Metabolism-Autotrophy

Keyword:Biol120Metabolism-Autotrophy

Topics

Autotrophy
Chloroplast review
Light properties and pigments
Photosystems
Light Reactions
Non-cyclic
Cyclic
Light-independent reactions
Calvin Benson cycle (CBC)
Photosysnthesis in ecosystems
Variations on the theme
C3 and C4 plants
CAM
Chemoautotrophy

Autotrophy autotrophs="self-nourishing" or "eating". Use sunlight (photoautotrophs) or some chemical substance (chemoautotrophs) as an energy source

Heterotrophs= "other eating"; get energy and carbon compounds from other organisms

Photosynthesis-primary pathway of autotrophy on this planet; most life on this planter depends directly or indirectly on this process

Chloroplast review

Light Properties and Pigments

Visible light only a small part of the electromagnetic spectrum that bombards the earth from space-varies from very short wavelength gamma rays to long wave length radio and microwaves

Photosystems

The photosystems (I and II) consist of light absorbing pigments, primarily chlorophyll (chl a and chl b) and other assorted pigments such as carotenoids
Each pigment absorbs particular wave lengths of light maximally






Photosystem I-reactive center=P700
Photosystem II-reactive center=P680
numbers refer to the wavelengths of light each absorb maximally

In some plants Photosystem I occurs alone, in other it is joined to Photosystem II via an ETS Light Reactions

energy from sunlight is used to drive build a proton gradient in the lumen, generate ATP and reduce NADP+

Two cycles within the Light Reaction: Cyclic and non-Cyclic photophosphorylation

Non-cyclic

Cyclic

Summary of the light reactions

• The pigments in the photosystems capture energy from sunlight, which they use to excite electrons
• These high energy electrons are passed along an ETS, where they give up energy at each step. This energy is used (depending on the system) to move protons into the lumen and/or to produce NADPH (good reducing agent)
• The protons are released from the lumen via CF1 channels. The energy these protons give up as they leave is stored in new phosphate bonds, as ATPs are made from ADPs

Significance of Photosystem II

• Perhaps most important legacy is not the production of ATP (possible with Photosystem I) but the production of oxygen Early earth had a reducing atmosphere-little if any free oxygen

  production of oxygen by plants enable the development of aerobic respiration, much more efficient means of metabolism which made larger, more complex organisms possible (more later) Single most important evolutionary step in earth's history ??

Light-Independent Reactions

production of glucose and other storage products in the stroma through the use of ATP, NADPH, CO2 (carbon dioxide) and enzymes

Process called the Calvin-Benson Cycle, after Melvin Calvin who won the Nobel prize for his work on the pathway

Calvin-Benson Cycle (CBC)



















































Summary: Six turns of the CBC gives one molecule of glucose and 10 G3Ps

• The G3Ps are returned to the cycle to power future cycles
• Six turns of the cycle cost 12 NADPH and 18 ATP to phosphorylate and reduce PG and to phosphorylate RP
• The ADP, NADP+ and the unassociated phosphate group (from the NADPH reduction of DPG) are returned to the thylakoids, where the light-dependent reactions recycle them to produce more ATP and NADPH.

Most life on earth is completely dependent on these processes of photosynthesis

• The molecules used (CO2 and H2O) are very stable (low free energy) and would not normally undergo the reactions above without large amounts of energy
• This energy is provided by sunlight-large scale extinctions and other disturbances in the past may be linked to changes in the quality or quantity of sunlight reaching the earth

Uses of glucose and CBC intermediates

• glucose: high energy bonds in glucose can be broken and used to form ATP, stored as starches, important in the production of cellulose
• G3P-used by plants to produce phospholipids, amino acids etc.

Photosynthesis in the ecosystem

• Terrestrial systems: large plants we're most familiar with
• Aquatic/marine systems: Huge numbers of single celled or colonial phytoplankton in the lakes, rivers and oceans. One of the bases of the aquatic food-web, extremely productive

Variations on the photosynthesis theme Photosynthesis is, like most reactions, affected by environmental conditions-the efficiency of the "standard" photosynthetic pathway we've described drops under hot, dry and intensely sunlit conditions

C3 vs. C4 plants








































The C4 plants avoid photorespiration










































CAM plants



















































Other types of autotrophy

Chemoautotrophy




















































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Last Modified: Sept. 4, 1995
All contents copyright (C) 1995.
Western Kentucky University.