Biological Molecules: Lipids

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Notas de aula

What Are Lipids?

Lipids are macromolecules containing carbon, hydrogen and oxygen.
Unlike carbohydrates, lipids contain a lower proportion of oxygen.
Lipids are non-polar and hydrophobic (they do not mix with water).
There are two main groups of lipids: triglycerides (the main component of fats and oils) and phospholipids (a major component of cell membranes).

Triglyceride Structure

Triglycerides are made from glycerol and fatty acids.
Glycerol is an alcohol; fatty acids have a carboxyl group (–COOH) at one end and a hydrocarbon chain (the R group) at the other.
The shorthand formula for a fatty acid is RCOOH.
Fatty acids vary in the length of the hydrocarbon chain and whether the chain is saturated or unsaturated.
Unsaturated fatty acids can be mono-unsaturated (one C=C double bond) or poly-unsaturated (more than one C=C double bond).
In cis fatty acids, hydrogen atoms are on the same side of the double bond and they can be metabolised by enzymes.
In trans fatty acids, hydrogen atoms are on opposite sides of the double bond; they cannot form enzyme–substrate complexes, so they are not metabolised and are linked with coronary heart disease.

Bonding in Triglycerides

Triglycerides are formed by esterification.
An ester bond forms when a hydroxyl (–OH) group on glycerol bonds with the carboxyl (–COOH) group of a fatty acid.
An H from glycerol combines with an OH from the fatty acid to make water, so this is a condensation reaction.
Three fatty acids join to one glycerol molecule to form a triglyceride.
For one triglyceride to form, three water molecules are released.

Properties and Functions of Triglycerides

Energy storage – triglycerides have a high calorific value and provide long-term energy storage.
Thermal insulation – they reduce heat loss by acting as an insulating layer beneath the skin (e.g. whale blubber).
Buoyancy – their low density helps aquatic organisms stay afloat.
Protection – they form a cushioning layer around vital organs, protecting them from physical damage.
Triglycerides are also part of the myelin sheath, insulating nerves and speeding up nerve impulse transmission.

Why Triglycerides Are Efficient Energy Stores

They have long hydrocarbon chains with many C–H bonds and little oxygen, making them highly reduced.
Oxidation of these bonds during respiration releases large amounts of energy for ATP production.
They provide more energy per gram (≈37 kJ g⁻¹) than carbohydrates or proteins (≈17 kJ g⁻¹).
Being hydrophobic, they do not cause osmotic water uptake, allowing efficient storage in cells.
Inside cells, triglycerides form insoluble droplets with hydrophobic fatty acids on the inside and glycerol on the outside.
Plants store triglycerides as oils in seeds and fruits; these are often liquid at room temperature due to unsaturated fatty acids with kinks.
Mammals store triglycerides in adipose tissue (e.g. for energy during hibernation).
Triglyceride oxidation releases metabolic water, important for desert animals and bird/reptile embryos in eggs.

Phospholipid Structure

Phospholipids are a major component of cell membranes.
They are amphipathic: they have a hydrophilic head and hydrophobic tails.
A typical phospholipid consists of one glycerol molecule, two fatty acid chains (non-polar and hydrophobic) and one phosphate group (polar and hydrophilic).
Sometimes an additional polar group (e.g. choline) is attached.
Fatty acids are bonded to glycerol by ester bonds; the phosphate group is attached via a phosphoester bond.

Phospholipids in Membranes

Due to their hydrophobic and hydrophilic parts, phospholipids form monolayers or bilayers in water.
Hydrophobic fatty acid tails create a hydrophobic core in a bilayer, acting as a barrier to water-soluble molecules.
Hydrophilic phosphate heads form H-bonds with water, allowing the membrane to compartmentalise cells and organise specific roles into organelles.
Membrane fluidity depends on phospholipid composition: mainly saturated tails make the membrane less fluid; mainly unsaturated tails make it more fluid.
Phospholipids control membrane protein orientation through weak hydrophobic interactions, holding proteins in the membrane while allowing movement within the layer.

