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MCAT Official Prep (AAMC)
Practice Exam 4 B/B Section Passage 9 Question 52
Intracellular junctions are different types of connections between cells.
Intercellular junctions are regions of contact between the plasma membranes of two or more adjacent cells. They are essential to any multicellular organism, providing the structural means by which groups of cells can adhere and interact. The junctions come in three classes: gap, tight and desmosomes.
Gap junctions are channels between neighboring cells that allow for the transport of ions, water, and other substances between cells. Gap junctions in vertebrates develop when a set of six membrane proteins called connexins form an elongated, donut-like structure called a connexon. When the pores, or “doughnut holes,” of connexons in adjacent animal cells align, a channel forms between the cells.
Not all junctions between cells produce cytoplasmic connections. Instead, tight junctions create a watertight seal between two adjacent animal cells. At the site of a tight junction, cells are held tightly against each other by many individual groups of tight junction proteins called claudins, each of which interacts with a partner group on the opposite cell membrane. The groups are arranged into strands that form a branching network, with larger numbers of strands making for a tighter seal. The purpose of tight junctions is to keep liquid from escaping between cells, allowing a layer of cells (for instance, those lining an organ) to act as an impermeable barrier.
Animal cells may also contain junctions called desmosomes, which act like spot welds between adjacent epithelial cells. A desmosome involves a complex of proteins. Some of these proteins extend across the membrane, while others anchor the junction within the cell. Cadherins are specialized adhesion proteins, are found on the membranes of both cells and interact in the space between them, holding the membranes together. Inside the cell, the cadherins attach to a structure called the cytoplasmic plaque (red in the image below), which connects to the intermediate filaments and helps anchor the junction. Desmosomes pin adjacent cells together, ensuring that cells in organs and tissues that stretch, such as skin and cardiac muscle, remain connected in an unbroken sheet.
Practice Questions
Key Points
• Gap junctions are channels between neighboring cells that allow for the transport of ions, water, and other substances using connexion proteins to form pores.
• Tight junctions create a watertight seal between two adjacent animal cells using claudin proteins.
• Desmosomes act like spot welds between adjacent epithelial cells joining them together using cadherins.
Key Terms
Gap junction: channels between neighboring cells that allow for the transport of ions, water, and other substances
Connexin: membrane protein that makes up a gap junction
Tight junctions: watertight seal between two adjacent animal cells
Claudins: proteins that make up tight junctions
Desmosomes: small spot connections between epithelial cells
Cadherins: specialized adhesion proteins that interact between cells
Epithelial: the outer layer of a cell surface
Adhesion proteins: glycoproteins that mediate cell-cell connections
