Holozoic nutrition
Monacia Williams, Contributor

I hope you had a good week and have already settled down to enjoy another good one. Remember to continue to do your studying in stages and not to leave everything for the last minute. If you continue to take it in stages, you will be sure to get top marks.

Today, we will continue our study of nutrition by looking at nutrition in humans. This is known as holozoic nutrition. We begin by taking an introductory look at the process. As we go through it, we will meet some words that are important to the process and, hence, must be learnt.

What happens to the food that we eat and where does it happen?

The alimentary canal is a long, muscular tube running from the mouth to the anus. The walls of this tube contain muscles, which are able to contract and expand in order to facilitate the passage of food. This type of movement is known as peristalsis. Along the tube, at specific places, there are special muscles which serve to block off the tube completely. These muscles are known as sphincter muscles. The alimentary canal also has special cells, which secrete mucus (goblet cells). Mucus helps the food to slide along easily.

The process by which food is taken into the body is known as ingestion. The food taken in usually contains large molecules. These large molecules cannot enter or leave the cells without being broken down. The breaking down process is known as digestion. In order for the food to be of use to the body, the molecules have to move from the alimentary canal to the blood stream. This process is known as absorption. The blood stream then transports the digested food to the tissues where assimilation takes place. Food which was not digested is egested.

Two types of digestion take place in the alimentary canal:

• Mechanical
• Chemical

The teeth, as we have seen in a previous lesson, do mechanical digestion. This mechanical digestion continues with the churning actions of the alimentary canal. Chemical digestion involves a chemical change of the molecule from a complex to a simple form. This chemical change is brought about using enzymes. Simple molecules such as water, minerals and vitamins are small molecules and do not need to be digested. We will need to take a closer look at enzymes in order to better appreciate what goes on in the digestion process.

Holozoic nutrition, therefore, includes the following processes:

•ingestion

•digestion

•absorption

•assimilation

•egestion

Let us now define these highlighted words with reference to this type of nutrition.

• Ingestion - the taking in of food through the mouth
• Digestion - the breaking up of large pieces of food by the teeth and the chemical conversion of large, complex molecules into simple ones.
• Absorption - the movement of these simple molecules into the blood stream
• Assimilation - the use of the absorbed molecules by the body cells.
• Egestion - the undigested food particles are removed from the body. This process must not be confused with excretion, which is the removal of metabolic wastes from the body.

We will be looking in more detail at these processes, but for today we will take a look at enzymes.

ENZYMES

Enzymes are organic catalysts. A catalyst is a substance that speeds up the rate of a chemical reaction while remaining unchanged after the reaction is complete. Enzymes are made by living organisms so they are organic. The following are the characteristics of enzymes:

• They are proteins and because they are proteins certain factors affect their structure. These include:

- Extremes of temperature. If the temperature is too low, the enzyme cannot work and is said to be deactivated. If the temperature is too high, the enzyme's structure is altered and it can no longer work, it is denatured. Denaturation is an irreversible change, but deactivation is not. If the temperature is increased, deactivated enzymes will begin to work. The temperature at which the enzyme works best is the optimum temperature.

- Enzymes are affected by changing pH; extremes of pH also destroy enzyme structure. Each enzyme has a particular pH at which it works best; this is known as the optimum pH.

• Enzymes are required in small amounts.
• They can be used over and over again since they are not destroyed by the reaction that they are catalysing.
• Enzymes are specific; they only break down one type of compound - this compound is called the substrate.

Your teacher should be doing experiments with you to show how enzymes work. A popular enzyme that is used is catalase. Catalase is found in living organisms, green leaves, Irish potato and fresh liver. The substrate that the enzyme acts on is hydrogen peroxide; it breaks it down into water and oxygen gas. The evolution of oxygen makes it very easy to see whether the reaction has taken place.

Hydrogen peroxide is something that most persons have in their medicine cabinets. It is used on cuts, for gargling, sore mouths and throats and for bleaching hair. This means that you can try these simple experiments at home. Grind a small piece of liver, any liver, chicken, goat or cow, and pour some hydrogen peroxide in a pill bottle (the bottle in which you get tablets from the pharmacy), add some of the ground liver and watch what happens. All that froth that you will see is the oxygen gas that is produced as a result of the action of catalase on hydrogen peroxide. You could also use your paper punch to make some holes in a piece of filter paper, collect the circles that are made and soak them in crushed leaves or raw, crushed Irish potato and put some hydrogen peroxide in the pill bottle. Drop the circles into the solution and watch the hydrogen bubbles carry them to the surface. Who said biology was not fascinating? See you next week!

Members of the victorious Cornwall College under-16 basketball team celebrate the ISSA/KFC trophy after completing a 2-0 series win in their best-of-three finals against Spot Valley High School at the Montego Bay Cricket Club courts. - photo by Adrian Frater

Monacia Williams teaches at Glenmuir High School.