How Animals Get Their Food
Every living thing needs energy to survive — and that energy comes from food. But unlike plants, which can make their own food using sunlight, animals depend entirely on eating other organisms. This is the fundamental difference that separates autotrophs (self-feeders, like plants) from heterotrophs (other-feeders, like us).
Class 7 students often ask: why can’t animals just make food like plants? The short answer — we lack chlorophyll and the cellular machinery for photosynthesis. So evolution gave us something else: a digestive system that breaks down complex food into simple, absorbable nutrients.
Nutrition in animals isn’t just about eating. It’s a complete process — ingestion, digestion, absorption, assimilation, and egestion. We’ll go through each one carefully, with examples from animals you already know (including yourself).
Key Terms and Definitions
Nutrition — the process by which an organism takes in food and uses it for energy, growth, and repair.
Ingestion — taking food into the body (eating or drinking).
Digestion — breaking down complex food molecules into simpler ones that can be absorbed. This happens both mechanically (chewing) and chemically (enzymes).
Absorption — the process by which digested nutrients pass from the digestive tract into the bloodstream.
Assimilation — cells using absorbed nutrients for energy, growth, and repair.
Egestion — removal of undigested food from the body (not to be confused with excretion, which removes metabolic wastes from cells).
Enzyme — a biological catalyst that speeds up chemical reactions in digestion. For example, amylase breaks down starch into sugars.
Villi — tiny finger-like projections in the small intestine that massively increase the surface area for absorption.
Many students write “excretion” when they mean “egestion.” Excretion removes metabolic wastes produced inside cells (like urea from the kidneys). Egestion removes undigested food that never entered the cells. CBSE frequently tests this distinction.
The Five Steps of Animal Nutrition
Step 1 — Ingestion
Ingestion simply means taking food in. Different animals do this differently:
- Humans and most mammals use teeth and a mouth
- Amoeba surrounds food particles with its body (a process called phagocytosis)
- Spiders inject digestive juices into prey before ingesting — they drink pre-digested liquid
- Pythons swallow prey whole
The mode of ingestion depends on the animal’s body plan and the type of food it eats.
Step 2 — Digestion
This is where food gets broken down. Two types:
Mechanical digestion — physical breaking down. Teeth grinding food, the stomach churning — no chemical change, just smaller pieces.
Chemical digestion — enzymes break molecular bonds. Starch → sugars. Proteins → amino acids. Fats → fatty acids + glycerol.
In humans, digestion begins in the mouth (salivary amylase acts on starch), continues in the stomach (pepsin acts on proteins in acidic conditions), and is completed in the small intestine.
Step 3 — Absorption
Digested food is absorbed mainly in the small intestine. The inner wall is folded into thousands of tiny villi, each covered with even tinier microvilli (together called the “brush border”). This design gives us a surface area of about 250 square metres — roughly the size of a tennis court — packed inside an organ just 6–7 metres long.
Simple sugars and amino acids pass directly into the blood capillaries inside each villus. Fatty acids and glycerol enter the lymph vessels (lacteals).
Step 4 — Assimilation
Once nutrients reach the cells via blood, the cells use them:
- Glucose → energy (cellular respiration)
- Amino acids → building proteins for growth and repair
- Fats → stored energy, cell membrane formation
Step 5 — Egestion
Whatever wasn’t digested or absorbed reaches the large intestine. Water is reabsorbed here. The remaining solid waste (faeces) is stored in the rectum and expelled through the anus.
Remember the sequence with: I Did A Assimilation Easy — Ingestion, Digestion, Absorption, Assimilation, Egestion. Five steps, always in this order.
The Human Digestive System — Organ by Organ
| Organ | Main Function | Key Enzyme/Juice |
|---|---|---|
| Mouth | Mechanical + chemical digestion | Salivary amylase |
| Oesophagus | Transport (peristalsis) | None |
| Stomach | Protein digestion, churning | Pepsin (+ HCl) |
| Small intestine | Final digestion + absorption | Bile, pancreatic enzymes |
| Large intestine | Water absorption | None |
| Rectum | Storage of faeces | None |
The Mouth
The tongue mixes food with saliva (produced by salivary glands). Saliva contains salivary amylase, which starts breaking down starch. The food is shaped into a soft ball called a bolus and swallowed.
