15/03/2016
Photosynthesis
Starter
1. Why do we need energy within the cell?
2. Where does this energy come from?
3. Why is photosynthesis so vital for all living
things?
4. Write down everything you can remember
about photosynthesis from GCSE.
5. Word and symbol equations!!
6. What features do you think a chloroplast
needs to effectively do it’s job?
Why is energy needed within
cells?
• Allows chemical reactions to take
place
• BUILD UP (synthesis) or
BREAKDOWN of molecules
• In order to do this, energy is
required to make and break bonds
Where does the energy come
from?
• The SUN is the ultimate source of
energy for nearly all living organisms
(the exceptions being a few deep sea
chemosynthetic bacteria)
What provides the energy within
cells?
• ATP…Adenosine Tri Phosphate
• Common to ALL living things
• Any chemical that interferes with the
production or breakdown of ATP is fatal to
the cell and therefore the organism
•Chemical energy
is stored in the
phosphate bonds
Photosynthesis
• All the food we eat has originated from plants
• Plants use energy from the sun and convert it
to chemical energy
• Chemical energy is used to synthesise large
organic molecules from inorganic molecules
Such as carbohydrates / sugars
Photosynthesis
Autotrophic or heterotrophic?
• Autotrophic – ability to synthesise
complex organic molecules from simpler
molecules and an energy source
• Heterotrophic – ability to digest complex
organic molecules and break them down
into simpler ones
Photosynthesis
Autotrophic organisms can be split into 2
groups:
1.Chemo-autotrophs – use the energy from
exergonic reactions for synthesis of complex
molecules
2.Photo-autotrophs - use the energy from
light for synthesis of complex molecules
Photosynthesis
6CO2 + 6H20 + light
C6H1206 + 02
• The products of photosynthesis are glucose
(C6H1206) and oxygen
• This can then be used by plants and animals
for aerobic respiration
C6H1206 + 602
6CO2 + 6H20 + some ATP
Task
• Describe the process of photosynthesis.
• You must refer:
– The reactants and products,
– How they enter and leave the leaf,
– What transport process is used
– Remember all your keywords from the
transport module from AS Biology
Plant cells undergo the biochemical processes of photosynthesis (in daylight) and
respiration.(at all times) :
Sun provides energy for
photosynthesis and evaporation
of water
Glucose produced in
chloroplasts in leaf palisade
cells by photosynthesis
Sugars and other organic
substances are transported
through sieve tubes (phloem)
Leaf
Organ adapted for photosynthesis – e.g.
•Flat – large surface area – absorbs
sunlight
•Thin – short distance for diffusion and
penetration of light
Carbon dioxide (CO2)
Required for photosynthesis
•Enters leaf through open stomata (pores)
Oxygen (O2 ) – waste; some used in
respiration
Removed through open stomata (pores)
– located mainly on underside of leaf
Water is lost through open stomata by
evaporation (transpiration) down a
water vapour potential gradient; some
water is lost through lenticels (in
stem)
Water enters root hair cells by osmosis,
down a water potential gradient
Minerals enter by active transport and
diffusion
Water and dissolved minerals are
transported upwards through xylem
vessels
Definition
• Write a detailed definition for
photosynthesis.
• It MUST NOT be the GCSE definition but
must contain more advanced scientific
words that you have learnt in AS Biology
when referring to the types of molecules
Photosynthesis (in chloroplasts in leaves)
Definition
Process whereby light energy from the Sun is transformed into chemical energy,
and used to synthesise larger (complex) organic molecules from inorganic
substances
Forms the basis of most food chains
Chemical equation (overall - simplified)
Light energy
6H2O + 6CO2
Chlorophyll
C6H12O6 + 6O2
Glucose
Respiration in plants & animals depends upon the products
of photosynthesis – i.e. organic substances and oxygen
Photosynthesis and Respiration
Respiration
C6H12O6 + 6O2
CO2 + 6H2O + Energy (ATP)
The Structure of the Leaf
• From your previous GCSE and AS
knowledge
• Write how the structure of the leaf is
related to its function in performing the
process of photosynthesis
Leaf - Organ of Photosynthesis - Adaptations
Flat – large surface area - maximum light
absorption
Thin – short diffusion distance between palisade
mesophyll cells & external environment (for CO2,
H2O and O2); palisade mesoophyll cells are near the
upper surface – maximises light absorption; upper
epidermal cells are transparent –allows light to
reach the palisade mesophyll cell
Waxy transparent cuticle – allows light to enter;
prevents loss of water for photosynthesis
Lower epidermis contain stomata (pores) – allows
gas exchange – intake of CO2 and release of O2
Leaf mosaic arrangement– exposure of maximum
number of leaves to light
Chloroplasts
Contain light absorbing pigments in membranes of
Thylakoids - chlorophylls (a and b) + carotenoids
+ xanthophylls
Pigments absorb light energy and convert it into
chemical energy (ATP) through
photophosphorylation
Contain enzymes for synthesis of hexose
sugars (carbohydrates)
Vascular (transport) tissue
Xylem – transports H2O (and minerals) to leaf
mesophyll cells (chloroplasts) for photosynthesis
Phloem transports organic molecules made in the leaf
to rest of the plant
Palisade mesophyll cells (upper layer)
Contain many chloroplasts – large amount of
chlorophyll;
Closely packed columnar cells arranged with long
axis perpendicular to surface – reduces number of
light absorbing cross walls and increases surface
area;
Chloroplasts moved by cytoskeleton (cyclosis) - to
absorb maximum light or to protect from excessive
light
Thin cell walls – reduces diffusion pathway; efficient
light penetration
Chloroplasts at periphery of cell – short diffusion
pathway
Non pigmented vacuole – allow light penetration
Spongy mesophyll (lower layer)
Spherical cells; less chloroplasts; larger intercellular
air spaces for movement of gases and H2O vapour);
store carbohydrates (and other organic substances)
made by photosynthesis – which are taken into the
phloem.
