Life processes – MRS GREN

Yayınlandı: 30 Kasım 2009 / Genel

Living things have certain life processes in common. There are seven things that they need to do to count as being alive. The phrase MRS GREN is a way to remember them:

M Movement All living things move, even plants
R Respiration Getting energy from food
S Sensitivity Detecting changes in the surroundings
G Growth All living things grow
R Reproduction Making more living things of the same type
E Excretion Getting rid of waste





Taking in and using food


It can be easy to tell if something is living or not. A teddy bear might look like a bear, but it can’t do any of the seven things it needs to be able to do to count as being alive.

What about a car? A car can move, it gets energy from petrol (like nutrition), it might have a car alarm (sensitivity), and it gets rid of waste gases through its exhaust pipe (excretion). But it can’t grow or make baby cars. So a car is not alive.

Living things have certain life processes in common. There are seven things that they need to do to count as being alive. The phrase MRS GREN is a way to remember them:

Respiration is a chemical reaction that happens in all living cells. It is the way that energy is released from glucose, for our cells to use to keep us functioning.

Remember that respiration is not the same as breathing (which is properly called ventilation).

Aerobic respiration

The glucose and oxygen react together in the cells to produce carbon dioxide and water. The reaction is called aerobic respiration because oxygen from the air is needed for it to work.

Here is the word equation for aerobic respiration:

glucose + oxygen   →   carbon dioxide + water (+ energy)

(Energy is released in the reaction. We show it in brackets in the equation because energy is not a substance.)

Now we will look at how glucose and oxygen get to the cells so that respiration can take place and how we get rid of the carbon dioxide.

Glucose from food to cells

Glucose is a type of carbohydrate, obtained through digestion of the food we eat. Digestion breaks food down into small molecules. These can be absorbed across the wall of the small intestine into the bloodstream.

Glucose is carried round the body dissolved in blood plasma, the pale yellow liquid part of our blood. The dissolved glucose can diffuse into the cells of the body from the capillaries. Once in the cell glucose can be used in respiration.

Oxygen from the air to cells

When we breathe in oxygen enters the small air sacs, called alveoli, in the lungs. Oxygen diffuses from there into the bloodstream.

Oxygen is not carried in the plasma, but is carried by the red blood cells. These contain a red substance called haemoglobin, which joins onto oxygen and carries it around the body in the blood, then lets it go when necessary. Like glucose, oxygen can diffuse into cells from the capillaries.

Red blood cells carry oxygen around the body

Carbon dioxide from cells to the air

The carbon dioxide produced during respiration diffuses out of the cells and into the blood plasma. The blood carries it to the lungs. It then diffuses across the walls of the alveoli and into the air, ready to be exhaled

The respiratory system

The human respiratory system contains the organs that allow us to get the oxygen we need and to remove the waste carbon dioxide we don’t need. It contains these parts:

  • lungs

  • tubes leading from the lungs to the mouth and nose

  • various structures in the chest that allow air to move in and out of the lungs.


Movements of the ribs, rib muscles and diaphragm allow air into and out of the lungs. Take care – this is called breathing or ventilation, not respiration. When we breathe in, we inhale. When we breathe out, we exhale.

Air passes between the lungs and the outside of the body through the windpipe, called the trachea. The trachea divides into two bronchi, with one bronchus for each lung.

Each bronchus divides further in the lungs into smaller tubes called bronchioles. At the end of each bronchiole, there is a group of tiny air sacs. These air sacs have bulges called alveoli to increase their surface area


We need to get oxygen from the air into the blood, and we need to remove waste carbon dioxide from the blood into the air. Moving gases like this is called gas exchange. The alveoli are adapted to make gas exchange in lungs happen easily and efficiently.

Here are some features of the alveoli that allow this:

  • they give the lungs a really big surface area

  • they have moist, thin walls (just one cell thick)

  • they have a lot of tiny blood vessels called capillaries.

The gases move by diffusion from where they have a high concentration to where they have a low concentration:

  • Oxygen diffuses from the air in the alveoli into the blood.

  • Carbon dioxide diffuses from the blood into the air in the alveoli

Some water vapour is also lost from the surface of the alveoli into the lungs – we can see this condensing when we breathe out on cold days.


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