Carbohydrates, Lipids and Minerals: Sugars, Fats and What Soil Supplies

Sort sugars by size, linkage and reducing power, tell starch from cellulose from chitin, classify lipids and fatty acids, and match each plant mineral to its role and deficiency symptom.

In the paper

Unit 1 carries 2 of the 40 botany marks; with two chapters sharing them, this one averages one (MEC publishes weights by unit, not by chapter). The MEC scope line: Structure, types and biological role of carbohydrates; Lipids; Minerals. Three ways it is asked:

Recall: the monomers of sucrose, lactose and maltose; the linkage in cellulose; the element at the centre of chlorophyll; the micronutrient nitrogenase needs.
Understanding: why sucrose fails Benedict's test; why humans cannot digest cellulose; why nitrogen deficiency shows on old leaves but iron deficiency on young ones.
Application: yellowing between green veins on the youngest leaves; an extract that reacts with Benedict's only after acid boiling.

Structure, types and biological role of carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones and their polymers, made of C, H and O, most fitting the formula Cn(H2O)n — "hydrates of carbon" (deoxyribose, C₅H₁₀O₄, does not fit). Monomers join by a glycosidic bond, made by removing water (condensation) and broken by adding it back (hydrolysis).

Monosaccharides

Single sugar units: sweet, crystalline, soluble. An aldose carries an aldehyde group (–CHO), a ketose a keto group (C=O).

Carbons	Name	Examples	Why it matters
3	Triose	Glyceraldehyde (aldose), dihydroxyacetone (ketose)	Phosphorylated forms run glycolysis and the Calvin cycle
5	Pentose	Ribose, deoxyribose, ribulose	RNA, DNA and ATP; ribulose bisphosphate accepts CO₂ in photosynthesis
6	Hexose	Glucose, galactose (aldoses); fructose (ketose)	Glucose is the main respiratory fuel

Glucose, fructose and galactose share the formula C₆H₁₂O₆ — they are isomers. In water they close into rings, and the –OH on carbon 1 of glucose can sit below the ring (alpha) or above it (beta). That one difference decides whether a glucose polymer is food (starch) or fibre (cellulose).

Haworth rings of alpha-glucose, with the –OH on carbon 1 below the ring, and beta-glucose, with it above; below, two alpha-glucose units lose water between carbon 1 of one and carbon 4 of the next to form maltose, joined by an alpha-1,4 glycosidic bond — A: the only difference between alpha- and beta-glucose is the side of the ring the –OH on carbon 1 lies on. B: that –OH and the H of the –OH on carbon 4 of the next glucose leave as water, and the bridging oxygen is the alpha-1,4 glycosidic bond of maltose; hydrolysis adds the water back. Hydrogen atoms on carbons 2 to 5 are not drawn. Diagram: IOM Mock.

Disaccharides

Disaccharide	Monomers	Bond	Reducing?	Where found
Sucrose	Glucose + fructose	alpha-1,2 (C1 of glucose to C2 of fructose)	No	Cane sugar; the phloem transport sugar
Maltose	Glucose + glucose	alpha-1,4	Yes	Malt; product of starch digestion by amylase
Lactose	Galactose + glucose	beta-1,4	Yes	Milk

A reducing sugar has a free aldehyde or keto group that turns blue Benedict's or Fehling's solution to a brick-red precipitate. All monosaccharides are reducing. Sucrose's reducing carbons are locked in its bond, so it is non-reducing until boiling with dilute acid splits it into glucose and fructose.

Polysaccharides

Hundreds to thousands of units; not sweet, mostly insoluble.

Polysaccharide	Monomer and linkage	Shape	Iodine	Role
Amylose (about 20–25 % of starch)	Glucose, alpha-1,4	Unbranched helix	Blue-black	Plant storage
Amylopectin (about 75–80 %)	Glucose, alpha-1,4 with alpha-1,6 branches	Branched	Red-violet	Plant storage
Glycogen	Glucose, alpha-1,4 and alpha-1,6	More branched than amylopectin — many chain ends, so glucose is released fast	Red-brown	Storage in animals and fungi
Cellulose	Glucose, beta-1,4	Straight chains hydrogen-bonded into microfibrils	None	Plant cell wall; the most abundant organic compound on Earth
Chitin	N-acetylglucosamine, beta-1,4	Straight chains	None	Fungal cell wall; insect exoskeleton
Inulin	Fructose	Chains	No blue	Storage in dahlia tubers

Walls also hold hemicellulose and pectin, which as calcium pectate forms the middle lamella.

Why the linkage matters. Amylases fit alpha bonds only. In cellulose every second glucose is flipped, so chains lie straight and bond into tough fibres no human enzyme can split; cattle digest grass only through cellulase-making rumen microbes.

