Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Plants have indeterminate growth because of undifferentiated cells in meristems, which can be apical (at the tips of roots and shoots) or lateral (within the stem).
Mitosis and cell division in the shoot apical meristem provide the cells needed for the extension of the stem and the development of leaves.
Plant hormones, specifically auxin (Indole-3-acetic acid or ), control growth by regulating cell elongation and changing patterns of gene expression.
Auxin efflux carriers, known as PIN proteins, establish concentration gradients of within plant tissues to control the direction of growth.
In phototropism, light causes auxin to move to the shaded side of the stem, promoting elongation and causing the plant to bend toward the light source.
The 'Acid Growth Hypothesis' explains that auxin stimulates proton pumps to transport ions into the cell wall, lowering the and activating enzymes called expansins.
Micropropagation is a technique used to produce large numbers of identical plants from stock plant tissues using sterile nutrient agar and specific ratios of auxin and cytokinin.
Plant cells are totipotent, meaning a single differentiated cell can still divide and differentiate into any cell type to generate a whole new plant.
📐Formulae
💡Examples
Problem 1:
Explain the role of ions in the elongation of plant cells during a phototropic response.
Solution:
Auxin () accumulates on the shaded side of the shoot. It triggers the activation of proton pumps in the plasma membrane, which move ions into the cell wall. This lowers the of the cell wall to approximately . The acidity activates expansin proteins, which break the hydrogen bonds between cellulose microfibrils. This increases the plasticity of the wall, allowing turgor pressure to expand the cell.
Explanation:
This mechanism demonstrates how a chemical signal (auxin) results in a physiological change (cell wall loosening) via manipulation and enzymatic activity.
Problem 2:
A researcher is using micropropagation to clone a rare orchid. If the ratio of is high, what part of the plant will primarily develop?
Solution:
A high ratio will promote the development of roots ().
Explanation:
In plant tissue culture, the balance of hormones determines the pathway of differentiation. A high ratio of auxin to cytokinin promotes root growth, whereas a low ratio (high cytokinin) promotes shoot growth (caulogenesis).