Module 1 · Protein

Protein from the atoms up: the four kinds of atom in an amino acid, the shared backbone and changing side chain, the peptide bond, all 20 amino acids drawn at one scale, how chains fold into working shapes, how you digest and use protein, and which foods give you all 9 essential amino acids. The figure is on the left and the lesson script on the right; the quiz is at the end.

Previous lesson: Module 1 · Other Lipids

Figures1 2 3 4 5 6 7 8 9

Figure 1

The Protein Family Tree

Amino acids build up into peptides and proteins; food supplies the amino acids.

Protein on a food label

Protein (grams)Every gram gives about 4 kcal, the same as carbohydrate. The grams are total protein, whatever the source.
%DV is often missingIn the US the % Daily Value for protein is optional unless the package makes a protein claim or the food is for young children; then it is adjusted for protein quality.
Not listed: which amino acidsThe label doesn’t say whether a protein is complete. Figure 7 shows how to tell from the food itself.

Methane, CH4: one carbon with four hydrogens

Ammonia, NH3: one nitrogen with three hydrogens

Histidine (His), C6H9N3O2: amine and carboxyl at the bottom, side chain pointing up; every hydrogen shown

Isoleucine (Ile), C6H13NO2: amine and carboxyl at the bottom, side chain pointing up; every hydrogen shown

Essential	What its side chain does	In your body
Leucine, isoleucine, valine	Branched carbon chains (the BCAAs); oily, so they sit inside proteins	Plentiful in muscle protein; leucine also signals muscle to build protein
Lysine	Long chain ending in NH₂, charged	Collagen cross-links, carnitine; often the lowest essential amino acid in grains
Methionine	Contains sulfur	Starts nearly every new protein chain; the body makes cysteine from it; often lowest in beans
Phenylalanine	Benzene ring	The body makes tyrosine from it. People with PKU can’t, which is why aspartame labels warn about it
Tryptophan	Largest: a double ring with nitrogen	Raw material for serotonin and melatonin, and some niacin (vitamin B3)
Threonine	Has an OH group	Part of mucus proteins that line the gut; collagen
Histidine	Ring with two nitrogens	Raw material for histamine; part of hemoglobin’s iron pocket

One heme in its pocket in hemoglobin, ball-and-stick with every hydrogen: a flat ring of four pyrroles joined by CH bridges, its four nitrogens (blue) holding an orange iron atom at the centre. Above the iron an oxygen molecule (two red atoms) is bound end-on and bent; the distal histidine E7 above it hydrogen-bonds the far oxygen (dashed line). Below, the proximal histidine F8 is bonded to the iron. Valine, phenylalanine and leucine side chains (sticks) line the pocket, and pale blue tubes trace the E and F helices of the protein chain.

The same heme seen edge-on: the ring is a flat line with the iron in it; the O2 sits straight above the iron, bent, with histidine E7 above it, and histidine F8 sits straight below.

Urea: a carbon with a C=O and two NH2 groups; every hydrogen shown

Alanine: amine group (NH2) on the left of the alpha carbon, methyl side chain on top, carboxyl group on the right; every hydrogen shown

Figure 7

Complete and Incomplete Protein

Which foods have all 9 essentials, and how to combine the rest.

Complete and incomplete foods

Food	Protein type	Lowest essential amino acid
Meat, poultry, fish	Complete	none
Eggs, milk, yogurt, cheese	Complete	none
Soybeans (tofu, tempeh, edamame)	Complete	none really (methionine a little low)
Quinoa, buckwheat	Complete, or close	none really
Beans, lentils, peas, chickpeas	Incomplete	methionine (and cysteine)
Wheat, rice, oats, corn	Incomplete	lysine
Nuts and seeds	Incomplete	lysine (most)

Putting plant proteins together

Limiting amino acidA protein is only as useful as its scarcest essential amino acid. Your body can’t build a protein when one is missing.
Beans + grainsBeans are low in methionine but rich in lysine; grains are the opposite. Together they cover each other: rice and beans, hummus and pita, peanut butter on whole-wheat bread.
Over the day is fineThe amino acid pool lasts for hours, so the foods don’t have to be eaten in the same meal, just over the course of the day.

