What Am I Made Of?

Picture a turkey sandwich on whole-wheat bread. Within hours of eating it, your body takes its large molecules apart into small pieces and uses many of those pieces to build parts of you. The bread came from wheat plants and the turkey from a bird, so the sandwich is made from the cells of plants and animals. Your body is made of cells too. So what are cells made of, and how can one living thing’s food become another living thing’s body?

Explore: build big molecules from small blocks

The simulation gives you four kinds of building block: a simple sugar, an amino acid, a nucleotide, and a set of fatty acids with glycerol. You pick one kind and snap the blocks together into a chain. The simulation names the large molecule you built, tells you its main job, and counts the water molecules released each time two blocks join.

1. Pick a kind of building block

Simple sugar Amino acid Nucleotide Fatty acids + glycerol

2. Build Remove last block Reset

Blocks

Links

Water released

Large molecule

Main job

Elements

In this sketch each block is one monomer drawn as a labelled box, not a molecule. A chain stops at 12 blocks to fit the screen; a real polymer has many monomers.

Rule to keep: joining two monomers releases one water molecule, so a chain of n blocks has n − 1 links and releases n − 1 waters. A common fat is one glycerol joined to three fatty acids, so it is not a true polymer.

Try this.

1. Predict how many water molecules will be released when you link 4 sugars in a row. Build the chain and check.
2. Now predict the count for 10 sugars. Can you write a rule that works for any number of blocks?
3. Switch to amino acids. Predict whether the name of the molecule changes, and whether your water rule still works. Then test.
4. Try fatty acids and glycerol. Predict whether you can keep adding blocks to make a very long chain, then try it.

Explain: small blocks, big chains

Think of a necklace of beads. Each bead is small, but a string of hundreds of them is long. Cells build their largest molecules the same way.

A monomer is a small molecule that can join to others like it. A polymer is a large molecule made of many monomers linked in a chain. A macromolecule is a very large molecule in a living thing, such as a polymer. The necklace analogy breaks in one place: real chains fold, coil and branch, and their shape matters for their job.

When a cell links two monomers, it removes a hydrogen from one and an oxygen and hydrogen from the other. Those atoms leave as one water molecule. This joining reaction is dehydration synthesis, which means “making something by removing water.” Breaking the link runs the other way. Hydrolysis splits a polymer into smaller pieces by adding water.

simple sugar + simple sugar → double sugar + water (dehydration synthesis)

C6H12O6 + C6H12O6 → C12H22O11 + H2O (two glucose molecules form maltose and water)

double sugar + water → simple sugar + simple sugar (hydrolysis)

Digestion is mostly hydrolysis. Your gut breaks the bread’s starch into sugars and the turkey’s proteins into amino acids. Your cells then use dehydration synthesis to build their own polymers from those pieces. A turkey protein does not become a human protein; its amino acids are reused in new chains.

Most of these molecules share carbon, hydrogen and oxygen. Cells can take the carbon, hydrogen and oxygen from sugar and join them with nitrogen to make amino acids. Plants make their amino acids in a similar way, using nitrogen they take up from the soil as nitrate or ammonium; in legumes such as peas, bacteria in root nodules also supply nitrogen. Your cells can make some amino acids, but about nine must come from food.

Explain: four classes, four kinds of job

Scientists sort the large molecules of life into four classes. Each has its own building blocks and main jobs.

A carbohydrate is a molecule made of simple sugars, such as glucose, starch or cellulose. A protein is a chain of amino acids folded into a shape. An amino acid is a small molecule containing nitrogen that is the building block of proteins. A nucleic acid is a chain of nucleotides, such as DNA or RNA. A nucleotide is a building block made of a sugar, a phosphate group and a base containing nitrogen. A lipid is a molecule that mostly does not mix with water, such as a fat, oil or phospholipid, which has one end that attracts water.

Class	Building blocks	Main jobs	Elements
Carbohydrates	simple sugars	quick energy; plant cell walls	C, H, O
Proteins	amino acids	building tissues; running cell processes	C, H, O, N (often S)
Nucleic acids	nucleotides	storing and passing on genetic information	C, H, O, N, P
Lipids (fats)	fatty acids and glycerol	long-term energy storage; insulation; cell membranes	mostly C, H, O

Two cautions. Not every lipid is a fat: phospholipids and steroids such as cholesterol are lipids too. And fats are not true polymers. A common fat is one glycerol joined to three fatty acids, not a long chain of repeating blocks.

