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Plants are awesome
They really are!
GCSE Knowledge
• Plant Cells – draw one now
• Plant growth – what effects it?
• Plant nutrition – what this mean?
• Plant distribution – what effects it?
• A2 – photosynthesis and hormones
Right let us begin
• Why do plants need energy?
• To do photosynthesis, DNA replication, growth, active
transport
• What processes do they use to create a store of energy?
• Photosynthesis, respiration (anaerobic and aerobic)
• Two equations write them now!
ATP
• What is it?
• How is it made?
• How is it used?
• What are it’s properties?
• Small amount of malleable usable energy, soluble, easily broken
down, transfers energy by phosphate groups, can’t leave the cell
so readily available
Plants and energy
• Both Respiration and Photosynthesis occur at the same time,
dependent on light intensity
• The light intensity at which both happen at the same time
this is the Compensation Point for light intensity
• You can show the compensation point because the net
production/use of oxygen will be zero
Questions
• Name three processes that require energy in a plant
• Outline the relationship between the raw materials and products
of photosynthesis and respiration
• What is the function of ATP?
• Describe the structure of ADP and ATP
• What is ATP broken down into by ATPase and what is the process
called
Next Steps
• Draw a chloroplast and describe the structures
• Lets see what you can remember
Chloroplast Structure
• Chloroplasts are little self contained organelles that act like
little autonomous cells in the plant cell
• They have their own DNA
• They have their own ribosomes
• They pair up with Mitochondria (they be needing their
glucose)
More structure
• Double membrane (so and inner and outer) called the chloroplast
envelope
• They contain loads of membrane bound sacs called THYLAKOIDS
these are stacked up into Grana (sg granum)
• Grana are inked together with lamella which are bits of thylakoid
membranes (pl lamellae)
• They contain pigments attached the proteins in the membranes –
chlorophyll a and b, carotene
• These harness light the protein and pigment is called a
photosystem
• There is also a cytoplasm like substance called stroma – this
stores Glucose in starch grains, enzymes, sugars and organic
acids
What it all looks like!
Reactions
• Simple little thingies these are they are the sites for two
reactions
• Light dependent – using water
• Light independent – using carbon dioxide
• Both are there to make one molecule - Glucose
Now how it all works!
• It is all really to do with chemistry!
• Remember Redox
• Oxidation is Loss of electrons or the gain of oxygen
• Reduction is Gain of electrons or the gained hydrogen or lost
oxygen
• It always happens in pairs
Important points
1. There are two photosystems in the light dependent
reaction PSI (700nm) and PSII (680nm)
2. The photosystems exist to excite electrons
3. The process involves the very important coenzyme NADP
(reduced NADPH) this molecule can oxidise (remove
Hydrogen) or reduce (add hydrogen)
Simple idea of the light dependent
reaction
I will warn you now this is complex!
Right
Simply put the light dependent reaction is there to
Make ATP from ADP
Make NADPH from NADP
Both of the above transfer energy to the light independent
reaction
• The electrons for this process come from Water and produce
Oxygen and protons (hydrogen ions) through photolysis (what do
you think this means?)
•
•
•
•
•
•
Non Cyclic Photophosphorylation
• Great word!
• First the basics remember we had how many photosystems?
• 2 – PSII (680nm) and PSI (700nm)
• They are both linked by electron carriers (proteins that
transfer electrons)
• Happy so far?
1. Light energy excites the electrons in
chlorophyll
• Light energy is absorbed by PSI
• This in turn excites electrons on Chlorophyll
• This creates a high energy electron
• These electrons move to PSI
2. Photolysis of water produces
protons, electrons and oxygen
• As the excited electrons leave PSII they must be replaced
• Light splits water into H+ ions (or protons) electrons and
oxygen at PSII
• Equation – H2O → 2H+ + ½ O2
3. Energy from the excited electrons
makes ATP
• The electrons lose energy as they pass along the transport chain
• This energy is used to pump more H+ ions into the thylakoid via
protein proton pumps
• Therefore more H+ in the thylakoid than without
• Hydrogen then moves down the gradient back out through ATP
synthase and the energy from this makes ATP
4. Energy from the excited electrons
generates NADPH
• Light is absorbed by PSI
• This excites the electrons even higher
• These higher energy electrons can reduce NADP to NADPH
(reduced NADP)
Starter
• On a fresh sheet of A3 – try to diagram the light dependent
reaction
• This is a revision task to see what you can remember
• Enjoy!!
Cyclic Photophosphorylation
• No electron chain
• Only one PS, PSI
• No NADPH made only ATP
• And not a lot of ATP
Light Independent – using the ATP and
NADPH
• The Calvin Cycle!!!!
• This is awesome and is how plants make glucose, even in the
dark people!
• It uses the energy stored from the light dependent stage
• Cool yes??