How much you need

Adults: 0.8 g per kgThe US RDA is 0.8 grams of protein per kilogram of body weight per day: about 56 g for a 70 kg (154 lb) adult.
More at some stagesPregnancy and breastfeeding need more; growing children need more per kilogram; some research suggests older adults benefit from a little more.
Found inMeat, poultry, fish, eggs, dairy, beans, lentils, peas, soy, nuts, seeds and whole grains.

Figure 8

Where protein is used in the body

Tap a number on the cell to see what that part does; the list below it gives every other place in the body.

Digested to amino acids, then built back into the proteins that do almost every job in the body.

Inside a cell

A typical animal cell, not to scale. Real cells hold thousands of ribosomes and hundreds of mitochondria.

Every cell builds its own proteins from amino acids. Here is where each step happens, in a typical cell.

DNA holds the gene for every protein. It is wound on spools of histone proteins, and a gene is copied into messenger RNA (mRNA) to send out.
Read the mRNA and join amino acids one by one with peptide bonds. Free ribosomes make the proteins that stay in the cytosol.
Ribosomes on it make proteins for export or for membranes. Inside, the chain folds and S–S links form.
Adds sugar tags, sorts the finished proteins and ships them in little sacs to the membrane or out of the cell, like albumin from liver cells.
The fluid inside the cell, crowded with enzyme proteins that run its reactions, such as breaking glucose down for energy.
Their enzymes burn fuel and their protein machines make ATP. The carbon from spare amino acids is burned here too.
Protein fibers that give the cell its shape and move things around inside it. In muscle cells, actin and myosin slide past each other to contract.
Channel and carrier proteins let amino acids, glucose and salts in and out; receptor proteins catch signals such as insulin.
Small sacs of enzymes that break worn-out proteins and cell parts back down to amino acids for reuse.

Everywhere else in the body

Each item, the places outside the cell where it is found or used, and what it does there.

Digesting protein

Stomach. Acid unfolds proteins and pepsin starts cutting them.
Pancreas. Sends trypsin, chymotrypsin and other enzymes into the small intestine.
Small intestine. Its lining finishes the job and absorbs amino acids and small peptides into the blood.

Proteins that move you

Skeletal muscle. Holds about 40% of body protein; actin and myosin slide past each other to contract.
Heart. Its muscle uses the same proteins for every beat.
Gut and blood vessel walls. Smooth muscle squeezes food along and sets vessel width.

Proteins that hold you together

Skin. Collagen gives it strength and elastin lets it stretch back.
Bones and teeth. A collagen framework holds the minerals.
Tendons, ligaments and cartilage. Mostly collagen.
Arteries and lungs. Elastin lets them stretch and recoil.
Hair, nails and outer skin. Made of keratin.
Eyes. Collagen keeps the cornea clear; crystallin proteins make the lens.

Proteins in the blood

Liver. Makes albumin, which holds water in the blood and carries things, and the clotting factors such as fibrinogen.
Red blood cells. Hemoglobin carries oxygen.
Blood. Carrier proteins move iron, fats and hormones.

Defense

Bone marrow, lymph nodes, spleen and gut lining. Plasma cells there make antibodies that tag germs.
Skin and mucus. Protein barriers and germ-killing enzymes such as lysozyme in tears and saliva.

Protein hormones

Pancreas. Insulin and glucagon.
Pituitary gland. Growth hormone, ADH and oxytocin.
Parathyroid glands. PTH, which raises blood calcium.
Kidneys. EPO, which tells the marrow to make red blood cells.
Fat tissue. Leptin, which signals how much fat is stored.
Gut. Gastrin, CCK and other digestion signals.

Amino acids made into other things

Brain. Serotonin from tryptophan, dopamine from tyrosine, and GABA and glutamate.
Adrenal glands. Adrenaline from tyrosine.
Thyroid. Thyroid hormones are built on tyrosine.
Skin and hair. Melanin, the pigment, from tyrosine.
Bone marrow. Heme is built from glycine.
Liver, kidneys and muscle. Creatine is made in the kidneys and liver and stored in muscle for quick energy.
Breast milk. Casein and whey proteins feed the baby.

