Sign up to access the complete lesson and track your progress!
Unlock This CourseAll living things must be able to detect changes in their surroundings and react appropriately to survive. They also need to remove harmful waste products that build up inside their bodies. These two characteristics - responding to the environment and excretion - are fundamental signs of life that separate living organisms from non-living things.
Key Definitions:
Imagine walking outside on a sunny day - your pupils automatically get smaller to protect your eyes. Or think about a plant growing towards a window - it's responding to light. These responses help organisms survive by finding food, avoiding danger and staying healthy.
Living organisms constantly monitor their surroundings and respond to changes. This ability to detect stimuli and produce appropriate responses is called sensitivity or irritability. Without this characteristic, organisms couldn't find food, escape predators, or reproduce successfully.
Organisms can detect many different types of environmental changes. These stimuli can come from outside the organism or from inside its own body.
Plants grow towards light sources (phototropism). Animals use light to navigate and find food. Your eyes detect light changes and adjust pupil size automatically.
Many animals migrate to warmer climates in winter. Plants may drop leaves when it gets cold. Humans shiver when cold and sweat when hot.
Animals use hearing to detect predators or prey. Birds respond to mating calls. Even some plants can respond to vibrations and sounds.
The Venus flytrap is a fascinating example of plant response. When an insect touches the trigger hairs inside the trap twice within 20 seconds, the plant responds by snapping shut in less than a second. This response helps the plant catch prey for nutrients in nutrient-poor soil.
Animals have developed sophisticated ways to detect and respond to their environment. They have specialised sense organs and nervous systems that allow rapid responses to stimuli.
Examples of animal responses:
Although plants don't have nervous systems like animals, they can still respond to environmental changes. These responses are usually slower than animal responses but are equally important for survival.
Types of plant responses (tropisms):
Excretion is the removal of toxic waste products that are produced during metabolism. It's crucial to understand that excretion is different from egestion - excretion removes waste made inside cells, whilst egestion removes undigested food that was never part of the organism.
Metabolic waste products can be poisonous if they build up in the body. For example, ammonia (produced when proteins break down) is highly toxic to cells. Organisms must remove these wastes quickly to prevent damage and death.
Different metabolic processes produce different types of waste that need to be removed from the body.
Produced during cellular respiration. Removed through lungs in mammals, gills in fish and stomata in plants. Too much CO2 makes blood acidic.
Made in the liver when amino acids are broken down. Less toxic than ammonia. Removed by kidneys in mammals and excreted in urine.
Excess water must be removed to maintain proper cell function. Too much or too little water can damage cells through osmosis.
Different types of organisms have evolved various methods to remove waste products effectively.
Mammals (including humans):
Plants:
Single-celled organisms:
Human kidneys are incredible organs that filter about 180 litres of blood every day! They remove urea, excess salts and water whilst keeping useful substances like glucose and proteins in the blood. When kidneys fail, people need dialysis machines to do this job artificially, showing just how vital excretion is for life.
It's important not to confuse excretion with other processes that remove materials from the body.
Excretion vs Egestion:
Excretion vs Secretion:
Response to environment and excretion work together to keep organisms healthy. For example, when you exercise (a response to your decision to be active), your body produces more carbon dioxide and heat. Your breathing rate increases and you start sweating - both excretory responses that help remove excess waste and cool your body down.
Similarly, when plants detect drought conditions, they may close their stomata to conserve water. This affects both their response to the environment (reducing water loss) and their excretion (limiting gas exchange).
Both response to environment and excretion are essential characteristics that all living organisms must have. They work together to help organisms survive, grow and reproduce in their ever-changing world.