AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/1/1-5)
Unit 1 · Topic 1.5
1.5 Lipids
Lipids are a varied group of mostly nonpolar, hydrophobic molecules: fats, phospholipids and steroids. Fats store energy, phospholipids build every cell membrane, and steroids act as hormones and, in the case of cholesterol, help stabilize animal cell membranes. Whether a fatty acid is saturated or unsaturated changes how tightly lipids pack, which affects whether a fat is solid or liquid.
Key terms
- hydrophobic
- saturated fatty acid
- unsaturated fatty acid
- phospholipid
- steroid
- cholesterol
What makes a lipid a lipid
Lipids aren't defined by a single monomer. What they share is that they're mostly made of carbon and hydrogen joined by nonpolar bonds, so they're hydrophobic: they don't dissolve in water. That's why oil and vinegar separate in salad dressing.
Lipids are built by dehydration synthesis, but they aren't polymers. A typical fat (a triglyceride) is one glycerol molecule bonded to three fatty acids. A fatty acid is a long chain of carbons with hydrogens attached (a hydrocarbon chain) and an acid group at one end.
Saturated vs. unsaturated fatty acids
In a saturated fatty acid, every bond between carbons in the chain is a single bond. The chain holds as many hydrogens as possible (it's 'saturated' with hydrogen) and is straight. Straight chains pack tightly together, so saturated fats such as butter and lard tend to be solid at room temperature.
An unsaturated fatty acid has at least one double bond between carbons. In natural fats, each double bond usually puts a kink, or bend, in the chain. Kinked chains can't pack closely, so unsaturated fats such as olive oil tend to be liquid at room temperature.
The more double bonds, the more unsaturated the fatty acid, and the more liquid the lipid is at room temperature. A fatty acid with several double bonds is called polyunsaturated.
Three kinds of lipids and their jobs
- Fats store energy. A gram of fat holds more than twice as much usable energy as a gram of carbohydrate. Fat also supports cell function, and in some animals, like seals and whales, a fat layer insulates the body against cold.
- Phospholipids form membranes. A phospholipid has a glycerol attached to two fatty acid tails and a phosphate-containing head. The head is polar and hydrophilic; the tails are nonpolar and hydrophobic. In water, phospholipids arrange themselves into a double layer, the lipid bilayer, with heads facing the water and tails hidden in the middle (2.3).
- Steroids have a structure of four fused carbon rings. Many are hormones: testosterone and estrogen affect growth and development, and others help control metabolism and homeostasis. Because steroid hormones are nonpolar, they can pass through membranes and bind receptors inside cells (4.2).
- Cholesterol is a steroid found in animal cell membranes, where it gives the membrane structural stability and helps keep its fluidity in a healthy range (2.3). It is also the starting material for steroid hormones.
What you won't be tested on
You don't need to draw or memorize the molecular structures of specific lipids. Know the parts (glycerol, fatty acids, phosphate head, four rings), whether each part is hydrophilic or hydrophobic, and how saturation changes the shape and behavior of the chains.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting membrane changes in the cold
A species of fish lives in Arctic water near 0 °C, and a related species lives in warm tropical water. Predict which fish has a higher proportion of unsaturated fatty acids in its cell membranes, and justify your prediction.
Show the solutionHide the solution
- Step 1: Recall what membranes need: a membrane must stay fluid, not stiff, so its proteins and lipids can move and work (2.3).
- Step 2: Recall the effect of temperature: cold makes lipid tails pack more tightly, which makes a membrane stiffer.
- Step 3: Recall the effect of saturation: kinks from double bonds in unsaturated tails keep them from packing tightly, which keeps the membrane fluid.
- Step 4: Predict: the Arctic fish should have more unsaturated fatty acids, which offset the stiffening effect of the cold.
Answer: The Arctic fish. More unsaturated (kinked) fatty acid tails can't pack tightly, which keeps its membranes fluid at low temperatures.
- Example 2
Phospholipid orientation (classic trap)
A student drops phospholipids into water and predicts they'll form a single layer with tails pointing out into the water. What will actually happen, and why?
Show the solutionHide the solution
- Step 1: Identify the parts: the phosphate head is polar and hydrophilic; the fatty acid tails are nonpolar and hydrophobic.
- Step 2: Apply 'like dissolves like': the heads can form attractions with polar water, but the tails can't.
- Step 3: The trap: hydrophobic tails don't 'want' to face water. They end up grouped together away from it.
- Step 4: Predict the arrangement: two layers form with tails facing each other in the middle and heads facing the water on both sides. This is a bilayer, the basic structure of a membrane.
Answer: They form a bilayer: hydrophilic heads face the water on both sides and hydrophobic tails face each other in the middle, away from water.
Common mistakes
- Calling lipids polymers. They're built by dehydration synthesis but are not chains of one repeating monomer.
- Reversing saturated and unsaturated. Saturated = all single bonds, straight chains, solid at room temperature. Unsaturated = at least one double bond, kinked chains, liquid at room temperature.
- Saying a whole phospholipid is hydrophobic. The head is hydrophilic and the tails are hydrophobic; that two-sided nature is what makes a bilayer form.
- Forgetting that steroids are lipids. They don't look like fats, but they're nonpolar and hydrophobic.
On the exam
- Expect prediction questions that link fatty acid saturation to membrane fluidity or to whether a fat is solid or liquid, often with temperature in the scenario.
- If asked how a steroid hormone enters a cell, connect its nonpolar structure to its ability to cross the lipid bilayer.
Connected topics
Videos
Check yourself
4 questions on 1.5 Lipids. Pick an answer to see if you got it, and why.
| Fatty acid | Number of carbons | Number of C=C double bonds | Melting point (°C) |
|---|---|---|---|
| Myristic acid | 14 | 0 | 54 |
| Palmitic acid | 16 | 0 | 63 |
| Stearic acid | 18 | 0 | 69 |
| Oleic acid | 18 | 1 | 13 |
| Linoleic acid | 18 | 2 | −5 |
| Linolenic acid | 18 | 3 | −11 |
Approximate values from standard chemistry reference tables
Which of the following claims is best supported by the data for the 18-carbon fatty acids?
Which of the following best explains the trend among the 18-carbon fatty acids?
Some fish live in very cold water. Based on the data, which of the following would most likely help keep their cell membranes fluid?
When phospholipids are mixed with water, they spontaneously form a double layer. Which of the following best explains this behavior?
0 of 4 answered