Spare amino acids

Liver. Removes their nitrogen as urea and turns the rest into glucose, fat or energy.
Kidneys. Send urea out in urine and use glutamine to balance acid.
Muscle. In a fast, breaks some protein down to supply amino acids.

Figure 9

Putting It All Together

The whole lesson in one table, plus a few extras.

The whole lesson in one table

Idea	In one line	See
Atoms	C, H, O and N (plus S): nitrogen is what sets protein apart from fat and carbohydrate	Figure 2
Amino acid	Amine + alpha carbon + carboxyl + one side chain	Figure 2
Peptide bond	Carboxyl of one joins the amine of the next; one water released	Figure 3
20 amino acids	9 essential, 6 conditionally essential, 5 non-essential	Figure 4
Shape	Primary, secondary, tertiary, quaternary; cooking unfolds it	Figure 5
Digestion	Back to amino acids; nitrogen leaves as urea	Figure 6
Food	Animal foods and soy are complete; combine beans and grains	Figure 7

Worth knowing

Protein powders are just protein

Whey and casein come from milk, and pea and soy powders from those plants. They give the same amino acids as the whole foods, without the fibre, vitamins and minerals that come along in the food.

Collagen is incomplete

Collagen, the most common protein in your body, is rich in glycine and proline but has almost no tryptophan, so on its own it’s an incomplete protein.

Gelatin is cooked collagen

Simmering bones and skin unfolds collagen into gelatin, which sets into a gel as it cools.

Protein quality scores

PDCAAS and DIAAS score proteins by their essential amino acids and how well they are digested. Milk, eggs and soy score near the top.

General education, not medical advice. Facts follow mainstream sources (US Dietary Guidelines, NIH Office of Dietary Supplements, standard biochemistry and food-science texts).

Nutrition reads this on your call. Tap Next as we go, or tell Nutrition the line number.

1.1
What Protein Is
Protein is the next macronutrient in this course. Like fat it gives energy, about four calories per gram, the same as carbohydrate. But energy isn't its main job. Protein is what your body is built from and what does most of its work: muscle, skin and hair, the enzymes that run every chemical reaction, the antibodies that fight infection, and the hemoglobin that carries oxygen in your blood.
In food and in the body
Enzymes. Enzymes are proteins that speed up chemical reactions without being used up. The lipases that break down fat in Fats Figure 9 are enzymes, and so are pepsin and trypsin, which digest protein in Figure 6.
Antibodies. Antibodies are Y-shaped immune proteins that latch onto germs and tag them for your immune system to destroy. Eating enough protein, plus zinc from foods like meat, shellfish and seeds, supports making them.
Hemoglobin. Hemoglobin is the iron-containing protein in red blood cells that carries oxygen. Your body builds it from dietary protein plus iron, from foods like red meat, liver, beans and spinach.
Title, then the chain down the middle, from Amino acids to Protein
1.2
Amino Acids
Every protein is built from smaller units called amino acids, the box at the top. There are twenty of them. Nine are essential, meaning your body can't make them, so they have to come from food. The other eleven your body can make, although six of those, called conditionally essential, can run short when you're ill, injured or growing fast.
Top: Amino acids, then Essential, Non-essential and Conditional on the left
1.3
Chains
Follow the arrows down the middle. Amino acids link together with peptide bonds, one after another, into a chain. A short chain, up to about fifty amino acids, is called a peptide. A longer chain folds into a working shape and becomes a protein. A peptide and a protein are the same kind of chain, just different lengths. Folded proteins make up your muscle and work as enzymes, antibodies and hemoglobin.
Down the middle: peptide bonds, Peptide, Protein, then the four boxes underneath
1.4
Protein in Food
On the right is food, where the amino acids come from. The protein you eat is digested back down into amino acids, the dashed arrow, and your body rebuilds them into its own proteins. A complete protein has all nine essential amino acids in good amounts. An incomplete one is low in at least one, but two incomplete proteins can cover each other's gaps. Each box links to the figure that explains it.
Right: Food, Complete, Incomplete, Combine them, and the dashed digestion arrow into Amino acids
1.5
On the Label
On a nutrition label, protein is just one line, in grams. In the US the percent Daily Value for protein is often left off; it's only required when the package makes a protein claim or the food is for young children. And the label won't tell you which amino acids are in there; Figure 7 shows how to tell from the food.
The three boxes under the tree

