Fishes are able to taken oxygen which is dissolved in water, through their gills.The gills of a fish are not equipped to take in oxygen from air.Hence, when a fish is taken out of water, it is unable to inhale oxygen rich air and so dies quickly.

Most of the carbon dioxide released during respiration exits from the plant cells through various openings. The plant cells through various openings. The carbon dioxide, required for photosynthesis is taken through stomata. So, even if a plant keeps on taking in oxygen and releasing carbon dioxide during the day, it does not mean that no photosynthesis is happening in the plant.

The plant which is kept in continuous light would live longer. The plant which is kept in the dark is unable to carry out photosynthesis and hence the container within a short span of time. Lack of oxygen in the container would kill the plant. On the other hand, the plant being kept in light would be able to carry out photosynthesis and thus would be able to convert the carbon dioxide into oxygen. Hence this plant would live for a longer duration.

Guard cells control the opening and closing of stomatal pore by change in turgidity. When water enters the guard cells, they become turgid and the stomatal pore opens. On the other hand, when water exits from the guard cells; they become flaccid and the stomatal pore opens.

This statement does not show the complete picture. In fact, plants carry out respiration throughout the day and night, like other living beings. Hence, they keep on emitting carbon dioxide all the time. Photosynthesis takes place only in the presence of light and hence, oxygen is released by plants only during the day.