Comprehensive Biology Notes: Coordination, Response, and Homeostasis

Coordination and Response: The Nervous System

  • Fundamental Coordination and Regulation: The nervous system is responsible for the coordination and regulation of body functions.
  • Electrical Impulses: These are the signals that travel along neurones to facilitate communication within the body.
  • Types of Neurones:
    • Sensory Neurone: Carries impulses from sense receptors toward the Central Nervous System (CNS).
    • Relay Neurone: Carries nervous impulses within the CNS to connect sensory and motor neurones.
    • Motor Neurone: Carries nervous impulses from the CNS to the effector.
  • Effectors: Any organ, muscle, or gland that carries out a physical response to a physical or chemical change in the environment.
    • Examples: Muscles and glands.

The Reflex Arc and Synaptic Transmission

  • Reflex Arc Process:
    • Stimulus Detection: A receptor detects a stimulus, such as heat from a flame.
    • Trigger: Once activated, the receptor triggers an electrical impulse.
    • Sensory Path: The impulse travels along the sensory neurone to the spinal cord.
    • CNS Processing: The electrical impulse passes across a synapse to the relay neurone.
    • Motor Path: The impulse passes across another synapse to the motor neurone.
    • Response: The motor neurone passes the electrical impulse to an effector, which reacts (e.g., muscle contraction to remove the organism from danger).
  • Reflex Action: An automatically and rapidly integrating process coordinating stimuli with effector responses (muscles and glands).
  • Synapse Definition: A junction between two neurones where a chemical is released by one neurone and picked up by the next.
  • Synapse Structure:
    • Vesicles: Tiny membrane-bound sacs containing neurotransmitter molecules.
    • Synaptic Gap: The physical space between neurones.
    • Receptor Proteins: Located on the receiving neurone; designed to fit specific neurotransmitters like a "lock and key."
  • Events at a Synapse:
    • Stimulation: An impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.
    • Diffusion: Neurotransmitter molecules diffuse across the gap due to a concentration gradient (high concentration near the sending neurone, low concentration near the receiving neurone).
    • Binding: Molecules bind with receptor proteins on the next neurone.
    • New Impulse: An impulse is then stimulated in the next neurone.
    • Directionality: Synapses ensure that impulses travel in one direction only.

Sense Organs: The Eye and Vision

  • Sense Organs Definition: Groups of receptor cells responding to specific stimuli, including light, sound, touch, temperature, and chemicals.
  • Anatomy and Function of the Eye:
    • Pupil: Controls how much light enters the eye.
    • Retina: Contains light-sensitive receptors.
    • Optic Nerve: Carries impulses from the eye to the brain.
    • Pupil Reflex: An involuntary reflex action that protects the retina from damage in bright light by changing the pupil diameter.
  • Light Intensity and Pupil Response:
    • Pupil diameter is inversely proportional to light intensity.
    • High Intensity: Pupil diameter decreases (constricts) to reduce light entering the eye and protect the retina.
    • Low Intensity: Pupil diameter increases (dilates) to allow more light in and improve vision.
  • Iris Muscle Dynamics:
    • Circular Muscles: Contract to constrict the pupil (make it small).
    • Radial Muscles: Contract to dilate the pupil (make it large).
    • Antagonistic Pair: These muscles work against each other.
    • RAD Mnemonic: Radial Antagonizes in Dim light (Radial muscles contract in low light).
  • Accommodation: The process by which the lens changes shape to focus clearly on objects at varying distances.
    • Distant Objects: Ciliary muscles relax; Suspensory ligaments become taut; Lens becomes thin (less convex); Bends light less strongly; Light rays are almost parallel.
    • Near Objects: Ciliary muscles contract; Suspensory ligaments become slack (loose); Lens becomes fat (more convex); Bends light more strongly; Light rays must be bent sharply.

