IGCSE Biology Edexcel Revision Notes: The Nature & Variety of Living Organisms

The Nature & Variety of Living Organisms

Characteristics of Living Organisms

• Living organisms must fulfill specific criteria, remembered by the acronym MRS C GREN:
  • Movement
  • Respiration
  • Sensitivity
  • Control
  • Growth
  • Reproduction
  • Excretion
  • Nutrition
• If something doesn't carry out all these processes, it's either dead or non-living.
• Viruses are non-living.

Nutrition

• Organisms need food for energy to carry out life processes (e.g., movement, respiration, excretion).
• Plants:
  • Use sunlight, carbon dioxide, and water to produce oxygen and glucose via photosynthesis.
  • Are autotrophic (make their own food).
• Animals:
  • Consume other living organisms for energy.
  • Break down complex molecules into simpler ones through digestion.
  • Are heterotrophic (obtain food from various sources).
• Autotrophs make their own food, while heterotrophs obtain it from other sources.
• Etymology can aid memory:
  • Autotroph: "auto" = "self," "trophic" = "feeding."
  • Heterotroph: "hetero" = "different," "trophic" = "feeding."

Respiration

• Respiration is a chemical reaction in all living organisms.
• Energy is released from glucose with (aerobic) or without (anaerobic) oxygen.
• The products are carbon dioxide and water.
• Energy is transferred as ATP.
• Aerobic respiration involves oxygen to release energy in the form of ATP.
• Gas exchange involves getting oxygen into cells and carbon dioxide out.

Excretion

• Metabolic reactions produce waste, some toxic, which must be removed.
• Excretion is the removal of toxic materials from organisms.
• In animals, waste products excreted include:
  • Carbon dioxide from respiration
  • Water from respiration and other chemical reactions
  • Urea (nitrogen from protein breakdown)
• In plants, waste products excreted include:
  • Oxygen from photosynthesis
  • Carbon dioxide from respiration
  • Water from respiration and other chemical reactions
• Excretion removes waste from cell chemical reactions, while egestion removes undigested food as faeces.

Response to Surroundings (Sensitivity)

• An organism's ability to detect and respond to stimuli in its environment.
• Enhances survival chances.

Sensitivity in Animals

• The nervous system uses receptors, neurons, and effectors to detect and respond to stimuli using electrical impulses.
• The endocrine system responds using chemical messengers (hormones) in the blood.

Sensitivity in Plants

• Responses are controlled by chemicals and are slower.
• Geotropism: response to gravity, roots grow down.
• Phototropism: response to light, shoots grow towards sunlight.

Movement

• An action causing a change of position or place.
• Locomotion: movement from place to place.
• Plants cannot move from place to place but can change orientation (e.g., sunflowers tracking the sun).

Control (Homeostasis)

• Living organisms must control their internal environment to maintain required conditions.
• Homeostasis: maintaining a stable internal environment.

Homeostasis in Humans

• Thermoregulation: control of body temperature (optimum is 37°C).
  • Mechanisms include sweating or vasodilation when body temperature increases.
• Other mechanisms include glucoregulation (blood glucose levels) and osmoregulation (water levels).

Homeostasis in Plants

• Transpiration maintains a suitable temperature.
• Water evaporates from stomata on the underside of the leaf, leading to heat loss.

Reproduction

• Production of more of the same kind of organism.
• Fundamental to the survival of a population/species.
• Types: sexual and asexual.

Sexual Reproduction

• Fusion of male and female gametes.
  • In humans: sperm and egg.
  • In plants: pollen grains and ovule.
• Offspring DNA composed of both maternal and paternal DNA

Asexual Reproduction

• Cells or organisms reproduce using mitosis.
• Involves one parent, producing an exact clone.
• Offspring DNA is identical to parental DNA.
• Plants can reproduce asexually through tubers, budding or runners.
• Single-celled organisms (bacteria, amoeba) reproduce asexually.

Growth

• Permanent increase in size.
• In animals, growth occurs between the zygote and adult stage.
• In plants, growth continues throughout their whole life with new shoots, leaves, branches etc forming year after year.

Common Features

• Living organisms are classified into five kingdoms: Animals, Plants, Fungi, Protoctists, Prokaryotes.
• Animals, plants, fungi, and protoctists are eukaryotes.
• Eukaryotic organisms can be multicellular or single-celled.
• Eukaryotic cells contain a nucleus with a distinct membrane.
• Prokaryotes are always single-celled and lack a nucleus.
• The nuclear material of prokaryotic cells is found in the cytoplasm.
• Bacteria are prokaryotic organisms that are substantially smaller than eukaryotic cells.

Animals

• Main Features:
  • Multicellular
  • Cells contain a nucleus with a distinct membrane
  • Cells lack cellulose cell walls
  • Cells lack chloroplasts (cannot perform photosynthesis)
  • Feed on organic substances made by other living things
  • Store carbohydrates as glycogen
  • Usually have nervous coordination
  • Able to move from place to place
• Animal cells contain structures such as a nucleus, cytoplasm, cell membrane, ribosomes and mitochondria.

