Edexcel IGCSE Biology
Topic 1: The Nature and Variety of Living
Organisms
Notes
Characteristics of living organisms
Functions of all living organisms (1.1)
Movement: can change position
Reproduction: can have offspring either sexually or asexually
Sensitivity: can detect stimuli, such as light, and respond to them
Control: can control their internal environment (homeostasis)
Growth: can increase mass
Respiration: can produce energy either aerobically or anaerobically
Excretion: can remove toxic or waste produced produced by reactions in the body
Nutrition: can absorb nutrients in order to use them for growth and repair
These can be remembered under the acronym of MRS C GREN.
Variety of living organisms
Eukaryotes (1.2)
Eukaryotes are organisms that have a nucleus and organelles that are found within a plasma
membrane. The functions of these subcellular structures will be described in more detail in Topic
2.
Subcellular structures found in plant and animal cells:
Structure Function
Nucleus ● Contains the genetic material, which codes for a particular protein
● Enclosed in a nuclear membrane.
Cytoplasm ● Liquid substance in which chemical reactions occur.
● Contains enzymes (biological catalysts, i.e. proteins that speed up the rate of
reaction).
● Organelles are found in it
Cell membrane ● Contain receptor molecules to identify and selectively control what enters and
leaves the cell
Mitochondria ● Where aerobic respiration reactions occur, providing energy for the cell
Ribosomes ● Where protein synthesis occurs.
● Found on a structure called the rough endoplasmic reticulum.
Plants
● E.g. cereals (such as maize) or herbaceous legume (such as peas)
● Multicellular organisms
● Cells contain chloroplasts which is the site of photosynthesis: chlorophyll pigments within
the chloroplast structure absorb light from the Sun
● Cellulose cell walls which provide strength to the cell
● Contain a permanent vacuole, which stores cell sap and improves the cell’s rigidity
● Store carbohydrates as starch or sucrose
Animals
● E.g. mammals (such as humans) and insects (such as flies)
● Multicellular
● Cannot photosynthesise
● Do not have cell walls
● Have nervous systems in order to coordinate movement
● Store carbohydrates as glycogen
Fungi
● Body is usually organised into a mycelium of thread-like structures called hyphae which
have many nuclei but some are single-celled
● E.g. Mucor has typical hyphal structure, yeast is single-celled
● Cell walls are made of chitin
● Feed by extracellular secretion of digestive enzymes which break it down into smaller
pieces, which can then be absorbed (saprotrophic nutrition)
● May store carbohydrates as glycogen
Protoctists
● Single-celled organisms
● Some have features like animals cells, such as Amoeba, that live in pond water
● Others are more like plants and have chloroplasts, such as Chlorella
Prokaryotes (1.3)
Prokaryotes do not have a nucleus or membrane-bound organelles
Bacteria
● E.g. Lactobacillus bulgaris (rod-shaped bacterium used to make yoghurt),
Pneumococcus (spherical bacterium that causes pneumonia)
● Single-celled and very small
● Have a cell wall, cell membrane, cytoplasm and plasmids
● Lack a nucleus but have circular chromosomes of DNA
● Some can carry out photosynthesis but they mainly eat off of other organisms, either dead
or alive
Pathogens (1.4)
Pathogens are disease-causing organisms and can be fungi, bacteria, protoctists or viruses.
Viruses
● E.g. tobacco mosaic virus which prevents chloroplast formation, influenza virus, HIV virus
leading to AIDS
● Viruses are small particles (much smaller than bacteria) - not living organisms
● Parasitic
○ Can only reproduce within living cells
○ Can infect every type of living organisms
○ Hijacks the cell mechanisms to create millions of copies of itself and then
spreads within the host by cell bursting
● They come in a wide variety of shapes and sizes
● Do not have a cellular structure but have one type of nucleic acid (either DNA or RNA) and
a protein coat
Bacteria
● E.g. Salmonella (food poisoning)
● Can reproduce many times through binary fission
● Produce toxins that can damage cells
Protists
● E.g. malaria
● Parasitic - use animals as their hosts to live in
Fungi
● E.g. Athlete’s foot
● Produce spores that can spread in the wind or between people
● Can treat with fungicides