The steps and stages
• All this happens in the stroma (that is where the ATP and
reduced NADP (NADPH) lives remember
• It makes a molecule called triose phosphate from CO2
• And it makes ribulose phosphate (5 carbons)
• This is such fun already
Step 1 – formation of glycerate 3
phosphate
• CO2 enters the leaf through the stomata
• In the stroma it is combined with Ribulose Bisphosphate (5
carbon) RuBP
• This makes an unstable 6 carbon molecule that breaks into
two Glycerate 3 Phosphate molecules GP
• This is catalysed by Ribulose bisphosphate Carboxylase
(RuBisCO)
Step 2 – Formation of Triose
Phosphate
• GP is reduced to a different 3 carbon compound called
• Triose Phosphate TP the energy comes from ATP
• The Hydrogens required come from reduced NADP (NADPH)
• The TP is then used in different reactions to make useful
sugars
Stage 3 – Regeneration of RuBP
• Five out of the six molecules of TP from each reaction
aren’t used to make glucose or other useful compounds
• They are used to generate more GP and TP
• More ATP is used to combine 2 GP molecules into one RuBP
and one single carbon
How many cycles
• The Calvin cycle needs to turn 6 times to produce one hexose sugar
• Each three turns make six 3 carbon GP molecules
• Five are used to regenerate RuBP
• So we need six turns to make the two needed for one hexose sugar
• So six turns – 6 CO2 and 18 ATP and 12 NADPH
• How many light dependent reactions do you need????
Here it is
Use of the 3c molecules
• Carbohydrates – hexose (6c) sugars are made from two TP
molecules and larger ones are made from joining hexose
sugars together
• Fats – Made using glycerol which is made from TP, and fatty
acids which are made from GP
• Amino Acids – some are made from GP
Recap – what covered so far and moving
on
• So far in photosynthesis we have covered the light and light
independent reactions
• Quick recap of those before we move on!
Autotrophs and Heterotrophs
Depending on their mode of nutrition, organisms can be classifies as autotrophs,or
heterotrophs
•An autotroph (termed a producer)
is an organism that makes complex organic compounds (“food”) from inorganic
molecules using energy (chemical or light)
plants,
A photoautotroph makes its own food using light energy and inorganic materials
(carbon dioxide, water and minerals) by the process of photosynthesis – e.g.
some bacteria, and some Protista (algae)
•A heterotroph (termed a consumer)
needs
is an organism that cannot make organic compounds from inorganic sources. It
a ready made supply of organic compounds (carbon compounds)
Almost
Heterotrophs obtain their organic compounds by consuming other organisms.
all animals, fungi and some Protista and bacteria
Note: All food (organic) molecules come ultimately from autotrophs
• The Sun is the ultimate source of energy for ALL living organisms.
• Photosynthesis is the only means available to use this energy..
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.
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 occurs in two stages:
Stage I
The Light Dependent Stage
In the thylakoids (granum) – involves
photosynthetic pigments and electron carriers –
located in thylakoid membranes
Photons of light absorbed by chlorophyll a cause
excited electrons to be ejected from chlorophyll
to a higher energy level
H2O is split using light energy (termed photolysis)
– H2O
H+ + electrons + oxygen
H ions combine with a hydrogen carrier,
NADP, to form reduced NADP
Some O2 is used for respiration (rest
diffuses out of the stomata)
Ejected electrons from chlorophyll are
accepted by an electron acceptor
Electrons are passed along a chain of
electron carriers – generating energy
Reduced H acceptor
(reduced NADP)
+
ATP
Energy is used to synthesise ATP by
photophosphorylation
ATP & reduced NADP are passed onto the light
independent reactions occurring in the stroma –
these reactions involve enzymes
To Calvin Cycle (Dark reaction)
Stage 2 The Light-Independent Stage (Calvin Cycle) – in the stroma – involves enzymes
CO2 is “fixed” – i.e. incorporated into the light independent reactions
CO2 combines with a 5C sugar (ribulose bisphosphate, RuBP) to from a 6C compound, in
a reaction catalysed by ribulose bisphosphate carboxylase (RuBisCo)
The 6C compound is unstable and splits into 2 x 3C compounds called glycerate 3phosphate (GP)
GP in the presence of ATP and reduced NADP is reduced to triose phosphate (a triose –
3C sugar)
Carbohydrate is produced at this stage in photosynthesis
RuBP (5C) is regenerated – requires ATP; 1C is used towards making glucose (6C)
6 cycles are required to produce
a molecule of glucose (6C) – a
hexose
Rubisco catalyses
CO2 fixation
Used for respiration
Converted to starch (storage polysaccharide)
Synthesis of cellulose (structural polysaccharide –cell wall)
Converted to amino acids, lipids
Synthesis of nucleotides, RNA, DNA
Photosynthetic Pigments
The initial requirement in photosynthesis is the trapping of sunlight energy by photosynthetic
pigments – two categories of light absorbing pigments are found in chloroplasts:
Primary pigments
2 forms of chlorophyll a – with slightly different
absorption peaks (680 nm and 700 nm)
Accessory pigments
Other forms of chlorophyll a
Chlorophyll b
Carotenoids
Xanthophylls
The pigments are arranged in light-harvesting clusters called PHOTOSYSTEMS (light
harvesting centres) - in the thylakoid membrane
Several hundred accessory pigment molecules surround a primary pigment molecule
Energy of the light absorbed by the different pigments is passed onto the primary pigment –
accessory pigments broaden the absorption spectrum and hence the action spectrum
Primary pigments act as reaction centres
The stroma is the fluid part of the chloroplast which contains the enzymes controlling the
carbon fixation reactions (affected by temperature).
