1.4 Carbohydrates
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Carbohydrate | 碳水化合物 | A biological molecule consisting of carbon, hydrogen, and oxygen, typically with a hydrogen-oxygen atom ratio of 2:1 (like water). |
| Monosaccharide | 单糖 | The simplest carbohydrate (simple sugar); the monomer used to build complex carbohydrates. |
| Polysaccharide | 多糖 | A polymer of many monosaccharides, formed by dehydration synthesis reactions. |
| Glycosidic Linkage | 糖苷键 | The specific type of covalent bond that connects two monosaccharides together. |
| Cellulose | 纤维素 | A structural polysaccharide of plant cell walls, consisting of glucose monomers joined by linear glycosidic linkages. |
| Starch | 淀粉 | A storage polysaccharide in plants, consisting entirely of glucose monomers. |
| Glycogen | 糖原 | An extensively branched glucose storage polysaccharide found in the liver and muscle of animals. |
1. Monomers and Polymers of Carbohydrates
Carbohydrates are the primary energy source for most living things and provide crucial structural support for cells. They are typically composed of Carbon, Hydrogen, and Oxygen in roughly a 1:2:1 ratio (the general formula is a multiple of \(\ce{CH2O}\)).
- The Monomers: The building blocks of carbohydrates are monosaccharides (simple sugars). The most common and important monosaccharide in biology is glucose (\(\ce{C6H12O6}\)).
- The Polymers: When multiple monosaccharides are joined together, they form polysaccharides (complex carbohydrates).
- The Connection: Just like we learned in the previous section, these monomers are connected by covalent bonds through dehydration synthesis (removing a molecule of \(\ce{H2O}\)). In carbohydrates, this specific covalent bond is called a glycosidic linkage.
AP Exam Exclusion Statement: Chemical Structures
The specific molecular structures (like memorizing the exact placement of \(-\ce{OH}\) groups in alpha vs. beta glucose rings) of carbohydrate polymers are beyond the scope of the AP Exam. You only need to know their general shape (linear vs. branched) and their biological function!
2. Structure Dictates Function: Linear vs. Branched
Complex carbohydrates can form different overall shapes depending on how the covalent bonds connect the monomers. They may be linear (straight chains) or branched (chains with side-branches). The 3D structure of the polysaccharide directly determines its biological function.
A. Energy Storage (Branched)
Organisms need to store excess sugar for later use. Polysaccharides used for storage tend to be branched. The branches create multiple "ends" on the molecule, allowing enzymes to quickly break off glucose monomers (via hydrolysis) from many points at once when energy is urgently needed.
- Starch: The storage polysaccharide used by plants. It consists of a mixture of linear and somewhat branched chains. Plants store starch granules within their chloroplasts and roots.
- Glycogen: The storage polysaccharide used by animals. It is highly branched, reflecting the higher and more rapid energy demands of animals compared to plants. Humans store glycogen primarily in liver and muscle cells.
B. Structural Support (Linear)
Organisms also use carbohydrates to build strong physical structures. Polysaccharides used for structure tend to be linear (unbranched).
- Cellulose: The major component of the tough walls that enclose plant cells. Because the chains are strictly linear, parallel cellulose molecules can lie close together. They form countless hydrogen bonds between the chains, creating strong, cable-like microfibrils.
- Note: Most animals (including humans) do not have the enzymes to hydrolyze (digest) the specific covalent bonds in cellulose. It passes through our digestive tract as "insoluble fiber."
Quiz
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