Topic Plant Nutrition Biology November 16, 2005

1. Introduction Nutrition involves obtaining energy, carbon, and other essential organic and inorganic compounds for metabolism Plants require sunlight, CO 2, water and certain minerals Sunlight & CO 2 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 CO 2 to enter, O 2 & H 2 O 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  6CO H 2 O + light energy  C 6 H 12 O 6 + 6O 2 6H 2 O 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 H 2 O  2H + + 2e / 2 O 2 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 CO 2 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  C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O + 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 CO 2 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 FADH 2 These carry electrons to the inner membrane where they are added to oxygen to release energy during oxidative phosphorylation Figure 9.6! O2O2 H2OH2O Link to Video

Comparing Photosynthesis & Cellular Respiration FeaturePhotosynthesisCellular Respiration WaterUsedProduced OxygenProducedUsed Carbon dioxideUsedProduced EnergyUsedProduced Organic moleculesProducedUsed When does it occursIn the lightAnytime Organelle involvedChloroplastMitochondrion 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