The 7 Characteristics of Living Things

Defining Life: Biology is the study of living things, but how do we distinguish living organisms from non-living objects? All living things share seven key characteristics that define what it means to be alive.

The MRS GREN Acronym: To remember these characteristics, biologists use the handy acronym MRS GREN (or sometimes MR GREN). This simple memory tool helps you recall all seven characteristics!

What is Movement in Living Things?

Definition: The ability of an organism to change position or place. Movement can be of the whole organism or parts of the organism.

• Animals: Most animals move their entire bodies from place to place (locomotion)
• Plants: Plants show movement too, but usually of their parts (leaves turning toward light, roots growing downward)
• Internal Movement: Movement inside organisms (blood circulation, food through digestive system)

CSEC Exam Tip: Remember that plants DO show movement, just differently from animals. Exam questions often test this distinction!

What is Respiration?

Definition: The chemical process that releases energy from food (usually glucose) in cells. This is NOT the same as breathing!

• Aerobic Respiration: Uses oxygen (most efficient) - Glucose + Oxygen → Carbon Dioxide + Water + Energy
• Anaerobic Respiration: Without oxygen (less efficient) - Glucose → Lactic Acid + Energy (in animals) or Alcohol + Carbon Dioxide + Energy (in yeast)
• Cellular Process: Occurs in mitochondria of cells

Did You Know? Plants respire too! They take in oxygen and release carbon dioxide at night when photosynthesis isn't occurring.

Responding to the Environment

Definition: The ability to detect and respond to changes in the environment (stimuli).

• Animals: Have specialized sense organs (eyes, ears, skin receptors)
• Plants: Respond to light (phototropism), gravity (geotropism), water, and touch
• Simple Organisms: Bacteria move toward nutrients (chemotaxis)

CSEC Exam Tip: Be prepared to give specific examples of stimuli and responses for different organisms.

What is Biological Growth?

Definition: A permanent increase in size and dry mass (weight without water) by cell division and/or cell enlargement.

• Cell Division: Mitosis produces new cells
• Cell Enlargement: Cells increase in size (common in plants)
• Differentiation: Cells develop into specialized types

Key Distinction: Living things grow from within (cell-based), while non-living things may increase in size by adding material to the outside (like crystals growing or dust accumulating).

Creating New Life

Definition: The production of offspring, either sexually or asexually, to continue the species.

Types of Reproduction

• Sexual Reproduction: Involves two parents, genetic variation, gametes (sex cells)
• Asexual Reproduction: One parent, genetically identical offspring, faster

CSEC Exam Tip: Remember that reproduction is essential for species survival but NOT for individual survival. An individual can live without reproducing.

Removing Waste Products

Definition: The removal of toxic waste products of metabolism (chemical reactions in cells).

Key Metabolic Wastes

• Carbon Dioxide: From respiration (removed by lungs in animals)
• Urea: From breakdown of excess proteins (removed by kidneys)
• Oxygen: Waste product of photosynthesis in plants (released through stomata)

Remember: Plants excrete too! They remove oxygen (photosynthesis waste) and other compounds through leaves, bark, or into soil.

Obtaining and Using Food

Definition: The taking in of nutrients (food) for energy, growth, and repair.

Types of Nutrition

• Autotrophic Nutrition: Organisms make their own food (photosynthesis in plants, chemosynthesis in some bacteria)
• Heterotrophic Nutrition: Organisms consume other organisms (animals, fungi, some bacteria)

CSEC Exam Tip: Know the difference between photosynthesis (making food) and respiration (releasing energy from food).

MRS GREN Summary Table

CSEC Exam Tips

• Common Mistake: Confusing excretion with egestion - remember EXCRETION = METABOLIC waste
• Question Style: "List FOUR characteristics of living things" - always give the full term AND an example
• Application: Be prepared to apply MRS GREN to specific organisms in exam questions
• Memory Aid: Create your own mnemonic or story using MRS GREN

Why MRS GREN Matters

All living things - from the smallest bacterium to the largest whale, from moss to mahogany trees - share these seven characteristics. This unity demonstrates the interconnectedness of life on Earth. Remember the acronym MRS GREN as your key memory tool for this fundamental biological concept.

Real-World Application

Understanding these characteristics helps scientists:

• Identify new species discovered in remote locations
• Determine if something found on other planets might be alive
• Study how organisms adapt to changing environments
• Develop medicines that target specific life processes

Next Steps in Your Biology Journey

Now that you've mastered the characteristics of living things, you're ready to explore:

• Cell Structure and Function - How these characteristics happen at cellular level
• Classification of Organisms - How we group living things based on similarities
• Ecosystems - How living things interact with each other and their environment