Coordination: Hormones & Homeostasis
Hormone definition; nervous vs hormonal comparison; adrenaline and fight-or-flight; insulin and glucagon; homeostasis definition; blood glucose regulation; Extended diabetes Types 1 and 2; skin temperature responses; Extended vasodilation/vasoconstriction; tropism definition; phototropism and gravitropism; auxin mechanism; Extended auxin in roots vs shoots.
Hormones
CORE EXTENDEDWhat is a Hormone?
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
Nervous vs Hormonal Coordination
| Feature | Nervous system | Hormonal system |
|---|---|---|
| Messenger type | Electrical impulse along neurones | Chemical hormone in blood |
| Speed | Very fast (milliseconds) | Slower (seconds to minutes) |
| Duration of effect | Short-lived | Longer-lasting |
| Target | Specific — one effector | Widespread — any cell with the receptor |
| Transmission pathway | Fixed nerve pathways | Blood circulation |
Adrenaline — The ‘Fight or Flight’ Hormone
Produced by: Adrenal glands (on top of each kidney)
Trigger: Stress, fear, excitement, danger — detected by the nervous system, which signals the adrenal glands to release adrenaline.
Effects (all prepare the body for immediate physical action):
- Increases heart rate — delivers more O₂ and glucose to muscles
- Dilates pupils — improves vision
- Increases breathing rate — more O₂ intake
- Causes glycogen to be converted to glucose in the liver — more fuel for muscles
- Diverts blood to muscles (away from gut and skin)
Insulin and Glucagon
These two hormones work together to regulate blood glucose concentration (covered in detail in Topic 14.4).
| Hormone | Produced by | Released when | Main effect |
|---|---|---|---|
| Insulin | β (beta) cells of the islets of Langerhans in the pancreas | Blood glucose is too high | Stimulates cells to absorb glucose; stimulates liver to convert glucose to glycogen (glycogenesis) → blood glucose decreases |
| Glucagon | α (alpha) cells of the islets of Langerhans in the pancreas | Blood glucose is too low | Stimulates liver to convert glycogen back to glucose (glycogenolysis) → blood glucose increases |
Which of the following correctly describes how hormones are transported in the body?
- A. Along nerve fibres from the gland to the target organ
- B. Dissolved in the blood from the gland to target organs throughout the body
- C. Through the lymphatic system from gland to target cell
- D. Directly between adjacent cells by diffusion through cell membranes
Homeostasis
CORE EXTENDEDDefinition and Importance
Homeostasis is the maintenance of a constant internal environment.
The body must keep the following within narrow limits for cells to function normally:
- Blood glucose concentration — to ensure cells always have a supply of glucose for respiration
- Body temperature — to maintain the optimum temperature for enzyme activity throughout the body
- Water content of blood — regulated by the kidneys (ADH, covered in Topic 13)
Blood Glucose Regulation — Core
| Situation | Hormone released | Effect on liver | Blood glucose |
|---|---|---|---|
| After a meal (blood glucose too high) | Insulin (from pancreas β cells) | Liver converts glucose → glycogen (glycogenesis); cells take up glucose for respiration | ↓ Decreases back to normal |
| During exercise / fasting (blood glucose too low) | Glucagon (from pancreas α cells) | Liver converts glycogen → glucose (glycogenolysis) and releases glucose into blood | ↑ Increases back to normal |
Diabetes Mellitus — Extended
| Feature | Type 1 diabetes | Type 2 diabetes |
|---|---|---|
| Cause | Immune system destroys β cells of the pancreas → no insulin produced | Body cells become resistant to insulin; β cells may also produce less insulin over time |
| Blood glucose | Dangerously high if untreated (no insulin to lower it) | Chronically elevated (cells do not respond adequately to insulin) |
| Management | Regular insulin injections (or insulin pump); dietary management; blood glucose monitoring | Diet (reduce refined carbohydrates); exercise; weight loss; oral medication; insulin injections in severe cases |
| Age of onset | Usually childhood/adolescence (autoimmune) | Usually adult onset; increasingly in younger people due to obesity |
Insulin is a protein. If swallowed, digestive enzymes (proteases) in the gut would break it down into amino acids before it reaches the bloodstream. It must be injected directly into the subcutaneous tissue to enter the blood intact.
