Module 6: Response to Stimuli Revision Notes

This page introduces the concept of stimuli and why organisms need to respond to them. It covers simple organism responses and plant tropisms.

Definition: A stimulus is a change in the internal or external environment that organisms respond to for survival.

Highlight: Simple organisms respond to stimuli through taxis (directional) and kinesis (non-directional) movements.

Plants respond to stimuli through tropisms, which are directional growth responses controlled by the plant growth factor Indoleacetic Acid (IAA), also known as auxin.

Example: Shoots exhibit positive phototropism (growth towards light) and negative geotropism (growth away from gravity), while roots show positive geotropism and positive hydrotropism (growth towards water).

The page explains the mechanism of positive phototropism and negative geotropism in shoots, detailing how IAA redistribution causes differential growth rates on opposite sides of the plant stem.

Vocabulary: IAA (Indoleacetic Acid) - A plant hormone that promotes growth in shoots and inhibits growth in roots.

Sensory Receptors: Pacinian Corpuscle and Retina

This page provides detailed information on the structure and function of two important sensory receptors: the Pacinian corpuscle and the retina of the eye.

Definition: The Pacinian corpuscle is a touch receptor found in the skin, fingers, and toes that responds to pressure.

The structure and process of the Pacinian corpuscle's response to pressure are explained, involving the opening of stretch-mediated Na+ channels.

The retina's role in detecting light and generating visual information is described, focusing on the two types of photoreceptor cells: cone cells and rod cells.

Vocabulary:

• Cone cells: Detect high light intensity, produce color images, and provide high visual acuity.
• Rod cells: Can detect low light intensity, produce black and white images, and have low visual acuity.

The properties of cone and rod cells are compared, including their pigments (iodopsin and rhodopsin), light sensitivity, and connections to neural pathways.

Highlight: The arrangement of cone cells (one-to-one connections with neurons) allows for high visual acuity, while the convergence of multiple rod cells onto single neurons enables detection of low light intensities through summation.