recap

7.5 recap

Cells can communicate with their neighbors through specialized cell junctions. In animals, these structures are gap junctions; in plants, they are plasmodesmata. The evolution of intercellular communication and tissue formation can be inferred from existing organisms, such as certain related green algae.

learning outcomes

  • Explain the biological advantages of gap junctions in animals.

  • Design investigations to answer questions about intercellular communication.

Question 1

List the characteristics of direct communication by cell junctions and chemical signaling between cells. What are the advantages of direct communication over chemical signaling?

Characteristics of direct communication by cell junctions: The size of signal molecules is limited by the size of openings between cells. It is not specific. It is fast. There can be cytoplasmic connection between cells.
Characteristics of receptor-mediated communication: The signal molecules can be larger. It is specific. It is slower. There is no direct cytoplasmic connection.
The advantage of direct communication over chemical signaling is that direct communication enables a rapid, coordinated response of many cells.

Question 2

How does the Volvocine line of green algae show possible steps in the evolution of cell communication and tissue formation?

From single-celled Chlamydomonas evolved larger and larger groups of cells with communicating junctions (Gonium, Pandorina, Eudorina). Even larger groups had some differentiation (Pleodorina and Volvox).

Question 3

The tiny invertebrate Hydra has an apical region with tentacles and a long, slender body. Hydra can reproduce asexually when cells on the body wall differentiate and form a bud, which then breaks off as a new organism. Buds form only at certain distances from the apex, leading to the idea that the apex releases a signal molecule that diffuses down the body and, at high concentrations (i.e., near the apex), inhibits bud formation. Hydra lacks a circulatory system, so this inhibitor must diffuse from cell to cell. If you have an antibody that binds to connexons and plugs up the gap junctions, how would you test the hypothesis that Hydra’s inhibitory factor passes through these junctions?

Experiments might involve applying a solution containing the antibody to the upper part of the Hydra body. The antibody would block diffusion of the signal molecule from the apex to the upper body and—if the hypothesis is correct—would allow a bud to form in the upper body. A sham experiment, in which the solution without antibody is applied, would be a control. In this case, a bud would not form in the upper body.

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