Ground substance and the hydrated gel
Ground substance is the jelly-like material between the cells and fibres of connective tissue, where glycosaminoglycans (GAGs, sugar-based chains) attract water to make a hydrated (water-filled) gel.
Remember
Glycosaminoglycans carry many negative charges that pull in water, and the hydrated gel this makes lets nutrients diffuse and resists compression (being squashed), while collagen resists pulling.
Memory aid
a water-filled gel cushion with rope threaded through it
The gel takes the squeeze, and the rope, which stands for collagen, takes the pull.
Ground substance is the clear, slippery, jelly-like material that fills the space between the cells and fibres of connective tissue. It is made mainly of proteoglycans, large molecules with a central core protein and many glycosaminoglycan (GAG) chains attached.
The GAG chains carry many negative charges, and these pull water in. The ground substance becomes a hydrated gel, a gel full of water.
That gel does two jobs. Because it is mostly water, nutrients and waste products can diffuse through it (spreading from where there is more of a substance to where there is less). The trapped water also pushes back when the tissue is squeezed, so the gel resists compression, which means being squashed.
Hyaluronic acid, a particular GAG, binds many proteoglycans together into very large aggregates (huge joined-together groups of proteoglycans). In articular cartilage (the smooth cartilage covering the ends of bones at moveable joints), which has no blood vessels, the chondrocytes (cartilage cells) depend on nutrients diffusing through the matrix (the ground substance and fibres between the cells).
The fibres embedded in the gel do different jobs. Collagen gives tensile strength (resistance to being pulled apart), and elastin gives recoil (the tissue returning to its original shape after it is stretched).
Key facts
- What it is
- a slippery material with a high water content, composed primarily of proteoglycans.
- Proteoglycan
- a large molecule made of a core protein with GAG chains attached.
- Why it holds water
- GAGs carry many negative charges, which attract water.
- What the gel does
- it permits diffusion but resists compression.
- Hyaluronic acid
- a particular GAG that binds many proteoglycans into very large aggregates.
- Articular cartilage
- it has no blood vessels, so chondrocytes depend on nutrients diffusing through the matrix.
- Fibres
- collagen gives tensile strength and elastin gives recoil, so neither job belongs to the gel.
Exam traps
- Tensile strength, the resistance to being pulled apart, comes from densely packed collagen fibres, which are a separate component embedded in the ground substance, not the ground substance itself.
- GAGs carry many negative charges, not positive ones, and these charges attract water; a molecule that repelled water could not form the hydrated gel that permits diffusion and resists compression.
- Recoil after stretch is the job of elastic fibres built from elastin, and it is not the same as resisting compression, so elastin does not explain how a tissue resists being squashed.
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