Nutrition and Excretion
2. Digestive Systems
Learning outcomes
- I can explain the purpose of digestion.
- I can identify the major organs of a digestive system.
- I can distinguish between mechanical and chemical digestion.
- I can describe the movement of food through the digestive tract.
- I can explain how digestive systems are adapted to different diets.
Why Is Digestion Necessary?
Animals need nutrients for:
- Energy release through cellular respiration.
- Growth and repair.
- Building proteins and other molecules.
- Producing enzymes and hormones.
- Maintaining cells and tissues.
- Storing energy for later use.
However, much of the food animals eat contains molecules that are too large and insoluble to pass through cell membranes and be absorbed directly.
The purpose of digestion is to break large, insoluble food molecules into smaller, soluble molecules that can be absorbed and used by the body.
Digestion therefore changes:
Large food molecules → smaller molecules that can be absorbed
For example:
- Starch → simple sugars such as glucose
- Proteins → amino acids
- Lipids → fatty acids and monoglycerides
Digestion and Absorption Are Different
Digestion is the breakdown of food into smaller molecules.
Absorption is the movement of digested nutrients from the digestive system into the body's internal transport system, such as the blood or lymph.
The sequence is:
Food → digestion → small molecules → absorption → transport → cells
This distinction is important. Food has not been absorbed simply because it has been digested.
The Digestive System
The human digestive system consists of the digestive tract and several accessory organs.
The digestive tract is a continuous tube running through the body.
The main pathway is:
Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus
Several other organs contribute substances needed for digestion:
- Salivary glands
- Liver
- Gallbladder
- Pancreas
The Mouth
Digestion begins in the mouth.
Several processes occur here.
Teeth
Teeth cut, crush, and grind food into smaller pieces.
This is mechanical digestion.
Breaking food into smaller pieces increases its surface area, allowing digestive enzymes to act more effectively.
Saliva
Salivary glands produce saliva.
Saliva:
- Moistens food.
- Helps food form into a mass that can be swallowed.
- Contains the enzyme amylase.
Amylase begins the chemical digestion of starch.
Tongue
The tongue:
- Moves food during chewing.
- Mixes food with saliva.
- Helps form a bolus.
- Pushes the bolus toward the back of the mouth for swallowing.
Mechanical Digestion
Mechanical digestion is the physical breakdown of food into smaller pieces without changing the chemical identity of the food molecules.
Examples include:
- Chewing with the teeth.
- Crushing food.
- Grinding food.
- Churning in the stomach.
Imagine breaking one large piece of food into many smaller pieces.
The total amount of food has not changed, but its surface area has increased.
This allows digestive enzymes to contact more of the food at the same time.
Chemical Digestion
Chemical digestion involves breaking large food molecules into smaller molecules through chemical reactions.
These reactions are usually controlled by digestive enzymes.
Examples include:
Starch → sugars
Proteins → amino acids
Lipids → fatty acids and monoglycerides
Chemical digestion changes the molecules themselves.
This is the major difference between mechanical and chemical digestion.
Comparing Mechanical and Chemical Digestion
| Mechanical digestion | Chemical digestion |
|---|---|
| Physically breaks food apart | Breaks chemical molecules apart |
| Does not change molecular identity | Produces different, smaller molecules |
| Includes chewing | Includes enzyme action |
| Includes stomach churning | Occurs throughout several parts of the digestive tract |
| Increases food surface area | Produces molecules that can be absorbed |
The two processes work together.
Mechanical digestion often makes chemical digestion more efficient.
The Oesophagus
After swallowing, food enters the oesophagus.
The oesophagus is a muscular tube connecting the mouth to the stomach.
Food does not simply fall through it.
Instead, waves of muscular contraction move food along the digestive tract.
This process is called peristalsis.
Peristalsis
During peristalsis, muscles in the wall of the digestive tract contract and relax in a coordinated sequence.
The contraction behind the food pushes it forward.
A simplified pattern is:
Relaxed region ahead of food → contraction behind food → food moves forward
Peristalsis occurs in several parts of the digestive tract, including:
- Oesophagus
- Stomach
- Small intestine
- Large intestine
This muscular movement helps ensure that food continues travelling through the digestive system.
The Stomach
The stomach is a muscular organ that temporarily stores food and continues digestion.
The stomach performs both mechanical and chemical digestion.
Mechanical Digestion
Muscular contractions churn food and mix it with gastric secretions.
