Characteristics of Living Organisms

All living organisms share the following characteristics: 

1. Nutrition: They absorb nutrients for growth and tissue repair.
2. Respiration: They release energy through respiration.
3. Excretion: They excrete wastes from metabolic activities or toxic materials.
4. Sensitivity: They detect stimuli and respond to their surroundings.
5. Movement: They change their position.
6. Control: They control their internal conditions, such as temperature, blood glucose level, or water content.
7. Reproduction: They create offspring either through sexual or asexual reproduction. 
8. 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.

Animal

• 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)

Plant

• 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)

Fungus

• 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)

Protoctist

• 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

• 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.

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