Control and Coordination
Complete chapter notes with key concepts, important definitions, labelled diagrams and quick revision points.
1 Introduction
Living things move because they respond to changes in their surroundings. For example, plants grow towards sunlight, and human beings respond to sounds, touch, heat and other changes around them. These movements are controlled and coordinated so that organisms can survive and carry out their life processes.
- All living organisms respond and react to changes in the environment around them.
- Stimuli: Changes in the environment to which organisms respond and react are called stimuli.
- Examples of stimuli include light, heat, cold, sound, smell and touch.
- Both plants and animals respond to stimuli, but they respond in different ways.
2 Animals – Nervous System
In animals, nervous tissue and muscular tissue are responsible for regulating and coordinating body functions. Specialised cells called receptors receive information from the surroundings. They are present in sense organs such as the eyes, ears, nose, tongue and skin.
Examples of receptors
- Gustatory receptors: Found on the tongue; help detect sweet, sour, salty and bitter tastes.
- Olfactory receptors: Located in the nose; detect different smells.
The nervous system consists of the brain, spinal cord and a network of nerves. It receives information, processes it and sends suitable instructions to different parts of the body.
Nervous tissue is made up mainly of specialised cells called neurons. A neuron is the basic structural and functional unit of the nervous system. A nerve impulse is an electrical signal that travels through a neuron.
Main parts of a neuron
- Dendrites: Branched structures that receive messages from other neurons and transmit them towards the cell body.
- Cell body: Contains a nucleus and controls the activities of the cell.
- Axon: A long, tube-like structure that carries the electrical impulse from the cell body towards the axon terminals.
- Synapse: The small junction or gap between the axon of one neuron and the dendrites of another neuron. Chemicals released at the synapse help transmit the signal.
Types and functioning of nerves
- Sensory nerves: Transmit messages from sense organs to the brain or spinal cord.
- Motor neurons: Carry messages from the brain or spinal cord to muscles and glands.
- When a nerve cell receives a signal at its tip, it creates an electrical signal.
- The signal moves from the dendrites to the cell body and then along the axon.
- At the end of the axon, the signal causes chemicals to be released.
- These chemicals cross the small gap at the synapse and start a new electrical signal in the next neuron.
- This process helps transmit messages through the body, including messages to muscles and glands.
3 What Happens in Reflex Actions?
A reflex action is a quick and automatic response to a stimulus. It takes place without conscious thinking. For example, when you touch a hot object, you immediately pull your hand away.
A reflex arc is the pathway followed by a nerve impulse during a reflex action from a receptor, through the central nervous system, to an effector. In many reflex actions, the spinal cord coordinates the immediate response, while the information is also sent to the brain.
Components of the reflex arc
- A sensory neuron transmits sensory impulses from the receptor to the central nervous system.
- The sensory impulses are processed or integrated in the central nervous system.
- The central nervous system sends motor information to the effector through a motor neuron.
- The effector, such as a muscle or gland, produces the required response to the stimulus.
4 Human Brain
Types of nervous system
1. Central Nervous System (CNS): Consists of the brain and spinal cord. It receives information from the body, processes it and sends instructions to specific organs.
2. Peripheral Nervous System (PNS): Includes the cranial nerves and spinal nerves emerging from the brain and spinal cord. It forms a network outside the CNS and connects the CNS with different parts of the body.
Major regions of the brain
The brain has three major regions: the forebrain, midbrain and hindbrain.
Parts and functions of the brain
- Forebrain: The most complex and highly specialised part; consists mainly of the cerebrum. It is the thinking part of the brain, controls voluntary actions, stores information and is associated with memory. It receives and integrates sensory impulses and contains centres associated with hunger and other important activities.
- Midbrain: Controls some involuntary actions, such as changes in the size of the pupil and reflex movements of the head, neck and trunk.
- Hindbrain: Has three main parts:
- Cerebellum: Controls posture, balance and the precision of voluntary actions.
