Hormones and the endocrine system — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 44What a hormone is

What a hormone is

Hormone

Hormone a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Chemical messengers

• Chemical messengers hormones carry information from one part of the body to another to bring about a change.

Endocrine system

• Endocrine system the hormone-producing glands together.

Section 2 of 44Transport and target cells

Blood supply

• Blood supply endocrine glands have a good blood supply, so hormones enter the blood plasma quickly.

Target receptors

• Target receptors a hormone affects only cells with complementary receptors, which may be on the cell membrane or inside the cell.

Liver

• Liver it regulates hormone levels by transforming or breaking down any excess.

Section 3 of 44Important hormones

Hormone	Source	Role	Effect
Adrenaline	Adrenal gland	Readies the body for fight or flight	Increases heart rate and breathing rate and dilates pupils
Insulin	Pancreas	Lowers blood glucose	Muscles and liver take up excess glucose and store it as glycogen
Glucagon	Pancreas	Raises blood glucose	Liver and muscles convert glycogen to glucose
Testosterone	Testes	Main male sex hormone	Development of male secondary sexual characteristics
Oestrogen	Ovaries	Main female sex hormone	Development of female secondary sexual characteristics and control of the menstrual cycle

Section 4 of 44Adrenaline

1Fight or flight

• Fight or flight adrenaline is produced when the body may be in danger.
• Flight means getting away rapidly and fight means physical combat if flight is not possible.

2Preparing for movement

• Preparing for movement it increases blood glucose concentration for more respiration in muscle cells.

3Heart and breathing

• Heart and breathing it increases pulse rate and breathing rate so glucose and oxygen reach muscles faster.

4Blood flow

• Blood flow it diverts blood towards the muscles and away from non-essential parts such as the alimentary canal.

5Pupils

• Pupils it dilates the pupils so more light reaches the retina.

Section 5 of 44Sequence of adrenaline action

1

A stimulus such as danger is detected.

2

Nerve impulses reach the adrenal glands.

3

The adrenal glands secrete adrenaline into the blood.

4

The blood carries adrenaline to the target organs.

5

Heart rate and breathing rate increase and the pupils dilate.

Section 6 of 44Nervous and hormonal control compared

Feature	Nervous system	Endocrine system
Made up of	Neurones, brain and spinal cord	Glands
Message	Electrical impulse	Chemical hormone
Speed	Very fast	Slower
Length of effect	Short, until impulses stop	Longer, until the hormone is broken down

Section 7 of 44Pulse rate practical

1

Measure the resting pulse at the wrist for 30 seconds and double it.

2

Repeat three times and take a mean.

3

Give a sudden harmless stimulus such as a loud clap.

4

Measure the pulse again straight away and every minute until it returns to resting.

Section 8 of 44More from the specification

More from the specification

Nervous system

• Receptors detect stimuli and the response is coordinated by the brain and spinal cord;
  • a reflex is a fast automatic response using a reflex arc.

Endocrine system

• Hormones are chemical messengers released into blood by glands and act on target organs, slower but longer lasting than nerves.

Homeostasis

• Maintaining a constant internal temperature and blood glucose by negative feedback.

Auxin

• A plant hormone that controls growth in shoots; it accumulates on the shaded side, causing phototropism.

Gravity response

• Shoots grow away from gravity and roots toward it, due to unequal auxin distribution.
  • (negative gravitropism)

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Temperature

• Average energy of the particles in an object.

Growth

• Permanent increase in size.

Homeostasis

• Maintaining a constant internal environment.

Gravity

• Force of attraction between objects with mass.

Section 9 of 44More on this topic

Feature	Nervous system	Endocrine system
Message	Electrical impulses	Chemical hormones
Route	Neurones	Blood
Speed	Very fast	Slower
Duration of effect	Short	Longer lasting
Target	Specific muscles or glands	Target organs with receptors

Section 10 of 44What a hormone is

What a hormone is

Hormone

Hormone a chemical substance made by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Messenger

• Messenger hormones carry information from one part of the body to another, bringing about a change.

