Biology  by Bradford
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IB BIOLOGY · THEME B · FORM AND FUNCTION

How sugars and fats fuel and build cells

Starch, wood, butter and the membrane round every cell are built from the same few atoms. Turn one OH group over and a food store becomes a plant fibre; add one double bond and a solid fat becomes an oil.

IB Biology B1.1SL and HL4 hours11 chaptersdry lab

Start learningRun the dry labTeach it

-glucose: a food storeTurn every other one overa plant fibreOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OHOOHOHOHHOCH2OH
One OH group, turned over, changes a food store into a plant fibre. Chapter 5 shows how.

01Learn

Eleven chapters, all standard level: B1.1 has no higher level content. Build the molecules yourself in the bond builder, and make each prediction before you open the answer.

  1. 1Chains, branches and rings
  2. 2Water out to build, water in to break
  3. 3Glucose: small, soluble and stable
  4. 4Starch and glycogen: energy packed tight
  5. 5Turn every other glucose over and the chain runs straight
  6. 6Sugar name tags on the cell surface
  7. 7Lipids keep their distance from water
  8. 8Every double bond puts a kink in the tail
  9. 9Fat: the lightest way to carry energy
  10. 10Half loves water, half does not
  11. 11Four rings, and straight through the membrane

B1.1.1 Carbon compounds

Chains, branches and rings

Every molecule in this guide is built round carbon atoms. To see why carbon is so good at it, start with how atoms join. A covalent bond is a pair of electrons shared by two atoms, and the shared pair holds them together.

A carbon atom has four electrons in its outer shell to share, so it can make up to four covalent bonds: four single bonds, or a double bond plus two single bonds. It bonds to other carbon atoms and to other non-metals: hydrogen, oxygen, nitrogen, phosphorus and sulfur.

Because carbon bonds to carbon, it builds skeletons. A carbon compound can have a chain, either unbranched, like the tail of a fatty acid, or branched; a single ring, like glucose; or several rings fused together, as in a steroid. Add other atoms to each skeleton and the number of possible molecules is enormous. That variety is why life, with all its forms, is built on carbon.

  • Carbon makes four bonds in total, and a double bond counts as two.
  • Three skeletons to recognise in a drawing: a chain (straight or branched), a single ring and fused rings.
Why every lab agrees on a nanometre Scientists can compare measurements only if a unit means the same everywhere. The SI, the international system of units, was adopted by agreement in 1960. Its prefixes scale a unit up or down in powers of ten: kilo, centi, milli, micro and nano. Biology uses all five, as the ladder below shows.
Watch out A double bond is not one bond: it uses two of carbon’s four. Count it twice when you check a drawing.

A drawing shows a carbon atom with a double bond to an oxygen, a single bond to another carbon and single bonds to two hydrogens. What is wrong with it?

HHshared pair of electronsTwo hydrogen atomsCHHHHmethane, CH4from carbonfrom hydrogenCCHHHHdouble bond: two shared pairsethene, C2H4unbranched chainbranched chaineach corner and end is a carbon atom; hydrogens are left outOone ringfour fused rings
  1. A covalent bond is a pair of electrons shared by two atoms.
  2. Carbon has four electrons to share, so it makes four bonds.
  3. A double bond shares two pairs: it uses two of carbon’s four.
  4. Carbon bonds to carbon, so it builds chains, straight or branched,
  5. and rings, one or several fused together.
kilok× 1000kJa food labelcentic÷ 100a potato, about 10 cmmillim÷ 1000the tube’s boremicroµ÷ 1 000 000starch grain andred blood cellnanon÷ 1 000 000 000microfibril andglucose molecule

Tap a prefix.

Five prefixes biology uses all the time, from a food label to a molecule.

B1.1.2 B1.1.3 Condensation and hydrolysis

Water out to build, water in to break

Many of life’s large molecules are polymers: long chains of similar small units called monomers. Very large molecules like these are macromolecules.

Monomers are joined by a condensation reaction. An OH group from one monomer and an H from the other leave together as a molecule of water, and a covalent bond forms between the two monomers. One water molecule is released for every bond made.