2.1
Four Kinds of Atom
Let's build an amino acid from scratch. It uses only a few kinds of atom, and each always makes the same number of bonds. Carbon makes four, like this methane. Nitrogen makes three, like ammonia. Oxygen makes two, like water. Hydrogen makes just one. Sulfur, which appears in two amino acids, makes two, like hydrogen sulfide.
The four small molecules, left to right
2.2
Nitrogen Is New
Fats and carbohydrates are made only of carbon, hydrogen and oxygen. Protein adds nitrogen. That's the atom that makes protein special, and later we'll see that your body has to do something with it when protein is burned for energy.
Nitrogen (blue) in ammonia
2.3
Glycine
Here's the simplest amino acid, glycine. On the left is an amine group: a nitrogen with two hydrogens. That's the amino part of the name. In the middle is a carbon called the alpha carbon. On the right is a carboxyl group: a carbon with a double-bonded oxygen and an OH. That's the acid part, because the OH can give up its hydrogen.
Markers 1, 2, 3 on glycine
2.4
The Side Chain
Now alanine. It has exactly the same amine group, alpha carbon and carboxyl group. The only difference is what hangs off the alpha carbon. In glycine it's just a hydrogen; in alanine it's a CH3, a methyl group. That hanging part is called the side chain, or R group, and it's the only thing that differs between all twenty amino acids.
Marker 4: the side chain
2.5
Charged in the Body
One small detail: drawn here, the amine and carboxyl groups are neutral. In your body's fluids the carboxyl usually gives up its hydrogen and the amine picks one up, so each end carries a charge. The atoms are the same, just one hydrogen moved.
Note

3.1
Head to Tail
Amino acids join head to tail. The carboxyl group of one lines up with the amine group of the next. The OH from the carboxyl and one hydrogen from the amine come off together, and that's a water molecule, H2O.
Markers 1 and 2, then the water
3.2
The Bond Itself
What's left is a new bond between the carbon of the first amino acid and the nitrogen of the second. That carbon-nitrogen link, next to its C=O, is the peptide bond. It's strong and flat, which is part of why protein chains fold the way they do. Two amino acids joined like this make a dipeptide, here glycyl-alanine.
Marker 3: the peptide bond
3.3
Adding More
Add a third, here serine, and you get a tripeptide with two peptide bonds, and another water released. The chain always has a free amine at one end, called the N-terminus, and a free carboxyl at the other, the C-terminus. New amino acids are added at the C-terminus end.
N, markers 1 and 2, then C
3.4
The Backbone
Look along the chain: nitrogen, carbon, carbon, nitrogen, carbon, carbon. That repeating pattern is the backbone, and the side chains stick out from it, alternating sides. Most proteins are a few hundred amino acids long, some many thousands. Digestion is the reverse: water is added back to split each peptide bond.
The N–C–C repeat along the zigzag

4.1
One Backbone, Twenty Side Chains
Here are all twenty, drawn at the same scale and the same way round: the amine and carboxyl along the bottom, the side chain pointing up. The bottoms all match. Only the tops differ, from glycine's single hydrogen to tryptophan's big double ring.
Compare the bottoms, then the tops
4.2
The Nine Essential
These nine are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Your body has no way to make their side chains, so every one has to come from food. Three of them, leucine, isoleucine and valine, have branched side chains and are called the branched-chain amino acids.
The nine marked essential
4.3
Conditionally Essential
The next six, arginine, cysteine, glutamine, glycine, proline and tyrosine, your body normally makes, but it can't always keep up, for example in premature babies, after serious injury or during illness. Some are made from essential ones: tyrosine from phenylalanine, and cysteine using the sulfur from methionine.
The six conditionally essential
4.4
The Non-Essential Five
The last five, alanine, asparagine, aspartic acid, glutamic acid and serine, your body makes easily from glucose and other amino acids. Non-essential doesn't mean unimportant; it only means you don't have to eat them.
The five non-essential
4.5
What the Side Chains Do
The side chains decide how a protein behaves. Oily ones like leucine and phenylalanine avoid water and end up inside a folded protein. Charged ones like lysine and glutamic acid face the water outside. Cysteine's sulfur can bond to another cysteine, a cross-link that holds shapes together, like the curls in hair.
The table: what each essential one does
4.6
Raw Materials
Some amino acids are also raw materials. Tryptophan becomes serotonin and melatonin, and a little niacin. Phenylalanine becomes tyrosine, which becomes adrenaline and the pigment melanin. Histidine becomes histamine. People with the inherited condition PKU can't process phenylalanine, which is why products with aspartame carry a phenylalanine warning.
In your body

