Integration of body systems — IB Diploma Biology HL

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. System integration and the hierarchy of subsystems

   System integration is the coordination of the component parts of a system so that they collectively perform an overall function. In a multicellular organism, cells, tissues, organs and body systems form a hierarchy of subsystems. Nerves and hormones send signals between organs, and the blood transports materials and energy between them, giving emergent properties.

2. The brain, spinal cord, neurons and nerves

   The brain is the central information integration organ: it processes information combined from several inputs and is the site of learning and memory. The spinal cord is an integrating centre for unconscious processes. Sensory neurons carry messages from receptor cells to the central nervous system, motor neurons carry impulses to muscles, and nerves are bundles of both.

3. Pain reflex arcs and the cerebellum

   A pain reflex arc is an involuntary response with skeletal muscle as the effector. A free nerve ending of a sensory neuron in the hand detects pain, the impulse passes to a single interneuron in the grey matter of the spinal cord, then to a motor neuron that makes a muscle contract. The cerebellum coordinates skeletal muscle contraction and balance.

4. Melatonin, epinephrine and control by the hypothalamus and pituitary

   Melatonin is a hormone secreted by the pineal gland in a diurnal pattern: secretion rises in the evening darkness and falls in the morning light, helping to establish a cycle of sleeping and waking. Epinephrine from the adrenal glands prepares the body for vigorous activity. The hypothalamus and pituitary gland control the rest of the endocrine system.

5. Feedback control of heart rate and ventilation rate

   Heart rate and ventilation rate are controlled by negative feedback. Baroreceptors monitor blood pressure and chemoreceptors monitor blood pH and the concentrations of oxygen and carbon dioxide. The medulla coordinates the response, sending nerve impulses to the heart to change heart rate and stroke volume, and to the diaphragm and intercostal muscles to change ventilation rate.

6. Control of peristalsis by the CNS and enteric nervous system

   Peristalsis is the wave of muscle contraction that moves food along the gut. Swallowing of food and egestion of faeces are under voluntary control by the central nervous system, but peristalsis between these points is under involuntary control by the enteric nervous system, a network of neurons in the gut wall that coordinates the passage of material.

7. Tropic responses in seedlings and positive phototropism

   PracticalHL only

   Positive phototropism is a directional growth response to lateral light in plant shoots: the shoot grows towards the light. Tropic responses can be observed in seedlings and recorded qualitatively with diagrams or quantitatively by measuring the angle of curvature. Precision, accuracy and reliability are improved by careful measurement, large samples and repeats.

8. Phytohormones, auxin efflux carriers and cell growth

   HL only

   Phytohormones are signalling chemicals that control growth, development and response to stimuli in plants. Auxin diffuses freely into plant cells but not out, so auxin efflux carriers positioned on one side of each cell actively transport it through the tissue and concentrate it. Auxin promotes cell growth by causing hydrogen ion secretion that loosens cell walls.

9. Auxin and cytokinin interactions, and ethylene in fruit ripening

   HL only

   Root tips produce cytokinin, which is transported to shoots, and shoot tips produce auxin, which is transported to roots; interactions between them integrate root and shoot growth. Ethylene stimulates the changes in fruit ripening, and ripening stimulates more ethylene production. This positive feedback makes ripening rapid and synchronized.