ADAPTATIONS IN C3, C4 AND CAM PLANTS

What variations have evolved in some plants

• The calvin cycle

How many groups can plants be organised into.

How are they organised

• Three groups

• Based on how they fix carbon into glucose

What are the groups called

• C3 plants

• C4 plants

• CAM plants

How do each of the groups fix carbon into glucose

• C3 plants: carry out the original Calvin cycle 

• C4 and CAM plants: have evolved a different variation of how the Calvin cycle operates

How much of terrestrial plants are C3 plants

• 85%

Where do C3 plants grow the best

• cool to temperate moist conditions

C3 plants examples

• wheat

• rice

• barley

• rye

• oats

• soybean

• sugar beet

• potato 

What is rubisco meant to do in C3 plants

• fix the CO2

What occurs once rubisco has fixed the CO2

• the immediate organic product in the Calvin cycle is a three-carbon molecule of phosphoglyceric acid (PGA)

Where does the entire pathway of the calvin cycle take place (carbon dioxide to glucose)

• stroma of the leaf mesophyll cells

What percent of plants are C4 plants

• 3%

Where do C4 plants thrive

• warm, temperate regions and tropical regions

How much of C4 plants account of global primary production

• 23%

C4 plant examples

• sugar cane

• sorghum

• Mitchell grass tussocks

What are the leaves of C4 plants anatomically different to

• C3 plants

What do the leaves of C4 plants have

• bundle sheath cells, each with many chloroplasts, enclose the vascular tissue in leaves 

• mesophyll cells that are arranged around the bundle sheath cells.

Where do CAM plants thrive

•  hot and arid environments

• in regions exposed to drought

How many CAM plants make up land plants

• 8%

CAM plant examples

•  cacti

• moulded wax agave

• pineapple

What is important that the C3, C4 and CAM plants are able to do

•  maximise photosynthesis in changing conditions, making the process as efficient as possible

What is an input and output for all three types

• Input: CO2

• Output: glucose

What cell do all three types use

• mesophyll cell

Where do C3 and CAM plants fix carbon compared to C4 plants

• C3 and CAM: fix carbon in the mesophyll cells

• C4: the only plant type that can fix carbon in the bundle sheath cell

What is photorespiration

• Where plants take up O2 rather than CO2 , resulting in less efficient photosynthesis

What is rubisco

• a critical enzyme in C3 plants that brings CO2 from the air into the Calvin cycle where the glucose is made

What can rubisco also bind with? Why?

• Oxygen

• The active site can accommodate either of the molecules.

What type of inhibitor is oxygen in this topic

• Competitive inhibitor

Rubisco adds whichever molecule it binds to, to what

• the five-carbon compound called (RuBP)

Why is photorespiration not a problem in C3 plants

• In the cool to temperate conditions in which C3 plants thrive, photorespiration is not a problem

• Rubisco will preferentially bind carbon dioxide

If Rubisco does bind oxygen rather than CO2 what is the result

• Photorespiration

What is rubiscos affinity for CO2 in mild temperatures

• 80x higher than its affinity for O2

What is rubiscos relationship with O2 rather than CO2 in high temperatures

• rubisco fixes oxygen more often

• CO2 becomes less soluble, so there is more oxygen available in the mesophyll cells than at lower temperatures

What occurs in prolonged high temperatures to the rate of photorespiration and photosynthesis

• the rate of photorespiration increases faster than the rate of photosynthesis

C3 plants do what to prevent water loss

• close their stomata

what does closing stomata do for C3 plants

• blocks entry of carbon dioxide. 

• limits exit of oxygen produced in the light-dependent stage of photosynthesis

  This creates a high oxygen and low carbon dioxide environment in mesophyll cells

  Rubisco will bind oxygen over carbon dioxide, therefore photorespiration rates increase

What does photorespiration produce?

• Carbon dioxide

• Photorespiration creates a product that cannot be used to make sugars

• Instead of photosynthesis producing glucose from carbon dioxide, photorespiration produces carbon dioxide

What does photorespiration reduce?

• reduces the efficiency of the Calvin cycle

• reduces levels of photosynthesis by up to ~40% in C3 plants, reducing energy yield in these plants

When CO2 binds with rubisco what does it produce

• Sugar

When O2 binds with rubisco what does it produce

• Wasted energy

• lost CO2

When does rubisco work most efficiently

• carbon dioxide levels in leaves are high 

• oxygen levels are low (as happens when water is freely available) 

• when temperatures are moderate

When does photorespiration occur

• CO2 : O2 ratio is low — that is, low CO2 and high O2

When does photorespiration occur? when does photorespiration increase?