Temperature Regulation — Core
Body temperature must be kept close to 37°C — the optimum temperature for most human enzymes. The skin plays a key role in temperature regulation.
| Situation | Response | How it helps |
|---|---|---|
| Too hot | Sweating increases; vasodilation (blood vessels near skin surface widen) | Sweat evaporates, removing heat; more blood flows near skin surface, radiating heat to environment |
| Too cold | Shivering; vasoconstriction (blood vessels near skin surface narrow); hairs stand on end (erector muscles contract) | Shivering generates heat by muscle contractions; vasoconstriction reduces heat loss from skin; hairs trap air layer (insulation — more effective in other mammals) |
Vasodilation and Vasoconstriction in Detail — Extended
Arterioles near the skin surface dilate (widen). More blood flows through capillaries close to the skin surface. Heat is lost by radiation, conduction, and convection from the skin. The skin appears flushed/red. Combined with sweating, this is highly effective at cooling the body.
Arterioles near the skin surface constrict (narrow). Less blood flows through superficial capillaries. Blood is diverted to deeper tissues, reducing heat loss from the skin surface. The skin appears pale. Blood is shunted through deeper vessels (arteriovenous anastomoses), keeping warm blood away from the cold surface.
A common error is saying “blood moves closer to or further from the skin.” More precisely: the arterioles (small arteries) change their diameter. The capillary beds near the surface receive more or less blood as a result. Blood does not physically move upward or downward in the skin.
After eating a large meal containing carbohydrates, blood glucose rises. Which hormone is released, and what effect does it have on the liver?
- A. Glucagon is released; it stimulates the liver to convert glycogen to glucose
- B. Insulin is released; it stimulates the liver to convert glucose to glycogen
- C. Insulin is released; it stimulates the liver to convert glycogen to glucose
- D. Glucagon is released; it stimulates the liver to convert glucose to glycogen
Tropic Responses in Plants
CORE EXTENDEDDefinition of a Tropism
A tropism is a directional growth response of a plant to a directional stimulus. Growth toward the stimulus is positive; growth away is negative.
| Tropism | Stimulus | Shoot response | Root response |
|---|---|---|---|
| Phototropism | Light (direction) | Positive — grows toward light | Negative — grows away from light (in most species) |
| Gravitropism (geotropism) |
Gravity | Negative — grows upward (against gravity) | Positive — grows downward (with gravity) |
Role of Auxin in Tropisms — Core + Extended
Auxin (IAA — indoleacetic acid) is a plant hormone produced in the shoot tip that controls cell elongation. It migrates away from its site of production and stimulates cells to elongate.
1. Auxin is produced in the shoot tip (apical meristem).
2. When light comes from one side, auxin migrates to the shaded side of the shoot.
3. Higher auxin concentration on the shaded side causes cells on that side to elongate more.
4. The shaded side grows longer than the lit side → shoot bends toward the light (positive phototropism).
Auxin Effects in Roots vs Shoots — Extended
Auxin has different effects at different concentrations:
In shoots: moderate-to-high auxin concentrations promote cell elongation.
In roots: the same concentrations that promote elongation in shoots inhibit cell elongation in roots (roots are more sensitive to auxin).
Gravitropism example: When a root is horizontal, gravity causes auxin to accumulate on the lower side. High auxin on the lower side inhibits root cell elongation there. Upper side (lower auxin) elongates more → root bends downward (positive gravitropism).
Classic experiment: Grow seedlings in a box with a hole on one side (unilateral light). Observe direction of bending over several days.
Control: Seedlings in a box with uniform light from all sides — no bending (grow straight up).
Variables to control: Temperature; water supply; same species of seedling; same initial seedling size.
Tip removal experiment: Remove the shoot tip → no bending occurs even with unilateral light, confirming auxin is produced in the tip.
Agar block experiment (Extended): Place an agar block containing auxin on one side of a decapitated (tip-removed) shoot → shoot bends away from the block side, confirming auxin causes differential elongation.
A seedling is placed on its side in the dark. After 24 hours the shoot has grown upward and the root has grown downward. What type of tropism is shown by the root?
- A. Negative gravitropism
- B. Positive phototropism
- C. Positive gravitropism
- D. Negative phototropism
Comprehensive Practice Questions
Mixed questions across Topics 14.3, 14.4, and 14.5.
On a hot day, which response in the skin helps to increase heat loss from the body?
