1 The nature and variety of living organisms

The following sub-topics are covered in this section.
(a) Characteristics of living organisms

(b) Variety of living organisms

(a) Characteristics of living organisms

Students should:

1.1 understand how living organisms share the following characteristics:

• they require nutrition

• they respire

• they excrete their waste

• they respond to their surroundings

• they move

• they control their internal conditions

• they reproduce

• they grow and develop.

(MRS GREN)

(b) Variety of living organisms

Students should:

1.2 describe the common features shown by eukaryotic organisms: plants, animals, fungi and protists

Plants: these are multicellular organisms; their cells contain chloroplasts and are able to carry out photosynthesis; their cells have cellulose cell walls; they store carbohydrates as starch or sucrose. Examples include flowering plants, such as a cereal (for example, maize), and a herbaceous legume (for example, peas or beans).

Animals: these are multicellular organisms; their cells do not contain chloroplasts and are not able to carry out photosynthesis; they have no cell walls; they usually have nervous coordination and are able to move from one place to another; they often store carbohydrates as glycogen. Examples include mammals (for example, humans) and insects (for example, houseflies and mosquitoes).

Fungi: these are organisms that are not able to carry out photosynthesis; their body is usually organized into a mycelium made from thread-like structures called hyphae, which contain many nuclei; some examples are single-celled; their cells have walls made of chitin; they feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic products; this is known as saprotrophic nutrition; they may store carbohydrate as glycogen. Examples include Mucor, which has the typical fungal hyphal structure, and yeast, which is single-celled.

Protists: these are microscopic single-celled organisms. Some, like Amoeba, that live in pond water, have features like an animal cell, while others, like Chlorella, have chloroplasts and are more like plants. A pathogenic example is Plasmodium, responsible for causing malaria.

Note format:

• Plants: Multicellular, contain chloroplasts for photosynthesis, have cellulose cell walls, store carbohydrates as starch or sucrose. Examples: maize and peas.

• Animals: Multicellular, do not contain chloroplasts and cell walls, usually have nervous coordination, move, and store carbohydrates as glycogen. Examples: humans (mammals) and houseflies and mosqitoes (insects).

• Fungi: Does not carry out photosynthesis, body organized into mycelium with thread-like hyphae, contains many nuclei, have chitin cell walls, use saprotrophic nutrition (feeds by secreting digestive enzymes onto food to absorb the organic products). Examples: Mucor (typical hyphal structure) and yeast (unicellular).

• Protists: Microscopic, single-celled. Examples include Amoeba (animal-like) and Chlorella (plant-like), with pathogenic examples like Plasmodium (malaria).

Students should:

1.3 describe the common features shown by prokaryotic organisms such as bacteria

Bacteria: these are microscopic single-celled organisms; they have a cell wall, cell membrane, cytoplasm and plasmids; they lack a nucleus but contain a circular chromosome of DNA; some bacteria can carry out photosynthesis but most feed off other living or dead organisms. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yogurt from milk, and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.

Note format:

Main features of animals: Multicellular; nucleus with membrane; no cell wall or chloroplasts; feeds on organic substances; store carbohydrates as glycogen; have nervous coordination and can move.

Main features of plants: Multicellular; nucleus with membrane; have cell walls of cellulose; contain chloroplasts; feeds by photosynthesis; store carbohydrates as starch or sucrose; lack nervous coordination.

Common cells in plants and animals: Nucleus, Cytoplasm, Cell membrane, Mitochondria, Ribosomes.

Unique to plant cells: Cell wall, Permanent vacuole, Chloroplast.

Prokaryotic organisms (bacteria), single-celled, have cell wall, membrane, cytoplasm, plasmids; lack nucleus, contain circular DNA. Some can photosynthesize, most feed on living or dead organisms. Examples: Lactobacillus and Pneumococcus.

1.4 understand the term pathogen and know that pathogens may include fungi, bacteria, protists or viruses

Viruses: These are not living organisms. They are small particles, smaller than bacteria; they are parasitic and can reproduce only inside living cells; they infect every type of living organism. They have a wide variety of shapes and sizes; they have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA. Examples include the tobacco mosaic virus that causes discolouration of the leaves of tobacco plants by preventing the formation of chloroplasts, the influenza virus that causes ‘flu’ and the HIV virus that causes AIDS.

Note format:

Pathogens: Disease-causing microorganisms include fungi, bacteria, protists, and viruses. Bacteria (cause illnesses like tuberculosis), fungi (athlete's foot), and protists (cause malaria).

Viruses: Not living; parasitic, reproduce in living cells; infect all living organisms; consist of protein coat and nucleic acid (DNA or RNA). Examples: Tobacco mosaic virus, HIV, and influenza virus. Does not have a nucleus, any organelles or a cell membrane.

Tobacco Mosaic Virus (TMV): Infects plants; causes discolored leaves, reduces yield; spread by contact and soil; no treatment available.

HIV: Leads to AIDS; initially presents as flu-like; spreads through bodily fluids; no cure, but antiretroviral drugs can control progression.

Influenza virus: Three types infect humans; cause flu symptoms like high temperature and fatigue. Influenza A causes the most cases globally.

Virus structure

They are not classified into one of the 5 kingdoms because they are not technically a living organism (they do not carry out all 7 life processes).

They are parasites and can only reproduce once inside another living cell. 

The cell they infect is called the host. 

Viruses are not made of cells.

A virus particle is a very simple structure, with no nucleus or cytoplasm.

The genetic material (DNA/RNA)is surrounded by a protein coat.

The genetic material is either DNA or RNA and is made up of only a few genes - the instructions for making new viruses when inside a hosts cell.  

Some viruses have membranes around the outside, these have been ‘stolen’ from the host cell when the newly made viruses burst out.

Virus replication

In order to replicate, viruses enter a host cell and take control of its genetic machinery, causing the host cell to produce new virus particles. A virus injects its genetic material into the cell; this material may be single-stranded, double-stranded, or either DNA or RNA. They are now put together to form a complete virus.

Once the host cell has produced a large number of viruses, the host cell bursts as the breaks loose and infects more host cells.

This is how the influenza (flu), cold, measles, mumps, polio, and rubella viruses all infect our cells and make us ill. But our immune systems can eventually destroy the virus particles and so stop their reproduction.

Antiretroviral drugs: This drug prevents virus from replication, so as long as people have access to these medication early on in their disease.

Variety of Living Organisms:

Eukaryotic Organisms:

• Plants: Multicellular, have chloroplasts for photosynthesis, cellulose cell walls, store carbohydrates as starch or sucrose (e.g., maize, peas).

• Animals: Multicellular, lack chloroplasts and cell walls, possess nervous coordination, move, store carbohydrates as glycogen (e.g., humans, houseflies).

• Fungi: Non-photosynthetic, organized into mycelium with hyphae, chitin cell walls, use saprotrophic nutrition (e.g., Mucor, yeast).

• Protists: Microscopic, single-celled (e.g., Amoeba, Chlorella, Plasmodium).

Prokaryotic Organisms (Bacteria):

• Single-celled, have cell wall, membrane, cytoplasm, plasmids; lack nucleus; contain circular DNA.

• Some are photosynthetic; most feed on living or dead organisms (e.g., Lactobacillus, Pneumococcus) they are called decomposers.