Four glucose polymers drawn as chains of hexagons: amylose as an unbranched helix, amylopectin as alpha-1,4 chains with a few alpha-1,6 branch points, glycogen with many more branches and chain ends, and cellulose as straight parallel chains with every second unit flipped and hydrogen bonds between the chains — Each hexagon is one glucose. Amylose is an unbranched alpha-1,4 helix (grey units lie at the back of the coil); amylopectin adds alpha-1,6 branch points (filled), and glycogen branches far more often, so it has many chain ends; in cellulose every second glucose is flipped (the stub shows which way), so the chains run straight and hydrogen-bond side by side into microfibrils. Schematic, not to scale. Diagram: IOM Mock.

Roles: fuel (glucose, about 17 kJ or 4 kcal per gram); storage (starch, glycogen, inulin); structure (cellulose, chitin, pectin); transport (sucrose, which is non-reducing and so travels unchanged); and building blocks (ribose and deoxyribose in nucleic acids and ATP; sugar chains as cell-recognition markers).

Lipids

Lipids are compounds insoluble in water but soluble in organic solvents (ether, chloroform, benzene); most are esters of fatty acids. They hold far less oxygen than carbohydrates and so yield about 37 kJ (9 kcal) per gram — roughly twice as much.

Fatty acids

A hydrocarbon chain ending in a carboxyl group (–COOH), usually with an even number of carbons.

	Saturated	Unsaturated
C=C bonds	None	One (monounsaturated) or more (polyunsaturated)
Chain	Straight, packs tightly	Cis double bonds kink it
At room temperature	Solid — fats	Liquid — oils
Examples	Palmitic (C16), stearic (C18)	Oleic (C18, one C=C), linoleic (two), linolenic (three)

Hydrogenation saturates the double bonds and turns an oil into a solid fat (vegetable ghee), also making harmful trans fats. Linoleic and linolenic acids are essential fatty acids that humans cannot make.

Classes

Class	Composition	Examples and role
Simple: fats and oils (triglycerides)	Glycerol + three fatty acids by ester bonds	Energy store in seeds (mustard, groundnut) and fat tissue
Simple: waxes	A long-chain fatty acid + a long-chain alcohol (not glycerol)	Leaf and fruit cuticle, beeswax; waterproofing
Compound: phospholipids	Glycerol + two fatty acids + a phosphate group (with choline in lecithin)	Amphipathic — hydrophilic head, hydrophobic tails; the bilayer of every membrane
Compound: glycolipids, lipoproteins	Lipid + sugar, lipid + protein	Membrane markers; lipid transport in blood
Derived: steroids	Four fused carbon rings, no fatty-acid chains	Cholesterol (animal membranes, steroid hormones); phytosterols (plants); ergosterol (fungi)
Derived: terpenes	Isoprene (C5) units	Carotenoids, the phytol tail of chlorophyll, gibberellins, rubber, essential oils

A triglyceride: glycerol whose three carbons are each joined by an ester bond to a fatty acid, two straight saturated chains and one kinked at a cis C=C; a phospholipid of choline, phosphate, glycerol and two fatty-acid tails, marked hydrophilic head and hydrophobic tails; and a phospholipid bilayer with heads facing water on both sides — A: each fatty acid is joined to one –OH of glycerol by an ester bond; the two straight chains are saturated, and the cis C=C in the bottom chain kinks it. B: a phospholipid keeps two fatty acids and carries a phosphate group, with choline in lecithin, on the third carbon. C: in water the heads face outward and the tails face each other, forming the bilayer of every membrane. Diagram: IOM Mock.

Roles: compact energy stores in seeds; membranes (phospholipid bilayer, with sterols regulating fluidity); waterproofing (the waxy cuticle cuts water loss); steroid hormones and the fat-soluble vitamins A, D, E and K. Tests: Sudan III stains lipid red; a grease spot leaves paper translucent. Boiling a fat with alkali gives glycerol and soap (saponification).

Minerals

Plants absorb minerals as ions dissolved in soil water. An element is essential (Arnon and Stout, 1939) if (1) without it the plant cannot complete its life cycle, (2) no other element can replace it, and (3) it is directly involved in metabolism. Essentiality is tested in hydroponics (soil-free nutrient solution, developed by Sachs and Knop around 1860) by leaving out one element at a time.

How many? The Grade 11 text and NCERT list 17 essential elements, split by the amount needed (NCERT's line is 10 mmol per kg of dry matter):

Macronutrients (9): C, H, O (from air and water) and N, P, K, Ca, Mg, S.
Micronutrients (8), or trace elements: Fe, Mn, Cu, Zn, B, Mo, Cl, Ni.

OpenStax Biology 2e counts about 20 because it adds cobalt, sodium and silicon, which are beneficial to some plants. Use 17 in the paper.