Photoreceptor Cells and Distribution

  • Photoreceptor Cells: Cells that detect light stimuli.
    • Rod Cells: Responsible for night vision; highly sensitive to light.
    • Cone Cells: Responsible for daytime and color vision; three types: red, green, and blue; less sensitive to light; provide sharpest/most detailed vision.
  • Cell Distribution:
    • Fovea (Center): High concentration of cones packed together for maximum visual acuity and color vision; Rods are absent.
    • Periphery (Outer edges): High concentration of rods; low concentration of cones.
    • Optic Disc (Blind Spot): Both rods and cones are absent.

Hormonal Control

  • Hormone Definition: A chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
  • Adrenaline: Secreted by endocrine glands in "fight or flight" situations.
    • Effects: Increased breathing rate, increased heart rate, and increased pupil diameter.
    • Blood Glucose Impact: Adrenaline travels to the liver and triggers liver cells to break down stored glycogen into glucose, releasing it into the bloodstream.
    • Heart Impact: Directly stimulates the heart to beat faster.
  • Nervous vs. Hormonal Control:
    • Speed of Action: Nervous control is very fast/instantaneous (electrical impulses); Hormonal control is slower (must travel through the circulatory system).
    • Duration of Effect: Nervous control is short-lived (stops once impulse passes); Hormonal control is long-lasting (continues until broken down by liver and excreted by kidneys).

Homeostasis and Blood Glucose Regulation

  • Homeostasis Definition: The maintenance of a constant internal environment.
  • Negative Feedback: The body's response to work against an initial change to return conditions back to a set point (ideal normal value).
    • Human Set Point: Human body temperature is approximately 37∘C37^{\circ}C.
  • Blood Glucose Concentration (BGC) Control:
    • When BGC is too high (after a meal): Pancreas detects the rise and secretes insulin. Insulin travels to the liver, stimulating cells to absorb glucose from the blood and convert it into insoluble glycogen.
    • When BGC is too low (during exercise): Pancreas detects the drop and secretes glucagon. Glucagon travels to the liver, stimulating cells to break down insoluble glycogen back into soluble glucose.
  • Glucagon vs. Glycogen:
    • Glucagon: The hormone (think: "glucose is gone, you need glucagon").
    • Glycogen: The substance stored in the liver.
  • Type 1 Diabetes: Caused by an autoimmune response where antibodies attack insulin-producing cells in the pancreas.
    • Treatment: Injecting insulin, monitoring blood glucose levels, regulating carbohydrate (sugar) intake, and regular exercise.

Thermoregulation

  • Managing High Internal Temperature (Too Hot):
    • Brain Role: Detects high blood temperature.
    • Vasodilation: Smooth muscle in the walls of the arterioles relaxes, the lumen dilates, allowing more blood to flow through capillaries near the skin surface.
    • Sweating: Starts to cool the body.
    • Insulation: Hair erector muscles relax, causing hair to flatten.
  • Managing Low Internal Temperature (Too Cold):
    • Brain Role: Detects low blood temperature.
    • Vasoconstriction: Smooth muscle in the walls of the arterioles contracts, the lumen narrows, allowing less blood to flow through surface capillaries.
    • Shivering: Starts to generate heat.
    • Insulation: Hair erector muscles contract, causing hair to stand upright (goosebumps) to trap heat.

Tropic Responses in Plants

  • Definitions:
    • Gravitropism: A response in which plant parts grow toward or away from gravity.
    • Phototropism: A response in which plant parts grow toward or away from a light source.
  • Response Patterns:
    • Roots: Positively gravitropic (grow down) and negatively phototropic.
    • Shoots: Negatively gravitropic (grow up) and positively phototropic (grow toward light).
  • Role of Auxin in Shoot Growth:
    • Production: Auxin is made in the shoot tips.
    • Movement: Auxin diffuses from the tip through the plant.
    • Distribution: Auxin is unequally distributed in response to light and gravity.
    • Action: Auxin stimulates cell elongation, causing the plant to grow in a specific direction.