biology unit 3: sac2

the process of photosynthesis

The process of converting light energy into chemical energy stored in glucose.

what does photosynthesis involve?

the conversion of simple inorganic molecules (CO2 and H2O) into complex organic molecules (glucose - C6H12O6).

where is the location of photosynthesis?

the chloroplast

chloroplast diagram

what are grana

stacks of thylakoid discs (membrane bound compartments) - containing green pigment chlorophyll

what is stroma

the gel like fluid between the grana in the chloroplast

equation of photosynthesis simplified

carbon dioxide + water (light) (chlorophyll) → glucose + oxygen

equation of photosynthesis using chemicals

6CO2 + 6H20 (light) (chlorophyll) → C6 H12 06 + 6O2

what to consider about photosynthesis equation

This chemical equation on its own won’t create glucose. It only tells us the inputs and outputs. Photosynthesis occurs in a series of biochemical pathways that involve many other compounds including enzymes and coenzymes.

COenzyme: ATP full name

Adenosine triphosphate

COenzyme: ATP loaded form

ATP

COenzyme: ATP unloaded form

ADP

COenzyme: ATP function

Transfers energy

Coenzyme: full name of NADH

Nicotine adenine dinucleotide

Coenzyme: loaded form of NADH

NADPH

Coenzyme: unloaded form of NADH

NAD⁺

Coenzyme: function of NADH

Transfer of electrons and hydrogen ions (protons)

Coenzyme: full name of NADPH

Nicotine adenine dinucleotide phosphate

Coenzyme: loaded form of NADPH

NADPH

Coenzyme: unloaded form of NADPH

NADP⁺

Coenzyme: function of NADPH

Transfer of electrons and hydrogen ions (protons)

Coenzyme: full name of FADH₂

Flavine adenine dinucleotide

Coenzyme: loaded form FADH₂

FADH₂

Coenzyme: unloaded form FADH₂

FAD

Coenzyme: function of FADH₂

Transfer of electrons and hydrogen ions (protons)

light dependent stage location

Grana of the chloroplast

light dependent stage inputs

H₂O (and ADP and Pi, and NADP⁺)

light dependent stage outputs

O₂ ATP and NADPH

light dependent stage description

• Light energy hitting the chlorophyll molecules imbedded in the grana causes water molecules to split into oxygen, electrons and hydrogen ions.

• Oxygen forms in O2 and leaves the chloroplast.

• The hydrogen ions and electrons are collected by NADP+ forming NADPH which moves on to the second stage in the stroma of the chloroplast.

• Energy released in the reactions is used to form ATP (from ADP and Pi) which moves on to the second stage in the stroma of the chloroplast.

Light independent stage of photosynthesis (Calvin cycle) location

stroma of the choloplast

Light independent stage of photosynthesis (Calvin cycle) inputs

CO₂ ATP, NADPH

Light independent stage of photosynthesis (Calvin cycle) outputs

C₆H₁₂O₆, H₂O (and ADP and Pi, and NADP⁺

Light independent stage of photosynthesis (Calvin cycle) description

• CO2, from the atmosphere, and NADPH (supplying hydrogen ions and electrons), from the first stage, react together to form glucose (C6H12O6).

• Energy for this reaction to occur is supplied by ATP from the first stage.

• Water (and unloaded coenzymes) is also a product of this reaction.

• A number of other molecules in the cycle (including the key enzyme RuBisCO) assist in the reaction.

what is chlorophyll

A green pigment located in the grana which absorbs and traps light energy.

absorption of chlorophyll

Chlorophyll absorbs/reflects different wavelengths of light. It is therefore more or less efficient depending on the colour of light.

worst = green

best = blue and red

what is Rubisco

an enzyme involved in the light independent stage (calvin cycle) of photosynthesis

role of rubisco

The role of rubisco is to bind CO₂ and ‘fix’ the carbon into organic molecules in order to ultimately produce glucose.

what does rubisco bind CO₂ with

with another compound called RuBP in order to make 3-PGA (a 3 carbon molecule).

what is 3- PGA converted too?

