Cells and control Study Guide

A reflex arc is a neural pathway responsible for an involuntary, rapid response to a stimulus. It does not involve conscious thought as the brain is bypassed or involved only after the response. The pathway typically involves three types of neurones: a sensory neurone, a relay neurone, and a motor neurone.

When a stimulus is detected by a receptor, a nerve impulse is generated and transmitted along a sensory neurone. This neurone carries the impulse from the receptor towards the central nervous system (CNS), specifically to the spinal cord. Within the spinal cord, the sensory neurone synapses with a relay neurone. The relay neurone, an interneurone, then processes the information and transmits the impulse to a motor neurone. In some reflex arcs, a relay neurone may not be present, and the sensory neurone synapses directly with the motor neurone. The motor neurone carries the nerve impulse from the CNS to an effector organ, such as a muscle or gland. The effector then produces the response, for example, contracting a muscle to withdraw a hand from a hot object. This rapid, automatic sequence protects the body from harm.

Mitosis is a fundamental biological process wherein a single eukaryotic cell divides to produce two genetically identical daughter cells. This process is crucial for several key biological functions. For growth, multicellular organisms increase in size by producing more cells through mitosis. Instead of cells simply getting larger, which would limit their surface area to volume ratio for efficient transport, new cells are generated. Repair of damaged tissues and replacement of old or worn-out cells also rely on mitosis, as new, identical cells are needed to substitute those that are no longer functional or have been lost. Furthermore, mitosis underpins asexual reproduction, allowing organisms to produce offspring without the involvement of gametes, resulting in genetically identical clones of the parent.