Topic 6

• What is tropism?


  A tropism is the response of a plant to a directional stimulus

  Plants respond to stimuli by regulating their growth.

  A positive tropism is growth towards a stimulus

  A negative tropism is growth away from a stimulus
• What is phototropism?


  Phototropism is the growth of a plant in response to light. Shoots are positively phototropic and grow towards light. Roots are negatively phototropic and grow away from light.
• what is gavitropism


  Gravitropism is the growth of a plant in response to gravity. Shoots are negatively gravitropic and grow upwards. Roots are positively gravitropic and grow downwards.
• why must plants respond to stimuli?


  Flowering plants, like animals, increase their chances of survival by responding to changes in their environment,

  e.g:

  • They sense the direction of light and grow towards it to maximise light absorption for photosynthesis.

  • They can sense gravity, so their roots and shoots grow in the right direction.

  • Climbing plants have a sense of touch, so they can find things to climb up and reach the sunlight.
• What are growth factors? and why is it important in plants?

  1) Plants respond to directional stimuli using specific growth factors — these are hormone-like chemicals that speed up or slow down plant growth.

  2) Growth factors are produced in the growing regions of the plant (e.g. shoot tips, leaves) and they move to where they're needed in the other parts of the plant.

  3) Growth factors called auxins stimulate the growth of shoots by cell elongation — this is where cell walls become loose and stretchy, so the cells get longer.

  4) High concentrations of auxins inhibit growth in roots though.
• What is indoleacetic acid? and why is it important?

  1) Indoleacetic acid (IAA) is an important auxin that's produced in the tips of shoots in flowering plants.

  2) IAA is moved around the plant to control tropisms — it moves by diffusion and active transport over short distances, and via the phloem over long distances.

  3) This results in different parts of the plant having different concentrations of IAA. The uneven distribution of IAA means there's uneven growth of the plant,
• what is the effects of IAA on phototropism in shoots and roots?

  IAA moves to the more shaded parts of the shoots and roots, so there's uneven growth.

  
  

  SHOOTS:

  IAA concentration increases on the shaded side — cells elongate and the shoot bends towards the light

  
  

  ROOTS:

  IAA concentration increases on the shaded side — growth is inhibited so the root bends away from the light
• what is the effect of IAA on gavitropism on shoots and roots?

  IAA moves to the underside of shoots and roots, so there's uneven growth.

  
  

  SHOOTS:

  IAA concentration increases on the lower side- cells elongate so the shoot grows upwards

  
  

  ROOTS:

  IAA concentration increases on the lower side- growth is inhibited so the root grows downwards
• What is taxes?


  the organisms move towards or away from a directional stimulus

  when an organism moves towards a stimulus its known as a positive taxis

  when an organism moves away from a stimulus its known as a negative taxis

  Bacteria can show positive chemotaxis- moves toward certain chemicals to aid survival

  FOR EXAMPLE:

  woodlice show a tactic response to light (phototaxis) — they move away from a light source. This helps them survive as it keeps them concealed under stones during the day (where they're safe from predators) and keeps them in damp conditions (which reduces water loss).
• what is kinesis?


  the organisms' movement is affected by a non-directional stimulus

  For example, woodlice show a kinetic response to humidity. In high humidity they move slowly and turn less often, so that they stay where they are. As the air gets drier, they move faster and turn more often, so that they move into a new area. This response increases the chance that a woodlouse will move to an area with higher humidity. This improves the survival chances of the organism — it reduces their water loss and it helps to keep them concealed.
• how does Responding to their Environment Helps Organisms Survive

  1) Animals increase their chances of survival by responding to changes in their external environment.

  2) They also respond to changes in their internal environment to make sure that the conditions are always optimal for their metabolism

  3) Plants also increase their chances of survival by responding to changes in their environment

  4) Any change in the internal or external environment is called a stimulus.
• what are the role of receptors?

  1) Receptors detect stimuli

  2) Effectors are cells that bring about a response to a stimulus, to produce an effect. Effectors include muscle cells and cells found in glands, e.g. the pancreas

  3) Receptors communicate with effectors via the nervous system or the hormonal system, or sometimes using both.
• what are the three neurones?

  Sensory neurones transmit electrical impulses from receptors to the central nervous system (CNS) — the brain and spinal cord.

  •Motor neurones transmit electrical impulses from the CNS to effectors.

  •Relay neurones transmit electrical impulses between sensory neurones and motor neurones

  A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone.

  When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters take the information across to the next neurone, which then sends an electrical impulse

  The CNS processes the information and sends impulses along motor neurones to an effector.
• What is the order of the reflex arc?

  stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response
• what is the CNS?

  brain and spinal cord
• what is the The peripheral nervous system?

  made up of the neurones that connect the CNS to the rest of the body.
• what is the The somatic nervous system?

  controls controls conscious activities
• What is the autonomic nervous system?


  controls unconscious activities
• what is the sympathetic and parasympathetic nervous system?