Incident Light
More cell wall to cross
Incident Light
Mosaic arrangement
Of leaves – maximum
number of leaves
exposed to sunlight
Columnar arrangement
Few cell wall to cross
H2O
O2
CO2
15/03/2016
Chloroplasts
Starter
1. What does a chloroplast do?
2. What features do you think a
chloroplast needs to effectively do it’s
job?
Objectives
• Explain how the structure of chloroplasts
enables them to carry out their functions
• Define the term “photosynthetic pigment”
• Explain the importance of photosynthetic
pigments in photosynthesis
Photosynthesis
• Takes place in specialised plant organelles
called chloroplasts that contain
photosynthetic pigments
• Absorb light energy & have their own peak of
absorption
• Occurs in 2 main stages
Light – dependant stage
Light – independent stage
Evolution of the chloroplast
• It is believed that
photosynthetic bacteria
were acquired by
eukaryotic cells
• By endocytosis (engulfing)
• To produce the first
algal/plant cell
• This is called the
“endosymbiont theory”
• They were then passed on
to the next generation
Current organisms trying
endosymbiosis to get chloroplasts
• Two types of marine molluscs
current engulf algae and incorporate
their chloroplasts to allow them to
make complex molecules
• However, they have not yet found a
way of passing these to the next
generation
Size and Shape
• Can vary
• Usually between 2-10µm in length
• Usually disc shaped
Chloroplast structure
Task
• You have got 10 minutes
• You need to annotate the diagram of the
chloroplast and think of how each
adaptations help it to carry out its function
Membranes
Double membrane (envelope);
• Outer membrane – permeable to many
small ions
• Inner membrane – less permeable and
has transport proteins embedded in it
Intermembrane space is 10–20nm wide
between the inner and outer membrane
Photosynthetic pigments – arranged in
photostems
Lamellae and thylakoids
• The inner membrane is folded into
lamellae (thin plates) aka thylakoids
• The lamellae (thylakoids) are stacked in
piles called granum (increases SA)
• Between the grana are intergranal
lamellae
Stroma (viewed with LM)
• Fluid-filled matrix
• The light-independent stage of
photosynthesis occurs here (contains
enzymes for this)
• Contains starch grains, oil droplets,
DNA and ribosomes
Grana (viewed with LM)
• Contains stacks of thylakoids
• Where the light-dependant stage of
photosynthesis takes place
• Absorb light and ATP synthesised
• Thylakoids are only seen under electron
microscope
Task
• Complete the table to describe each
function of the chloroplast and how it is
related to its function
How chloroplasts are adapted
Adaptation
How it helps
Inner membrane with
transport proteins
Controls molecules travelling between the cells
cytoplasm and the stroma
Many grana (consisting
of up to 100 thylakoids)
Large SA for photosynthetic pigments, electron
carriers and ATP synthase enzyme needed in LDS
Photosynthetic pigments
Arranged in photosystems, allows max. absorbtion of
light
Proteins embedded in
grana
Hold photosystem in place
Fluid-filled stroma
Contains enzymes needed for LIS
Grana surrounded by
stroma
Products made in LDS in grana can pass into stroma to
be used in LIS
Chloroplast DNA and
ribosomes
Can make some proteins needed for photosynthesis
Chloroplast
Intermembrane space
Outer membrane
Permeable
Inner membrane
Selectively permeable
Transport proteins present
Lipid droplet
Intergranal lamella
Starch grain (storage)
Storage polysaccharide (made of
glucose)
Thylakoid membrane
• Increase surface area
• Pigments arranged in clusters
termed photosystems (PS)
• Allow maximum absorption of
light
• Electron carriers present
• Proton pumps present
• ATP synthase complex (for
ATP synthesis by
photophosphorylation)
• Photolysis (splitting) of water
• Products of light-dependant
reactions (ATP + reduced
NADP + O2) pass into stroma
Circular DNA
