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Decay – Revision Pack (B4)
Decay:
Detritivores are organisms that feed on dead and decaying material (detritus) –
examples include woodlice, maggots and earthworms.
Detritivores increase the rate of decay by breaking up the detritus. This increases the
surface area for further microbial breakdown.
Rate of decay can also be increased by: increasing the temperature, amount of
oxygen and water.
Raising the temperature to 37oC will increase the rate of respiration for bacteria OR
raising it to 25oC will increase the rate of respiration for fungi.
If you increase the amount of oxygen, bacteria will use aerobic respiration to grow
and reproduce faster.
Increasing the amount of water will allow for material to be digested and absorbed
more easily and increase the growth and reproduction of fungi and bacteria.
A saprophyte is an organism that also feeds on dead and decaying material – an
example is fungus.
Saprophytes, like fungus, break material down in a few simple steps:
STEP 1 – The saprophyte produce enzymes
STEP 2 – These enzymes digest food outside their cells
STEP 3 – The saprophyte then reabsorbs the simple soluble substances
This type of digestion is called extracellular digestion.
Preserving Food:
In canning, the foods are heated to kill any
bacteria present and then sealed with a vacuum
to prevent the entry of oxygen and any microbes
like bacteria.
Cooling foods (in fridges for
example) slows down bacterial
and fungal growth and
reproduction.
Freezing foods (in freezers for
example) kills some bacteria and
fungi and slow their growth and
reproduction.
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Drying foods removes water so
bacteria cannot feed and
grow.
Adding vinegar will produce
very acidic conditions, thus
killing most bacteria and fungi.
Adding salt or sugar will kill some
bacteria and fungi – this is because the
high osmotic concentration will remove
water from them (meaning that they
cannot feed and grow).
Additional NOTES:
Decay will provide minerals for plants; for more information on this, see the ‘Plants
Need Minerals’ Revision Pack.
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Diffusion:
Diffusion is the movement of particles in a liquid or gas from an area of high
concentration to an area of low concentration. It happens because of the random
movement of individual particles.
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Diffusion explains how molecules like carbon dioxide, water and oxygen can get
into and out of cells via the cell membrane. For example, if a plant is using up
carbon dioxide then the concentration is low, so carbon dioxide will enter via
diffusion.
Leaves are adapted to increase the rate of diffusion of CO2 and O2 by:
-
Having a large surface area
Having stomata (pores) which are spaced out
Having gaps between spongy mesophyll cells
The rate of diffusion can be increases by:
-
Having a short distance for molecules to travel
Having a greater surface area for the molecules to diffuse into or out of
Having a steeper concentration gradient (see below)
In this instance, the concentration goes from an
area of higher concentration, to an area of
lower concentration.
To increase the steepness of this gradient, the
difference between the higher concentration
and lower concentration must be greater.
Osmosis:
Osmosis is a type of diffusion. It
needs a semi-permeable
membrane that allows small
molecules like water through, but
doesn’t permit larger molecules
like sugar through.
Osmosis is the movement of
water across a semi-permeable
membrane from an area of high
water concentration (dilute) to
an area of low concentration
(concentrated).
Osmosis happens because of the random movement of water molecules, which are not
restricted (like sugar is) by a semi-permeable membrane. The movement of water molecules
Water
in Cells:
will
be from
an area where there is lots to an area where there is few.
When lots of water enters a
cell, the
pressure
If we know the concentration of water inside and outside a cell, we plant
can predict
the
pushing
on
the
cell wall is
movement of those water molecules.
high. This is a high turgor
pressure, which supports
the cell – stopping it, and
the whole plant, from
collapsing.
When too much water
leaves a plant then it has a
low turgor pressure and the
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wall)
A plant cell that is full of water is
called turgid.
When a plant cell loses water we
call the cell flaccid.
NOTE – when a cell is plasmolysed,
the cytoplasm is pulled away from
the wall.
Animal cells react in the same way as plant cells do towards water loss and water
intake. When too much water is lost, animal cells will shrink and collapse. When too
much water enters an animal cell, the cell will also swell up.
Unlike plant cells, animal cells (like red blood
cells) do not have a supporting cell wall. This
means that when too much water enters an
animal cell, they swell up and burst (image 3)
– this is called lysis.
