What you'll learn
- How autotrophic, heterotrophic, saprotrophic, holozoic and parasitic nutrition differ.
- How gut design becomes more specialised from Amoeba to Hydra to mammals.
- Why humans, herbivores, carnivores and parasites have different adaptations for feeding.
- How to use microscope and specimen evidence to interpret nutritional adaptations.
The big idea: nutrition is about obtaining usable molecules
All organisms need substances for growth, repair, respiration and reproduction. Nutrition is the process of obtaining these substances from the environment.
Large biological molecules, such as starch, proteins and lipids, are often too large or insoluble to be absorbed directly. Digestion is the breakdown of large, insoluble molecules into smaller, soluble molecules that can cross cell membranes.
Autotrophic and heterotrophic nutrition
Autotrophic and heterotrophic nutrition
- Autotrophic organisms make organic molecules from inorganic substances, especially carbon dioxide.
- Heterotrophic organisms obtain organic molecules by feeding on other organisms, their remains, or their products.
Autotrophs are not “organisms that do not need energy”. They still respire. The key difference is their carbon source.
Autotrophs can be:
- Photoautotrophic — use light energy to make organic molecules, for example plants, algae and cyanobacteria.
- Chemoautotrophic — use energy released from chemical reactions involving inorganic substances, for example some bacteria that oxidise ammonia or hydrogen sulfide.
Heterotrophs include animals, fungi, protoctists such as Amoeba, and parasites such as tapeworms and lice.
Autotrophs still respire
Photosynthesis and chemosynthesis make organic molecules, but autotrophs still need respiration to release energy from those molecules.
Types of heterotrophic nutrition
Saprotrophic or saprobiotic nutrition
Saprotrophic nutrition is feeding on dead or decaying organic matter. The term saprobiotic is often used for the decomposer organisms or processes involved.
Saprotrophs, such as fungi, do not ingest large pieces of food. Instead, fungal hyphae secrete enzymes onto the food source. These enzymes carry out external digestion, meaning digestion outside the organism’s cells. The soluble products, such as glucose and amino acids, are then absorbed into the fungus.
Saprotrophic sequence
Saprotrophic nutrition involves enzyme secretion → external digestion → absorption of soluble products.
Holozoic nutrition
Holozoic nutrition involves taking in organic food, usually by ingestion, followed by internal digestion. Ingestion means taking food into the body or cell. Absorption means taking soluble digestion products across a membrane into cells or the blood.
Animals are holozoic. Amoeba also shows holozoic feeding because it takes in food particles, although its digestion happens inside food vacuoles rather than in a gut.
Parasitic nutrition
A parasite obtains nutrients from a host organism, usually causing harm but often not killing the host immediately. Parasites are highly specialised because they live in close association with their host.
Classifying nutrition from evidence
An unfamiliar organism uses carbon dioxide as its carbon source and gains energy by oxidising hydrogen sulfide. Another organism secretes enzymes onto a dead leaf and absorbs sugars. A third lives on a mammal and feeds on blood.
- The first organism uses carbon dioxide, so its carbon source is inorganic. Because the energy comes from chemical oxidation rather than light, it is chemoautotrophic.
- The second organism digests dead organic matter outside its body using secreted enzymes, then absorbs soluble products. This is saprotrophic nutrition.
- The third organism obtains food from a living host and benefits at the host’s expense. This is parasitic nutrition.
Increasing complexity in gut design
Unicellular organisms do not need a specialised gut because each cell can exchange substances directly with its surroundings. Larger multicellular organisms need specialised surfaces and regions because diffusion distances are greater and diets are more complex.
Amoeba: intracellular digestion
Amoeba is a unicellular protoctist. It surrounds food particles using extensions of cytoplasm called pseudopodia. The food is enclosed in a food vacuole, which is a membrane-bound sac inside the cell.
Digestive enzymes enter the food vacuole and break down the food. This is intracellular digestion, meaning digestion inside a cell. Soluble products then pass into the cytoplasm.
Hydra: a sac-like gut
Hydra is a small multicellular animal with a gastrovascular cavity, a simple internal space used for digestion and distribution of nutrients.
It has only one opening, which acts as both mouth and anus. Food enters through this opening, digestion begins extracellularly in the cavity, and some final digestion occurs intracellularly in lining cells.
This is more advanced than Amoeba because there is a digestive cavity, but it is still limited because food and waste must use the same opening.
Tube guts: separate openings and specialisation
A tube gut has two openings: a mouth for ingestion and an anus for egestion. Egestion is the removal of undigested material, not the same as excretion.
A tube gut allows one-way movement of food and specialisation along its length. Different regions can carry out different roles, such as mechanical digestion, chemical digestion, absorption and water reabsorption.
Why a tube gut is efficient
A tube gut allows food to move in one direction through specialised regions, so different food substances can be digested and absorbed under suitable conditions.
Human gut adaptations for an omnivorous diet
Humans are omnivores, meaning we eat both plant and animal material. The human gut is adapted to digest carbohydrates, proteins and lipids, but it is not specialised for digesting large amounts of cellulose.
Important regions include:
- Mouth — teeth break food into smaller pieces; saliva contains amylase for starch digestion.
- Stomach — muscular churning; hydrochloric acid provides acidic conditions; pepsin digests proteins.
- Duodenum — first part of the small intestine; receives bile and pancreatic enzymes.
- Ileum — later part of the small intestine; main site of absorption.
- Large intestine — absorbs water and forms faeces.
Different enzymes need different conditions
Efficient digestion requires different enzymes because enzymes are substrate-specific. A carbohydrase digests carbohydrates, a protease digests proteins, and a lipase digests lipids.