• Increases with increasing temperature 

• Occurs frequently on hot, dry days when C3 plants close their stomata to prevent water loss

C4 and CAM plants have…

•  evolved mechanisms to minimise or prevent photorespiration

What mechanisms are used in C4 and CAM plants to minimise/prevent photorespiration

• Separating the process of carbon dioxide fixation from the process of glucose production by the Calvin cycle

C4 plants

• carrying out these processes in different cell types

CAM plants

• carrying out these processes at separate times (day and night)

What does pep carboxylase only bind to

• Carbon dioxide molecules

Minimising photorespiration in C4 plants pathway

The pathway from carbon dioxide to glucose occurs in two stages that take place in two different cell types

First stage

• carbon dioxide to malic acid — occurs in leaf mesophyll cells

• C4 plants use PEP carboxylase instead of rubisco to fix carbon

• PEP carboxylase can only bind carbon dioxide molecules, so photorespiration cannot occur

Second stage

• glucose production via the Calvin cycle — occurs in bundle sheath cells

Why is photorespiration not a problem in C4 plants

• The use of PEP carboxylase by C4 plants eliminates photorespiration. 

• In the bundle sheath cells, malic acid is continuously converted to pyruvate and carbon dioxide

• This steady production of CO2 into the bundle sheath cells means that the rubisco enzyme will preferentially bind carbon dioxide, not oxygen.

malic acid → pyruvate + carbon dioxide equation

• (C4H6O5 ) → (C3H4O3 ) + (CO2)

Most plants open…

• their stomata during the day

When are dangerous amounts of water lost

• In arid environments

• If the stomata are open during the hot, dry days.

Less water is lost by…

• opening the stomata at night

CAM plants night vs day (hint: malate)

• CAM plants take in CO2 at night and store it in the form of a four-carbon acid called malate. 

• Malate is released during the day, converted to CO2 and enters the carbon cycle.

Why is photorespiration not a problem in CAM plants - night time

• Carbon fixation stage takes place only at night when stomata are open

• Reaction catalysed by PEP carboxylase

• Malic acid is stored in vacuoles in the plant cells until daytime

Why is photorespiration not a problem in CAM plants - day time

• The Calvin cycle occurs only during the day when stomata are closed (no water loss!). 

• Steady production of CO2 creates a high concentration environment that increases rubisco’s affinity for CO2 . 

• Rubisco can bind to carbon dioxide easily, so photorespiration is reduced.

C3, C4 and CAM enzyme to fix carbon dioxide from the air

C3

• rubisco

C4

• pep carboxylase

CAM

• pep carboxylase

C3, C4 and CAM plant acceptor molecule of CO2 from air

C3

• RuBP (ribulose biphosphate)

C4

• PEP (phosphoenol pyruvate)

CAM

• PEP (phosphoenol pyruvate)

C3, C4 and CAM plants first product of carbon fixation

C3

• phosphoglyceric acid (PGA), a #C molecule

C4

• oxaloacetic acid (OAA), a 4C molecule

CAM

• at night: OAA

• by day: PGA

C3, C4 and CAM plants location and number of carbon fixation events

C3

• one in mesophyll cells

C4

• two, in different cell types first in mesophyll cells

• second in bundle sheath cells

CAM

• two, both in mesophyll cells first by night

• second by day

C3, C4 and CAM plants location of calvin cycle

C3

• mesophyll cells

C4

• bundle sheath cells

CAM

• mesophyll cells

C3, C4 and CAM plants enzyme to start calvin cycle

C3

• rubisco

C4

• rubisco

CAM

• rubisco

C3, C4 and CAM plants presence of chloroplasts in bundle sheath cells

C3

• No

C4

• Yes

CAM

• No

C3, C4 and CAM plants open stomata required for efficient photosynthesis

C3

• yes

C4

• no

CAM

• yes, at night only

C3, C4 and CAM plants photorespiration in high temps and low CO2 concentrations

C3

• high

C4

• low to zero

CAM

• low to zero

C3, C4 and CAM plants optimal temperature range

C3

• 15-25 degrees celsius

C4

• 30-40 degrees celsius

CAM

• >40 degrees celsius

What is important for C3, C4 and CAM plants are able to do

• maximise photosynthesis in changing conditions, therefore making the process as efficient as possible.

RuBP role

• recycles carbon

Rubisco role

• Carbon fixation

• Occurs in the mesophyll cells

When does photorespiration occur in C3 plants

• when rubisco can bind to oxygen rather than carbon dioxide, leading to a loss of energy and lower efficiency of photosynthesis

When does photorespiration occur in C4 plants

• pep carboxylase, fixes carbon dioxide, eliminating the cause of photorespiration (where rubisco binds to oxygen)

• carbon dioxide moves into the bundle sheath so rubisco will bind to this preferentially

When does photorespiration occur in CAM plants

• pep carboxylase, fixes carbon dioxide, eliminating the cause of photorespiration (where rubisco binds to oxygen)

• (extra steps), which allow rubisco to more easily bind to CO2

C3, C4 and CAM plant comparison diagram