i) Substances like CO₂ and water move in and out of the cell via the phenomenon of diffusion and osmosis respectively.

ii) Gases like CO₂ are very small and hence can easily diffuse through the plasma membrane. However, the concentration of such gases on the inside and outside of the cell plays a very important role in their movement. Gases move from a region of higher concentration to a region of lower concentration.

iii) Generally, the concentration of CO₂ inside the cell is higher than the outside. This makes the CO₂ diffuse from the inside of the cell to the outside. When the concentration of CO₂ becomes higher on the outside, the CO₂ simply diffuses from the outside of the cell to the inside.

iv) In case of water, the movement occurs as a result of osmosis. Water moves from a region of lower solute concentration to a region of higher solute concentration.

v) When concentration of water is more on the outside of the cell, the water will move from the outside to the inside of the cell. This occurs as the inside of the cell has less water than the outside but has more concentration of solutes.