BIOL 2390 - Topic 6 (part 3)

Alternative photosynthetic pathways

Some plant species have alternative photosynthetic pathways that helps reduce water loss, such as

1.) C4 photosynthesis

2.) CAM photosynthesis

C4 photosynthesis

1.) It occurs in 3% of plants

2.) It is usually seen in warm climates, such as the tropics or subtropics

3.) Commonly seen in grasses, corn, sugar cane, etc

CAM photosynthesis

1.) It occurs in 7% of plants

2.) It is usually seen in hot + dry environments

3.) Commonly seen in cacti, pineapples, and other epiphytes (i.e. plants that live on top of plants)

CAM and C4 photosynthesis

It adds an extra step, such that it converts CO2 into an organic acid, before it enters the Calvin cycle (i.e. dark reactions)

Reduction of water loss in C4 and CAM photosynthesis

It reduces water loss, per unit of CO2 gained

Photosynthesis in C3 plants

Both stages of photosynthesis, i.e. the light and dark reactions, occurs in the mesophyll cells

Light reactions

It captures the light energy and then funnels it into high energy carriers, creating O2 in the process

Dark reactions

It converts CO2 into sugars, using the energy provided from the high energy carriers produced from the light reactions

Role of Rubisco

The enzyme that is responsible for carrying out the calvin cycle, i.e. turning CO2 into simple sugars

Major limitation of C3 plants

1.) When O2 concentrations are high and CO2 concentrations are low, rubisco can accidentally bind O2, thereby causing photorespiration

2.) This causes a complete loss of all the captured light energy, with no sugars being made

When does photorespiration occur

It occurs when the stomata are closed, resulting in low CO2 and high O2

C4 plants

They have a different leaf anatomy, with spatial specialization that separates light and dark reactions, such that they are carried out in different cells

Light reactions in C4 plants

1.) It occurs in the mesophyll cells

2.) It fixes CO2 into an organic acid, preventing any O2 binding that may occur

3.) The organic acid is then shipped of into the bundle sheath cells

Dark reactions in C4 plants

1.) It occurs in bundle sheath cells

2.) Once the organic acid is inside the bundle sheath cell, it is broken down into CO2 once again, concentrating the amount of CO2 present for rubisco to use

3.) No light reactions occur in the sheath cells, reducing the chance of any O2 binding

Significance of concentrating CO2 in the sheath cells

It leads to a higher rate of activity for the enzyme

Why are C4 plants not the norm?

1.) It leads to very fast growth rates, but it requires high temps and high levels of light

2.) This is why C4 plants are not generally seen in Manitoba

Bundle sheath cells location

It makes up the tissue surrounding the vascular tissue, depositing the sugars made right into the vascular tissue

Vascular tissue in plants

It transports water and sugars to the rest of the plant

CAM plants

They have temporal specialization, rather than spatial specialization, such that certain processes occurs at a specific time

CAM plants at night

At night the stomata is open, allowing CO2 to come in and build a pool of organic acids

CAM plants during the day

The stomata are closed and the pool of organic acids are made into CO2 once again, to be used for photosynthesis

CAM plants advantage

Water loss is minimized by keeping the stomata closed, thereby improving water-use efficiency

Why are CAM plants not the norm?

CAM plants are slow and inefficient, because the stored carbon depletes quickly, as you can only store so much CO2/organic acid