Overview of Living Organisms

Overview

This lecture provides an overview of animals, plants, fungi, protists, bacteria, and viruses, highlighting their key features and main differences.

Animals, plants, fungi, protists (also called protocists), and bacteria are the five kingdoms of life.
Viruses are not classified as living organisms and are not part of any kingdom.

Animals, plants, fungi, and protists are eukaryotes, meaning their cells contain a nucleus with DNA arranged in chromosomes.
Bacteria are prokaryotes, lacking a nucleus and with DNA loose in the cell.
Eukaryotic cells are 10–100 times bigger than prokaryotic cells.

Animals are multicellular (many-celled) and heterotrophs (obtain energy by consuming other organisms).
Most animals reproduce sexually.
Estimated 5–10 million animal species exist.

Plants are multicellular and autotrophs (make their own energy via photosynthesis).
There are about 300,000 species of plants.

Fungi can be multicellular (e.g., mushrooms) or unicellular (e.g., yeast).
Fungi are heterotrophs and mainly use saprotrophic nutrition (digest food outside their bodies).
The body of multicellular fungi is made of mycelium, which consists of thread-like hyphae.
Some fungi can act as pathogens (cause disease), e.g., athlete's foot.

Protists are mostly unicellular but are very diverse in form and function.
Some can photosynthesize (like plants), while others consume organisms (like animals).
Some protists are pathogens, e.g., Plasmodium causes malaria.

Bacteria are single-celled prokaryotes found almost everywhere.
Some bacteria can photosynthesize, but none have chloroplasts.
Most feed off living or dead organisms; some are pathogens (e.g., Salmonella), but many are beneficial (e.g., gut bacteria).

Viruses are non-living particles much smaller than bacteria or cells.
Structure: protein coat surrounding genetic material (DNA or RNA).
Viruses can only reproduce inside living cells (host organisms).
All viruses are parasites and pathogens, causing harm to their hosts (e.g., influenza, HIV, COVID-19).

Eukaryote — organism with cells containing a nucleus and chromosomes.
Prokaryote — organism with cells lacking a nucleus; DNA is free in the cell.
Multicellular — made of many cells.
Unicellular — made of a single cell.
Heterotroph — organism that gets energy by eating other organisms.
Autotroph — organism that produces its own food, usually via photosynthesis.
Saprotroph — organism that digests food outside its body and absorbs nutrients.
Pathogen — organism or agent that causes disease.
Parasite — organism that depends on a host for growth and reproduction, causing harm.