The Digestive System

Overview Animals are multicellular organisms that rely on specialized cells and organs to carry out specific tasks. One of the systems that you will learn about is the digestive system. You will familiarize yourself with the organs of the digestive system. As well, you will understand the role that accessory organs play in digestion. There are four phases to digestion: ingestion, digestion, absorption, and excretion.

Ingestion You may recall that all living things need to consume essential nutrients in order to survive. Remember, autotrophs, like photosynthetic protists and plants, manufacture carbohydrates to satisfy their nutritional needs. Heterotrophs need to ingest organic materials from their surroundings. Organisms use various means to ingest their food. Unicellular organisms, like amoeba, take in food by the process of phagocytosis. More complex multicellular animals are divided into fluid feeders, filter feeders or chunk feeders.

The Process of Phagocytosis The cell membrane envelopes the food particle creating a phagosome

Fluid Feeders Fluid feeders pierce and suck fluid from the surface of a plant or animal. They may inject special enzymes that prevent blood from clotting while they feed. Fluid feeders include mosquitoes, leeches, and insects.

Filter Feeders Filter feeders filter water through specialized structures, like a beak or gills, to collect organisms. Their specialized structures work like the strainer you have in your kitchen. Water filters through and the good stuff stays behind in the strainer. Flamingos, whales, and sponges are examples of filter feeders.

Chunk Feeders Although the name is not appealing, chunk feeders, is a good description of how most animals eat. Chunk feeders consume large amounts of food at one time. Some animals can go a couple of days without consuming any food. Elephants, lions, and snakes are all examples of chunk feeders.

Digestion Now that you know the different ways organisms ingest their food, you will now look at what happens to the food. Digestion involves the breakdown of complex molecules into smaller components. This can be achieved through a mechanical process and/or a chemical process.

Mechanical breakdown involves a physical action that breaks, crushes, or smashes larger food particles into smaller particles. In higher level vertebrates, this is done by teeth. Incisors, canines, and molars all have a distinct structure and shape to break down food.

Chemical digestion involves the release of enzymes that act on specific nutrients. These enzymes are released from glands located in the mouth, stomach and small intestine. Salivary glands release saliva that contains an enzyme that breaks down starch into glucose. In addition, saliva moistens food so that it will slide easily down the esophagus.

Parts of the Human Digestive Tract

The Mouth The oral cavity is the location where mechanical and chemical breakdown of food begins. The teeth, tongue, and salivary glands aid in the breakdown. When food leaves the mouth, it is called a bolus. The Esophagus The esophagus is a flat thin wall tube that connects the mouth to the stomach. In adults, the average esophagus is about 25 cm long and 2 cm wide. The esophagus contains circular and longitudinal muscles that relax and contract rhythmically to move food downwards in a process called peristalsisperistalsis

In addition, mucus is secreted along the walls to help food slide down the tube. A valve called a sphincter connects the esophagus to the stomach. The esophageal sphincter prevents food moving back up into the esophagus after it has entered the stomach. No digestion or absorption occurs in the esophagus.

Homework Read through P208 – 212 to recap on the importance of digestion and the process of ingestion (food travelling from the mouth to the stomach)