects chemical reactions of the body when taken in. Many drugs  are useful to us like antibiotics, painkillers and caffeine.
Some drugs however are abused by users to  feel relaxed, or reach euphoria. Euphoria is a state of mind at which the  abuser feels extremely happy and relaxed. These drugs include alcohol and  heroin.
 Alcohol:  Alcohol  is a depressant drug. This means that it reduces the activity of the brain and  slows down the nervous system and reflex actions. Alcohol can be extremely dangerous  when the user is in a situation in which they need fast reflex actions.  Alcohol is addictive. The more you drink it  the more you need it. The user may reach a point where they cannot do without  alcohol, this is when they become alcoholics. Alcohol is broken down into fats  by the liver. If the abuser drinks too much alcohol, the cells of the kidney  may die shortening their life.
  Heroine:  Heroine is a narcotic drug. This means that  it relieves pain and induces sleep. Heroine is extracted from a plant called  opium poppy. Most heroine abusers become addicts. For the addicts heroine become the number one priority in their lives. They would do anything to get  the drug even become criminals and possess a threat to their society. If not  rehabilitated, a heroine abuser will end up homeless or dead. Some heroine  users inject the drug in their veins by an unsterilized, shared needle, this  increases the risk of getting AID/HIV.
 The Endocrine System:
You have previously learned that messages  are delivered around body as electrical impulses by the nervous system. Another  way messages are transported around the body is by chemicals called hormones secreted  by the endocrine system.
Hormones are chemical substances produced  by a gland, carried by the blood, which alters the activity of one or more  specific target organs and is then destroyed by the liver.
Hormones are  produced in organs called endocrine glands which make up the endocrine system.  The following diagram shows the glands of a human body.
Glands are organs made of secretory cells  which’s function is to produce hormones and secret them into the bloodstream.  Glands have a dense network of blood capillaries in them to secret the hormones in. hormones are carried around the plasma like all other content of the blood  but certain organs are able to use them, these are target organs.
 
 
GlandHormone producedFunction of hormone
Adrenal glandAdrenalinePrepares the body for activities that need energy and quick reflex actions
Pancreas InsulinMakes liver reduce blood glucose level
GlucagonMakes liver increase blood glucose level
TestisTestosteroneProduces male secondary sexual characteristics
Ovary Oestrogen Produces female secondary sexual characteristics
Progesterone Helps control menstrual cycle and maintain pregnancy
   Adrenaline:  When you get a fright you feel some changes  in your body like a sudden increase in heart beat rate, blood flowing quickly  in veins and your breath becomes deeper and faster. This is because the fright  you got caused the brain to send electrical impulses to the adrenal glands making them secrete adrenaline hormone in your bloodstream. Adrenaline is a  hormone that is secreted from the adrenal glands to prepare the body for  situations that need lots of energy and fast reflex action, like fights or  running away for example. Adrenaline’s main objective is to increase your  metabolic rate so that you have enough energy for fighting or running away etc.  This is why adrenaline is called the three Fs hormone (Fight, fright, flight).  One of adrenaline’s target organs is the heart. When adrenaline reaches the  heart it causes the cardiac muscle to contract and relax much rapider so that  oxygen and glucose reach the muscles of the body faster. Adrenaline also makes  the liver convert glycogen into glucose and secret it in the blood to be used  in respiration. When adrenaline reaches the diaphragm and the intercostals  muscles of the ribs, they make it contract and relax faster too to increase  rate of breathing. These changes cause an increase in the respiration rate so  that lots of energy is being released. Generally, adrenaline is secreted when  you are nervous or anxious.
 Use of Hormones in Food Industries:  Technologies and science have advanced  enough that we can now gut much more money out of farming and animal keeping.  Hormones are now being used in farms to increase milk yields in cows and growth  rate in cattle and fish.
In farms, the cows are being injected with  a hormone called Bovine Somatotropin or BST. BST is a hormone that is naturally  produced in cows. The function of BST is to produce milk. Injecting cows with extra BST will boost milk production and bring in more money for the farmers.  Some people however are against the use of BST and claim it is safer for both  the cows and the consumer to keep it natural and keep more cows if we want an  increased milk yield.
Growth hormones are also being mixed with  the food fed to cattle to increase their growth rate and make them grow larger.  But again many people are against this and prefer buying meat and fish that  were naturally grown.
 
