Living Creatures: Exploring Their Characteristics

Characteristics of Living OrganismsVertebrates and Invertebrates

Characteristics of Living Organisms

How do we know whether something is living or non-living? A car can move and a river can flow, yet neither is alive; a seed lies still, yet it is living. We decide by checking for a set of features that all living things share, called the characteristics of living organisms. A thing is living if it shows these life processes:

  • Growth — living things grow in size, increasing from within. A seed grows into a plant; a kitten grows into a cat.
  • Need for food (nutrition) — living things need food for energy and growth. Plants make their own food; animals take in food.
  • Respiration — living things breathe; they take in air and use the oxygen in it to release energy from food.
  • Response to stimuli — living things react to changes around them. A change that causes a reaction is a stimulus. We pull our hand away from a hot object; a plant bends towards light; a touch-me-not (Mimosa) folds its leaves when touched.
  • Movement — living things move on their own. Animals move from place to place; plants show slower movements, such as roots growing down and shoots growing up.
  • Excretion — living things get rid of the waste their bodies make.
  • Reproduction — living things produce more of their own kind (young ones, eggs or seeds).

Living things show all these features; a non-living thing may show one (a car moves) but not the others. Finally, all living things have a definite life span — they are born, grow, and one day die.

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Worked Example
Example 1: A car can move and uses fuel, yet it is non-living. Give two reasons why.
Solution

Living things must show all the life processes, not just one.

  • A car cannot grow on its own.
  • It cannot reproduce (make young cars) or respond to stimuli by itself.
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Worked Example
Example 2: What is a stimulus? Give one example of how a plant responds to one.
Solution

A stimulus is a change that causes a reaction.

  • A stimulus is a change in the surroundings that a living thing reacts to.
  • Example: a plant bends/grows towards light (light is the stimulus).
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Worked Example
Example 3: Name the life process by which living things produce more of their own kind.
Solution

Recall the characteristic that continues a species.

  • Producing young ones, eggs or seeds is reproduction.

Key Points

    • Living things share characteristics: growth, nutrition, respiration, response to stimuli, movement, excretion and reproduction.
    • A stimulus is a change that a living thing reacts to (light, touch, heat).
    • Non-living things may show one feature (a car moves) but not all.
    • All living things have a definite life span — they are born, grow and die.
✎ Quick Check — 2 questions0 / 2
Q1.Which of these is a characteristic of all living things?
Explanation: All living things grow and reproduce, among other life processes.
Q2.A change in the surroundings that a living thing reacts to is called a:
Explanation: A stimulus is a change that causes a response.

Vertebrates and Invertebrates

The animal world is huge and varied. To study animals, scientists divide them into groups. One of the most useful divisions is based on the backbone — the row of small bones (the spine) running down the middle of the back. Animals are grouped as:

  • Vertebrates — animals that have a backbone. They usually have a well-developed skeleton inside the body. Examples: fish, frog, snake, sparrow, cat, human.
  • Invertebrates — animals that do not have a backbone. Examples: ant, butterfly, spider, earthworm, snail, jellyfish. Most animals on Earth are actually invertebrates.

Vertebrates themselves are divided into five well-known groups, each with its own features:

  • Fish — live in water, breathe with gills, have fins and a body covered with scales.
  • Amphibians — such as frogs; can live both in water and on land.
  • Reptiles — such as snakes and lizards; crawl and have dry, scaly skin.
  • Birds — have feathers, wings and a beak; most can fly.
  • Mammals — such as cats, cows and humans; usually have hair on the body and feed their young on milk.

Grouping animals like this helps us see how they are similar and different, and makes the great variety of animal life easier to study and understand.

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Worked Example
Example 1: Classify as vertebrate or invertebrate: sparrow, ant, snake, snail.
Solution

Check each for a backbone.

  • Sparrow and snake have a backbone → vertebrates.
  • Ant and snail have no backbone → invertebrates.
2
Worked Example
Example 2: To which group of vertebrates does an animal with feathers, wings and a beak belong?
Solution

Match the features to the group.

  • Feathers, wings and a beak are the features of birds.
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Worked Example
Example 3: Why is the presence of a backbone a useful feature for grouping animals?
Solution

A clear, common feature makes a good basis for grouping.

  • Every animal either has a backbone or does not — a clear yes/no.
  • So it neatly divides all animals into two big groups for easy study.

Key Points

    • Animals are grouped by the backbone: vertebrates (with backbone) and invertebrates (without).
    • Examples — vertebrates: fish, frog, snake, bird, human; invertebrates: ant, spider, earthworm, snail.
    • Vertebrates form five groups: fish, amphibians, reptiles, birds and mammals.
    • Most animals on Earth are invertebrates.
✎ Quick Check — 2 questions0 / 2
Q1.Animals that do NOT have a backbone are called:
Explanation: Invertebrates are animals without a backbone.
Q2.A frog, which can live both in water and on land, belongs to which group of vertebrates?
Explanation: Amphibians, such as frogs, can live both in water and on land.