By the end of this section, you will be able to:

  • What are the differences among the different types of endocytosis: (phagocytosis, pinocytosis, and receptor-mediated endocytosis) and exocytosis?

Connection for AP® Courses

Diffusion, osmosis, and active transport are used to transport fairly small molecules across plasma cell membranes. However, sometimes large particles, such as macromolecules, parts of cells, or even unicellular microorganisms, can be engulfed by other cells in a process called phagocytosis or “cell eating.” In this form of endocytosis, the cell membrane surrounds the particle, pinches off, and brings the particle into the cell. For example, when bacteria invade the human body, a type of white blood cell called a neutrophil will remove the invaders by this process. Similarly, in pinocytosis or “cell drinking,” the cell takes in droplets of liquid. In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to a specific receptor protein on the external surface of the cell membrane (e.g., hormones and their target cells) before under going endocytosis. Some human diseases, such as familial hypercholesterolemia, are caused by the failure of receptor-mediated endocytosis. Exocytosis is the process of exporting material out of the cell; vesicles containing substances fuse with the plasma membrane and the contents are released to the exterior of the cell. The secretion of neurotransmitters at synapses between neurons is an example of exocytosis.

Information presented and the examples highlighted in the section support concepts and learning objectives outlined in Big Idea 2 of the AP® Biology Curriculum Framework. The learning objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.B Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments.
Essential Knowledge 2.B.2 Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes.
Science Practice 1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective 2.12 The student is able to use representations and models to analyze situations or solve problems qualitatively and quantitatively to investigate whether dynamic homeostasis is maintained by the active movement of molecules across membranes.
Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.D Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment.
Essential Knowledge 2.D.4 Plants and animals have a variety of chemical defenses against infections that affect dynamic homeostasis.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Science Practice 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 2.30 The student can create representations or models to describe nonspecific immune defenses in plants and animals.

In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see Table 5.2 for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.

Endocytosis

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane.

Phagocytosis

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil will remove the invaders through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 5.21).

This illustration shows a plasma membrane forming a pocket around a particle in the extracellular fluid. The membrane subsequently engulfs the particle, which becomes trapped in a vacuole.
Figure 5.21 In phagocytosis, the cell membrane surrounds the particle and engulfs it. (credit: Mariana Ruiz Villareal)
 

In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with a protein called clathrin, which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

SCIENCE PRACTICE CONNECTION FOR AP® COURSES

ACTIVITY

Create a representation/diagram to describe how a neutrophil, a type of human white blood cell, attacks and destroys an invading bacterium. What cellular organelles are involved in this process?

Pinocytosis

A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome (Figure 5.23).

This illustration shows a plasma membrane forming a pocket around fluid in the extracellular fluid. The membrane subsequently engulfs the fluid, which becomes trapped in a vacuole.
Figure 5.23  In pinocytosis, the cell membrane invaginates, surrounds a small volume of fluid, and pinches off. (credit: Mariana Ruiz Villareal)
 

A variation of pinocytosis is called potocytosis. This process uses a coating protein, called caveolin, on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Receptor-mediated Endocytosis

A targeted variation of endocytosis employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances (Figure 5.24).

This illustration shows a part of the plasma membrane that is clathrin-coated on the cytoplasmic side and has receptors on the extracellular side. The receptors bind a substance, then pinch off to form a vesicle.
Figure 5.24 In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds to the receptor on the external surface of the cell membrane. (credit: modification of work by Mariana Ruiz Villareal)
 

In receptor-mediated endocytosis, as in phagocytosis, clathrin is attached to the cytoplasmic side of the plasma membrane. If uptake of a compound is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by the failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear LDL particles from their blood.

Although receptor-mediated endocytosis is designed to bring specific substances that are normally found in the extracellular fluid into the cell, other substances may gain entry into the cell at the same site. Flu viruses, diphtheria, and cholera toxin all have sites that cross-react with normal receptor-binding sites and gain entry into cells.

LINK TO LEARNING

See receptor-mediated endocytosis in action, and click on different parts for a focused animation.

Salmonella is one of the most common food borne illnesses. When a white blood cell engulfs salmonella bacteria during phagocytosis, it secretes a protein that prevents the fusion of the encased bacteria with the lysosome of the cell. What effect would this have?
  1. The bacteria will be destroyed and will not cause any illness.
  2. The bacteria will survive and will definitely result in illness.
  3. The bacteria will be destroyed, but will still cause illness.
  4. The bacteria will survive and possibly will cause illness.

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure 5.25). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

This illustration shows vesicles fusing with the plasma membrane and releasing their contents to the extracellular fluid.
Figure 5.25 In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal)
 
Methods of Transport, Energy Requirements, and Types of Material Transported
Transport MethodActive/PassiveMaterial Transported
Diffusion Passive Small-molecular weight material
Osmosis Passive Water
Facilitated transport/diffusion Passive Sodium, potassium, calcium, glucose
Primary active transport Active Sodium, potassium, calcium
Secondary active transport Active Amino acids, lactose
Phagocytosis Active Large macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosis Active Small molecules (liquids/water)
Receptor-mediated endocytosis Active Large quantities of macromolecules
Table 5.2