1.1Unit 1 · Living Organisms & Cells
Characteristics of Living Organisms — Unit 1 — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 4 parts Lesson 2 of 6 1 quiz
Lesson summary
List the characteristics shared by all living organisms.All living organisms show eight characteristics (MRS C GREN), including control of internal conditions.Eukaryotesplants, animals, fungi and protoctists.Prokaryotesbacteria, with circular chromosomal DNA and plasmids.Viruses …
- Unifying characteristics of living organisms
- Common features of eukaryotic organisms — plants
- Common features of prokaryotic organisms
- The seven life processes, individually
- Cells and nutrition across the groups
- Why a virus is not placed in a living group
Section 1 of 4Characteristics of Living Organisms — Unit 1
Unifying characteristics of living organisms:
all organisms need nutrition, respire to release energy, excrete waste, respond to their surroundings (sensitivity), move, control internal conditions, reproduce and grow and develop.Common features of eukaryotic organisms — plants:
multicellular, with chloroplasts for photosynthesis, cellulose cell walls, storing carbohydrate as starch or sucrose.Animals:
- multicellular, no chloroplasts, no cell walls, usually have nervous co-ordination and can move;
- store carbohydrate as glycogen.
- multicellular, no chloroplasts, no cell walls, usually have nervous co-ordination and can move;
Fungi:
- cannot photosynthesise;
- body made of hyphae forming a mycelium;
- cell walls of chitin;
- feed saprotrophically by secreting extracellular enzymes;
- store glycogen.
- Yeast is single-celled.
- cannot photosynthesise;
Protoctists:
- microscopic single-celled eukaryotes;
- some have chloroplasts (Chlorella), others are more like animals (Amoeba).
- microscopic single-celled eukaryotes;
Common features of prokaryotic organisms:
bacteria are single-celled with a cell wall, cell membrane, cytoplasm and plasmids but no nucleus; some have a flagellum.- Some photosynthesise but most feed on other organisms, living or dead. Lactobacillus bulgaricus is used to make yoghurt.
Pathogens:
- disease-causing organisms, including fungi, bacteria, protoctists and viruses.
- Viruses are particles with a protein coat and genetic material;
- they take over host cells to make more virus particles.
Organisms:
- the word covers every living thing, from single bacteria to multicellular plants and animals;
- classification places organisms into these groups using shared features.
- the word covers every living thing, from single bacteria to multicellular plants and animals;
The seven life processes, individually
- Movement an action that causes a change of position or place.
- Respiration the breakdown of nutrients in cells to release energy.
- Sensitivity detecting a stimulus and making an appropriate response.
- Growth a permanent increase in size and dry mass.
- Reproduction making more organisms of the same kind.
- Excretion removing toxic materials, waste products of metabolism and any excess substances.
- Nutrition taking in materials for energy, growth and development.
- An organism is described as living only when it shows these processes — not when it shows one of them.
Cells and nutrition across the groups
- Animals
- multicellular cells with a nucleus and no cell wall;
- feed on organic substances.
- multicellular cells with a nucleus and no cell wall;
- Plants
- multicellular, with chloroplasts and a cellulose wall;
- feed by photosynthesis.
- multicellular, with chloroplasts and a cellulose wall;
- Fungi
- usually multicellular, with a non-cellulose (chitin) wall;
- feed saprophytically or parasitically.
- usually multicellular, with a non-cellulose (chitin) wall;
- Protoctists
- mostly single-celled with a nucleus;
- some photosynthesise, such as Chlorella.
- mostly single-celled with a nucleus;
- Prokaryotes (bacteria)
- no nucleus and no mitochondria;
- single-celled.
- no nucleus and no mitochondria;
- Animals
Why a virus is not placed in a living group
- A virus is genetic material (RNA or DNA) inside a protein coat.
- It is not a cell and does not carry out respiration, nutrition, growth or excretion.
- It reproduces only by taking over a host cell, which is why it sits outside the living groups.
Control of internal conditions:
Living things keep their internal environment within set limits (homeostasis).- Humans hold body temperature near 37 °C by sweating or widening skin blood vessels, and also regulate water content (osmoregulation);
- plants lose heat as water evaporates from stomata.
- Humans hold body temperature near 37 °C by sweating or widening skin blood vessels, and also regulate water content (osmoregulation);
Excretory products:
Animals excrete CO2 and water from aerobic respiration and urea, a nitrogen-containing waste formed when spare amino acids are broken down.- Plants excrete CO2 from respiration and O2 from photosynthesis.
- Faeces are egested, not excreted, because they never took part in cell reactions.
