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Intercellular Junctions

6 April, 2016 - 17:26

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).

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Figure 3.17
There are four kinds of connections between cells. (a) A plasmodesma is a channel between the cell walls of two adjacent plant cells. (b) Tight junctions join adjacent animal cells. (c) Desmosomes join two animal cells together. (d) Gap junctions act as channels between animal cells.
 

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.

Table 3.1 Components of Prokaryotic and Eukaryotic Cells and Their This table provides the components of prokaryotic and eukaryotic cells and their respective functions.

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