With ATP and NADPH from the light dependent stage, 3-PGA is converted into another molecule G3P.

what are G3P molecules further converted too

into glucose

what is photorespiration

when Rubisco sometimes binds O₂ rather than CO₂

why is photorespiration bad

The binding of O₂ by rubisco wastes energy and reduces the rate of photosynthesis, therefore decreasing the efficiency of the plant.

what impacts the rate of photorespiration vs photosynthesis

impacted by the temperature and CO2:O2 ratio.

what happens when temp increases for photorespiration

• As temperature of the plant increases, rubisco binds O2 more readily, increasing photorespiration rate.

what happens when CO2:O2increases for photorespiration

As O2 concentration in the leaf increases, relative to CO2, rubisco binds O2 more readily, increasing photorespiration rate.

how do plants reduce the rates of photorespiration in normal conditions

Plants open their stomata to take up CO₂ from the atmosphere and release O₂, reducing photorespiration rate.

what happens to reduce photorespiration in hot dry climates

• plants may lose too much water through open stomata.

• Closing stomata reduces water loss, but also increases O2 concentration in the leaf.

• Along with the direct effect of the high temperature, this increases photorespiration rate

what adaptions do C3 plants have to reduce photorespiration

normal’ plants – have no features to reduce photorespiration. ‘Typical’ Calvin cycle (as per previous slides).

what adaptions do C4 plants have to reduce photorespiration

separate Calvin cycle into 2 stages occurring in 2 different cell types.

what adaptions do CAM plants have to reduce photorespiration

separate Calvin cycle into 2 stages occurring at different times of the day.

What defines a C3 plant?

• Most common type of plant (~85%)

• First product of carbon fixation = 3-PGA (3-carbon molecule)

• Carbon fixed by Rubisco during the Calvin cycle

where does C3 photosynthesis occur

• Occurs in mesophyll cells of leaves

• Calvin cycle happens here

limitations of C3 plants

• Photorespiration can occur

• More common in hot / dry conditions

• Reduces photosynthesis efficiency

e.g of c3 plants

• Rice

• Wheat

• Fruits & vegetables

• Trees

c3 quick summary

• Use Rubisco to fix CO₂ → forms 3-PGA

• Happens in mesophyll cells

• Common, but affected by photorespiration

What defines a C4 plant?

• First product = malate (4-carbon compound)

• Carbon initially fixed by PEP carboxylase

• Called C4 because first product has 4 carbons

Where does C4 photosynthesis happen?

Two different cells involved:

• Mesophyll cells → initial carbon fixation

• Bundle-sheath cells → Calvin cycle continues

How is CO₂ fixed in C4 plants?

• CO₂ fixed in mesophyll cells

• Enzyme = PEP carboxylase

• Forms malate (4C)

• PEP carboxylase does NOT bind O₂

What happens to malate?

• Malate moves to bundle-sheath cells

• Releases CO₂

• Rubisco uses CO₂ in the Calvin cycle

why are C4 plants better in hot conditions?

• PEP carboxylase doesn’t bind O₂

• Little/no photorespiration

• More efficient in hot, dry climates

How do C4 plants minimise photorespiration?

• Maintain high CO₂ concentration in bundle-sheath cells

• Gives Rubisco more CO₂ to bind

• Less chance of Rubisco binding O₂

• Minimises photorespiration

What is a disadvantage of C4 photosynthesis?

• Uses ATP to move/fix carbon

• More energy expensive than C3 plants

What environment suits C4 plants?

• Best in hot, humid environments

• Efficient photosynthesis under high temperatures

examples of C4 plants?

• corn

• sugarcane

What does CAM stand for?

• Crassulacean Acid Metabolism

• Special photosynthetic adaptation for dry environments

How is CAM photosynthesis separated?

• Both stages occur in mesophyll cells

• Happens at different times of day

• Separation is by time, not by cell

(Memory trick: C4 = different cells, CAM = different times)

what do CAM plants do at night?

• Stomata open

• CO₂ enters leaf

• PEP carboxylase fixes CO₂

• Forms malate

What happens in CAM plants during daylight?

• Stomata close → conserves water

• Malate releases CO₂

• Rubisco uses CO₂ in Calvin cycle

How is photorespiration reduced in CAM plants?

• High CO₂ concentration around Rubisco during day

• Even with stomata closed

• Less chance of Rubisco binding O₂

• Reduces photorespiration

What is a disadvantage of CAM plants?

• Uses ATP

• More energy expensive than C3

What environment suits CAM plants?

• Hot, dry environments

• Conserves water efficiently

Examples of CAM plants?

• cacti

• pineapples

factors effecting rate of photosynthesis

• CO₂ concentration

• light intensity

• light colour

• water availability

• temp

• pH

• enzyme inhibators

How does CO₂ concentration affect photosynthesis?

• CO₂ is an input of the light-independent stage (Calvin cycle)

• More CO₂ = faster photosynthesis rate

• Rate increases until saturation point is reached

Why does photosynthesis level off at high CO₂?