Three families of polymer are built this way. Polysaccharides are monosaccharides joined by glycosidic bonds. Polypeptides are chains of amino acids (B1.2) and nucleic acids are chains of nucleotides (A1.2).

Hydrolysis runs the other way: hydro means water and lysis means splitting. A water molecule is split, its H joins one monomer and its OH joins the other, and the bond between them breaks. Digestion is hydrolysis: it turns the polymers in food back into monomers. In the amylase lab you can watch an enzyme hydrolyse starch.

In cells, enzymes catalyse both reactions. Building polymers by condensation is part of anabolism, and breaking them down by hydrolysis part of catabolism (C1.1). Not every large molecule is a polymer: triglycerides are also made by condensation, but they are not chains of repeating units (chapter 7). Why water behaves as it does is in A1.1.

Watch out Condensation releases water and hydrolysis uses it. And in hydrolysis the water is not simply added: it is split, and its H and OH end up in the two new ends.

Ten glucose molecules are joined into one unbranched chain. How many water molecules were released?

OHOHHOHOHHHHOHCH2OH123456OHOHHOHOHHHHOHCH2OH123456alpha glucosealpha glucoseHHOwater formswaterOHHOHOHHHHOHCH2OH123456OHOHHOHOHHHHCH2OH123456Oglycosidic bondmaltoseOHHOHHOHHOHHOHHOwater leaves at every linkmonomerpolymerHHOwater is split: its OH goes to one end and its H to the other
  1. Two monomers: alpha glucose. Watch the OH groups that face each other.
  2. Condensation: an OH and an H leave together as a molecule of water.
  3. A covalent glycosidic bond now joins them.
  4. Repeat it and a chain grows: a polymer. Polysaccharides, polypeptides and nucleic acids are all built like this.
  5. Hydrolysis runs it backwards: water is split, its H to one monomer and its OH to the other.
OHHOHOHHHHOHCH2OH123456OHOHHOHOHHHOHCH2OH123456+OHHOHHOHHOHOHHHHOHCH2OH123456OHOHHOHOHHHHCH2OH123456OmaltoseHHOalpha glucose + alpha glucosemaltose + watercondensationhydrolysis
  1. Two alpha glucose molecules, their carbons numbered.
  2. The OH on carbon 1 of one and the H from the OH on carbon 4 of the other face each other.
  3. They leave as water, and a 1,4 glycosidic bond joins the rings: condensation, making maltose.
  4. Add water and the bond splits again, the H to one sugar and the OH to the other: hydrolysis.
Join two alpha (α) glucose, then add a third. Then split one bond with water.

B1.1.4 Monosaccharides

Glucose: small, soluble and stable

A monosaccharide is one sugar unit, the monomer of carbohydrates. In cells most monosaccharides exist as rings. They are named by their number of carbon atoms: a pentose has five (ribose, deoxyribose) and a hexose has six (glucose, fructose).

To recognise one from a ring diagram, count every carbon atom, including any outside the ring in CH2OH groups. The ring also holds one oxygen atom, which is not a carbon.

Glucose comes in two forms with the same formula, C6H12O6: alpha (α) glucose and beta (β) glucose. They are isomers. They differ only in the OH on carbon 1: it is below the ring in α-glucose and above it in β-glucose, on the same side as the CH2OH. That one difference decides whether glucose is built into starch or into cellulose, as the next two chapters show.

Glucose is the sugar cells run on, and four properties explain why:

  • Soluble. Its many OH groups form hydrogen bonds with water (A1.1), so it dissolves in cytoplasm and in blood plasma.
  • Transportable. Dissolved in plasma, it is carried to every cell.
  • Chemically stable. It does not react or break down by itself, so it keeps while it is stored and carried, until an enzyme acts on it.
  • Yields energy. Oxidised in respiration to carbon dioxide and water, it releases about 16 kJ per gram (C1.2).

Remember it ABBA: which way carbon 1’s OH points

  • AlphaBelowthe OH on carbon 1 points down, away from the CH2OH
  • BetaAbovethe OH on carbon 1 points up, on the same side as the CH2OH

Remember it Why glucose is the sugar on the move

  • SweetSolubleits OH groups hydrogen-bond with water
  • TreatsTransportabledissolved, it travels in blood plasma
  • StayStableit keeps until an enzyme acts on it
  • EnergeticEnergy yieldoxidised in respiration, it releases energy
Watch out α-glucose and β-glucose are not different sugars with different formulas. Both are C6H12O6: only the OH on carbon 1 differs.

