Every cell is surrounded by a cell surface membrane (plasma membrane), and in eukaryotic cells membranes also surround and form many organelles. All of these membranes have the same basic structure, a phospholipid bilayer with proteins, about 7 to 10 nm thick.Functions of the cell surface membrane
• It is partially permeable: it separates the cell contents from the outside and controls which substances enter and leave.
• It carries receptors that detect hormones and other signalling molecules, so cells can communicate.
• It carries glycoproteins and glycolipids that act as antigens, allowing cell recognition, for example by the immune system.
• It allows cells to stick together to form tissues.Functions of membranes inside the cell
• Compartmentalisation: membranes surround organelles such as the nucleus, mitochondria and lysosomes, keeping different reactions and enzymes apart (for example, keeping hydrolytic enzymes inside lysosomes).
• Providing a large surface on which reactions happen and enzymes are attached, for example the folded inner mitochondrial membrane (cristae) in respiration and the thylakoid membranes in photosynthesis.
• Forming vesicles that transport substances around the cell, for example from the rough ER to the Golgi apparatus.
• Controlling what enters and leaves each organelle, such as mRNA leaving the nucleus through pores in the nuclear envelope.The structure that allows membranes to do all this is explained by the fluid mosaic model.
• It is partially permeable: it separates the cell contents from the outside and controls which substances enter and leave.
• It carries receptors that detect hormones and other signalling molecules, so cells can communicate.
• It carries glycoproteins and glycolipids that act as antigens, allowing cell recognition, for example by the immune system.
• It allows cells to stick together to form tissues.Functions of membranes inside the cell
• Compartmentalisation: membranes surround organelles such as the nucleus, mitochondria and lysosomes, keeping different reactions and enzymes apart (for example, keeping hydrolytic enzymes inside lysosomes).
• Providing a large surface on which reactions happen and enzymes are attached, for example the folded inner mitochondrial membrane (cristae) in respiration and the thylakoid membranes in photosynthesis.
• Forming vesicles that transport substances around the cell, for example from the rough ER to the Golgi apparatus.
• Controlling what enters and leaves each organelle, such as mRNA leaving the nucleus through pores in the nuclear envelope.The structure that allows membranes to do all this is explained by the fluid mosaic model.