Endocrine system

• Endocrine system the hormone-producing glands together form it.

Section 11 of 44Transport and target cells

Blood supply

• Blood supply endocrine glands have a good blood supply, so hormones enter the plasma quickly and travel around the body.

Target receptors

• Target receptors only cells with receptors complementary to the hormone respond.
• The receptors may be on the cell membrane or inside the cell.
• This explains why a hormone can affect a distant organ but not the cells next to the gland.

Removal

• Removal the liver breaks down or transforms excess hormones, which stops the effect.

Section 12 of 44Sex hormones

Hormone	Gland	Role
Testosterone	Testes	Main male sex hormone; development of male secondary sexual characteristics
Oestrogen	Ovaries	Main female sex hormone; development of female secondary sexual characteristics and control of the menstrual cycle

• Puberty
  • these hormones are made by the gonads and cause the secondary sexual characteristics to appear in adolescence.

Section 13 of 44Nervous and hormonal control compared

Feature	Nervous system	Endocrine system
Made up of	Neurones, brain and spinal cord	Glands
Type of message	Electrical impulse	Chemical hormone
Speed	Very fast	Slower
Length of effect	Short, until impulses stop	Longer, until the hormone is broken down
Area affected	Precise, e.g. one muscle or gland	Widespread, wherever target cells have receptors

Section 14 of 44Speed and duration of hormonal effects

Delay

• Delay a hormone must be made, released and carried in the blood, so the effect begins after seconds or minutes.

Duration

• Duration effects can last hours or years, for example the changes at puberty.

Section 15 of 44Use of each system

Use of each system

Nervous control

• Nervous control rapid reflex actions and precise movements.

Hormonal control

• Hormonal control slower, longer-term changes such as growth, puberty and control of blood glucose.

Adrenaline and the eye

• Adrenaline and the eye it widens the pupils when the body is in danger.

Section 16 of 44More from the specification

1Endocrine Control of Blood Glucose

• The hormone insulin is secreted by the pancreas to lower blood glucose concentration when levels rise, prompting liver and muscle cells to synthesize glycogen.

2Negative Feedback Set-Point Control

• Homeostatic mechanisms ensure that core body temperature and blood glucose levels return to equilibrium via negative feedback loops.

3Auxin and Phototropism in Shoots

• Auxin stimulates differential cell elongation;
  • unilateral light causes auxin to migrate to the shaded side, curving shoots towards the light source.

4Comparison of Animal and Plant Chemical Signals

• Animal hormones like insulin are distributed via the circulatory system,
• whereas plant regulators like auxin diffuse locally through tissues.

5Retina

• The layer at the back of the eye with light receptors.

6Synapse

• A junction between two neurones.

7Ciliary muscle

• Muscle that changes the lens shape

8Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

9Negative feedback

• A control that reverses a change to restore the set point.

10Motor neurone

• A neurone carrying impulses from the CNS to an effector.

11Temperature

• Average energy of the particles in an object.

12Insulin

• The hormone that lowers blood glucose.

13Concentration

• Amount of solute dissolved in a volume of solution.

14Homeostasis

• Maintaining a constant internal environment.

15Glycogen

• The storage carbohydrate in animals.

16Gravity

• Force of attraction between objects with mass.

Section 17 of 44Insulin and glucagon

Feature	Insulin	Glucagon
Made in	Pancreas	Pancreas
Released when	Blood glucose is high, usually after a meal	Blood glucose is low
Target cells	Liver and muscle cells	Liver and muscle cells
Action	Glucose is taken up and converted to glycogen for storage	Stored glycogen is converted to glucose and released into the blood
Effect on blood glucose	Falls	Rises

Section 18 of 44Glucagon and glycogen

Glucagon

Glucagon the hormone.

Glycogen

Glycogen the polysaccharide in which animals store glucose.

Section 19 of 44Type 1 diabetes

Type 1 diabetes

Cause

• Cause the insulin-secreting cells of the pancreas cannot make insulin.