Sugar C in the second figure has a five-sided ring. Is it a pentose or a hexose?

OOHOHOHHOCH2OH123456OOHOHOHHOCH2OH123456OH-glucose, dashedC6H12O6the same atoms in every state-glucose-glucose-glucose, solid
One glucose ring, with carbon 1 ringed in pink. Switch between the two forms, or lay one over the other.
OHOHHOHOHHHHOHCH2OHOHOHHOHOHHHHOHCH2OH123456AOOHHHOHHHOHHOH2COOHHHOHHHOHHOH2C1'2'3'4'5'BOOHHOHHHOHCH2OHHOH2COOHHOHHHOHCH2OHHOH2C234516COOHHHOHOHHHHOHCH2OHOOHHHOHOHHHHOHCH2OH123456DOOHHHHHHOHHOH2COOHHHHHHOHHOH2C1'2'3'4'5'E

Tap a sugar. Pentose or hexose?

Count every carbon, the ones outside the ring too. The ring itself holds one oxygen.

B1.1.5 Starch and glycogen

Starch and glycogen: energy packed tight

Plants store energy as starch and animals (and fungi) store it as glycogen. Both are polymers of α-glucose, built by condensation.

Starch is a mixture of two polysaccharides. Amylose is an unbranched chain that coils into a helix. Amylopectin is a coiled chain with branches coming off it. Glycogen is built like amylopectin, but it branches far more often.

Coiling and branching make both stores compact: a great deal of glucose in a small space. Starch is packed into grains inside plant cells, and glycogen into granules in liver and muscle cells.

Each molecule is also huge, thousands to tens of thousands of glucose units, so it barely dissolves. Being relatively insoluble, a store stays where it is put. It does not draw water into the cell, as the same glucose dissolved in the cytoplasm would (osmosis, B2.1).

α-glucose is easy to add by condensation, to build the store, and easy to take off by hydrolysis, to use it. Enzymes do both at the ends of the chains, one glucose unit at a time.

Watch out Insoluble does not mean hydrophobic. Starch and glycogen are covered in OH groups: they barely dissolve because each molecule is so large.
Example A potato tuber is roughly a sixth starch by mass. An adult human liver holds about 100 g of glycogen and the muscles about 400 g more, roughly a day’s energy.

An amylose molecule and a glycogen molecule hold the same number of glucose units. A sprinter’s muscles suddenly need glucose. Which store can release it faster?

working ends: 1working ends: 5working ends: 18one glucose unita working end, where glucose goes on or comes offreal molecules: thousands of glucose units
Each bead is one glucose unit. Switch between the three stores and compare their shapes and their ends.
Pick a store, then Build or Use: an enzyme works at every free end at once. Compare how fast the glucose comes off amylose and glycogen.

B1.1.6 Cellulose

Turn every other glucose over and the chain runs straight

Cellulose is an unbranched polymer of β-glucose and the main material of plant cell walls. β-glucose does not join to itself the way α-glucose does. Try it in the bond builder under this chapter, then read on.

In β-glucose the OH on carbon 1 points up, but the OH on carbon 4 points down. To join two of them, the second must be turned over, so in the chain each unit is upside down compared with the one before it: alternate units point opposite ways.

The alternation keeps the chain straight rather than coiled. Straight chains lie side by side, and OH groups on neighbouring chains form hydrogen bonds that cross-link them into bundles called microfibrils.

One hydrogen bond is weak, but there are thousands along every chain, and together they make a microfibril very strong when pulled: it has high tensile strength. In a cell wall, layers of microfibrils lie at different angles. The wall resists the pressure of the cell’s contents and helps hold the plant up (D2.3).

Few organisms make an enzyme for the β links, and humans do not, so cellulose passes through us as fibre. Amylase cannot hydrolyse it (C1.1).

