Common Misconceptions

Misconception: "Plants only photosynthesise during the day and only respire at night."

Reality: Plants respire continuously — 24 hours a day, 7 days a week — just like animals. Respiration never stops because cells always need energy. During daylight, plants also photosynthesise, and the rate of photosynthesis normally exceeds respiration, so there is a net release of oxygen. At night, without light, photosynthesis stops but respiration continues, so plants take in oxygen and release CO2. The mistake is thinking these processes are opposites that switch on and off alternately.

Misconception: "Photosynthesis and respiration are just opposites of each other and cancel out."

Reality: Although the equations look like reverses of each other, photosynthesis and respiration serve entirely different purposes and occur in different organelles. Photosynthesis (in chloroplasts) stores energy in glucose. Respiration (in mitochondria) releases that stored energy for use. They do not simply cancel out — a plant that photosynthesises more than it respires is growing and storing biomass. If they always cancelled perfectly, plants could never grow.

Misconception: "Plants get their food (nutrients) from the soil through their roots."

Reality: Plants make their own food (glucose) through photosynthesis — they are autotrophs (self-feeders). What plants absorb from soil through roots are water and mineral ions (such as nitrates, magnesium), not food. These minerals are needed for specific processes (e.g., nitrates for making amino acids, magnesium for making chlorophyll), but the energy-containing organic molecules — glucose, starch, cellulose — are all manufactured by the plant itself using CO2, water, and light.

Misconception: "Increasing light intensity always increases the rate of photosynthesis."

Reality: Increasing light intensity only increases photosynthesis rate if light is the limiting factor. Once light is no longer limiting (i.e., the plant has more light than it can use), increasing it further has no effect. At that point, CO2 concentration or temperature becomes the new limiting factor, and further improvement requires increasing whichever of those is now the bottleneck.