Characteristics of Living Organisms (Edexcel IGCSE Biology)

The Unifying Characteristics of Living Organisms

• In order for something to be considered 'living', it must fulfil specific criteria that are true of all living organisms

• These criteria can be remembered using the acronym MRS C GREN, which stands for:

  • Movement

  • Respiration

  • Sensitivity

  • Control

  • Growth

  • Reproduction

  • Excretion

  • Nutrition

• If something does not carry out all of these life processes, it is either dead or non-living

• Viruses are a good example of non-living particles/agents

Nutrition

• Organisms must obtain food to provide energy

  • Energy is necessary to carry out life processes e.g. movement, respiration and excretion

Nutrition in plants

• Plants use sunlight, carbon dioxide and water to produce oxygen and glucose in the process of photosynthesis

• Because plants create their own food for energy, they are described as being autotrophic

Nutrition in animals

• Animals consume other living organisms in order to obtain the energy they require

• They break down larger complex molecules into simpler molecules through the process of digestion

• As animals obtain their food from a range of different sources, they are described as being heterotrophic

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Autotrophs make their own food whereas heterotrophs obtain it from a range of food sources.

Respiration

• Respiration is a chemical reaction carried out in all living organisms

• Energy is released from glucose either in the presence of oxygen (aerobic respiration) or the absence of oxygen (anaerobic respiration)

• The reactions ultimately result in the production of carbon dioxide and water as waste products

• Energy is transferred in the form of ATP

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The equation for aerobic respiration

Excretion

• Chemical reactions that take place inside living cells are described as metabolic reactions

• Metabolic reactions produce waste products, some of which may be toxic

• These toxic products must be eliminated from the body

• Excretion is the removal of toxic materials and substances from organisms

Excretion in animals

• Waste products excreted by animals include:

  • Carbon dioxide from respiration

  • Water from respiration and other chemical reactions

  • Urea which contains nitrogen resulting from the breakdown of proteins

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Excretion in humans, the waste products and organs involved.

Excretion in plants

• Waste products excreted by plants include:

  • Oxygen from photosynthesis

  • Carbon dioxide from respiration

  • Water from respiration and other chemical reactions

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Excretion in plants, the waste products and the difference between day and night.

Response to Surroundings

• The sensitivity of an organism refers to its ability to detect and respond to stimuli in its surroundings

• Responding to the environment around them gives an organism the best chances of survival

Sensitivity responses in animals

• In humans, the nervous system provides a complex system of receptors, neurones and effectors which detect and respond to different stimuli using electrical impulses

• The endocrine system also allows a response to stimuli using chemical messengers, which travel in the blood, called hormones

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The nervous system and endocrine system allow humans to respond to their environment.

Sensitivity responses in plants

• In plants, responses are controlled by chemicals and are usually much slower

  • Geotropism describes a plants response to gravity which causes the roots to grow down into the soil

  • Phototropism describes a plants response to light which causes shoots to grow towards sunlight

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Phototropism and geotropism allow plants to respond to their environment.

Movement

• Movement is an action by an organism causing a change of position or place

• The movement of an organism from place to place is called locomotion

• Plants cannot move from place to place but can change their orientation

  • For example, sunflowers track the sun and so change their orientation throughout the day

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Sunflowers track the sun throughout the day.

Control

• Living organisms must control their internal environment in order to keep conditions within required limits

• This is called homeostasis

Homeostasis in humans

• Thermoregulation refers to the control of body temperature

• The optimum human body temperature is 37°C

  • If body temperature increases e.g. during exercise, mechanisms for control will be initiated to return the temperature back to the optimum

  • Mechanisms include sweating or vasodilation

• Other homeostatic mechanisms in humans include glucoregulation (control of blood glucose levels) and osmoregulation (control of water levels)

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Thermoregulation is an example of homeostasis required to maintain a body temperature of 37°C.

Homeostasis in plants

• Plants use transpiration to maintain a suitable temperature

• Water evaporates from the stomata on the underside of the leaf, leading to heat loss

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Plants maintain an optimum temperature through transpiration.

Reproduction

• Reproduction is the process that leads to the production of more of the same kind of organism

• Reproduction is fundamental to the survival of a population and ultimately, the species

• There are different types of reproduction: sexual and asexual

Sexual reproduction

• In this type of reproduction, the male and female gametes fuse together

  • In humans, the male gamete is the sperm and the female gamete is the egg

  • In plants, the male gamete is in the pollen grains and the female gamete is the ovule

• The DNA of the offspring is composed of both maternal and paternal DNA

Sexual reproduction involves the fusing of two gametes to form a zygote that contains DNA from both parents.

Asexual reproduction

• Cells or whole organisms can also reproduce using asexual reproduction

• Mitosis is an example of asexual reproduction

• There is only one parent involved so an exact clone is produced

• The DNA of offspring is identical to parental DNA

  • Plants can reproduce asexually through tubers, budding or runners

  • Single-celled organisms such as bacteria or amoeba reproduce asexually

Asexual reproduction in bacteria involves creating exact copies of the parent cell.

Growth

• Growth is defined as a permanent increase in size

• In animals, an individual grows larger between the zygote and adult stage with changes in proportion or shape

• In plants, an individual grows larger throughout their whole life with new shoots, leaves, branches etc. forming year after year