Biochemical Test for Lipids: The Emulsion Test

The emulsion test determines if a sample contains lipids.
It is qualitative, so it does not give a quantitative value of how much lipid is present.
Lipids dissolve in organic solvents such as ethanol.
Method: add ethanol to the sample, shake to mix, then add the mixture to a test tube of water.
If lipids are present, a milky emulsion forms (the solution appears cloudy); the more lipids present, the more obvious the milky colour.
If no lipid is present, the solution remains colourless.

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CADASTRE-SE — GRÁTIS ENTRAR

Questões de prática

Prévia grátis — 8 de 62 perguntas. Cadastre-se para ver todas.

1.Which of the following is the correct shorthand chemical formula for a fatty acid?
Easy
- ARCOOH
- BRCOH
- CRCH3
- DROH
2.Lipids contain a lower proportion of oxygen than carbohydrates.
Easy
True or false?
3.Which of the following are functions of triglycerides in organisms? (select all that apply)
Medium
- AEnergy storage
- BThermal insulation
- CBuoyancy
- DProtection of organs
- ECatalysis of reactions
4.Match each lipid component with its correct description.
Medium
- Glycerol
- Fatty acid
- Phospholipid
- Triglyceride
- An alcohol with three hydroxyl groups
- Contains a carboxyl group and a hydrocarbon chain
- Contains a phosphate group and two fatty acid chains
- Contains three fatty acid chains and one glycerol
5.Which bond is formed when a fatty acid joins to glycerol in a triglyceride?
Easy
- AEster bond
- BPeptide bond
- CGlycosidic bond
- DPhosphoester bond
6.Which of the following best defines a saturated fatty acid?
Medium
- AA fatty acid with no carbon-to-carbon double bonds
- BA fatty acid with one or more carbon-to-carbon double bonds
- CA fatty acid with a phosphate group attached
- DA fatty acid that is hydrophilic
7.Phospholipids are amphipathic, meaning they have both hydrophilic and hydrophobic parts.
Easy
True or false?
8.Which of the following statements about triglycerides are correct? (select all that apply)
Medium
- AThey are non-polar and hydrophobic
- BThey are formed by a condensation reaction
- CThey contain a phosphate group
- DThey release approximately 37 kJ of energy per gram when oxidised
- EThey are polymers of fatty acids only

