Introduction
Okay, let's talk about what actually happens to your lunch. You eat a sandwich, and a few hours later it's... you. Muscle, brain fuel, the energy to scroll your phone at 1am. That's not magic, that's your digestive system doing chemistry 24/7 without you thinking about it once.
We're going to break down what a balanced diet actually means, follow food from your mouth to your small intestine (physical digestion, then chemical digestion, two totally different jobs), and land on how your gut turns "food" into molecules small enough to actually get into your blood. Let's get into it 🍽️
The Balanced DietC7.1
A balanced diet isn't "eat salad forever", it's having all the nutrient groups, in roughly the right amounts. Carbs (bread, rice, pasta) are your main energy source. Protein (meat, fish, beans) is growth and repair. Fat is a backup energy store and keeps you insulated. Fibre keeps your gut moving. Then the small-but-mighty crew: vitamin C (citrus, peppers, healthy gums and healing), vitamin D (sunlight, oily fish, lets your body actually use calcium), iron (red meat, leafy greens, makes haemoglobin), calcium (dairy, bones and teeth), and water (literally every reaction in this chapter needs it).
Skip one of these long-term and your body sends a very specific error message. No vitamin C long-term? Scurvy, swollen bleeding gums, slow-healing wounds. No vitamin D long-term? Rickets, soft, weak bones (often bowed legs in kids), because vitamin D is what lets your body absorb and use calcium, it's a vitamin D problem wearing a calcium costume.
> 🔑 Key idea: balanced diet = all nutrient groups + right amounts. Missing vitamin C long-term → scurvy (gums, healing). Missing vitamin D long-term → rickets (weak bones, calcium use).
Worked Example: What's the Definition Everyone Forgets?C7.1
Problem What is meant by the term balanced diet?
Step 1 Two things need to be true at once: every food group has to be present, and each has to be in roughly the right amount.
Step 2 Rule out the wrong-shaped options: "as much food as possible from every group" isn't balance, it's overeating. "Only carbohydrate" or "only protein" misses every other group entirely.
Step 3 So the answer is: a diet with all the food groups in the correct proportions to supply sufficient energy. Notice it's not "eat everything you can", it's "eat the right amounts of the right things."
The Digestive System: Your Gut's Org ChartC7.2
Food's journey has a fixed route, and four checkpoint words you need to keep straight: ingestion (food goes in your mouth), digestion (breakdown into smaller stuff), absorption (small stuff crosses into your blood), egestion (leftovers exit at the anus). There's a fifth word too: assimilation, cells actually using the nutrients once they're in the blood, a different step from absorption.
The route: mouth (teeth + amylase) → oesophagus (transport tube) → stomach (churns food, starts protein digestion) → small intestine (bile + pancreatic enzymes finish digestion, then absorption happens) → large intestine/colon (soaks up remaining water) → rectum (stores the leftovers) → anus (egestion). Three glands feed into this pipeline: the liver (makes bile), the gall bladder (stores bile), and the pancreas (makes amylase, protease, and lipase). Quick flag: bile is not an enzyme, it just emulsifies fat.
> 🔑 Key formula: ingestion (mouth) → digestion (mouth, stomach, small intestine) → absorption (small intestine) → assimilation (cells) → egestion (anus). Liver makes bile, gall bladder stores it, pancreas makes the enzyme trio.
Worked Example: Egestion Isn't ExcretionC7.2
Problem A student claims that passing out faeces is a form of excretion. Which statement correctly explains why this claim is wrong?
Step 1 Excretion removes waste your body's own cells actually made. Egestion removes food that was never digested or absorbed in the first place.
Step 2 Faeces are mostly undigested food, your cells didn't make that stuff, it just never got broken down and absorbed.
Step 3 So the claim is wrong because faeces are mainly undigested food, so passing them out is egestion, not excretion. Two different words for two totally different origins of "stuff leaving your body."
Teeth: The OG Physical Digestion SquadC7.3
Physical digestion is just breaking food into smaller pieces, no chemical change to the molecules, that's chemical digestion's job (coming up). The whole point: smaller pieces = way more surface area = enzymes act way faster later.