5.1
Primary
A protein's shape is described in four levels. The primary structure is simply the order of amino acids along the chain, drawn here as beads. That order is set by your genes, and it decides everything that follows. Change a single amino acid and the protein can change: sickle-cell hemoglobin differs from normal hemoglobin by just one.
Level 1: the bead chain
5.2
Secondary
Secondary structure is how short stretches coil or line up. The two common shapes are the alpha helix, a spiral, and the beta sheet, strands lying side by side. Both are held by hydrogen bonds: weak attractions between the C=O of one peptide bond and the N–H of another.
Level 2: the helix close-up (each C=O dashed to the N–H four along), then the sheet (C=O on one strand to N–H on the next)
5.3
Tertiary
Tertiary structure is the whole chain folded into its final shape. Oily side chains tuck inside, away from water, and water-loving ones face out, with a few sulfur cross-links holding it steady. The shape decides the job: an enzyme's pocket fits only the molecule it works on.
Level 3: the folded chain, then the close-up: oily side chains inside, water-loving outside, the two S–S links, and the enzyme-pocket inset
5.4
Quaternary
Some proteins are teams of chains, and that's quaternary structure. Hemoglobin is four chains, each holding a heme group with an iron atom that grabs oxygen. Collagen, the most common protein in your body, is three chains wound into a rope.
In food and in the body
Hemoglobin. Hemoglobin needs iron. Red meat, liver and shellfish give heme iron, which your body absorbs well; beans, lentils and spinach give non-heme iron, which is less well absorbed. Inside red blood cells, hemoglobin carries oxygen from your lungs to your tissues.
Collagen. Collagen comes from skin-on chicken, bone broth, gelatin and the connective tissue in meat. Your gut breaks it down to amino acids like any other protein, and your body builds its own collagen from them, using vitamin C. It is the main structural protein of skin, tendons, ligaments, bone and cartilage.
Level 4: hemoglobin (two alpha and two beta chains, each heme with its iron and O2), then the close-up of one heme in its pocket (iron in the flat ring, bent O2 above it, His E7 above, His F8 below, side view edge-on), then collagen's three-chain rope
5.5
Denaturing
The folds are held by weak bonds, so heat, acid or whipping can undo them. That's denaturing. When egg white turns from clear to white, or milk curdles with lemon juice, the proteins are unfolding and tangling. The peptide bonds don't break, so the amino acids are all still there, and cooked protein is usually easier to digest.
Note

