92020 · Microorganisms and their environment

• The main groups, and what actually separates them

  Bacteria are single cells with no nucleus - their DNA sits free in the cytoplasm as a loop, inside a cell wall. Fungi do have a nucleus: yeasts are single-celled, while moulds grow as branching threads called hyphae. Protists are single-celled organisms with a nucleus, some of which photosynthesise and some of which engulf or absorb other organisms. Viruses are in a category of their own - genetic material in a protein coat, with no cell structure at all. Size follows roughly the same order: a bacterium is about 1-5 µm across, a yeast cell about 5-10 µm, and a virus 20-300 nm, often ten times smaller across than a bacterium and far too small for a school light microscope.

• Viruses are not cells, and they replicate only inside a host

  Because a virus has no cytoplasm, no ribosomes and no respiratory enzymes, it cannot take in nutrients, release energy, grow or excrete. It reproduces by attaching to a host cell, getting its genetic material inside, and using the host's own machinery to build new virus particles, which are then released - often destroying the cell. That is why a virus needs a living host and why an antibiotic, which targets bacterial structures, does nothing to it. In an answer, describing precisely what a virus can and cannot do is far more useful than trying to settle the argument about whether it is alive.

• Nutrition: three genuinely different ways of getting food

  Some microorganisms make their own food: cyanobacteria and many protists photosynthesise, using light energy to build glucose. Others are saprotrophs, or decomposers - fungi and many bacteria release enzymes out onto dead material, digest it outside their body, and then absorb the small soluble molecules. This is called extracellular digestion, and it is why a mould spreads across the surface of an orange rather than swallowing it. A third group takes nutrients from other living things: some protists engulf smaller organisms whole, while parasites live on or in a host. These are different processes, not different words for the same thing.

• Respiration: aerobic, anaerobic and fermentation

  Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water, releasing a large amount of energy. Without oxygen, microorganisms respire anaerobically and release much less energy from the same glucose. In yeast this is alcoholic fermentation: glucose → ethanol + carbon dioxide, which is what raises bread dough and what makes beer and wine. In lactic acid bacteria - the ones used in yoghurt, cheese and silage - it is glucose → lactic acid. The energy released in every case is used for growth, movement and building new cell material; the products are simply what is left over.

• Growth and reproduction: binary fission, and why numbers explode

  In microbiology, growth usually means the population getting bigger rather than one cell getting larger. Bacteria reproduce asexually by binary fission: the cell copies its DNA loop, grows, and splits into two genetically identical daughter cells. In good conditions this can happen every 20-40 minutes, so numbers double each generation - 1, 2, 4, 8, 16. Ten doublings is about a thousand-fold increase and twenty doublings is about a million-fold, which is why food left in a warm kitchen becomes unsafe so quickly. Yeasts usually reproduce by budding, moulds by releasing spores, and viruses not at all on their own - they are replicated by a host cell.

• Excretion: waste that changes the surroundings

  Every life process leaves waste behind. Respiring microorganisms release carbon dioxide; fermenting yeast releases ethanol; lactic acid bacteria release lactic acid; decomposers release ammonia-containing compounds as they break protein down. None of this waste is neutral. It changes the pH, the oxygen level and sometimes the temperature of the surroundings, and as it builds up it eventually becomes toxic to the organism that produced it. That is the first half of the two-way relationship, and it is the reason a culture in a sealed flask always stops growing in the end.

• The five conditions that control growth

  Temperature: the rate rises as it warms, up to an optimum, then falls sharply because the enzymes doing the work are denatured. pH: each species has an optimum, with most bacteria preferring close to neutral and many fungi tolerating acid conditions. Moisture: microorganisms can only take in nutrients dissolved in water, so drying, salting or heavy sugaring makes water unavailable and stops growth. Oxygen: aerobes need it, obligate anaerobes are harmed by it, and facultative organisms use it when it is there. Nutrients: a source of carbon and energy, plus nitrogen and minerals. Whichever condition is furthest from the optimum is the limiting factor - warming a dried-out compost heap does nothing until you add water.

