Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Plastids are double-membrane bound organelles found in all plant cells and in euglenoids. They are easily observed under the microscope as they are large in size.
Based on the type of pigments, plastids are classified into three types: Chloroplasts, Chromoplasts, and Leucoplasts.
Chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy () essential for photosynthesis.
Chromoplasts contain fat-soluble carotenoid pigments like carotene and xanthophylls, giving parts of the plant a yellow, orange, or red color.
Leucoplasts are colorless plastids that store nutrients: Amyloplasts store carbohydrates (starch), Elaioplasts store oils and fats, and Aleuroplasts store proteins.
The chloroplast is divided into the outer membrane and inner membrane. The space limited by the inner membrane is called the stroma.
Inside the stroma, organized flattened membranous sacs called thylakoids are present. Thylakoids are arranged in stacks like piles of coins called grana (singular: granum).
Stroma contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes ().
Chloroplasts are considered semi-autonomous organelles because they possess their own genetic material and protein-synthesizing machinery ( ribosomes).
📐Formulae
💡Examples
Problem 1:
A student observes a colorless plastid in a potato tuber cell that turns blue-black when treated with Iodine solution. Identify the plastid and the substance it stores.
Solution:
The plastid is an Amyloplast, and it stores starch.
Explanation:
Leucoplasts are colorless plastids. Amyloplasts are a specific type of leucoplast that store carbohydrates (starch). The blue-black color with Iodine is a positive test for the presence of starch.
Problem 2:
Calculate the difference in Svedberg units () between the ribosomes found in the stroma of a chloroplast and those found in the surrounding cytoplasm of a typical mesophyll cell.
Solution:
Explanation:
Cytoplasmic ribosomes in eukaryotic plant cells are , while the ribosomes found within the chloroplast (stroma) are , similar to prokaryotic ribosomes. The difference is .
Problem 3:
If a mesophyll cell has chloroplasts and each chloroplast contains grana, how many total grana are present in the cell?
Solution:
Explanation:
The total number of grana is calculated by multiplying the number of chloroplasts by the number of grana per chloroplast: .