Plant Metabolism & Productivity Tutorial

PhotosynthesisWhere does the energy come from that sustains all life at the earth’s surface?

The Sun!!

Summarizing reaction of photosynthesis!

C6H12O6

How do plants acquire these starting reactants?

What do plants do with these products?

Thought Questions:

Record your answers to all of the though questions posed in this tutorial in the homework for Lab #4 in you lab manual. Review the entire tutorial before you attempt to answer any questions!

Who photosynthesizes?

Plants!

Who photosynthesizes?

Marine and Freshwater phytoplankton – single celled

and colonial algaeNon-planktonic multicellular algae

Coccolithophore

Diatom

Volvox

Zooplankton (an animal)

Rhodophyta (red algae)

Chlorophyta (green algae)

Phaeophyta (brown algae)

Who photosynthesizes?

Cyanobacteria

Found in the hot springs of Yellowstone N.P.

Anatomy of Photosynthesis in Plants

Photosynthesis occurs in the chloroplasts which are located in the cells of plant leaves. It is here that important light harvesting molecules called photosynthetic pigments are located.

The anatomy of a plant cell

Note that plant cells have both mitochondria for cell respiration and chloroplasts for photosynthesis!

Euglena – single cell freshwater phytoplankton

Phytoplankton also undergo both cell respiration and photosynthesis!

mitochondrion

chloroplast

nucleus 5 µm

The concept map on the following slide summarizes the important connections between cellular respiration and photosynthesis within and between plant cells.

As you toggle through this map see if you can determine where the sun’s energy is being used to facilitate the production of new plant tissues needed for plant growth. Also see if you can determine what energy is not available to sustain the growth of new tissues.

Chloroplast

Mitochondrion

CO2 from atmospherePl

ant

Leaf

Mes

ophy

ll Ce

ll

Energy from sunlight

Glucose (C6H12O6)

Fuels cell respiration

ATP

Amino acids

Cellulose

starch

Exported to other plant cells to build

H2O from soil

Also converted to

sucrose

nucleotides

Fatty acids

Exported to fuel respiration in other plant cells lacking chloroplasts

i.e. root cell

ATP

Note that these leaf cells, like many cells in plants, contain BOTH chloroplasts for photosynthesis and mitochondria for cellular respiration!

CO2 CO2

O2

Provides the material resources to build plant cells and tissues to support plant growth and reproduction

Productivity is a measure of the amount of the sun’s energy captured by plants which is used to sustain the energy demands of life and provide energy for growth and reproduction.

We can estimate productivity by measuring something that is taken up by a plant which is proportional to energy uptake.

What do you think this is?Carbon dioxide! - Used in the Calvin Cycle in photosynthesis to make carbohydrates. The rate of uptake of CO2 is directly proportional to the uptake of light energy from the sun.

Carbon dioxide is also a by-product of cellular respiration, so the rate of release of CO2 is directly proportional to how much energy is ultimately used by plants to run cellular processes.

The difference between these two provides a measure which is proportional to the amount of energy that is left over to fuel growth and reproduction!

Gross Primary Productivity (GPP) is…

The total amount of the sun’s energy a plant converts into chemical energy subtracting out the amount of chemical energy a plant uses to fuel cellular respiration.

Net Primary Productivity (NPP)

…the total amount of the sun’s energy that a plant converts into chemical energy (stored in the bonds of organic molecules).

Think of it like your total earnings from a job!

Total Pay - rent & living expenses!

=or Gross (Total) Productivity – Cellular Respiration

NPP or Energy available for growth and

reproduction

=Money left over

to grow your bank account

Like…

Let’s say that we put some plant leaves in a sealed bottle and exposed it to 10 minutes of light and then 10 minutes of dark. During this time we used a CO2 sensor to measure changes in the concentration of CO2 in the bottle. The following results were obtained.

Time (minutes)0 10 20

CO2 c

once

ntra

tion

(*pp

m)

Slope = -35.0 ppm/minSlope = 20 ppm/min

The next 3 slides summarize the changes in CO2 in the bottle with respect to the metabolic processes occurring in the plant during the light and dark phases of the experiment.