Conditions also matter. Pepsin works best in the acidic stomach. Pancreatic enzymes work best in the alkaline small intestine. Bile helps by neutralising acidic chyme from the stomach and emulsifying lipids into smaller droplets.
Bile is not an enzyme
Bile does not digest lipids chemically. It emulsifies lipid droplets, increasing surface area for lipase, and helps produce alkaline conditions in the small intestine.
Duodenum and ileum under the microscope
Histology is the study of tissues using a microscope. In this topic, you may examine prepared transverse sections, written as T.S., of the duodenum and ileum.
Both duodenum and ileum have villi, which are finger-like projections that increase surface area for absorption. Each villus has a thin epithelial lining, blood capillaries for absorbing glucose and amino acids, and a lacteal, which is a lymph vessel that absorbs lipid products.
The duodenum has Brunner’s glands, which secrete alkaline mucus to protect the wall and help neutralise acid. The ileum often has many goblet cells, which secrete mucus, and Peyer’s patches, which are lymphoid tissue involved in immune defence.
Calculating actual size from magnification
A villus on a microscope image is 36 mm long. The image was taken at a magnification of 80. Find the actual length of the villus in micrometres.
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Use the magnification relationship and rearrange it:
actual size=image sizemagnification\text{actual size} = \frac{\text{image size}}{\text{magnification}}actual size=magnificationimage size -
Substitute the values, keeping the image size in millimetres:
actual size=36 mm80=0.45 mm\text{actual size} = \frac{36\ \text{mm}}{80} = 0.45\ \text{mm}actual size=8036 mm=0.45 mm -
Convert millimetres to micrometres. Since 1 mm is 1000 micrometres, 0.45 mm is 450 micrometres. The villus is 450 µm long.
Slide identification
If you see Brunner’s glands in the submucosa, think duodenum. If you see Peyer’s patches, think ileum.
Herbivores and carnivores
Diet strongly affects gut structure and dentition. Dentition means the type, number and arrangement of teeth.
Herbivores and ruminants
Herbivores eat plant material, which is often high in cellulose. Cellulose is a structural polysaccharide in plant cell walls. Mammals do not produce cellulase, so many herbivores rely on mutualistic microorganisms to digest cellulose.
A ruminant is a herbivore with a specialised four-chambered stomach:
- Rumen — microbial fermentation of cellulose.
- Reticulum — helps trap particles; involved in regurgitation of cud.
- Omasum — absorbs water and mineral ions.
- Abomasum — true acidic stomach with enzymes.
Ruminants regurgitate partially digested food as cud and chew it again, increasing surface area for microbial digestion.
Herbivore dentition usually includes broad ridged molars for grinding, a diastema or gap for manipulating food, and reduced canines.
Carnivores
Carnivores eat animal tissue, which is usually high in protein and easier to digest than cellulose-rich plant material. They tend to have shorter guts than herbivores.
Carnivore dentition includes large canines for gripping and killing prey, sharp incisors for cutting, and carnassial teeth for shearing flesh.
Herbivores do not make cellulase
In ruminants, cellulose digestion is mainly carried out by mutualistic microorganisms in the rumen, not by enzymes produced by the mammal itself.
Using gut length ratios
Two mammals have similar body masses. Mammal A has a body length of 1.2 m and a gut length of 24 m. Mammal B has a body length of 1.0 m and a gut length of 5 m. Which is more likely to be a herbivore?
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Calculate the gut length to body length ratio for Mammal A:
24 m1.2 m=20\frac{24\ \text{m}}{1.2\ \text{m}} = 201.2 m24 m=20 -
Calculate the ratio for Mammal B:
5 m1.0 m=5\frac{5\ \text{m}}{1.0\ \text{m}} = 51.0 m5 m=5 -
Mammal A has the much larger ratio, so food remains in its gut for longer. This supports microbial fermentation of cellulose, so Mammal A is more likely to be a herbivore.
Parasites: highly specialised nutrition
Taenia tapeworm
Taenia is an endoparasitic tapeworm, meaning it lives inside its host, usually in the intestine. It is adapted for absorbing already-digested nutrients from the host.
Key adaptations include:
- A scolex, or head region, with hooks and suckers for attachment.
- No digestive system, because soluble nutrients are absorbed across the body surface.
- A resistant outer tegument, which protects against host enzymes.
- Many reproductive segments called proglottids, producing large numbers of eggs.
When observing specimens or slides of Taenia, look for the scolex, suckers or hooks, proglottids, and the absence of a gut.
Pediculus louse
Pediculus is an ectoparasite, meaning it lives on the outside of the host. It feeds on blood.
Its adaptations include piercing-sucking mouthparts, claws for gripping hairs or fibres, a flattened body, and eggs called nits that attach firmly to hair.
Parasite evidence
Do not classify an organism as parasitic just because it eats another organism. A parasite lives in or on a host for an extended time and gains nutrition at the host’s expense.
In the exam
- When classifying nutrition, identify the carbon source, whether enzymes are secreted externally, and whether there is a host relationship.
- For gut adaptations, link structure to function: surface area, enzyme conditions, retention time, attachment or absorption.
- In microscope questions, name visible features and use magnification carefully, converting units such as millimetres to micrometres.
- In comparison questions, avoid vague phrases like “better adapted”; state the specific feature and explain the nutritional advantage.
Check yourself
- Why does a ruminant need microorganisms in the rumen?
- How would you distinguish duodenum from ileum on a prepared slide?
- What features of Taenia show that it is adapted to parasitic nutrition?
