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CIE IGCSE: Characteristics and Classification of Living Organisms
Transcript of CIE IGCSE: Characteristics and Classification of Living Organisms
All living organisms are classified into 5 Kingdoms.
What are they?
Which is one is living and which one is non-living? Why do you think that?
M R S G R E N
There is a mnemonic to remember the 7 factors that all living organisms share which you learned in year 7.
M is for Movement
R is for respiration
Respiration is the process by which living organisms get energy from the breakdown of organic molecules (e.g. sugars, protein or fats).
Respiration is NOT the same as breathing.
S is for sensitivity
Sensitivity is the response of an organism to a change in its environment
G is for growth
R is for Reproduction
E is for Excretion
Excretion is the process of getting rid of waste materials produced by the body's metabolism
This includes carbon dioxide and urea in most animals and mainly carbon dioxide and oxygen in plants as well as waste chemicals stored in the leaves.
N stands for Nutrition
All living organisms need to take up nutrients. This can be through the roots as in plants or through feeding as in animals.
Are the following alive or not? Note down all the letters from MRS GREN that each has:
Movement can be a change in position or a change in place.
Organisms can either move their whole bodies or just parts.
Oxygen is often needed for respiration to occur leading to the following word equation
Glucose + oxygen carbon dioxide + water +
A change like this is a stimulus. Many organisms have sense organs to sense a stimulus.
Growth is the permanent increase in size and dry mass of an organism.
Growth might be the result of cell division, an increase in cell size or both.
Reproduction is when organisms make new individuals
This can be through asexual reproduction
or sexual reproduction where two individuals produce gametes (sperm and egg or pollen and ovum), which combine to produce offspring which is not identical to either parent
Plants produce their own nutrition through photosynthesis
Animals need to ingest their food.
Excretion is not the same as egestion. Egestion is the removal of undigested food from the gut through faeces. This material has never truly been inside the body so is not classified as a waste product.
In Biology, classification involves putting organisms into groups depending on their shared characteristics.
The largest grouping is a Kingdom.
Each kingdom is split into multiple phyla.
Each phylum can be split into multiple classes.
Each class can be split into multiple orders.
Each order can be split into multiple families
Each family can be split into multiple genera.
Each genus can be split into multiple species which are given a trivial name.
Biologists use the binomial system of classification to make sure that a particular species is known by the same name across the world.
A species name consists of the genus name (which is first and capitalised) and a trivial name (which is second and starts with a lower-case letter)
The species name for a human is:
Homo is the name of our genus.
sapiens is our trivial name. We are the only currently existing Homo species, but other species used to exist such as
Homo neanderthalensis or Homo habilis.
What are our Kingdom, Phylum and Class?
Chordates are a phylum that consists of some invertebrates (for example lancets and sea cucumbers) as well as all vertebrates (fish, amphibians, reptiles, birds and mammals) .
All have a spinal cord, though only the vertebrates have a spine to protect this cord.
Draw a table which compares the 5 classes of vertebrates on the following features:
Type of egg
Hair, scales etc.
How do they breathe?
Homeothermic or poikilothermic (can maintain its own body temperature or not)
Other identifying features
Plants are split into 4 main groups (phyla).
Mosses and liverworts
Flowering plants and ferns are those you need to know in more detail.
As all plants, flowering plants are multicellular.
Many of their cells contain chloroplasts. Chloroplasts contain chlorophyll which absorb light for photosynthesis.
Flowering plants have stems, roots and leaves.
They also have transport vessels running through these organs.
Xylem vessels take water and dissolved minerals from the roots to the other organs.
Phloem vessels transport dissolved food and other biological molecules from places of high concentration (e.g leaves) to areas of low concentration (e.g roots).
The shoot is the above ground part of the plant. it is made up of a stem, bearing leaves, buds and flowers.
The apical bud is where the plant grows new leaves and lengthens its stem.
The stem contains vascular tissues, which are the xylem and phloem. These allow for the transport of materials up and down the plant.
The root is below ground. They do not contain chlorophyll and anchor the plant into the soil. Roots absorb water and minerals.
There are two types of flowering plant: Dicotyledons and Monocotyledons.
Monocotyledons include the grasses and cereals.
Dicotyledons include roses and buttercups.
Scientists use dichotomous keys to identify living organisms quickly.
Dichotomous means dividing into two
Let's try an example.
Now try for yourself
Animal and Plant Cells
Animal and plant cells share certain components, but not all of them are the same.
As you can see cells are made up out of a number of different components. We will looked at the shared components first.
The cell membrane forms the barrier between the cell and the outside. It controls what can go in and out of the cell; oxygen and other small molecules can move through it freely, larger molecules cannot. It is partially permeable
The cytoplasm is the liquid that fills the cell, and in the case of animal cells, gives it its shape. Substances can move throughout the cytoplasm and many reactions take place here.
