Here's a breakdown:
* Structure: The cell membrane is a thin, flexible layer made of phospholipids (fatty molecules) arranged in a double layer. It also contains proteins embedded within it.
* Selective Permeability: The cell membrane is selectively permeable, meaning it chooses what can pass through. This selectivity is determined by:
* Size: Small molecules like water, oxygen, and carbon dioxide can easily pass through. Larger molecules, like proteins, cannot.
* Charge: Charged molecules like ions (e.g., sodium, potassium) need help from membrane proteins to cross.
* Solubility: Lipid-soluble molecules can pass through the membrane more easily than water-soluble molecules.
* Methods of Transport:
* Passive Transport: Movement of molecules across the membrane without energy expenditure. This includes:
* Diffusion: Movement of molecules from a high concentration area to a low concentration area.
* Osmosis: Movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration.
* Facilitated Diffusion: Movement of molecules across the membrane with the help of transport proteins (no energy required).
* Active Transport: Movement of molecules across the membrane against their concentration gradient (from low to high concentration). This requires energy, typically provided by ATP.
In summary: The cell membrane controls what moves in and out of the cell by being selectively permeable and using different mechanisms of transport to move molecules across its structure.
