Study notes
Biological Molecules
What carbohydrates, fats, and proteins are made of and what they do in the body, how food tests identify them, and how enzymes speed up reactions.
Learn it step by step
Carbohydrates, fats and proteins are built from different elements
Carbohydrates and fats are both made only of carbon, hydrogen and oxygen. Proteins are made of carbon, hydrogen, oxygen, AND nitrogen (and often a little sulfur too). Nitrogen being present is what marks a molecule out as a protein rather than a carbohydrate or fat.
Each nutrient group has its main biological role
Carbohydrates serve mainly as an immediate energy source. Fats are used for insulation and as a longer-term energy store. Proteins are used mainly for growth and repair of body tissues, not primarily as an energy source.
Large molecules are built by joining many small repeating units
Starch, glycogen, and cellulose are all large molecules built by joining together many glucose units. Proteins are built by joining together many amino acid units. Fats are built from one glycerol molecule joined to (usually three) fatty acid molecules. In every case, a big molecule is a chain (or branch) of many small repeating building blocks.
Food tests: iodine test for starch
Adding iodine solution to a food sample: a colour change from browny-orange to blue-black shows starch is present. No colour change means starch is absent.
Food tests: Benedict's test for reducing sugars
Adding Benedict's solution to a food sample and heating it: a colour change from blue to brick-red (with a precipitate) shows a reducing sugar is present. Staying blue on heating means no reducing sugar was detected.
Food tests: biuret test for protein
Adding biuret solution (or sodium hydroxide followed by dilute copper sulfate) to a food sample: a colour change to purple (lilac) shows protein is present. No colour change (staying blue) means protein is absent.
Food tests: emulsion test for fats
Shaking a food sample with ethanol, then pouring the mixture into water: a cloudy white emulsion forming shows fat is present. The mixture staying clear means fat is absent.
Enzymes speed up reactions by lowering the energy needed to start them
An enzyme is a biological catalyst: it speeds up a specific reaction by lowering the activation energy needed, without itself being used up or permanently changed. A substrate fits into the enzyme's active site (forming an enzyme-substrate complex) the way a key fits a specific lock, which is why each enzyme normally works on only one kind of substrate.
Temperature and pH change an enzyme's rate by changing its shape
Raising temperature or pH toward the enzyme's optimum speeds up the reaction, up to a point. But temperatures too high, or a pH too far from the optimum, permanently change (denature) the enzyme's active site shape, so the substrate can no longer fit: this is a permanent structural change, not simply the enzyme working 'more slowly'.
Worked examples
- Read the iodine result: it stayed browny-orange, meaning no colour change, so starch is absent.
- Read the Benedict's result: it turned brick-red on heating, which is the positive result, so a reducing sugar IS present.
- Read the biuret result: it stayed blue, meaning no colour change, so protein is absent.
- Read the emulsion result: the mixture stayed clear, meaning no cloudy white emulsion formed, so fat is absent.
- Only one test gave a positive result (Benedict's), so the sample's macronutrient content is a reducing sugar, a type of carbohydrate, and none of starch, protein, or fat.
- Recall the correct iodine test result for starch: a positive result is a colour change to blue-black, not purple.
- So a purple colour cannot be a positive iodine/starch result at all; the student has mismatched a colour to the wrong test.
- Recall which test does give a purple (lilac) result: the biuret test, which is the test for protein, not starch.
- The likely explanation is that the student either used the wrong reagent (biuret instead of iodine) or is misremembering which colour belongs to which test; the fix is to redo the test correctly and match colour to reagent: blue-black = iodine/starch, brick-red (after heating) = Benedict's/reducing sugar, purple/lilac = biuret/protein, cloudy white = emulsion/fat.
- Between 20 degC and the enzyme's optimum temperature (here, 37 degC), rising temperature gives particles more kinetic energy, so enzyme and substrate collide more often and the reaction speeds up.
- Beyond the optimum, the rise in temperature starts to disrupt the precise shape of the enzyme's active site rather than just adding useful collision energy.
- Once the active site's shape is sufficiently disrupted, the substrate can no longer fit into it to form an enzyme-substrate complex, so the reaction rate falls sharply: this is denaturation.
- Denaturation is a PERMANENT change to the enzyme's shape, not a temporary slowdown, which is exactly why cooling the enzyme back down to 37 degC afterward does not restore the original active-site shape or the original reaction rate.
- The key misconception to avoid is describing this as the enzyme 'working more slowly' at high temperature; past the optimum, the enzyme is being structurally destroyed, not merely delayed.
- Recall what happens to the substrate: it binds to the enzyme's active site, is chemically converted into product(s), and leaves as something different from what it started as, so the original substrate molecule is indeed used up.
- Recall what happens to the enzyme in that same reaction: it forms a temporary enzyme-substrate complex, helps the reaction happen by lowering the activation energy, and then releases the product(s) unchanged itself.
- Because the enzyme comes out of the reaction in the same form it went in, it is immediately free to bind another substrate molecule and catalyse the same reaction again, many times over.
- So the claim wrongly treats the enzyme like a substrate; the correct description is that an enzyme is a reusable catalyst, not a reactant that gets consumed, which is also why only a small amount of enzyme is needed to process a much larger amount of substrate.
Mind map
Mind map for Biological Molecules.
- Elements and building blocks
- carbohydrates & fats: carbon, hydrogen, oxygen
- proteins: also nitrogen (+ often sulfur)
- starch/glycogen/cellulose: made of glucose units
- proteins: made of amino acid units
- fats: glycerol + fatty acids
- Main biological roles
- carbohydrates: immediate energy
- fats: insulation, long-term energy store
- proteins: growth and repair
- Food tests
- iodine: starch -> blue-black
- Benedict's (heated): reducing sugar -> brick-red
- biuret: protein -> purple/lilac
- emulsion (ethanol + water): fat -> cloudy white
- Enzymes
- lower activation energy, not used up
- active site + substrate = enzyme-substrate complex
- specificity: lock and key
- extreme temperature/pH: denatures (permanent shape change)