Characteristics of living organisms
- Movement
- Respiration
- Sensitivity
- Growth
- Reproduction
- Excretion
- Nutrition
Building on the concept of 'life', these processes distinguish living things from non-living matter. For example, a car moves and consumes fuel (nutrition), but it does not grow by adding cells or excrete metabolic waste in the biological sense.
| Definition |
|---|
| An action by an organism or part of an organism causing a change of position or place. |
| The chemical reactions in cells that break down nutrient molecules and release energy for metabolism. |
| The ability to detect and respond to changes in the internal or external environment. |
| A permanent increase in size and dry mass. |
| The processes that make more of the same kind of organism. |
| The removal of the waste products of metabolism and substances in excess of requirements. |
| The taking in of materials for energy, growth and development. |
- Breathing is a physical process of moving air in and out of the lungs. It facilitates gas exchange but is not respiration itself.
- Respiration is a chemical process occurring inside cells. It involves breaking down glucose to release energy.
The equation for aerobic respiration is:
\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} + \text{Energy}
Examiners accept descriptions that mention the release of energy or the breakdown of nutrients. Simply saying 'taking in oxygen' is insufficient because it describes breathing, not the metabolic process of respiration.
- Excretion is the removal of waste products of metabolism. Examples include carbon dioxide (from respiration), urea (from protein breakdown), and excess water/salts.
- Egestion is the removal of undigested food (faeces) from the digestive tract. This material was never part of the organism's cells or metabolism; it simply passed through.
Therefore, faeces are not excreted. Only metabolic wastes like CO_2 and urea are excreted.
Biological growth is defined as a permanent increase in size and dry mass.
- Dry mass is the mass of an organism after all water has been removed. This is important because organisms can swell with water (temporary increase in size) without actually growing biologically.
- Growth involves the synthesis of new cells and materials, not just getting bigger due to fluid intake.
Students often mistakenly believe that plants do not move or excrete. Let's look at how plants demonstrate these characteristics:
- Movement: While plants are rooted, parts of them move. For example, roots grow towards water (hydrotropism) and shoots grow towards light (phototropism). The Venus flytrap closes its leaves rapidly to trap insects.
- Excretion: Plants produce carbon dioxide during respiration. They also produce oxygen during photosynthesis. Both are waste products of metabolism in different contexts and must be removed. Plants excrete excess water through transpiration and store some metabolic wastes in vacuoles or shed leaves (which contain waste products).
- Sensitivity: The Venus flytrap closing is a response to touch (sensitivity). Sunflowers turning towards the sun is a response to light.
| Characteristic Demonstrated |
|---|
| Sensitivity (response to light) and Growth (permanent increase in length). |
| Respiration (breaking down sugar to release energy and CO_2). |
| Nutrition (taking in materials) and Growth (increase in size/dry mass). |
The Correction: Remember that excretion is strictly about metabolic waste. Undigested food has never entered the cells to be metabolized; it remains in the gut lumen. Therefore, removing undigested food is egestion, not excretion. If a question asks for a characteristic of all living organisms, 'egestion' is incorrect because plants do not have a digestive tract to egest from, but all living things must excrete metabolic waste.
The Correction: All living organisms respire 24/7 to stay alive. Plants respire all the time. During the day, they photosynthesize (taking in CO_2, releasing O_2) AND respire (taking in O_2, releasing CO_2). At night, they only respire. When asked to identify respiration, look for keywords like 'release of energy', 'breakdown of glucose', or 'carbon dioxide production' as a metabolic result.
Why this is accepted: Examiners want to ensure you understand that excretion deals with internal chemical by-products (like urea or CO_2), not external matter like undigested food. Mentioning 'metabolism' links the process directly to cellular activity.
Correct Usage Example: 'Excretion is the removal of toxic waste products produced during metabolism, such as carbon dioxide and urea.'
Incorrect Usage: 'Excretion is getting rid of faeces.' (This is egestion).
Context: This is often tested with diagrams of yeast fermenting sugar or animals breathing. Note that while plants release CO_2 during respiration, they also take in CO_2 for photosynthesis. However, the process of breaking down nutrients to release energy is always respiration.
Why this is accepted: Respiration is defined by the chemical breakdown of nutrients to release energy. The production of CO_2 and water are the direct results of this metabolic reaction.
Correct Usage Example: 'The yeast is carrying out respiration because it is breaking down sugar to release energy, producing carbon dioxide as a waste product.'
B) Excretion
C) Respiration
D) Nutrition
B) Sensitivity
C) Growth
D) Reproduction
- Growth (permanent increase in size/dry mass)
- Nutrition (taking in materials/water/minerals)
(Other acceptable answers: Sensitivity, Respiration)