Comprehensive theory, key formulas, diagrams, and memory aids for Control and Coordination.
In the previous chapter, we discussed life processes involved in maintenance functions. But living organisms must also respond to their environment. If you touch a hot pan, you immediately pull your hand away. If a plant is kept near a window, it bends towards the light. These movements, in response to stimuli, must be carefully coordinated. In animals, this is achieved by the nervous and endocrine (hormone) systems. In plants, which lack a nervous system, chemical coordination takes center stage.
The nervous system consists of a highly organized network of specialized cells called neurons (nerve cells). It is designed for receiving information from the environment (through receptors), processing it, and responding to it (via effectors like muscles or glands).
The neuron is the structural and functional unit of the nervous system. It has three main parts: 1. Dendrites: Short, branched fibers extending from the cell body. They acquire information (receive signals). 2. Cell Body (Soma): Contains the nucleus and cytoplasm. 3. Axon: A long fiber that transmits the electrical impulse away from the cell body to the next neuron. * Synapse: The microscopic gap between the nerve ending of one neuron and the dendrite of the next. Electrical impulses cannot jump this gap; instead, chemicals called neurotransmitters are released, which cross the synapse and start a similar electrical impulse in the next neuron.
A reflex action is a sudden, involuntary, and almost instantaneous movement in response to a stimulus (e.g., pulling your hand away from a flame). * Reflex Arc: The pathway taken by nerve impulses in a reflex action. * Receptor (e.g., skin detects heat) → Sensory Neuron → Spinal Cord (Relay Neuron) → Motor Neuron → Effector (e.g., muscle in arm contracts). * Why do we have reflex arcs? The thinking process of the brain is not fast enough for emergencies. The reflex arc is formed in the spinal cord itself to ensure an immediate response, protecting the body from harm.
The brain is the highest coordinating center in the body. The brain and spinal cord together constitute the Central Nervous System (CNS). The nerves connecting the CNS to the rest of the body constitute the Peripheral Nervous System (PNS).
The brain has three major parts: 1. Fore-brain: The main thinking part of the brain. It consists of the cerebrum, which is responsible for voluntary actions, memory, learning, and sensory reception (hearing, smell, sight). 2. Mid-brain: Connects the fore-brain and hind-brain. It controls involuntary actions like the change in pupil size and reflex movements of the head and neck. 3. Hind-brain: Consists of: * Pons: Regulates respiration. * Medulla: Controls involuntary actions like blood pressure, salivation, and vomiting. * Cerebellum: Responsible for the precision of voluntary actions and maintaining the posture and balance of the body (e.g., riding a bicycle, walking in a straight line).
Protection: The brain is a delicate organ protected by a bony box (the skull) and is surrounded by cerebrospinal fluid, which provides further shock absorption.
Plants do not have a nervous system or muscles. How do they respond to stimuli? They use chemical coordination (plant hormones or phytohormones) and change cellular water content to effect movement.
These are chemical compounds synthesized in tiny amounts that help coordinate growth, development, and responses to the environment. * Auxin: Synthesized at the shoot tip. It helps cells grow longer. When light comes from one side, auxin diffuses to the shady side, causing those cells to grow longer and the plant to bend towards the light. * Gibberellins: Help in the growth of the stem. * Cytokinins: Promote cell division. They are present in greater concentrations in areas of rapid cell division (fruits, seeds). * Abscisic Acid (ABA): The 'stress hormone'. It inhibits growth. Its effects include the wilting of leaves and closing of stomata during drought.
While the nervous system provides rapid, point-to-point coordination, it is short-lived and cannot reach every single cell. To provide sustained and widespread coordination, animals use the Endocrine System.
Endocrine glands are ductless glands that secrete chemicals called hormones directly into the bloodstream, which carries them to specific target organs.
Hormones must be secreted in precise quantities. The timing and amount of hormone released are regulated by feedback mechanisms. For example, if blood sugar levels rise, they are detected by the cells of the pancreas, which respond by producing more insulin. As the blood sugar level falls, insulin secretion is automatically reduced.
To survive, organisms must constantly adjust to their surroundings. Animals employ a dual system: a rapid, electrical nervous system centered around the brain and spinal cord for immediate reactions, and a slower, chemical endocrine system for sustained physiological changes. Plants rely entirely on chemical hormones and water-driven cellular changes for their tropic and nastic movements. Together, control and coordination ensure that an organism functions as a unified whole rather than a chaotic collection of individual cells.