Chapter 2 - Chemical Components of Cells (Part D)

1. Sugars （糖类）

Monosaccharides 单糖
- General formula: (CH₂O)ₙ (e.g., glucose 葡萄糖, C₆H₁₂O₆)
- Isomers 异构体: same formula, different structures
- D-form and L-form: optical isomers 光学异构体
Polysaccharides 多糖
- Disaccharides 二糖 (e.g., sucrose 蔗糖 = glucose + fructose 果糖)
- Oligosaccharides 寡糖: 2–10 monosaccharides
- Polysaccharides 多糖: hundreds–thousands of monosaccharides
- Glycosidic bond 糖苷键 & Condensation reaction 缩合反应 vs. Hydrolysis 水解反应
Biological roles
- Energy storage: glycogen 糖原 (animals), starch 淀粉 (plants)
- Structural: cellulose 纤维素 (plant cell walls), chitin 几丁质 (insect/fungal cell walls)
- Glycoproteins 糖蛋白 and glycolipids 糖脂 in membranes (cell recognition, adhesion)

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Source: Campbell Biology

Structure
- Hydrophobic hydrocarbon tail 疏水烃链
- Hydrophilic carboxyl group 亲水羧基
- Amphipathic 两亲性
Types
- Saturated fatty acids 饱和脂肪酸: no double bonds
- Unsaturated fatty acids 不饱和脂肪酸: one or more double bonds (cis “kinks”)
Storage and derivatives
- Fatty acids are stored in the cytoplasm of many cells in the form of fat droplets composed of triacylglycerol molecules
  - Triacylglycerol 三酰甘油/甘油三酯: glycerol 甘油 backbone + 3 fatty acids
- Lipids 脂类: loosely defined as molecules that are insoluble in water, but soluble in organic solvents such as benzene.
  - Examples of lipids: fatty acids (and their derivatives, including triacylglycerols)
- Phospholipids 磷脂: hydrophilic head + two hydrophobic tails → lipid bilayer 脂质双层
- Other lipids: Steroids 类固醇, polyisoprenoids 多异戊二烯类化合物

Two categories of lipids

Lipid Category	Sub-class	Key Examples	Built From	Notes
Fatty acid–based lipids	Triacylglycerols (fats, oils)	Storage fat in adipose tissue	Glycerol + 3 fatty acids	Main energy reserve
	Phospholipids	Phosphatidylcholine, phosphatidylethanolamine	Glycerol + 2 fatty acids + phosphate group	Cell membrane bilayer
	Sphingolipids	Sphingomyelin, cerebrosides	Sphingosine + fatty acid	Found in nerve tissue, myelin
Isoprenoid (non–fatty acid) lipids	Steroids	Cholesterol, cortisol, estrogen, testosterone	Isoprene units (C₅H₈), cyclized into 4-ring steroid nucleus	Membrane fluidity, hormones, bile acids
	Polyisoprenoids	Dolichol, natural rubber, ubiquinone (CoQ)	Long chains of isoprene units	Electron transport, glycoprotein synthesis
	Carotenoids & Vitamins	Vitamin A, E, K, carotene	Isoprene units	Pigments, antioxidants, cofactors

<aside> 💡

Summary:

Fatty acid–based lipids = energy storage + membranes.
Isoprenoid lipids = signaling, cofactors, pigments, and structural roles. </aside>

General structure
- α-carbon connected to: amino group 氨基, carboxyl group 羧基, side chain R基团
- Exist as D- and L-forms (only L-form used in proteins)
Peptide bond 肽键
- Formed by condensation reactions between amino and carboxyl groups
- Polypeptide 多肽: directionality with N-terminus and C-terminus

amino acid condensation 1.png

Diversity
- 20 standard amino acids with different R groups
- Properties: polar 极性, nonpolar 非极性, acidic 酸性, basic 碱性