The Stomach
The stomach is a muscular bag. It produces gastric juice containing:
- Pepsin — digests proteins
- Hydrochloric acid (HCl) — creates acidic environment for pepsin, kills bacteria
The stomach churns food for 2–4 hours, turning it into a semi-liquid called chyme.
Students often say “stomach digests everything.” The stomach only digests proteins significantly. Carbohydrate digestion (starch) begins in the mouth but actually stops in the stomach because the acid deactivates salivary amylase. It resumes in the small intestine.
The Small Intestine
This is where most digestion is completed and almost all absorption happens. Three parts: duodenum (receives bile and pancreatic juice), jejunum, and ileum (main absorption site).
Bile from the liver (stored in the gall bladder) does not contain enzymes — it emulsifies fats, breaking large fat droplets into smaller ones, giving enzymes more surface area to work on.
Pancreatic juice contains enzymes for all three food types: amylase (carbs), trypsin (proteins), and lipase (fats).
The Large Intestine
No significant digestion here. The large intestine absorbs water and some minerals from undigested matter. The remaining waste becomes faeces.
Nutrition in Special Cases
Amoeba — Holozoic Nutrition
Amoeba is a single-celled organism. It feeds by phagocytosis:
- Amoeba detects food (e.g., a bacterium)
- It extends its body projections (pseudopodia) around the food
- The food gets trapped in a food vacuole
- Digestive enzymes enter the vacuole and digest the food
- Nutrients are absorbed into the cytoplasm
- Undigested waste is expelled
This entire process happens within a single cell — no separate organs needed.
Grasshopper — Herbivorous Insect
Grasshoppers have mandibles (jaw-like mouthparts) that cut and grind plant material. Their digestive system has a crop (temporary storage), a gizzard (grinding), and a midgut (digestion and absorption). Very efficient for processing tough plant matter.
Pitcher Plant — Insectivorous Plant
Wait — plants can eat animals? Yes. The pitcher plant traps insects in its leaf (shaped like a pitcher), drowns them in digestive fluid, and absorbs the nutrients. It does this because it grows in nitrogen-poor soil — insects provide the nitrogen it can’t get from soil.
CBSE Class 7 Science exam (Chapter 2) frequently asks about the pitcher plant as an example of an organism that is both autotrophic (makes food via photosynthesis) and heterotrophic (digests insects). It’s a classic two-mark question — don’t forget both aspects.
Solved Examples
Example 1 — CBSE Level (Easy)
Question: Name the enzyme present in saliva and state its function.
Solution:
The enzyme present in saliva is salivary amylase (also called ptyalin).
Its function: it breaks down starch into simpler sugars (maltose) during digestion in the mouth.
Why it matters for your exam: This is a direct one-mark question. Write the name correctly — “salivary amylase” or “ptyalin” — and state it acts on starch.
Example 2 — CBSE Level (Easy)
Question: What is the role of HCl in the stomach?
Solution:
Hydrochloric acid (HCl) in the stomach:
- Creates an acidic environment (pH ~2) needed for pepsin to work
- Kills harmful bacteria that enter with food
- Deactivates salivary amylase (which works in alkaline/neutral conditions)
One mark for each point — this is typically a 3-mark question in Class 7 exams.
Example 3 — CBSE Level (Medium)
Question: Why is the small intestine so long? What structural feature helps in absorption?
Solution:
The small intestine is long (6–7 metres in adults) to provide enough time and space for complete digestion and absorption.
The inner walls have villi — tiny finger-like projections that dramatically increase the surface area available for absorption. Each villus contains:
- Blood capillaries (absorb glucose and amino acids)
- Lacteals — lymph vessels (absorb fatty acids and glycerol)
The large surface area means nutrients can be absorbed quickly and efficiently before food passes further along.
Example 4 — CBSE Level (Medium-Hard)
Question: Differentiate between egestion and excretion with one example of each.
