It's the cell membrane!
The cell membrane envelopes the entire cell, separating the interior from the external environment. Organelles inside of the cell such as the cytoskeleton or even things outside (like cilia in bacteria or the cell wall in plants) are attached to the cell membrane. Protein channels scattered across the membrane can let certain molecules in and out of the cell such as oxygen, water, nutrients, and carbon dioxide. The structure is extremely complex, as you can see in the diagram to the right. It is constructed mainly of lipids and proteins, such as carbohydrates and cholesterol.
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