- A. Vasoconstriction — arterioles near skin narrow, reducing blood flow
- B. Shivering — muscles contract rapidly, generating heat
- C. Vasodilation — arterioles near skin widen, increasing blood flow and heat loss by radiation
- D. Hair erection — hairs stand up, trapping an insulating air layer
(a) Define homeostasis. [1 mark]
(b) Describe the role of insulin and glucagon in regulating blood glucose concentration. [4 marks]
(c) Describe how auxin causes a shoot to bend toward light. [3 marks]
- (a) Homeostasis is the maintenance of a constant internal environment [1 mark]
- (b) When blood glucose is too high, insulin is released from the pancreas [1 mark]; insulin stimulates the liver to convert glucose to glycogen (glycogenesis) and cells to take up glucose → blood glucose falls [1 mark]; when blood glucose is too low, glucagon is released from the pancreas [1 mark]; glucagon stimulates the liver to convert glycogen to glucose (glycogenolysis) → blood glucose rises [1 mark]
- (c) Auxin is produced in the shoot tip and migrates to the shaded side [1 mark]; higher auxin concentration causes cells on the shaded side to elongate more [1 mark]; the shaded side grows longer → shoot bends toward the light [1 mark]
(a) Compare Type 1 and Type 2 diabetes, including causes and management. [4 marks]
(b) Explain why auxin causes positive gravitropism in roots even though auxin promotes cell elongation. [3 marks]
- Type 1: caused by destruction of β cells in the pancreas (autoimmune) → no insulin produced; managed by insulin injections and blood glucose monitoring [2 marks]
- Type 2: caused by cells becoming resistant to insulin; managed by diet and exercise (weight loss); oral medication; insulin in severe cases [2 marks]
- Gravity causes auxin to accumulate on the lower side of a horizontal root [1 mark]
- Roots are more sensitive to auxin than shoots — the concentration on the lower side is high enough to inhibit (not promote) cell elongation in root cells [1 mark]
- Upper side has lower auxin → more elongation; lower side inhibited → less elongation → root bends downward (positive gravitropism) [1 mark]
High-Frequency Mistakes — Topics 14.3, 14.4 & 14.5
- 🔄Confusing insulin and glucagon — reversing their effectsInsulin lowers blood glucose (released when glucose too HIGH — after a meal). Glucagon raises blood glucose (released when glucose too LOW — during fasting/exercise). Many students reverse these. Memory: Insulin = In glucose (brings glucose into cells/storage); Glucagon = Go get glucose (releases it).
- 🏥Saying the pancreas produces insulin from the whole glandInsulin is produced only by the β (beta) cells of the islets of Langerhans — specialised regions within the pancreas. The pancreas also produces digestive enzymes (exocrine function) from different cells. Don’t say “the pancreas produces insulin” without mentioning beta cells for full marks at Extended level.
- 📅Saying hormones travel through nervesHormones are chemical substances transported in the blood — not through nerves. Nerve impulses travel along nerve fibres. These are two completely separate systems.
- 🌞Ext: Saying vasodilation means blood moves closer to the skinVasodilation means the arterioles widen, allowing more blood to flow through capillaries near the skin surface. Blood does not physically “move toward” the skin. Use precise language: “arterioles dilate, increasing blood flow through skin capillaries, increasing heat loss by radiation.”
- 🌿Saying auxin causes the shoot to grow toward light directlyAuxin does not pull the shoot toward light. It causes differential elongation — cells on the shaded side elongate more because auxin accumulates there. The difference in elongation between the two sides creates the bending. The mechanism must be stated: auxin → unequal elongation → bending.
- 🌿Ext: Saying auxin always promotes growth in rootsIn roots, the same auxin concentrations that promote cell elongation in shoots actually inhibit elongation in roots (roots are far more sensitive to auxin). This is how positive gravitropism in roots works — auxin accumulates on the lower side and inhibits elongation there, causing the root to bend downward.
- 🧠Ext: Saying Type 1 diabetics don’t produce enough insulinType 1 diabetics produce no insulin (not just less) because their immune system has destroyed the β cells entirely. Type 2 involves cells becoming resistant to insulin (insufficient response), and β cells may also decline over time. The distinction matters for exam marks.
Highest-yield Core items: the definition of homeostasis; insulin vs glucagon — which is released when and what effect on the liver (glycogenesis vs glycogenolysis); skin responses to hot and cold (sweating + vasodilation vs shivering + vasoconstriction); tropism definitions; phototropism — auxin migrates to shaded side → differential elongation → bending toward light. For Extended: the diabetes comparison table (Type 1 vs 2 — causes and management); vasodilation/vasoconstriction mechanism (arterioles); why insulin cannot be taken orally (protein, digested); and the auxin dual effect (promotes shoots, inhibits roots at same concentration) for gravitropism. Blood glucose regulation and diabetes are near-certain Paper 4 targets.