The cell nucleus is surrounded by a membrane. It separates the DNA (which is packaged in chromosomes) from the cytoplasm to protect it. It controls the cell's activities and the way it develops.
Mitochondria are organelles ("little organs") inside cells that carry out aerobic respiration. This is the breakdown of glucose in presence of oxygen releasing energy.
Ribosomes are responsible for turning messages from the nucleus into amino acid sequences for proteins.
Plants have some components that animal cells do not.
The cell wall gives shape to the plant cell and is made of cellulose. It stops the cell from bursting when it fills with water. Substances can move freely through the cell wall.
Chloroplasts are organelles surrounded by a membrane where photosynthesis takes place.
Starch is also stored here
The vacuole contains cell sap. This is a mixture of water, sugars and other substances. Plants cells use it for storage.
It also helps maintain the shape and turgidity (firmness) of the cell.
Multicellular organisms have cells that adapted for specific tasks.
These cells are known as specialised cells.
When specialised cells divide they make more of the same type of cell (so a muscle cell makes two muscle cells rather than say a muscle cell and a liver cell).
Cells that can turn into other cell types are known as stem cells.
An early embryo is made up out of embryonic stem cells which can turn into any type of tissue.
Bone marrow contains cells that can turn into the various blood cell types.
Look up different specialised cells (you should know about ciliated epithelial cells, nerve cells, red blood cells, sperm cells, egg cells and muscle cells in animals and root hair cells, palisade mesophyll cells and xylem vessels in plants).
Find an image and label the cell's characteristics and how it is different from the generic cell that we saw earlier.
A Multicellular Organism
Cone cells in the eye
Adipose (fat) cells
Microorganisms contain the prokaryotes, fungi and protoctists.
Viruses do not consist of cells and are often not considered living organisms and so do not belong to any kingdom.
1m = 10 m
1mm = 10 m
1um = 10 m
1nm = 10 m
Bacterial DNA is packaged in single circular loop of DNA. Many bacteria will also have little loops of DNA known as plasmids in their cell, which carry only a few genes.
Many bacteria and fungi are decomposers, breaking down waste and dead tissue.
Microorganisms that can cause disease are known as pathogens.
Bacteria reproduce asexually by splitting into two.
As they can do this very rapidly if enough food is available, a single bacteria can turn into billions of bacteria in a day. The initial bacteria will have formed a colony of its clones.
Bacteria digest food by secreting enzymes to digest the food outside of the cell and absorbing the digested materials through its cell membrane.
Some bacteria produce spores which can withstand very harsh conditions. Bacterial spores have been found from tens of thousands of years ago that could still grow into new colonies.
Fungal cells are eukaryotic and are a similar size to our cells.
Most fungi, except for yeasts, are multicellular.
Fungal cells have nucleus and a cell wall which is made of chitin, rather than cellulose as in plants.
The hyphae release enzymes which digest the food outside the fungus which will then absorb the broken down molecules into its cells.
Most fungi are saprotrophs, which feed on dead or decaying matter, but a few are parasitic and live on living organisms (for example the fungus that causes athlete's foot).
Viruses are often classed as non-living disease-causing agents.
Whilst they can replicate inside of a host cell they don't show many of the signs of life outside of a host.
Viruses consist of a protein capsid that surrounds its genetic material (which can be DNA or RNA). There is no cytoplasm and most viruses do not have a membrane surrounding them.
There are a number of reasons why they are often not classed as living organisms.
They can be crystallised
They can only reproduce inside a host cell
They have no metabolism of their own. They rely on the host.
You could argue that they are alive because:
They have genes that code for proteins
They respond to natural selection
They can reproduce and infect more cells.
Some scientists argue that viruses should therefore be classified as being "on the edge of life".
Viruses do not look like cells. They are much smaller with even the largest ones being ten times smaller than a bacterium and some of the smaller ones being only 20nm in size.
Magnification and size of specimens
Images such as the cells we saw earlier have been magnified.
If we know the magnification then we can work out the actual size of the specimen we are looking at.
Often you will have to convert units from millimetres to micrometres or rearrange the formula.
Organisation of multicellular organisms
Multicellular organisms consist of many different cell types.
A group of similar cells that carries out a particular task is known as a tissue.
So a number of muscle cells make up muscle tissue or a number of nerve cells make up nervous tissue.
Organs are made from a number of tissues working together to carry out a specific task.
So for example your heart is made up out of muscle tissue, nervous tissue, fibrous tissue and blood.
Different organs work together to form part of an organ system.
The heart forms part of the circulatory system, together with the blood vessels.
Other organ systems include:
The digestive system
The excretory system
The nervous system
The reproductive system
You need to be aware of the structure of one plant organ: the leaf.
How does each tissue in the leaf help it photosynthesise?