Where it leads Because cellulose is so hard to break down, wood and peat store carbon for centuries or longer, and coal formed from ancient plant matter. Stores like these are carbon sinks (C4.2, D4.3).
Watch out Cellulose is the plant’s structure, not its energy store: it is built to stay in the wall.

Cellulose chains lie side by side in bundles. What holds neighbouring chains together?

OOHHHOHOHHHHOHCH2OHOOHHHOHOHHHHOHCH2OHbeta glucosebeta glucoseOHHOHOHHHHOHCH2OHOOHHHOHOHHHHCH2OHOglycosidic bondHHOwaterthe second unit is turned overcellulose: a chain of beta glucose unitsevery other unit is turned overthe dot marks each ring’s oxygen: top, then bottom, then top againthree chains side by sideone chain, seen end-onmicrofibril: a bundle of chainscell wall, cut awayeach layer: microfibrils side by side, all in one direction
  1. Two beta glucose molecules. Their OH groups point opposite ways.
  2. Turn one over and they can join: water leaves and a bond forms.
  3. Every other unit is turned over, so the chain runs straight.
  4. Neighbouring chains are linked by many hydrogen bonds.
  5. Bundles of chains form microfibrils.
  6. Layers of microfibrils give the cell wall its strength.
OHHOHOHHHHOHCH2OH123456OOHHHOHOHHHOHCH2OH123456+OHHOHHOHHOHOHHHHOHCH2OH123456OOHHHOHOHHHHCH2OH123456OdisaccharideHHObeta glucose + beta glucosedisaccharide + watercondensationhydrolysis
  1. Two beta glucose molecules, their carbons numbered.
  2. The OH on carbon 1 of one points up, and the OH on carbon 4 of the other points down: they cannot meet.
  3. Turn the second over and they meet. They leave as water, and a 1,4 glycosidic bond joins the rings, as in cellulose: condensation.
  4. Add water and the bond splits again, the H to one sugar and the OH to the other: hydrolysis.
Try to join them. What has to change?

B1.1.7 Glycoproteins

Sugar name tags on the cell surface

A glycoprotein is a protein with carbohydrate chains attached. In the plasma membrane the protein sits in the membrane and its sugar chains face outwards, away from the cell.

Carbohydrate chains can differ in which monosaccharides they contain, in the order of those sugars and in how the chain branches, so they can take a huge number of forms. That makes them good identity tags, and other cells read them: this is cell-cell recognition. Cells of one tissue recognise each other and stick together, and immune cells tell the body’s own cells from foreign ones.

The ABO blood groups are the standard example. Red blood cells carry a short sugar chain on glycoproteins in their membrane. In group O the chain ends at its base. Group A cells add one extra sugar to its tip, group B cells add a different extra sugar, and group AB cells carry both kinds, on different chains. The A and B forms are antigens. A person whose cells lack one of them treats it as foreign, which is why transfused blood must match the recipient’s group (the antibody side is C3.2).

Which extra sugar a person adds depends on the alleles they inherit, IA, IB and i (D3.2). Glycoproteins also take part in the structure of the plasma membrane (B2.1).

Watch out Group O cells are not bare. They carry the same base sugar chain: they only lack the extra A or B sugar on its tip.

What makes a group A red blood cell different from a group B one?

plasma membranered blood celloutside the cellinside the cell

Tap a chain tip, or the protein.

Part of a red blood cell membrane. Pick a blood group, then tap the tips of the sugar chains.

B1.1.8 B1.1.9 Lipids, triglycerides and phospholipids

Lipids keep their distance from water

Lipids are substances made by living things that dissolve in non-polar solvents, such as hexane, but only sparingly in water. They are grouped by this behaviour, not by a shared monomer. Lipids include fats, oils, waxes and steroids.

The reason is in their atoms. Lipids are mostly carbon and hydrogen, and C–H bonds are non-polar, so there are no partial charges for water to attract (A1.1). Water molecules stay bonded to each other and push the lipid out into droplets or a separate layer: lipids are hydrophobic. Wax on a leaf and oil on a feather keep water off for this reason. A wax is a fatty acid joined to a long-chain alcohol, and its long hydrocarbon chains make a waterproof coat, as on a leaf surface or in beeswax.

