Science notes, chapter 3

Coordination and Response

Textbook Science Form 1 (DLP), pages 70–87

By the end of this chapter you can

Explain the meaning of homeostasis (3.1)
Describe how the human body regulates water content and body temperature, and how animals respond to temperature changes (3.1)
Explain how plants regulate water content through transpiration and the stoma (3.1)
Explain why homeostasis is important to humans and other living things (3.1)

3.1Homeostasis in Living Things

Homeostasis is the maintenance of the internal environment in the body of an organism, such as temperature, water content, pH and blood pressure, in a balanced and stable condition. The word comes from two Greek words: homeo ("similar") and stasis ("stable").

When the inside of the body stays balanced, all the living processes in the body work well. If the internal conditions are not balanced, for example the temperature becomes too high, the cells of the organism may die.

Our normal body temperature is about 37 °C. Other animals have their own normal temperature: a magpie is about 39 °C and a porcupine about 31 °C. If body temperature moves more than about 6 °C away from normal, a person can die. That is why a patient with a high fever must be watched closely.

An everyday comparison: the oven thermostat

An oven has a thermostat. If it detects that the oven is too hot, it switches the heater off and the temperature falls. If the oven is too cold, it switches the heater on and the temperature rises. Your body works in a similar way to keep its temperature and water content steady.

The homeostatic control process

Flow chart: normal range, change detected by the control centre, corrective mechanism, back to the normal range — Any change away from the normal range is detected by the control centre (in the brain), and a corrective mechanism brings it back. (Textbook Figure 3.1, p. 73)

A condition inside the body (for example temperature) increases or decreases away from its normal range.
The control centre in the brain detects the change.
A corrective mechanism takes place.
The condition returns to its normal range.

The two most important kinds of homeostasis in humans are the regulation of water content and the regulation of body temperature.

Regulation of water content

Systems involved: the excretory system and the endocrine system (the hormone system). Organs involved: the kidneys and the brain.

	Water content too high	Water content too low
Cause	We drink a lot of water	We sweat a lot (hot day, exercise)
Detected by	The brain	The brain
Corrective mechanism	The brain stimulates the secretion of a hormone so that the kidneys increase urine production. More urine is produced.	The brain stimulates the secretion of a hormone so that the kidneys decrease urine production. Less urine; we feel thirsty.
Result	Water content decreases to the normal level	Water content increases to the normal level

Based on textbook Figure 3.2, p. 74.

Sweat and urine work like a see-saw. When you sweat a lot, you lose water through the skin, so the kidneys save water by making less urine.

Ali drank three large glasses of water during recess. What will happen to the amount of urine he produces? Which organ detects the change?

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He will produce more urine. The change is detected by the brain, which stimulates a hormone so that the kidneys make more urine.

Regulation of body temperature

Systems involved: the excretory system and the endocrine system. Organs involved: the skin, the brain and the skeletal muscles. The normal body temperature is 37 °C.

	Hot day (body temperature rises)	Cold day (body temperature falls)
Blood vessels in the skin	Dilate (widen) — more blood flows close to the skin, so more heat is lost	Constrict (narrow) — blood flows away from the skin, so less heat is lost
Hairs	Lie flat — little air is trapped, so heat escapes easily	Stand erect — a layer of air is trapped and acts as a heat insulator
Sweating	Increases — sweat evaporates and cools the skin	Decreases — less heat lost by evaporation
Skeletal muscles and hormones	Muscle activity and certain hormones reduce	Muscles contract and relax actively, so we shiver; hormones increase metabolism to make heat
Urine	Less urine (water is lost as sweat)	—

Based on textbook Figure 3.3, p. 75.

Skin on a hot day: hairs lie flat, blood vessels dilate, sweat glands produce sweat — Left: the skin at a higher surrounding temperature. Right: the skin at a lower surrounding temperature. (Textbook Figure 3.4, p. 76)

Skin on a cold day: hairs stand erect, blood vessels constrict — Left: the skin at a higher surrounding temperature. Right: the skin at a lower surrounding temperature. (Textbook Figure 3.4, p. 76)

Mixing up dilate and constrict. Hot → blood vessels dilate (that is why your face looks red after exercise). Cold → blood vessels constrict (that is why you look pale when cold).

Saying "blood vessels move closer to the skin". The blood vessels do not move. They widen, so more blood flows near the skin surface.

Wearing a few thin layers of clothing keeps you warmer than one thick layer: air is trapped between the layers, and air is a good heat insulator — just like erect hairs.

Experiments on biological responses

Experiment 3.1 — Do we sweat in a hot or cold condition? (p. 76–77)

Hypothesis: we sweat in a hot condition.
Manipulated variable: surrounding temperature (fans off = hot, fans on = cool). Responding variable: presence of sweat. Fixed variable: time taken (10 minutes each).
Result: we sweat when the fans are off (hot), not when they are on (cool). The hypothesis is accepted.

