Plasma Membrane
The plasma membrane is a dynamic component of cell structure and surrounds all living cells. The plasma membrane separates the inner workings of the cell from the environment. It maintains homeostasis, the constant internal condition needed for life. The plasma membrane is composed primarily of lipids and proteins. It is a semi-permeable or selectively permeable membrane. The selective permeability of the membrane maintains homeostasis by controlling what enters and what leaves the cell. The cell membrane and pertinent transport mechanisms have been discussed in the transport and diffusion lab. Further discussion will not occur here.
The plasma membrane exhibits modifications that better equip the cell for its function. Epithelial cells
of the small intestine function in nutrient absorption. Their apical surface is covered by microvilli. These microvilli are nipple-like or fingerlike projections that dramatically increase the surface area of the cell. More area means more effective absorption of nutrients. Epithelial cells of the trachea, nasal passageways and oviducts have cilia on their apical surfaces. Cilia are fine, short extensions of the membranes that move substances like mucus or the ovum past the surface of the cell.
Cytoplasm
The cytoplasm is found within the plasma membrane. It is the cell 'jelly' within which the cell organelles are suspended and cell solutes are dissolved. Metabolism occurs in the cytoplasm and in the organelles suspended within the cytoplasm. Some texts define the cytoplasm as everything inside the cell except for the nucleus. Cytosol is another term frequently used in discussions of cell anatomy. Cytosol is the liquid component of the cytoplasm, i.e., the cytoplasm without organelles.