  The sympathetic nervous system gets the body ready for action. It's the 'flight or fight' system. (increase in heart rate)

  The parasympathetic nervous system calms the body down. It's the 'rest and digest' system. (decrease in heart rate)
• how is the Nervous System Communication is Localised, Short-lived and Rapid?

  1) When an electrical impulse reaches the end of a neurone, neurotransmitters are secreted directly onto target cells

  so the nervous response is localised.

  2) Neurotransmitters are quickly removed once they've done their job, so the response is short-lived.

  3) Electrical impulses are really fast so the response is rapid this allows animals to react quickly to stimuli.
• Receptors are Specific to One Kind of Stimulus

  1) Receptors are specific.

  2) There are many different types of receptor that each detect a different type of stimulus.

  3) Some receptors are cells,

  4) W hen a nervous system receptor is in its resting state there's a difference in charge between the inside and the outside of the cell — this is generated by ion pumps and ion channels. This means that there's a voltage across the membrane.

  • The potential difference when a cell is at rest is called its resting potential. When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell — altering the potential difference. The change in potential difference due to a stimulus is called the generator potential.

  • A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference — so a bigger generator potential is produced. • If the generator potential is big enough it'll trigger an action potential — an electrical impulse along a neurone. An action potential is only triggered if the generator potential reaches a certain level called the threshold level. Action potentials are all one size , so the strength of the stimulus is measured by the frequency of action potentials

  • If the stimulus is too weak the generator potential won't reach the threshold, so there's no action potential.
• What is a pacinian corpuscle?

  1) Pacinian corpuscles are mechanoreceptors — they detect mechanical stimuli, e.g. pressure and vibrations They're found in your skin.

  2) Pacinian corpuscles contain the end of a sensory neurone, imaginatively called a sensory nerve ending. The sensory nerve ending is wrapped in loads of layers of connective tissue called lamellae.

  3) When a Pacinian corpuscle is stimulated, e.g. by a tap on the arm, the lamellae are deformed and press on the sensory nerve ending.

  4)This causes the sensory neurone's cell membrane to stretch, deforming the stretch-mediated sodium ion channels. The channels open and sodium ions diffuse into the cell, creating a generator potential.

  5) If the generator potential reaches the threshold, it triggers an action potential.
• What are photoreceptors? and how does light enter?

  1) Light enters the eye through the pupil. The amount of light that enters is controlled by the muscles of the iris.

  2) Light rays are focused by the lens onto the retina, which lines the inside of the eye. The retina contains photoreceptor cells — these detect light.

  3) The fovea is an area of the retina where there are lots of photoreceptors.

  4) Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones. Where the optic nerve leaves the eye is called the blind spot — there aren't any photoreceptor cells, so it's not sensitive to light.
• How does Photoreceptors Convert Light into an Electrical Impulse?

  1) Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive optical pigments.

  2) Light bleaches the pigments, causing a chemical change and altering the membrane permeability to sodium ions.

  3) A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.

  4) Bipolar neurones connect photoreceptors to the optic nerve, which takes impulses to the brain.

  5) The human eye has two types of photoreceptor — rods and cones.

  6) Rods are mainly found in the peripheral parts of the retina, and cones are found packed together in the fovea.

  7) Rods and cones contain different optical pigments making them sensitive to different wavelengths of light.

  8) Rods only give information in black and white, but cones give information in colour. There are three types of cones, each containing a different optical pigment — red-sensitive, green-sensitive and blue-sensitive. When they're stimulated in different proportions you see different colours.
• describe cone and rods sensitivity?


  • Rods are very sensitive to light (they fire action potentials in dim light). This is because many rods join one neurone, so many weak generator potentials combine to reach the threshold and trigger an action potential.

  • Cones are less sensitive than rods (they only fire action potentials in bright light). This is because one cone joins one neurone, so it takes more light to reach the threshold and trigger an action potential.
• What is visual acuity?

  the ability to tell apart points that are close together

  1)Rods give low visual acuity because many rods join the same neurone, which means light from two points close together can't be told apart. Cones give high visual acuity because cones are close together and one cone joins one neurone. When light from two points hits two cones, two action potentials (one from each cone) go to the brain — so you can distinguish two points that are close together as two separate points.
• describe the distribution of rods and cones?

  uneven

  light is focused on the fovea which receives the highest light intensity

  most cones are located near the fovea as they only respond to high light intensities and rod cells are further away as these can respond at lower light intensities
• What is the sinoatrial node? and where is it located?

  located in the right atrium

  and is known as the pacemaker
• What is the atrioventricular node? and where is it located?

  located near the border of the right and left ventricles within the atria
• Describe how the cardiac cycle is controlled by the SAN and the AVN?