Light Dar

k

*ppm = part per million

Remember…

As we calculate productivity the sign of the slope (rate of change of [CO2]) for each measurement is important because they tell us whether CO2 is being taken up or released by the plant in your chamber!

• A negative slope (i.e. -55 ppm/min/g) means the plant is TAKING UP CO2 from the chamber.

• A positive slope (i.e. 34 ppm/min/g) means the plant is RELEASING CO2 into the chamber.

CO2 uptake sugars Photosynthesis

Some amount of CO2 released from cell respiration

CO2 uptake CO2 release

-60 -50 -40 -30 -20 -10 10 20 30 40 50 60

Light

Rate of Change in CO2 concentration (ppm/min)

Remember that the plant is doing both photosynthesis & cell respiration in the light!

-35.0 ppm/min = the rate of change of [CO2] due to BOTH respiration and photosynthesis which is Net

Photosynthetic Productivity (NPP)

?

The plant is actually taking up more CO2 than this through photosynthesis, but we cannot “see” this in the light because CO2 is also being released through cell respiration at the same time, and we don’t know how much!

?

CO2 uptake CO2 release

-60 -50 -40 -30 -20 -10 10 20 30 40 50 60

Light

Rate of Change in CO2 concentration (ppm/min)

-35.0 ppm/min (rate of change of [CO2] in the light)

Dar

k

During the dark, the plant is only doing cell respiration so….

…these should be equal!

So…to calculate gross (or total) primary productivity (the whole green box)……, we need to add the dark [CO2] change to the light, but in the negative direction.

We do this by subtracting the dark from the light!

-35.0 – 20.0 = -35.0 + (-20.0) = -55.0 ppm/min

?

Here is another possibility!

Time (minutes)0 5 10

CO2 c

once

ntra

tion

(ppm

)

Slope = 10.0 ppm/minSlope = 30.0 ppm/m

in

Light Dar

k

In the light some amount of CO2 is used in photosynthesis…

…but more CO2 is given off through respiration

CO2 uptake CO2 release

-60 -50 -40 -30 -20 -10 10 20 30 40 50 60

Light

Rate of Change in CO2 concentration (ppm/min)

10.0 ppm/min = the rate of change of [CO2] in the light due to BOTH respiration

and photosynthesis

Dar

k

We know that this plant is photosynthesizing because the rate of release of CO2 is greater in the dark than in the light!

To get gross primary productivity (GPP)……again subtract the dark slope from

the light slope

10.0 – 30.0 = -20.0

?

+30dark slope

This plant is using up more of the energy it harvested from the sun to fuel cellular respiration than it is gaining through photosynthesis. This is indicated by the positive slope in the light!

Using our earnings analogy from earlier…this is like the person who’s rent and living expenses are GREATER than their total pay!

This person is spending more than he is earning, and therefore must be depleting his savings in the bank!

Like this plant which is using more energy just to stay alive than it is bringing in through photosynthesis! To stay alive, in the short term, this plant will have to use energy stored in organic molecules like starch.

A plant may be using more energy to fuel life processes (cell respiration) than it is taking in through photosynthesis either because its cell respiratory rate is elevated or because photosynthetic rate is diminished.

Team think and share…

What is one potential reason why a plant may be forced to elevate its cell respiratory rate?

What is one potential reason for why a plant may experience diminished photosynthetic rates?

Record your answer in your homework for Lab #4!

Thought Question!Calculate the gross and net photosynthetic productivity for the plant in this experiment. What do you think is happening in this plant with regard to the relationship between cellular respiration and photosynthesis, and why might a living plant be exhibiting these metabolic characteristics?

Time (minutes)0 5 10

CO2 c

once

ntra

tion

(ppm

)

Slope = 10.0 ppm/min

Slope = 10.0 ppm/min

Light Dar

k

Record your answer in your homework for Lab #4!