6-1 Internal and external stimuli are detected and lead to a response

Question 1

What is a stimulus?

Answer 1

• This is something that can be detected by an organism.
• It can be either internal in multicellular organisms or external in any organism.

Question 2

What is a receptor?

Answer 2

• A receptor is an organ or specialised cell that can detect the change that is causing the stimulus.

Question 3

What is a response?

Answer 3

• As a result of the stimulus that is detected by the receptor a response is caused.
• This may be movement of the organism or a change in behaviour.

Question 4

What is a taxis?

Answer 4

• A taxis is a response that involves movement in a specific direction.
• Therefore, positive taxis is towards the stimulus and negative taxis is away from the stimulus.
• An example of a positive chemotaxis is mobile bacteria moving to an area where there is a higher concentration of glucose.

Question 5

What is a kinesis?

Answer 5

• A kinesis is a response that involves movement, but this time in random directions.
• Both the speed and frequency of direction change increase.
• The response is carried out in order to increase the change that the organism will enter different conditions more rapidly.
• An example is if you place a woodlouse in a dry area it will speed up and change direction more frequently in order to increase the change it enters a damp region which are its favoured conditions.

Question 6

What is a tropism?

Answer 6

• Growth responses controlled by a direction stimulus.

Question 7

What is a phototropism?

Answer 7

• Direction of growth is determined by the direction of light.

Question 8

What is gravitropism?

Answer 8

• Direction of growth is with gravity.

Question 9

What controls plant growth?

Answer 9

• Indoleacetic acid (IAA) which is an important auxin produced in the tips and shoots of flowering plants.
• The distribution of IAA around the plant controls tropisms.
• For instance, if IAA is unevenly distributed, it causes uneven growth of the plant to occur.

Question 10

How can the distribution of IAA cause bending of the shoot?

Answer 10

• When the shoot is illuminated from all sides, the auxins are distributed evenly and move down the shoot tip thus causing elongation of cells across the zone of elongation.
• Whereas if the shoot is only illuminated from one side, the auxins move towards the shaded part of the shoot thus causing elongation of the shaded side only which results in the bending of the shoot towards the light.

Question 11

How does IAA in the roots differ to in the shoots?

Answer 11

• IAA will build up on the lower side of the root.
• In roots IAA inhibits growth, therefore causing the cells on the upper side to grow faster, causing the root to bend downwards.

Question 12

What are reflexes?

Answer 12

• Reflexes are rapid automatic responses that can protect an organism from harmful stimuli and therefore can help them to survive and avoid danger.
• These are fast responses which bypass the brain meaning that no decision has to be made.

Question 13

What is the general path of a reflex arc?

Answer 13

• Stimulus – > Receptor – > Sensory Neurone – > Intermediate Neurone – > Motor Neurone – > Effector – > Response

Question 14

What is a sensory neurone?

Answer 14

• Carries the nerve impulse from the receptor to the spinal cord.

Question 15

What is an intermediate neurone?

Answer 15

• This is located entirely in the spinal cord and relays the nerve impulse from the sensory neurone to the motor neurone.

Question 16

What is a motor neurone?

Answer 16

• Carries the nerve impulse from the spinal cord to the effector which can be a muscle or gland.

Question 17

Where are the Pacinian Corpuscles found?

Answer 17

• Located deep in the skin.
• Mostly found on fingers, soles of the feet as well as external genitalia.
• They are also found in joints, tendons and ligaments.

Question 18

What are the Pacinian Corpuscles?

Answer 18

• Pacinian Corpuscles have a single sensory neurone, located in the centre of connective tissue called lamellae which forms layers separated by a gel.
• Contains stretch mediated sodium channels in the cell surface membrane.
• When not under pressure these channels are closed, however under pressure these become deformed.
• As a result, they open and allow the rapid influx of sodium ions to occur.
• The positive charge on the sodium ions changes the membrane potential, causing the membrane to become depolarised.
• This results in a generator potential being created which goes on to create an action potential in the axon.

Question 19

What are photoreceptors?

Answer 19

• Photoreceptors are light receptors in the eye.
• The light enters the eye through the pupil and the amount of light entering is controlled by muscles in a structure called the iris.
• The lens of the eye focuses the light on the retina where the photoreceptors are located, specifically the fovea.
• Subsequently, the nerve impulses received by the photoreceptor’s cells are then carried via the optic nerve to the brain.
• The point where the optic nerve leaves the eye is known as the blind spot as there are no photoreceptor cells located here.

Question 20

What are the two types of photoreceptors?

Answer 20

• Cone cells
  o Colour vision
  o Present at greatest density in the fovea of the eye and contain the pigment iodopsin.
  o Not sensitive to light and therefore require qute bright light in order to work.
  o Three different types for red, green or blue.
  o Provide good visual acuity because each cone cell has its own synapse via a bipolar neurone which connects to the optic sensory neurone.
• Rod cells
  o Monochromatic vision
  o Highest density outside of the fovea
  o Contain a pigment rhodopsin.
  o Very sensitive to light and therefore stimulated in low light conditions.
  o Low visual acuity as more than one rod cell shares the same synapse with a bipolar cell.
  o As a result multiple rods need to be stimulated to cause the creation of a generator potential.

Question 21

Why is the heart referred to as myogenic?

Answer 21

• The heart initiates its own contraction.

Question 22

What is the sinoatrial node and where is it located?

Answer 22

• Region of specialised fibres which is the pacemaker of the heart.
• In the right atrium.

Question 23

How does the sinoatrial node work?

Answer 23

• Initiates a wave of electrical stimulation which causes the atria to contract at roughly the same time.
• The ventricles do not start contracting until the atria have finished due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation.
• The electrical wave eventually reaches the atrioventricular node located between the two atria which passes on the wave of excitation to ventricles, down the bundle of His to the apex of the heart.
• The bundle of His branches into Purkyne fibres which carry the wave upwards.
• This causes the ventricles to contract, thus emptying them.

Question 24

What two nerves is the sinoatrial node connected to from the medulla oblongata in the brain?

Answer 24

• Accelerator nerve
  o Part of the sympathetic nervous system
  o Delivers a higher frequency of impulses to the SAN to increase the heart rate.
• Vagus nerve
  o Part of the parasympathetic nervous system
  o Delivers a slower frequency of impulses to the SAN to slow down the heart rate.

Question 25

What are the factors which increase the heart rate?

Answer 25

• Changes in pH
  o Caused by high CO2 concentration.
  o Detected by chemoreceptors located in carotid arteries, aorta and the bran.
  o The receptors send impulses to the medulla oblongata more frequently via the sympathetic pathway.
  o As a result, more frequent impulses are sent to the SAN, which results in an increase in heart rate.
  o This consequently speeds up blood flow to the lungs where the CO2 can be expelled.
• Changes in blood pressure
  o Monitored by baroreceptors in the sinus.
  o If blood pressure increases, then an increased frequency of impulses are sent from the medulla oblongata via the parasympathetic pathway to the SAN.
  o This causes the heart rate to decrease, lowering the blood pressure.