- Medulla: Controls involuntary actions such as blood pressure, salivation, swallowing and vomiting.
- Pons: Helps regulate respiration and carries out other involuntary functions.
Protection of the brain and spinal cord
- The brain is enclosed in the cranium or skull.
- Cerebrospinal fluid surrounds the brain and acts as a shock absorber.
- The spinal cord is enclosed and protected by the vertebral column or backbone.
- The spinal cord transmits sensory information to the brain, carries messages from the brain to muscles and coordinates many reflex actions.
How does nervous tissue cause action?
- Nerve impulse transmission: Nervous tissue collects and processes information. A nerve impulse is sent from the nervous system to a muscle.
- Muscle activation: The nerve impulse reaches the muscle. Muscle fibres respond by changing their shape and shortening.
- Protein interaction: Special proteins in muscle cells change their arrangement in response to a nerve impulse. This causes the muscle to contract and produce movement.
5 Coordination in Plants
Plants do not have a nervous system or muscles like animals. However, they respond to stimuli in different ways. The leaves of a sensitive plant fold quickly when touched, while a growing seedling can move in a particular direction.
Plant movements can be classified into movements independent of growth and movements dependent on growth.
Immediate response to a stimulus
- A sensitive plant responds to touch without growth being involved.
- Plants can detect stimuli even though they do not have a nervous system.
- Information about a stimulus is communicated through the plant.
- Plants use electrical and chemical signals to convey information between cells.
- Plant cells can change shape by altering their water content. This causes cells to swell or shrink.
Types of plant movements
| Type of movement | Explanation | Example |
|---|---|---|
| Independent of growth | Immediate response; cells change shape by changing the amount of water in them. | Leaves of touch-me-not plant fold when touched. |
| Dependent on growth | Tropic or directional movements caused by different growth rates in different parts of a plant. | Shoots growing towards light. |
Tropic movements
Tropic movements are directional movements that may occur towards or away from a stimulus.
- Phototropism: Growth or movement in response to light. Shoots usually grow towards light (positive phototropism); roots generally grow away from light (negative phototropism).
- Geotropism: Growth or movement in response to gravity. Roots usually grow towards gravity (positive geotropism), while shoots grow away from gravity (negative geotropism).
- Chemotropism: Growth in response to chemicals. Example: the pollen tube grows towards the ovule.
- Hydrotropism: Growth or movement in response to water. Roots generally grow towards water.
Tendril movement
When a tendril comes in contact with an object, the part away from the object grows more rapidly than the part near the object. This causes the tendril to coil around the object.
6 How Is Information Communicated in Multicellular Organisms?
Limitations of electrical impulses
- Electrical impulses can reach only those cells that are connected by nervous tissue.
- After sending an impulse, a cell needs some time to reset before it can send another impulse.
Chemical signalling
Because of these limitations, most multicellular organisms also use chemical signals for communication. A cell releases a chemical signal that can travel to nearby cells. Cells with specific molecules called receptors on their surfaces can detect the chemical and respond to it.
- Chemical signalling is slower than electrical impulses, but it can reach cells that are not directly connected by nerves.
- Chemical signals can provide communication for a longer duration.
- Hormones are chemical messengers that help control and coordinate activities in multicellular organisms.
- Plant hormones coordinate growth, development and responses to the environment. They may be produced in one part of a plant and transported to another part where they are needed.
Plant hormones
| Plant hormone | Function |
|---|---|
| Auxin | Synthesised mainly at the shoot tip. Promotes cell growth and is involved in phototropism. |
| Gibberellin | Promotes the growth of the stem and supports other growth processes. |
| Cytokinins | Promote cell division and are present in higher concentrations in fruits and seeds. |
| Abscisic acid | Inhibits growth and may cause wilting and falling of leaves. Also called the stress hormone. |
7 Hormones in Animals
Endocrine system
The endocrine system consists of several endocrine glands. A ductless gland is called an endocrine gland because it releases its secretions directly into the bloodstream. The chemical messengers released by endocrine glands are called hormones.
