IB Biology · Theme B: Form and function · SL and HL
B1.1 Carbohydrates and lipids
A one-page summary of B1.1 Carbohydrates and lipids, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
In what ways do variations in form allow diversity of function in carbohydrates and lipids?
How do carbohydrates and lipids compare as energy storage compounds?
What B1.1 covers
Carbon chemistry: building and breaking macromolecules
- B1.1.1Carbon builds everything
- B1.1.2Building polymers, one water molecule at a time
- B1.1.3Digestion is hydrolysis at scale
Carbohydrates: form and function
- B1.1.4One formula, many sugars
- B1.1.5Starch and glycogen: coiled for storage
- B1.1.6Cellulose: the same sugar, a different job
- B1.1.7Sugar tags for cell recognition
Lipids: structure, storage and membranes
- B1.1.8Lipids: built to repel water
- B1.1.9One glycerol, two different lipids
- B1.1.10Kinks control melting point
- B1.1.11Fat: the densest energy store
- B1.1.12Membranes build themselves
- B1.1.13Steroids slip straight through
B1.1 Carbohydrates and lipids: summary
Carbon & macromolecules
- Carbon: four covalent bonds → chains, branches, rings.
- Condensation builds polymers; hydrolysis breaks them.
Monosaccharides
- α/β-glucose differ only in C1 –OH orientation.
- Ribose/deoxyribose (RNA/DNA); galactose/fructose (glucose isomers).
Storage vs structural carbs
- Starch/glycogen: coiled α-glucose, energy storage.
- Cellulose: straight β-glucose, H-bonded microfibrils, structural.
Glycoproteins & lipids
- Glycoproteins tag cells for recognition (ABO antigens).
- Lipids are hydrophobic: fats, oils, waxes, steroids.
Triglycerides & fatty acids
- Triglyceride = glycerol + 3 fatty acids; energy + insulation.
- Saturation (0/1/2+ C=C bonds) sets melting point.
Membranes & steroids
- Phospholipids (amphipathic) self-assemble into bilayers.
- Non-polar steroids cross membranes unassisted; polar hormones can't.
Key terms
- Condensation reaction
- Joins two monomers into a larger molecule, releasing water.
- Hydrolysis reaction
- Adds water across a bond to split a polymer into monomers.
- Monosaccharide
- A single sugar unit, the simplest carbohydrate.
- Polysaccharide
- A polymer of many monosaccharides linked by glycosidic bonds.
- Glycoprotein
- A protein with carbohydrate chains attached, used in cell recognition.
- Triglyceride
- Glycerol joined to three fatty acids by ester bonds.
- Phospholipid
- Glycerol with two fatty acids and a phosphate group; amphipathic.
- Steroid
- A small, non-polar lipid that can cross membranes unassisted.
Sample exam questions
Three of the 40 multiple-choice questions for B1.1. Try each one before opening the answer.
Question 1. The general formula for a carbohydrate is (CH2O)n. What does this formula indicate about the ratio of elements?
- Carbon and water are present in equal amounts
- Carbon, hydrogen and oxygen are present in a 1:2:1 ratio
- Hydrogen and oxygen are present in a 2:1 ratio only in monosaccharides
- The carbons are all hydrated individually
Show the answer
Answer: B. The formula (CH2O)n means that for every carbon atom there are two hydrogen atoms and one oxygen atom — a 1:2:1 ratio. This is why they are called 'carbo-hydrates': carbon plus the components of water.
Question 2. Starch and cellulose are both polymers of glucose, yet humans can digest starch but not cellulose. What explains this difference?
- Cellulose contains nitrogen, which human enzymes cannot break down
- Starch is a smaller molecule than cellulose and therefore easier to digest
- Starch has α-1,4 glycosidic bonds; cellulose has β-1,4 glycosidic bonds, and human enzymes only recognise the α form
- Cellulose is only found in animal cells, which humans lack the enzymes to break down
Show the answer
Answer: C. The difference lies in the glycosidic bond geometry. Starch uses α-1,4 bonds (OH group below the ring plane), which human amylase recognises. Cellulose uses β-1,4 bonds (OH above the plane), and every other glucose monomer is flipped — human enzymes cannot hydrolyse this linkage.
Question 3. Glucose, galactose and fructose all have the molecular formula C6H12O6. What is the relationship between them?
- They are identical in both structure and function
- They are polymers — repeating units of a smaller molecule
- They are isotopes — same number of protons, different number of neutrons
- They are structural isomers — same formula, different arrangement of atoms
Show the answer
Answer: D. Glucose, galactose and fructose are structural isomers: they share the molecular formula C6H12O6 but differ in the arrangement of functional groups around asymmetric carbon atoms. Glucose and galactose are aldoses (carbonyl at end); fructose is a ketose (carbonyl in middle).
Linking questions
Questions that connect B1.1 to other parts of the course, the kind that come up in Paper 2.
- The solvent properties of water (A1.1) are essential for the transport of glucose in blood plasma. How does the polarity of glucose enable its dissolution and transport? (see A1.1)
- How does the structure of phospholipids, introduced in B1.1, underpin the formation and selective permeability of cell membranes? (see B2.1)
- Protein channels and pumps embedded in the phospholipid bilayer are responsible for membrane transport. How do the properties of amino acid R-groups determine whether a protein region sits within the hydrophobic core of the membrane or faces the aqueous environment? (see B1.2, B2.1)
- During prolonged starvation, the body shifts from carbohydrate metabolism to lipid metabolism. Explain how this shift relates to the different energy-storage roles of glycogen and adipose tissue, and why it is a survival adaptation. (see C1.2)
Practise B1.1
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