Four tooth types, four jobs: incisors (chisel-shaped, front, cut and bite), canines (pointed, tear), premolars (broad, ridged, crush), molars (broadest, flattest, at the back, grind and chew). Structure, outside to inside: enamel (super hard outer shell), dentine (bone-like, most of the tooth), pulp cavity (nerves and blood vessels, keeps it alive). The gum wraps the neck of the tooth and holds it in place with the jawbone.
> 🔑 Key idea: physical digestion = smaller pieces, same molecules, more surface area for enzymes. Incisors cut, canines tear, premolars/molars grind. Enamel → dentine → pulp, outside to inside.
Worked Example: Identify the Tooth LayerC7.3
Problem A cross-section through a molar tooth shows four regions: a hard pale outer layer, a thick layer immediately beneath it that forms the bulk of the tooth, a soft central cavity with nerves and blood vessels, and the root. Which region is the dentine?
Step 1 The hard pale outer layer is enamel, straight away, no debate there.
Step 2 "Just beneath the enamel, forms the bulk of the tooth" is dentine's exact job description.
Step 3 So dentine is the thick layer immediately beneath the hard outer layer, forming the bulk of the tooth. The soft centre is pulp, and the lower embedded part is the root, neither of those matches.
Stomach and Surface Area: Chewing's Whole PointC7.3
The stomach does its own physical digestion move: its muscular walls churn food, mixing and breaking it into smaller pieces, separately from the chemical digestion also happening there.
Here's the actual reason physical digestion matters at all: breaking food into smaller pieces increases its surface area, so enzymes can act on it faster. Enzymes work at the surface, a huge lump hides most of its molecules from the enzyme, tiny crumbs expose way more of them at once, same total amount of food, way more contact points.
> 🔑 Key idea: physical digestion (chewing, churning) = smaller pieces = more surface area = enzymes act faster. It's the same reason chopped veggies cook faster than a whole potato.
Worked Example: What Actually Increases Surface Area?C7.3
Problem What increases the surface area of food so that enzymes can act on it more quickly?
Step 1 Absorption doesn't touch surface area, that's about molecules crossing the gut wall, a different step entirely.
Step 2 Active transport and respiration are cell-level processes, nothing to do with breaking food into pieces.
Step 3 Physical digestion breaks food into smaller pieces, for example by chewing, and smaller pieces have more surface area. Answer: physical digestion.
Chemical Digestion and Amylase: Where Molecules Actually ChangeC7.4
Chemical digestion is the actual glow-up: enzymes break large, insoluble molecules down into small, soluble molecules, so they can be absorbed. Opposite of physical digestion, physical just changes size and shape, chemical actually changes the molecule.
Amylase digests starch into maltose. It's made in two spots (salivary glands, pancreas) and acts in two spots (mouth, small intestine). You can actually test for starch: iodine solution turns blue-black if starch is present, and stays yellow-brown if it isn't, useful for checking whether digestion actually happened in an experiment.
$$\text{starch} \xrightarrow{\text{amylase}} \text{maltose}$$
> 🔑 Key formula: chemical digestion = large insoluble molecules → small soluble molecules, so they can be absorbed. Amylase: starch → maltose. Iodine test: blue-black = starch present, yellow-brown = starch gone.
Worked Example: What Does Amylase Actually Do?C7.4
Problem Which statement correctly describes what amylase does in chemical digestion?
Step 1 Amylase's substrate is starch, a large carbohydrate, not protein and not fat.
Step 2 Its product is a sugar, specifically maltose.
Step 3 So the answer is it breaks down starch into a sugar such as maltose, not protein into amino acids (that's protease's job) and not fat into fatty acids and glycerol (that's lipase's job).
Protease: Two Sites, One JobC7.4
Protease digests protein into amino acids, and it's made in two places: the stomach lining secretes one that acts right there in the stomach, and the pancreas secretes another that acts in the small intestine. Same job, two different sites, working one after the other as food moves through your gut.
