Life On Earth – The Main Source Of Energy
The ultimate source
of nearly all energy for life
on Earth is the Sun.
The energy that powers our
muscles, allows birds to fly,
and drives the many
chemical reactions that are
characteristic of living cells,
originates from the Sun.
The Nature of Photosynthesis
The light energy from the sun must first be transformed
into chemical energy before it can be used by living organisms.
ENERGY
FROM
THE SUN
The process that transforms light energy into chemical energy and
uses this to manufacture organic food is called photosynthesis.
Autotrophs and Heterotrophs
The living organisms that inhabit our planet can be
classed as either autotrophs or heterotrophs.
Autotrophs are organisms that are capable of synthesising their
own complex organic food molecules from simpler inorganic ones.
The term autotroph means “self-feeding” and organisms in this
group make use of an external, non-living supply of energy to
drive their “self-feeding” way of life.
The vast majority of autotrophs harness the energy of sunlight to
manufacture their own food during the process of photosynthesis.
These are the photoautotrophs
ENERGY
FROM
THE SUN
Photoautotrophs (1)
Photoautotrophs capture the sun’s energy and use it to
convert simple inorganic molecules, such as carbon dioxide
and water, into complex, energy-rich organic food.
The various species of green plants
in this woodland are photoautotrophs
and manufacture their own food by the
process of photosynthesis:
6CO2 + 6H2O
Carbon dioxide + Water
Light
Light Energy
C6H12O6 + 6O2
Glucose + Oxygen
Energy
Photoautotrophs (2)
There is a huge diversity of photoautotrophs in the many ecosystems.
Diatoms – microscopic algae
Giant Redwoods of California
The various species of photoautotrophs
range in size from the Giant Redwoods
of California to the microscopic algae
(Diatoms) that are the major producers
of food for marine animals.
Chemoautotrophs (1)
Light is not the only source of energy utilised by autotrophs.
Various species of bacteria have evolved mechanisms for
manufacturing their own food through utilising the energy contained
in certain inorganic molecules.
These are the chemoautotrophs
ENERGY
FROM
inorganic
molecules
Chemoautotrophic bacteria obtain the energy they need for food
manufacture by oxidising inorganic molecules such as
ammonia and hydrogen sulphide.
Chemoautotrophs (2)
Species of chemoautotrophic bacteria known as the
nitrifying bacteria play an essential part in the nitrogen cycle.
Nitrifying bacteria oxidise ammonium and nitrite ions to nitrates.
NH4+
NO2
energy
The energy released from these
oxidation reactions is used by the
bacteria to manufacture their own food.
NO3
energy
The nitrates are absorbed by green
plants and the nitrogen is
incorporated into nitrogen-containing
organic compounds.
Chemoautotrophs (3)
In habitats devoid of light such as caves and deep ocean beds, these
archaean and bacterial organisms are the primary producers that
supply the energy that supports an entire community of organisms.
A dark cave - chemosynthetic bacteria supply
energy to support a community of organisms.
The deep ocean bed
- devoid of light.
Chemoautotrophs - Extremophiles
Most of the Archaea are tolerant to extreme
environmental conditions, i.e. they are
extremophiles. Halophiles (thrive in high levels
of salt), piezophiles (thrive under high
pressure), thermophiles (thrive in high
temperatures) and acidophiles (thrive in acidic
conditions), etc.
In the deepest parts of the oceans (no sunlight),
where tectonic plates meet, there may be cracks
in the ocean floor into which cold sea water
seeps. The water is super-heated, mixes with
sulfur compounds and may be released via
hydrothermal vents. Chemosynthetic bacteria
and Archaea convert the heat, methane, and
sulfur compounds (particularly hydrogen
sulfide) into energy by chemosynthesis.
‘Black smoker’
Chemoautotrophs – Supporting An Ecosystem
AN OCEAN BED FOOD CHAIN IN THE PACIFIC OCEAN
H2S (Hydrogen Sulphide from the earth’s core)
Archaea and Chemosynthetic bacteria (primary producers)
barnacles
clams
anemones
mussels
small worms
Eaten by large crabs and fish
All of the living things are extremophiles as they have to thrive in
high temperatures (thermophiles) and high pressures (piezophiles).