When too much water leaves an animal cell,
it shrinks into a scalloped shape (image 1) –
this is called crenation.
A normal animal cell is actually more flaccid than it is turgid (image 2).
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Distribution of Organisms:
An ecosystem is made up of all of the plants and animals that live there and their
surroundings.
A habitat is simply where an animal or plant lives.
The community, just like in humans, is made up of all of the different plants and
animals living in a habitat. The number of a particular plant or animal in that habitat
is called its population.
If London, local community is diverse, it means it houses a variety of different people
from all walks of life. In natural ecosystems, like lakes or woodland, there is a variety
of plants and animals living there – this is known as biodiversity. Most artificial
(unnatural or man-made) ecosystems have poor biodiversity as they house one or
two types of plant or animal.
In artificial ecosystems, like fish farms, humans protect ONLY one species and
generally take any other organisms out of its habitat that could:
-
Complete with it
(and in doing so) Reduce the yield of that organism
A transect line is used as to map the distribution of
organisms in a specific habitat.
STEP 1 – a long piece of string is laid out across an
area, like a sea shore (as seen to the left)
STEP 2 – At regular intervals along the line,
quadrats (square frames) are placed on the string
STEP 3 - The amount of animals can be counted
that are present within the area, and the
(average) percentage cover can be calculated
for plants
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The information collected can be put in a kite
diagram (right). This highlights the distribution of
organisms. The larger the surface area of each
kite, the more organisms there were in that
area.
Zonation in the habitat is shown in the diagram
to the right. This zonation is not caused by
anything biological but more abiotic (or
physical) factors like availability of water,
exposure and pH.
For example, the mosses in the diagram to the
right can survive in a variety of areas and can
An organism like(dependent
a fern lives away
from the
Organisms
ecosystems
are self-supporting and interdependent
on
withstand
dryinand
poor conditions.
footpath
and
thrive
in
wetter
and
more
each other for survival). In food chains, all animals depend on plants both directly
protected
areas
from the
footpath.
and indirectly – energy is transferred from one organism
to the
next.away
The gases
in the
air are balanced because of photosynthesis (which removes carbon dioxide and
given off oxygen) and respiration (which removes oxygen and gives off carbon
dioxide). The only thing that ecosystems need from outside is the sun as its energy
source.
Population size can be estimated by obtaining data from a small
Population Size:
sample and scaling up. For example, quadrats (see left) are often
used to determine a larger value of organisms in a specific area by
scaling up.
E.g. Callum has a very large front garden. He wants to find out how
many daffodils are in his garden. Firstly, he uses a suitable sample of
quadrats (perhaps 10) and places these at random areas around
his garden by throwing them with his eyes closed. Next he
measures the amount of daffodils in each of these 1m2 quadrats.
There are 3, 6, 4, 2, 4, 5, 3, 4, 3 and 5 daffodils in each of these
quadrats respectively. The mean is therefore 3.4 or 3 daffodils per
1m2. His garden is 30m2 so 3 x 30 = 90. This means there’s
approximately 90 daffodils in his garden.
Capturing animals can be done in a number of ways. For example, the first picture above (from left to
right) is called a pitfall trap – this is where there’s a small container buried in the ground collects small
insects and animals, these are then counted and identified. The second is a pooter, whereby insects
can be sucked up via a collecting tube into the chamber, counted and identified. The third is simply a
net – this is used to collect air-borne insects like butterflies and animals as well, like fish. The fourth is the
capture-recapture method; in which you follow the following steps:
STEP 1 – You capture insects or animals using the most appropriate technique
STEP 2 – You then count how many there are and put a dot of paint on them (as seen in the image)
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Additional NOTES:
The bigger quadrats you use or the more samples you take make the estimations
more accurate.
The capture-recapture method estimate may be slightly unreliable because:
-
The method assumes that no new plants/animals have been born and that
no plants/insects have died in that area in between the two samples
The markings may affect the survival of the insect when it is released,
meaning it will NOT return
Identical sampling methods MUST be used for both the original and second sample.
