chapter 15 pt

Question 1

homeostasis

Answer 1

The maintenance of stable/constant conditions in the body.

The mechanism by which levels in the body are brought back to normal by a series of processes.

Question 2

Receptors and effectors p1

Answer 2

Receptors and effectors are vital for the body to maintain this dynamic equilibrium.
As you have seen, sensory receptors detect changes in the internal and external environment of an organism.
In homeostasis, it is essential to monitor changes in the internal environment, for example, the pH of the blood, core body temperature, and concentrations of urea and sodium ions in the blood.

Question 3

Receptors and effectors p2

Answer 3

Information from the sensory receptors is transmitted to the brain and impulses are sent along the motor neurones to the effectors to bring about changes to restore the equilibrium in the body.
Effectors are the muscles or glands that react to the motor stimulus to bring about a change in response to a stimulus.
Both are vital in a homeostatic system - detecting change is no use without the means to react to that change, but effectors cause chaos unless responding to a need.

Question 4

Feedback systems:

Answer 4

Homeostasis depends on sensory receptors detecting small changes in the body, and effectors working to restore the status quo.
These precise control mechanisms in the body are based on feedback systems that enable the maintenance of a relatively steady state around a narrow range of conditions.

Question 5

Negative feedback systems

Answer 5

Most of the feedback systems in the body involve negative feedback.
A small change in one direction is detected by sensory receptors.
As a result, effectors work to reverse the change and restore conditions to their base level.
Negative feedback systems work to reverse the initial stimulus. You have seen negative feedback in action in the control of blood sugar levels by insulin and glucagon.
Negative feedback systems are also important in many other aspects of homeostasis including temperature control and the water balance of the body.

Question 6

general principles of negative feedback systems

Answer 6

Question 7

Positive feedback systems:

Answer 7

There are relatively few positive feedback systems in the body. In a positive feedback system, a change in the internal environment of the body is detected by sensory receptors, and effectors are stimulated to reinforce that change and increase the response.

Question 8

Positive feedback systems examples

Answer 8

One example occurs in the blood clotting cascade.
When a blood vessel is damaged, platelets stick to the damaged region and they release factors that initiate clotting and attract more platelets.
These platelets also add to the positive feedback cycle and it continues until a clot is formed.

Another example of a positive feedback mechanism is seen during childbirth.
The head of the baby presses against the cervix, stimulating the production of the hormone oxytocin.
Oxytocin stimulates the uterus to contract, pushing the head of the baby even harder against the cervix and triggering the release of more oxytocin.
This continues until the baby is born

Question 9

thermoregulation

Answer 9

An important aspect of homeostasis in many animals is the maintenance of a relatively constant core body temperature to maintain optimum enzyme activity.
This process is known as thermoregulation.

Question 10

Organisms are constantly heating up and cooling down as a result of their surroundings. These changes depend on a number of physical processes. These include:

Answer 10

• Exothermic chemical reactions.
• Latent heat of evaporation - objects cool down as water evaporates from a surface.
• Radiation - the transmission of electromagnetic waves to and from the air, water, or ground.
• Convection - the heating and cooling by currents of air or water, warm air or water rises and cooler air or water sinks setting up convection currents around an organism.
• Conduction - heating as a result of the collision of molecules. Air is not a good conductor of heat but the ground and water are.

Question 11

In many cases, the balance between

Answer 11

heating and cooling determines the core temperature of the organism.
Animals can be classified as ectotherms or endotherms depending on how they maintain and control their body temperature.

Question 12

ways in which an animal warms up and cools down

Answer 12

Question 13

Ectotherms:
p1

Answer 13

Most animals are ectotherms and use their surroundings to warm their bodies (ectotherm literally means ‘outside heat’).
Their core body temperature is heavily dependent on their environment.
Ectotherms include all the invertebrate animals, along with fish, amphibians, and reptiles.
Many ectotherms living in water do not need to thermoregulate.

Question 14

Ectotherms:
p2

Answer 14

• The high heat capacity of water means that the temperature of their environment does not change much.
• Ectotherms that live on land have a much bigger problem with temperature regulation.
• The temperature of the air can vary dramatically both between seasons and even over a 24-hour period from the middle of the day to the end of the night.
• As a result ectotherms have evolved a range of strategies that enable them to cool down or warm up.