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
Accessory pigments (chlorophylls, carotenoids and
xanthophylls) funnel light photons to the chlorophyll a
molecules in the reaction centre
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
Carbon Fixation – Light Independent Stage (Calvin Cycle)
•
The carbon fixation stage occurs in the stroma and results in the production of
glucose.
•
It is a result of an enzyme controlled sequence of reactions requiring ATP and
hydrogens (from reduced NADP) from the light stage, and carbon dioxide
(“fixed” from the air).
•
It involves the reduction of carbon dioxide, that is the addition of hydrogen
(from reduced NADP), to form carbohydrate.
•
CO2 is accepted by the 5C compound ribulose 1,5-biphosphate (RuBP) to
form an unstable 6C compound.
•
The 6C compound formed immediately splits into two molecules of a 3C
compound called glycerate 3-phosphate (GP).
•
ATP and reduced NADP is used to convert the two GP molecules into two
molecules of triose phosphate (TP), a 3 carbon compound.
•
TP’s are used in the formation of carbohydrate (glucose) and to regenerate
RuBP
•
ATP is required to regenerate RuBP
Rubisco
Regeneration of
RuBP
TP
Triose phosphates (TPs) are used to form glucose.
3C (TP)
+
3C (TP)
1C (x6)
Hexose (6C)
5C
Regenerate RuBP
6C
ATP
•`6 cycles are required to form 1 molecule of glucose
• RuBP is then joined with carbon dioxide to re-start the cycle.
1C (6 cycles)
5C RuBP) + 1C (CO2)
6C
2 x 3C (TP)
Hexose
6C
5C
Regenerate RuBP
The Limiting Factors of Photosynthesis
Photosynthesis requires
Photosynthetic pigment (internal)
CO2 (external)
Water (external)
Light energy (external)
Enzymes (internal)
The rate of any process which depends on a series of reactions is limited by the slowest reaction
in the series. If a process is affected by more than one factor, the rate will be limited by the
factor which is nearest its lowest value
There are three main external limiting factors in photosynthesis:
Lack of CO2
•If there is no CO2 available RuBP cannot be converted into GP.
•As a result the RuBP starts to build up and no more glucose will be produced.
Low temperatures
•These limit photosynthesis since the enzymes controlling the reactions are below their
optimum temperature.
•Too high a temperature will denature enzymes and stop photosynthesis altogether
Lack of light
In the absence of light
•Neither ATP or reduced NADPH will be produced and so the GP cannot be converted into
glucose.
•This results in the GP building up and the RuBP being used up.
Draw graphs to represent the limiting
factors of photosynthesis
Experiment 1: Light Intensity – at Constant Temperature
At constant
temperature
Region C
Any increase in light intensity does not increase
photosynthetic rate.
Therefore, in region C, light is not a limiting factor.
Other limiting factors are involved – e.g. temperature or CO2
supply.
Regions A and B
Any increase in light intensity increases photosynthesis.
Therefore light is a limiting factor.
At A and B, photosynthesis is controlled by the intensity of
light. If it increases, the rate also increases.
Experiment 2: Constant Light Intensity – Different Temperatures
At high light intensities the rate of photosynthesis
increases as the temperature is increased over a limited
range
At low light intensities, increasing the temperature has little
effect on the rate of photosynthesis
Photochemical reactions not generally affected by temperature
Temperature affects the rate of photosynthesis
There must be two sets of reactions in photosynthesis
-a light dependent photochemical stage (expt 1)
-a light independent temperature-dependent stage (expt 2)
Experiment 3: Increasing Light Intensity – Increasing CO2
Increasing CO2 concentration increases the rate of
photosynthesis
The rate at which the leaf can be supplied with CO2 also
affects the rate – this depends on the steepness of the
diffusion gradient and the permeability of the leaf
Measuring Photosynthesis
Measurements
Uptake of substrate
Appearance of product
To measure rate
Volume of O2 produced per unit time - has limitations
- some O2 may be used by the plant
- some dissolved nitrogen may be present in the gas collected
Rate of uptake of CO2
Rate of increase in dry mass of plants