Mais perguntas

9.Place the steps of the emulsion test for lipids in the correct order.
Medium
- Add ethanol to the sample and shake
- Add the ethanol mixture to a test tube of water
- Observe for a milky emulsion
10.What is the common name of the alcohol that is a monomer of triglycerides?
Easy
- AGlycerol
- BEthanol
- CMethanol
- DPropanol
11.What is the name of the bond that forms between glycerol and a fatty acid in a triglyceride?
Medium
- AEster bond
- BPeptide bond
- CGlycosidic bond
- DPhosphodiester bond
12.A triglyceride is formed from glycerol and three fatty acids. How many water molecules are released in total?
Medium
- A1
- B2
- C3
- D6
13.A phospholipid bilayer forms in water. Which part of the phospholipid is responsible for the hydrophobic core of the bilayer?
Medium
- AThe phosphate group
- BThe glycerol molecule
- CThe fatty acid tails
- DThe choline group
14.A sample of lipid is tested using the emulsion test. After adding ethanol and shaking, the mixture is poured into water. A milky emulsion forms. What does this result indicate?
Medium
- AThe sample contains protein
- BThe sample contains lipid
- CThe sample contains starch
- DThe sample contains reducing sugar
15.In an experiment, lipase is added to a beaker containing lipids. The pH of the mixture is monitored. Which change in pH would you expect and why?
Hard
- ApH increases because fatty acids are produced
- BpH decreases because fatty acids are produced
- CpH increases because glycerol is produced
- DpH decreases because glycerol is produced
16.Three cell membranes, A, B and C, are made from phospholipids containing fatty acid chains A, B and C respectively. Chain A is fully saturated, chain B has one cis double bond, and chain C has two cis double bonds. Which membrane would be the most fluid?
Hard
- AMembrane A
- BMembrane B
- CMembrane C
- DAll would be equally fluid
17.Triglycerides are polymers.
Easy
True or false?
18.The emulsion test for lipids is a quantitative test that indicates how much lipid is present.
Medium
True or false?
19.Match each lipid property with its correct explanation.
Medium
- High energy storage
- Thermal insulation
- Buoyancy
- Low density helps aquatic organisms float
- Many C–H bonds release large amounts of energy when oxidised
- Reduces heat loss beneath the skin
20.Using the data in Figure 1, what is the relationship between omega-3 intake and relative risk of coronary heart disease?
Medium
- AAs omega-3 intake increases, relative risk increases
- BAs omega-3 intake increases, relative risk decreases
- CThere is no relationship between omega-3 intake and relative risk
- DRelative risk is constant regardless of omega-3 intake
21.A student carries out the emulsion test on a food sample. After adding ethanol to the sample, what is the next step?
Medium
- AAdd the ethanol mixture to a test tube of water
- BAdd water directly to the food sample
- CHeat the ethanol mixture to boiling
- DAdd dilute hydrochloric acid to the mixture
22.Which of the following correctly describes the emulsion test for lipids?
Medium
- AIt is qualitative and gives a rough indication of lipid concentration by the intensity of the milky colour
- BIt is quantitative and gives an exact mass of lipid present
- CIt is qualitative and can only detect triglycerides, not phospholipids
- DIt is quantitative and requires a colorimeter to measure the colour change
23.A student tests a food sample for lipids using the emulsion test. They add ethanol, shake, then pour into water. The solution remains colourless. What is the most likely explanation?
Medium
- AThe sample contains no lipid
- BThe sample contains a small amount of lipid
- CThe student forgot to add ethanol
- DThe sample contains protein instead of lipid
24.In an experiment investigating the effect of lipase concentration on the hydrolysis of lipids, the pH of the mixture is monitored. Which of the following best explains why the pH decreases?
Hard
- ALipase hydrolyses triglycerides into fatty acids and glycerol, releasing H⁺ ions
- BLipase hydrolyses triglycerides into fatty acids and glycerol, releasing OH⁻ ions
