1. Size and Polarity:
* Carbon dioxide (CO2) and water (H2O): These molecules are small and relatively non-polar (meaning they don't have a strong positive or negative end). They can easily slip through the phospholipid bilayer, the main component of the cell membrane.
* Starch and proteins: These molecules are much larger and more complex. They are also polar, meaning they have regions with distinct positive and negative charges. This makes it very difficult for them to pass through the hydrophobic (water-fearing) interior of the cell membrane.
2. Membrane Transport Mechanisms:
* Simple diffusion: Carbon dioxide and water can pass through the membrane by simple diffusion. This means they move from an area of high concentration to an area of low concentration, without the help of any membrane proteins.
* Facilitated diffusion or active transport: Larger molecules like starch and proteins require special membrane proteins called transporters or channels to help them move across the membrane. These proteins can bind to the molecule and help it move through the membrane.
In summary:
* The small size and non-polar nature of carbon dioxide and water allow them to pass through the cell membrane easily via simple diffusion.
* Starch and proteins are too large and polar to pass through the membrane on their own. They require specialized transport mechanisms to cross the membrane.
This selectivity of the cell membrane is crucial for maintaining the cell's internal environment and carrying out its functions.
