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Unit 1 · Living Organisms & Cells

Unit 1 — Living Organisms and Cells — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions

5 min read 7 parts Lesson 1 of 6 Unit quiz

📚 Edexcel IGCSE Biology Modular - UNIT 1 Hub / Topical Revision
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Lesson summary

Living Organisms and Cells: Overview — EDEXCEL MODULAR - OL notes: Summarise the characteristics of living organisms, compare prokaryotic and eukaryotic cells and name the main groups of organisms.To be classed as living, an organism must show all the characteristics shared by living things.

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Prokaryote
  • Saying excretion is the removal of faeces - that is egestion

Section 1 of 7What makes something alive

To be classed as living, an organism must show all the characteristics shared by living things. They are remembered with the mnemonic MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.

CharacteristicMeaningExample
MovementChange of position or placeAnimals walk; plants move parts, e.g. sunflowers track the Sun
RespirationChemical reactions that release energy from glucose, with oxygen (aerobic) or without it (anaerobic)Glucose + oxygen to carbon dioxide + water + energy
SensitivityDetecting and responding to changes in the environmentRoots grow towards water
GrowthPermanent increase in size or number of cellsA seedling becomes a tree
ReproductionProducing offspringBacteria divide; mammals have young
ExcretionRemoval of waste products of metabolic reactionsCarbon dioxide breathed out
NutritionTaking in materials for energy, growth and repairPlants photosynthesise; animals eat

Excretion is often confused with egestion. Excretion removes wastes that originate from chemical reactions inside cells (for example carbon dioxide and urea). Egestion removes faeces, which are the undigested remains of food that were never absorbed into the body's cells. Plants regulate their temperature partly through transpiration.

Section 2 of 7Cells: eukaryotic and prokaryotic

All living organisms are made of cells. Eukaryotic cells (animals, plants, fungi, protoctists) have a distinct nucleus enclosed by a membrane, plus other membrane-bound organelles. Prokaryotic cells (bacteria) are always single-celled, are much smaller and have no nucleus: their genetic material is a single circular chromosome of DNA lying free in the cytoplasm, often with small rings of DNA called plasmids.

FeatureProkaryotic cellEukaryotic cell
NucleusAbsentPresent, with membrane
DNASingle circular chromosome plus plasmidsLinear chromosomes in the nucleus
SizeSmallLarger
OrganisationAlways single-celledSingle-celled or multicellular
ExamplesBacteriaAnimals, plants, fungi, protoctists

Section 3 of 7The main groups of organisms

Organisms are classified into groups based on shared features: animals, plants, fungi, protoctists, bacteria (prokaryotes) and viruses.

1
The four eukaryote groups differ from the prokaryotes.
2
Plants have cell walls of cellulose, store carbohydrate as starch or sucrose, and have no nervous coordination.
3
Fungi can be single-celled (yeast) or multicellular with thread-like hyphae forming a mycelium, store carbohydrate as glycogen,
  • and many feed on dead matter.
4
Animals are multicellular, have nervous coordination and store glycogen.
5
Protoctists are a diverse group including amoeba, Paramecium, Plasmodium and Chlorella.
6
Viruses are not made of cells.

A pathogen is an organism or virus that causes disease in another organism, the host. Pathogens include certain bacteria, fungi, protoctists and viruses. Not every member of these groups causes disease. Match each disease to its pathogen type: Pneumococcus is a bacterium that causes pneumonia, Plasmodium is a protoctist that causes malaria, and some fungi cause plant diseases.

Section 4 of 7Linking cells to the variety of life

The study of living organisms begins with the cell, because every characteristic of life is carried out by cells.

1
Unicellular organisms such as bacteria, yeast and amoeba perform all seven life processes within a single cell.
2
In multicellular organisms cells become specialised and are organised into tissues, organs and organ systems, for example the digestive system.
3
Nutrition may be autotrophic (plants make food by photosynthesis) or heterotrophic (animals, most fungi and many bacteria take in food),
  • and respiration can occur with or without oxygen: aerobic respiration releases much more energy per glucose molecule than anaerobic.
4
Always match a statement to the correct group by thinking about cell type, cell wall, carbohydrate store and nutrition.

Section 5 of 7Worked examples

  1. State the seven characteristics of living organisms. [4]4 marks

  2. Distinguish between excretion and egestion. [2]2 marks

  3. Describe two differences between a bacterial cell and an animal cell. [2]2 marks

  4. Name the pathogen type for pneumonia and for malaria. [2]2 marks

Section 6 of 7Key terms

TermMeaning
MRS GRENMnemonic for the seven characteristics of living organisms.
EukaryoteAn organism whose cells have a membrane-bound nucleus.
ProkaryoteA single-celled organism whose cells have no nucleus.
PathogenAn organism or virus that causes disease.
ExcretionRemoval of metabolic waste products from the body.
EgestionRemoval of undigested food as faeces.
PlasmidA small ring of DNA in bacteria.

Section 7 of 7Exam technique

1
Saying excretion is the removal of faeces - that is egestion.
2
Assuming all bacteria cause disease - most do not.
3
Thinking prokaryotes have no genetic material - they have a circular chromosome in the cytoplasm.
4
Saying viruses are living cells - they have no cellular structure.

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