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Pesticides:
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Pesticides are used to kill harmful insects or organisms to protect the well-being of
other plants or animals. Examples of pesticides are herbicides, fungicides and
insecticides – these all have disadvantages, for example:
-
They enter and accumulate in food chains, causing lethal doses to predators
They can harm other organism living nearby which are NOT pests
Some take a long time to break down and become harmless
Organic Farming:
Organic farming does NOT use any artificial
fertilisers or pesticides. Instead, organic farmers
use manure and compost. These farmers will
also use crop rotation (where they change
what they grow, on the same piece of land,
every few months) to avoid a build-up of soil
pests. Nitrogen-fixing crops are used in these
rotations (so put nitrogen into the soil). These
farmers will also vary their planting times to
prolong crop time and avoid coincide with
times when certain insect pests are not in
abundance.
Organic farming does avoid expensive artificial fertilisers and pesticides and the
disadvantages that come with them, but the crops are often very small and the
produce (what’s made) is often very expensive. Many people believe that organic
crops taste better and are healthier – this is a false statement.
Biological Control:
Biological control uses living organisms to control pests; it acts as an alternative to
pesticides. For example, many gardeners will introduce ladybirds to their gardens to
kill off any aphids which feed on and damage plants.
Biological control avoids the use of artificial insecticides. As living organisms are
used, they generally do not need to be replaced.
However many attempts at using biological control have failed miserably. This is
because the new (introduced) species often eat other useful species, rather than
just the pest. Some show a rapid increase in their population and they then become
pests and spread out into other areas!
Introducing a new species into a habitat to kill another species can affect the food
sources of other organisms in a food web, causing unexpected results.
Hydroponics and Intensive Farming:
Intensive farming makes use of artificial pesticides and fertilisers. Intensive farming is
very efficient in producing high crop yield and does this cheaply!
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However, this
method raises
questions about
animal cruelty
because animals are
kept in very small
spaces (left).
There are also further concerns about the effects of extensively using chemicals on
soil structure and other organisms (top right).
However, intensive farming improves the efficiency of energy transfer in food chains
involving humans – they do this by removing or reducing competing organisms such
as animal pests and weeds. Furthermore, by using sheds or barns in battery farming,
the animals use less energy moving and keeping warm and more energy on growth
(in animals like cattle) or egg production (in birds like hens).
Plants can be grown without
soil using hydroponics. This
system uses a regularly
recycled flow of aerated
water (water with air)
containing minerals. The
process usually takes place in
glasshouses or polytunnels.
Tomatoes are commonly
grown using a hydroponic
system.
Hydroponics is a type of intensive farming that is generally used in areas where there
is little rainfall or barren soil.
Being soil free means that hydroponic farmers have better control over mineral
levels and disease – they can manipulate the mineral levels to increase productivity.
Also, many plants can be grown in one (relatively) small space. As there is nothing to
hold plants in place in hydroponics, artificial fertilisers are used.
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Leaf Structure:
Sunlight will enter here
The wax cuticle
layer is there to
protect the leaf
without blocking
out sunlight
The above diagram shows the specialised cells in a green leaf – you should know
how to label a diagram like the one above. These cells are adapted to do certain
jobs:
Cell Adaption
The outer epidermis is transparent
because it lacks chloroplast (and as
such chlorophyll which is a green
pigment)
Purpose
They allow light to reach the palisade
layer; they do not act as an obstacle to
light
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Leaf Adaption
Broad
Thin
Purpose
To maximise surface area so they can
get as much light as possible
Contain a variety of pigments (e.g.
chlorophyll a, b, carotene etc.)
1) So that gases (like CO2) can
diffuse through easily
2) So that light can get to ALL cells
This allows the plant to absorb light from
a broad range of the light spectrum
They have loads of vascular bundles (or
veins)
This allows support and transport of
chemicals like water and glucose
Specialist guard cells
(see below)
These control the opening and closing
of the stomata, thus regulating the flow
of carbon dioxide, oxygen and water
loss
Because this is where most of the light
from the sun will be received, it allows
the plant to absorb all of this light
1) This allows the diffusion of gases
between the cells and the
atmosphere to happen
2) It also creates a large surface
area to volume ratio – this means
that large amounts of gases can
enter and exit the cells
Upper palisade layer contains most of
the leafs chloroplasts
The spongy mesophyll cells are loosely
packed (there’s lots of air space)
Leaf adaptations for Photosynthesis:
Leaves are adapted so that photosynthesis is VERY efficient.
