IB Biology Topic B4.2.4: Holozoic Nutrition in Animals Notes

Holozoic Nutrition: Animals Take Food In and Digest It Inside the Body

1. Animals are heterotrophic, so they take in organic compounds made by other organisms.
2. In holozoic nutrition the food is ingested and then digested inside the body.
3. It runs through five stages: ingestion, digestion, absorption, assimilation and egestion.
4. Different animals ingest different foods, but they all use the same internal sequence.

The Five Stages: From Food to Body Materials

Ingestion: Taking Food Into the Body

1. Ingestion is the intake of food into the gut, ready for digestion.
2. Different animals take in different foods, from a cow grazing grass to a lion feeding on prey.

Digestion: Breaking Food Into Smaller Molecules

1. Mechanical digestion physically breaks food into smaller pieces.
   1. Examples include chewing in the mouth and churning in the stomach.
2. Chemical digestion uses enzymes to break large molecules into simpler ones.
   1. Amylase breaks carbohydrates into sugars such as maltose and glucose.
   2. Proteases such as pepsin break proteins into amino acids.
   3. Lipase breaks lipids into fatty acids and glycerol.

Absorption: Moving Nutrients Into the Body

1. Villi are finger-like projections that increase the surface area for absorption.
   1. Microvilli on each villus increase the surface area further.
2. Glucose and amino acids are absorbed into the blood, often by active transport.
3. Fatty acids and glycerol are absorbed into the lymphatic system before reaching the blood.

Assimilation: Using Nutrients for Growth and Repair

1. Amino acids are used to build proteins such as enzymes and muscle fibres.
2. Glucose is used in cell respiration to produce ATP.
3. Fatty acids are built into cell membranes or stored as fat.

Egestion: Removing Undigested Material

1. In the large intestine, water is reabsorbed from the remaining material.
2. The compacted waste is then egested through the rectum and anus.

Why Holozoic Nutrition Matters for Animals

1. It supplies energy, since glucose fuels cell respiration.
2. It supplies building blocks, since proteins and lipids build and repair tissues.

External Digestion: Where Holozoic Ends

1. Some animals digest food outside the body, so they are not holozoic.
2. A spider injects digestive enzymes into its prey, liquefies the tissues externally, then sucks up the digested liquid.