Result

• Result blood glucose is often far too high.

Treatment

• Treatment injecting insulin lets the liver convert glucose to glycogen.

Section 20 of 44Adrenaline

• Fight or flight hormone
  • it is produced when the body may be in danger and prepares it for movement.
  • It raises blood glucose for faster respiration in muscle cells.
  • It increases pulse and breathing rate to deliver glucose and oxygen and remove carbon dioxide faster.
  • It diverts blood towards the muscles and away from non-essential parts such as the alimentary canal.
  • It dilates the pupils.

Section 21 of 44Homeostasis and negative feedback

Homeostasis

Homeostasis maintenance of a constant internal environment.

Conditions controlled

• Conditions controlled temperature, blood pressure, water concentration and glucose concentration must stay within set limits so cell reactions can function.

Negative feedback

• Negative feedback a change from the set point switches on a control system that returns the level to it.
• If a level rises, the system lowers it.
• If a level falls, the system raises it.

Section 22 of 44Temperature control by nerves

1Mainly nervous

• Mainly nervous hormones act more slowly, so temperature is corrected by nervous reflexes.

2Pathway

• Pathway skin receptors send impulses via sensory neurones to the brain, which signals effectors in the skin.

Section 23 of 44Blood glucose after a meal and during fasting

1

Glucose is absorbed after a meal and blood glucose rises above the set point.

2

The pancreas releases insulin.

3

Liver and muscle cells convert glucose to glycogen and blood glucose falls.

4

During fasting or exercise blood glucose falls below the set point.

5

The pancreas releases glucagon and glycogen is converted to glucose.

• Emergency
  • adrenaline also raises blood glucose so muscles can respire faster.

Section 24 of 44More from the specification

More from the specification

Endocrine Glands and Hormones

• Endocrine glands secrete chemical messenger hormones directly into blood vessels to reach target organs,
• regulating growth, metabolic activity,
• and blood solute levels.

Plant Tropisms and Auxin Mechanism

• Auxin is a plant growth hormone produced in shoot tips that diffuses down to stimulate cell elongation in shoots while inhibiting cell elongation in roots.

Phototropism and Gravitropism

• In plant shoots, auxin accumulates on shaded sides causing positive phototropic bending toward light;
  • in roots, auxin accumulates on lower sides in response to gravity, directing downward growth.

Comparison of Motor and Tropic Responses

• Animal sense organs trigger rapid motor neurone impulses for instantaneous muscular responses,
• whereas plant growth responses mediated by auxin develop gradually over extended periods.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Growth

• Permanent increase in size.

Gravity

• Force of attraction between objects with mass.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Section 25 of 44More on this topic

Feature	Insulin	Glucagon
Released when	Blood glucose is high, most often after a meal	Blood glucose is low
Target cells	Liver and muscle cells	Liver and muscle cells
Action	Cells take up excess glucose and convert it to glycogen	Stored glycogen is converted to glucose and released
Effect on blood glucose	Falls	Rises

Section 26 of 44Hormones and glands

Hormones and glands

Hormone

Hormone a chemical substance made by a gland and carried by the blood, which alters the activity of specific target organs.

Endocrine system

• Endocrine system all the hormone-producing glands together.

Target cells

• Target cells only cells with complementary receptors respond, and the receptors may be on the membrane or inside the cell.

Liver

• Liver it breaks down excess hormones to regulate their level.

Section 27 of 44Main endocrine glands

Gland	Hormone	Effect
Pituitary	Many, e.g. FSH and LH	Controls other glands and the menstrual cycle
Thyroid	Thyroxine	Controls metabolic rate
Pancreas	Insulin and glucagon	Control blood glucose
Adrenal glands	Adrenaline	Prepares the body for fight or flight
Ovaries	Oestrogen and progesterone	Female characteristics and the menstrual cycle
Testes	Testosterone	Male characteristics and sperm production

Section 28 of 44Adrenaline

• Fight or flight hormone
  • adrenaline is released from the adrenal glands when the body may be in danger, and it prepares the body for movement.
  • It raises blood glucose for faster respiration in muscle cells.
  • It increases pulse rate and breathing rate to deliver glucose and oxygen and remove carbon dioxide faster.
  • It diverts blood to the muscles and away from non-essential parts such as the alimentary canal.
  • It dilates the pupils.