Question 15

Endotherms:

Answer 15

• Mammals and birds are endotherms.
• They rely body temperature regardless of the temperature of the environment (endotherm literally means ‘inside heat’).
• They have adaptations which enable them to maintain their body temperature and to take advantage of warmth from the environment.
• As a result, endotherms survive in a wide range of environments.
• Keeping warm in cold conditions and cooling down in hot conditions are both active processes.
• The metabolic rate of endotherms is around five times higher than ectotherms, so they need to consume more food to meet their metabolic needs than ectotherms of a similar size.

Question 16

Temperature regulation in ectotherms:

Answer 16

Ectotherms cannot control their body temperature using their metabolism - however, they have evolved a range of behavioural responses that enable them to overcome the limitations imposed by the temperature of their surroundings.

Question 17

Behavioural responses in ectotherms p1

Answer 17

• Sometimes they need to warm up to reach a temperature at which their metabolic reactions happen fast enough for them to be active.
• They may bask in the Sun, orientate their bodies so that the maximum surface area is exposed to the Sun, and even extend areas of their body to increase the surface area exposed to the Sun.
• For example, lizards often bask for long periods of time to get warm enough to move fast and hunt their prey, and insects such as locusts and butterflies orientate themselves for maximum exposure to the Sun and spread their wings to increase the available surface area to get warm enough to fly.

Question 18

Behavioural responses in ectotherms p2

Answer 18

• Ectotherms can increase their body temperature through conduction by pressing their bodies against the warm ground.
• They also get warmer as a result of exothermic metabolic reactions.
• Galapagos iguanas will contract their muscles and vibrate increasing cellular metabolism to raise their body temperature.
• Similarly, moths and butterflies may vibrate their wings to warm their muscles before they take flight.

Question 19

Behavioural responses in ectotherms p3 - cooling down

Answer 19

• Ectotherms sometimes need to cool down to prevent their core temperature reaching a point where enzymes begin to denature.
• To cool down, many of the warming processes are reversed.
• Ectotherms shelter from the sun by seeking shade, hiding in cracks in rocks, or even digging burrows.
• They will press their bodies against cool, shady earth or stones, or move into available water or mud.
• They orientate their bodies so that the minimum surface area is exposed to the sun, and minimise their movements to reduce the metabolic heat generated.

Question 20

Physiological responses to warming in ectotherms p1

Answer 20

• Dark colours absorb more radiation than light colours so Lizards living in colder climates tend to be darker coloured than lizards living in hotter countries so that they get warmer.
• alter their heart rate to increase or decrease the metabolic rate and sometimes to affect the warming or cooling across the body surfaces.

Question 21

Physiological responses to warming in ectotherms p2

Answer 21

• Ectotherms are always more vulnerable to fluctuations in the environment than endotherms.
• However, by using a variety of behavioural and physiological strategies many of them can maintain relatively stable core temperatures.
• They need less food than endotherms as they use less energy regulating their temperatures, and so they can survive in some very difficult habitats where food is in short supply.

Question 22

The Namaqua chameleon - a highly adapted ectotherm
* The Namaqua chameleon lives in the Namib desert, one of the most inhospitable hot and waterless environments on Earth.
* Several observations have been made on this rare and extremely well-adapted ectotherm:

Answer 22

• It is black in the morning.
• It may even appear black on the side exposed to the sun and pale grey on the other side of the body.
• It orientates its body sideways to the Sun.
• It has an increased heart rate early in the morning when basking.
• It inflates its body in the early morning.
• It presses its body to the desert sand in the morning.
• During the day the chameleon deflates its body.
• The animal becomes a very pale grey.
• It holds itself well away from the desert surface.
• The heart rate slows down.
• The chameleon opens its mouth and pants in the middle of the day.

Question 23

Thermoregulation in endotherms

Answer 23

Endotherms can regulate their body temperature within a very narrow range in a wide variety of external conditions.
Human beings, like all mammals, have a number of physiological responses that make this thermoregulation possible.

Question 24

Detecting temperature changes:
in endotherms p1

Answer 24

• In any homeostatic system receptors are needed to detect a change in the internal environment.
• The peripheral temperature receptors are in the skin and detect changes in the surface temperature.
• Temperature receptors in the hypothalamus detect the temperature of the blood deep in the body.
• The temperature of the skin is much more likely to be affected by external conditions than the temperature of the hypothalamus.

Question 25

Detecting temperature changes:
in endotherms p2

Answer 25

• The combination of the two gives the body great sensitivity and allows it to respond not only to actual changes in the temperature of the blood but to pre-empt possible problems that might result from changes in the external environment.
• The temperature receptors in the hypothalamus act as the thermostat of the body, controlling the responses that maintain the core temperature in a dynamic equilibrium to within about 1 °C of 37°C.