Characteristics of Living Organisms

What counts as a living organism?

• Biologists separate everything in the natural world into living and non-living things. A rock is not alive; a mouse is. But what exactly is the test that decides?
• Every living organism, from a bacterium to a blue whale, carries out the same eight life processes. If something does any one of them only sometimes, or relies on another organism's cellular machinery to do them, it is not classed as living
• The most popular acronym for the eight processes is MRS C GREN:

• All eight must be present for something to be alive. Viruses fail most of these criteria. They can only reproduce inside a host cell's machinery, which is why most biologists do not class viruses as living organisms

M — Movement

Movement is any action by an organism (or by part of it) that shifts its position or orientation

• When the whole organism moves from one place to another, the special term is locomotion. Animals (and many bacteria) achieve locomotion using legs, fins, wings or flagella
• Plants cannot uproot themselves and walk away, but they can reorient. A sunflower head tracks the Sun across the sky during the day (a movement called heliotropism); roots grow downwards and shoots grow upwards in response to gravity and light

R — Respiration

Respiration is the chemical process by which energy is released from glucose inside every living cell

• This is not the same as breathing (gas exchange). Breathing moves oxygen into the lungs; respiration uses that oxygen inside the cells to release energy
• Two forms:
  • Aerobic respiration happens with oxygen present and produces carbon dioxide and water as waste products
  • Anaerobic respiration happens without oxygen, produces less energy, and gives different waste products (lactic acid in animals; ethanol and carbon dioxide in yeast)
• The energy released is captured by the cell in the chemical bonds of a molecule called ATP (adenosine triphosphate), the "energy currency" of every cell. ATP then powers everything else the cell does, including movement, building new molecules and transporting substances

S — Sensitivity

Sensitivity is the ability of an organism to detect changes in its surroundings (stimuli) and respond to them in a way that helps its survival

• A stimulus might be light, sound, temperature, touch, gravity, a chemical, or the presence of food or a predator
• Animals use two coordinating systems:
  • The nervous system uses fast electrical impulses carried by neurones (receptors → nervous system → effectors)
  • The endocrine system uses slower chemical messengers called hormones, carried in the blood
• Plants respond more slowly and use chemicals only. Two important plant responses are:
  • Phototropism: shoots grow towards the light so that the leaves get maximum sunlight for photosynthesis
  • Geotropism (gravitropism): roots grow downwards in the direction of gravity, anchoring the plant and reaching water; shoots grow upwards against gravity

C — Control

Control (also called homeostasis) is the maintenance of the internal environment of the body within narrow limits, despite changes in the outside world

• Living cells only function properly within a narrow range of temperature, pH, water content and dissolved-substance concentrations. Homeostasis keeps these conditions stable
• Examples in humans:
  • Thermoregulation: keeping body temperature at around 37 °C (sweating and vasodilation cool you down when too hot; shivering and vasoconstriction warm you back up when too cold)
  • Osmoregulation: keeping the water and salt content of the blood at a constant level (the kidneys do this, helped by the hormone ADH)
  • Blood glucose regulation: keeping blood sugar within a narrow range using insulin and glucagon
• Plants also keep their internal conditions within tight limits. Transpiration (the evaporation of water from leaf stomata) helps cool a plant on a hot day, because the water vapour leaving the leaf carries heat away with it

G — Growth

Growth is a permanent increase in size (and dry mass) of an organism

• Animals grow rapidly between the zygote stage and the adult stage, after which growth stops once the adult size is reached. The body shape often changes too: a baby's head is proportionally much larger than an adult's
• Plants tend to grow throughout their whole life, adding new shoots, leaves, branches and roots year after year. A 1000-year-old oak tree is still growing
• Growth is fuelled by nutrition (raw materials) and respiration (energy to build them into new tissue)

R — Reproduction

Reproduction is the process by which an organism produces offspring of the same species

• Without reproduction the species would die out. Reproduction is the only way that one generation passes its genes (and its species' continued existence) to the next
• Two types:
  • Sexual reproduction: two parents, two gametes (sex cells) fuse to form a zygote; offspring inherit a mix of DNA from both parents and are genetically different from either. In humans the male gamete is the sperm and the female gamete is the egg (ovum). In flowering plants, the male gamete is inside the pollen grain and the female gamete is inside the ovule
  • Asexual reproduction: one parent only; offspring are genetically identical clones of the parent. Mitosis is the cell-division process behind asexual reproduction
• Examples of asexual reproduction:
  • Bacteria dividing into two by binary fission
  • Amoeba (a protoctist) splitting in half
  • Strawberry plants sending out runners that root and form new plants
  • Gardeners taking cuttings to grow new plants

E — Excretion

Excretion is the removal of the toxic waste products of metabolism from the body

• "Metabolism" means all the chemical reactions happening inside the cells. Many of these reactions produce waste that would poison the cell if it built up
• Wastes excreted by animals:
  • Carbon dioxide from aerobic respiration, removed via the lungs
  • Water from respiration, removed via the lungs (water vapour in breath), the skin (sweat) and the kidneys (urine)
  • Urea from the breakdown of excess amino acids in the liver, removed by the kidneys in urine
• Wastes excreted by plants:
  • Oxygen during the day, produced by photosynthesis
  • Carbon dioxide at night, produced by respiration (during the day, the plant's own photosynthesis uses up its respiratory CO₂ before it can leak out)
• Excretion is not the same as egestion. Egestion is the removal of undigested food in the faeces, which never entered the cells in the first place and is therefore not a metabolic waste

N — Nutrition

Nutrition is the taking in of materials needed for energy, growth and repair

• Two strategies:
  • Autotrophic nutrition means building your own food out of simple inorganic raw materials. Green plants are autotrophs: they combine carbon dioxide with water, powered by sunlight absorbed by chlorophyll, to manufacture glucose and release oxygen as a by-product. This process is photosynthesis:

6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂ (in the presence of sunlight and chlorophyll)

- **Heterotrophic nutrition** means taking in ready-made food from other organisms. **Animals**, **fungi** and many protoctists are heterotrophs. Animals digest large food molecules into small ones in the gut, and the small molecules are absorbed into the bloodstream and delivered to every cell