WJEC (Wales) Biology GCSE Topic 2.5: Regulation and Response Study Guide

Structure and Function of the Nervous System

• Composition of the Nervous System:     * The nervous system consists of the Central Nervous System (CNS), which comprises the brain and the spinal cord.     * It also includes specialised nerves that function to carry information as impulses into and out of the CNS.
• Mechanism of Control:     * The nervous system regulates movement by transmitting electrical signals, known as nerve impulses, through a network of specialised nerve cells.
• Neurones:     * These are considered the 'functional units' of the nervous system.     * Neurones allow an organism to react rapidly to both internal and environmental changes.
• Types of Neurones:     * Sensory Neurone: Responsible for carrying impulses from receptors (sense organs) to the CNS.     * Relay Neurone: Located within the CNS; it carries impulses from sensory neurones to motor neurones.     * Motor Neurone: Carries impulses from the CNS to effectors, which include muscles and glands.
• The Synapse:     * A synapse is defined as a small gap existing between neurones.     * Nerve impulses are transmitted across this gap via chemical messengers called neurotransmitters.

Sense Organs and Reflex Actions

• Sense Organs:     * These consist of groups of receptor cells designed to detect specific stimuli (changes in the environment or internal conditions, such as temperature and sound).     * Information is sent from these organs to the CNS along neurones.     * The eye is a primary example of a sense organ that responds to the stimulus of light.
• Reflex Actions:     * A reflex is an automatic, involuntary response to a stimulus by the body.     * It does not involve the conscious portion of the brain and functions primarily as a protective mechanism.
• Examples of Reflexes:     * Withdrawal Reflex: Pulling away from a stimulus; for instance, initiated when touching a hot object to prevent burns.     * Pupil Reflex: The constriction of pupils to prevent damage to the eye when exposed to bright light.     * Blink Reflex: Provides protection for the eyes against foreign bodies.
• The Reflex Arc:     * The reflex arc is the specific nerve pathway involved in a reflex action.     * It requires five essential components: Stimulus, Receptor, Coordinator, Effector, and Response.     * Coordinator Function: This component coordinates information from the receptors and transmits the resulting impulses to the effectors.     * Pathway Sequence: Stimulus $\rightarrow$ Receptor $\rightarrow$ Sensory neurone $\rightarrow$ Relay neurone $\rightarrow$ Motor neurone $\rightarrow$ Effector $\rightarrow$ Response.

Structure and Function of the Eye

• Components of the Eye:     * Lens: A transparent bi-convex structure that is flexible. Through a process called accommodation, it changes shape to refract light and focus it specifically onto the retina.     * Pupil: The hole located in the centre of the iris that allows light to enter the interior of the eye.     * Iris: A pigmented ring composed of circular and radial muscles. Its function is to control the size of the pupil, thereby altering the amount of light entering the eye.     * Cornea: The transparent outer covering of the eye that refracts light as it first enters.     * Retina: A light-sensitive layer containing photoreceptors. It converts light energy into neural signals sent to the brain via the optic nerve.     * Choroid: A black pigmented layer that absorbs light to prevent internal reflection within the eyeball.     * Sclera: An opaque, white protective outer layer. It is transparent at the front and serves to maintain the shape of the eyeball.     * Optic Nerve: Located at the back of the eye, it transmits nerve impulses from the retina to the brain.     * Blind Spot: The specific point where the optic nerve leaves the eye. It contains no photoreceptor cells.

Homeostasis and Hormonal Control

• Definition of Homeostasis:     * Homeostasis is the maintenance of a stable internal environment within the body, despite fluctuations in external and internal conditions.
• Importance:     * It ensures optimum conditions for enzymes and various cellular processes to function correctly.     * Key variables that must be maintained include temperature, blood glucose concentration, and water levels.
• Hormones:     * A hormone is a cell signalling molecule produced by endocrine glands.     * Hormones are released into the blood and travel to a target organ where they bind and initiate a specific response.

