COORDINATION

Question 1

SYNAPSE

Answer 1

Question 2

sense organ

Answer 2

Question 3

hormone homeostatsis gravitropism gravitropism

Answer 3

Question 4

nervous control in humans

Answer 4

Question 5

neurone

Answer 5

Question 6

nervous structure function

Answer 6

Question 7

reflex action

Answer 7

Question 8

reflex arc

Answer 8

Question 9

volantary involantary

Answer 9

Question 10

synapse long expntn

Answer 10

At a synapse, a branch at the end of one fibre is in close contact with the cell body or dendrite of another neurone (Figure 14.5).

When an impulse arrives at the synapse, vesicles in the cytoplasm release a tiny amount of the neurotransmitter substance. It rapidly diffuses across the synaptic gap (also known as the synaptic cleft) and binds with neurotransmitter receptor molecules in the membrane of the neurone on the other side of the synapse. This then sets off an impulse in the neurone.

Synapses control the direction of impulses because neurotransmitter substances are synthesised on only one side of the synapse, while receptor molecules are present only on the other side. They slow down the speed of nerve impulses slightly because of the time taken for the chemical to diffuse across the synaptic gap. Many drugs produce their effects by interacting with receptor molecules at synapses.

Heroin, for example, stimulates receptor molecules in synapses in the brain, triggering the release of dopamine (a neurotransmitter), which gives a short-lived ‘high’.

Question 11

sense organs

Answer 11

Question 12

normal eye function

Answer 12

The eyebrow stops sweat running down into the eye. Eyelashes help to stop dust blowing on to the eye. Eyelids can close automatically (blinking is a reflex) to prevent dust and other particles getting on to the surface of the cornea. Blinking also helps to keep the surface moist by moving liquid secretions (tears) over the exposed surface. Tears also contain enzymes that have an antibacterial function.

Question 13

structure of eye

Answer 13

Question 14

part eye function

Answer 14

Question 15

puplic reflex

Answer 15

Question 16

reflex angiostic muscles

Answer 16

Question 17

accomodation

Answer 17

Question 18

chemical control of metabolic activity by adrenaline

Answer 18

Question 19

rodes and cones

Answer 19

Question 20

duod structure

Answer 20

Question 21

hormones

Answer 21

Question 22

comparing nervous and hormonal system

Answer 22

Question 23

homeostasis expl

Answer 23

Question 24

skin structure

Answer 24

Question 25

temp regulation

Answer 25

Question 26

sweating

Answer 26

Sweat is a liquid made up of water, salts and some urea. Sweat glands in the skin secrete sweat through pores on to the skin surface. As the water in the sweat evaporates, it removes heat from the skin, cooling it down. When we are too hot, the volume of sweat produced increases. If we get too cold, the amount of sweat produced is reduced, so less heat is lost through evaporation.

Question 27

insulation

Answer 27

The skin has a layer of fatty tissue that has insulating properties – it reduces heat loss from the skin surface.

Question 28

Shivering

Answer 28

Uncontrollable bursts of rapid muscular contraction in the limbs release heat as a result of respiration in the muscles. However, its effectiveness in temperature control is questionable.

Question 29

Role of the brain

Answer 29

The brain plays a direct role in detecting any changes from normal by monitoring the temperature of the blood. A region called the hypothalamus contains a thermoregulatory centre in which temperature receptors detect temperature changes in the blood and co-ordinate a response to them. Temperature receptors are also present in the skin. They send information to the brain about temperature changes.

Question 30

Role of negative feedback in homeostasis

Answer 30

Homeostasis is the control of internal conditions within set limits. A change from normal, for instance an increase in temperature, triggers a sensor, which stimulates a response in an effector. However, the response – in this case an increase in sweating and vasodilation of arterioles – would eventually result in temperature levels dropping below normal. As temperature levels drop, the sensor detects the drop and instructs an effector (the skin) to reduce sweating and reduce vasodilation of arterioles. This is negative feedback – the change is fed back to the effector.

