By the end of this lesson
- Name and define the eight shared characteristics of living organisms.
- Distinguish respiration from breathing and excretion from egestion.
- Use several kinds of evidence to judge whether something is living.
BIG QUESTION
How can something be alive when it appears to do nothing?
A dormant seed, a virus and a flame reveal why biologists use a pattern of evidence rather than one visible action.
Life is recognised from a pattern
Biologists do not decide that something is alive from one dramatic action. Fire spreads, consumes fuel and releases waste, while a dormant seed may appear to do almost nothing. The stronger judgement comes from a connected pattern of cellular processes: nutrition, respiration, excretion, response, movement, control of internal conditions, reproduction, and growth and development.
The familiar mnemonic MRS C GRENA memory aid for the characteristics of living organisms: movement, respiration, sensitivity, control of internal conditions, growth, reproduction, excretion, nutrition. is a memory aid, not a definition of life. The characteristics are linked. Nutrients provide materials; respiration releases energy; control systems keep conditions suitable; and growth or reproduction depends on all of the others.
Processes that supply matter and energy
nutritionObtaining the materials an organism needs for energy, growth and repair, by feeding (animals) or by photosynthesis plus mineral uptake (plants). is obtaining the materials needed for energy release, growth and repair. Animals obtain organic nutrients by feeding. Green plants manufacture glucose by photosynthesis, but still require water and mineral ions. Nutrition therefore does not simply mean “eating”.
respirationA series of enzyme-controlled chemical reactions in cells that release energy from nutrient molecules, such as glucose; it is not the same as breathing. is the set of chemical reactions in cells that releases energy from nutrient molecules for useful work. Breathing ventilates gas-exchange surfaces; it is not respiration. A plant, fungus or bacterium respires even though it has no lungs.
excretionRemoval from the body of waste products of metabolism, such as carbon dioxide and urea, and of substances in excess of requirements. is the removal of waste products made by metabolism and substances in excess of requirements. Carbon dioxide from aerobic respiration and urea formed from excess amino acids are examples. Removing undigested food from the gut is egestionRemoval of undigested food from the gut as faeces; it is not excretion because the material was never part of metabolism in cells., because that material was never absorbed into cells and is not metabolic waste.
Processes that keep an organism functioning
sensitivityThe ability to detect a stimulus (a change in the environment) and make a response. means detecting a stimulus and making a response. A shoot growing towards light, a bacterium moving along a chemical gradient and a person withdrawing a hand from a hot surface are different examples of the same broad characteristic.
movementAn action by an organism or part of an organism that causes a change of position or place. can mean movement of the whole organism or a change in position of part of it. Sessile organisms do not travel from place to place, but plant shoots, roots, leaves and cells can still change position or orientation.
control of internal conditionsKeeping conditions inside an organism, such as temperature, water content and glucose concentration, within limits that allow cells to function., or homeostasisKeeping the internal environment relatively constant, with factors such as body temperature and blood glucose concentration held within limits suitable for cell function., keeps factors such as temperature, water content and glucose concentration within limits compatible with cell function. The exact variables and mechanisms differ among organisms.
Processes that continue an individual or a lineage
growth and developmentA permanent increase in size or dry mass, by cell division and cell enlargement, together with cells becoming specialised. involve a permanent increase in size or dry mass, often through cell division, cell enlargement and differentiation. Temporary swelling after taking in water is not necessarily growth.
reproductionThe production of new organisms of the same species, by sexual or asexual reproduction. produces new organisms. It is a characteristic of living systems and populations, but not every individual must reproduce. A sterile worker bee or an infertile animal is still alive because it belongs to a living lineage and carries out the other coordinated life processes.
A dormant seed remains living even when most activity is greatly reduced. Its cells maintain a very low metabolic rate and can resume growth when water, oxygen and a suitable temperature are available. Observations must therefore be made over an appropriate timescale.
Viruses test the boundary
A virus contains genetic material inside a protein coat, sometimes with a lipid envelope, but it is acellularNot made of cells: viruses are acellular, consisting of genetic material inside a protein coat.. It has no cytoplasm or ribosomes, does not respire and cannot reproduce independently. New virus particles are assembled only after the virus enters a host cell and redirects the host machinery.
