0610 · 1.1Characteristics of living organisms

Cells, metabolism and Mrs Gren

0:00Here is a question that sounds easy until you try to answer it. What is the difference between a cat and a car? Both of them move. Both of them take in fuel, food for the cat and petrol for the car. Both of them give out waste gases. Some cars can even sense when another car is too close and brake on their own. So why do we say the cat is alive and the car is not?

0:21By the end of this video you will be able to describe the seven characteristics that all living organisms share, give the exact meaning of each one, and use them to decide whether something is alive. You will also see the mistakes that cost students marks on this topic, because a few of these words mean something more precise in biology than they do in everyday life. Before we start, there are two ideas you need.

0:41The first is that every living organism is made of one or more cells. A bacterium is a single cell, and you are made of trillions of them. The second is the word metabolism. Metabolism means all of the chemical reactions that happen inside the cells of an organism. Several of the definitions in this video use that word, so keep it in mind. Biologists agree that every living organism shows seven characteristics: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.

1:09A handy way to remember all seven is the name Mrs Gren, where each letter stands for one characteristic. M is movement, R is respiration, S is sensitivity, G is growth, R is reproduction, E is excretion and N is nutrition. The name helps you list them, but the exam also expects you to describe each one properly, so let us go through them carefully.

Movement and respiration

1:31The syllabus describes movement as an action by an organism or part of an organism causing a change of position or place. Notice the words part of an organism. The whole organism does not have to go anywhere. When you walk across a room, your whole body changes place. But when your heart beats, or your arm bends, only part of you is moving, and that still counts. This matters for plants.

1:52A plant cannot walk anywhere, so students sometimes say that plants do not move. They do. Leaves turn to face the light, flowers open in the morning and close at night, and the leaves of some plants fold up when they are touched. These movements are slow, but they are real changes of position, made by parts of the plant. Next is respiration. The syllabus describes respiration as the chemical reactions in cells that break down nutrient molecules and release energy for use in metabolism.

2:17The nutrient molecule is usually glucose. Every cell needs a constant supply of energy to stay alive, to build new molecules, to move and to grow, and respiration is how it gets that energy. This is the most common mistake on the whole topic. Respiration is not breathing. Breathing is the movement of air into and out of the lungs. Respiration is a set of chemical reactions that happens inside cells, in every living cell of every organism, including plants and bacteria that have no lungs at all.

2:45Breathing helps by bringing in oxygen for respiration in many animals, but the two are different processes. Notice also that respiration releases energy. It does not make energy. Energy cannot be created, so in an exam, say that respiration releases energy from nutrient molecules, never that it produces or creates energy.

Sensitivity, growth and reproduction

3:06The third characteristic is sensitivity. The syllabus describes sensitivity as the ability to detect and respond to changes in the internal or external environment. There are two parts here. The organism must detect a change, and it must respond to it. If you touch a hot pan, you detect the heat and you pull your hand away. That is a response to a change in the external environment, the world outside your body.

3:28The internal environment means conditions inside the body. When you get too hot, your body detects the rise in your own temperature and responds by making you sweat. Plants are sensitive too. A seedling on a windowsill detects light coming from one side and grows towards it. Plant roots detect gravity and grow downwards. The fourth characteristic is growth. The syllabus describes growth as a permanent increase in size and dry mass.

3:52Every word in that definition is doing a job, so let us look at why. First, the increase has to be permanent. If you drink a large glass of water, your mass goes up for a while, but you have not grown. A dry sponge dropped into water swells up and gets heavier, but it is not alive and it is not growing. It has just soaked up water, which it can lose again.

4:13Second, it is dry mass that must increase. The dry mass is the mass of an organism after all of its water has been removed. The amount of water in an organism goes up and down all the time. A plant can lose a lot of water on a hot afternoon and take it back up at night. So to measure real growth, biologists dry the organism and weigh what is left.

4:30If that dry mass has gone up, new material has been built, and the organism has grown. The fifth characteristic is reproduction. The syllabus describes reproduction as the processes that make more of the same kind of organism. Ducks make more ducks, oak trees make more oak trees, and a bacterium splits into two bacteria. It does not matter whether the organism needs a partner or does it alone. Both of those count as reproduction, and you will meet them in detail later in the course.

4:56One thing to notice is that not every individual has to reproduce. A pet dog that has been neutered will never have puppies, but it is still alive. The characteristic belongs to the kind of organism, and every kind of living organism has a way of making more of itself.

