Characteristics of Living Organisms (Edexcel IGCSE Biology)

• 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

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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

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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

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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.

• 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 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.

• 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 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