Section 29 of 44Sex hormones

Hormone	Gland	Role
Testosterone	Testes	Main male sex hormone; development of male secondary sexual characteristics
Oestrogen	Ovaries	Main female sex hormone; development of female secondary sexual characteristics and control of the menstrual cycle

Section 30 of 44Nervous and hormonal control compared

Feature	Nervous system	Endocrine system
Made up of	Neurones, brain and spinal cord	Glands
Type of message	Electrical impulse	Chemical hormone
Speed	Very fast	Slower
Length of effect	Short, until impulses stop	Longer, until the hormone is broken down
Area affected	Precise, e.g. one muscle or gland	Widespread, wherever target cells have receptors

Section 31 of 44Medical uses of hormones

Medical uses of hormones

Insulin

• Insulin injected to treat type 1 diabetes.

Thyroxine

• Thyroxine tablets treat an underactive thyroid.

FSH and LH

• FSH and LH fertility drugs stimulate eggs to mature.

Oestrogen and progesterone

• Oestrogen and progesterone hormonal contraceptives prevent ovulation.

Anabolic steroids

• Anabolic steroids synthetic hormones related to testosterone, banned in sport because they increase muscle growth but have harmful side effects.

Section 32 of 44Using both systems together

Using both systems together

Reflex

• Reflex a quick response such as withdrawing from a hot object relies on neurones.

Long-term change

• Long-term change growth and puberty rely on hormones.

Fright

• Fright the nervous system acts at once and adrenaline keeps the body prepared for longer.

Section 33 of 44More from the specification

More from the specification

Principles of homeostasis and negative feedback

• Homeostasis maintains a constant internal environment using negative feedback mechanisms that counteract deviations from set points.
  • (e.g. core body temperature and blood glucose levels)

Hormonal regulation of blood glucose

• When blood glucose concentration rises, insulin stimulates liver and muscle cells to convert glucose into glycogen;
• when glucose levels fall, glucagon stimulates glycogen breakdown.

Plant tropisms and auxin redistribution

• In plant shoots, auxin accumulates on the shaded side, stimulating differential cell elongation that causes shoots to bend towards directional light.
  • (positive phototropism)

Thermoregulation in endotherms

• Maintaining body temperature near 37 deg C involves physiological effectors controlled by the hypothalamus,
• including vasodilation, sweating, vasoconstriction,
• and shivering.

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Negative feedback

• A control that reverses a change to restore the set point.

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Temperature

• Average energy of the particles in an object.

Concentration

• Amount of solute dissolved in a volume of solution.

Homeostasis

• Maintaining a constant internal environment.

Glycogen

• The storage carbohydrate in animals.

Gravity

• Force of attraction between objects with mass.

Section 34 of 44More on this topic

• Source
  • both hormones are made in the pancreas.

Section 35 of 44Main hormones

Hormone	Gland	Main effect
Adrenaline	Adrenal glands	Prepares the body for fight or flight
Insulin	Pancreas	Lowers blood glucose
Glucagon	Pancreas	Raises blood glucose
Testosterone	Testes	Male secondary sexual characteristics and sperm production
Oestrogen	Ovaries	Female secondary sexual characteristics and control of the menstrual cycle
Progesterone	Ovaries	Maintains the uterus lining
ADH	Pituitary	Increases water reabsorption in the kidneys

Section 36 of 44Adrenaline effects

Source

• Source the adrenal glands, above the kidneys, release it when a person is frightened, excited or stressed.

Heart

• Heart it beats faster and harder, increasing blood flow to the muscles.

Breathing

• Breathing the rate increases, bringing in more oxygen.