Question 31

Control of glucose content in blood

Answer 31

The liver is a homeostatic organ – it controls the levels of a number of materials in the blood, including glucose. Two hormones – insulin and glucagon – control blood glucose levels. Both hormones are secreted by the pancreas and are transported to the liver in the bloodstream. Excess glucose is stored in the liver and muscles as the polysaccharide glycogen (animal starch). When glucose levels drop below normal, glycogen is broken down to glucose, which is released into the bloodstream.

Question 32

Type 1 diabetes

Answer 32

There are two types of diabetes. Type 1 is the less common form, the cause of which is outlined in Chapter 10. It results from a failure of cells of the pancreas to produce sufficient insulin. The outcome is that the patient’s blood is deficient in insulin. This form of the disease is, therefore, sometimes called ‘insulin-dependent’ diabetes. The patient is unable to regulate the level of glucose in the blood. Symptoms include feeling tired, feeling very thirsty, frequent urination and weight loss. Weight loss is experienced because the body starts to break down muscle and fat. Blood glucose may rise to such a high level that it is excreted in the urine, or may fall so low that the brain cells cannot work properly and the person goes into a coma. Treatment: diabetics need a carefully regulated diet to keep blood sugar within reasonable limits. They also need to take regular exercise and have regular blood tests to monitor their blood sugar levels. The patient needs regular injections of insulin to control blood sugar level and thus lead a normal life.

Question 33

vasodilation constriction

Answer 33

Heat is transported around the body in the bloodstream (see Chapter 9). When blood passes through blood vessels near the skin surface, heat is lost by radiation. Arterioles (small arteries) have muscle in their walls. When we are too hot, these muscles relax, creating a wide lumen through which lots of blood can pass to the skin surface capillaries (the skin of a hot person may look red). This is called vasodilation. More heat is radiated, so we cool down. When we are too cold, the muscles contract, creating a narrow lumen through which little blood can pass (the skin of a cold person may look very pale). This is called vasoconstriction. Less heat is radiated to conserve heat.

Question 34

Control of plant growth by auxins

Answer 34

Auxins are plant growth substances. They are sometimes referred to as hormones, but this is not very accurate because they are not secreted by glands and are not transported in blood. They are produced by the shoot and root tips of growing plants. An accumulation of auxin in a shoot stimulates cell growth by the absorption of water. However, auxins have the opposite effect in roots – when they build up, they slow down cell growth.

Question 35

light

Answer 35

When a shoot is exposed to light from one side, auxins that have been produced by the tip move towards the shaded side of the shoot (or the auxins are destroyed on the light side, causing an unequal distribution). Cells on the shaded side are stimulated to absorb more water than those on the light side, so the unequal growth causes the stem to bend towards the light.

Growth of a shoot towards light is called positive phototropism. If a root is exposed to light in the absence of gravity, auxins that have been produced by the tip move towards the shaded side of the root. Cells on the shaded side are stimulated to absorb less water than those on the light side, so the unequal elongation causes the root to bend away from the light. Growth of a root away from light is called negative

Question 36

gravity

Answer 36

Shoots and roots also respond to gravity. If a shoot is placed horizontally in the absence of light, auxins accumulate on the lower side of the shoot, owing to gravity. This makes the cells on the lower side elongate more quickly than those on the upper side, so the shoot bends upwards. This is called negative gravitropism. If a root is placed horizontally in the absence of light, auxins accumulate on the lower side of the root, owing to gravity. However, this makes the cells on the lower side elongate more slowly than those on the upper side, so the root bends downwards. This is called positive gravitropism. Shoots and roots that have their tips removed will not respond to light or gravity because the part that produces auxins has been cut off. Shoots that have their tips covered with opaque material grow straight upwards when exposed to one-sided light because the auxin distribution is not influenced by the light.

Question 37

Effects of synthetic plant hormones used as weedkillers

Answer 37

Synthetic plant hormones are chemicals, similar to auxins, that have been manufactured. If they are sprayed on to plants, they can cause rapid, uncontrolled growth and respiration, resulting in the death of the plant. One effective weedkiller is the synthetic plant hormone 2,4-D. When sprayed on a lawn, it affects the broad-leaved weeds (e.g. daisies and dandelions) but not the grasses. (It is called a ‘selective weedkiller’.) Among other effects, it distorts the growth of weeds and speeds up their rate of respiration so much that they exhaust their food reserves and die. Another term for a weedkiller is a herbicide.