Active Transport, Endocytosis, and Exocytosis

Section 2: Active Transport, Endocytosis, and Exocytosis

In plant cells, animal cells, and microbial cells alike, many substances undergo transmembrane transport against concentration gradients. For instance, the concentration of amino acids and glucose in intestinal fluid is much lower than in the epithelial cells lining the intestine, yet these substances are still absorbed by the intestinal epithelial cells. The concentration of K+ in human red blood cells is 30 times higher than in plasma, while in certain algae cells, the concentration of K+ can be 63 times higher than in the surrounding water environment. How do these substances transport against concentration gradients?

Active Transport When ions like Na+, K+, Ca2+, and other molecules undergo transmembrane transport against concentration gradients, they first bind to specific sites on carrier proteins embedded in the membrane. Due to differences in size and properties of different ions or molecules, carrier proteins exhibit significant structural variations, with each carrier protein typically suited to binding with specific ions or molecules. Once bound, the energy released from chemical reactions within the cell drives a change in the spatial structure of the carrier protein, facilitating the transport of the bound ion or molecule across the membrane from one side to the other, where it is released. Subsequently, the carrier protein returns to its original state, ready to transport similar substances again. This process, where substances are transported against concentration gradients with the assistance of carrier proteins and the energy released from internal chemical reactions, is known as active transport.

Active transport is widespread in both animal and plant cells as well as microbial cells. It allows cells to selectively absorb necessary substances, expel metabolic wastes and harmful materials, thus fulfilling the metabolic needs essential for cellular and organismal life activities.

Endocytosis and Exocytosis While transport proteins facilitate the passage of many ions and small molecules across the cell membrane, they are ineffective for transporting large biological molecules such as proteins and polysaccharides.

Protozoa ingest organic particles from water, presenting a challenge for the entry of large molecules into cells.

Proteins synthesized by mammary gland cells, protein hormones secreted by endocrine gland cells, and digestive enzymes secreted by digestive gland cells all need to be released from cells. In fact, most cells can ingest and excrete specific large molecules. How do these large molecules enter and exit cells?

When cells ingest large molecules, the molecules first bind to proteins on the membrane, causing a portion of the cell membrane to invaginate and form small vesicles that envelop the large molecules. Subsequently, these vesicles separate from the cell membrane to form vacuoles that enter the interior of the cell—a process known as endocytosis. For large molecules that need to be excreted from cells, vacuoles containing these molecules form inside the cell, move to the cell membrane, fuse with it, and release the large molecules outside the cell—a phenomenon known as exocytosis (Figure 4-8). Both endocytosis and exocytosis are common in transmembrane transport of substances and require energy derived from cellular respiration.

In summary, besides small, uncharged molecules that can freely diffuse into and out of cells, the transmembrane transport of ions and small organic molecules (such as glucose and amino acids) necessitates the assistance of transport proteins. This underscores the role of proteins as facilitators of cellular activities. Each transport protein is typically suited to transport specific substances, and thus, the types and quantities of transport proteins on the cell membrane or changes in their spatial structure play a decisive role in transmembrane transport of many substances, forming the structural basis for the selective permeability of cell membranes. Large biological molecules like proteins enter and exit cells through endocytosis or exocytosis processes, which also rely on membrane proteins and the fluidity of the phospholipid bilayer on the membrane.