Liver

• Liver glycogen is converted to glucose, raising blood glucose for respiration.

Other changes

• Other changes the pupils dilate and blood is diverted from the digestive system to the muscles.

Section 37 of 44Speed and spread of hormonal control

Feature	Nervous	Hormonal
Transmitted by	Neurones	Blood
Speed	Rapid	Slower
Response length	Brief	Longer
Area affected	Precise	Widespread

Section 38 of 44Duration of hormone effects

Tiny amounts

• Tiny amounts hormones are produced in very small quantities.

Breakdown

• Breakdown the liver breaks them down, so effects wear off and adrenaline acts only briefly.

Pituitary gland

• Pituitary gland the master gland regulates long-term hormones such as growth hormone and the sex hormones.

Section 39 of 44Puberty

Testosterone in boys

• Testosterone in boys it causes facial hair, a deeper voice and muscle development.

Oestrogen in girls

• Oestrogen in girls it causes breast development, wider hips and the start of the menstrual cycle.

Control

• Control hormone levels are usually controlled by negative feedback.

Section 40 of 44Hormones in medicine

1Insulin

• Insulin given to people with type 1 diabetes, taken with meals and checked by testing blood glucose.

2Contraception and fertility

• Contraception and fertility hormones are used in the contraceptive pill and in fertility treatment with FSH.

3Hormone therapy

• Hormone therapy sex hormones can be taken to replace missing hormones.

4Misuse

• Misuse anabolic steroids build muscle but have harmful side effects.

Section 41 of 44Other hormones and plant hormones

1Thyroxine

• Thyroxine controls metabolic rate.

2Growth hormone

• Growth hormone it controls growth in childhood; too little causes dwarfism and too much causes gigantism.

3Auxin

• Auxin a plant hormone that controls growth responses, and it is used in rooting powder and as a selective weedkiller.

4Ethene

• Ethene a plant hormone gas that speeds fruit ripening, used when fruit is transported unripe.

5Plant hormones

• Plant hormones they need no glands and are made in tissues such as shoot tips.

Section 42 of 44More from the specification

More from the specification

Endocrine System and Hormonal Control

• Endocrine glands secrete chemical hormones directly into the bloodstream,
• which travel to target cells or organs to elicit slow, long-lasting physiological changes, in contrast to the rapid electrical impulses of the nervous system.

Direction of Impulse in a Reflex Arc

• In a rapid protective reflex arc, nerve impulses travel away from peripheral sensory receptors, towards the spinal cord and brain via sensory neurones,
• and then out towards effector muscles or glands.

Homeostatic Thermoregulation in Mammals

• The hypothalamus in the brain monitors core blood temperature and coordinates responses such as vasodilation, sweating,
• and shivering to regulate thermal balance against external temperature fluctuations.

Tropic Responses in Plant Shoots

• In growing plant shoots, a higher concentration of auxin accumulating on the shaded or lower side stimulates extended cell elongation,
• causing the shoot to bend towards light and away from gravity (negative gravitropism).
  • (positive phototropism)

Endocrine System and Hormonal Control

• Endocrine glands secrete chemical hormones directly into the bloodstream,
• which travel to target cells or organs to elicit slow, long-lasting physiological changes, in contrast to the rapid electrical impulses of the nervous system.

Direction of Impulse in a Reflex Arc

• In a rapid protective reflex arc, nerve impulses travel away from peripheral sensory receptors, towards the spinal cord and brain via sensory neurones,
• and then out towards effector muscles or glands.

Homeostatic Thermoregulation in Mammals

• The hypothalamus in the brain monitors core blood temperature and coordinates responses such as vasodilation, sweating,
• and shivering to regulate thermal balance against external temperature fluctuations.

Tropic Responses in Plant Shoots

• In growing plant shoots, a higher concentration of auxin accumulating on the shaded or lower side stimulates extended cell elongation,
• causing the shoot to bend towards light and away from gravity (negative gravitropism).
  • (positive phototropism)

Retina

• The layer at the back of the eye with light receptors.