  1) The process starts in the sinoatrial node, which is in the wall of the right atrium.

  2) The SAN is like a pacemaker — it sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls.

  3) This causes the right and left atria to contract at the same time.

  4) A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles.

  5) Instead, these waves of electrical activity are transferred from the SAN to the atrioventricular node.

  6) The AVN is responsible for passing the waves of electrical activity on to the bundle of His. But, there's a slight delay before the AVN reacts, to make sure the atria have emptied before the ventricles contract.

  7) The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity between the ventricles to the apex (bottom) of the heart. The bundle splits into finer muscle fibres in the right and left ventricle walls, called the Purkyne tissue.

  8) The Purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously, from the bottom up.
• The cardiac cycle is controlled by the sinoatrial node (SAN) and the atrioventricular node (AVN). Describe how.

  (exam model answer)

  1. SAN initiates heartbeat / acts as a pacemaker / myogenic;

  2. (SAN) sends wave of electrical activity / impulses (across atria) causing atrial contraction;

  3. AVN delays (electrical activity / impulses);

  4. (Allowing) atria to empty before ventricles contract / ventricles to fill before they contract;

  5. (AVN) sends wave of electrical activity / impulses down Bundle of His / Purkyne fibres;

  6. (Causing) ventricles to contract (from base up) / ventricular systole;
• how is the brain and autonomic nervous system involved in the heart rate?

  1) The sinoatrial node generates electrical impulses that cause the cardiac muscles to contract.

  2) The rate at which the SAN fires is unconsciously controlled by a part of the brain called the medulla oblongata.

  3) Animals need to alter their heart rate to respond to internal stimuli

  4) Stimuli are detected by pressure receptors and chemical receptors:

  • There are pressure receptors called baroreceptors in the aorta and the carotid arteries. They're stimulated by high and low blood pressure.

  • There are chemical receptors called chemoreceptors in the aorta, carotid arteries and in the medulla. They monitor the oxygen level in the blood and also carbon dioxide and pH Electrical impulses from receptors are sent to the medulla along sensory neurones. The medulla processes the information and sends impulses to the SAN along sympathetic or parasympathetic neurones (which are part of the autonomic nervous system).
• Stimuli Detected by Pressure Receptors Cause Heart Rate to Slow Down. How?

  1) Stimulus: High blood pressure.

  2) receptor: baroreceptors detect this high blood pressure and are stretched

  3) coordination: more electrical impulses sent to medulla oblongata and then impulses sent via parasympathetic nervous system to SAN to decrease frequency of electrical impulses

  4) effector: cardiac muscle - SAN tissues

  5) response: reduced heart rate
• Stimuli Detected by Pressure Receptors Cause Heart Rate to speed up. How?

  1) Stimulus: decrease blood pressure.

  2) receptor: baroreceptors detect this low blood pressure and are not stretched

  3) coordination: more electrical impulses sent to medulla oblongata and then impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses

  4) effector: cardiac muscle - SAN tissues

  5) response: increased heart rate
• stimuli detected by chemoreceptors can cause the heart rate to speed up. How?

  1) Stimulus: decrease PH

  2) receptor: chemoreceptors detect this low PH

  3) coordination: more electrical impulses sent to medulla oblongata and then impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses

  4) effector: cardiac muscle - SAN tissues

  5) response: increased heart rate to deliver blood to lungs rapidly to remove carbon dioxide
• stimuli detected by chemoreceptors can cause the heart rate to slow down How?


  1) Stimulus: increase PH

  2) receptor: chemoreceptors detect this high PH

  3) coordination: more electrical impulses sent to medulla oblongata and then impulses sent via parasympathetic nervous system to SAN to decrease frequency of electrical impulses

  4) effector: cardiac muscle - SAN tissues

  5) response: decreased heart rate
• How is a resting potential maintained?

  1) In a neurone's resting state , the outside of the membrane is positively charged compared to the inside. This is because there are more positive ions outside the cell than inside.

  2) So the membrane is polarised — there's a difference in charge across it.

  3) The voltage across the membrane when it's at rest is called the resting potential .

  4) The resting potential is created and maintained by sodium potassium pumps and potassium ion channels in a neurone's membrane:

  • The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn't permeable to sodium ions, so they can't diffuse back in. This creates a sodium ion electrochemical gradient because there are more positive sodium ions outside the cell than inside.

  • The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels. • This makes the outside of the cell positively charged compared to the inside.
• what happens when a stimulus is detected?

  this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
• what happens at depolarisation?

  1) If potential difference reaches the threshold.

  2) More sodium ion channels open.

  3) So more sodium ions diffuse into the neurone.
• what happens at repolarisation?

  1) Sodium ion channels close.

  2) Potassium ion channels open.

  3) So potassium ions diffuse out of the neurone, down their concentration gradient.

  4) This starts to get the membrane back to its resting potential.