AQA A-level Notes on Digestion and Absorption: Detailed Overview

Breaking Down Food: The Digestive Process

Your digestive system uses two main strategies to tackle food. Mechanical digestion physically breaks food into smaller pieces through chewing, stomach churning, and the wave-like muscle contractions called peristalsis that push food along your digestive tract.

Chemical digestion is where the real magic happens - enzymes slice large molecules into tiny, absorbable units. Your mouth kicks things off with amylase in saliva, which starts breaking down carbohydrates whilst mucus lubricates the food and bicarbonate creates the perfect alkaline environment for the enzyme to work.

In your stomach, gastric juice containing hydrochloric acid and pepsin begins protein digestion, creating a soupy mixture called chyme. After 1-2 hours, this moves into the small intestine where pancreatic enzymes and bile from the liver finish the job.

Quick Fact: Your small intestine's ileum absorbs 95% of nutrients thanks to its massive surface area created by length, villi, and microvilli - like having a football pitch folded inside your belly!

Carbs and Proteins: Getting the Good Stuff Out

Carbohydrate digestion follows a clear pathway: amylase converts starch to maltose, then specific enzymes like maltase, lactase, and sucrase break disaccharides into simple sugars. Glucose and galactose enter intestinal cells via co-transport with sodium, whilst fructose uses facilitated diffusion.

About 65% of adults produce minimal lactase, explaining why many people struggle with unfermented dairy products. Your genetics determine whether you'll be reaching for lactose-free milk or not!

Protein digestion relies on two enzyme types: endopeptidases (like pepsin and trypsin) slice proteins internally, whilst exopeptidases nibble amino acids from the ends. This tag-team approach ensures proteins become individual amino acids ready for absorption.

Amino acids enter cells through sodium-dependent transporters - several different types handle various amino acids. Some even transport small peptide chains that get their final breakdown inside the cell.

Study Tip: Remember that infants produce rennin to curdle milk - this disappears as we grow up, which is why babies handle milk better than many adults!

Fats: The Tricky Customer

Lipid digestion presents a unique challenge because fats and water don't mix - like trying to dissolve butter in a glass of water. Your body solves this with bile salts that emulsify fats, breaking large droplets into tiny ones that lipase enzymes can actually attack.

The digestion products - monoglycerides and fatty acids - form microscopic packages called micelles (just 4-8 nm diameter) that can approach intestinal cells. Unlike other nutrients, these lipids enter enterocytes through simple diffusion rather than special transporters.

Inside the intestinal cells, something fascinating happens. The endoplasmic reticulum and Golgi apparatus reassemble the fats into triglycerides and package them with proteins into particles called chylomicrons. These are too large for blood capillaries, so they enter the lymphatic system through special vessels called lacteals.

This explains why fat absorption takes longer than carbs or proteins - your body needs time for all this repackaging and rerouting through the lymph system before fats finally reach your bloodstream.

Remember: Fat digestion is the only nutrient absorption that bypasses direct entry to blood - it takes the scenic route through your lymphatic system first!