1aCharacteristics of living organisms

Every living organism — bacterium, fungus, protoctist, plant or animal — carries out all eight of these processes. If something only does some of them (a virus, a car engine, a flame), it is not classed as a living organism.

A useful memory hook is Mrs Gren, but learn the proper definitions because the exam wants the biology, not the mnemonic. The table below runs in Mrs Gren order.

Mrs Gren gives you seven. Edexcel examines eight. Control of internal conditions — homeostasis — has no letter in the mnemonic, so it is the one candidates forget. Recite Mrs Gren, then add homeostasis.

Points that gain the marks

• Excretion is not egestion. Excretion removes waste made by the organism's own reactions (urea, carbon dioxide). Egestion removes undigested food that was never absorbed — faeces. Faeces is not excretion.
• Respiration is not breathing. Respiration is a chemical reaction inside cells that releases energy. Breathing (ventilation) just moves air.
• Nutrition is not just eating. Plants require nutrition too — they make their own food and take up mineral ions such as nitrate from the soil.
• Movement does not have to mean walking. Plants move — growth movements of roots and shoots, opening of flowers — so movement still counts for a plant.
• Growth means a permanent increase in size and dry mass. A balloon inflating gets bigger but is not growing; a plant that has just taken up water has gained mass temporarily, not grown.

Examiner tip. Read every word of the question stem. If it asks for two characteristics shown by the data, do not write everything you know about Mrs Gren — examiners report that nervous candidates latch onto one key word and rush into an answer that never addresses what was asked.

These are the specific errors examiners keep reporting when candidates write about the characteristics of living organisms.

1. Respiration is not photosynthesis

Respiration does not need light. It does not absorb light, it does not happen only in the dark, and it does not stop in the light.

Common mistake: writing that "plants start to respire in the dark when photosynthesis stops". Plants respire continuously, day and night. In the dark you only notice it because photosynthesis is no longer masking it. Also never say respiration "absorbs light".

2. Excretion is not egestion

Only excretion is one of the characteristics in the list. "They excrete faeces" is the quickest way to throw this mark away.

3. "Nutrition" does not only mean eating

Every living organism requires nutrition, but there are two ways of getting it:

• Plants make their own food (glucose) using light energy.
• Animals take in ready-made food from their surroundings.

Common mistake: writing "all living things eat food". A plant does not eat — and a candidate who says every organism eats has just argued that plants are not alive.

4. "Controlling internal conditions" is not "responding to surroundings"

These are two separate characteristics on the list, and examiners report them being swapped:

• Control of internal conditions — keeping things inside the body steady: body temperature, water content, blood glucose concentration.
• Response to surroundings — reacting to a change outside the body (a stimulus): light, touch, heat, a chemical.

Shivering when you get cold is controlling internal conditions. Snatching your hand off a hot pan is responding to the surroundings.

If a question asks you to explain how an organism controls internal conditions, name the condition being held steady and say that it is kept near a constant level, whatever is happening outside.

5. Movement and growth are wider than you think

• Movement does not have to mean travelling from place to place. A plant moves parts of itself, slowly — so "plants don't move, therefore they aren't alive" is wrong.
• Growth is a permanent increase in size, from making new cells and new cell material. A wilted plant swelling up after watering has not grown — it has only taken in water.
• Development means becoming more specialised as the organism gets bigger. Growth and development are listed together for that reason, so don't treat "it got bigger" as the whole answer.

6. Answer the question that was set

If a question asks what happened to a population, describe the change shown by the data and nothing else. Examiners report marks lost when candidates write everything they know instead of the one thing asked for.

Examiner tip. The single most reported error in this area is confusing respiration with photosynthesis. Fix two sentences in your head: respiration happens in all living cells all of the time and never needs light; photosynthesis happens only in cells with chlorophyll and only in the light. Then check you can separate excretion from egestion, and controlling internal conditions from responding to the surroundings.

Examiners report that many lost marks have nothing to do with not knowing the biology — the candidate knew it and answered the wrong sort of question.

Command words

Common mistake: confusing describe and explain. If you have quoted the data but never used the word because, you have described, not explained. If the question says explain, quoting figures alone scores nothing.

Equally, if a question asks you to use data to support a conclusion, don't just list the differences in the table — link each difference to the requirement the question sets out.

Variables

Experiments always ask about variables. Get these straight:

Common mistake: when asked for the dependent variable, candidates give the independent variable, or reel off a list of control variables. Ask yourself: what did I have to measure and write in the results table? That is the dependent variable.

Also be careful that the variable you name really is controlled in that experiment — examiners report candidates naming variables such as pH that were nowhere near the investigation in front of them.

Two other technical terms

• Reliable — repeats give similar results, so you can trust them. Improve reliability by repeating and calculating a mean. It does not mean "accurate" or "a good experiment".
• Graphs — both axes must have a linear scale, even if the data were collected at uneven time intervals (e.g. 0, 5, 15, 40 minutes). Plot the times in their correct positions on an evenly divided axis; do not simply space the readings equally.

Reading the question

If a question asks for the dry mass of the seedlings kept in the dark on day 10, that is three conditions — the right measurement, the right group, the right day. Slow down and underline every condition in the stem before you write.

Examiner tip. Before writing, circle the command word and count the conditions in the question. "Describe" = what happens (data, trends). "Explain" = why it happens (because…). Answering with the wrong one is the most commonly reported reason for scoring zero on a question the candidate actually understood.