Plants cannot move. This limitation causes plants to develop various mechanisms for adapting to their surroundings, attracting and repelling other organisms, absorbing and releasing nutrients and chemicals, and using these organisms and chemicals for their benefit and sustenance.
A plant’s interaction with the environment happens at the surface. Substances enter and exit through diffusion or active transport by transport proteins embedded in the surface membrane of the plant. The border control operation is mediated by the selective binding affinity of transport proteins and also by a membrane potential that exists across the membrane. This membrane potential drives directional flow, providing a pulling force.
At the surface are other sensory receptors that activate a protein kinase chain reaction to produce a particular response in this part of the plant. These chain reactions often multiply a signal to initiate a systematic response in other parts of the plant as well.
The mechanism for carrying substances or chemical signals to other parts of the plant is the vascular network of xylems and phloems. Water and minerals enter into the root system and travel upwards through the xylem into other parts of the plant. Sugars are produced in the leaves through photosynthesis and move from the point of sugar production to sugar storage via the phloem.
vascular network of a plant
The xylem sap, which contains water and minerals, is pulled upward through the xylem by water escaping through the stomata located at the surface of the plant. The water escapes when sunlight strikes the surface of the plant, causing the water to evaporate through the stomata. As the water evaporates, the water curves around the mesophyll cells, increasing surface tension, which causes increased water loss from the mesophyll cells. This water loss causes mesophyll cells to bring in more water from the xylem, which leads to the negative pressure pull of water into the roots. When the water comes in through transport proteins embedded in the epidermis of the root system, it also brings in the nutrients and substances necessary for the plant to survive.
When sunlight strikes the surface of a leaf, it also activates an electron donated by a water molecule, which flows down an electron transport chain in the thylakoid membrane of chloroplasts found in mesophyll cells. The electron transport chain ends with the the conversion of NADP into NADPH and the release of O2. The Calvin Cycle then combines NADPH with CO2 and converts it to glyceraldehyde 3-phosphate, a 3-carbon sugar, that is recombined to form sucrose and starch. Starch is stored as reserve energy for the plant, and sucrose is immediately transported to other parts of the plant for use and storage.
Sucrose is transported throughout the plant via phloem. The phloem consists of sieve tubes and companion cells. The sieve tubes are made up of sieve cells that have plasmodesmata at each end to allow substances through. The companion cells regulate the substances that enter and exit the sieve tubes. Substances move through the sieve tube by positive pressure flow, a push factor caused by density. A high concentration of organic substances at the source creates a diffusion gradient that draws water into the cells. This sugar-water sap moves by bulk flow through the sieve tube from the sugar source to the sugar sink.
In a living system, the architecture is found in the thickness of the wall, through the interactions between the elements that make up the thickness, and through the communication that happens in the void between the opposite walls. Communication happens by channeling forces that already exist, and making these forces do all the work. The intelligence is found not only in the complexity of the system, but in how little effort is required for the system to sustain itself over time.