12th Class Biology Chapter 1 Notes: Thermoregulation and Osmoregulation (Punjab Board)

Chapter at a glance

In the new Punjab 12th class Biology book this is Chapter 1. Some websites number the 12th class chapters after the 11th class chapters, so you may see it listed as Chapter 13. The topics are the same. Your teacher may mark a few details as not included in your school, so follow your own textbook for the final list.

Key concepts and definitions

• Homeostasis: the maintenance of a relatively stable internal environment in an organism, such as temperature, water content and salt concentration, even when the outside conditions change.
• Feedback control: a homeostatic system has a receptor that detects a change, a control centre that compares it with a set point, and an effector that brings the value back. In negative feedback the response reverses the change.
• Thermoregulation: the process by which an organism keeps its body temperature within a tolerable range.
• Ectotherms (also called poikilotherms) get most of their body heat from the environment, so their body temperature varies with it. Examples: fish, frogs, lizards, snakes, insects.
• Endotherms (also called homeotherms) produce heat by their own metabolism and keep a fairly constant body temperature. Examples: birds and mammals.
• Osmoregulation: the control of the water and solute concentration of body fluids so that cells neither swell nor shrink.
• Osmoconformers let their body fluids match the surroundings. Osmoregulators keep their body fluids at a steady concentration, whatever the surroundings are.

Thermoregulation in animals

How heat is gained and lost

• Gain: metabolism inside the body, and heat from sunlight or hot surfaces.
• Loss: radiation, conduction, convection, and evaporation of water from skin and lungs.

Ectotherms

Ectotherms control temperature mainly by behaviour. A desert lizard basks in the morning sun to warm up, moves into shade or a burrow at noon, and becomes active again when the ground cools. Some animals enter hibernation in winter or aestivation in hot, dry seasons to survive extreme conditions.

Endotherms: the mammalian thermostat

Temperature receptors in the skin and in the hypothalamus detect changes. The hypothalamus is the control centre. It compares the blood temperature with the set point, which is near 37 °C in humans, and signals the effectors.

When the body is too hot

1. Receptors detect the rise in temperature.
2. The hypothalamus sends signals to the skin and sweat glands.
3. Skin arterioles widen (vasodilation), so more warm blood flows near the surface and heat is lost by radiation.
4. Sweat glands release sweat. Evaporation of sweat takes heat from the skin and cools it. Dogs lose heat by panting.
5. Body temperature falls to the set point and the signals stop (negative feedback).

When the body is too cold

1. Receptors detect the fall in temperature.
2. The hypothalamus signals the skin, muscles and glands.
3. Skin arterioles narrow (vasoconstriction), so less heat is lost from the surface. Sweating stops.
4. Skeletal muscles contract rapidly (shivering) and release heat. Hairs stand up and trap air as insulation.
5. Hormones such as thyroxine and adrenaline increase the metabolic rate, which makes more heat.
6. Body temperature rises to the set point and the signals stop.

When control fails

• Fever happens when chemicals made during infection raise the set point of the hypothalamus.
• Hypothermia is a dangerous fall in core temperature, below about 35 °C.
• Heat stroke occurs when the cooling mechanisms fail in very hot weather and the core temperature rises above about 40 °C. It is an emergency that needs medical care.

Thermoregulation in plants

Plants cannot move away from heat or cold, so they depend on structure and chemistry.

• High temperature: transpiration cools the leaf as water evaporates. Small or divided leaves, hairs, a waxy shiny surface and leaf orientation reduce heat gain.
• Low temperature: deciduous plants drop their leaves, seeds and buds become dormant, and cells accumulate sugars and other solutes that lower the temperature at which ice forms in the cell sap.

Osmoregulation in animals

Water moves by osmosis from a region of lower solute concentration to a region of higher solute concentration through a semipermeable membrane. So an animal in water gains or loses water depending on how its body fluid compares with the surroundings.

Marine invertebrates such as jellyfish are mostly osmoconformers. Desert animals, such as the camel, save water by making concentrated urine and by losing little water. The hump of a camel stores fat, not water.

Osmoregulation in plants

Salty soil has a very low water potential, so even though water is present, a plant may find it hard to absorb it. That is why halophytes build up solutes in their cells.

Common mistakes

• Writing that sweat cools the body because it is cold. It cools because it takes heat from the skin as it evaporates.
• Mixing up vasodilation (hot weather) and vasoconstriction (cold weather).
• Saying the pituitary or the cerebellum controls temperature. It is the hypothalamus.
• Saying ectotherms have no temperature control. They control it by behaviour.
• Reversing hypertonic and hypotonic in fish. A freshwater fish is hypertonic to its water; a marine bony fish is hypotonic to sea water.
• Saying a camel stores water in its hump.
• Mixing hydrophytes with halophytes because the two names look alike.
• Writing diagrams without labels or a title, which loses easy marks.