- CLipase breaks down lipids into amino acids, which are acidic
- DLipase breaks down lipids into glucose, which lowers pH
25.Three cell membranes, A, B and C, are made from phospholipids containing fatty acid chains A, B and C respectively. Chain A is fully saturated, chain B has one cis double bond, and chain C has three cis double bonds. Which membrane would be the least fluid?
Hard
- AMembrane A
- BMembrane B
- CMembrane C
- DAll membranes would have the same fluidity
26.Which of the following statements about the emulsion test for lipids are correct? (select all that apply)
Medium
- AThe test is qualitative
- BA milky emulsion indicates the presence of lipid
- CThe test requires the use of a colorimeter
- DEthanol is used to dissolve the lipid
- EThe test gives an exact quantitative value for lipid concentration
27.A scientist is investigating the effect of lipase concentration on the hydrolysis of lipids. Which of the following variables should be controlled to ensure accurate and reliable results? (select all that apply)
Hard
- ATemperature
- BpH
- CVolume of lipid substrate
- DConcentration of lipase
- ETime of incubation
28.In the emulsion test, a rubber bung should be used to shake the test tube rather than a thumb.
Easy
True or false?
29.The emulsion test can be used to determine the exact concentration of lipid in a sample.
Medium
True or false?
30.Match each step of the emulsion test with its correct description.
Medium
- Step 1
- Step 2
- Step 3
- Add ethanol to the sample and shake
- Pour the ethanol mixture into water
- Observe for a milky emulsion
31.Which three elements are contained in all lipids?
Easy
- ACarbon, hydrogen and oxygen
- BCarbon, hydrogen and nitrogen
- CCarbon, oxygen and nitrogen
- DHydrogen, oxygen and nitrogen
32.Unlike carbohydrates, lipids contain:
Easy
- AA lower proportion of oxygen
- BA higher proportion of oxygen
- CNo oxygen at all
- DA higher proportion of hydrogen than carbon only
33.Lipids are non-polar and hydrophobic molecules.
Easy
True or false?
34.Which of the following are the two main groups of lipids?
Easy
- ATriglycerides and phospholipids
- BTriglycerides and polypeptides
- CPhospholipids and nucleotides
- DMonosaccharides and disaccharides
35.Which of the following are monomers used to build triglycerides? (select all that apply)
Medium
- AGlycerol
- BFatty acids
- CPhosphate groups
- DAmino acids
- ECholine
36.What is the correct shorthand chemical formula for a fatty acid?
Easy
- ARCOOH
- BRCOH
- CRCH₂OH
- DRCOOR
37.A fatty acid has hydrogen atoms on opposite sides of its carbon-to-carbon double bond. What type of fatty acid is this?
Medium
- AA trans-fatty acid
- BA cis-fatty acid
- CA saturated fatty acid
- DA polyunsaturated cis-fatty acid
38.Trans-fatty acids cannot form enzyme-substrate complexes and so are not metabolised.
Medium
True or false?
39.Which statement correctly describes the difference between cis- and trans-fatty acids?
Medium
- AIn cis-fatty acids the H atoms are on the same side of the double bond, while in trans-fatty acids they are on opposite sides
- BIn cis-fatty acids the H atoms are on opposite sides of the double bond, while in trans-fatty acids they are on the same side
- CCis-fatty acids are saturated, while trans-fatty acids are unsaturated
- DCis-fatty acids contain no double bonds, while trans-fatty acids contain one double bond
40.Which of the following is the process by which triglycerides are formed?
Medium
- AEsterification
- BHydrolysis
- CPhosphorylation
- DOxidation
41.Name the alcohol that is a monomer of triglycerides.
Easy
42.How many moles of water are released when 5 moles of glycerol react with excess fatty acids to form triglycerides?
Medium
43.Complete the sentence about the emulsion test.
Easy
In the emulsion test for lipids, the sample is first mixed with ____ and then added to water; a milky ____ indicates the presence of lipids.
44.The triglyceride stearin is made up of glycerol and stearic acid in a 1:3 ratio. Calculate how many moles of stearic acid are required to combine fully with 5 × 10⁻³ moles of glycerol.
Medium
