This is a topic that should give pupils a knowledge and understanding
of the structure, functioning and uses of plants. It also takes into
account the concept of the whole organism in its environment and seeks to
strike a balance between the descriptive natural history tradition, to
which modern biology owes so much, and the rigorous, detailed study of
plant structure and physiology. In such an approach, the attitude to
living things is important and sensitivity to the needs of plants can be
encouraged within this topic.
The main focus for attention within the topic is on flowering
plants. Some aspects of the topic have links with other subjects in the
curriculum, e.g. uses and economic importance of plants and their
products. It is hoped that this will encourage an awareness of an
interest in plants that may be further developed through leisure pursuits.
Introducing
Plants
In this sub-topic, plant variety should be demonstrated
and related to practical applications in various industries. Reduced
variety could result in a great loss of a natural resource.
Plants are essential for the well being of our world ecosystem:
Plants are the basis of all food chains.
Plants are the source of the world’s oxygen.
Plants make a significant contribution to weather.
Plants prevent erosion.
Plants provide us with many medicines.
There are many types of plants in the world. There are many
advantages to this:
There are new medicines waiting to be discovered.
New crops waiting to be discovered also.
Plants contain useful genes that can be combined with current crops
to improve them.
It is aesthetically pleasant to see a wide variety of
plants – compare native woodland to Forestry Commission forests.
There are some very specialised uses of crops e.g.
Malting Barley
The barley is allowed to germinate for a few days
In this process the starch in the grain is converted to maltose
sugar
After malting the sugar can be extracted and yeast can then
convert the sugars to alcohol by the process of fermentation.
The relationship between structure and function are emphasised rather
than details of plant anatomy for its own sake. The important process of
sexual reproduction is described in the context of seed production and
propagation of new varieties. Employing forms of artificial propagation
can preserve important strains of plants; natural asexual reproduction is
also studied.
Flowering plants divide into two main classes – the dicotyledons
and the monocotyledons.
The dicotyledons have two “seed leaves” in the seed
Swollen with starch they provide food for the dormant and
germinating embryo.
The structure of monocotyledons is not considered in this
course.
The structure of the dicotyledonous seed is shown opposite –
be able to state the structure and function of:
The seed coat
The food store
The embryo plant
Germination is the process that follows dormancy:
Dormancy is the period where the seed is not growing in any
way.
Germination is when the seed begins to grow.
Several things can affect germination
The plant requires:
Oxygen for respiration (photosynthesis requires leaves)
A suitable temperature
Water
All these factors can limit germination when in short supply
When you count successful germination over a range of
temperatures you get a bell shaped curve like that shown opposite.
This suggests that enzymes are involved in germination
Sexual reproduction in plants
Pollination
Flowers are produced to attract pollinators.
Pollination is the process by which pollen (the male gamete) is
transferred to the stigma of a suitable plant.
Pollinators are insects, birds or animals which carry pollen from
plant to plant.
Plants that use insects to bring about pollination are called
insect pollinated plants.
Insect Pollinated Plants
The structure of an insect pollinated flower is shown opposite:
The parts are:
Sepal, protects the developing bud.
The brightly coloured petals attract insects.
The stamen is the male part of the plant, at the end the anther produces the pollen.
As the insect reaches into the flower to get the sweet
nectar pollen is brushed over its body.
The pollen is spiky and stays on the insect.
As the insect tries to get the nectar from the next plant
some pollen is transferred to the sticky stigma.
The stigma is part of the female part of the flower, the
pollen must be placed on this to fertilise the ovum.
The ovary, contains the ovum.
The ovum is the egg cell.
The nectary produces sweet, sticky nectar to attract
insects.
Wind pollinated Plants
The structure of a wind pollinated plant is shown opposite:
There is no need for petals, the stamen and stigma are
protected by hard bracts.
The anthers hang out into the airflow.
The pollen is light and often has air bags.
The stigma is sticky and hangs out into the wind to increase
its chances of collecting pollen.
The stigma is feathery to offer the largest cross sectional
area to the wind to catch the pollen.
There are a advantages and disadvantages of the two forms of pollination:
Advantages of insect pollination:
The habits of insects make it very likely that the pollen will
reach a suitable plant.
Less pollen needs to be made.
Disadvantages of insect pollination:
Petals have to be grown which uses energy and resources.
Nectar also has to be made which uses energy and resources.
Advantages of wind pollination:
There is no need to make petals or nectar.
This saves a lot of energy.
Disadvantages of wind pollination:
It is far less likely that the pollen will meet a suitable
stigma.
Therefore pollen has to be made in vast numbers
Fertilisation
Fertilisation occurs after pollination:
It is necessary for the nucleus from the male gamete (the pollen) to meet and fuse with nucleus from the female gamete (the ovum)
To do this the pollen grows a pollen tube which grows down the stigma and style and finds the ovum
The male nucleus travels down this tube and fertilises the ovum.
Fruit formation
After fertilisation the seed and sometimes fruit are formed.
The ovule forms the seeds
The ovary wall often thickens and produces sugars to become
the fruit.
