Humboldt State University ® Department of Chemistry

Richard A. Paselk

VUB Biology

Fall 2001

Lecture Notes:: 22 October

© R. Paselk 2001
 
     
 

PHOTOSYNTHESIS

Look at chloroplast first. (Figure 10.2, p 170) [overhead, P 19.2] Note the similarities to a mitochondrian. As with mitochondria have a very tight inner membrane with a permeable outer membrane, and store energy as a proton gradient across the inner membrane. However, whereas mitochondria have invaginated the inner membrane to form cristae, the chloroplast invaginates inner membrane and then pinches it off to form thylacoid membranes (thylacoids). Note how the thylacoids are stacked, with granal lamellae between stacked thylacoids, and stromal lamellae facing the plastisol or stroma (chloroplast "matrix"). The components of the photosynthetic ETS will be differentially distributed between these two lamellae. Most of protons will be pumped into the lumen of the thylacoid system. (The important thing here is that the protons are being pumped out of the plastisol.)

 

PHOTOSYNTHESIS - The Z Scheme

for the Light Reactions

Let's look a bit at chlorophyll (Figure 10.8, p 175) and its interaction with light.[overheads] Find that there are many chlorophylls/active center. That is, only a small portion of the chlorophyll pool is actually involved in using the light energy (1/300 in Chlorella). What do the rest do? Act together as an antenna (Figure 10.10, p 176) [overhead, L]. The transfers between molecules take <10-10 sec with an efficiency of >90%. Higher plants also have b-carotenes to absorb other light frequencies. Aquatic plants have different dyes, since only blue-green light penetrates to depth.

So what is light energy used for? Look at the "Z" scheme for photosynthetic electron transport. (Figure 10.11, p 177) [overhead] Note the three major complexes are not directly connected. Like the mitochondrial complexes they are connected by carriers which diffuse between them. The cytochrome b6/cytochrome f complex is analogous to Complex III in mitochondria: same electron path and a (plastoquinone) Q cycle for proton pumping.

The initial removal of electrons from water to give oxygen uses a manganese complex.

Note the stoichiometry: one O2 : 2 H2O : 4 e- : 8 H+ pumped : 2 ATP : 2 NADPH

The overall proton pumping and ATP synthesis process is summarized in (Figure 10.15, 180) [overhead]. Note that cyclic photophosphorylation is also shown on this figure, involving PSI and

Finally, note the distribution of the components of the Z scheme (Figure 10.11, p 177) [overhead]

 

PHOTOSYNTHESIS - THE DARK REACTIONS

(CALVIN CYCLE)

We have just looked at the so-called Light Reactions of photosynthesis: the process whereby the energy in sunlight is converted by plants into biological useful energy (ATP) and reducing equivalents (NADPH). Now we want to look at how plants use ATP and NADPH to capture ("fix") carbon dioxide and make glucose: the Calvin Cycle.

Most of the reactions of the Calvin Cycle are familiar. Thus on the Calvin Cycle pathway diagram reactions 1-5 (numbered in bold italics) are from Gluconeogenesis/Glycolysis, and reactions 6-8 (numbered in outline font) are from the Pentose Phosphate Shunt, while reactions 9 & 11 are variations of familiar enzyme catalyzed reactions. The only really new reaction is catalyzed by Ribulose-1,5-bis phosphate carboxylase.

 

C4 Plant Photosynthesis

Background on plant strategies under high heat/light low water conditions

C4 Plants: These plants thrive in environments with high temperatures and low humidities where the stomata in the leaves must be closed. Under these circumstances carbon dioxide concentrations fall in the leaves while oxygen rises, favoring photorespiration over photosynthesis and greatly reducing productivity. (In photorespiration oxygen binds competitively with carbon dioxide at the active site of RuBisCo: the net result is that Ru-1,5-bis P is oxidized and energy and carbon are lost instead of gained.) In C4 plants the photosynthesizing cells are protected from the atmosphere by a layer of mesophyll cells. In these cells the PEP carboxylase reaction is used to capture carbon dioxide, with the resulting oxaloacetate carbons transported to the photosynthesizing cell. [Figure 10.17, p 183] {overhead, H} The first compound incorporating the carbon dioxide thus has four carbons and hence the name (unlike in the Calvin cycle where the first labeled compound, PGA is C3). The carbon dioxide is then released and used in the Calvin cycle. Note that these plants are investing extra energy from ATP to concentrate carbon dioxide. However, they tend to live in high light environments where cyclic Photophosphorylation can be used to make up this extra ATP with little trouble. A variety of transport mechanisms exist in different plant groups. In another mechanism, CAM carbon dioxide is taken up at night and incorporated into malate. The malate is then used the next day to make PEP. Thus the plants can keep their stomata closed during the day.

 

The Cell Cycle & Mitosis

An absolutely critical function of living organisms is replication, and since the cell is the functional unit of living organisms the replication of cells is a critical function of all living organisms. All cells can be said to go through a "cell cycle" involving growth and replication. For cells these phases are referred to as Interphase and mitosis.

Interphase consists of three stages:

Mitosis is commonly divided into 5 stages as noted in the table below:

Mitosis

 Late Interphase (G2)
c = centrosomes with centrioles.
Nucleus is well defined, containing one or more nucleoli, and fibrous chromatin. The centrosome has duplicated (in animals each centrosome has two centrioles). Small asters may be visible around the centrosomes.
 
 Prophase
a = early aster (mitotic spindle)
The mitotic spindle begins to form as the centromers move apart along the surface of the nucleus. The chromatin fibers condense into recognizable pairs of chromosomes. The nucleoli dissappear.
 
 Prometaphase
Fragmentation of nuclear membrane, microtubules interact with chromatin, connecting to kinetochores in centromere regions of chromatids. Spindle fibers extend between centrosomes. Chromatids dance around, approaching metaphase plate.
 
 Metaphase
ep = metaphase plate
Chromatids lined up on metaphase plate, an imaginary disk centered between the centromers. Bundles of microtubules are attached to each chromatid and the centromer on that side of the metaphase plate.
 
 Anaphase
if = spindle fibers between chromatids
n = nucleolus
Anaphase is initiated when the centromers of each chromosome separates as the chromatids are pulled towards their respective centromeres.
 
 Telophase & Cytokinesis
The non-kinetochore microtules elongate the cell, and daughter nuclei form at teh two poles. Nuclear membrane arises from fragements of the old nuclear envelope. In animals the cell is pinching in two (cytokinesis) via a cleavage furrow, driven by an actin filament based contractile system.
 
Figures scanned from: Edmund B. Wilson (1906) The Cell in Development and Inheritance, 2nd ed. Columbia University Press, The MacMillan Co. New York .

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Last modified 22 October 2001
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