45.The recommended daily reference intake of lipids should make up 25% of total energy intake. The recommended energy intake for most adult men is 8100 kJ day⁻¹. The energy content of lipids is 38 kJ g⁻¹. Calculate the recommended lipid intake per day for adult men, in grams. Give your answer to 1 decimal place.
Medium
46.Complete the sentence about the emulsion test for lipids.
Medium
In the emulsion test, the sample is first mixed with ____ and then added to water. A positive result is indicated by the formation of a ____ emulsion.
47.Name the type of bond that forms between glycerol and a fatty acid in a triglyceride.
Easy
48.The energy content of lipids is approximately 37 kJ g⁻¹ and that of carbohydrates is approximately 17 kJ g⁻¹. Calculate how much more energy, in kJ, is released by respiring 10 g of lipid than 10 g of carbohydrate.
Medium
49.The recommended daily reference intake of lipids should make up 25% of total energy intake. The recommended energy intake for most adult men is 8100 kJ day⁻¹ and the energy content of lipids is 38 kJ g⁻¹. Calculate the recommended lipid intake per day for adult men. Give your answer in grams.
Medium
50.Complete the sentences about the structure of a fatty acid.
Easy
A fatty acid has a methyl group at one end of a hydrocarbon chain known as the ____ group, and a ____ group at the other end. The shorthand chemical formula for a fatty acid is ____.
51.Complete the sentences about bonding in triglycerides.
Medium
Triglycerides are formed by ____. An ester bond forms when a hydroxyl group on ____ bonds with the carboxyl group of a fatty acid. Because a small molecule of ____ is released, this is a ____ reaction.
52.Complete the sentences about the results of the emulsion test for lipids.
Medium
The emulsion test is a ____ test, so it does not give a quantitative value for how much lipid is present. Lipids dissolve in the organic solvent ____. After shaking the sample with this solvent, the mixture is added to water; if lipid is present a ____ emulsion forms. If no lipid is present, the solution remains ____.
53.Describe the main structural difference between a saturated fatty acid and an unsaturated fatty acid.
Hard
54.Describe the two main differences between a triglyceride and a phospholipid.
Hard
55.Describe and explain a sample preparation step that a scientist should take before performing the emulsion test on a sample of dried pasta. Do not include health and safety precautions in your answer.
Hard
56.Using your knowledge of the structure of triglycerides, explain why the solution goes cloudy when water is added in the emulsion test.
Hard
57.Using the data in Figure 1, evaluate whether coronary heart disease can be prevented by increasing omega-3 intake.
Hard
58.How many moles of water are released when 5 moles of glycerol react with excess fatty acids to form triglycerides?
Medium
- A17 moles
- B16 moles
- C15 moles
- D14 moles
59.The triglyceride stearin is made up of glycerol and stearic acid in a 1:3 ratio. Calculate how many moles of stearic acid are required to combine fully with 5 × 10⁻³ moles of glycerol.
Medium
- A2.015 moles
- B0.015 moles
- C-0.985 moles
- D1.015 moles
60.The recommended daily reference intake of lipids should make up 25% of total energy intake. The recommended energy intake for most adult men is 8100 kJ day⁻¹. The energy content of lipids is 38 kJ g⁻¹. Calculate the recommended lipid intake per day for adult men, in grams. Give your answer to 1 decimal place.
Medium
- A53.3 g
- B58.6 g
- C63.9 g
- D48 g
61.The energy content of lipids is approximately 37 kJ g⁻¹ and that of carbohydrates is approximately 17 kJ g⁻¹. Calculate how much more energy, in kJ, is released by respiring 10 g of lipid than 10 g of carbohydrate.
Medium
- A180 kJ
- B240 kJ
- C200 kJ
- D220 kJ
62.The recommended daily reference intake of lipids should make up 25% of total energy intake. The recommended energy intake for most adult men is 8100 kJ day⁻¹ and the energy content of lipids is 38 kJ g⁻¹. Calculate the recommended lipid intake per day for adult men. Give your answer in grams.
Medium
- A53.3 g
- B48 g
- C63.9 g
- D58.6 g

