Q1.
Animals can obtain food from their surroundings, however, plants synthesise food in the presence of sunlight. But where do plants synthesise their food and obtain energy for cellular activities?
Answer
Plants make food in chloroplasts, a type of plastid, and then release energy from that food in mitochondria, exactly as animals do.
sunlight + CO₂ + H₂O → (in the chloroplast, photosynthesis) → sugar
sugar + O₂ → (in the mitochondrion, cellular respiration) → ATP + CO₂ + H₂O
sugar + O₂ → (in the mitochondrion, cellular respiration) → ATP + CO₂ + H₂O
- Inside the chloroplast is a semi-fluid substance, the stroma. Within it are disc-shaped membrane structures containing chlorophyll, the green pigment that absorbs sunlight.
- The sugars made during photosynthesis are stored in the stroma along with starch granules.
- Chloroplasts are double-membrane-bound, just like mitochondria.
- Other plastids do other jobs: chromoplasts carry yellow, orange or red pigments; leucoplasts have no pigment and store starch, oils or proteins.
Why it happens: the question hides a common mistake — that plants use sunlight directly to run their cells. They cannot. Sunlight is only captured, and its energy is stored in the chemical bonds of sugar. Every activity of a plant cell, day or night, root or leaf, is then paid for with ATP made by mitochondria breaking that sugar down. That is why a plant cell contains both organelles, and why a root cell — which never sees light and has no chloroplasts — still has plenty of mitochondria.
Did you know? Both mitochondria and plastids have their own DNA and ribosomes and can make some of their own proteins — features shared with bacteria, which suggests both organelles share an evolutionary history with single-celled organisms.