Hormones are secreted in very small amounts by specialised tissues. They travel through the blood and act on particular target tissues or organs, which may be far away from the gland that produced them. Hormonal control is generally slower than nervous control, but its effects may last longer and reach many parts of the body.
Exocrine glands
Exocrine glands release their secretions through ducts. These ducts open onto an epithelial surface or into a body cavity. For example, digestive glands release their secretions through ducts.
Major endocrine glands in humans
| Gland | Location / description | Hormone and function |
|---|---|---|
| Pituitary gland | Pea-sized gland at the base of the brain. Often called the master gland. | Secretes growth hormone (GH). Under-secretion in children causes dwarfism; over-secretion causes gigantism. |
| Thyroid gland | Butterfly-shaped gland in the neck. | Secretes thyroxine, which regulates body metabolism. Iodine is required for its synthesis; iodine deficiency leads to goitre. |
| Pancreas | Leaf-shaped gland behind the stomach in the abdomen. Acts as both endocrine and exocrine gland. | Produces insulin and glucagon to regulate blood sugar. As an exocrine gland, releases digestive enzymes. Insufficient insulin secretion leads to diabetes. |
| Adrenal glands | Occur in pairs above each kidney. | Secrete adrenaline, which prepares the body for emergency situations (fight-or-flight response). |
| Gonads | Gamete-producing organs: testes in males and ovaries in females. | Testes produce testosterone; ovaries produce oestrogen and progesterone. They regulate gamete formation, secondary sexual characteristics and pregnancy maintenance. |
8 Feedback Mechanisms
The body must release hormones in the correct amount and at the correct time. The levels and timing of hormone release are controlled by feedback mechanisms.
- Hormones must be released in suitable amounts to perform their functions properly.
- Feedback mechanisms monitor changes inside the body.
- These mechanisms adjust hormone secretion according to the needs of the body.
Example: Regulation of blood sugar level
- When the blood sugar level rises, specialised cells in the pancreas detect the change.
- In response, the pancreas produces more insulin.
- Insulin helps body cells take up glucose, causing the blood sugar level to fall towards normal.
- When the blood sugar level returns towards normal, the pancreas reduces insulin secretion.
✓ Quick Revision – Important Terms
| Term | Meaning |
|---|---|
| Stimulus | A change in the environment to which an organism responds. |
| Receptor | A specialised cell that receives information from the surroundings. |
| Neuron | Basic structural and functional unit of the nervous system. |
| Nerve impulse | An electrical signal that travels through a neuron. |
| Synapse | Small junction or gap between two neurons. |
| Reflex action | A quick and automatic response to a stimulus. |
| Reflex arc | Pathway followed by a nerve impulse during a reflex action. |
| Tropic movement | Directional plant movement in response to a stimulus. |
| Hormone | A chemical messenger that helps control and coordinate activities. |
| Endocrine gland | A ductless gland that releases its secretions directly into the bloodstream. |
| Feedback mechanism | A mechanism that monitors internal changes and adjusts hormone secretion. |
Important differences at a glance
| Nervous control | Hormonal control |
|---|---|
| Uses electrical impulses along nervous tissue. | Uses chemical messengers called hormones. |
| Generally rapid. | Generally slower. |
| Impulses reach cells connected by nervous tissue. | Hormones travel through blood to target tissues or organs. |
| Effects are generally short-lived. | Effects may last longer. |
Show a short chapter recap
Animals coordinate activities through the nervous system and hormones. Neurons carry nerve impulses; reflex arcs enable quick automatic responses. The brain and spinal cord form the central nervous system. Plants respond to stimuli through movements that may or may not involve growth. Plant and animal hormones coordinate growth and other activities. Feedback mechanisms regulate hormone secretion, as shown by blood sugar control through insulin.
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