Explain: shape decides the job

A key only opens a lock its shape fits. Molecules work in a similar way, although the analogy breaks because molecules bend and move. Here is one example from each class.

• Carbohydrate: cellulose. Long chains of glucose lie side by side and hold together in strong fibers. Those fibers stiffen the walls of plant cells. Much of the fiber in whole-wheat bread comes from plant cell walls.
• Protein: an enzyme. An enzyme is usually a protein that speeds up a reaction. Its folded shape forms a pocket where one kind of molecule fits. If heat or acid changes that shape, the molecule may no longer fit. You will test this in Enzymes at Work.
• Nucleic acid: DNA. DNA has two strands whose bases pair in a fixed way. Each strand can therefore guide the copying of the other, so the information can be passed to new cells. DNA: Structure and Copying shows how.
• Lipid: a phospholipid. Each phospholipid has a head attracted to water and two tails that avoid it. In water they form a double layer, heads out and tails in. That layer is the basic structure of the cell membrane.

Together these molecules carry out life processes. Proteins and lipids build new cells for growth and repair. DNA passes information on in reproduction. A protein in red blood cells carries oxygen for gas exchange. Enzymes run metabolism (the chemical reactions in cells), and proteins in membranes detect signals so cells can respond.

Elaborate: what is in the sandwich and its neighbors

These values come from a food database. Each row is grams of nutrient in 100 grams of a reference food, so you can compare foods fairly. Carbohydrate already includes the fiber.

Food (100 g)	Carbohydrate (g)	Protein (g)	Fat (g)	Fiber (g)
Whole-wheat bread	42.71	12.45	3.5	6
Turkey breast, roasted	0	30.13	2.08	0
Peanut butter, smooth	22.31	22.21	51.36	5
Apple, raw, with skin	13.81	0.26	0.17	2.4
Whole milk	4.8	3.15	3.25	0
Olive oil	0	0	100	0

Source: USDA FoodData Central, SR Legacy reference foods (FDC IDs 172688, 171496, 174266, 171688, 171265, 171413), published April 2019. These are averaged reference values, not one sample from one store or lab. This table does not include nucleic acids. Food scientists estimate energy in kilocalories (kcal) with general factors of about 4 kcal per gram for carbohydrate and protein and about 9 kcal per gram for fat.

1. Which two foods have the most protein per 100 grams? Is one of them a plant food?
2. Bread lists 42.71 grams of carbohydrate, including 6 grams of fiber. How many grams are not fiber? Compare with the apple.
3. Using the energy factors, about how much energy is in the fat of 100 grams of peanut butter? How does that compare with the energy estimated for its carbohydrate?

Design problem

Your teacher gives you three tests used in many school labs. Iodine solution turns blue-black with starch. Biuret solution turns violet with protein. A drop of fat leaves a see-through spot on brown paper that stays after the paper dries. Plan an investigation to find which of these molecules are in bread, turkey and apple.

One way to do it

The independent variable is the food. The dependent variable is the color change or spot. Keep the amount of food, the amount of test solution and the waiting time the same. Use water as a negative control, and a known starch, protein or oil sample as a positive control for each test. Run each test three times. Based on the table, you might expect starch in the bread and protein in the turkey and bread. The apple has very little protein or fat, and a ripe apple may show little starch. Treat these as predictions to test. Wear goggles, never taste lab samples, and handle iodine and biuret solution with care.

Evaluate: check your thinking

The sandwich holds carbohydrates, proteins and fats, and, because its ingredients came from cells, nucleic acids too. Your digestive system splits those large molecules into sugars, amino acids, nucleotides, fatty acids and glycerol. Your cells then join those pieces into new large molecules with shapes that fit your own body’s jobs.

Science practices you used: making and testing predictions with a model, naming where an analogy breaks, reading a data table on a fair 100-gram basis, calculating with energy factors, and planning a test with controls and repeated trials.