Transcript Plants and Photosynthesis

Plants and photosynthesis
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What is photosynthesis?
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How do plants grow?
Many years ago, people
thought that plants ate soil
and that made them grow.
But think about it, if that
really happened then there
wouldn’t be any soil left!
Plants actually grow by
making their own food,
not by eating soil.
So where does the stuff
that plants are made of
come from?
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How do plants make their own food?
One of the raw materials that plants need to make food
does come from the soil, the other comes from the air.
What are these two raw materials?
carbon dioxide
(from the air)
water
(from the soil)
Plants use carbon dioxide and water to make their own food
in a chemical reaction. What is the name of this reaction?
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What is the name of the process?
Plants make their food from carbon dioxide and water in
a chemical reaction called photosynthesis.
The food made by photosynthesis is the sugar, glucose.
carbon dioxide
(from the air)
glucose
water
(from the soil)
oxygen
Oxygen gas is also made as a by-product of photosynthesis.
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Where does the energy come from?
Plants need energy for photosynthesis to take place.
The energy for photosynthesis comes from the Sun.
carbon dioxide
(from the air)
glucose
water
(from the soil)
oxygen
Where in a plant does photosynthesis take place?
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Where does photosynthesis happen?
Plant cells in the upper surface of leaves have chloroplasts
which contain the green pigment called chlorophyll.
chlorophyll
carbon dioxide
(from the air)
glucose
water
(from the soil)
oxygen
It is chlorophyll, which absorbs light energy from the
Sun, that enables plants to carry out photosynthesis.
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What is the equation?
Plants make their own food through photosynthesis.
In this chemical reaction, chlorophyll in plant cells absorbs
light energy to change carbon dioxide and water into
glucose and the by-product oxygen.
What is the word equation for photosynthesis?
carbon
dioxide
light energy
+
water
glucose
+
oxygen
chlorophyll
This equation can be read as:
“Carbon dioxide and water, in the presence of light energy
and chlorophyll, react to form glucose and oxygen”.
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What is needed for photosynthesis?
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What is the word equation?
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Photosynthesis and the atmosphere
Before plants the atmosphere
was mainly made up of carbon
dioxide and water vapour.
Gradually carbon dioxide was
converted into oxygen as
photosynthesis in plants and
algae evolved.
The increased level of oxygen
slowly made the planet more
suitable for other forms of life.
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Leaves and glucose
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How are leaves adapted?
Leaves can be thought of as small ‘factories’ that produce
food for plants by photosynthesis.
Leaves are adapted so that
photosynthesis can take place.
Plants need carbon dioxide,
water, sunlight and chlorophyll to
carry out this important process.
What features of leaves make
them suitable for photosynthesis?
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What are the useful features of leaves?
Leaves often have many features that make them suitable
for photosynthesis, including:
 A leaf is broad and flat to
capture lots of sunlight.
 Veins carry water to the leaf and
take food from the leaf to the
rest of the plant. Veins also help
to support the leaf.
 Certain plant cells contain
chloroplasts filled with
chlorophyll.
 Small holes called stomata in
the underside of a leaf allow
gases in and out.
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Inside a leaf
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What do plants use glucose for?
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Testing leaves for starch
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Does light affect photosynthesis?
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Photosynthesis
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Photosynthesis
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Roots and water
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How does water enter a plant?
Water is one of the raw materials
needed for plants to carry out
photosynthesis.
How does water enter a plant?
Water from the soil enters a plant
through the roots.
You can’t normally see them but roots
are a very important part of a plant.
Why are roots branched and spread out through the soil?
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How are roots adapted?
Roots are branched and spread out for
two reasons:
 to absorb water (and mineral salts)
from a large amount of soil
 to anchor the plant in the soil.
Taking a closer look, roots are
covered in root hair cells.
Root hair cells have thin walls
and a large surface area to
help them absorb lots of water.
How are roots adapted to their job?
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Why do plants need water?
Water is needed by plants for photosynthesis but this
important liquid is also used in many other ways:
 to provide dissolved minerals that
keep the plant healthy
 to transport substances around the
plant
 to keep the plant rigid and upright
 to keep the plant cool
 to allow other chemical reactions to
occur in plant cells.
What happens to a plant if it does not get enough water?
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Humans in food chains
Humans eat many different foods and
so are at the top of many food chains.
What are the food chains for the
ingredients in a slice of pizza?
tomato
human
onion
human
wheat
(bread)
human
grass
cow’s milk
(cheese)
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Plants as producers
All food chains start with a producer.
Plants are called producers because they produce the
food that humans and all other consumers depend on.
How do plants make their food?
Plants use light energy to carry out photosynthesis:
carbon
dioxide
light energy
+
water
glucose
+
oxygen
chlorophyll
The glucose produced by plants is converted into starch for
storage or used to make proteins, fats and other substances.
Which parts of plants can be eaten?
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Food from plants
Plants provide food in many different forms – some foods
are the leaf, the stem or the root of a plant, others are the
seed or the fruit.
Which plant parts are these foods from?
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Food from plants
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Other uses of plants
Plant material can be used to make other products, including:

