(1a) What defines life? What are the characteristics of living organisms?

Why are some 'things' referred to as alive! and how chemically complex are cells?

Before looking at cellular structures, how do we define what we mean by a 'living organism', what are their characteristics?

Listed below are typical characteristics of simple (e.g. unicellular) or very complex organisms (e.g. human beings).

BUT, note that not every organism exhibits every characteristic listed here, but usually 7/8 !!!

Reproduction is the processes that makes more of exactly the same kind of organism, otherwise the species becomes extinct..

It can be simple cell division like binary fission, or much more complicated for multicellular organisms with gestation periods or complicated life cycles.

Movement is a physical action by an organism or part of an organism causing a change of position or place.

e.g. seeking and capturing food, hunting or escaping a predator, but many organisms do not move from one place to another e.g. lichen on walls.

Respiration are the chemical reactions in cells that break down nutrient molecules and release energy for metabolism, usually in the cell's mitochondria.

Metabolism is all the chemical processes of organisms that involve breaking down large molecules to small molecules or making larger molecules from small ones (as well as the respiration reactions), and lots of other reactions too..

Sensitivity - able to respond to surroundings - the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses for survival.

e.g. respond to changes in the environment like light, temperature, movement and substances in the environment.

Growth and development is the permanent increase in size and dry mass by an increase in cell number or cell size or both.

Excretion is removal from organisms of the waste products of metabolism from chemical reactions in cells including carbon dioxide from respiration, toxic materials, and substances in excess of requirements e.g. ions or nitrogen compounds like amino acids.

Nutrition as taking in of substances for energy, growth and development; plants require light, carbon dioxide, water and ions; animals need organic compounds and ions and usually need water.

Control of internal conditions - the organisms must be able to maintain stable conditions in the cells - to keep the unicellular or multicellular organism stable i.e. keep alive to reproduce!

e.g. keeping the pH of the cell or body fluids constant at the appropriate value,

or for warm blooded animals, maintaining the correct temperature (thermoregulation)

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Summary of 'life' characteristics

What defines life as we understand it, as living organisms ourselves?

What are the characteristic features and functions of 'thing' that we refer to them as living organisms that exhibit these characteristics?

Be able to describe living organisms in terms of:

their means of reproduction, able to move, respiration for energy production to power cell chemistry,

be sensitive and detect changes in environmental surroundings and adapt to changing conditions,

be able to grow and develop (e.g. to maturity),

be able to excrete residual waste products not needed by the organism, have a means of feeding to provide nutrition and control of internal conditions of the cell or cells to maintain the biological stability of the organism.

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(1b) Summary of types of organisms and their principal characteristics

Be able to describe the common features shown by eukaryotic organisms such as plants, animals, fungi and protoctists

Plants are multicellular organisms; some of their cells contain chloroplasts which carry out photosynthesis; plant cells have cellulose cell walls; plants can store carbohydrates as starch or sucrose. Common examples include flowering plants including cereals like maize and a herbaceous legumes like peas or beans.

Animals are multicellular organisms, but their cells do not contain chloroplasts and so not able to carry out photosynthesis; animal cells have no cell walls; animal organisms usually have s nerve system for co-ordination and are usually able to move from place to place; they often store carbohydrate as glycogen. Examples include mammals like cattle, humans and insects like houseflies and mosquitos.

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

Protoctists are a varied collection of microscopic single-celled organisms. Some protoclist cells, like amoeba, that live in pond water and have features like an animal cell, but others, like chlorella, have chloroplasts and are more like plants because they can perform photosynthesis. Plasmodium is a pathogen responsible for causing malaria.

Note: Protoctista (protists) is a term used modern biological classifications for any eukaryotic unicellular or simple multicellular organism belonging to the kingdom Protoctista, which includes protozoans, algae, and slime moulds

Be able to describe the common features shown by prokaryotic organisms such as bacteria

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

Understand the term pathogen and know that pathogens may include fungi, bacteria, protoctists or viruses.

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Key points - Summary of ideas

These notes cover the fundamental characteristics that define a living organism and the different types of living organisms.

Characteristics of Living Organisms

All living organisms share certain characteristics

1. Movement – Organisms move in response to stimuli or changes in their environment. For example:

   • Animals use muscles to move.

   • Plants grow towards light sources (phototropism).

2. Respiration – The process of releasing energy from food molecules. There are two types:

   • Aerobic respiration (uses oxygen):
     Glucose + Oxygen → Carbon Dioxide + Water + Energy (ATP)

   • Anaerobic respiration (without oxygen):
     Glucose → Lactic Acid + Energy (ATP) (in animals)
     Glucose → Ethanol + Carbon Dioxide + Energy (ATP) (in microorganisms)

3. Sensitivity – Organisms detect and respond to changes in their surroundings.

   • Humans react to temperature changes by sweating or shivering.

   • Plants respond to light, water, and gravity.

4. Growth – Organisms increase in size and complexity. Growth occurs through:

   • Cell division (mitosis)

   • Increase in number or size of cells

5. Reproduction – Producing offspring to ensure species survival.

   • Asexual reproduction (single parent, identical offspring)

   • Sexual reproduction (two parents, genetically varied offspring)

6. Excretion – The removal of waste products produced during metabolism.

   • Humans excrete CO₂ (via lungs) and urea (via kidneys).

   • Plants remove oxygen (from photosynthesis) and CO₂ (from respiration).

7. Nutrition – Organisms take in substances needed for energy, growth, and repair.

   • Autotrophic (plants make their own food via photosynthesis)

   • Heterotrophic (animals obtain food by consuming other organisms)

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Types of Living Organisms

Living organisms are classified into different groups based on their characteristics.

Animals

• Multicellular organisms.

• No cell walls or chloroplasts.

• Heterotrophic (feed on other organisms).

• Store carbohydrates as glycogen.

• Examples: Humans, fish, insects.

Plants

• Multicellular organisms.

• Cell walls made of cellulose.

• Contain chloroplasts for photosynthesis.

• Store carbohydrates as starch.

• Examples: Trees, grasses, flowering plants.

Fungi

• Can be unicellular (yeast) or multicellular (mushrooms).

• Cell walls made of chitin.

• Do not photosynthesize.

• Saprotrophic nutrition (release digestive enzymes to break down food externally).

• Store carbohydrates as glycogen.

• Examples: Yeast, mold, mushrooms.

Protoctists

• Mostly unicellular organisms.

• Some have characteristics similar to plants (e.g., Chlorella, photosynthetic).

• Some have characteristics similar to animals (e.g., Amoeba, moves like an animal).

• Examples: Plasmodium (causes malaria), Chlorella.

Bacteria

• Unicellular organisms.

• No nucleus (DNA found in a circular chromosome).

• May have flagella for movement.

• Some bacteria photosynthesize, while others feed off other organisms.

• Examples: Lactobacillus (used in yogurt production), Pneumococcus (causes pneumonia).

Viruses (Not considered living organisms)

• Viruses do not exhibit the characteristics of a living organism on their own.

• Smaller than bacteria.

• Genetic material (DNA or RNA) surrounded by a protein coat.

• Can only reproduce inside host cells.

• Examples: Influenza virus, HIV.

• Make sure you understand why viruses are not classified as living organisms.

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