Solution:
| Egestion | Excretion | |
|---|---|---|
| Definition | Removal of undigested food from the body | Removal of metabolic wastes produced by cells |
| Material removed | Food that was never absorbed | Urea, CO₂, excess salts |
| Example | Defaecation (passing faeces) | Urination (passing urine) |
| Organ | Anus | Kidneys, lungs, skin |
Key distinction: egested material was never part of the body’s metabolism. Excreted material was produced inside cells as a result of chemical reactions.
Exam-Specific Tips
CBSE Class 7 — Board Pattern: Chapter 2 (Nutrition in Animals) typically carries 5–8 marks in the annual exam. Focus areas:
- The five steps of nutrition (1-2 marks, direct question)
- Organs and their functions (table format — 3-5 marks)
- Amoeba’s feeding (2 marks)
- Differences: egestion vs excretion, digestion vs absorption
- Pitcher plant (2 marks — it’s a favourite)
Draw and label the human digestive system — this is almost always a 3-mark diagram question.
For diagram questions: Practice drawing the digestive system with these labels: mouth, oesophagus, stomach, liver, gall bladder, pancreas, small intestine, large intestine, rectum, anus. The liver and pancreas above the small intestine often get misplaced by students.
For definition questions: Use the exact NCERT language. CBSE markers check for specific terms — “peristalsis”, “villi”, “bolus”, “chyme” — these single words often carry full marks.
Common Mistakes to Avoid
Mistake 1: Confusing egestion with excretion Faeces is egested, not excreted. Urine is excreted. This comes up every single year in CBSE exams. Nail the distinction now.
Mistake 2: Saying bile digests fats Bile does NOT contain enzymes. It emulsifies fats — breaks large fat globules into tiny droplets. The actual digestion is done by lipase from the pancreas. “Bile digests fats” will cost you marks.
Mistake 3: Forgetting the stomach deactivates salivary amylase Students learn that starch digestion starts in the mouth — correct. But they forget it stops in the stomach (acidic pH deactivates salivary amylase) and resumes in the small intestine. This is tested in fill-in-the-blank questions.
Mistake 4: Confusing absorption and assimilation Absorption = nutrients enter the bloodstream from the intestine. Assimilation = cells use those nutrients. Two separate steps — don’t merge them in answers.
Mistake 5: Drawing the liver on the wrong side The liver is the largest gland in the body and sits on the right side of the abdomen. In diagrams, many students draw it on the left. The gall bladder sits underneath the liver. Get this right before your exam.
Practice Questions
Q1. What is a bolus?
A bolus is the soft, moist ball of chewed food formed in the mouth after mixing with saliva. The tongue shapes the bolus and pushes it to the back of the throat for swallowing.
Q2. Name the organ where maximum absorption of digested food occurs. What structural adaptation helps this process?
Maximum absorption occurs in the small intestine, specifically the ileum.
The structural adaptation is villi — tiny finger-like projections on the inner wall that increase surface area enormously (to ~250 m²). Each villus has blood capillaries (for glucose and amino acids) and lacteals (for fatty acids and glycerol).
Q3. How does Amoeba obtain its nutrition? Name the process.
Amoeba feeds by phagocytosis (also called holozoic nutrition):
- It detects food (bacteria, algae)
- Extends pseudopodia around the food particle
- Traps it in a food vacuole
- Digestive enzymes enter the vacuole
- Nutrients are absorbed into the cytoplasm
- Undigested waste is expelled
This entire process occurs within a single cell.
Q4. Why does the pitcher plant feed on insects even though it is a plant?
The pitcher plant grows in nitrogen-poor, marshy soil where it cannot get adequate nitrogen from the ground. Insects provide the nitrogen-containing compounds (proteins) the plant needs.
The plant still performs photosynthesis for energy (so it is autotrophic for carbon) but is heterotrophic for nitrogen — getting it by digesting trapped insects with digestive juices in its pitcher-shaped leaf.
Q5. What is peristalsis? Where does it occur?
Peristalsis is the wave-like muscular contractions of the wall of the digestive tube that push food along the digestive tract.
It occurs throughout the oesophagus (pushing bolus from mouth to stomach), the stomach (churning food), the small intestine, and the large intestine. It is an involuntary process — you cannot control it consciously.
Q6. Gastric juice contains HCl. How does this help in digestion, and doesn’t it damage the stomach itself?