Plants

• Main Features:
  • Multicellular
  • Cells contain a nucleus with a distinct membrane
  • Cells have cell walls made of cellulose
  • Cells contain chloroplasts (can perform photosynthesis)
  • Feed by photosynthesis
  • Store carbohydrates as starch or sucrose
  • Lack nervous coordination
• Plant cells contain structures such as a nucleus, cytoplasm, cell membrane, ribosomes, mitochondria, cell wall, vacuole, and chloroplasts.

Fungi

• Main Features:
  • Usually multicellular, some single-celled (e.g., yeast)
  • Multicellular fungi made of thread-like hyphae (containing many nuclei) organized into a mycelium network
  • Cells contain a nucleus with a distinct membrane
  • Cells have cell walls made of chitin
  • Cells lack chloroplasts (cannot perform photosynthesis)
  • Feed by secreting extracellular digestive enzymes and absorbing digested molecules (saprotrophic nutrition).
  • Some fungi are parasitic and feed on living material
  • Store carbohydrates as glycogen
  • Lack nervous coordination
• Examples: moulds, mushrooms, yeasts.
• A typical fungal cell contains structures such as a nucleus, cytoplasm, cell membrane, ribosomes, mitochondria, cell wall, and vacuole.
• The typical structure of a multicellular fungus e.g. Mucor (bread mould).

Protoctists

• Main Features:
  • Diverse kingdom; does not fit into other eukaryotic kingdoms (animals, plants, fungi)
  • Mainly microscopic and single-celled, some aggregate into colonies or filaments
  • Cells contain a nucleus with a distinct membrane
  • Some protoctists are more like animal cells e.g. Plasmodium (the protoctist that causes malaria)
  • Some protoctists are more like plant cells for example green algae, such as Chlorella
  • Some photosynthesise; some feed on organic substances made by other living things
  • Lack nervous coordination
• Examples: amoeba, Paramecium, Plasmodium, Chlorella

Prokaryotes

• Prokaryotes differ from eukaryotes as they are single-celled and lack a nucleus.
• Nuclear material is found in the cytoplasm.

Bacteria

• Bacteria characteristics:
  • Microscopic single-celled organisms
  • Have a cell wall (not cellulose), cell membrane, cytoplasm, and plasmids
  • Lack a nucleus but contain a circular chromosome of DNA
  • Lack mitochondria and other membrane-bound organelles
• Examples: Lactobacillus (used in yoghurt production), Pneumococcus (causes pneumonia).
• Bacteria feed in different ways:
  • Some bacteria can carry out photosynthesis
  • Most feed on other living or dead organisms (saprobionts or decomposers)
• A typical bacterial cell contains structures such as a cell wall, cell membrane, cytoplasm, ribosomes, plasmids, and a circular chromosome of DNA.

Pathogens

• A pathogen is any microorganism that causes disease in another organism (e.g., in plants or animals).
• Include bacteria, fungi, protoctists, and viruses.
• Not all species within these groups (except viruses) are pathogens; many are harmless.
• All viruses are pathogenic (can only exist by living inside cells of other organisms).

Pathogenic Bacteria

• Do not always infect host cells; can remain within body cavities or spaces.
• Examples:
  • M. tuberculosis: causes tuberculosis (TB) in humans; infects lungs, causing chronic cough and bloody mucus.
  • N. meningitidis: causes bacterial meningitis in humans; inflammation of the meninges causes fever, headache, neck stiffness, and a characteristic rash.

Pathogenic Fungi

• Fungal diseases are more common in plants than animals.
• Examples:
  • Cattle ringworm and athlete's foot: fungal diseases of the skin surface
  • Black Sigatoka: fungal disease in bananas; spreads through leaves, reducing photosynthesis.

Pathogenic Protoctists

• Examples:
  • Plasmodium falciparum: causes severe malaria in humans; spread by mosquitoes.
  • P. infestans: causes potato blight.
    • Transmitted via spores.
    • Destroys potato and tomato crops

Viruses

• Not considered alive as they do not carry out the 8 life processes for themselves.
• They can only reproduce by taking over a host cell’s metabolic pathways in order to make multiple copies of themselves.
• Characteristics:
  • Small particles (smaller than bacteria)
  • Parasitic and can only reproduce inside living cells
  • Infect every type of living organism
  • No cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA
• Examples:
  • Tobacco mosaic virus (TMV): causes discolouring of leaves on tobacco plants by preventing the formation of chloroplasts.
  • HIV virus: causes AIDS.
  • Influenza virus: causes the flu.
• Three different influenza viruses infect humans to cause the flu, influenza A, influenza B, and influenza C.
• Influenza A is the virus that causes the most cases of flu globally.