Cells can also communicate with each other by direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata (singular = plasmodesma) are junctions between plant cells, whereas animal cell contacts include tight and gap junctions, and desmosomes.
In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell walls surrounding each cell. Plasmodesmata are numerous channels that pass between the cell walls of adjacent plant cells, connecting their cytoplasm and enabling signal molecules and nutrients to be transported from cell to cell (Figure 3.17 a).
A tight junction is a watertight seal between two adjacent animal cells (Figure 3.17 b). Proteins hold the cells tightly against each other. This tight adhesion prevents materials from leaking between the cells. Tight junctions are typically found in the epithelial tissue that lines internal organs and cavities, and composes most of the skin. For example, the tight junctions of the epithelial cells lining the urinary bladder prevent urine from leaking into the extracellular space.
Also found only in animal cells are desmosomes, which act like spot welds between adjacent epithelial cells (Figure 3.17c). They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.
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 3.17 d). Structurally, however, gap junctions and plasmodesmata differ.
|
Cell Component |
Function |
Present in Prokaryotes? |
Present in Animal Cells? |
Present in Plant Cells? |
|
Plasma membrane |
Separates cell from external environment; controls passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell |
Yes |
Yes |
Yes |
|
Cytoplasm |
Provides structure to cell; site of many metabolic reactions; medium in which organelles are found |
Yes |
Yes |
Yes |
|
Nucleoid |
Location of DNA |
Yes |
No |
No |
|
Nucleus |
Cell organelle that houses DNA and directs synthesis of ribosomes and proteins |
No |
Yes |
Yes |
|
Ribosomes |
Protein synthesis |
Yes |
Yes |
Yes |
|
Mitochondria |
ATP production/cellular respiration |
No |
Yes |
Yes |
|
Peroxisomes |
Oxidizes and breaks down fatty acids and amino acids, and detoxifies poisons |
No |
Yes |
Yes |
|
Vesicles and vacuoles |
Storage and transport; digestive function in plant cells |
No |
Yes |
Yes |
|
Centrosome |
Unspecified role in cell division in animal cells; organizing center of microtubules in animal cells |
No |
Yes |
No |
|
Lysosomes |
Digestion of macromolecules; recycling of worn-out organelles |
No |
Yes |
No |
|
Cell wall |
Protection, structural support and maintenance of cell shape |
Yes, primarily peptidoglycan in bacteria but not Archaea |
No |
Yes, primarily cellulose |
|
Chloroplasts |
Photosynthesis |
No |
No |
Yes |
|
Endoplasmic reticulum |
Modifies proteins and synthesizes lipids |
No |
Yes |
Yes |
|
Golgi apparatus |
Modifies, sorts, tags, packages, and distributes lipids and proteins |
No |
Yes |
Yes |
|
Cytoskeleton |
Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently |
Yes |
Yes |
Yes |
|
Flagella |
Flagella Cellular locomotion |
Some |
Some |
No, except for some plant sperm. |
|
Cilia |
Cellular locomotion, movement of particles along extracellular surface of plasma membrane, and filtration |
No |
Some |
No |
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