HCl in gastric juice:
- Creates the acidic pH (~2) needed for pepsin to activate and digest proteins
- Kills most bacteria that enter with food
The stomach protects itself with a thick layer of mucus secreted by the stomach lining. This mucus acts as a barrier between the acidic gastric juice and the stomach wall. When this protection breaks down (due to stress, certain medicines, or H. pylori infection), the result is a peptic ulcer — a painful erosion of the stomach lining.
Q7. What is the function of the large intestine? Is any digestion done there?
The large intestine does NOT perform significant digestion. Its main function is:
- Absorbing water from undigested food remains
- Absorbing some minerals (like sodium)
- Forming and storing faeces in the rectum until egestion
Some bacteria in the large intestine can break down certain plant fibres, and they also produce vitamin K — but this is minor compared to the small intestine’s role.
Q8. Distinguish between mechanical and chemical digestion with one example of each.
| Mechanical Digestion | Chemical Digestion | |
|---|---|---|
| Definition | Physical breakdown of food into smaller pieces — no change in chemical composition | Enzymatic breakdown — food molecules are broken into simpler molecules |
| Example | Chewing food with teeth in the mouth | Salivary amylase breaking starch into maltose |
| Result | Smaller pieces of the same food | New, simpler molecules (sugars, amino acids, fatty acids) |
Both types work together — mechanical digestion increases surface area, making chemical digestion more efficient.
FAQs
Why can’t animals make their own food like plants?
Animals lack chlorophyll and the cellular structures (chloroplasts) needed for photosynthesis. Photosynthesis converts light energy + CO₂ + water into glucose — a complex biochemical process that required millions of years of evolution to develop in plants and algae. Animals evolved along a different path — developing mobility and digestive systems to find and process food instead.
What happens if we don’t chew food properly?
Swallowing large food chunks means the stomach and intestine have less surface area to work on. Digestion becomes slower and less complete. Large undigested particles irritate the gut lining. Chewing also mixes food thoroughly with salivary amylase — skipping this step means carbohydrate digestion starts later. Short answer: chew your food.
Why does the stomach not digest itself?
The stomach lining secretes thick mucus that protects the stomach wall from the corrosive HCl and pepsin. The cells lining the stomach also replace themselves rapidly (every few days). When this protection breaks down — due to NSAIDs, alcohol, or Helicobacter pylori bacteria — the result is an ulcer.
Is the liver part of the digestive system?
Yes — the liver is an accessory digestive organ. It produces bile, which is essential for fat digestion. Bile is stored in the gall bladder and released into the duodenum (first part of small intestine) when fatty food arrives. The liver also processes absorbed nutrients — converting excess glucose to glycogen, detoxifying harmful substances, and producing blood proteins.
What is the difference between a herbivore, carnivore, and omnivore?
- Herbivores eat only plants (cow, deer, rabbit) — their digestive systems are adapted for tough cellulose
- Carnivores eat only animals (tiger, eagle, shark) — shorter intestines, strong stomach acid
- Omnivores eat both (humans, bears, pigs) — versatile digestive system
- Insectivores eat mainly insects (pitcher plant, sundew, frogs)
Why does digested food need to be absorbed? Why can’t we just eat sugar directly?
We can eat sugar directly — and it does get absorbed faster. But complex foods (starch, protein, fat) cannot cross the intestinal wall in their original form — the molecules are too large. Digestion breaks them into small units (glucose, amino acids, fatty acids) that can pass through the intestinal cells into the bloodstream. It’s essentially a size filter.
Can stress affect digestion?
Yes — significantly. Stress activates the “fight or flight” response, which slows or halts digestion (the body redirects blood flow to muscles and brain). This is why stress causes nausea, stomach pain, diarrhoea, or constipation. For CBSE Class 7, you don’t need this detail — but understanding the nervous system’s role in digestion becomes relevant in Class 10 Biology.
What is the difference between a digestive gland and a digestive organ?
A digestive organ is part of the main tube food passes through (stomach, intestine). A digestive gland produces secretions that help in digestion but food does not pass through them — the liver, pancreas, and salivary glands are digestive glands. They deliver their secretions into the digestive tube via ducts.