AQA Biology A-Level: Organisms Respond to Changes

Stimuli and Response

• Stimulus: Change in internal/external environment.
  • Receptor: Detects stimulus.
  • Coordinator: Formulates response.
  • Effector: Produces response.
• Receptors:
  • Specific to one type of stimulus.
• Nerve Impulses vs. Hormones:
  • Nerve impulses: Rapid, short-lived, localized.
  • Hormones: Slow, long-lasting, widespread.
• Plants: Use hormone-like substances.

3.6.1 Detection and Response to Stimuli

3.6.1.1 Survival and Response

• Organism Response: Increases survival by adapting to environmental changes.
• Flowering Plants:
  • Growth factors regulate growth in response to stimuli (e.g., IAA & cell elongation).
  • Gravitropism & phototropism explanations.
• Simple Responses:
  • Taxes & kineses maintain favorable environments.
  • Simple reflexes (three-neurone reflex).
• Skills Development:
  • Investigating effects of IAA on root growth.

3.6.1.2 Receptors

• Pacinian Corpuscle:
  • Example of a receptor: Responds to specific stimuli.
  • Establishes a generator potential.
• Structure:
  • Deformation of sodium ion channels leads to generator potential.
• Human Retina:
  • Differences in light sensitivity, color sensitivity, visual acuity due to rods and cones.
• Skills Development:
  • Investigate temperature, touch, and resolution of receptors in skin.

3.6.1.3 Control of Heart Rate

• Heart Stimulation: Myogenic, via SAN, AVN, Purkyne tissue.
• Chemoreceptors & Pressure Receptors: Control heart rate via autonomic nervous system.
• Skills Development:
  • Investigate effects on pulse rate.
  • Calculate cardiac output using CO = RV.

3.6.2 Nervous Coordination

3.6.2.1 Nerve Impulses

• Motor Neurone Structure:
  • Myelinated vs. non-myelinated.
• Resting Potential:
  • Differential permeability, electrochemical gradients.
• Action Potential:
  • Depolarization, all-or-nothing principle.
  • Refractory period and impulse transmission.
• Conduction Speed: Influenced by myelination, axon diameter, temperature.
• Skills Development:
  • Calculate impulse conduction frequency.

3.6.2.2 Synaptic Transmission

• Synapse & Neuromuscular Junction:
  • Detailed structure.
• Transmission Events:
  • Unidirectionality, summation, inhibition.
• Drug Effects:
  • Predict and explain drug effects on synapses.

3.6.3 Skeletal Muscles

• Muscle Structure:
  • Antagonistic pairs, myofibrils.
• Contraction:
  • Roles of actin, myosin, calcium ions, ATP.
• Muscle Fibre Types: Slow and fast.
• Skills Development:
  • Examine muscle slides.
  • Investigate muscle fatigue.

3.6.4 Homeostasis

3.6.4.1 Principles of Homeostasis

• Physiological Control Systems:
  • Maintain internal environment.
• Importance:
  • Stable temperature, blood pH, glucose concentration.
  • Negative feedback for system restoration.

3.6.4.2 Control of Blood Glucose

• Glucose Regulation:
  • Liver roles in glycogenesis, glycogenolysis, gluconeogenesis.
  • Insulin & glucagon action.
  • Adrenaline's role and second messenger model.
• Diabetes:
  • Causes of type I & II, control methods.
• Skills Development:
  • Evaluate health positions.
  • Glucose concentration investigation.

3.6.4.3 Control of Blood Water Potential

• Osmoregulation:
  • Hypothalamus, pituitary, ADH roles.
• Nephron Function:
  • Glomerular filtrate, reabsorption processes.
• Skills Development:
  • Investigate nephron roles and osmoregulation.

These notes summarize key concepts covered in the AQA A-Level Biology syllabus related to organisms responding to changes in their environments. The topics include stimuli response, nervous coordination, muscular contraction, and homeostasis, providing a comprehensive overview for A-Level studies.