Mobility decides where symptoms appear. Mobile elements (N, P, K, Mg) are withdrawn from old leaves and sent to growing ones, so shortage shows first on older, lower leaves. Immobile elements (Ca, S, Fe, B, Cu) stay where first laid down, so young leaves and tips suffer first. Chlorosis is yellowing from lost chlorophyll; necrosis is death of tissue.

Element	Main roles	Deficiency
N	Amino acids, proteins, nucleic acids, chlorophyll	Stunting; chlorosis starting in older leaves
P	ATP, nucleic acids, phospholipids	Dark or purplish leaves, poor roots, late maturity
K	Opens stomata, activates enzymes, ion balance	Scorched margins of older leaves, weak stems that lodge
Ca	Calcium pectate of the middle lamella, mitotic spindle, membranes	Death of shoot and root tips; blossom-end rot of tomato
Mg	Central atom of chlorophyll; activates RuBisCO and ATP enzymes	Interveinal chlorosis of older leaves
S	Cysteine and methionine, coenzyme A, thiamine, biotin	Chlorosis of young leaves
Fe	Cytochromes, ferredoxin; needed to make chlorophyll, though not part of it	Interveinal chlorosis of young leaves
Mn	Splitting of water in photosynthesis; enzyme activator	Chlorosis with grey spots (grey speck of oats)
Zn	Carbonic anhydrase; needed for auxin synthesis	Little leaf, rosetting; khaira disease of rice
Cu	Plastocyanin, cytochrome oxidase	Die-back of shoots
B	Calcium uptake, pollen germination, sugar movement	Death of tips; brown heart of turnip and swede
Mo	Nitrogenase, nitrate reductase	Whiptail of cauliflower; needed in the smallest amount
Cl	Splitting of water (with Mn); ion balance	Wilted leaf tips
Ni	Urease	Urea builds up and burns leaf tips

Numbers and names to memorise

Item	Value
Sucrose / maltose / lactose	Glc + Fru, alpha-1,2 / Glc + Glc, alpha-1,4 / Gal + Glc, beta-1,4
Starch	Amylose 20–25 %, amylopectin 75–80 %
Branch points (amylopectin, glycogen)	alpha-1,6
Cellulose; chitin	beta-1,4 glucose; beta-1,4 N-acetylglucosamine
Energy per gram	Carbohydrate about 17 kJ (4 kcal); fat about 37 kJ (9 kcal)
Triglyceride; phospholipid	Glycerol + 3 fatty acids; glycerol + 2 fatty acids + phosphate
Essential elements	17 = 9 macro + 8 micro (Grade 11, NCERT); about 20 in OpenStax
Mobile / immobile	N, P, K, Mg / Ca, S, Fe, B, Cu
Water splitting	Mn and Cl

Traps

Sucrose is non-reducing; maltose and lactose are reducing.
Starch and cellulose are both glucose polymers; they differ in linkage (alpha versus beta-1,4), not in monomer.
Glycogen is not a plant store. Plants store starch (and inulin); animals and fungi store glycogen.
Chitin contains nitrogen; cellulose does not.
A wax is not a triglyceride: its alcohol is a long-chain alcohol, not glycerol. Steroids contain no fatty acids yet count as lipids by solubility.
Mg is part of chlorophyll; Fe is not. Both cause chlorosis — Mg on old leaves, Fe on young ones.
17 or 20 essential elements? 17 in the Grade 11 text; OpenStax's larger count adds beneficial Co, Na and Si.

Quick check

0 of 5 answered

Pick an answer to see whether you are right, and why.

1
Which sugar gives no brick-red precipitate when boiled directly with Benedict's solution?
2
Humans digest starch but not cellulose, although both are glucose polymers, because cellulose:
3
Mustard oil is treated with hydrogen over a nickel catalyst until it sets solid at room temperature. The change was:
4
In a maize field the youngest leaves show yellow tissue between green veins while older leaves stay green. The most likely deficiency is:
5
The micronutrient needed by both nitrogenase and nitrate reductase, and required in the smallest amount of all, is:

Sources

OpenStax, Biology 2e, 3.2 Carbohydrates and 3.3 Lipids (consulted), read 2026-09-15.
OpenStax, Biology 2e, 31.1 Nutritional Requirements of Plants (Table 31.1 and the macronutrient roles), read 2026-09-15.
Wikipedia: Plant nutrition (17 elements, mobility, Arnon and Stout 1939, zinc and little leaf), Starch (amylose 20–25 %), Reducing sugar, Glycosidic bond, Hydroponics (Sachs and Knop), read 2026-09-15.
NCERT Biology Class 11 (2006 edition), ch. 9 Biomolecules and ch. 12 Mineral Nutrition; Nepal CDC Grade 11 Biology — consulted for scope, terminology and the deficiency list.
Figures: credited in each caption (original diagrams).