Characteristics of Living Organisms (Edexcel IGCSE Biology: Double Science)

• 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

• 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

Use this image

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

• 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

Use this image

The equation for aerobic respiration

• 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

Use this image

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

Use this image

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

• 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

Use this image

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

Use this image

Phototropism and geotropism allow plants to respond to their environment.

• 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

Use this image

Sunflowers track the sun throughout the day.

• 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)

Use this image

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

Use this image

Plants maintain an optimum temperature through transpiration.

• 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 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