1. Nature and variety of living organisms

Characteristics of living organisms

All living organisms perform several key functions, which can be remembered using the acronym MRS C GREN:

  • Movement: This means they can change their position, either by moving their whole body from one place to another or by moving parts of their body.

  • Reproduction: Organisms can create new individuals (offspring), either sexually (involving two parents and the combination of genetic material) or asexually (involving one parent producing identical offspring).

  • Sensitivity: They can detect changes in their surroundings (called stimuli, like light, temperature, or touch) and react to them.

  • Control: Living things can keep their internal body conditions stable, such as temperature or water levels. This process is called homeostasis.

  • Growth: They can increase in size and mass, becoming larger and more complex.

  • Respiration: Organisms produce energy to live by breaking down food. This can happen aerobically (using oxygen) or anaerobically (without oxygen).

  • Excretion: They get rid of waste materials or harmful substances produced by their body's processes.

  • Nutrition: They take in and use food or nutrients to get energy, grow, and repair themselves.

Eukaryotes

Eukaryotes are organisms whose cells have a clear nucleus containing their genetic material, and also contain small compartments called organelles (like tiny organs) which are enclosed within a protective outer membrane. The specific jobs of these organelles will be looked at in more detail in a later topic.

Subcellular structures found in plant and animal cells

  • Nucleus

    • This is the control center of the cell, holding the cell's genetic blueprint (DNA), which contains instructions for making specific proteins.

    • It is surrounded by a protective covering called the nuclear membrane.

  • Cytoplasm

    • This is a jelly-like substance that fills the cell, where many vital chemical reactions take place.

    • It contains enzymes, which are special proteins that act like catalysts, speeding up these chemical reactions without being used up themselves.

  • Cell membrane

    • This flexible outer layer of the cell has special receptor molecules that help the cell recognize and respond to signals.

    • It also acts as a gatekeeper, carefully choosing what substances can enter and leave the cell.

  • Mitochondria

    • Often called the 'powerhouses' of the cell, these are where aerobic respiration occurs to generate energy for the cell's activities.

  • Ribosomes

    • These are tiny structures responsible for building proteins (a process called protein synthesis).

    • They can often be found attached to a network of membranes called the rough endoplasmic reticulum.

Plants

  • Examples include cereals (like corn) or legumes (like peas).

  • They are multicellular, meaning they are made up of many cells.

  • Their cells contain chloroplasts, which are special structures that capture light energy from the Sun using a green pigment called chlorophyll. This is where photosynthesis (the process of making food) happens.

  • They have rigid cellulose cell walls around their cells, which provide strong support and protection.

  • They also have a large, permanent vacuole which stores water and dissolved substances (called cell sap). This helps maintain the cell's firmness and rigidity.

  • Plants store their food energy, primarily carbohydrates, in forms like starch or sucrose.

Animals

  • Examples include mammals (like humans) and insects (like flies).

  • They are also multicellular organisms.

  • Unlike plants, animals cannot photosynthesise; they must get their food by eating other organisms.

  • Their cells do not have cell walls.

  • They possess complex nervous systems that allow them to sense, communicate, and coordinate their movements and actions.

  • Animals store their carbohydrate energy as glycogen.

Fungi

  • The body of most fungi is typically arranged into a network of tiny threads called hyphae, which collectively form a mycelium. These hyphae often contain many nuclei. Some fungi, like yeast, are single-celled.

  • Examples include Mucor (a type of mold with thread-like hyphae) and yeast (a single-celled fungus).

  • Their cells have cell walls made of chitin, a tough substance.

  • Fungi obtain nutrients by saprotrophic nutrition: they release digestive enzymes outside their bodies to break down food into smaller pieces, which they then absorb.

  • They may store carbohydrates as glycogen.

Protoctists

  • Protoctists are mostly single-celled organisms.

  • Some protoctists behave like animal cells, such as Amoeba, which lives in pond water and moves by changing its shape.

  • Others are more plant-like, possessing chloroplasts and performing photosynthesis, such as Chlorella.

Prokaryotes (1.3)

Prokaryotes are simple organisms whose cells do not have a nucleus or any other membrane-bound organelles. Their genetic material floats freely in the cytoplasm.

  • Bacteria

    • Examples include Lactobacillus bulgaris (a rod-shaped bacterium used in making yogurt) and Pneumococcus (a spherical bacterium that causes pneumonia).

    • They are single-celled and extremely small.

    • Bacteria have a cell wall, a cell membrane, cytoplasm, and small loops of DNA called plasmids.

    • While they lack a true nucleus, their main genetic material is a single circular piece of DNA.

    • Some bacteria can photosynthesise (make their own food), but most get their nutrients by feeding on other living or dead organisms.

Pathogens (1.4)

Pathogens are any organisms that cause disease. These can include fungi, bacteria, protoctists, or viruses.

  • Viruses

    • Examples include the tobacco mosaic virus (which harms plants by preventing chloroplast formation), the influenza virus (causing flu), and the HIV virus (which leads to AIDS).

    • Viruses are much smaller than bacteria and are not considered living organisms because they cannot carry out life processes independently.

    • They are parasitic, meaning they depend entirely on other living cells to survive and reproduce.

    • Viruses can only reproduce inside living cells. They invade a host cell, take over its machinery, and force it to make millions of copies of the virus. These new viruses then spread by bursting out of the host cell.

    • Viruses come in many different shapes and sizes.

    • They do not have a cellular structure; instead, they consist of just one type of genetic material (either DNA or RNA) enclosed within a protective protein coat.

  • Bacteria

    • An example is Salmonella, which can cause food poisoning.

    • They can reproduce very quickly by a process called binary fission, where one bacterium simply splits into two identical copies.

    • Some bacteria produce toxins (poisonous substances) that can damage host cells and cause illness.

  • Protists

    • An example is the protist that causes malaria.

    • These are often parasitic, meaning they live inside and feed off animal hosts to complete their life cycle.

  • Fungi

    • An example is the fungus that causes Athlete’s foot.

    • Fungi reproduce by producing spores that can be spread easily by wind or direct contact between people.

    • Fungal infections can often be treated with medicines called fungicides.