Fats and oils are mostly triglycerides: one glycerol (three carbons, each with an OH group) joined to three fatty acids. A fatty acid is a hydrocarbon chain with a carboxyl group, COOH, at one end. Each one joins glycerol by condensation: the OH of the carboxyl group and the H of one of glycerol’s OH groups leave as water, and an ester bond forms. Three fatty acids mean three ester bonds and three molecules of water.

A phospholipid is glycerol joined to two fatty acids and one phosphate group, also by condensation. (In most phospholipids a further small polar group is attached to the phosphate, which we leave out.)

A triglyceride is not a polymer: it is three fatty acids on one glycerol, not a chain of repeating units.

Watch out A fat is not a chain of “fat molecules”, and it is not a polymer: it is one glycerol carrying three fatty acids.

You have salted butter but need it unsalted. What could you do?

waterdrops of oiloil, floatingoil shaken with wateroil on water: two layershexaneoil in hexane: one clear layersalted butterfoambutterfatwatery layer,with the saltsalted butter melting in a bath of warm waterafter melting and standing: the fat, with the salt left behind in the water
  1. Shake oil with water,
  2. and it separates and floats: it does not dissolve.
  3. In a non-polar solvent such as hexane, it dissolves.
  4. Butter is mostly fat, with some water and, in salted butter, salt.
  5. Melted, the fat separates from the watery layer, and the salt stays dissolved in the water.
CHHOHCHOHCHHOHOOOOHOOHOOHO+CHHOOCHOOCHHOOthree ester bondsHHOHHOHHO3 water moleculesreleasedCHHOHCHOHCHHOHOOOOHOOHOPOHOOHOH+CHHOOCHOOCHHOPOO−OHester bond to the phosphatehydrophilic headhydrophobic tailsHHOHHOHHO3 water moleculesreleasedchains drawn short: most real fatty acids have 14 to 20 carbons
Each join releases one water molecule: an OH from the acid group and an H from glycerol. Switch between the two products and count the waters.
CHHOCHOCHHOOOOHHHHOHOHOHHHHOHOHOCHHOCHOCHHOOOOtriglycerideHHOHHOHHOglycerol + 3 fatty acidstriglyceride + 3 watercondensationhydrolysis
  1. Glycerol, three fatty acids.
  2. The OH of each fatty acid's carboxyl group and the H of one of glycerol's OH groups face each other.
  3. They leave as three water molecules, and three ester bonds join them: condensation, making a triglyceride.
  4. Add water and each ester bond splits again: hydrolysis, back to glycerol and three fatty acids.
Join three fatty acids to glycerol and count the water. Then start again and try the phosphate.

B1.1.10 Saturated and unsaturated fatty acids

Every double bond puts a kink in the tail

The hydrocarbon tail of a fatty acid may or may not contain C=C double bonds. A saturated fatty acid has none: each carbon holds as many hydrogen atoms as it can, so the chain is saturated with hydrogen. An unsaturated fatty acid has at least one C=C. With one it is monounsaturated, and with two or more it is polyunsaturated.

In natural fatty acids the double bonds are almost all cis: the chain continues on the same side at both ends of the double bond, so it bends there, by about 30°. Every double bond puts one more kink in the tail. (A trans double bond, made when oils are partially hydrogenated, leaves the chain almost straight, so trans fats pack and melt much like saturated ones.)

Kinks decide the melting point. Straight saturated chains pack closely side by side, so the attractions between neighbouring molecules are strong and more heat is needed to pull them apart: a higher melting point, and a fat that is solid at room temperature. Kinked chains cannot pack closely, the attractions are weaker and the melting point is lower: an oil, liquid at room temperature. The more double bonds, the lower the melting point. Melting only separates whole molecules from each other. No covalent bond breaks.

The guidance asks about plants and endotherms. A plant is the temperature of its surroundings, and its seeds may sit in cold soil, so plants store oils rich in unsaturated fatty acids. These stay liquid when cold, so the store can still be used. An endotherm keeps its body warm, about 37 °C in a mammal, so its fat can hold a far larger share of saturated fatty acids: often a third to a half, against about a tenth in plant oils. Solid at room temperature, it is still soft at body temperature.

