Seven characteristics of living organisms
Monacia Williams, Contributor

Welcome to another in our series of lessons. Over the last weeks, we have been looking at the background material that is required to enable us to better understand the seven characteristics of living things. Do you remember that I told you initially that the study of biology revolves around the seven characteristics of living organisms? Today, we will take the first step in our study of these characteristics. The characteristic that begins our journey is that living organisms feed. Feeding comes under the topic nutrition.

Two types of living organisms

First, let me remind you that the two types of living organisms that we will be studying are plants and animals. Would you like to take a guess as to which living organism we will commence our study with? Did you guess plants? If you did, you are correct! I can just hear someone asking why this is correct. Do you remember when we studied feeding relationships? Where did the food chain begin? With the plant. Remember why? Because plants are the only organisms that can manufacture their own food, they are the only organisms that can use the sun's energy to convert inorganic raw materials to organic end products that can be used to sustain life. Do you now agree that the study of nutrition should begin with plants?

This type of nutrition, the conversion of inorganic raw materials into organic end products, is said to be autotrophic. This is simply translated to mean self-feeding; auto, meaning self, and trophic, meaning feeding. The other type of nutrition where organisms eat food that is ready-made by autrotrophs is known as heterotrophic, where hetero means different or other.

Photosynthesis

The conversion takes place through a series of reactions in a process which is known as photosynthesis. I am sure that this is a word that you have heard before; perhaps you have been told that you can break down the word into two parts - photo, meaning light, and synthesis, meaning putting together or blending. The word summarises the process quite nicely, photosynthesis involves the putting together or blending of inorganic raw materials in the presence of light.

What are these inorganic materials? They are carbon dioxide, CO2, and water, H2O. What are the organic end products that are formed? They are glucose, C6H12O6, and oxygen, O2. We already know that light must be present, but is there anything else that must be present for the process to take place? Yes, there is! Photosynthesis takes place only in green plants, so there must be something found in green plants that is absent from non-green ones. That 'something' is the green pigment, chlorophyll. We can, therefore, conclude that for photosynthesis to take place, the following must be available:

• Two raw materials:
  • Carbon dioxide
  • Water
• Two conditions:
  • Sunlight
  • Chlorophyll

We can also add that two end products are formed

• Two end products
  • Glucose
  • Oxygen

The process can be summarised using the following equation:

Word equation

sunlight
Carbon dioxide + water
glucose + oxygen
chlorophyll

Symbol equation

sunlight
CO₂ + H2O
C₆H₁₂O₆ + O₂

chlorophyll

The process takes place in two stages. These are:

The light-dependent stage

In this stage, the chlorophyll in the chloroplasts traps the sunlight. The energy provided from this is used to slit the water molecule to produce oxygen, which is released, and hydrogen atoms, which are bonded to another molecule, to be used in the second stage of the reaction.

The light-independent stage

This is sometimes known as the dark reaction because it does not need sunlight to take place. In this stage, the hydrogen from the water, as well as the energy generated in the process, is used in the conversion of the carbon dioxide to glucose.

Next week, we will look at the leaf as we continue our study of photosynthesis. See you then!

These Cornwall College students engage Roger Bent, of the University of the West Indies, Mona, in a discussion about life at university, during the Western Jamaica campus' schools fair in Montego Bay, recently. - Tashieka Mair photo

Monacia Williams teaches at Glenmuir High School.