drugs – medicines, such as aspirin, are made from plants.

construction – wood is an important building material for
houses and furniture.

clothing – materials,
such as cotton, are made
from plants. Some dyes
are also made from plant
extracts.

paper – wood pulp is
used to make paper.
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Healthy plant growth
Plants need carbon dioxide and water for photosynthesis,
but they also need small amounts of mineral salts for
healthy growth.
Where do plants get mineral salts from?
Minerals are dissolved in water in the soil and so plants
absorb these nutrients in the water they take in from the soil.
The three main elements in mineral salts are:
 nitrogen (N) – needed for healthy leaves
 phosphorus (P) – needed for healthy roots
 potassium (K) – needed for healthy flowers and fruit.
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Healthy plant growth
What happens when plants use up all the minerals in the
soil, what do they look like?
low
low
low
low
phosphorous potassium magnesium nitrogen
Can you explain why potted plants often look sickly?
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Healthy plant growth
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Fertilizers and plant growth
Plants take in mineral salts from the soil and over time
there are fewer nutrients available in the soil.
Farmers and gardeners often add
manure or chemicals, called fertilizers,
to soil to make sure that their crops get
enough mineral salts.
Manufactured fertilizers help crops to grow
well and so increase the farmer’s crop
yield, but they can be very expensive.
If fertilizers aren’t used responsibly they can
run into rivers, causing water pollution.
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Competition between plants
Organisms that share a
habitat have to compete
with each other for limited
living resources.
A weed is any plant that is
growing in the wrong place.
Weeds are a problem for
farmers as they compete
with the crops for resources
such as light, water, living
space and mineral salts.
How does competition affect the growth of crops?
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Controlling weeds
Weeds compete with crops for living resources and this can
reduce crop yield. What can farmers do to control weeds?
One way farmers can control
weeds is to use chemicals
called herbicides (or
weedkillers).
Weeds are part of the food
web. How might using
herbicides affect other
organisms in the food web?
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Types of herbicides
Non-selective herbicides kill most of the plants that they
are sprayed on. This type of herbicide can be used to clear
large patches of ground.
Selective herbicides only kill specific types of plants.
Many of the weeds that are killed using herbicides are
important sources of food or shelter for consumers like
insects and butterflies.
As the number of weeds
decline, competition between
the consumers increases.
Is this thistle a weed to a farmer?
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Controlling insects
Insects can cause a huge amount of damage to crops. They
are thought of as pests as they compete with humans for food.
One way farmers can control
insects is to use chemicals called
insecticides. Insecticides can kill
the adult insect or prevent insect
larvae hatching.
However, nearly all types of
insecticides can cause damage
to local ecosystems and many
are toxic to humans.
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Alternative farming methods
Organic farming is an alternative farming method. It is
thought that this method may have less impact on the
environment as fewer artificial chemicals are used.
 Artificial fertilizers can be replaced with natural fertilizers
and crop rotation to keep the soil rich in nutrients.
 As an alternative to insecticides
farmers may encourage the insects’
natural predators, such as birds
and other insects, to feed on them.

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Instead of using herbicides weeds are
removed by hand or mechanically.
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The pros and cons of organic farming
There are both advantages and disadvantages of organic
farming.
Using fewer artificial chemicals may decrease the risk of toxic
substances remaining on food and could reduce the impact
farming has on the environment.
However, organic farming
is more labour-intensive and
may result in a lower yield.
This means that organic
farming requires more labour
and space, making organic
food more expensive.
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Organic food
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Which chemical?
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Summary activities
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Glossary
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Anagrams
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Multiple-choice quiz
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