When the stoma is open, the
guard cells are full of water
and are turgid.
When the stoma is closed, the
guard cells lose water and
become flaccid. They would
normally only close when it is
dark when no carbon dioxide
is needed for photosynthesis.
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NOTE – Through having a variety of pigments (these being: chlorophyll a, chlorophyll
b, carotene and xanthophylls), the plants cells can maximise the use of the suns
energy. Each of these pigments absorbs light of different wavelengths.
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The Chemistry behind Photosynthesis:
+
Chlorophyll
This is taken from
the air
Taken from the
soil
(GLUCOSE) – Stored
as starch in the leaf
Released
back into the
environment
Simple sugars like glucose can be used in a number of ways; for example:




In respiration, releasing energy
Can be converted into cellulose to make cell walls
Can be used to make proteins for growth and repair
Can be converted into starch, fats and oils for storage
Starch is insoluble so it is used for storage. Glucose can affect the water
concentration of cells and cause osmosis; starch does NOT do this and doesn’t
move from where it is being stored.
Photosynthesis happens in a few simple steps:
STEP 1 – Water (H20) is split up
STEP 2 – This releases oxygen gas and hydrogen ions
STEP 3 – Carbon dioxide (CO2) combines with the hydrogen ions forming glucose
(and water)
The History of Photosynthesis:
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Many Greek scientists just assumed that plants took
ALL nutrients and minerals out of the soil and this
helped them to grow and gain mass.
A man called Van Helmont (pictured above) planted a willow tree with 90kg of soil.
He let
it grow
andscientist
addedwho
water
regularly but
DIDunderstanding
NOT change the
Priestley
was
another
conributed
to the
of soil. After 5 years,
the
willow
tree
had
increased
in
mass
by
54kg
and
there
was
basically
photosynthesis. His experiment showed that plants must produce oxygen. the same
amount of soil. Van Helmont concluded that the growth couldn’t just be due to the
uptake of soil minerals – he thought that it was due to the water alone!
A more modern experiment was conducted using a green alga (plant) and an
isotope of oxygen (O18). This formed part of a water molecule. The experiment
showed that the light energy is used to split up the water, rather than the carbon
dioxide. The oxygen gas made was O18 while the oxygen present in glucose was
normal oxygen (O16). An isotope is a different form of a certain element.
The Rate of Photosynthesis and Limiting Factors:
Generally, three things can increase the rate of photosynthesis; these are: more
carbon dioxide, more light and a higher temperature (increases enzyme action).
Photosynthesis only happens in day time because it needs light. Respiration however
continues to happen at all times – this is because plants are living organisms; this
means they are releasing energy at all times. REMEMBER – During respiration takes in
oxygen and releases carbon dioxide.
During the day (in light), photosynthesis takes place – this is basically the same gas
exchange as respiration but in reverse. Photosynthesis takes in carbon dioxide and
releases oxygen. By comparison, the rate of gas exchange is a lot higher for
photosynthesis than it is for respiration. Respiration is only really noticed at night.
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Here, light intensity is
the limiting factor
Since photosynthesis
Past Papers: depends on light intensity, carbon dioxide concentration and temperature, if
one of these is lacking it limits the rate at which photosynthesis can take place. When one factor limits
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the rate,
we call it the limiting factor. For example, for the light intensity graph, when it begins to
plateau (level off) it means that the rate is being limited by either the temperature or the CO2 conc.