Chemoautotrophs - Thermophiles
The optimum growth temperature range for extreme thermophiles is
between 80ºC and 115ºC.
Archaea produce the bright colours in the hot springs in Yellowstone
Park USA. Some Archaea can fix nitrogen at 90°C. A strain of
Methanopyrus kandleri can even reproduce at 122°C (252°F) - the
highest recorded temperature tolerated by any living thing.
Heterotrophs (1)
The majority of living species are unable to carry out
either photosynthesis or chemosynthesis.
These are the heterotrophs
Energy from
already
manufactured
organic
materials
Heterotrophs must consume already manufactured organic
materials as a source of energy for their life activities.
Heterotrophs are ultimately dependent upon the
autotrophs for their supply of organic food.
Energy is transferred from autotrophs to
heterotrophs through the food chain.
Heterotrophs (2)
Caterpillars obtain organic food material from the products of
photosynthesis locked up in photosynthesising plants.
Caterpillars are a source of food for small birds, which are in turn
eaten by larger birds and other carnivores.
The energy locked up in the organic material of this autotrophic
producer is passed along the food chain.
General Structure of a Leaf (Dicotyledon)
The palisade
layer of cells
located close
to the upper
epidermis are
the principal
photosynthetic
cells of the leaf.
A section through the leaf
from upper to lower surface
reveals a characteristic
arrangement of tissues.
Palisade cells
are packed
with chloroplasts.
A single chloroplast
(the site of photosynthesis).
The Chloroplast (1)
The chloroplast, the site of photosynthesis, is surrounded by an envelope of
two membranes and contains a jelly-like matrix called the stroma.
Envelope
Stroma
The Chloroplast (2)
Many of the sugar molecules formed during photosynthesis are
stored as starch and, starch grains can be found growing close to the
grana.
Envelope
Stroma
Thylakoids
Circular
DNA
molecule
Starch
Grain
Lipid
droplet
Ribosomes
Many of the thylakoids
are stacked to form
grana.
A single granum
The stroma also contains a circular DNA molecule, numerous
ribosomes and lipid droplets.
The Chloroplast (3)
The photomicrograph (false colour) below details chloroplast
structure as viewed with a transmission electron microscope (TEM).
Chloroplast envelope
Stroma containing
numerous small
ribosomes
Lipid
droplet
Starch
grain
A single
granum
Inter granal thylakoid
(connects different grana)
Leaf Structure
This typical dicotyledonous
sycamore leaf is the major
photosynthetic organ
for the sycamore tree.
Dicotyledonous leaves are
structurally adapted for
their photosynthetic role.
THE MAJOR PHOTOSYNTHETIC
ORGAN OF GREEN PLANTS
How are Leaves Adapted for Photosynthesis?
Most leaves are wide and flat which gives them a large surface area
for the absorption of light energy and gaseous exchange.
Most leaves are also thin, which allows
light to penetrate right through them.
Leaves have a dense network of
veins containing xylem vessels
which carry the water needed for
photosynthesis up from the roots.
lamina
The midrib and stalk of a leaf connect its xylem and phloem
vessels with those in the stem.
Dicotyledon Leaf - Internal Structure (1)
waxy cuticle
upper epidermis
Layer of palisade
mesophyll cells –
main photosynthetic layer.
xylem vessel
Spongy mesophyll layer layer of irregular shaped cells
with numerous air spaces.
substomatal
air space
lower epidermis
stoma
guard cell
thin cuticle
Dicotyledon Leaf - Internal Structure (2)
Drag each label to the correct position on the diagram
to produce a correctly labelled section of a leaf...
Click here to view
the animation…
Dicotyledon Leaf - Internal Structure (3)
palisade
cells
upper epidermis
xylem
stoma
and guard
cells
spongy
mesophyll
cells
phloem
intercellular
air spaces
parenchyma
(packing tissue)
lower epidermis
Section of midrib
area of a typical
dicotyledonous leaf.
Dicotyledon Leaf - Internal Structure (4)
leaf vein
palisade
cells upper epidermis
guard
cell
spongy
stoma
mesophyll
cells
Section of
intercellular
air spaces
lower epidermis
lamina area of a typical dicotyledonous leaf.
Dicotyledon Leaf - Adaptations For Photosynthesis (1)
The colourless flattened cells of the epidermis and the transparent
protective waxy cuticle readily allow light to pass through the leaf
surface to the photosynthetic tissue.
