Characteristics of Living Organisms
All living organisms share the following characteristics:
- Nutrition: They absorb nutrients for growth and tissue repair.
- Respiration: They release energy through respiration.
- Excretion: They excrete wastes from metabolic activities or toxic materials.
- Sensitivity: They detect stimuli and respond to their surroundings.
- Movement: They change their position.
- Control: They control their internal conditions, such as temperature, blood glucose level, or water content.
- Reproduction: They create offspring either through sexual or asexual reproduction.
- Growth: They grow and develop by increasing cell size or cell number.
Now, we will take a look at how each group of living organisms differs from one another.
- Animals are eukaryotes: eukaryotes have membrane-bound organelles, such as the nucleus.
- Animals are multicellular organisms: multicellular organisms are made of many cells that work together.
- Since animal cells do not contain chloroplasts, animals cannot carry out photosynthesis. Since animals cannot make their own nutrients, they obtain the nutrients through eating other organisms.
- Animal cells have no cell walls.
- Animals usually have nervous system for coordination and response.
- Animals often store carbohydrate as glycogen.
- Examples: mammals (e.g. humans), insects (e.g. housefly and mosquito)
- Plants are eukaryotes.
- Plants are multicellular organisms.
- Plant cells contain chloroplasts, so plants can carry out photosynthesis and make their own food.
- Plant cells have cell walls made of cellulose.
- Plants store carbohydrate as starch or sucrose.
- Examples: flowering plants(e.g. maize), herbaceous legume (e.g. bean)
- Fungi are eukaryotes.
- Fungi can be either single-celled organisms or multicellular organisms.
- Some fungi are pathogens; a type of fungi injection causes athletes' foot.
- Fungi cannot carry out photosynthesis. Instead, they use sapotrophic nutrition: they secrete digestive enzymes outside of their body, directly onto the food. Then, they absorb the digested organic products.
- Fungal body is usually organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei.
- Fungal cells have cell walls made of chitin.
- Fungi store carbohydrate as glycogen.
- Examples: Mucor (typical hyphal structure), yeast (single-celled)
- Protoctists are eukaryotes.
- Protoctists are microscopic single-celled organisms.
- Some protoctists are pathogens; Plasmodium causes malaria.
- Some protoctists, such as Amoeba, have features like animal cells. Some protoctists, such as Chlorella, have features like plant cells and have chloroplasts.
- Bacteria are prokaryotes: prokaryotes do not have membrane-bound organelles, such as the nucleus.
- Bacteria are microscopic single-celled organisms.
- Some bacteria are pathogens; Pneumococcus causes pneumonia.
- Bacteria have circular chromosome that is not bound by the nucleus and floats freely in the cytoplasm.
- In addition to the chromosome, bacteria often contain plasmids, which are small circular DNA molecules. Plasmids contain genes that are beneficial for survival, such as antibiotic resistance.
- Bacteria have cell walls made of polysaccharide and amino acids.
- Some bacteria can carry out photosynthesis, but most bacteria feed on living or dead organisms.