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Uses of Minerals:
Elements from the soil minerals are used to make useful compounds:
Mineral
Nitrogen (N) contained in nitrates
Phosphorous (P) contained in
phosphates
Potassium (K) compounds
Magnesium (Mg) compounds
Why it’s needed
It’s used to make amino acids, which
combine to make a variety of proteins –
this is used for cell growth
It’s used to make the cell’s DNA which
contains its genetic code and cell
membranes
It’s used to help enzyme action in
photosynthesis and respiration (enzymes
speed up chemical reactions)
It’s used to make chlorophyll which is
essential in photosynthesis
Mineral Deficiency:
The lack of specific minerals has specific symptoms for plants:
Mineral
Deficiency Symptoms
Result
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Nitrate Deficiency
Poor plant growth and
yellow leaves
Phosphate Deficiency
Poor root growth;
stunted plant and
discoloured (purple)
leaves
Potassium Compounds
Deficiency
Poor flower and root
growth; yellowed leaves
with brown spots
(discoloured leaves)
Magnesium Compounds
Deficiency
Yellow leaves (especially
on the lower leaves)
Mineral uptake:
Minerals are normally present in soil at very low concentrations; they normally move
around the soil in solution.
Minerals are taken up by root hair cells via active transport – they CANNOT be
transferred via osmosis or diffusion (which are both passive transport).
Systems of carriers transport the minerals
across the cell membrane inside the cell.
Active immunity allows minerals to enter the
root hair cells.
The uptake of these minerals goes AGAINST
the concentration gradient – we expect
materials to travel from an area of high
concentration to an area of low
concentration; but in this instance it goes from
a low concentration to a high concentration.
(See below)
Energy is required for active transport; this
energy comes from respiration.
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The energy allows us to go
against the concentration
gradient.
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Xylem and Phloem Cells:
Xylem and phloem are made
up of specialist plant cells. Both
types of tissue are continuous
from the root, through the stem
to the leaf. Both xylem and
phloem form vascular bundles
in broad-leaved plants.
-
Xylem Cells
Carry water and minerals from
the roots to the leaves – and are
therefore involved in transpiration
Xylem cells are called vessels.
These cells are dead, and
therefore do not have a living
cytoplasm, but have a hollow
lumen instead
-
-
Phloem Cells
Carry food substances (like
sugar) up and down stems to
growing and storage tissues – this
transporting of food is called
translocation
Phloem cells are living cells, and
are arranged in columns
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-
Their cellulose walls have an
extra thickening of lignin which
gives the xylem great strength
and support
Transpiration:
Transpiration is the evaporation and diffusion of water from inside leaves. This loss of
water from the leaves helps to create a continuous flow of water from the roots to
the leaves via the xylem cells.
Root hairs come off of root hair cells and produce a large surface area for water
uptake via osmosis in the soil.
Transpiration ensures that plants have water for cooling (through evaporation),
photosynthesis and for transport of minerals. They also support cells’ turgor pressure.
The structure of the leaf is adapted to prevent too much water loss, which could
cause the plant to wilt (or go limp). Water loss is reduced by having waxy cuticles
which cover the outer epidermal cells. Furthermore, the stomatal openings are
situated on the shaded lower surface.
Plant leaves are adapted for
efficient photosynthesis by having
the stoma for the entry and exit of
gases. The spongy mesophyll layers
(above the stoma) are also
covered with a film of water in
which gases can be dissolved. This
water can therefore readily escape
via the stomata.
The stoma will generally close when
it is dark (when no CO2 is needed
for photosynthesis.
The rate of transpiration can be increased in a number of ways:
Way to increase the rate of transpiration
Increase the light intensity
Increase the temperature
Increase air movement
Decrease the humidity (the amount of
water vapour in the atmosphere)
How it increases the rate of transpiration
Results in the stomata being open
Causes an increase in the evaporation
of water
Blows away air that contains a lot of
evaporated water
Allows more water to evaporate
The structure of the leaf is also adapted to reduce water loss. Its guard cells are able
to change the size of the stomatal openings. The guard cells contain chloroplasts, so
photosynthesis (being in the presence of water and light) will produce sugars,
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increasing the turgor pressure of the guard cells and swelling them up. Due to
varying thickness of their walls, the guard cells curve, and as such open the stoma,
allowing gases in and out.
Other ways a leaf reduces the amount of water loss is through having less or smaller
stomata.
As one water molecule evaporates, it pulls on
a column of water molecules upwards from
the root of the plant. This is called the
transpiration stream – the water goes against
gravity!
REMEMBER – the water first enters in the root
hair cell (see image below) via osmosis.
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The water
concentration in
the root hair cell
is low. The
concentration in
the soil is high. So
water diffuses in
via osmosis.
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