The 5 Kingdoms are: Plants, Animals, Fungi, Protoctists and Prokaryotes.
If you research these you might see another way of classifying all living organisms into 3 domains: Eubacteria, Archaea and Eukaryota
Living organisms all share 7 characteristics which identify them as living.
Plants and some bacteria produce their own food using photosynthesis.
All other organisms rely on these organisms for their organic molecules.
The energy released by respiration is used to power all metabolic processes in the organisms. These include making new cells, repairing cells, movement, making enzymes etc.)
The actions taken by the organisms as a result of the stimulus is known as the response.
You can identify what kingdom a particular organism belongs to by looking at their cells.
We will look at cells in more detail later in the course.
The kingdom of the prokaryotes contains the bacteria and the archaea.
They are very small, with the typical length of a prokaryote's cell being 100 times smaller than that of our cells (0.5-10 micrometres)
All prokaryotes are unicellular though they often live in groups called colonies or films.
Prokaryotic cells do not have a nucleus and their DNA is in a loose circle in their cytoplasm. Prokaryotes do not have parts of the cell that are surrounded by membranes such as mitochondria, chloroplasts or a nucleus.
The protoctists (or protozoa) are a very diverse group of organisms that share certain characteristics.
All protoctists are unicellular and are eukaryotic.
This means that they have organelles in their cells that are surrounded by a membrane, such as a nucleus or mitochondrion.
Fungi are also eukaryotic but can be unicellular (such as yeast) or multicellular (such as mould)
Their cells are surrounded by a cell wall which is made of chitin and they feed by secreting enzymes on to their surroundings, absorbing the available nutrients.
Fungi are almost all decomposers.
Plants are multicellular eukaryotes that produce their own food using photosynthesis.
This means that at least some of their cells contain chloroplasts.
Plant cells have cell walls made out of cellulose.
Animals are also multicellular eukaryotes, but cannot produce their own food.
Animals cells never have chloroplasts and do not have a cell wall
Classification of arthropods
Arthropods are a phylum of the animal kingdom.
It is the only invertebrate phylum you need to know about in detail.
The arthropod phylum contains a number of different classes.
These are the insects, arachnids, myriapods and crustaceans.
All arthropods have a tough exoskeleton, a segmented body and jointed legs.
The different arthropod classes can be identified by a number of different features.
Draw a table comparing the following features for the four classes of arthropods:
Number of legs
Number of body segments (and which segments are present)
Presence of wings
Type of eyes
Presence and number of antennea
Method of breathing
Ferns are non-flowering plants that have a stem-like root called a rhizome.
They have phloem and xylem vessels.
Their leaves are known as fronds and roll out from a bud to form a pinnate frond consisting of a fan-like spread of smaller leaves
Ferns reproduce by spores which mature on the underside of the frond.
The spores contain both male and female gametes and fertilise when there is enough moisture in the soil.
Up until now we have been looking at easily observable features to classify different organisms. These are usually morphological (dependent on the shape) or behavioural features.
Looking at the morphological features, which two animals do you think are most closely related?
It turns out that the golden mole is more closely related to the elephant than to the mole, even though it resembles the mole a lot more.
This is an example of convergent evolution where the same adaptations have appeared in many unrelated animals (there's a marsupial mole as well, again unrelated to the other two moles).
What morphological features have evolved in a convergent manner in these four species from 4 different classes?
Just looking at morphological and behavioural characteristics means you can mistakenly classify organisms.
A more precise method of looking at the evolutionary relationships between organisms (which is called cladistics) is to look at DNA and protein.
DNA is the information carrying molecule in the cell. Parts of the DNA in the cell that code for specific proteins are known as genes.
DNA consists (as we will see in more detail later) of 4 bases (A, T, G and C) arranged randomly into long sequences (the human genome has 3 billion bases).
Over time the sequence can change due to mutation. Two species that are very closely related will have fewer changes in the sequence of a particular gene than those that more distantly related.
When you line the sequences up you can spot the differences. Here you can see that there are far more differences between the human and the mouse than between the human and chimpanzee. Humans are therefore likely to be more closely related to chimps than mice.
The DNA between more distantly related species can be hard to compare, however almost all species share certain proteins. Proteins consists of a long chain of amino acids. There are 20 different types of amino acids and most proteins are 100s of amino acids long.
A gene codes for a protein because every three bases (a triplet codon) together code for a specific amino acid. If the codon changes due to a mutation the amino acid it codes for can change as well.
The number of differences in the sequence of either the DNA or a protein can tell you how long ago different species shared a common ancestor because we know the rate of mutation (the number of mutations that build up in a certain amount of time).
You can then create a phylogenetic tree.
The rough endoplasmic reticulum is used by the cell to produce proteins that are transported out of the cell. It is a membrane with many ribosomes embedded into it.