Coordinated Science
Reading: Photosynthesis and Cellular Respiration
Green plants absorb light energy using chlorophyll in their leaves. They use it to react
carbon dioxide with water to make a sugar called glucose. The glucose is used in
respiration, or converted into starch and stored. Oxygen is produced as a by-product.
This process is called photosynthesis. Temperature, carbon dioxide concentration and
light intensity are factors that can limit the rate of photosynthesis.
Photosynthesis summary
Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first
step towards making food - not just for plants but ultimately every animal on the planet.
During photosynthesis:



Light energy is absorbed by chlorophyll, a green substance found in chloroplasts in some
plant cells and algae
Absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil)
into a sugar called glucose
Oxygen is released as a by-product
This equation summarizes what happens in photosynthesis:
Some glucose is used for respiration, while some is converted into insoluble starch for storage.
The stored starch can later be turned back into glucose and used in respiration.
Factors limiting photosynthesis
Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration
and temperature.
Light intensity
Without enough light, a plant cannot photosynthesize very
quickly, even if there is plenty of water and carbon dioxide.
Increasing the light intensity will boost the speed of
photosynthesis.
1
Coordinated Science
Reading: Photosynthesis and Cellular Respiration
Carbon dioxide concentration
Sometimes photosynthesis is limited by the
concentration of carbon dioxide in the air. Even if
there is plenty of light, a plant cannot
photosynthesize if there is insufficient carbon dioxide.
Temperature
If it gets too cold, the rate of photosynthesis will
decrease. Plants cannot photosynthesize if it gets
too hot.
If you plot the rate of photosynthesis against the levels of these three limiting factors, you get
graphs like the ones above.
Economics of greenhouses
Farmers can use their knowledge of factors limiting the rate of photosynthesis to increase crop
yields.
This is particularly true in greenhouses, where the conditions are more easily controlled than in
the open air outside:



The use of artificial light allows photosynthesis to continue beyond daylight hours. Bright lights
also provide a higher-than-normal light intensity.
The use of artificial heating allows photosynthesis to continue at an increased rate.
The use of additional carbon dioxide released into the atmosphere inside the greenhouse also
allows photosynthesis to continue at an increased rate.
However, the additional cost of providing extra lighting, heat and carbon dioxide has to be
weighed against the increased crop yield and the extra income it will provide. The cost of should
not exceed the additional income it generates for the farmer.
In practice, the farmer will need to find the optimum growing conditions for the crop, given
the costs of providing extra lighting, heat and carbon dioxide.
Paraffin lamps have traditionally been used in greenhouses. Their use increases the rate of
photosynthesis because as well as the light generated from the lamps, the burning paraffin
produces heat and carbon dioxide too.
2
Coordinated Science
Reading: Photosynthesis and Cellular Respiration
Storage and use of glucose
The glucose produced in photosynthesis may be used in various ways by plants and algae.
Storage
Glucose is needed by cells for respiration. However, it is not produced at night when it is too
dark for photosynthesis to happen. Plants and algae store glucose as insoluble products. These
include:


Starch
Fats and oils
Use
Some glucose is used for respiration to release energy. Some is used to produce:


Cellulose - which strengthens the cell wall
Proteins - such as enzymes and chlorophyll
Plants also need nitrates to make proteins. These are absorbed from the soil as nitrate ions.
Respiration releases energy for use in life processes. There are two types of
respiration, aerobic (uses oxygen) and anaerobic (does not use oxygen). Respiration
can produce useful products that have uses in a range of industries.
Energy from respiration
Glucose is produced during photosynthesis. This energy is released from cells by a series of
chemical reactions. This process is called respiration. Energy from respiration is used in reactions
that produce different molecules. Examples of these molecules include starch and cellulose,
which are polymers of glucose required by plant cells.
Aerobic respiration
Respiration is not the same thing as breathing. Breathing is more properly called ventilation.
Instead, respiration is a chemical process in which energy is released from food substances,
such as glucose - a sugar.
Aerobic respiration needs oxygen to work. Most of the chemical reactions involved in the
process happen in tiny objects inside the cell cytoplasm, called mitochondria.
This is the equation for aerobic respiration:
glucose + oxygen → carbon dioxide + water (+ energy)
C6H12O6 + 6O2 → 6CO2 + 6H2O
The energy released by respiration is used to make large molecules from smaller ones. In plants,
for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids
can then join together to make proteins. The energy is also used:
3
Coordinated Science
Reading: Photosynthesis and Cellular Respiration


To allow muscles to contract in animals
To maintain a constant body temperature in birds and mammals
Anaerobic respiration
Anaerobic respiration is a type of respiration that does not use oxygen. It is used when there is
not enough oxygen for aerobic respiration. It can be summarized by the following equation:
glucose → lactic acid (+ energy released)
This type of respiration may be used when, for example, an animal is being chased by a
predator.
Anaerobic respiration also takes place in plants and some microbial cells in the presence of little
or no oxygen. Examples of this include the roots of plants in waterlogged soils and bacteria in
puncture wounds.
Anaerobic respiration in plant cells and some microorganisms (such as yeast) produces ethanol
and carbon dioxide, as opposed to lactic acid. It can be summarized by the following equation:
glucose → ethanol + carbon dioxide (+ energy released)
Aerobic respiration releases more energy per glucose molecule than anaerobic respiration.
Useful products from respiration
Anaerobic respiration of microorganisms such as yeast can lead to the production of useful
commodities. During anaerobic respiration sugars are converted into ethanol. This is called
fermentation.
When fermentation is performed on a larger scale using yeast, water, sugar and some other
nutrients bioethanol can be made that can be used as a fuel.
Sustainability
Sustainability is the idea of using resources to meet the needs of the present without damaging
the Earth or using up resources people might need in the future.
Biofuel is seen as a renewable resource that would seem to be more sustainable. However, there
is more to it. The large areas of land used to grow crops on for biofuel could be used to grow
food. As well as this, trees can be cut down to make space to grow crops for biofuel.
Biogas
Biogas is a fuel manufactured using animal manure (or sometimes human waste). Biodigesters
are used to convert the manure into biogas. Bacteria inside the biodigester break down parts of
the manure and produce methane gas. This gas can then be used as a fuel to run generators
and heat buildings. This method of fuel production is more commonly used in developing
countries.
From:
http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/photosynthesis/photosynthesisrev1.shtml
and http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_21c/life_processes/energyrev1.shtml
4