Synapse

• A junction between two neurones.

Pupil reflex

• The automatic change in pupil size in response to light.

Ciliary muscle

• Muscle that changes the lens shape

Suspensory ligaments

• Fibres joining the lens to the ciliary muscles

Motor neurone

• A neurone carrying impulses from the CNS to an effector.

Temperature

• Average energy of the particles in an object.

Impulse

• Electrical signal along a neurone

Concentration

• Amount of solute dissolved in a volume of solution.

Homeostasis

• Maintaining a constant internal environment.

Gravity

• Force of attraction between objects with mass.

Section 43 of 44Key terms

Term	Meaning
Hormone	A chemical messenger made by a gland and carried in the blood.
Endocrine gland	A gland that releases hormones into the blood.
Target organ	An organ whose cells have receptors for a hormone.
Adrenaline	The hormone that prepares the body for fight or flight.
Insulin	The hormone that lowers blood glucose.
Glucagon	The hormone that raises blood glucose.
Testosterone	The main male sex hormone.
Oestrogen	The main female sex hormone.

Term	Meaning
Endocrine gland	Gland that releases hormones into the blood
Target organ	Organ with cells that respond to a hormone
Testosterone	Male sex hormone made in the testes
Oestrogen	Female sex hormone made in the ovaries
Receptor	Protein that binds a hormone
Secondary sexual characteristics	Features that develop at puberty

Term	Meaning
Insulin	Hormone that lowers blood glucose
Glucagon	Hormone that raises blood glucose
Pancreas	Gland making insulin and glucagon
Glycogen	Animal storage carbohydrate
Adrenaline	Fight or flight hormone
Set point	Ideal value of a condition

Term	Meaning
Hormone	Chemical messenger made by a gland and carried in the blood
Endocrine gland	Gland that releases hormones into the blood
Target organ	Organ whose cells respond to a hormone
Thyroxine	Thyroid hormone controlling metabolic rate
Adrenaline	Fight or flight hormone from the adrenal glands
Anabolic steroid	Synthetic hormone related to testosterone

Term	Meaning
Adrenaline	Hormone preparing the body for fight or flight
ADH	Hormone that increases water reabsorption in the kidneys
Progesterone	Hormone that maintains the uterus lining
Pituitary gland	Master gland controlling other glands
Auxin	Plant hormone controlling growth
Ethene	Plant hormone gas that speeds ripening

Section 44 of 44Exam technique

1

Saying hormones travel in nerves — they travel in the blood plasma.

2

Saying every cell responds to a hormone — only cells with the right receptors respond.

3

Saying hormonal effects are faster than nervous ones — they are slower but longer lasting.

4

Mixing up insulin and glucagon — insulin lowers blood glucose and glucagon raises it.

1

Saying hormones are carried in lymph — they are carried in the blood plasma.

2

Saying hormones work on all cells — only cells with complementary receptors respond.

3

Saying the testes make oestrogen — the ovaries do.

4

Believing endocrine glands have ducts — they release hormones directly into the blood.

1

Calling glycogen a hormone — it is a storage carbohydrate.

2

Saying insulin converts glycogen to glucose — glucagon does.

3

Saying the pancreas stores glucose — the liver and muscles store glycogen.

4

Describing type 1 diabetes as caused by eating too much sugar — it is a lack of insulin.

1

Saying hormones are carried by neurones — they are carried by the blood.

2

Saying a hormone affects every cell — only cells with matching receptors respond.

3

Confusing the pituitary with the pancreas — the pituitary controls other glands, the pancreas makes insulin.

4

Saying nervous responses last longer than hormonal ones — hormonal effects last longer.

1

Confusing insulin with glucagon — insulin lowers and glucagon raises blood glucose.

2

Saying adrenaline slows the heart — it speeds it up.

3

Saying plant hormones are made in glands — they are made in tissues such as shoot tips.

4

Saying hormonal effects are instant — they are slower than nervous ones.