• Another limiting factor becomes the problem, such as:

  • Light intensity

  • Enzyme concentration

  • Water availability

Which plants cope better in low CO₂ conditions?

• C4 and CAM plants are better adapted

• Maintain high CO₂ around Rubisco

• Reduce photorespiration

• More efficient than C3 plants in low CO₂ conditions

CO₂ concentration — summary

• More CO₂ → higher photosynthesis rate

• Increases until saturation point

• Then another factor limits rate

• C4 + CAM cope better than C3 in low CO₂ conditions

How does light intensity affect photosynthesis?

• Light is needed for the light-dependent stage

• Used to split water (photolysis)

• More light intensity = faster photosynthesis rate

• Increases until saturation point is reached

Why does photosynthesis stop increasing at high light intensity?

Another limiting factor takes over, such as:

• CO₂ concentration

• Enzyme concentration

• Water availability

Light intensity — summary

• Light powers light-dependent reactions

• More light → higher photosynthesis rate

• Rate rises until another factor limits it

How does light colour affect photosynthesis?

• Chlorophyll absorbs some wavelengths better than others

• Different light colours provide different amounts of usable energy

• This changes the rate of photosynthesis

Which light colours are most effective for photosynthesis?

• Blue light → highly absorbed → high photosynthesis

• Red light → highly absorbed → high photosynthesis

• Green light → mostly reflected → low photosynthesis

How does water availability affect photosynthesis?

• Low water / high temperature → stomata close

• Helps reduce water loss

• Less CO₂ enters leaf

• Photosynthesis rate decreases

Why does low water lower photosynthesis?

• Closed stomata stop CO₂ uptake

• O₂ builds up inside plant

• Rubisco binds O₂ instead of CO₂

• Causes photorespiration → lowers photosynthesis

Which plants are better adapted to low water conditions?

• C4 and CAM plants

• Better at maintaining high CO₂ around Rubisco

• Reduce photorespiration

• More efficient than C3 plants in dry conditions

Why do enzyme-related factors affect photosynthesis?

• Photosynthesis is enzyme controlled

• Enzymes such as Rubisco help reactions occur

• Anything affecting enzyme activity affects photosynthesis rate

How does temperature affect photosynthesis?

• Temperature increases → photosynthesis rate increases

• Reaches an optimal temperature (best rate)

• Above optimum → rate declines

• Enzymes denature at high temperatures

How does pH affect photosynthesis?

• Photosynthesis enzymes have an optimal pH

• At optimal pH → highest photosynthesis rate

• Too acidic / too alkaline → enzyme activity decreases

• Photosynthesis rate drops

How do enzyme inhibitors affect photosynthesis?

• competitive and non-competitive inhibitors reduce enzyme activity

• Prevent enzymes from binding to their substrate properly

• This slows photosynthesis

How do herbicides affect photosynthesis?

• Many herbicides act as enzyme inhibitors

• They block enzymes involved in photosynthesis

• This reduces photosynthesis rate

CRISPR-Cas9

A genome editing tool which cuts DNA at precise locations in order to:

• delete genes from a cell (knock-out)

• insert new genes/alleles into an organism’s genome (knock-in) 

Why is improving photosynthesis important?

• World population is growing → increased food demand

• Climate change makes growing conditions harder

• Need to increase crop efficiency and yield

What is crop yield?

Crop yield = amount of product harvested from crops

How can crops be modified to improve photosynthesis?

• Use CRISPR-Cas9 gene editing

• Alters plant DNA

• Creates genetically modified crops

• Can improve photosynthesis efficiency and crop yield

Why do crops need improved photosynthetic efficiency?

• Most crops are C3 plants

• Susceptible to photorespiration

• Worse in hot / dry conditions

• This lowers photosynthesis efficiency

How can CRISPR-Cas9 improve photosynthesis?

• Can modify Rubisco efficiency

• Helps reduce photorespiration

• Makes C3 plants act more like C4/CAM plants

• Improves photosynthesis rate

How can CRISPR-Cas9 improve crop yield?

• Alters plant DNA to improve traits

• Creates higher-yielding crops

• Increases amount of harvestable product

Examples of CRISPR methods to improve crop yield?

• Increase disease resistance

• Improve shelf life of fruits/vegetables

• Create compact plants

• Increase grain size (e.g. wheat)

• Increase flowering / fruiting timespan

What is Adenosine triphosphate?

• Cell’s portable energy package

• Stores energy for intracellular use

• Provides energy for cellular processes quickly

How does ATP release energy?

• Breaks off one phosphate group

• Energy is released quickly

• ATP becomes ADP