In this chapter you need to become familiar with the following concepts:
- homeostasis (maintaining a stable internal environment in terms of temperature, hydration, pH, blood pressure and volume, oxygen and carbon dioxide concentration in the blood and the concentration of various ions, including sodium and glucose).
- stimulus-response and sensors-effectors
- positive and negative feedback mechanisms
- glands and hormones, including phermones
- signal transduction and signaling molecules
it is important to understand that lipid soluble hormones (lipophilic or hydrophobic) can pass through the cell membrane directly, but water-soluble hormones (hydrophilic or lipophobic) bind to a receptor molecule embedded in the cell membrane, which results in the activation of a secondary protein or messenger. This secondary messenger causes the cell to initiate a response. Signal transduction is a series of events (sometimes called a ‘cascade’) that changes the signal received by the cell.
“The growth and development of a plant are influenced by genetic factors, external environmental factors, and chemical hormones inside the plant. Plants respond to many environmental factors such as light, gravity, water, inorganic nutrients, and temperature.” ~ Biology Online ~ Growth and Plant Hormones.
How does a seed ‘know’ when to germinate? How does a tree ‘know’ when to start changing leaf colour and drop it’s leaves for winter? How does a plant ‘know’ which direction to grow leaves and roots and where the light is? How does a plant ‘know’ when to produce flowers and what colour they should be? Instead of having a brain, central nervous system and endocrine system that co-ordinate communication, every plant cell can produce hormones (growth substances) that are transported throughout the plant via diffusion from cell to cell and through the xylem and phloem. (How mobile are plant hormones?).
Botanists recognize five major groups of hormones:
- abscisic acids
Although plants usually appear not to move, time-lapse photography shows that plants do, in fact, move and sometimes quite quickly. There are different types of movement, tropisms and nastic movements:
- geotropism (or gravitropism)
- thigmonastic movements
- seasonal responses
Draw up a table with three columns, with each of the key concepts above in the first column, a definition in the second column and an example in the third column. You can get your information from these resources:
Use digital tools to create a labelled image showing the different plant hormones, where they act (leaves, stems, roots, fruits, flowers etc) and what they do. Copy and paste a link to the image in the comment section below.