Questions to practise

Short questions

1. Define homeostasis and negative feedback.
2. Differentiate between ectotherms and endotherms with two examples of each.
3. What is the role of the hypothalamus in thermoregulation?
4. How does sweating help in cooling the body?
5. What is the difference between vasodilation and vasoconstriction?
6. Differentiate between hibernation and aestivation.
7. Define osmoregulation. Differentiate between osmoconformers and osmoregulators.
8. Why does a freshwater fish not drink water?
9. List four adaptations of xerophytes.
10. How do mangroves survive in salty water?

Long questions

1. Describe thermoregulation in mammals. Explain the responses of the body to heat and to cold with the help of flow charts.
2. Describe osmoregulation in freshwater and marine bony fish with labelled diagrams.
3. Classify plants on the basis of water availability and describe the adaptations of each group.
4. Explain how plants cope with high and low temperatures.

Practice MCQs

Try these first. The answer key is at the end of the article.

1. Which part of the brain acts as the thermostat of the body?
   A) Cerebellum
   B) Medulla oblongata
   C) Hypothalamus
   D) Cerebrum
2. In cold weather the blood vessels of the skin
   A) dilate
   B) constrict
   C) burst
   D) remain unchanged
3. Which animal is an ectotherm?
   A) Sparrow
   B) Lizard
   C) Goat
   D) Human
4. A freshwater fish removes the excess water that enters its body by
   A) drinking more water
   B) passing a large amount of dilute urine
   C) passing a small amount of concentrated urine
   D) sweating
5. A feature of xerophytes is
   A) sunken stomata and a thick cuticle
   B) no cuticle and large air spaces
   C) roots only at the surface in water
   D) stomata only on the upper surface of floating leaves
6. An osmoconformer is an animal whose body fluid
   A) always stays at the same concentration
   B) changes with the concentration of its surroundings
   C) is always more dilute than its surroundings
   D) has no salts
7. The normal core body temperature of a healthy human is about
   A) 27 °C
   B) 32 °C
   C) 37 °C
   D) 42 °C
8. Shivering helps to
   A) lose heat by evaporation
   B) produce heat by muscle contraction
   C) remove salt
   D) widen skin blood vessels

Preparation strategy

1. Learn the feedback loop as a flow chart: stimulus, receptor, control centre, effector, response. Draw it twice, once for heat and once for cold.
2. Make two comparison tables from memory: ectotherm and endotherm; freshwater fish and marine fish.
3. Memorise the four plant groups with three adaptations each.
4. Practise the labelled diagrams until you can draw them in about five minutes.
5. End with the chapter MCQs and short questions. Check any wrong answer against your textbook.

See also the 12th class Biology textbook guide and the 12th class Biology pairing scheme.

Diagram ideas

• Negative feedback loop for body temperature. Boxes for stimulus, receptors, hypothalamus, effectors and response, with two arrows for hot and cold. Alt text: "Flow chart of temperature control in a mammal from receptors to the hypothalamus to the effectors and back to the normal temperature."
• Skin in hot and cold conditions. Two sections of skin side by side. Label the epidermis, dermis, sweat gland, hair, erector muscle, arteriole and capillaries. Alt text: "Two diagrams of mammal skin, one with widened blood vessels and sweating, one with narrowed blood vessels and raised hairs."
• Freshwater fish and marine fish. Two fish outlines with arrows for water and salt at the gills, mouth and urine. Alt text: "Freshwater fish gaining water and losing salts, and marine fish losing water and drinking sea water, with arrows showing each movement."
• Plant groups. Four small sketches: water lily, mesophyte, cactus and mangrove with pneumatophores. Alt text: "Four plants side by side showing a hydrophyte, a mesophyte, a xerophyte and a halophyte."
• Leaf cross-section of a xerophyte. Label the thick cuticle, hairs, sunken stoma and water-storing tissue. Alt text: "Cross-section of a dry-habitat leaf showing a thick cuticle, hairs and a sunken stoma."

MCQ answer key

1. C. The hypothalamus compares body temperature with the set point and sends signals to effectors.
2. B. Vasoconstriction in the cold reduces blood flow to the skin and so reduces heat loss.
3. B. A lizard gets heat mainly from its surroundings. Sparrow, goat and human are endotherms.
4. B. The fish is hypertonic to fresh water, so water enters. The kidneys remove it as a large volume of dilute urine.
5. A. Thick cuticle and sunken stomata reduce water loss in dry habitats. The other options describe hydrophytes.
6. B. Osmoconformers let their body fluid concentration follow that of the surroundings.
7. C. Normal human core temperature is about 37 °C.
8. B. Shivering is rapid muscle contraction that releases heat.

Frequently asked questions