Control of Blood Glucose Concentration

• Rationale for Control:     * If blood glucose rises too high, the body faces a risk of dehydration.     * If blood glucose becomes too low, the rate of cellular respiration decreases.
• Role of the Pancreas:     * The pancreas secretes two primary hormones: insulin and glucagon.
• Hormonal Effects:     * Insulin: Causes liver and muscle cells to increase their uptake of glucose. It facilitates the conversion of glucose into glycogen (a storage molecule).     * Glucagon: Causes the breakdown of glycogen back into glucose in the liver, which is then released into the blood.
• Negative Feedback Mechanism:     * This is a corrective mechanism allowing only small fluctuations around a specific set point.     * When blood glucose exceeds the set point: The pancreas secretes insulin and stops producing glucagon. Liver cells convert glucose to stored glycogen, causing blood glucose to decrease back to normal levels.     * When blood glucose falls below the set point: The pancreas secretes glucagon and stops producing insulin. Liver cells convert glycogen into glucose for release into the blood, causing blood glucose to increase back to normal levels.

Diabetes

• Condition Overview: Diabetes occurs when the homeostatic control of blood glucose levels fails to work properly.
• Type 1 Diabetes:     * Cause: The immune system attacks and destroys insulin-producing cells, resulting in a pancreas that does not produce sufficient insulin.     * Treatment: Involves daily insulin injections at meal times, managing diet (limiting refined sugar intake), and regular testing of blood glucose levels.
• Type 2 Diabetes:     * Cause: The person develops insulin resistance, which is closely linked to obesity.     * Treatment: Involves managing diet, regular exercise, and drugs such as metformin.

Control of Body Temperature

• Biological Necessity:     * Enzymes work best at an optimum temperature of $37^\circ C$.     * Deviations from this temperature decrease the rate of enzyme-controlled reactions.
• Organ Involvement: The skin is the primary organ responsible for temperature control through negative feedback.
• Responses to Increased Temperature (Above $37^\circ C$):     * Vasodilation: Blood vessels near the skin surface dilate, allowing more blood to flow closer to the surface, resulting in greater heat loss to the surroundings.     * Sweating: Heat energy is used to evaporate sweat, increasing heat transfer from the skin to the environment.     * Erector Muscles: These muscles relax, causing hairs to lie flat.     * Shivering: No shivering occurs.
• Responses to Decreased Temperature (Below $37^\circ C$):     * Vasoconstriction: Blood vessels near the skin surface constrict, so less blood flows near the surface, reducing heat loss.     * Sweating: Little to no sweat is produced.     * Erector Muscles: These muscles contract, causing hairs to stand on end. This creates pockets of air between hairs that act as a layer of insulation.     * Shivering: Involuntary muscle contractions generate heat energy through respiration.

Lifestyle Choices and Impact

• Obesity: Directly increases the risk of developing type 2 diabetes.
• Drugs: These affect chemical processes within the body and can lead to damaging side effects.
• Alcohol: Known to decrease reaction times and can cause conditions such as liver damage and cardiovascular disease.

Plant Responses

• Plant Tropisms: These are growth responses to stimuli.     * Positive Tropism: Growth towards a stimulus.     * Negative Tropism: Growth away from a stimulus.
• Auxin: A plant hormone that controls tropisms. It stimulates growth in plant shoots but inhibits growth in plant roots.
• Phototropism (Response to Light):     * Shoots: Positively phototropic (grow towards light).     * Roots: Negatively phototropic (grow away from light).
• Gravitropism (Response to Gravity):     * Shoots: Negatively gravitropic (grow away from gravity/upwards).     * Roots: Positively gravitropic (grow towards gravity/downwards).
• Importance for Survival:     * Enables plants to respond to environmental changes.     * Shoot growth towards light maximises light absorption for photosynthesis.     * Root growth downwards increases the uptake of water and minerals from the soil and ensures the anchorage of the plant body.