Water is drawn into the guard cells by osmosis because of the osmotic gradient which develops as the glucose increases. Guard cells become fully turgid , and because of their specialised structure, they 'bend' apart . The stomata are open - gas diffusion is possible. That's all very well, if it's early morning and the air is still relatively humid. Water diffuses out of the guard cells in response to the decreasing water content of the air adjacent to them.
Guard cells become flaccid .
Stomatal pores close . Gas exchange, including water loss , through stomata is shut down . Photosynthesis can still provide the inputs for cellular respiration to continue in the cells inside the sealed leaf whilst the stomata are closed. In preparing for this topic in your CAT 1 exam, you should ensure that you have read widely about different plant adaptations to environmental stress. Questions have appeared in the past where this topic is merged with a question testing your interpretation of experimental data given in the introduction to the question.