Label the polar and non-polar ends of the lipid representation.

Problem Analysis:

The image contains several questions related to lipids, their components, and their functions. Let's solve each part step by step.

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#### Question 12: Label the polar and non-polar ends of the representation of a lipid below.

The diagram shows a simplified representation of a lipid molecule:
- The red sphere represents the polar head (hydrophilic end).
- The long black lines represent the non-polar tails (hydrophobic fatty acid chains).

Solution:
- Polar end (head): The red sphere.
- Non-polar end (tails): The black lines extending from the red sphere.

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#### Question 13: What are the two main functions of lipids in humans?

Lipids have several functions in the human body, but the two primary ones are:

1. Energy storage: Lipids, especially triglycerides, serve as an efficient form of stored energy. When the body needs energy, it can break down these lipids into fatty acids and glycerol, which can be metabolized for energy production.
2. Structural components: Lipids are essential components of cell membranes. Phospholipids form the bilayer structure of cell membranes, providing structural integrity and regulating the movement of substances in and out of cells.

Solution:
1. Energy storage
2. Structural components of cell membranes

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#### Question 14: Lipids include triglycerides (fats), phospholipids, and waxes. One of the key parts that make up a lipid is glycerol (a type of alcohol). An alcohol is recognizable by its hydroxyl (-OH) groups.

##### Part (a): Draw a box around each OH group.

The structure of glycerol is shown in the image. Glycerol has three carbon atoms, and each carbon atom has a hydroxyl (-OH) group attached to it.

Solution:
- Draw a box around each -OH group in the glycerol structure.

##### Part (b): The molecular formula for glycerol is C\_\_ H\_\_ O\_\_.

To determine the molecular formula of glycerol:
- Glycerol has 3 carbon atoms (C).
- Each carbon atom is bonded to hydrogen atoms and oxygen atoms through hydroxyl groups (-OH).
- Counting all the atoms:
- Carbon (C): 3
- Hydrogen (H): 8 (each carbon has 2 hydrogens, plus 1 hydrogen per -OH group)
- Oxygen (O): 3 (one oxygen per -OH group)

Thus, the molecular formula of glycerol is C₃H₈O₃.

Solution:
- C₃ H₈ O₃

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#### Question 15: Another key component of lipids is a fatty acid. A fatty acid is recognizable by its carboxyl group.

##### Part (a): Draw a box around the carboxyl group on each fatty acid.

The carboxyl group (-COOH) is the characteristic functional group of fatty acids. It consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl (-OH) group.

Solution:
- For butyric acid, caproic acid, and lauric acid, draw a box around the -COOH group at the beginning of each fatty acid chain.

##### Part (b): The molecular formula for butyric acid is C\_\_ H\_\_ O\_\_.

Butyric acid has the structure:
```
CH₃ - CH₂ - CH₂ - COOH
```
- Carbon (C): 4
- Hydrogen (H): 8
- Oxygen (O): 2 (one from the -COOH group and one from the -OH group)

Thus, the molecular formula of butyric acid is C₄H₈O₂.

Solution:
- C₄ H₈ O₂

##### Part (c): The molecular formula for caproic acid is C\_\_ H\_\_ O\_\_.

Caproic acid has the structure:
```
CH₃ - CH₂ - CH₂ - CH₂ - CH₂ - COOH
```
- Carbon (C): 6
- Hydrogen (H): 12
- Oxygen (O): 2 (one from the -COOH group and one from the -OH group)

Thus, the molecular formula of caproic acid is C₆H₁₂O₂.

Solution:
- C₆ H₁₂ O₂

##### Part (d): The molecular formula for lauric acid is C\_\_ H\_\_ O\_\_.

Lauric acid has the structure:
```
CH₃ - (CH₂)₁₀ - COOH
```
- Carbon (C): 12
- Hydrogen (H): 24
- Oxygen (O): 2 (one from the -COOH group and one from the -OH group)

Thus, the molecular formula of lauric acid is C₁₂H₂₄O₂.

Solution:
- C₁₂ H₂₄ O₂

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Final Answers:

1. Question 12:
- Polar end: Red sphere
- Non-polar end: Black lines

2. Question 13:
- 1. Energy storage
- 2. Structural components of cell membranes

3. Question 14:
- (a) Box around each -OH group in glycerol.
- (b) Molecular formula: C₃H₈O₃

4. Question 15:
- (a) Box around the -COOH group in each fatty acid.
- (b) Butyric acid: C₄H₈O₂
- (c) Caproic acid: C₆H₁₂O₂
- (d) Lauric acid: C₁₂H₂₄O₂

Final Answer:
\boxed{C_3H_8O_3, C_4H_8O_2, C_6H_{12}O_2, C_{12}H_{24}O_2}