Core Concepts Summary

Self-Assessment

1. Name three types of cytoskeletal element, the subunits they are composed of, their relative sizes, and the major functions of each type.

   Self-Assessment 1 Answer

   Three types of cytoskeletal elements are microtubules, microfilaments, and intermediate filaments. Microtubules are made up of α- and β-tubulin dimers; microfilaments are made up of actin monomers; and intermediate filaments are made up of intermediate filament protein subunits. Microtubules have the largest diameter (25 nm); microfilaments have the smallest diameter (7 nm); and intermediate filaments have an intermediate diameter (10 nm). Microtubules help to maintain cell shape, provide tracks for vesicles and other cargo inside of cells, and make up the spindles that attach to chromosomes during cell division. Microfilaments also help to maintain cell shape and associate with myosin to cause muscle contraction. Intermediate filaments provide cells with mechanical strength.

2. Explain how the dynamic nature of microtubules and microfilaments is important for their functions.

   Self-Assessment 2 Answer

   Microtubules and microfilaments are dynamic in structure because they can become longer or shorter with the addition or subtraction of their subunits. This addition or deletion is influenced by many factors, including the concentration of free subunits and the activity of regulatory proteins. The dynamic nature of microtubules in spindles allows them to explore the space of a cell and find chromosomes during cell division. The dynamic nature of microfilaments is important for some forms of cell movement.

3. Describe the functions of the three major motor proteins and state which cytoskeletal element each interacts with.

   Self-Assessment 3 Answer

   The three major motor proteins are myosin, kinesin, and dynein. Myosin binds to microfilaments in the cell and can cause these filaments to move relative to each other, as in muscle cell contraction. Myosin can also attach to various types of cellular cargo and move along a microfilament, transporting these materials from one part of the cell to another. Kinesin transports cargo toward the plus end of microtubules, while dynein transports cargo in the opposite way, toward the minus end of the microtubule. Dynein also associates with microtubules to cause movement of cilia and flagella.

4. Describe three major types and functions of cell junctions.

   Self-Assessment 4 Answer

   The three major types of cell junctions are anchoring junctions (adherens junctions, desmosomes, and hemidesmosomes); barrier junctions (tight junctions); and communicating junctions (gap junctions and plasmodesmata). Anchoring junctions allow cells to adhere to one another. Barrier junctions prevent the passage of substances between cells. Communicating junctions permit substances to flow from the cytoplasm of one cell to the cytoplasm of a neighboring cell.

5. Identify the cytoskeletal element that interacts with adherens junctions, desmosomes, and hemidesmosomes.

   Self-Assessment 5 Answer

   Adherens junctions interact with microfilaments. Desmosomes and hemidesmosomes interact with intermediate filaments.

6. Predict the effects of interfering with the function of cadherins and integrins.

   Self-Assessment 6 Answer

   Cadherins are transmembrane proteins that are clustered together in adherens junctions and desmosomes. The extracellular domains of the cadherins associate with cadherins from another cell, attaching one cell to another. In adherens junctions, the cytosolic domains of these cadherins associate with microfilaments. This establishes a physical connection among the actin cytoskeletons of all cells present in the tissue. Unlike adherens junctions, the cytosolic domains of the cadherins in desmosomes bind to intermediate filaments of the cytoskeleton, also holding together adjacent cells. Interfering with the function of adherens junctions and desmosomes would lead to defects of cell‒cell adhesion.

   Hemidesmosomes anchor epithelial cells to the basal lamina. They are composed of transmembrane proteins called integrins. The integrin extracellular domain binds to extracellular matrix proteins in the basal lamina, and the integrin cytosolic domain binds to intermediate filaments. This results in a firmly anchored cell. Defects in integrins would interfere with the ability of cells to adhere to the basal lamina.

7. Name two places where the extracellular matrix can be found in plants and animals.

   Self-Assessment 7 Answer

   In plants, the extracellular matrix makes up the cell wall, which is composed of proteins like cellulose and lignin. The interconnected cell walls of the plant support the entire organism. In animals, one example of the extracellular matrix is connective tissue. Here, proteins like collagen, elastin, and laminin provide support and protection to the tissues surrounding them. Another example is the basal lamina. This is a specialized layer of extracellular matrix that is present beneath all epithelial tissues and provides a structural foundation.

8. Describe two effects that the extracellular matrix can have on the cells that synthesize it.

   Self-Assessment 8 Answer

   The extracellular matrix can influence the shape of cells that grow in or on it. For example, fibroblasts take on different shapes depending on whether they are grown on a two-dimensional or three-dimensional extracellular matrix. The extracellular matrix can also affect the expression of genes of the cells that grow in or on it. For example, the expression of the milk-protein β-casein by mammary epithelial cells grown in cell culture is strongly influenced by the type and composition of the extracellular matrix.