6.1
In the Stomach
Protein digestion starts in the stomach. Stomach acid denatures the proteins, unfolding them so enzymes can reach the peptide bonds, and an enzyme called pepsin starts cutting the chains into shorter pieces.
Mouth, then Stomach
6.2
In the Small Intestine
In the small intestine, the pancreas adds more enzymes, trypsin and chymotrypsin, and enzymes on the gut wall finish the job, down to single amino acids and some pairs and threes. These are absorbed into the blood and go first to the liver.
Small intestine, then Into the blood
6.3
The Amino Acid Pool
From there they join your body's amino acid pool, a supply that cells draw on all day. Your body breaks down and rebuilds a few hundred grams of its own protein every day, far more than you eat, and reuses most of the amino acids. Nothing is stored as spare protein, though, so you need a steady supply.
The amino acid pool
6.4
What It Builds
The pool builds muscle, skin, hair and the collagen in bone; enzymes; antibodies; carriers like hemoglobin; and hormones such as insulin. Only when there's more than you need for building are amino acids burned for energy, or turned into glucose or fat.
The three boxes
6.5
Urea
Burning an amino acid leaves its nitrogen behind as ammonia, which is toxic. Your liver joins two nitrogens with a carbon and an oxygen to make urea, this small molecule, and your kidneys send it out in urine. That's why a very high protein intake means a little more water.
Marker 1: the two nitrogens on urea; then the alanine diagram: the blue nitrogen lane (ammonia, urea, out in urine) and the grey carbon lane (pyruvate and its three fates)
6.6
Too Little, Too Much
Too little protein over time leads to muscle loss, slow healing, weaker immunity and thinning hair; in severe shortage, especially in children, swelling from fluid, a condition called kwashiorkor, and poor growth. Extra protein beyond your needs is burned or stored as fat; very high intakes are mainly a concern for people with kidney disease.
In your body

7.1
The Limiting Amino Acid
Your body builds each protein from a full set of amino acids. If one essential amino acid runs short, building slows down, however much of the others there is. The scarcest one in a food is called its limiting amino acid.
First box: Limiting amino acid
7.2
Complete Foods
Animal foods, meat, poultry, fish, eggs and dairy, are complete proteins: all nine essentials in good amounts. Among plants, soy is complete, and quinoa and buckwheat are close.
The first four rows
7.3
Incomplete Foods
Most plant proteins are incomplete. Beans, lentils and peas are low in methionine. Grains, wheat, rice, oats and corn, are low in lysine, and so are most nuts and seeds.
The last three rows
7.4
Combining
But their gaps are different, so they cover each other. Beans are rich in lysine and grains in methionine, which is why rice and beans, hummus with pita, or peanut butter on whole-wheat bread give a complete set. They don't have to be in the same meal; over the day is enough.
Beans + grains, then Over the day
7.5
How Much
The US recommended amount for adults is 0.8 grams of protein per kilogram of body weight per day, about fifty-six grams for someone weighing seventy kilograms. Needs go up in pregnancy and breastfeeding, and growing children need more per kilogram.
First box, then More at some stages
7.6
Found In
Good whole-food sources are meat, poultry, fish, eggs, milk, yogurt and cheese, beans, lentils, peas, soy foods like tofu and tempeh, nuts, seeds and whole grains.
In your bodyFound in

