Primarily, horse chestnuts receives nutrition from the photosynthesis it undergoes.  The complexity of photosynthesis can be summarized in one simple equation:

So, where does this happen?

The conversion in this equation takes place in the chloroplasts of the plant.  This organelle is easily identified by the green pigment, chlorophyll, located within.  The chloroplasts are each made up of a double membrane, containing an inner and an outer layer, stroma and thylakoids.  Thylakoids consist of membranous sacs that contain the thylakoid space.  These membranes include massive amounts of chlorophyll.  The stroma is simply the fluid in between the thylakoids.  Multiple chloroplasts come together to make up what is called a mesophyll cell, which consequently make up the mesophyll layer of the leaf.

So, how do these chloroplasts obtain Carbon dioxide, water and light?

Carbon dioxide is obtained from the surrounding atmosphere and taken in through pores in the epidermis of the leaves called stomata.  This carbon dioxide passes through the stomata based on its partial pressure gradient, fundamentally the difference in the amount of carbon dioxide in the atmosphere and inside the leaf.  The main complication in obtaining this gas exchange is the unavoidable loss of water that occurs due to the stomata opening.  However, this process is relatively simple due to the small distance that has to be traveled by carbon dioxide to the chloroplasts. 

In contrast, water is primarily obtained from the ground by the roots of the tree and is forced to travel to sites with chloroplasts.  The first step in this process for water is obtaining access to the transport vessels (xylem).  The water starts by entering the roots through extensions of the epidermis called root hairs or simple diffusion across the epidermal layer.  The water molecules then continue into the cortex of the root until it reaches the Casparian strip located in the endodermis cell layer of the root.  This waxy strip forces the water molecules to cross the membrane of the endodermis cells.  The water molecules are forced to take the symplastic route, a very important aspect to ensure that no air bubbles can gain entry into the xylem.  The second step in the process is for the water to move successfully from the roots to the rest of the tree.  Four factors are responsible for this movement: transpiration , adhesion, cohesion, and tension.  Water molecules hydrogen bond with themselves and the walls of the xylem, resulting in a strand of connected water.  As water is lost from the leaves a negative pressure is created that pulls the column of water molecules up the vessel.  Lastly, light is simply obtained from either genuine or artificial sunlight. 

So, how exactly are sugar and oxygen produced?

The complex process of photosynthesis can be divided up into two cycles, the light reactions and the Calvin cycle.  The purpose of the light reactions is to convert the solar energy that is acquired by the plant into chemical energy that can be used towards the Calvin cycle.  First, water molecules are each split into two hydrogen ions, 2 electrons and an oxygen molecule.  When light hits the chlorophyll in Photosystem II, the supplied electrons are excited and passed down membrane proteins to Photosystem I.  When light hits the chlorophyll in Photosystem I, the recieved electrons are again excited and continue through more membrane proteins until it reaches NADP+ reductase.  At this point the excited electrons are joined by NADP+ and a hydrogen ion from the Calvin cycle to form NADPH.  At the same time the hydrogen ions from water splitting along (along with hydrogen ions from other sources) form a concentration gradient inside the thylakoid space.  These ions exit the thylakoid through an enzyme called ATP synthase, allowing for the creation of ATP.  The byproduct of this reaction is oxygen and the generated ATP and NADPH are utilized in the next cycle. 

In a nutshell the Calvin cycle uses carbon dioxide from the air and ATP and NADPH from the light reactions to create glucose.  The diagram to the right shows more detail to each phase, but this cycle can be divided up into three phases.  The first phase is carbon fixation.  Each carbon dioxide molecule is attached to RuBP, a five carbon sugar, by an enzyme called rubisco.  The process utilizes 6 ATP and 6 NADPH before it forms two molecules of 3-phosphoglycerate.The next phase, reduction, actually creates the three-carbon sugar used to form the sugar.  During this phase each molecule of 3-phosphoglycerate receives a phosphate group and a pair of electrons.  Each carbon dioxide molecule yields 6 of these three-carbon sugars, but 5 of them are recycled into the third phase without being processed into glucose.  This last phase utilizes 3 ATP to reform the RuBP that is attached to carbon dioxide in the first phase. 

So, how is this generated sugar transported throughout the tree?

The generated sugars are carried as part of the phloem sap along with amino acids, hormones, and minerals.  This sap moves from sites of sugar production, called sources, to sites of sugar use or storage, called sinks.  From the chloroplast, the sugar enters into the sieve tube members of the phloem.  In this process, the increase of sugar inside the sieve tube creates a water gradient, forcing water to enter the sieve-tube at the site of the source.  The positive pressure that is consequently generated functions to transport the sugar to an area of lower sugar concentration, the sink.  Therefore, this movement of sugar from source to sink cells using the phloem can occur in any direction in the tree.  The sieve-tube elements lack a nucleus at maturity and therefore need other cells to aid in the function of the phloem.  Therefore to the left you will see a diagram of phloem also including nucleated companion cells and parenchyma cells.
If you would like to learn about another photosynthetic organism visit Cymbopogon citratus. Also! If you would like to explore an organism with a completely different method for obtaining nutrients visit Brevibacterium linens.




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