Codes for proteins (enzymes)
- e.g. rubisco
Stroma (fluid)
Enzymes for light-independent
(dark) reactions – Calvin cycle
Products – glucose + NADP + ADP
Granum
Stack of thylakoids (~ 100)
Large surface area
Site of light-depemdent reactions
Products – ATP + reduced NADP + O2
Biconvex shape
Increases surface area
Ribosome (70S)
Site of protein synthesis
Photosynthesis
Photosynthetic pigments are arranged in structures
called photosystems
Photosystems – found in the
thylakoid membranes
Photosynthetic pigments
• Absorb certain wavelengths of light
• Reflect other wavelengths (these are
the colours we see)
Chlorophyll
Mixture of pigments, all have a similar structure:
• Long hydrocarbon chain & pophyrin ring
• Similar to the haem group in haemoglobin but
has magnesium (Mg) rather than iron (Fe)electrons are excited
Chlorophyll
•
•
•
•
•
•
•
•
•
•
Found within chloroplasts
Absorb and capture light
Made up of a group of five pigments
Chlorophyll a
Chlorophyll b
Carotenoids; xanthophyll and carotene
Phaetophytin
All absorb light at different wavelengths
Chlorophyll a is the most abundant
Proportions of other pigments accounts for varying
shades of green found between species of plants
Photosystem I and Photosystem
II
• These are distinct chlorophyll
complexes
• Each contains a different combination
of chlorophyll pigments
Chlorophylls
• Chlorophyll a
– Is in the “Primary pigment reaction centre”
– Two forms
• P680 – in photosystem 2
• P700 – in photosystem 1
– Appears yellow-green
– Absorbs red light (and blue at 450nm)
– contains a Mg atom – when light hits this, a
pair of electrons become excited
Chlorophyll a
Found at the base of the photosystem in the primary
pigment reaction centre
p680 – in photosystem II-particles found on the
grana
p700 – in photosystem I- particles found on the
intergranal lamellae
Arrangement of Pigments in Thylakoid Membrane
Pigments are arranged in clusters (photosystems) in
the thylakoid membranes
There are two types of photosystems – with each
containing a reaction centre containing the principal
light absorbing pigment (the primary acceptor) – i.e.
chlorophyll a
P700 (PS I) – Absorbs orange light
Absorption peak ~ 700 nm
P680 (PS II) – Absorbs red light
Absorption peak ~ 680 nm
Absorption of light by chlorophyll a causes electrons to
be excited and move to a higher energy level
The electrons are accepted by an electron acceptor
and passed onto electron carriers
Depending on the photosystem, the electrons have
different fates
Chlorophyll
•
•
•
•
•
Accessory pigments
Other forms of chlorophyll a
Chlorophyll b
Carotenoids
Xanthophylls
Accessory Pigments
Absorb the wavelengths of light that are not easily
absorbed by chlorophyll and funnel the photons to
chlorophyll a at the base of the photosystem
(reaction centre)
Don’t contain a porphyrin group – not directly
involved in the light dependent reactions
Accessory pigments
• Chlorophyll b
– Absorbs light at wavelengths between 500-640nm
– It appears blue-green
– Is one of the accessory pigments
• Carotenoids
– Absorb blue light
– Reflects yellow (xanthophyll) and orange
(carotene) light
– They absorb light not normally absorb by
chlorophylls – then pass
Light
Chlorophyll solution
Energy transferred to surrounding as:
Electrons excited to a higher
energy level
Heat, and,
Electrons return to their
original (ground) state
Light at a longer (less energetic) wavelength
– seen as red fluorescence
Light
Chlorophyll in leaf
Electrons excited to a higher
energy level
Electrons accepted by an
electron acceptor and passed
onto electron carriers arranged
at progressively lower energy
levels
Energy generated by electron transfer is used
to phosporylate ADP to form ATP – by
PHOTOPHOSPHORYLATION – through
chemiosmosis
Light energy is converted to a usable form of
chemical energy (ATP)
ATP used for biosynthesis and physiological
processes