Chemical Digestion
The stomach produces substances including:
- Hydrochloric acid
- Protein-digesting enzymes such as pepsin
The acidic environment helps pepsin function and also helps kill many microorganisms entering with food.
Food eventually becomes a partially digested mixture called chyme.
Why Doesn't the Stomach Digest Itself?
The stomach contains strong acid and protein-digesting enzymes.
Its tissues therefore require protection.
The stomach lining produces a protective mucus layer containing bicarbonate that helps protect cells from acid and enzymes.
The stomach lining is also continually renewed.
Damage to these protective mechanisms can contribute to conditions such as stomach ulcers.
The Small Intestine
Most chemical digestion and nutrient absorption occur in the small intestine.
Despite its name, the small intestine is several metres long in an adult human.
It has three main regions:
- Duodenum
- Jejunum
- Ileum
The first part, the duodenum, receives digestive substances from the pancreas and liver.
The Pancreas
The pancreas produces digestive enzymes that enter the small intestine.
These include enzymes involved in digesting:
- Carbohydrates
- Proteins
- Lipids
The pancreas also releases bicarbonate into the small intestine.
Bicarbonate helps neutralise acidic material arriving from the stomach.
This creates conditions better suited to many intestinal enzymes.
The Liver and Gallbladder
The liver produces bile.
Bile is stored and concentrated in the gallbladder before being released into the small intestine.
Bile helps with lipid digestion by breaking large fat droplets into smaller droplets.
This process is called emulsification.
Emulsification increases the surface area available for lipase enzymes.
Importantly, bile is not an enzyme.
It assists digestion but does not itself enzymatically break lipid molecules apart.
Absorption in the Small Intestine
After digestion, small nutrient molecules must be absorbed.
The inner surface of the small intestine contains millions of finger-like projections called villi.
Each villus is covered with even smaller projections called microvilli.
These structures provide a very large surface area for absorption.
Villi also have:
- A thin surface.
- A good blood supply.
- Capillary networks.
- A lymph vessel called a lacteal.
These features allow digested nutrients to be absorbed efficiently.
What Happens to Absorbed Nutrients?
Different nutrients follow somewhat different pathways.
Glucose and amino acids enter blood capillaries in the villi.
Many products of lipid digestion enter cells of the intestinal lining, are repackaged into lipid-containing particles, and enter the lymphatic system through lacteals before eventually reaching the bloodstream.
The circulatory system can then transport absorbed nutrients around the body.
Cells may use them for:
- Respiration.
- Growth.
- Repair.
- Storage.
- Producing new biological molecules.
The Large Intestine
Material that has not been digested or absorbed passes into the large intestine.
The large intestine includes the colon.
Its important functions include:
- Absorbing water.
- Absorbing some ions.
- Housing large populations of microorganisms.
- Forming faeces from remaining material.
The Gut Microbiome
The digestive tract contains an enormous community of microorganisms known collectively as the gut microbiome.
These microorganisms can:
- Break down some substances that human enzymes cannot digest.
- Produce certain useful compounds.
- Interact with the immune system.
- Compete with potentially harmful microorganisms.
The relationship between animals and their gut microorganisms is especially important in many herbivores.
The Rectum and Anus
Material remaining after digestion and absorption forms faeces.
Faeces are temporarily stored in the rectum.
They eventually leave the digestive tract through the anus.
This removal of undigested material is called egestion.
Egestion should not be confused with excretion.
Egestion removes undigested material from the digestive tract.
Excretion removes metabolic waste produced by cells, such as carbon dioxide or urea.
Following Food Through the Human Digestive System
The complete journey can be summarised as:
Mouth
Food is chewed and mixed with saliva.
↓
Oesophagus
Peristalsis moves the bolus toward the stomach.
↓
Stomach
Food is churned and mixed with acid and digestive enzymes.
↓
Small intestine
Most chemical digestion is completed and nutrients are absorbed.
↓
Large intestine
Water and some ions are absorbed.
↓
Rectum
Faeces are stored.
↓
Anus
Undigested material leaves the body.
Digestive Systems Are Adapted to Diet
Not all animals have digestive systems identical to humans.
An animal's digestive system is influenced strongly by the type of food it eats.
Different foods present different digestive challenges.
Herbivore Digestive Systems
Plant material can be difficult to digest because plant cell walls contain cellulose.
Most vertebrate animals do not produce their own cellulase enzyme.
Many herbivores solve this problem by maintaining symbiotic microorganisms that can break down cellulose.
