Thylakoid

Eating the sun... —Oliver Morton

(CC) Diagram: Emmanuel Boutet
Schematic of a chloroplast: 1. outer membrane; 2. intermembrane space; 3. inner membrane (1+2+3: envelope); 4. [stroma (aqueous fluid); 5. thylakoid lumen (inside of thylakoid); 6. thylakoid membrane; 7. granum (stack of thylakoids); 8. thylakoid (lamella); 9. Starch; 10. Ribosome; 11. plastidial DNA; 12. plastoglobule (drop of lipids).

Inside plant cells and other eukaryotic cells that perform photosynthesis, tiny, bacteria-sized organelles, called chloroplasts, contain, within the inner membrane of their dual membrane structure, an extensive system of single-membrane-bound disk-shaped sacs called thylakoids, their interior spaces (lumens) interconnected, their membranes housing the pigment molecules that absorb the energy of photons of particular frequencies emitted by the sun, an event that initiates the physico-chemical sequence of steps culminating in the products of photosynthesis. A semifluid matrix within the chloroplasts, called the stroma, bathes the thylakoids, the disk-shaped sacs clustering in numerous stacks, called grana, their lumens interconnected by extensions, called lamellae. [See accompanying illustration at right.]

(PD) Drawing: Genome Management Information System, Oak Ridge National Laboratory

Prokaryotic cells that carry out photosynthesis (e.g., cyanobacteria) do not contain chloroplasts; their thylakoid membranes reside within the cytoplasm of the cells, as shown in the accompanying figure at left. 

In general terms, the photosynthetic process enables the energy of photons radiated from the sun to energize electrons in special photon-absorbing pigment molecules embedded in the thylakoid membranes, electrons ultimately supplied by the splitting of water molecules in a reaction that also converts water's oxygen atoms to molecular oxygen for release into the atmosphere and for use by the plant.

The thylakoid membranes also contain molecular clusters that transfer the energy of the energized electrons to energy-carrier molecules—ATP and NADPH—the energy therein used in metabolic reactions in the stroma to synthesize organic compounds using the inorganic carbon compound, carbon dioxide, as the carbon source starting material. In generating ATP, some molecules in the thylakoids' electron transfer chains pump protons from the stroma to the thylakoid lumen, making the lumen more acidic. The higher lumen concentration of protons compared to that in the stroma leads to protons passing back into the stroma through a pathway in the enzyme, ATP synthase, a complex molecule in the thylakaloid membrane that utilizes the energy inherent in the proton concentration gradient to synthesize ATP, converting ADP to ATP. ATP is a key energy source for synthesizing organic compounds from carbon dioxide in the stroma, a phase of the photosynthetic process called the Calvin cycle.

(PD) Drawing: http://genomics.energy.gov/. Genome Management Information System, Oak Ridge National Laboratory. http://genomicscience.energy.gov/roadmap/ Genomics:GTL Roadmap, U.S. Department of Energy Office of Science, August 2005. See text in preceding paragraph and legend-box at right.

Figure legend: In the chloroplast's thylakoid membranes, electrons in a pigment-protein complex, called photosystem II (PS-II), excited by PS-II-captured photons, are shuttled to the next step in an electron-chain of proteins, the transported electrons replaced by PS-II-splitting (oxidation) of water molecules (H2O), releasing protons (H+ ions) and oxygen into the thylakoid lumen. ATP and NADPH contribute energy and electrons (reducing equivalents) for the conversion of carbon dioxide to carbohydrate, in the stroma of the chloroplasts in eukaryotes, in the cytoplasm in prokaryotes. In some circumstances, green algae and cyanobacteria can use electrons to produce hydrogen gas (H2) by reduction of protons. Molecular complexes that mediate electron flow from water to carbohydratess or hydrogen gas are present the electron-transport chain found in the thylakoid membranes of cyanobacteria and some green algae.

The Internet has numerous images of thylakoids of particular instructive value. You will find links to those images on the Gallery subpage.