The plasma membrane of a cell is scientifically defined as a barrier between the cell and its external surroundings. The plasma membrane is semipermeable, which means its pores allow ions and organic molecules to pass in and out of the cell through diffusion but not other particles.
The job, or use, of the plasma membrane is to regulate what enters and leaves the cell. More specifically, the cell wants only nutrients entering and waste exiting.
The plasma membrane's function would be considered the method by which it ensures that only nutrients enter and waste products exit the cell. The plasma membrane functions due to its phospholipids and proteins.
Two layers of phospholipids create the protective barrier of the plasma membrane. Phospholipids are composed of a fatty acid head and two fatty acid tails. The head is polar, meaning magnetic, and hydrophilic, meaning water-loving. The tails are nonpolar, meaning non-magnetic, and hydrophobic, meaning water-fearing. Since living organisms are dominantly composed of water, the phospholipids layer up to construct the plasma membrane so that the hydrophobic tails of the phospholipids are sandwiched between two hydrophilic heads, allowing the heads to be exposed to the water of the cell's surrounding environment.
The phospholipids are also surrounded by many different embedded proteins. These proteins help keep the phospholipids in place and determine what comes in and out of the cell. The proteins themselves also have both hydrophobic and hydrophilic surfaces that bond with the corresponding parts of the phospholipids, allowing the proteins to extend either inside the cell or outside into the cell's surrounding environment. The proteins function to select what particles will enter or leave the cell by receiving and transmitting information.
Mitochondria (mitochondrion singular) are often called the cell's powerhouses. Mitochondria are organelles within the cell's plasma, outside of the cell's nucleus, and are essentially cells within a cell in that they are made up of inner membranes, outer membranes, and a fluid called a matrix. Ribosomes and DNA also float in the matrix. It should also be noted that mitochondria are not few and far between--a single animal cell can typically have between 1,000 and 2,000 mitochondria.
The use of a mitochondrion is to create energy for the cell by taking in nutrients and using oxygen to break nutrients down into chemical energy molecules, a process known as aerobic respiration.
The aerobic respiration process of the mitochondrion to convert nutrients into energy is the function of a mitochondrion. Aerobic respiration first requires the process of oxidative phosphorylation, completed in the matrix of the mitochondrion. In the matrix, enzymes work together through a chemical reaction to form reduced nicotinamide adenine dinucleotide (NADH). Enzymes in the inner membrane of the mitochondrion then convert NADH into adenosine triphosphate (ATP), which is the cell's chemical energy. We call the process of producing NADH oxidative phosphorylation and the process of converting NADH into ATP aerobic respiration.