What the Heck is a Chloroplast?

A question from Peter Greiner, who is a 7th-grader in Ms. Arlene Hicks' class at Pierce Middle School, Grosse Pointe, Michigan. Peter asked what some important parts were of plant cells. I chose the chloroplast.

Plant cells have everything all living cells have, e.g., membranes, ribosomes, a nucleus containing the chromosomes (DNA), storage material, and like bacteria, plants have a cell wall, only a plant cell wall is made of cellulose, and some other really funky things, while a bacterial cell wall is made of amino acids connected to sugars - the chemical name for the bacterial cell wall is, peptidoglycan (peptido, meaning peptides; and glycan, meaning sugar)

But to me, the most interesting thing about plant cells is the presence of chloroplasts (in leaves and sometimes in other parts of the plant). The chloroplast is a complex structure made of fat (lipid) and protein membranes all folded-around in a complicated way. The structural organization of the chloroplast is very important to its ability to function. Present within the matrix of the chloroplast are enzymes and a special chemical compound called chlorophyll. Chlorophyll is a complicated chemical which has the ability to absorb certain wavelenghts of visible light.

The chloroplast works in the following way: Light is actually both a physical particle and a wave of energy. The particle is called a photon. Chlorophyll embedded within the chloroplast structure (or if isolated and put into a bottle for that matter) is able to absorb the right "kind" of photons (as determined by the wavelength of the light encountered). However, the amazing reactions which later occur as a result of this photon absorption, can't occur without the chloroplast structure.

When chlorophyll in the chloroplast absorbs the photons, the energy of the photons is used to "knock" electrons from the chlorophyll (chlorophyll has a few kind of "loosely-held" electrons available) - and - these electrons jump onto a water molecule and "split" the water (H2O) molecule into 2 hydrogen (H) atoms and one oxygen (O) atom. This reaction is absolutely amazing! When 2 oxygen atoms become available, molecular oxygen (O2) forms. Therefore, a plant releases Oxygen (O2) whenever sunlight shines on it, and we breathe the oxygen - yaaaay for plants!

Now, the electrons associated with the hydrogens are released from the hydrogen atoms by this "splitting" of the water molecule. These electrons then "fall" down an energy hill (are shuttled along a complex series of enzyme/protein-controlled reactions). Some of the energy from the "fall" of these electrons is trapped in the form of ATP (adenosine triphosphate). The energy of these electrons is used to form a chemical bond between ADP and a phosphate group (both of which are already present within the cell). This ATP is very important to the life of a cell (is a source of energy that can be used in many different reactions). Further, some of these electrons are added, along with some hydrogen ions (H+), to NADP (nicotinamide adenine dinucleotide phosphate - oxidized form) to make another very important chemical called NADPH (nicotinamide adenine dinucleotide phosphate - reduced form). Therefore, the energy from sunlight has been transferred to ATP and has also provided the energy to get electrons from water to reduce NADP to NADPH, by these reactions.

Now, the plant is also capable of what are called light-independent reactions. In these reactions carbon dioxide (CO2), which the plant has taken inside via little structures called stomata becomes a part of a sugar molecule (sugar is nothing but carbon, hydrogen and oxygen arranged in a particular way). The sugar is made in the chloroplast in structures called stroma - and the CO2-addition reactions use the energy from the ATP and NADPH to work. These CO2-addition reactions are called CO2 "fixation". So, a plant is _continuously_ making sugar (its own food), by this process. Some plants make a _lot_ of sugar (sugar cane, sugar beets), and we isolate the sugar and use it to sweeten things. This ability to make sugar from CO2 (the synthesis part) the "splitting" of the water molecule using the energy of light (the photo part), is called Photosynthesis. And any organism which can do these reactions, e.g., algae (blue-green algae), certain bacteria (cyanobacteria - the oldest fossil yet found - over 3-billion years old - and alive ones are still here!), and plants, is called a photosynthetic organism.

The sugar the plant is making by CO2-fixation is ultimately broken-down in a many-step process inside the cytoplasm (the chloroplast does not participate specifically in these reactions - is mainly responsible for the "splitting" of water and sugar synthesis) to provide the energy to make more ATP fuel. This break-down of the sugar allows the energy present in the bonds which hold the atoms together in the sugar, to be made available for transfer to ATP (sort of like when you burn a piece of paper - you can feel the release of much of this energy, as heat). Now, this energy from the sugar molecules is transferred in the form of electrons, to ultimately allow the synthesis of LOTS more ATP. This ATP is THEN used all around the cell as a source of energy to make all of the things a plant needs to stay alive.

So, whenever you eat a plant, you are tasting sunlight! And whenever you exhale, you are helping a plant to stay alive! Were it not for plants, algae and bacteria, we would not have life as we know it on earth.

Book: Don't Touch That Doorknob!

Copyright John C. Brown, 1995
[ Top of Page | What the Heck?? | "Bugs" | My HomePage | KU Microbiology ]