Question

Why are some plants called air purifiers and kept indoors at night
(class 10 life processes)

Answer 1

The effectiveness of plants as air purifiers is not really proven. It's a common buzzword on almost all houseplant sites on the internet, but if you actually read the research, as opposed to information blogs and curated sites, you'll find that two big problems are that the original experiments were done in closed and controlled situations rather than real life environments, and the number of plants needed to affect the air quality rose over the years of investigating the phenomenon from 1 6" plant per 100 cubic feet to 10 10" plants per 100 cubic feet, and even more. (The measurement refers to pot diameter, not plant height.) If you want to learn more about this, here are a few references: and

Another problem is how the "air cleaning" actually happens. Some research indicates that the soil, or microbes in the soil, may be as important, or more important, than the leaves' absorption of volatile gases. .

But wait, there's more. The other responders' answers brought up some points that I found disconcerting, to say the least.

In the mean time, this is what I think happened. Somebody somewhere read about CAM photosynthesis, misunderstood it, and wrote about it as meaning that some plants produce oxygen at night. After that, this wrong explanation was picked up by producers of curated "information" sites, and has been passed around ever since.

This is not what happens. Plants don't give off oxygen at night, at least not in any significant amounts. Oxygen is a by-product, a waste product if you will, of photosynthesis. Thus, it takes place only in daylight, when the photosynthesis is going on. A small amount of the oxygen is used in metabolizing (burning) sugars for the plant's fuel, and in some other of the plant processes. The majority of it is outgassed through the cell walls as it is produced by the photosynthetic processes. Some of it does pass through the stomata, but only incidentally.

CAM photosynthesis is a defense against arid conditions. It apparently has been evolved independently by plants in a number of families. Some plants that use this process are many orchids, bromeliads, some cacti and euphorbias, some other succulents such as aloe, crassula, and portularia, sanseveria (snake plant,) and many tradescantia.

Different plants use it in different ways, and to different degrees, but the process basically allows them to keep their stomata closed during the day, to limit moisture evaporation, thus giving them a defense against dry environments. At night, when it's much cooler, they open the stomata to allow the air - which contains carbon dioxide - to enter. However, because the photosynthesis factories are closed for the night, they have to store the carbon dioxide in special cells, and send it to the factories (they're actually called chloroplasts) the next day. If you want to study this in detail, a good place.

Note that water vapor is one of the by-products of respiration. This has to exit the plant through the stomata; it can't diffuse through the cell walls like carbon dioxide can. You can see the advantage of conserving the available water (by closing the stomata) if you're in a dry place, then letting the extra go when the sun goes down. This is what many plants do by using the CAM photosynthesis.

Photosynthesis is the carbon fixation process by which plants use the energy of the sun to power the breaking apart and reassembly of carbon dioxide and water into sugars (also called glucose and carbohydrates) and oxygen. Because it is powered by the sun, the process occurs only during the day. The waste oxygen can be dispelled through the stomata, but if they are closed (as in CAM plants,) the oxygen can diffuse out of the plant through the cell walls. The oxygen isn't stored in the plant, which is why it isn't given off at night, when has stopped.

Stomata are the microscopic pores on the undersides of leaves. They can open and close, and admit air (oxygen, carbon dioxide, nitrogen) into the leaf, and expel water vapor and extra carbon dioxide and oxygen out. They are not, however, analagous to noses or nostrils in animals. Although they look wonderfully like they are breathing, and info sites often say "plants breathe through their leaves," it's not the same thing.

They don't inhale oxygen as they close, and exhale carbon dioxide as they open, or vice-versa. The opening and closing is regulated by a variety of environmental factors like temperature, humidity, soil moisture, and light level, and gases can be absorbed and expelled through the cell walls of the leaves.

When all is said and done, plants are a wonderful addition to your home. Whether or not they do much "purifying," they are beautiful, therapeutic, and easy. You need to know just a few basic things about taking care of them, and you will reap the rewards of getting close to Mother Nature far beyond what you could have imagined.

Answer 2

Normally during the night plants respire, i.e. they break down the sunlight energy with the evolution of carbon dioxide. So generally you are advised to not rest under trees at night.
However there are some plants that act upon the energy absorbed in the night, and hence the carbon dioxide evolved is utilised in this process to release oxygen. These plants are air purifiers and can be kept indoors at night.