Simulation · Biology · Class 11
Two membranes, two organelles
From the lesson Plastids in Cell: The Unit of Life. Change the values and watch what happens.
Two membranes, two organellesBiology · Class 11
The idea behind it
NCERT §8.5.5
- Plastids occur in all plant cells and in euglenoids. They are easy to see under the microscope because they are large and carry specific pigments that give plants their colours.
- Based on pigment type, plastids are chloroplasts, chromoplasts or leucoplasts.
- Chloroplasts contain chlorophyll and carotenoid pigments and trap light energy for photosynthesis.
- Chromoplasts contain fat-soluble carotenoids such as carotene and xanthophylls, which give yellow, orange or red colour.
- Leucoplasts are colourless storage plastids: amyloplasts store starch (as in potato), elaioplasts store oils and fats, and aleuroplasts store proteins.
- In green plants, the mesophyll cells of the leaves hold most of the chloroplasts. Their shape varies: discoid, oval, spherical, lens-shaped or even ribbon-like; they measure about 5–10 μm in length and 2–4 μm in width. A cell may have just one, as in Chlamydomonas, or 20–40, as in mesophyll.
- Chloroplasts are double-membraned, and the inner membrane is relatively less permeable. The space inside is the stroma, holding flattened sacs called thylakoids stacked into grana; flat membranous tubules called stroma lamellae link thylakoids of different grana.
- The stroma holds the enzymes for synthesising carbohydrates and proteins, and small, double-stranded circular DNA and ribosomes. Chlorophyll pigments are located in the thylakoids.
- Chloroplast ribosomes are 70S, smaller than the 80S cytoplasmic ribosomes.