Sensitivity in animals and plants:
Animals respond to stimuli using the nervous system (receptors, neurones, effectors carrying electrical impulses) and the endocrine system (hormones carried in the blood).- Plants respond by tropismsshoots grow towards light (phototropism) and roots grow downwards with gravity (geotropism).
Movement in plants:
Moving from place to place is called locomotion and is mostly seen in animals.- Plants cannot travel, but parts of them still move by changing direction or orientation, for example flowers or leaves turning to follow the Sun during the day.
Sexual and asexual reproduction:
Sexual reproduction needs two parents whose gametes fuse to form a zygote, so offspring carry a mix of both parents' DNA.- Asexual reproduction involves one parent and gives genetically identical offspring (clones), e.g. bacteria and Amoeba dividing, or plants producing runners.
Tobacco mosaic virus:
TMV stops chloroplasts forming in infected leaf cells, giving a mottled, discoloured leaf.- With less chlorophyll the plant photosynthesises less, so it grows poorly and crop yield falls.
- It spreads by contact between plants and is controlled by field hygiene and resistant varieties.
Viruses infect all groups:
- Every kind of organism, from bacteria to plants and animals, can be infected by viruses.
- Each virus contains only one type of nucleic acid (DNA or RNA) and has no cytoplasm, membrane or ribosomes of its own.
Section 2 of 4Process & sequence
- Check whether it is made of cells.
- If cells, check for a nucleus (eukaryote) or none (prokaryote).
- For eukaryotes, check for chloroplasts and cellulose walls (plant).
- Check for hyphae and chitin walls (fungus).
- Otherwise single-celled eukaryote (protoctist) or multicellular animal.
Classifying an organismwrite these steps in this order; each step is a marking point.
- Is it made of cells?
- Nucleus present or absent?
- Chloroplasts and cellulose wall?
- Hyphae and chitin wall?
- Assign to kingdom
Section 3 of 4Compare
| Group | Key features | Examples |
|---|---|---|
| Plants | Multicellular; chloroplasts; cellulose walls; store starch / sucrose | Maize, peas, beans |
| Animals | Multicellular; no chloroplasts or walls; nervous co-ordination; store glycogen | Humans, mosquitoes |
| Fungi | Hyphae forming mycelium; chitin walls; saprotrophic; store glycogen | Mucor, yeast (single-celled) |
| Protoctists | Mostly microscopic single cells; some plant-like, some animal-like | Chlorella, Amoeba, Plasmodium |
| Bacteria (prokaryotes) | Single-celled; no nucleus; cell wall; plasmids; some have flagella | Lactobacillus bulgaricus, Pneumococcus |
| Viruses | Not cells; protein coat; DNA or RNA; parasitic | Influenza, HIV, tobacco mosaic virus |
| Process | Full definition to write | Answer that loses the mark |
|---|---|---|
| Movement | An action causing a change of position or place | "Moving around" only — no change of place stated |
| Respiration | Breakdown of nutrients in cells to release energy | "Breathing" — that is gas exchange, not respiration |
| Sensitivity | Detecting a stimulus and making an appropriate response | "Responding" without detecting the stimulus |
| Growth | A permanent increase in size and dry mass | "Getting bigger" — temporary swelling is not growth |
| Reproduction | Making more organisms of the same kind | "Having babies" — asexual reproduction must be included too |
| Excretion | Removal of toxic waste products of metabolism and excess substances | "Passing out faeces" — egestion is not excretion |
| Nutrition | Taking in materials for energy, growth and development | "Eating" — no function stated |
Section 4 of 4Worked examples
Explain why viruses are not usually classed as living organisms.
- Viruses are not made of cells and cannot carry out life processes such as respiration, nutrition or growth on their own.
- They can only reproduce inside a living host cell.
- Deciding whether something is living
- A fungal thread lengthens as it grows through bread.
- Apply the definitions: growth is a permanent increase in size and dry mass, and nutrition is taking in materials — both are shown,
- so this is a living process.
- A pot of water freezes and the ice increases in size.
- Size increases, but dry mass does not and no nutrient is taken in or respired, so this is not growth.
- The answer is built by naming the process being tested, then quoting the defining words (permanent, dry mass, energy release).
- A fungal thread lengthens as it grows through bread.
- Two-mark define question
- "Define respiration." Award both marks for breakdown of nutrients + to release energy, and note that it happens in cells.
- Writing "breathing in oxygen" scores zero for this command word.
- Viruses are not made of cells and cannot carry out life processes such as respiration, nutrition or growth on their own.
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