Characteristics of living organisms

1.1: CHARACTERISTICS OF LIVING ORGANISMS

LEARNING OBJECTIVES

• Describe the seven characteristics of living organisms.

• Define movement as a change of position or place.

• Define respiration as chemical reactions releasing energy.

• Define sensitivity as detecting and responding to changes.

• Define growth as a permanent increase in size and dry mass.

• Define reproduction as making more of the same organism.

• Define excretion as removing waste products of metabolism.

• Define nutrition as taking in materials for energy and growth.

The Seven Life Processes

All living organisms share seven characteristics that distinguish them from non-living things. Movement, respiration, sensitivity, growth, reproduction, excretion, and nutrition are the processes every organism carries out — whether it is a bacterium, a plant, or a human.

These seven characteristics apply to every living organism, not just animals. A common exam error is forgetting that plants also move, respire, and excrete. Each characteristic has a precise biological definition, and examiners expect you to use it exactly.

Mnemonic: MRS GREN — Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.

Movement

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

Animals move their whole bodies from one place to another — this is called locomotion. Plants also show movement, but usually only parts of them move. For example, roots grow downward through the soil, and shoots bend towards light. Because plant movement is slow and does not involve a change of place for the whole organism, students sometimes forget it counts. Movement in biology does not require legs or muscles; it simply requires a change of position.

MisconceptionMany students think only animals show movement. Plants also move — roots grow towards water, and shoots grow towards light. These are genuine biological movements. Exam cue: If asked "Do plants move?", always say yes and give a specific example.

Respiration

Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism.

Respiration happens inside every living cell, all the time. Cells need a continuous supply of energy to drive processes such as growth, movement, and the transport of substances. Nutrient molecules — usually glucose — are broken down by chemical reactions, and the energy stored in those molecules is released for the cell to use. This is metabolism [the sum of all chemical reactions in an organism].

Respiration is not the same as breathing. Breathing is the physical movement of air into and out of the lungs, which is only one step in getting oxygen to cells. Respiration itself is a chemical process that takes place inside cells, not in the lungs.

MisconceptionStudents frequently confuse respiration with breathing. Breathing is a mechanical process (moving air in and out). Respiration is a chemical process inside cells that releases energy from nutrients. Exam cue: Always write "respiration occurs in cells" to show you mean the chemical process.

Examiner InsightExaminers look for three key parts in a definition of respiration: (1) chemical reactions, (2) in cells, and (3) release energy. Missing any one of these loses a mark. Exam cue: Memorise all three parts and include every one when defining respiration.

Sensitivity

Sensitivity is the ability to detect and respond to changes in the internal or external environment.

Every living organism must sense what is happening around it and inside it, then respond appropriately. A change that an organism detects is called a stimulus [anything that causes a response], and the organism's reaction is called a response. For example, a human pulling a hand away from a hot surface is a response to the stimulus of heat. Plants also show sensitivity — a seedling growing towards light is responding to the stimulus of light direction.

Sensitivity applies to internal changes too. When blood sugar rises after a meal, the body detects this change and responds by releasing insulin. Without sensitivity, organisms could not maintain stable internal conditions or avoid danger.

Growth

Growth is a permanent increase in size and dry mass.

The word "permanent" is critical here. An animal that drinks water and temporarily becomes heavier has not grown — it has simply gained water mass. True biological growth involves the production of new cells and new biological material, which increases the organism's dry mass [the mass remaining after all water has been removed]. Because water content fluctuates, biologists measure growth using dry mass rather than wet mass to get an accurate picture.

Growth occurs when an organism builds more cells through cell division, or when individual cells increase in size. Both processes add permanently to the organism's structure.

MisconceptionStudents sometimes state that any increase in mass is growth. Temporary increases (e.g. from drinking water) do not count. Growth must be a permanent increase, and dry mass is the most reliable measure.

Reproduction

Reproduction is the processes that make more of the same kind of organism.

Every species must reproduce to survive over time. Without reproduction, a species would die out as individuals aged and died. Reproduction can be sexual (involving the fusion of gametes from two parents) or asexual (involving only one parent producing genetically identical offspring). Both forms result in new individuals of the same species.

The key phrase is "the same kind of organism." Offspring produced by reproduction belong to the same species as the parent or parents.

Excretion

Excretion is the removal of the waste products of metabolism and substances in excess of requirements.

Excretion specifically deals with waste substances produced by chemical reactions inside cells. For example, carbon dioxide is a waste product of respiration, and urea is produced when the liver breaks down excess amino acids. The body must remove these substances because they would become toxic if allowed to build up.

Excretion is not the same as egestion. Egestion is the removal of undigested food from the body through the anus. Undigested food was never absorbed into cells, so it was never part of metabolism. Excretion only involves substances that were produced by or used in metabolic reactions.

MisconceptionStudents commonly confuse excretion with egestion. Egestion removes undigested food (which never entered cells). Excretion removes metabolic waste products (made inside cells), such as CO₂ and urea. Exam cue: If a question asks about excretion, never mention faeces — that is egestion.

Examiner InsightExaminers frequently test whether students can distinguish excretion from egestion. Questions may list substances and ask which are excreted. Carbon dioxide and urea are the two most commonly examined excretory products. Exam cue: Learn CO₂ (from respiration) and urea (from amino acid breakdown) as key examples.

Nutrition

Nutrition is the taking in of materials for energy, growth and development.

All organisms need a supply of substances from their environment to fuel their life processes. Autotrophs [organisms that make their own food], such as green plants, take in simple inorganic molecules like carbon dioxide and water and use light energy to build organic nutrients through photosynthesis. Heterotrophs [organisms that consume other organisms for food], such as animals, take in complex organic molecules by eating other living things.

Regardless of whether an organism makes its own food or consumes it, nutrition provides the raw materials needed for releasing energy through respiration, building new cells for growth, and supporting development.