These microorganisms live in specialised regions of the digestive tract.
Ruminant Digestion
Cattle, sheep, deer, and giraffes are examples of ruminants.
Ruminants have a highly specialised stomach with several compartments.
The largest compartment, the rumen, contains huge populations of microorganisms.
These microorganisms ferment plant material and help break down cellulose.
Ruminants may also regurgitate partially digested food called cud, chew it again, and swallow it.
Repeated chewing further breaks down tough plant material.
Hindgut Fermentation
Not all herbivores use a rumen.
Animals such as horses and rabbits rely heavily on microbial fermentation farther along the digestive tract.
A large caecum and parts of the large intestine can contain microorganisms that ferment plant material.
This is called hindgut fermentation.
Therefore, different herbivores have evolved different solutions to the challenge of digesting cellulose.
Carnivore Digestive Systems
Animal tissue is generally easier to digest than cellulose-rich plant material.
Many carnivores therefore have digestive tracts that are relatively shorter and less specialised for fermentation than those of many herbivores.
Carnivores may also have:
- Strong stomach acidity.
- Powerful protein-digesting enzymes.
- Digestive systems suited to relatively nutrient-rich animal tissues.
Their feeding structures and digestive systems work together.
Sharp teeth capture and process food, while the digestive tract chemically breaks down the nutrients.
Omnivore Digestive Systems
Omnivores consume both plant and animal material.
Humans and pigs are examples.
Their digestive systems tend to be capable of processing a broad range of foods rather than being extremely specialised for one particular diet.
However, humans cannot efficiently digest cellulose because we do not produce cellulase and lack the specialised fermentation system of animals such as cattle.
Cellulose therefore contributes to dietary fibre in the human diet.
Comparing Digestive Systems
| Feature | Many herbivores | Many carnivores | Many omnivores |
|---|---|---|---|
| Main diet | Plant material | Animal tissue | Plant and animal material |
| Cellulose challenge | Major | Minor | Variable |
| Microbial fermentation | Often extensive | Usually less extensive | Some microbial fermentation |
| Digestive tract | Often relatively long | Often relatively shorter | Intermediate/generalised |
| Specialised chambers | Common in some groups | Less common | Usually limited |
| Major adaptation | Processing tough plant material | Processing animal tissue | Dietary flexibility |
These are broad patterns rather than absolute rules. Digestive anatomy varies greatly among species.
Structure and Function
Digestive systems provide many examples of the relationship between structure and function.
| Structure | Function |
|---|---|
| Teeth | Mechanical digestion |
| Muscular stomach | Churning food |
| Digestive enzymes | Chemical digestion |
| Long small intestine | Provides time and area for digestion and absorption |
| Villi | Increase absorption surface area |
| Microvilli | Increase surface area even further |
| Capillaries | Carry absorbed nutrients away |
| Rumen | Provides a chamber for microbial fermentation |
| Long herbivore gut | Provides more time for processing plant material |
| Large caecum | Supports microbial fermentation in some herbivores |
The structure of an organ often provides clues about its function.
Worked Example: Why Villi Improve Absorption
Imagine two tubes of equal length.
Tube A has a completely smooth inner surface.
Tube B has thousands of folds and finger-like projections.
Tube B has much more surface area in contact with its contents.
The small intestine uses this same principle.
Folds, villi, and microvilli dramatically increase the area available for nutrients to cross into the body.
Therefore:
Greater surface area → greater opportunity for absorption
Worked Example: Identifying Diet from a Digestive System
Scientists examine the digestive system of an unfamiliar mammal.
They discover:
- A very long digestive tract.
- A large fermentation chamber.
- Large populations of cellulose-digesting microorganisms.
- Extensive grinding teeth.
These features strongly suggest that the animal consumes large amounts of plant material.
The evidence points toward a herbivorous diet because the digestive system is highly adapted for processing cellulose-rich food.
Mechanical and Chemical Digestion Work Together
Suppose a person eats a piece of bread.
Mouth
Teeth mechanically break the bread apart.
Salivary amylase begins chemically digesting starch.
Stomach
Muscular contractions churn the food.
Small Intestine
Enzymes continue chemical digestion until absorbable molecules are produced.
Absorption
Small nutrient molecules cross the intestinal wall.
This shows that digestion is not one event.
It is a coordinated sequence of mechanical processes, chemical reactions, movement, and absorption.
