 This video covers the second part of introduction to cytology for human anatomy. As we go, we will cover the following study objective, describe the structure of the plasma membrane and its overall functions. Plasma membrane is the semipermeable barrier surrounding and containing the cell, separates the extracellular fluid from intracellular fluid, and it's selectively permeable, enabling the cell to control what enters and exits. There are also cell markers and receptors on the surface on the plasma membrane that function for cell recognition in order for the leukocytes of the immune system to recognize what cells in our body are normal cells and what cells are foreign, pathogenic bacteria or what cells are healthy and what cells are cancerous or infected with viruses. The receptors bind hormones or other chemical messages like neurotransmitters that are important for cell communication and there are also proteins on the surface that bind to the extracellular materials or other cells in order to stabilize the position of the cell and its environment. The structure of the plasma membrane is mostly a bilayer of phospholipids, a phospholipid has a hydrophilic head made of phosphate and glycerol, and this hydrophilic head, hydrophilic means water loving, so the hydrophilic head will orient towards the water in the extracellular and intracellular fluid. This is a bilayer, so both the inner and the outer surface of the plasma membrane has the hydrophilic heads facing the cytosol and extracellular fluid. And then the hydrophobic tails are fatty acids that are attached to the glycerol of the hydrophilic head. The hydrophobic tails orient inward. This is a bilayer, so there's the tails of the outer layer facing the tails of the inner layer forming a lipid interior to the phospholipid bilayer. Here we see an illustration of the plasma membrane, which is mostly a phospholipid bilayer, but also contains many proteins that are embedded in the phospholipid bilayer. Integral membrane proteins are proteins that are embedded in the membrane and span the entire bilayer. There are also peripheral proteins that are embedded in the membrane facing the extracellular or intracellular fluid, the cytosol or the extracellular fluid, and some of the integral membrane proteins function as receptors in order to detect hormones or neurotransmitters or other chemical messages and then relay signals into the cytosol in order to regulate the function of the cell. The phospholipid bilayer of the plasma membrane creates a semipermeable barrier that prevents water soluble chemicals from being able to freely diffuse in and out of the cell. However, there are channel proteins embedded in the plasma membrane that enable water soluble chemicals to pass through. However, these channel proteins allow the cell to regulate what chemicals are able to cross the plasma membrane. You can also see here there are glycoproteins, which are proteins with carbohydrates attached, and glycolipids, which are lipids with a carbohydrate attached. Together, these form the glyco-calyx, which is an important marker on the surface of the cell recognized by leukocytes of the immune system to recognize our own cells and to distinguish foreign cells that are potentially pathogens that could make us sick. Another chemical found in the plasma membrane is cholesterol. Cholesterol is a lipid that binds to the hydrophobic tails, will associate with the hydrophobic tails and stabilize the plasma membrane. The transport of chemicals across the plasma membrane can involve passive diffusion. A small non-polar molecule will diffuse directly through the lipid bilayer of the plasma membrane because the interior hydrophobic tails are not a barrier to non-polar chemicals. Oxygen is an example of a small non-polar molecule that would just diffuse from high concentration to low concentration across the plasma membrane. This is passive transport because the cell does not have to spend energy in the form of ATP in order to move oxygen into the cell. It just diffuses down its concentration gradient. Water soluble chemicals that are not able to diffuse through the plasma membrane can diffuse across the plasma membrane through a channel protein. This is called facilitated diffusion where a protein that's embedded in the plasma membrane creates a pore or a channel through that allows a water soluble chemical to diffuse an area of higher concentration to lower concentration. Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. We can see here an example of a semipermeable membrane on the left where there's a higher concentration of solute on one side than the other. There's a high concentration of solute on the right and a low concentration of solute on the left. Then what would happen in this situation is that water would move from the low concentration of solute to where the solute concentration is high. Water would move by osmosis to the right. Now we have an equal concentration of solute on both sides of the semipermeable membrane as a result of osmosis. We can see the example of osmosis through the cell membrane. See what happens if we take a red blood cell, an erythrocyte, and place it into solutions with different concentrations. An isotonic solution is a solution that has a solute concentration that is equal to the inside of the cell. The cytosol and the surrounding solution have an equal concentration of water to dissolve solutes. The movement of water in and out of the cell is balanced. In this situation, the cell doesn't increase in volume or decrease in volume because the amount of water moving in and out is balanced. However, if we were to place a cell in a hypertonic solution, a hypertonic solution is a solution that has a concentration of solutes that's higher than the cytosol, if we place the erythrocyte in a hypertonic solution, water will move out of the cell from the cytosol into the extracellular fluid, and this will cause the cell to shrink. The opposite situation is if we place the erythrocyte in a hypotonic solution, a hypotonic solution has a solute concentration that's lower than the cytosol, and so water will move into the cytosol by osmosis, causing the erythrocyte to grow, to expand, and if too much water enters the cell more than the volume of the cell can handle, it will rupture or cause the cell to lace or rupture open. Active transport requires ATP to move solutes across the membrane. There are transport pumps, solute pumps that are proteins embedded in the plasma membrane that use ATP in order to pump solutes across the plasma membrane. One example is the sodium-potassium ATPase pump. The sodium-potassium pump is found in the plasma membrane of many cells and is important for creating a high concentration of potassium in the cytosol and a low concentration of sodium. So this sodium-potassium pump will move sodium out of the cytosol and potassium into the cytosol, and every time that it moves three sodium ions out of the cytosol and two potassium ions into the cytosol, one molecule of ATP is broken down into ADP and a phosphate group. ATP stands for adenosine triphosphate. ATP is an energy currency used by proteins inside cells. In order to release the energy from ATP, one of the three phosphate groups is removed, producing adenosine diphosphate and a free phosphate. And the energy released from breaking ATP down into ADP and a phosphate is used to pump three sodium ions out of the cell and two potassium ions into the cell. And this concentration gradient that's created by the sodium-potassium pump is then often used by other membrane transporters in order to facilitate the movement of ions across the membrane. For example, the movement of glucose can be facilitated by the movement of sodium as sodium moves down its concentration gradient. It can bring glucose in through a glucose transport protein. Endocytosis is the process that moves extracellular material into the cell, forming a small package of membrane, where the plasma membrane buds inward and wraps around some of the extracellular fluid and particles in the extracellular fluid, and then buds inward, forming a small chamber. Here we can see three different forms of endocytosis. There is phagocytosis, literally translated as cell eating, where the plasma membrane is wrapped around a large particle, such as a bacterial cell, and that particle is brought into the cell to form, we can see here what's called a vacuole. Another example is called pinocytosis. In pinocytosis, the cell takes in a sample of the extracellular fluid with small particles, and the in-budding formed from the pinocytosis we see here is called a vesicle. A vesicle is a small membrane-bound package, a small compartment that's surrounded by membrane. The membrane surrounding the vesicle is very similar to the structure of the plasma membrane and forms from the plasma membrane of the cell when it pinched inward in pinocytosis. Another example of endocytosis is called receptor-mediated endocytosis, where a cell takes in a particle with a specific structure that binds to receptors on the surface of the plasma membrane. When those chemicals with a very specific shape, which are called ligands, bind to the receptor, the plasma membrane buds inward and forms a vesicle. This vesicle is called a coated vesicle because there's a protein coat of the proteins that are important for recognizing the particle binding to the receptor and then forming the inward-budding structure. Exocytosis is essentially the opposite of endocytosis and is another active transport process in exocytosis. A vesicle merges with the plasma membrane, spilling its contents into the extracellular fluid. Exocytosis is a common mechanism for cells to release hormones or neurotransmitters. Here we can see an example of pancreatic cells that have secretory vesicles. Those secretory vesicles inside of the pancreas secrete hormones and also enzymes. This example is showing secretory vesicles that are filled with enzymes that are secreted from the pancreas out through ducts that connect into the small intestine. Those enzymes are important for digesting food to break down the chemicals in our food, releasing nutrients that we can absorb. Exocytosis is used in order to release secretory vesicles full of digestive enzymes from the pancreas into ducts that carry those enzymes out into the intestines.