Meanwhile lipase (made in the pancreas, acting in the small intestine) digests fat into fatty acids and glycerol, rounding out the three-enzyme squad: amylase (starch → maltose), protease (protein → amino acids), lipase (fat → fatty acids + glycerol).
> 🔑 Key idea: protease = protein → amino acids, made in the stomach lining AND the pancreas, acting in the stomach AND the small intestine. Lipase = fat → fatty acids + glycerol, pancreas → small intestine only.
Worked Example: Why Does Protease Even Matter?C7.4
Problem Protease is found in the human digestive system. Which statement best describes the importance of protease in digestion?
Step 1 Protease's substrate is protein, a large molecule that can't cross into the blood as-is.
Step 2 It breaks that protein down into amino acids, small enough and soluble enough to actually be absorbed.
Step 3 So the answer is it is an enzyme that breaks down proteins into small soluble amino acids that can be absorbed, not a hormone, not an acid, and definitely not something that acts on starch.
Hydrochloric Acid: Two Jobs, Not an EnzymeC7.4
Your stomach's hydrochloric acid (HCl) does double duty: it kills off pathogens lurking in your food, and it gives the stomach's protease the acidic pH it needs to actually work. That's it, two jobs, no more. HCl isn't an enzyme, it doesn't digest anything by itself.
> 🔑 Key idea: HCl kills pathogens + sets the acidic pH the stomach's protease needs. HCl is NOT an enzyme, it doesn't digest anything by itself.
Worked Example: HCl's Real Job DescriptionC7.4
Problem What is one function of the hydrochloric acid in the stomach?
Step 1 Options claiming HCl "digests starch" or "neutralises the food to make it alkaline" both get it backwards, HCl makes things more acidic, and it doesn't digest anything directly.
Step 2 Emulsifying fats is bile's job, not the acid's, that's a whole different substance.
Step 3 The real answer: it kills many harmful microorganisms taken in with food. (Its second job, giving protease its acidic pH, is the other half of the pair.)
Absorption: The Small Intestine's Big MomentC7.5
Absorption is where all that digestion pays off: small, soluble molecules (glucose, amino acids, fatty acids, glycerol, vitamins, minerals, water) finally cross from your gut into your blood. The small intestine is the main site for this, by a huge margin.
Water gets absorbed mostly in the small intestine too, with a bit more mopped up afterward in the colon (large intestine), which is exactly why what reaches your rectum is solid, not soup.
> 🔑 Key idea: small intestine = main absorption site for digested food AND most water. Colon = extra water absorption, no digested food.
Worked Example: Colon vs Small IntestineC7.5
Problem In which part of the gut is most of the water absorbed?
Step 1 Water gets absorbed in two places on its journey through the gut: the small intestine and the colon.
Step 2 Compare the amounts: the small intestine handles the bulk of it, the colon mops up the rest.
Step 3 So the answer is the small intestine, with the colon absorbing the smaller, remaining amount.
Key Points to Remember
- Balanced diet = all nutrient groups + right amounts. Missing vitamin C long-term → scurvy. Missing vitamin D long-term → rickets.
- Digestive system route: mouth → oesophagus → stomach → small intestine (digestion finishes, then absorption) → large intestine/colon → rectum → anus. Liver makes bile, gall bladder stores it, pancreas makes amylase/protease/lipase.
- Physical digestion = smaller pieces, same molecules, more surface area for enzymes. Teeth (incisors cut, canines tear, premolars/molars grind), tooth layers (enamel, dentine, pulp), stomach churning.
- Chemical digestion = enzymes break large insoluble molecules into small soluble ones. Amylase: starch → maltose. Protease (stomach + pancreas): protein → amino acids. Lipase (pancreas → small intestine): fat → fatty acids + glycerol.
- HCl kills pathogens and sets the acidic pH the stomach's protease needs. It is NOT an enzyme.
- Absorption happens mainly in the small intestine. Most water is absorbed there too, with extra absorbed in the colon.