# 1.3–1.4 Carbohydrates and Lipids

F.3 Biology · Topic I · Lesson 2

Three groups of carbohydrates, three food tests, and the jobs of lipids.

## Figure

Triglyceride = glycerol + 3 fatty acids. Phospholipid = glycerol + 2 fatty acids + phosphate.

## Key points

- **Carbohydrates**: organic substances of C, H and O with H : O usually **2 : 1**, as in water, so the general formula is $C_m(H_2O)_n$.
- **Monosaccharides ／ 單醣** (glucose, fructose, galactose) are the simplest carbohydrates: **soluble in water**, **sweet**, all **reducing sugars**. Glucose is oxidized in respiration for energy.
- **Condensation ／ 縮合**: two monosaccharides combine, with the **removal of a water molecule**, to form a **disaccharide**. **Hydrolysis ／ 水解** (reacting with water) splits it again; inside organisms, enzymes catalyze both.
- **maltose** = glucose + glucose · **sucrose** = glucose + fructose · **lactose** = glucose + galactose. **Sucrose** (transported in plants) is the one disaccharide that is **non-reducing**.
- **Polysaccharides** are many monosaccharides condensed together: **insoluble**, **tasteless**, non-reducing. **Starch** stores energy in plants, **glycogen** in animals (liver and muscles); **cellulose** builds the plant **cell wall**. Being compact and insoluble, these stores do not upset the osmotic balance of cells.
- **Lipids** (also C, H, O): **insoluble in water**, **soluble in organic solvents** (ether).
- **Triglycerides ／ 三酸甘油酯** (fats and oils) are **energy reserves** in **adipose tissues**: **subcutaneous fat** reduces heat loss, fat around internal organs absorbs shock; they transport the fat-soluble **vitamins A, D, E and K**. Saturated fat: **no double bonds** in its fatty acids, mostly animal fats, solid. Unsaturated fat: one or more unsaturated fatty acids, mostly plant oils, liquid.
- **Phospholipids ／ 磷脂** are the main component of **cell membranes** (phosphate end attracts water, fatty-acid end repels it). **Cholesterol** (a steroid) is also in membranes and makes **bile salts**, **sex hormones** and **vitamin D**; excess is deposited on arterial walls (heart attack, stroke).

## Must know

| Food test | Tests for | Method | Positive result |
|-------|------|-----------|-----------|
| **Benedict's test** | reducing sugars | add Benedict's solution and **heat** in a boiling water bath | blue → **brick-red precipitate** |
| **Iodine test** | starch | add iodine solution, **no heating** | brown → **blue-black** |
| **Grease spot test** | lipids | drop on filter paper, let it dry | **persistent translucent spot**, removed by an organic solvent |

## Common mistakes

- ✗ "Sucrose is a sugar, so Benedict's gives brick-red." ✓ Sucrose is a disaccharide but **not a reducing sugar**: Benedict's solution **stays blue**.
- ✗ "Add Benedict's solution and shake at room temperature; brick-red shows glucose." ✓ Benedict's test needs **heating in a boiling water bath**, and brick-red shows **a reducing sugar** (any monosaccharide, maltose or lactose), not glucose in particular.
- ✗ "Benedict's stayed blue, so there is no carbohydrate." ✓ Blue means **no reducing sugar**: starch and sucrose also stay blue. The iodine test detects **starch only**.
- ✗ "Cellulose is the food store of plants." ✓ **Starch** is the storage carbohydrate of plants and **glycogen** of animals. Cellulose is **structural** (cell wall), not a food store.
- ✗ "Disaccharides are formed by hydrolysis." ✓ **Condensation** joins two monosaccharides and **removes** a water molecule. **Hydrolysis** is the reverse: water is a reactant and the disaccharide splits.

## Quick check

1. Which sugar is a disaccharide but gives a negative Benedict's test? Name its two monosaccharide units.
2. A student adds Benedict's solution to a drink and shakes it at room temperature. What step is missing, and what does a brick-red precipitate actually show?
3. Name the storage carbohydrate of plants, the storage carbohydrate of animals, and the carbohydrate of the plant cell wall.
4. Which two of the three food tests on this card need no heating? Give the positive result of each.
5. A triglyceride is built from which molecules? State two functions of triglycerides in the body.

## Answers

1. Sucrose; glucose + fructose.
2. Heating in a boiling water bath. Brick-red shows a reducing sugar (any monosaccharide, maltose or lactose), not glucose in particular.
3. Starch; glycogen; cellulose.
4. Iodine test: brown → blue-black (starch). Grease spot test: persistent translucent spot (lipids).
5. Glycerol + 3 fatty acids. Any two: energy reserve; subcutaneous fat reduces heat loss; fat around internal organs absorbs shock; transport of fat-soluble vitamins A, D, E and K.

## Did you know?

A gram of fat stores about 38 kJ, more than twice the 17 kJ in a gram of carbohydrate — one reason migrating birds fuel up on fat.