While collagen provides tensile strength to resist stretching, a completely different group of macromolecules in the extracellular matrix of animal tissues provides the complementary function, resisting compression. These are the glycosaminoglycans (GAGs), negatively charged polysaccharide chains made of repeating disaccharide units.

Chains of GAGs are usually covalently linked to a core protein to form proteoglycans, which are extremely diverse in size, shape, and chemistry. Typically, many GAG chains are attached to a single core protein that may, in turn, be linked to another GAG, creating a macromolecule that resembles a bottlebrush.

In general, GAGs are strongly hydrophilic and tend to adopt highly extended conformations, which occupy a huge volume relative to their mass. Thus GAGs act as effective “space fillers” in the extracellular matrix of connective tissues.

Even at very low concentrations, GAGs form hydrophilic gels: their multiple negative charges attract a cloud of cations, such as Na + , that are osmotically active, causing large amounts of water to be sucked into the matrix. This gives rise to a swelling pressure, which is balanced by tension in the collagen fibers interwoven with the GAGs. When the matrix is rich in collagen and large quantities of GAGs are trapped in the mesh, both the swelling pressure and the counterbalancing tension are enormous. Such a matrix is tough, resilient, and resistant to compression. The cartilage matrix that lines the knee joint, for example, has this character: it can support pressures of hundreds of kilograms per square centimeter.

正确

错误

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正确

错误

相互拮抗的肌肉的交替的等张收缩

接着作主动肌的等张收缩

拮抗肌与主动肌同时的等张收缩

增加运动模式中无力成分的活动范围与耐力

拮抗肌的等张收缩、松弛"

大力发展生产力

实现共产主义

实现人民当家作主

正确处理人民内部矛盾

腊玛古猿

南方古猿

能人

直立人

智人

①②③

①②③④

④⑤

②④⑤

腊玛古猿

南方古猿

能人

直立人

智人

正确处理社会主义社会基本矛盾

正确处理阶级矛盾

正确处理人民内部矛盾

正确处理敌我矛盾