Excretion and nutrition

5:09The sixth characteristic is excretion. The syllabus describes excretion as the removal of the waste products of metabolism and substances in excess of requirements. Let us take that in two halves. Waste products of metabolism are substances made by the chemical reactions in cells that the organism does not need, and which could be harmful if they built up. Carbon dioxide is one. It is made by respiration in every cell, and in humans it is removed through the lungs every time we breathe out.

5:37Urea is another. It is made in the liver when the body breaks down proteins it does not need, and the kidneys remove it in urine. Plants excrete waste products too. During photosynthesis, a leaf makes oxygen, and the oxygen that the plant does not use is waste that leaves through the leaf. Substances in excess of requirements means more of something than the body needs. Water and salts are useful, but if you take in more than you need, the extra has to be removed, and the kidneys remove it in urine too.

6:03Here is the trap. Getting rid of faeces is not excretion. Faeces are mostly undigested food, such as fibre, that passed straight through the gut. That food was never absorbed into the cells and never took part in metabolism, so it is not a waste product of metabolism. Removing faeces is called egestion. In an exam, if you say that excretion is getting rid of faeces, you will lose the mark.

6:25The last characteristic is nutrition. The syllabus describes nutrition as the taking in of materials for energy, growth and development. Animals do this by eating food. The food gives them nutrient molecules to respire for energy, and materials such as proteins to build new cells as they grow and develop. Plants make their own food by photosynthesis, so students sometimes think that plants do not need nutrition. But look at the definition again.

6:50Nutrition is the taking in of materials. A plant takes in carbon dioxide through its leaves, and water and mineral ions through its roots. It uses them to make glucose and everything else it is built from. So plants carry out nutrition too, just in a different way from animals.

Testing life characteristics

7:05Now let us go back to the cat and the car, and test the car properly against all seven characteristics. A biologist does not ask whether something shows one or two of them. A living organism shows all seven. Movement? The car does move. Respiration? The car burns petrol to release energy, which looks a bit like respiration. But respiration is a set of reactions inside cells, and a car has no cells.

7:28Sensitivity? A car with sensors can detect another car and brake, but only because people built it that way. Growth? A car never gets permanently bigger by building new material of its own. Reproduction? Two cars parked in a garage will never make a third car. Excretion? The car gives out exhaust gases, but they do not come from metabolism in cells. And nutrition? The car takes in petrol, but it does not use it to grow or develop.

7:52So the car fails several characteristics completely, especially growth and reproduction, and it has no cells at all. It is not alive. The cat shows all seven, so it is. Try one more yourself. Crystals of salt can grow larger in a beaker of salty water. Does that make a crystal alive? Pause and think. The answer is no. The crystal gets bigger because more salt sticks to its outside, but it does not respire, it is not sensitive, it does not excrete and it does not take in nutrition.

8:20Showing one characteristic, like growth, is never enough. In the exam, this topic is usually tested in two ways. The first is to give you a definition, or an example, and ask you to name the characteristic. For example, a plant shoot bends towards a window. That is sensitivity, because the shoot is detecting light and responding to it. A student weighs some dried seedlings each week and the mass goes up.

8:42That is growth, and the word dried tells you it is dry mass. A fish releases carbon dioxide from its gills. That is excretion, because carbon dioxide is a waste product of respiration. The second way is to ask you to describe a characteristic, and here the marks are in the key words. For growth, you need permanent, and you need dry mass. For respiration, you need chemical reactions, in cells, and release energy.

9:07For excretion, you need waste products of metabolism, and substances in excess of requirements. Learn the definitions closely, because the examiner is looking for those exact ideas. Let us recap. All living organisms show seven characteristics, which you can remember as Mrs Gren. Movement is an action by an organism or part of an organism causing a change of position or place. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for use in metabolism.

9:35Sensitivity is the ability to detect and respond to changes in the internal or external environment. Growth is a permanent increase in size and dry mass. Reproduction is the processes that make more of the same kind of organism. Excretion is the removal of the waste products of metabolism and substances in excess of requirements. And nutrition is the taking in of materials for energy, growth and development. Remember the three traps.

9:59Respiration is not breathing. Excretion is not getting rid of faeces. And plants do all seven too, they just do them more slowly and more quietly than animals. In the next video, we will use the features that organisms share to sort them into groups, which is what biologists call classification.