Common Mistakes
Saying Digestion Releases All the Energy from Food
Digestion breaks food into smaller molecules.
Much of the usable energy is transferred later through metabolic processes such as cellular respiration.
Confusing Digestion and Absorption
Digestion breaks molecules down.
Absorption moves digested nutrients into the body's internal transport systems.
Saying Mechanical Digestion Changes Molecules
Mechanical digestion changes the physical size of food but does not break chemical bonds within nutrient molecules.
Saying All Digestion Occurs in the Stomach
Digestion begins in the mouth and continues mainly through the stomach and small intestine.
Saying Bile Is an Enzyme
Bile helps emulsify lipids but is not a digestive enzyme.
Saying Food Falls Down the Oesophagus
Food is moved by muscular contractions called peristalsis.
Confusing Egestion and Excretion
Egestion removes undigested food.
Excretion removes metabolic wastes produced by cells.
Assuming All Animals Have the Same Digestive System
Digestive systems vary considerably and are often adapted to an animal's diet.
Check Your Understanding
1. Explain why digestion is necessary.
2. What is the difference between digestion and absorption?
3. List the major organs food passes through from the mouth to the anus.
4. Define mechanical digestion.
5. Define chemical digestion.
6. Explain why chewing can make chemical digestion more efficient.
7. What is peristalsis?
8. Describe two functions of the stomach.
9. Why is the small intestine particularly important in digestion?
10. Explain how villi are adapted for absorption.
11. What is the role of bile in lipid digestion?
12. Explain why microorganisms are important in the digestive systems of many herbivores.
13. Compare the digestive systems of a typical herbivore and carnivore.
14. Explain why a ruminant benefits from having a large fermentation chamber.
15. An animal has a long digestive tract, large fermentation chambers, and extensive populations of cellulose-digesting microorganisms. Predict its likely diet and justify your answer.
Key Terms
- Digestion – the breakdown of large food substances into smaller molecules that can be absorbed.
- Mechanical digestion – physical breakdown of food without changing the chemical identity of its molecules.
- Chemical digestion – chemical breakdown of large molecules into smaller molecules.
- Absorption – movement of digested nutrients into the body's internal transport system.
- Digestive tract – continuous tube through which food passes during digestion.
- Enzyme – biological catalyst that increases the rate of a chemical reaction.
- Bolus – mass of chewed food mixed with saliva.
- Peristalsis – coordinated waves of muscular contraction that move material through the digestive tract.
- Chyme – partially digested mixture leaving the stomach.
- Bile – digestive fluid produced by the liver that helps emulsify fats.
- Emulsification – breaking large fat droplets into smaller droplets to increase surface area.
- Villus – finger-like projection in the small intestine that increases absorption surface area.
- Microvilli – microscopic projections on intestinal cells that further increase surface area.
- Lacteal – lymphatic vessel within a villus involved in transporting absorbed lipids.
- Egestion – removal of undigested material from the digestive tract.
- Cellulose – structural carbohydrate found in plant cell walls.
- Ruminant – herbivore with a specialised multi-compartment stomach supporting microbial fermentation.
- Rumen – large fermentation chamber in the digestive system of ruminants.
- Caecum – pouch associated with the intestine that supports microbial fermentation in some animals.
- Hindgut fermentation – microbial digestion occurring mainly in the caecum and large intestine.
Key Takeaways
- Digestion breaks large food substances into smaller molecules that can be absorbed.
- Digestion and absorption are different processes.
- The main human digestive pathway is mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.
- Mechanical digestion physically breaks food into smaller pieces.
- Chemical digestion uses chemical reactions, usually involving enzymes, to break large molecules apart.
- Mechanical digestion increases surface area and can make chemical digestion more efficient.
- Peristalsis moves food through the digestive tract.
- The stomach performs both mechanical and chemical digestion.
- Most chemical digestion and nutrient absorption occur in the small intestine.
- Villi and microvilli provide a very large surface area for absorption.
- The pancreas supplies digestive enzymes and bicarbonate to the small intestine.
- The liver produces bile, which helps with lipid digestion through emulsification.
- The large intestine absorbs water and contains an important microbial community.
- Digestive systems differ among animals because they are adapted to different diets.
- Many herbivores rely on microorganisms to digest cellulose.
- Ruminants use specialised stomach chambers for microbial fermentation.
- Carnivores generally require less extensive fermentation of their food.
- The structures of digestive systems provide strong evidence of the relationship between diet, structure, and function.