The rule is really about temperature. Tropical plants such as coconut and cocoa store more saturated fat, which is liquid in the tropics, and fish in cold seas, which are not endotherms, store very unsaturated oils.

Remember it Why unsaturated fats are oils

  • DoubleC=C double bondone in a monounsaturated chain, two or more in a polyunsaturated one
  • KinksKinked chaineach cis double bond bends the chain
  • LeaveLoose packingbent chains cannot lie close, so they attract each other less
  • LiquidsLower melting pointless heat separates them: an oil at room temperature
Watch out Melting does not break double bonds or any other covalent bond: it separates whole molecules.
Watch out Saturated means saturated with hydrogen, not with fat.

Olive oil’s fatty acids are mostly monounsaturated, and ordinary sunflower oil’s are mostly polyunsaturated. Both are liquid at room temperature. You put a bottle of each in the fridge at 5 °C. Which turns cloudy and thick?

OHOstearic acidno C=Cpacks closelysix copiesOHOoleic acidone C=Cpacks looselysix copiesOHOlinoleic acidtwo C=Cpacks very looselysix copies
Coral marks each C=C double bond. Under each fatty acid, six copies of it packed as they would sit in a fat or oil.

B1.1.11 Adipose tissue

Fat: the lightest way to carry energy

Adipose tissue is made of cells that each hold a large droplet of triglyceride, under the skin and round organs. Four properties suit triglycerides to long-term energy storage.

  • Energy dense. Fat releases about twice the energy of carbohydrate for the same mass: about 37 kJ a gram, against about 17 kJ.
  • Stored without water. Triglycerides are hydrophobic, so fat is stored dry. Glycogen is stored with about three times its own mass of water, so the same energy in fat weighs several times less. That matters to anything that moves, flies or migrates.
  • Insoluble. Even a large store has no osmotic effect on the cell.
  • Chemically stable. Fat keeps for months.

Why keep glycogen at all? Glucose can be released from glycogen quickly, carried in the blood and respired with or without oxygen. Fat is mobilised more slowly, and fatty acids are respired only with oxygen. Glycogen suits quick, short-term needs, and fat the long term.

Triglycerides also give thermal insulation. Fat conducts heat only about half as well as muscle. An endotherm loses heat to colder surroundings faster the bigger the difference, and far faster in water than in air (A1.1, the ringed seal). So endotherms in cold habitats, above all in polar seas, have thick layers of fat called blubber, and tropical mammals have thin ones. An ectotherm makes too little heat for insulation to be worth much.

Why more energy a gram? A fatty acid chain is almost all C–H bonds, with very little oxygen, so there is more to oxidise. Glucose already carries an oxygen on almost every carbon: it is partly oxidised before respiration starts (C1.2).
Carbohydrate or lipid? How the two stores compare.
GlycogenTriglyceride
Energy per gramabout 17 kJabout 37 kJ
Water stored with itabout 3 g for every gramnone
Releasequickslower
Respired without oxygenyes, as glucoseno
Usual roleshort-term store in animalslong-term store, and insulation
Watch out Not every carbohydrate store is short-term. Plants keep starch for months in seeds and tubers: “short-term” describes glycogen in animals.
Example A camel’s hump is fat, not water.

A seal and a cod swim in the same polar sea. Which needs a thick layer of fat to insulate it?

0204060801.9 kg4.1 kg16.5 kgfatglycogen, dryglycogen withits waterkilograms to store the energy

7

Assumes 10 000 kJ a day, 37 kJ g−1 for fat, 17 kJ g−1 for glycogen, and 3 g of water stored with every gram of glycogen.
body 37 °Cfat layerseal, cross-sectionsea 28 °Csea 12 °Csea −2 °Carrows: heat lost

The wider the arrow, the faster the heat leaves. Change the sea, then thicken the fat.

B1.1.12 The phospholipid bilayer

Half loves water, half does not

A phospholipid is amphipathic: one part of the molecule is attracted to water and another part is not. The phosphate group carries a negative charge and attracts water, and with the glycerol it forms the hydrophilic head. The two fatty acid tails are non-polar, so they are the hydrophobic tails.

