Good question. Plants sort of breathe through holes in their leaves called stomata. If they have more carbon dioxide than they can use immediately for photosynthesis, it just exits through the stomata.

The stomata are a problem for plants in dry climates because when they are open to allow carbon dioxide and oxygen to move into or out of the leaf, water also evaporates. When the plant gets dry, the cells that “guard the door” of the stomata shrink and they close. When there’s enough water, these cells swell up and open the stomata again.

Too much oxygen from photosynthesis can be a problem because if it builds up, it blocks the enzyme that should be grabbing onto carbon dioxide to complete photosynthesis. Some plants have special adaptations that allow them to avoid this problem. Check out C₄ plants for more information. Some plants, like cacti, only open their stomata at night, when the air is cooler. This saves on water loss, but means they have less time to do gas exchange.

So there’s a whole complex balance of water, gas exchange, cellular respiration, and photosynthesis. Here’s another wrinkle, water evaporating through stomata is what pulls water up from the roots. If plants can’t pull water up, what else will they not be able to get?

During photosynthesis green plants manufacture the sugar molecules fructose and glucose. Green plants use energy from sunlight to build sugar molecules from carbon dioxide and water. ... During cellular respiration animal cells combine oxygen with food molecules to release energy to live and function.

The plant releases the CO₂ into the atmosphere, the same thing you do with CO₂ that is produced as a product of cellular respiration. Generally, if a plant is respiring, then it's not photosynthesizing (usually this is because it's nighttime and so not enough light to photosynthesize), which means that it's an oxygen-breathing CO₂-producing organism just like you are.

(Note to the above: many desert plants do their carbon fixation at night because it costs them less water to do so. These plants may use the CO₂ from cellular respiration if they must).