8.1
Where Protein Goes
Here's where protein is used: first inside a single cell, then everywhere else in the body. Protein is digested to amino acids, and your cells build them back into the proteins that do almost every job in the body.
The cell drawing and the list of places
8.2
Inside a Cell
Zoom in to a single cell and protein is everywhere. The recipe sits in the nucleus as DNA. A copy, called messenger RNA, travels out to the ribosomes, which link amino acids into a chain. Proteins headed out of the cell are made on the rough ER, then finished and shipped by the Golgi. Inside, enzyme proteins run the cell's chemistry, protein fibers hold its shape, and membrane proteins act as doors and doorbells. Worn-out proteins go to the lysosomes, which break them back into amino acids to use again.
In food and in the body
Histones. Not something you eat as such: every cell makes its own histone proteins, rich in the amino acids lysine and arginine, to package its DNA.
Amino acids. From the protein you eat, digested and carried in the blood; cells also make the 11 non-essential ones themselves.
Enzymes. Enzymes in food are digested like any protein; your cells build their own from amino acids.
Insulin receptor. A protein in the membrane of muscle, fat and liver cells. When insulin binds it, the cell takes up glucose.
The cell drawing and its numbered parts
8.3
Digesting Protein
Pepsin starts digesting protein in the stomach, and trypsin from the pancreas carries on in the small intestine, which absorbs the amino acids into the blood. The pancreas also makes insulin and glucagon, which are proteins too.
In food and in the body
Pepsin and trypsin. Not eaten: your stomach makes pepsin and your pancreas makes trypsin, both protein enzymes. They cut the peptide bonds in the protein you eat.
Insulin. A small protein hormone made by the pancreas. It tells cells to take up glucose after a meal.
In the list: stomach, pancreas and small intestine
8.4
Building and Moving
Muscle holds about 40% of your body's protein; actin and myosin slide past each other to make it contract. Collagen gives strength to skin, bone, tendons and cartilage, and keratin makes hair and nails.
In food and in the body
Actin and myosin. Found in all meat, which is muscle. Your body digests them to amino acids and builds its own actin and myosin from them.
Keratin. Hair, nails and feathers are keratin, which you can’t digest, so it isn’t food. Your skin cells build their own keratin from amino acids to make hair, nails and the outer skin.
In the list: muscle, skin and hair, joints and bones
8.5
Carrying and Defending
The liver makes albumin, the main blood protein, and the clotting factors. Red blood cells carry oxygen on hemoglobin. Immune cells, in the spleen and elsewhere, make antibodies that tag germs, and the brain makes neurotransmitters such as serotonin from amino acids.
In food and in the body
Albumin. Egg white is mostly a related protein, ovalbumin. Your liver makes albumin, which carries fatty acids and hormones in the blood and holds water in it.
Serotonin. Not eaten as such: the brain and gut make it from the amino acid tryptophan, found in turkey, eggs, cheese and nuts.
In the list: liver, blood, spleen and brain
8.6
The Leftovers
Your body doesn't store spare amino acids. The liver removes their nitrogen as urea, which the kidneys send out in urine, and burns the rest for energy or turns it into glucose or fat.
In food and in the body
Urea. Not something you eat: your liver makes it from the nitrogen of spare amino acids. Your kidneys filter it out, and it is the main nitrogen waste in urine.
In the list: liver and kidneys
8.7
Simplified
This is simplified. Every cell is full of proteins, so the list gives each place's main job, not every detail.
NoteThe note under the list

9.1
One Table
Here's the whole lesson in one table: the atoms, the amino acid, the peptide bond, the twenty amino acids, the four levels of shape, digestion, and protein in food. Each row links back to its figure.
The table, row by row
9.2
Worth Knowing
A few extras. Protein powders are just protein from milk, peas or soy, without the rest of the food. Collagen is rich in glycine and proline but has almost no tryptophan, so on its own it's incomplete; gelatin is collagen that has been cooked. And scores like PDCAAS and DIAAS rate proteins by their essential amino acids and digestibility; milk, eggs and soy score near the top. That's protein. Try the quiz below.
Worth knowing, then down to the quiz

→ next line · ← previous · Space pause or continue · Shift + ← → figures · click a line to jump

Quiz

Tap a question to see the answer

1. How many kcal does a gram of protein give?

About 4, the same as carbohydrate (Figure 1).

2. Which atom do proteins have that fats and carbohydrates don’t?

Nitrogen, in the amine group of every amino acid (Figure 2).

3. What three parts do all amino acids share, and what part differs?

An amine group, an alpha carbon and a carboxyl group are shared; the side chain (R group) differs (Figure 2).

4. What is released when a peptide bond forms?

One water molecule: an OH from one amino acid’s carboxyl and an H from the next one’s amine (Figure 3).

5. How many amino acids are essential? Name three.

Nine: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine (Figure 4).

6. Which essential amino acid contains sulfur?

Methionine (cysteine, a conditionally essential one, has sulfur too) (Figure 4).

7. What does cooking an egg do to its proteins?

It denatures them: the folds unravel, but the peptide bonds and amino acids stay (Figure 5).

8. Where does protein digestion start, and which enzyme begins it?

In the stomach, where acid unfolds proteins and pepsin starts cutting them (Figure 6).

9. What happens to the nitrogen when amino acids are burned for energy?

It comes off as ammonia, the liver turns it into urea, and the kidneys remove it in urine (Figure 6).

10. Why do rice and beans go well together?

Grains are low in lysine and beans in methionine; each supplies what the other lacks (Figure 7).

11. What is the US RDA for protein for adults?

0.8 grams per kilogram of body weight per day, about 56 g for a 70 kg adult (Figure 7).

12. Is collagen a complete protein?

No; it has almost no tryptophan (Figure 9).

Next lesson: Module 1 · Carbohydrates →