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6.
Food as fuel
All substances from which energy is derived is termed as fuel. Thus food
that we consume is also fuel for our body. This fuel is needed to keep us
alive. The fuel is used for our breathing, for our heart to beat, our brains
to think, digestion, walking, talking and all the activities that we perform,
whether in a state of waking or sleep. The food that we eat is digested or
broken down into its smallest component. The glucose or sugar content in
food is the main fuel for our bodies. This is then absorbed by blood and
taken to various tissues that need the fuel or the energy. The oxygen present in the blood oxidizes the food. This reaction
releases energy along with carbon dioxide and water. The cells in the tissue
utilize the energy and the carbon dioxide is exhaled out. Water finds other
use in the body as a component of the blood itself. Excess water may be
given off as urine.
The equation below shows what happens when glucose is
oxidized.
There is one basic difference between food burnt as
fuel in our bodies and other fuels used in our day to day lives. Food burns
due to respiration. It is a slow process when compared to combustion. In
fact respiration is also known as slow combustion. Although the basic
process of burning by oxidation is same for both respiration and combustion,
respiration occurs inside a living cell and combustion process occurs in the
laboratory.
| The differences between respiration and combustion
are :
|
-
Respiration
is a slow process.
Combustion is relatively a fast process.
-
Respiration
takes place at the
Combustion takes place at a high
ignition temperature.
body temperature.
-
Fuel
for oxidation reaction is food.
Fuel for oxidation reaction is mainly hydrocarbons.
-
The
oxidation reaction is always The oxidation reaction may not be complete
and the
complete
and the reaction products reaction products may cause
pollution due to the
are
CO2, water and energy.
presence of unburned carbon particles and CO.
-
Respiration takes place
inside a Combustion
takes place outside a cell.
biological cell.
-
Respiration
takes place at all times Combustion is not a continuous process.
when the cell is alive.
|
| Animals eat plants and
/ or other animals, which is the
basic source of food or fuel. Thus plants are the most basic source of food.
Plants convert sun’s energy into sugar by the process of photosynthesis.
Photosynthesis takes place in the presence of a catalyst called chlorophyll,
which is the green colour in leaves. The basic photosynthesis reaction is as
follows :
|
Thus the energy from the sun and CO2 from
air gets converted into sugar. The plants release oxygen. The sugar is
stored in the plant’s fruits, flowers, seeds and leaves, which when
consumed release energy within a body.
Photosynthesis and respiration are reactions that are
opposite to each other. Photosynthesis is a process where food or sugar is
formed by absorption of energy and respiration is a process where food or
sugar is broken down and energy is released.
| The differences between photosynthesis and
respiration are :
|
-
Photosynthesis
produces food
Respiration breaks down food
-
In
photosynthesis carbon dioxide
In respiration glucose is broke down into
combines
with water to produce glucose
carbon dioxide and water
-
Photosynthesis
absorbs sun’s energy Respiration releases energy trapped in
food or
glucose
-
Photosynthesis
takes place in the presence
No catalyst is needed for respiration
of a catalyst chlorophyll
-
CO2
is absorbed in photosynthesis
CO2 is released in respiration
-
Oxygen
is released in photosynthesis
Oxygen is absorbed in respiration
-
Photosynthesis
has to take place in sunlight Respiration does not need sunlight
-
Photosynthesis
occurs only during daytime
Respiration occurs at all times during the lifetime
of
the living cell
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7.
Why the fire is
hot
When a fuel is burnt in oxygen, carbon dioxide and water is produced. The bonds
between the hydrocarbons are broken and new bonds are formed. Carbon
from the hydrocarbon reacts with oxygen, forming carbon dioxide.
Hydrogen from the hydrocarbons reacts with oxygen, giving rise to water.
The energy stored between the bonds in the hydrocarbons is broken and new
bonds are formed with oxygen. The bond energy in the carbon dioxide and
water is less than that stored in the hydrocarbon. The combustion reactions
are an exothermic reaction. The
excess bond energy appears as heat energy during the oxidation reaction of
the fuel. Thus combustion of fuels gives rise to heat. To put it in other
words, a fire is hot because heat given out while making new bonds in
products such as carbon dioxide and water, is greater than the amount of heat
used
up in breaking bonds within the fuel molecules and oxygen molecules. |
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