This layer of carbohydrate helps protect the cell surface from mechanical damage. And because the oligosaccharides and polysaccharides attract water molecules, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels.

They have an important role in cell–cell recognition and adhesion. Transmembrane proteins called lectins are specialized to bind to particular oligosaccharide side chains. The oligosaccharide side chains of glycoproteins and glycolipids, although short (typically fewer than 15 sugar units), are enormously diverse. Unlike proteins, in which the amino acids are all joined together in a linear chain by identical peptide bonds, sugars can be joined together in many different arrangements, often forming elaborate branched structures. Using a variety of covalent linkages, even three different sugars can form hundreds of different trisaccharides.

The carbohydrate layer on the surface of cells in a multicellular organism serves as a kind of distinctive clothing, like a police officer’s uniform. It is characteristic of each cell type and is recognized by other cell types that interact with it. Specific oligosaccharides in the carbohydrate layer are involved, for example, in the recognition of an egg by sperm.

Similarly, in the early stages of a bacterial infection, carbohydrates on the surface of white blood cells called neutrophils are recognized by a lectin on the cells lining the blood vessels at the site of infection; this recognition causes the neutrophils to adhere to the blood vessel wall and then migrate from the bloodstream into the infected tissue, where they help destroy the invading bacteria.

卵巢固有韧带

子宫圆韧带

宫骶韧带

卵巢悬韧带

阔韧带

主韧带

卵巢悬韧带

卵巢固有韧带

宫既韧带

子宫圆韧带

The number of cell phones in use exploded in the late 1980s and part of the 1990s．

The number of traffic deaths was reduced in the late 1980s and part of the 1990s due to eell phone use．

Cell phone users are likely to be involved in traffic accidents．

The use of cell phones has a life—saving effect for pedestrians and drivers．

It is one of the two main parts of the cell.

It is surrounded by the cytoplasm.

It forms an impassable wall to all materials.

It only allows certain materials in and out.

Although actin is found throughout the cytoplasm of a eukaryotic cell, in many cells it is highly concentrated in a layer just beneath the plasma membrane. In this region, called the cell cortex, actin filaments are linked by actin-binding proteins into a meshwork that supports the plasma membrane and gives it mechanical strength.

In human red blood cells, for example, a simple and regular network of fibrous proteins—including actin and spectrin filaments—attaches to the plasma membrane, providing the support necessary for the cells to maintain their simple discoid shape.

In other animal cells, the cortex includes a much denser network of actin filaments that are cross-linked into a three-dimensional meshwork.

The rearrangements of actin filaments within the cortex provide much of the molecular basis for changes in both cell shape and cell locomotion.

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