In water, phospholipids arrange themselves so that their tails avoid water and their heads touch it. The result is two layers, with the tails pointing inwards and the heads facing the water on both sides: a phospholipid bilayer.

Nothing places the molecules. No enzyme arranges them and no energy is spent: the bilayer is simply what happens when molecules with hydrophilic and hydrophobic regions meet water. Its core is hydrophobic, so it is a barrier to ions and polar molecules (chapter 11 and B2.1), and it is the basis of every cell membrane (B2.1).

Watch out Cells do not build the bilayer by placing each phospholipid with an enzyme. Once the molecules are made, water arranges them.

Phospholipids are stirred into a beaker of water, well below the surface. What do they form?

CHHOOCHOOCHHOPOO−OHhydrophilic headhydrophobic tailsiconhydrophilichydrophobicamphipathicwaterwaterhydrophilic headshydrophobic tailshydrophilic heads
  1. One molecule, two natures: amphipathic.
  2. Mixed into water,
  3. the tails turn away from water and the heads face it,
  4. forming a bilayer, with water on both sides.
Nothing places the molecules: watch where the tails end up, then turn the labels off and say what each part is.

B1.1.13 Steroids

Four rings, and straight through the membrane

Steroids are lipids built on four fused rings: three rings of six carbons and one of five, sharing edges. Different groups attached to the rings make different steroids. To identify a steroid from a diagram, look for that four-ring block, whatever is attached to it.

Cholesterol, which sits in animal cell membranes (B2.1), is a steroid, and so are the sex hormones oestradiol and testosterone. The hormones are made from cholesterol.

Steroids are mostly hydrocarbon, so they are non-polar. Oestradiol has two OH groups and testosterone an OH and a C=O, but the rings dominate. They dissolve into the hydrophobic core of the bilayer and diffuse straight through it, with no transport protein. (In blood they travel bound to carrier proteins, because they barely dissolve in plasma.) Glucose and ions, although smaller, are polar or charged. They cannot cross the core and need proteins (B2.1).

Inside the target cell these hormones bind receptors that act on genes (C2.1), and oestradiol takes part in the menstrual cycle (D3.1).

Watch out Not every lipid is a steroid, and not every steroid is a hormone: cholesterol is a steroid that sits in membranes.

Testosterone crosses a membrane with no transport protein. Glucose cannot. Why?

OHOAOHOHHOHOHHHHOHCH2OHBCDOHHOECHHOOCHOOCHHOOF

Which are steroids? Decide, then tap.

Look for the four-ring block, three rings of six and one of five, sharing edges.
outside the cellcytoplasmhydrophobic coreOHOtestosteroneOHHOoestradiolOOHOHOHHOCH2OHglucosepolar+sodium ioncharged
  1. The bilayer’s core is hydrophobic.
  2. Non-polar testosterone dissolves into the core,
  3. and diffuses out the other side.
  4. Oestradiol crosses the same way.
  5. Polar glucose and charged ions cannot cross the core: they need proteins (B2.1).

02Dry lab: Fats on the bench

Freeze fatty acids and kitchen fats, warm them slowly in a bath and catch the moment each one melts. Then find out what double bonds do to the melting point.

Your questionHow does the number of C=C double bonds in a fatty acid affect its melting point? And do the fats that plants and endotherms store fit the same pattern?
What you can changeThe sample (four fatty acids with 18 carbons and 0 to 3 double bonds, saturated fatty acids with 12 to 20 carbons, a mystery acid, and eight fats and oils from the kitchen), the bath's starting temperature, how fast it warms, whether it is stirred, and how many repeats you do.
What to recordFor each run: the sample, the settings, the temperature at which the first liquid appears and the temperature at which the sample turns completely clear, its melting point. Repeat each sample at least three times and take a mean.

