Do plants respire, or do they only photosynthesise?
Short answer: All living plant cells respire all the time. In daylight photosynthesis just outpaces it.
The trap
The wrong idea
“Plants photosynthesise instead of respiring.”
It sounds reasonable, and a lot of NEET aspirants carry it into the exam. Here's a one-question check, then the explanation that makes it click.
Quick check
Try it before you peek
Plant cells carry out respiration:
- Only at night
- Day and night
- Never
Show the answer
B. Day and night
All living plant cells respire all the time. In daylight photosynthesis just outpaces it.
The click
Plants do both. Respiration is how every living cell releases usable energy (ATP) from food, and plant cells need ATP all the time for growth, transport and repair. So aerobic respiration, glucose + O₂ → CO₂ + H₂O + energy, goes on in the mitochondria of living plant cells continuously, day and night.
Photosynthesis is a different process: it makes food (glucose) using light energy, in chloroplasts, and it happens only in the light and only in green cells. Root cells, for example, never photosynthesise, but they respire all the time.
Why the confusion? In bright light, a leaf's photosynthesis runs much faster than its respiration. The leaf takes in more CO₂ than it releases and gives out more O₂ than it uses, so the net gas exchange looks like photosynthesis alone. Respiration is still happening, just hidden. At night, with no photosynthesis, respiration shows up as a net release of CO₂. The light intensity at which the two exactly balance, with no net gas exchange, is called the compensation point.
A classic experiment shows it: a potted plant kept in the dark under a bell jar with a dish of lime water turns the lime water milky, because the plant releases CO₂. Germinating seeds do the same. Plants have no special breathing organs: gases move in and out by diffusion, mainly through stomata and lenticels.
Got this one? The Knot Scan finds the misconception you don't know you have, in 8 questions.