 Comparing Nervous and Endocrine Systems:
Nervous SystemEndocrine System
Information sent in form of electrical impulsesInformation sent in form of chemical hormones
Information travel neuronesInformation travel in bloodstream
Information travels extremely rapidly Information travels relatively slow
Information is headed to one target (effector)Information may be used by several targeted organs
Electrical signals have an effect that ends quicklyHormones have a longer lasting effect
 
 Coordinates and Responses in Plants:  Plants cannot move themselves to areas of  preferable conditions. This is why plants have the ability to detect a stimulus and respond to it by growing or bending in its direction or away from it. These  responses are called tropisms. For example a plant tends to grow its stem in  the direction of sunlight for more photosynthesis, this is a tropism. There are  two types of tropism, these are phototropism and geotropism.
 
  • Phototropism: the response in which a plant  grows towards or away from the direction from which light is coming.
  • Geotropism: the response in which a plant  grows towards or away from gravity.
A tropism can be either positive or  negative. If a tropism is in the direction of the stimulus, it is positive. If  the tropism is away from the stimulus it is negative.
For example, a plant’s shoot tends to grow  in the direction of sunlight, this is positive phototropism. But the plant’s  root grows in the opposite direction deeply into the soil, this is negative phototropism. However, positive phototropism can also be described as negative  geotropism because it involves the plant growing in the direction opposite to  gravity. And negative photo tropism can be described as positive geotropism  because it involves the plant growing towards gravity.
 Auxins:  Tropisms are controlled by a chemical  called Auxin. Auxin is a plant hormone. It is produced by cells at the tip of  roots and shoots of plants. At the tip of a shoot, there is an area in which cells are being produced by dividing so that the shoot grows. Old cells do not  divide, but they grow longer instead. The growth of these cells longer is  controlled by auxins. Auxins is what makes the plant grows this is why a plant  doesn’t grow if you cut it’s tip off.
 Auxins’ Role in Phototropism:  If the sun shines  on the right side of a plant’s shoot, auxins will accumulate on the dark  opposite left side. Auxins accumulating there makes the cells on the left side  grow much faster than the cells on the right side. When the left side of the  shoot starts growing faster than the right side, the shoot will start to bend  to the right side towards sunlight. This is phototropism.
Auxins’ Role in Geotropism:  Auxins tend to settle at the bottom end of  the root. However, this does not make the sells of the tip of the root grow  longer. Instead, auxins prevent the cells at the bottom tip of the root from growing, making the cells at the middle of the root grow faster. When the cells of  the middle of the root grow faster, they push the root deeper into the soil and  the root gets longer. The root grows in the direction of the gravitational  pull. This is geotropism.
Roots show positive geotropism and negative  phototropism because they grow towards gravity and away from sunlight at the  same time. Shoots show positive phototropism and negative geotropism because  they grow towards the sunlight and away from gravity at the same time.
 Advantages of Positive Phototropism:
  • Leaves exposed to more sunlight and are  able to do more photosynthesis,
  • Flowers can be seen by insects for pollination,
  • The plant gets higher for better seed  dispersal.
Advantages of Positive Geotropism:
  • By growing deeply into the soil, the root  fixes the plant into the ground firmly,
  • Roots are able to reach more water,
  • Roots have a larger surface area for more diffusion  and osmosis.
Selective Weed Killers:  Auxins can be used to kill weeds that grow  over grass or cereal crops. If weed grows on crops, auxins are sprayed  everywhere. Weeds absorb auxins faster than crops or grass. Auxins accumulate  in the weeds making them grow very rapidly. Fast growth of weed kills it leaving the crops or grass alive. Auxins are used ass selective weed killers.
 
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