Find out

  • Double bonds. Stearic, oleic, linoleic and α-linolenic acid, three runs each at 1 °C a minute, stirred. Plot the mean melting point against the number of C=C.
  • Chain length. Lauric to arachidic acid, three runs each at the same settings. Plot the mean melting point against the number of carbons. Which matters more: one double bond, or two extra carbons?
  • Heating rate. Stearic acid at 0.5, 1, 2 and 4 °C a minute, three runs each, stirred and then unstirred. What happens to the readings?
  • The mystery acid. It has 16 carbons and one C=C. Predict its melting point from your two graphs, then measure it.
  • Kitchen fats. All eight, three runs each. Plot the melting point against the mean C=C per fatty acid, and compare the plant and animal fats.

Record

  • For every run: the sample, the starting temperature, the heating rate, stirring, and the first liquid and clear temperatures.
  • Change one setting at a time and keep the rest the same.
  • Repeat each sample at least three times and take a mean; give the range as its uncertainty.
  • Use Graph your results under the lab, or download the CSV and plot it yourself.

Heating at 4 °C a minute gave readings about 1.5 °C higher than at 0.5 °C a minute, every time. What kind of error is this?

Coconut oil is about 90 % saturated, yet it melts lower than butter or lard. Why?

Olive, rapeseed and sunflower oil come from plants of cool and temperate climates; coconut and cocoa from the tropics. What explains the difference in their fats?

03Check

A fresh paper from the exam bank on this guide's topics each time: past-paper style questions, marked as you go, and saved to your Exam practice progress when you are signed in.

Exam practice

Exam questions on B1.1, marked

Start 15 marks of exam questionsYour exam practice

B1.1 revision mapB1.1 exam questionsB1.1 multiple choice quizB1.1 Tarsia puzzleB1.1 dominoesB1.1 pub quizBefore: A1.1 waterNext: A1.2 nucleic acids

04Downloads

Take the guide with you, or into a classroom with no internet.

DOCXNote templateFor students to fill in alongside the guide: a page per chapter with cues, gaps for the key terms and a summary box. Edit it in Word or Google Docs (A4). 1.6 MB PDFNote template, to printThe same note template, ready to print (A4). 2.3 MB PDFRevision guideThis guide as a printed book: every chapter, every step of every figure, the key terms, the predictions with answers at the back, and a link to the Check. 4.9 MB HTMLThe guide, offlineOne file that opens in any browser with no internet: the figures, the animations and the dry lab all work. 1.1 MB

The whole IB Biology revision guide, every guide written so far in one PDF.

05Teacher resources

The guide as four one-hour lessons. Each has a plan to print (the timings, what to project and where to stop, the activities with what they need, what students get wrong) and slides to teach it from, in the same order, with the teacher's moves in the notes.

LessonCoversFocusChaptersLesson planSlides
1Building blocks B1.1.1 to B1.1.460 min Carbon skeletons, condensation and hydrolysis, and telling the sugars apart 1, 2, 3 Lesson planPDF 792 KB SlidesPPTX 1.1 MB
2Stores, fibres and name tags B1.1.4 to B1.1.760 min Starch and glycogen as stores, cellulose as a fibre, and sugar name tags on cells 4, 5, 6 Lesson planPDF 687 KB SlidesPPTX 2.6 MB
3Lipids B1.1.8 to B1.1.1260 min Why lipids avoid water, how fats and phospholipids are built, and how fats store and insulate 7, 8, 9, 10 Lesson planPDF 765 KB SlidesPPTX 4.8 MB
4Steroids, the dry lab and review B1.1.10 and B1.1.1360 min Steroids crossing the bilayer, the melting points of fats, and a review of the whole subtopic 11, dry lab Lesson planPDF 660 KB SlidesPPTX 883 KB
PDFEvery lesson planAll four plans in one PDF, with this overview at the front. 1.4 MB KEYNote template, filled inThe class’s note template with every gap filled. 1.6 MB

The note template, the revision guide and the offline copy are in Downloads, for you and the class.

PRACTISE AND REVISE

Make it for your class, or set it

Build for your class

Already on the site for B1.1

Card sets on splycr

Open a set to host it as a class game, or make a copy to edit.

How this was made. The course structure, the activities and the teaching choices come from my own classroom, my presentations and my years of teaching IB and AP Biology. I use Claude, an AI, to help write and build the interactive guides, lesson plans, slides and practice questions, and to draw the diagrams and code the interactives. I direct the work and review it against the IB and AP specifications before it goes live.