Unit 2: Nutrition                                                                  1   2   3 

2.1. Digestion

Digestion includes all the processes which break down foods to obtain from them the essential substances for life (nutrients) and the transformations that these nutrients suffer to change into simpler substances able to be absorbed by the organism.

a) Stages of the digestive process:

   1^st) Ingestion

          It is the intake of food into the body through the mouth.

          It is also called feeding.

   2^nd) Digestion

           It is the transformation of food into nutrients which the body

           can absorb.

           There are two types of digestion:

- Intracellular digestion.

   It takes place inside the cells.
           It appears in Sponges, the most primitive animals,

   and in unicellular organisms such as protozoa, too.

          - Extracellular digestion.

             It takes place outside the cells, within the digestive tube.

             It is characteristic of the rest of animals.

             The transformation of food is both mechanical and chemical.

- Mechanical digestion includes chewing and crushing

   which break down food in smaller pieces.

- Chemical digestion involves chemical transformation

   of complex nutrients into simpler ones through the action

   of the enzymes present in the digestive juices.

          Most part of animals carry out digestion into their body  

          (internal digestion), but some of them, such as spiders carry out it
          outside their body ingesting the nutrients already digested.

          This kind of digestion is called external digestion.

  3^rd) Absorption

          It is the passage of simple nutrients from the digestive tube into the

          blood.

  4^th) Egestion

         It is the elimination of undigested and waste products from the

         digestive tube. Most part of animals transform these products into

          faeces.This type of egestion is called defecation.

b) Types of digestive systems:

   - Animals without digestive system:

There is only an Animal phylum that does not have digestive system, Poriferans.

Their bodies are bag-shaped and they are hollow. The inner space is called atrium. The body wall is full of pores. They have a bigger opening called osculum.

        They feed by filtration. Water enters by the pores and exits

        though the osculum.

Nutritive particles of water are caught by the cells that covered the atrium (choanocytes) and digested (intracellular digestion). These cells have a flagellum that is responsible for creating the water flow through the sponge.

     - Animals with digestive system:

   The digestive system can be:

         - Gastrovascular cavity.

  It is a body cavity lined by specialised cells with only one

  opening to the exterior that works at the same  time as mouth

  and anus. This opening is surrounded by tentacles with

  stinging cells (cnidocytes) which  functions are to capture prey

  and to defend from predators.

            The specialised cells which lined the gastrovascular cavity produce

  enzymes that are released into the inside and act over the food,

  digesting it.

  These cells also absorb the nutrients and expel the waste substances.

            It is characteristics of animals with extracellular digestion,

            such as Cnidarians (anemones, jellyfish and corals) and

            Platyhelminthes (planarian, taenia, etc)

         - Digestive tube.

  It is a tube that runs through the body. It

  has an opening through which food enters the body (mouth)

  and an opening through which feaces are expelled to the exterior

  (anus). 

  The tube is divided into different parts with different functions

  (oesophagus, pharynx, stomach, small and large intestine).

  Along the tube there are accessory glands (salivary glands,

  pancreas and liver) that produce digestive juices which contain

  enzymes.

            This system is characteristic of the rest of animals that have

  extracellular digestion.

            The different types of feeding have provoked the development

            of adaptations:

                  - to catch food, such as tentacles, tongues, lips, etc.

                  - to ingest it such as teeth, beaks, etc.

                  - to digest it, such as the specialized stomach of ruminants

                    that have four chambers.

2.3. Why is necessary that animals digest the food they take in?

2.4. What is the difference between:

a. Excretion - Egestion

        b. Mechanical digestion – Chemical digestion

2.5. Pictures represent an anemone digesting a prey.

a. What type of digestion is it?

b. Explain the process.

c. How is called this kind of digestive system? Why?