Resumos de revisão

## Lipids - **Macromolecules** containing **C, H, O** — but with a **lower proportion of oxygen** than carbohydrates. - **Non-polar** and **hydrophobic**. - Two main groups: **triglycerides** (main component of fats and oils) and **phospholipids** (major component of cell membranes). ## Triglycerides: structure - Monomers are **glycerol** (an alcohol) and **fatty acids**. - Fatty acid = **R group** (hydrocarbon chain, typically 4–24 carbons) + **carboxyl group**. Shorthand formula: **RCOOH**. - Fatty acids vary by: **length of R group**, and whether **saturated or unsaturated**. - **Unsaturated** can be **mono- or poly-unsaturated**: - **cis** — H atoms on the same side of the double bond; metabolised by enzymes. - **trans** — H atoms on opposite sides; cannot form enzyme–substrate complexes, so are not metabolised; linked with **coronary heart disease**. - Formed by **esterification**: the **-OH** group on glycerol bonds with the **-COOH** group of the fatty acid. An **H** from glycerol combines with an **OH** from the fatty acid to make **water** — a **condensation reaction**. - **Three fatty acids + one glycerol → triglyceride + three water molecules.** ## Properties of triglycerides - **Energy storage** — high calorific value; long hydrocarbon chains with many **C–H bonds** and little oxygen, making them **highly reduced**. Oxidation releases large amounts of energy for ATP. Provide **≈37 kJ g⁻¹** vs **≈17 kJ g⁻¹** for carbohydrates or proteins. - **Hydrophobic** — no osmotic water uptake; form **insoluble droplets** inside cells (fatty acids inside, glycerol outside). - **Thermal insulation** — adipose tissue under the skin reduces heat loss (e.g. whale blubber); part of the **myelin sheath**, speeding up nerve impulse transmission. - **Buoyancy** — low density helps aquatic organisms stay afloat. - **Protection** — cushions and protects internal organs. - Oxidation releases **metabolic water** — important for desert animals and bird/reptile embryos in eggs. - Plants store triglycerides as **oils in seeds and fruits**, often liquid due to unsaturated fatty acids with **kinks**. ## Phospholipids - **Amphipathic**: **hydrophilic head** + **hydrophobic tails**. - Structure: **one glycerol**, **two fatty acid chains** (non-polar, hydrophobic), **one phosphate group** (polar, hydrophilic), sometimes with an additional polar group (e.g. **choline**). - Fatty acids bonded by **ester bonds**; phosphate attached via a **phosphoester bond**. - In water they form **monolayers or bilayers**. - Hydrophobic tails create a **hydrophobic core** — a barrier to water-soluble molecules. Hydrophilic phosphate heads form **H-bonds with water**, allowing compartmentalisation into organelles. - Fluidity: mainly **saturated** tails → **less fluid**; mainly **unsaturated** tails → **more fluid**. - Weak hydrophobic interactions hold membrane proteins in place while allowing movement. ## Biochemical test: emulsion test - **Qualitative** — does not give a quantitative value of how much lipid is present. - Lipids dissolve in **organic solvents such as ethanol**. - Add **ethanol** to the sample, **shake to mix**, then add the mixture to a **test tube of water**. - Lipids present → **milky emulsion** (solution appears cloudy); more lipid → more obvious milky colour. No lipid → solution remains **colourless**.

Lipid: A macromolecule containing carbon, hydrogen and oxygen, with a lower proportion of oxygen than carbohydrates; non-polar and hydrophobic.
Triglyceride: A lipid formed from one glycerol molecule and three fatty acids; the main component of fats and oils.
Glycerol: An alcohol that acts as a monomer in triglycerides.
Fatty acid: A molecule with a methyl group at one end of a hydrocarbon chain (the R group) and a carboxyl group at the other; shorthand formula RCOOH.
R group: The hydrocarbon chain of a fatty acid, typically 4 to 24 carbons long.
Saturated fatty acid: A fatty acid whose hydrocarbon chain has no carbon-to-carbon double bonds.
Unsaturated fatty acid: A fatty acid containing one or more carbon-to-carbon double bonds; can be mono- or poly-unsaturated.
cis-fatty acid: An unsaturated fatty acid with H atoms on the same side of the double bond; metabolised by enzymes.
trans-fatty acid: An unsaturated fatty acid with H atoms on opposite sides of the double bond; cannot form enzyme-substrate complexes, so is not metabolised; linked with coronary heart disease.
Esterification: The condensation reaction forming an ester bond, when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of a fatty acid.
Ester bond: The bond formed between glycerol and a fatty acid during esterification.
Phospholipid: An amphipathic lipid made of one glycerol, two fatty acid chains and one phosphate group; a major component of cell membranes.
Amphipathic: Having both a hydrophilic head and hydrophobic tails.
Phosphoester bond: The bond attaching the phosphate group to glycerol in a phospholipid.
Hydrophobic: Non-polar; repels water (e.g. the fatty acid tails of phospholipids).
Hydrophilic: Polar; attracts water (e.g. the phosphate head of a phospholipid).
Emulsion test: A qualitative test for lipids: add ethanol to the sample, shake to mix, then add to water; a milky emulsion indicates lipid is present.
Qualitative: A test that does not give a quantitative value as to how much of a substance is present.
Adipose tissue: Tissue in which mammals store triglycerides, e.g. for energy during hibernation and for insulation.
Metabolic water: Water released by the oxidation of triglycerides; important for desert animals and bird and reptile embryos within eggs.

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