The closely packed palisade cells with
their numerous chloroplasts maximise
light absorption for photosynthesis.
Numerous transport tissues permeate
the leaf structure allowing water to be
efficiently delivered to the
photosynthetic cells.
The extensive network of air spaces in the spongy mesophyll layer
provides for an easy passage of gases to and from the palisade cells
and an efficient gas exchange system via the stomata.
Stomata are the sites of gas exchange and their opening and closing
is controlled by specialised epidermal cells called guard cells – such
regulation allows for efficient gas exchange while, at the same time,
reducing water loss as a result of transpiration.
Dicotyledon Leaf - Adaptations For Photosynthesis (2)
The leaf is very thin and presents a large surface area for
maximising light absorption and gas exchange.
According to Fick’s Law:
Rate of diffusion =
surface area x difference in concentration
thickness of the diffusion barrier
The large surface area presented by the leaves of dicotyledons and
the large differences in concentration for O2 and CO2 between the
leaf interior and the external environment, produce a high value for
the top line of this equation.
The thinness of the leaf, providing short diffusion paths for gases
from the environment to the photosynthesising cells of the palisade
layer, produces a low value for the bottom line of the equation.
The consequences of these leaf features allow for a rapid rate of
diffusion of CO2 gas to the photosynthesising cells – a feature
essential for efficient photosynthesis.
More About the Palisade Cells
Palisade cells are well suited to their role as the principal photosynthesising
cells of the leaf.
The column-shaped
nature of the cells allows
for close packing of the
palisade layer close to the
surface of the leaf, and
each cell possesses
numerous chloroplasts.
The chloroplasts are
able to move freely
around the cytoplasm.
vacuole
containing
cell sap
cell wall
chloroplasts
cell surface
membrane
The chloroplasts have the ability to
orientate themselves in positions
that maximise light absorption in
dim light and reduce potential
damage when light intensities are
very high.
nucleus
A Palisade Cell
More About the Palisade Cells
Dim light
In dim light, chloroplasts tend
to aggregate at the top surface
of the cell and to orientate
themselves so as to display a
large proportion of their
surface area to the
incoming light rays.
Intense light
In intense light, chloroplasts
aggregate at the lower end
of the cell and orientate
themselves in a vertical
position - this reduces the
chances of damage to the
chloroplasts through bleaching.
The Stomata – Structure (1)
Stomata are the sites of gas exchange between the leaf and the
environment.
Stomata are the pores that allow for exchange of gases, and their size
is regulated by specialised cells called guard cells.
epidermal
cells
guard cells
containing
chloroplasts
closed stoma
open stoma
Stomata are mostly found in a leaf’s lower epidermis although they
may be present, in fewer numbers, in the upper epidermis.
The large number of stomata, together with the regulation of their
size by guard cells, allows for efficient gas exchange at the interface
between leaf and the environment. This facilitates rapid uptake of
CO2 for use by the photosynthetic cells during daylight hours.
The Stomata – Structure (2)
thick, inelastic inner
wall of guard cell
stomatal
pore
thin, elastic outer
wall of guard cell
epidermal cell
Epidermis of Tulip leaf showing stomata (x400)
The Stomata – Structure (3)
mesophyll cell
below the stoma
Scanning electron micrograph (SEM) of a single stoma - false colour.
Adaptations of Sun and Shade Plants
The leaves of many typical sun and shade plants show adaptations
that allow them to thrive in their respective environments.
Normal Plant
Shade Plant
thick cuticle reduces
transpiration losses
in more intense light
thin cuticle
more than one
palisade layer with
smaller chloroplasts
thicker leaf - greater distance between
upper and lower epidermis
single palisade
layer with larger
chloroplasts
to maximise light
absorption
thinner leaf - shorter distance
between upper and lower epidermis
Autotrophs - Summary
Autotrophs manufacture their own
food using simple inorganic sources
and, in doing so, provide the resources
on which heterotrophs depend.
Approximately 600 billion tonnes
of carbon dioxide is converted
into organic food by the
autotrophs each year, with 400
billion tonnes of oxygen being
released into the environment.
All life is centred around the
activities of the autotrophs,
with photosynthesis being the
central process for powering
our own existence.