• The bacterial growth curve

  Grown in a closed container with a fixed amount of food, a bacterial population passes through four phases. In the lag phase the cells are busy - making enzymes and adjusting to the medium - but numbers barely change. In the log or exponential phase conditions are good and the population multiplies by the same factor in each equal time interval, which is a straight line on a logarithmic scale. In the stationary phase nutrients run short and waste builds up, so the rate of division falls until it matches the death rate and the total stops rising. In the death or decline phase deaths outnumber divisions and the count falls. The curve is really a record of the environment changing, not of the bacteria getting tired.

• Aseptic technique, and the reason behind each step

  Working aseptically means keeping everything except your intended microorganism out of the culture, and keeping the culture out of you. Sterilise the inoculating loop in a flame until it glows and let it cool before touching the agar. Work close to the Bunsen so rising warm air carries airborne spores away from the open dish. Lift the Petri dish lid only slightly, at an angle, rather than taking it off. Tape the lid in two places so it cannot fall open, but do not seal it right round - a fully sealed dish creates the low-oxygen conditions some harmful bacteria prefer. Incubate school plates at no more than about 30 °C, so organisms that grow best at human body temperature are not favoured, never reopen a plate after incubating, and sterilise everything before it is thrown out. Being able to give the reason for a step is worth more than reciting the list.

• The two-way relationship

  Microorganisms change their environment, and the changed environment then decides which microorganisms can live in it. In a wrapped silage bale, lactic acid bacteria ferment the sugars in the cut grass and the lactic acid they release drops the pH to around 4 - which preserves the feed by stopping the moulds and spoilage bacteria that would otherwise grow, and eventually slows the lactic acid bacteria themselves. In a compost heap, respiring microorganisms release enough heat to push the centre past 60 °C, and that heat replaces the community that started the process with heat-tolerant ones. Neither half of the loop makes sense on its own, and this standard asks you to describe both directions and the mechanism in each.

Internal · marked per part.

This is an internal achievement standard. Whetū does not offer a sit-down Exam paper for it. Learn and Practise stay available.

3 authored Learn units for this standard.

• Microorganisms and their life processes

  This page is about the microorganisms themselves: what they are, the life processes they carry out, and how to read a structure or a growth curve as evidence. Standard 92020 asks you to demonstrate understanding of the relationship between a microorganism and its environment, but that work only starts once you can name the organism's group and one process it is actually doing.

• Environmental factors and microbial growth

  A named life process only runs as fast as its surroundings allow. This page is about those surroundings: how to sort a condition as abiotic or biotic, how to name it with the standard's vocabulary, and how to read an optimum-style data set without inventing a universal number.

• The two-way relationship

  This page is the standard's assessed object: a two-way relationship. One direction is an abiotic or biotic factor affecting a named life process. The other direction is that same life process changing a factor of the interconnected environment. The two sentences are not a longer version of each other.

32 Practise questions in “Microorganisms and their life processes”. Feedback here is formative and is not an official NCEA grade.

• Groups of microorganism

  Tell bacteria, fungi, protists and viruses apart by structure.

• Life processes

  Describe how microorganisms feed, respire, grow, reproduce and excrete.

• Conditions for growth

  Link temperature, pH, moisture, oxygen and nutrients to growth rate.

• The growth curve

  Name and explain the four phases of a population in a closed container.

• The environment acting on microbes

  Explain how a change in conditions changes which microorganisms grow.

• Microorganisms at work

  Explain how people use microorganisms to make and preserve food.

• Investigating microorganisms

  Design and judge a fair test on microbial growth.

• Aseptic technique

  Give the reason behind each safe-working step, not just the step.

• Interpreting growth data

  Read a colony count, a curve or a pH trace and say what it shows.

• The two-way relationship

  Describe both directions: microbes change conditions, conditions change microbes.

Sample questions

1. Which structural feature separates bacteria from fungi and protists?
2. Match each group to the feature that defines it.
3. In a sentence or two, explain why a virus can only make copies of itself inside a living host cell.
4. Which products should you expect the yeast to release?
5. Put the stages of binary fission in a bacterium into order.
6. Explain how the mould obtains the nutrients it needs.
7. Why does the growth rate of a bacterial culture fall sharply once the temperature rises above its optimum?
8. Which explanation fits best?

Practise 92020 in Whetū