For this course, viruses are treated as non-cellular particles and pathogens rather than living organisms. The important exam argument is evidence-based: they have genetic information and evolve, but they do not independently carry out the coordinated cellular processes shared by living organisms.
The evidence pattern
Use this as a reasoning framework, not as eight isolated labels.
| Characteristic | Evidence to look for | Do not assume |
|---|---|---|
| Movement | A change in position of an organism or part of it. | Movement need not be locomotion. |
| Respiration | Cell reactions release energy from nutrients. | Respiration is not breathing. |
| Sensitivity | A stimulus is detected and produces a response. | The response may be slow. |
| Control | Internal conditions are regulated within limits. | This is homeostasis. |
| Growth and development | A permanent increase and organised change. | Water uptake alone may be temporary. |
| Reproduction | New organisms arise sexually or asexually. | Not every individual must reproduce. |
| Excretion | Metabolic waste or excess substances are removed. | Undigested food leaves by egestion. |
| Nutrition | Materials are obtained or manufactured for cells. | Plants manufacture glucose but still need resources. |
Language that earns precision
- metabolism
- All the chemical reactions occurring in a cell or organism.
- homeostasis
- The regulation of internal conditions within limits suitable for cell function.
- stimulus
- A detectable change in the internal or external environment.
- excretion
- Removal of metabolic waste and substances in excess of requirements.
- egestion
- Removal of undigested material that was not absorbed from the gut.
- acellular
- Not made of cells.
Common misconceptions
Claim: Anything that moves is alive.
Correction: Movement alone is weak evidence. Machines and flames move, while dormant or sessile organisms may show little visible movement.
Claim: Respiration means breathing.
Correction: Respiration is a chemical process in cells. Breathing moves air into and out of lungs.
Claim: Excretion means removing faeces.
Correction: Faeces contain mainly unabsorbed material and leave by egestion. Excretion removes products of metabolism or excess substances.
Claim: Every living individual must reproduce.
Correction: Reproduction characterises living lineages. Sterile individuals remain living organisms.
Match the answer to the command word
Give the requested characteristic or definition directly. Do not add an unrelated list.
Use evidence and a biological reason. Link the observed feature to the underlying cellular process.
Consider evidence on both sides, then reach a justified conclusion rather than relying on one characteristic.
Recall, apply, then justify
Recall, apply, then justify (continued)
State two characteristics shared by living organisms.
Hint
Choose any two distinct characteristics. A bare list is enough for “state”.
Suggested marking points
- Any two from: movement, respiration, sensitivity, control of internal conditions, growth and development, reproduction, excretion, nutrition.
Explain why removing faeces is not normally described as excretion.
Hint
Identify where the material came from and contrast it with metabolic waste.
Suggested marking points
- Faeces contain material that was not digested or absorbed.
- It was not produced by metabolism in cells, so its removal is egestion rather than excretion.
A dry seed shows no visible growth for several months. Explain why this observation does not prove that the seed is non-living.
Hint
Think about timescale, dormancy and what happens when conditions become suitable.
Suggested marking points
- The seed may be dormant with a greatly reduced metabolic rate.
- Its living cells can continue very low levels of respiration or maintenance.
- It can resume growth and development when water, oxygen and a suitable temperature are available.
A student says that a virus is living because it contains genetic material and makes copies of itself. Evaluate this conclusion.
Hint
Credit the evidence that supports the claim, then test whether the virus carries out life processes independently.
Suggested marking points
- Viruses contain genetic material and populations can evolve.
- They can produce copies only inside a host cell.
- They are acellular and lack their own cytoplasm and ribosomes.
- They do not independently respire or carry out the full coordinated pattern of life processes; therefore the conclusion is not supported for this course.
Design a fair set of observations that could distinguish a dormant seed from a similarly sized stone without cutting either one open.
Planning hint
Use suitable conditions, a comparison, repeated observations and more than one sign of life.
What a strong design includes
- Place matched samples under controlled conditions including water, oxygen and suitable temperature.
- Keep relevant variables such as temperature, water supply and observation time consistent.
- Measure evidence such as oxygen uptake, carbon dioxide production, mass change associated with germination, or emergence of a root.
- Include replicates and a suitable control experiment, then use the combined evidence rather than a single observation.