Intercellular Junctions
Cells can also communicate with one another via direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata are junctions between plant cells, whereas animal cell contacts include tight junctions, gap junctions, and desmosomes.
Plasmodesmata
In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell wall that surrounds each cell (Figure 4.8b). How then, can a plant transfer water and other soil nutrients from its roots, through its stems, and to its leaves? Such transport uses the vascular tissues, xylem and phloem, primarily. There also exist structural modifications called plasmodesmata (singular = plasmodesma): numerous channels that pass between cell walls of adjacent plant cells, connect their cytoplasm, and enable materials to be transported from cell to cell, and thus throughout the plant (Figure 4.28).
Tight Junctions
A tight junction is a watertight seal between two adjacent animal cells (Figure 4.29). The cells are held tightly against each other by proteins; predominantly two proteins called claudins and occludins.
This tight adherence prevents materials from leaking between the cells; tight junctions are typically found in epithelial tissues that line internal organs and cavities, and comprise most of the skin. For example, the tight junctions of the epithelial cells lining your urinary bladder prevent urine from leaking out into the extracellular space.
Desmosomes
Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 4.30). Short proteins called cadherins in the plasma membrane connect to intermediate filaments to create desmosomes. The cadherins join two adjacent cells together and maintain the cells in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.
Gap Junctions
Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 4.31). Structurally, however, gap junctions and plasmodesmata differ.
Gap junctions develop when a set of six proteins, called connexins, in the plasma membrane arrange themselves in an elongated donut-like configuration called a connexon. When the pores, or doughnut holes, of connexons in adjacent animal cells align, a channel between the two cells forms. Gap junctions are particularly important in cardiac muscle: The electrical signal for the muscle to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in tandem.
Link to Learning
To conduct a virtual microscopy lab and review the parts of a cell, work through the steps of this interactive assignment.
Explain two similarities and two differences between plant and animal cells that can be seen under a microscope.
- Plant cells have cell walls, which provide structure to the plant, and also chloroplasts, which allow for photosynthesis. Animal cells do not have either of these structures. Both cells have nuclei, the command center of the cell, and cytoplasm, the gel-like solution that fills the cell.
- Plant cells and animal cells have cell walls as well as nuclei. Plant cells have chloroplasts as well as plasmodesmata, which are lacking in animal cells.
- Plant cells have cell walls, which provide structure to the plant, and also chloroplasts, which allow for photosynthesis. Animal cells do not have either of these structures. Animal cells and plant cells both have glyoxysomes as well as cytoplasm.
- Plant cells and animal cells both have a rigid plasma membrane as well as cytoplasm, which is the gel-like solution that fills the cell. Plant cells have cell walls, which provide structure to the plant, and also chloroplasts, which allow for photosynthesis. Animal cells do not have either of these structures.