Experiment 3.2 — Does the pulse count increase during heavy tasks? (p. 77–78)

Hypothesis: pulse count increases when doing heavy tasks.
Manipulated variable: type of activity (at rest, walking, jogging for 5 minutes). Responding variable: pulse count. Fixed variable: time taken.
Count the pulse for one minute with two fingers on the wrist. The pulse is lowest at rest and highest after jogging, so the hypothesis is accepted. After heavy tasks, students also sweat and breathe faster: their muscles produce heat, so sweat is produced to cool the body.

In Experiment 3.2, why must every activity last the same time (5 minutes)?

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Time is the fixed (constant) variable. Keeping it the same makes the test fair, so any change in pulse count is caused only by the type of activity.

Homeostasis in animals

Cats and dogs have no sweat glands except on the soles of their feet, so they cool down in other ways.

A cat licking its fur, a dog with its tongue out, and a cat with fur standing up — Responses of animals to changes in surrounding temperature. (Textbook Photograph 3.4, p. 78)

Animal	How it keeps homeostasis
Cat	Licks its fur — the saliva evaporates and lowers its body temperature. When cold, its fur stands erect to trap heat.
Dog	Hangs its tongue out so that water evaporates and its temperature decreases.
Lizard (reptile)	Cold surrounding: body activities, muscles and movements become slower; metabolism rate and body temperature decrease. Hot surrounding: heart beats faster; movements become faster; metabolism rate and body temperature increase.
Snail	Loses a lot of water by evaporation from its skin, so it produces fluid and looks for humid places to reduce water loss.
Bee	Has a waxy skin layer; it loses water vapour through its spiracles, so it closes its spiracles between breathing movements to reduce water loss.

Based on textbook Photographs 3.4–3.7, pp. 78–79.

A lizard's body temperature changes with the surroundings. It cannot keep a steady 37 °C like we do, so it becomes slow when it is cold.

Homeostasis in plants

Transpiration is the process in which plants lose water from their leaves, as water vapour, to the surroundings through the stoma (plural: stomata).

Transpiration involves the transport system of the plant. Almost 90% of the water absorbed by the roots is lost through transpiration.

A plant showing water flowing from the root hairs, up the stem, to the leaf and out through a stoma — Water flows from the soil into the root hairs, up the stem and out of the leaves as water vapour. (Textbook Figure 3.5, p. 80)

The roots (root hairs) absorb water and minerals from the soil.
Transpiration at the leaves produces a pulling force that draws water up the stem.

Why transpiration is useful:

It helps the plant absorb and carry water and minerals from the soil to all parts of the plant.
Evaporation of water from the leaves cools the plant on hot days.

The stoma

Guard cells control the opening and closing of the stoma.

During the day	When the temperature is too high

The stoma opens for gas exchange. More water evaporates from the leaves, which increases the water intake by the roots.	The stoma closes to reduce the water lost through transpiration.

Stoma drawings: textbook Figure 3.6, p. 81.

Some plants have other tricks. Banana leaves roll up in the hot afternoon to avoid losing too much water, and open again in the cooler evening (Photograph 3.8, p. 80).

On a very hot afternoon, a plant's stomata close. Give one advantage and one disadvantage of this.

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Advantage: less water is lost by transpiration, so the plant does not wither. Disadvantage: less gas exchange, and less water and minerals are drawn up from the roots (and less cooling by evaporation).

Importance of homeostasis

Homeostasis gives the optimum conditions in the body so that cells can carry out metabolism efficiently. All chemical reactions in living cells are controlled by enzymes, and enzymes are sensitive to changes such as temperature. In humans, enzymes work best at about 37 °C.

Without homeostasis, body temperature could keep rising or falling and water could be lost without control. People would become very ill, and plants would wither and die. We should be grateful that our bodies regulate themselves automatically.

Why do people who exercise need to drink water before they feel thirsty?

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During exercise the body loses a lot of water as sweat. By the time we feel thirsty, the water content is already below normal. Drinking early replaces the water lost and keeps the water content balanced.

Summary

Remember	Key fact
Homeostasis	Keeping the internal environment (temperature, water, pH, blood pressure) balanced and stable
Control process	Change → detected by the control centre (brain) → corrective mechanism → back to normal range
Water regulation	Excretory + endocrine systems; kidneys and brain. Too much water → more urine. Too little → less urine, thirsty
Temperature regulation	Skin, brain, skeletal muscles. Hot: vessels dilate, hairs flat, more sweat. Cold: vessels constrict, hairs erect, less sweat, shivering
Normal body temperature	37 °C
Animals	Cat licks fur, dog hangs tongue out, lizard slows down in the cold, snail seeks humid places, bee closes spiracles
Plants	Transpiration through the stoma; guard cells open the stoma by day and close it when too hot
Why it matters	Enzymes need optimum conditions (37 °C in humans) so that cells work efficiently