Topic 14.2 - Plant Nutrition

Biology 1001 - November 16, 2005

1. Introduction Nutrition involves obtaining energy, carbon, and

other essential organic and inorganic compounds for metabolism

Plants require sunlight, CO2, water and certain minerals

Sunlight & CO2 are obtained from the aerial environment through the shoot, especially leaves

Water & minerals are obtained from the subterranean environment, through the roots

Plants perform photosynthesis to synthesize organic molecules

They perform cellular respiration to derive energy from these organic molecules

Photoautotrophs

2. Chloroplasts: The Sites of Photosynthesis Chloroplasts are located in mesophyll cells

of the leaf (30-40 chloroplasts per cell) Chloroplast internal structure includes a

dense fluid called stroma and stacks of thylakoid sacs called grana

Chlorophyll in the thylakoid membrane absorbs the light energy necessary for photosynthesis

Green wavelength light is reflected, which is why chloroplasts & leaves are green

Stomata on the leaf surface allow CO2 to enter, O2 & H2O to leave the leaf

Vascular veins transport water & minerals to the leaf and the organic products of photosynthesis away from the leaf

Figure 10.3!!

3. Overview of Photosynthesis Overall equation of photosynthesis

An anabolic pathway of endergonic reactions 6CO2 + 12H2O + light energy C6H12O6 + 6O2 6H2O

About the process Water is consumed and new molecules of water are formed Water is split during photosynthesis, not carbon dioxide The enzymes required for photosynthesis are located in the

stroma or embedded in the thylakoid membrane Photosynthesis does not occur as the reverse of cellular

respiration, but as two sets of many reactions each

The Two Stages of Photosynthesis The Light Reactions

The “photo” part of photosynthesis, these reactions occur in the thylakoid membrane

Light energy drives a transfer of electrons and hydrogen from water to NADP+, producing NADPH

H2O 2H+ + 2e- + 1/2O2

NADP+ + H+ + 2e- NADPH This is reducing power

Light energy also phosphorylates ADP to ATP ADP + P ATP This is chemical energy

Water is split and oxygen released during these reactions ATP and NADPH are used in Calvin cycle

The Two Stages of Photosynthesis The Calvin Cycle

The “synthesis” part of photosynthesis, occurs in the stroma The reactions of the Calvin Cycle do not directly require light, but they

require the products of the light reactions, so they generally occur in the light

Carbon enters the Calvin Cycle as CO2 and leaves as sugar It first gets added to already existing organic molecules, a process called

carbon fixation It then gets reduced to a carbohydrate using the reducing power of

NADPH and the chemical energy of ATP The carbohydrate produced by photosynthesis is actually the three

carbon sugar glyceraldehyde-3-phosphate, not glucose

ADP and NADP+ released from the Calvin Cycle are returned to the thylakoid membrane and used again in the light reactions

The cooperation of the light reactions and the Calvin cycle - Figure 10.5!,!!

4. The Relationship of Photosynthesis to Cellular Respiration Energy enters the ecosystem as sunlight

and gets stored as chemical energy in the organic products of photosynthesis which also releases oxygen as a by-product

During cellular respiration the organic molecules react with oxygen and are broken down to release the energy that phosphorylates ATP and powers cellular work - some of the energy dissipates as heat

The products of cellular respiration are used as the reactants of photosynthesis

Note the recycling of molecules but the flow of energy through the ecosystem

Figure 9.2!

An Overview of Cellular Respiration A catabolic pathway of exergonic reactions

Mostly occurs in the mitochondria; enzymes for cellular respiration are embedded in the mitochondrial membranes

Cellular respiration is the combustion of organic compounds by oxygen Organic compounds + oxygen carbon dioxide + water + energy C6H12O6 + 6O2 6CO2 + 6H2O + Energy (ATP & heat)

Fermentation is a partial degradation of organic compounds that does not involve oxygen Glucose carbon dioxide + ethanol + energy (ATP & heat)

The three stages of cellular respiration are glycolysis, the citric acid cycle and oxidative phosphorylation ATP is produced at each stage, but mostly by oxidative phosphorylation

An Overview of Cellular Respiration

Glycolysis, which occurs in the cytosol, converts glucose to pyruvate

Pyruvate is converted to CO2 during the citric acid cycle which occurs in the matrix of the mitochondria

Also during the citric acid cycle, electrons are added to NAD and FAD converting them to NADH and FADH2

These carry electrons to the inner membrane where they are added to oxygen to release energy during oxidative phosphorylation

Figure 9.6!

O2

H2O

Link to Video

Comparing Photosynthesis & Cellular Respiration

Feature Photosynthesis Cellular Respiration

Water Used Produced

Oxygen Produced Used

Carbon dioxide Used Produced

Energy Used Produced

Organic molecules Produced Used

When does it occurs In the light Anytime

Organelle involved Chloroplast Mitochondrion

Which organisms use it?

Photoautotrophs including plants and algae

Most (all?) eukaryotes

Certain prokaryotes

Compare and contrast photosynthesis and cellular respiration for each of the following characteristics