The plasma membrane is a selectively permeable membrane enclosing the cytoplasm of a cell. This is the outer layer in animal cells. Other organisms have a cell wall as the outer layer and the plasma membrane is between the cell wall and the cell's cytoplasm. The cell wall is the outer covering of most bacteria, algae, fungi, and plant cells. In eubacterium, which is a prokaryotic microorganism, the cell wall contains peptidoglycan.
The plasma membrane surrounds a eukaryotic cell and serves as a barrier between the inner cell and its environment. The cytoplasmic membrane is composed of proteins and lipids. Carbohydrates are used to uniquely identify the cell to other cells. Lipids, known as sterols, help prevent the destruction of the cell when there is an increase in osmotic pressure and are mainly used for stability. Lysis is the destruction of a cell. Prokaryotic cytoplasm lacks certain features that are found in eukaryotic cytoplasm, such as a cytoskeleton. In a eukaryotic microorganism, the cytoskeleton provides support and shape for cells and helps transport substances through the cell.
The plasma membrane of a eukaryotic cell functions like the plasma membrane of a prokaryotic cell, which you learned about previously in this chapter. That is, substances enter and leave the cell through the cytoplasmic membrane by using simple diffusion, facilitated diffusion, osmosis, and active transport.
Eukaryotic cells extend parts or sections of plasma membrane. The extensions of the plasma membrane are called pseudopods. The word pseudopod means "false foot," and these "feet" enable the cell to have amoeboid motion. An amoeboid motion consists of muscle-like contractions that move the cell over a surface. Pseudopods are used to engulf substances and bring them into the cell, which is called endocytosis (a type of active transport). There are two types of endocytosis. These are phagocytosis (eat) and pinocytosis (drink). In phagocytosis, solid particles are engulfed by the cell. An example is when a white blood cell engulfs and destroys a bacteria cell. In pinocytosis, liquid particles are brought into the cell. An example is when extracellular fluid containing a substance is destroyed by the cell.
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