Through the process of diffusion or osmosis, it is possible for molecules to cross the membrane. Diffusion is the movement of molecules from an area high concentration to an area of lower concentration until they are evenly distributed while osmosis is the diffusion of water molecules through a selective permeable membrane. Diffusion allows substances such as water, carbon dioxide and oxygen to pass in and out of the cell. Osmosis enables water to move in and out of the cell in response to differing solute concentrations.
With respect to osmosis, animal cells, which do not have cell walls, absorb water from a hypotonic solution (concentration of solute lower than that found inside the cell) and will swell until they burst. In a hypertonic solution (concentration of solute higher than that found inside the cell), the animal cells lose water to the solution and will shrink and appear lumpy or jagged. Plant cells have a large central vacuole that holds excess water and a cell wall that opposes the pressure built up by the uptake of water from a hypotonic solution. In a hypotonic solution, the plant cell absorbs water and moves into the central vacuole causing it to expand the push the cytoplasm into a thin layer just below the plasma membrane. Plant cells build up turgor pressure despite having cell walls which help them stay rigid and thus prevents further absorbance of water by the cell. In a hypertonic solution, water is lost from the cell and the size of the vacuole decreases. This causes a loss in turgor pressure and the plant droops. Eventually, the central vacuole (plasmolysis) collapses causing the cytoplasmic membrane to pull away from the cell wall (Biology Department, 2011).
The surface area to volume ratio is directly related to the amount of water, nutrients or waste that can diffuse through the cytoplasmic membrane into the cell. This ratio affects the cell’s ability to meet its nutritional needs and is a major factor in limiting the...