Dispersal
The ground on which the unaided seed falls is often already in use
by the parent plant.
Many plants disperse their seeds
In dispersal the plant distributes its seeds a distance away from
itself
There are several methods plants use to disperse their seeds.
Wind dispersal
The seed has wings like sycamore or parachutes like dandelions
The wind catches these and carries the seeds long distances.
Animal internal
The animal is attracted by the fruit
The seeds have waxy coats to enable them to pass harmlessly through
the animals alimentary canal
The seed is deposited with useful nutrients.
Examples are apples, blackberries and strawberries.
Animal external
The seed has a hooked surface like burdock or “sticky willies”
The seed catches in the animal’s coat
As the hooks dry they become brittle and break off
The seed falls to the ground.
Advantage of sexual
reproduction to the plant
There is a lot of variability within the species due to the
exchange of genetic information, this helps the plants adapt to new
conditions.*see
footnote
Asexual reproduction in plants
Asexual reproduction in plants can occur naturally or as a
consequence of propagation by growers:
Natural asexual reproduction
Here new plants are produced without the need for
fertilisation
This occurs either by runners or tubers
A runner is an aboveground horizontal stem.
Where it contacts the ground a new plant is produced
Spider plants and strawberries reproduce asexually by
runners.
A tuber is a belowground horizontal, swollen stem.
New stems arise from “eyes” on the tuber.
Potatoes are tubers.
Plants produced by asexual reproduction are clones:
They are genetically identical to the parent.
Advantages of asexual reproduction to the plant:
Reproducing asexually produces many new plants very quickly.
There in no need to produce flowers or gametes saving energy.
There is no risk of failure to pollinate or fertilise.
Artificial propagation
There are several methods growers can use to produce large numbers
of plants very quickly:
Cuttings
Many small pieces of the plant is taken.
With care an entire new plant can be grown from each piece.
Pelargonium commonly called the household geraniums is a
good plant to propagate by cuttings.
Grafting
Here many buds from a desirable plant is made to grow on the
roots of a more vigorous and common relative.
So many plants can be derived from a single good parent.
This sub-topic examines plant requirements and how these are obtained
and used in making food. In the examination of leaves, the emphasis is on
gas exchange surfaces.
Many of the structures in the plant are there to assist in
photosynthesis.
Photosynthesis is:
A chemical process.
Which changes light energy into chemical energy.
Occurring in the chloroplasts of plant cells.
Carbon dioxide and water are needed.
Sugar and oxygen are produced.
The word equation for photosynthesis is:
CO2 + H2O + sunlight Sugar +
O2.
Carbon dioxideandwaterusing the energy insunlightis converted tosugarandoxygen
The sugar can then be used for many purposes:
As an energy source for other chemical reactions.
As a building block to make other chemicals:
Starch for energy storage.
Cellulose for cell walls.
As a base for making other chemicals like fats and proteins.
From the word equation you can see that the following will limit how
fast photosynthesis can operate:
The intensity of the light.
The mass of CO2 available.
The volume of water available.
The temperature (this is an enzyme controlled reaction)
If any of these is in short supply the rate of photosynthesis will
be reduced.
The leaf and photosynthesis
The leaf is the main site of photosynthesis.
Below is a diagrammatic representation of a transverse cross section of a leaf:
The palisade mesophyll is the main site of photosynthesis and
is packed with chloroplasts.
The spongy mesophyll is the gas exchange surface
This is loosely packed to give a large surface area.
It has a moist surface like the surface of the lungs for
carbon dioxide to dissolve in.
It is close to the palisade mesophyll for rapid diffusion of
the gasses.
The stoma (pl stomata) allows gasses into and out of
the leaf
Since the spongy mesophyll is moist this means that the
plant loses water vapour out of the stomata.
The stomata can open and close to control water loss.
They also close at night when photosynthesis is not
occurring.
The xylem carries water from the roots to the leaves.
The phloem carries the sugar from photosynthesis to the rest
of the plant.
Transport systems, xylem and phloem
Plants need a transport system to get water and minerals from the soil
up to the leaves and to move sugar from photosynthesis around the plant.
Longitudinal
The transport system consists of bundles of tubes running up and down
the plant.
Xylem vessels carry water from the soil up to the leaves
where the water is needed for photosynthesis:
Xylem vessels are empty, tube shaped cells.
Their cytoplasm has been removed by the plant.
Their walls are strengthened and thickened with
lignin.
The lignin-strengthened tubes also help support the plant by
giving rigidity to the xylem.
Minerals from the soil are also carried in the Xylem:
These are needed by the plant in many of its chemical
reactions.
Phloem (sieve) tubes carry sugar around the plant.
Phloem Cells are alive
They have cytoplasm.
They have (sieve) plates at their ends with pores to let the
sugar through:
They have no nucleus, the cytoplasm is controlled by the
companion cell nucleus.
The companion cell nuclei tend to be large because of
this.
The vacuoles of the tube are joined and sugary sap can
flow along them.
alcohol + carbon dioxide + energy
