Chlorophyll is what gives plants their green color. The chlorophyll located in the chloroplasts captures the light energy that drives the synthesis of food molecules in the chloroplasts—photosynthesis.

Chloroplasts are found mainly in the cells of the mesophyll, which is the tissue in the interior of the leaf. Stomata are the pores that allow carbon dioxide to enter and oxygen to exit the leaf. The stroma is the dense fluid content of the chloroplast.

Chloroplasts are the organelles that contains chlorophyll. Mitochondria produce ATP and are not directly involved in capturing light and photosynthesis. The Golgi apparatus is involved in packaging substances, and Smooth endoplasmic reticulum are involved in lipid production.

A stack of thylakoids is known as granum. Stroma is the region outside the thylakoid membranes, but still inside the chloroplast. Mitochondria is the organelle that produces ATP, and there is no such organelle called thylakoidum.

During the light dependent reaction, protons are pumped from the thylakoid stroma into the lumen. Then, these protons flow down their electrochemical gradient (from high concentration to low concentration), through the ATP synthase protein, producing ATP.

Chlorophyll is used to capture sunlight during the light dependent reaction; the excited electrons then flow down the electron transport chain located on the thylakoid membrane.

During the light dependent reaction, protons are pumped from the thylakoid stroma into the lumen. Then, these protons flow down their electrochemical gradient (from high concentration to low concentration), through the ATP synthase protein, producing ATP.

As described in the beginning of this question, collenchyma cells are found in young stems and petioles (leaves) and function to provide flexible support to the plant. This is because chollenchyma cells lack secondary cell walls and do not produce lignin to harden them—this protein is characteristic of sclerenchyma cells, which are also used to provide support/strength to the plant. 

Due to their lack of rigidity, collenchyma cells a also capable of elongating with the stems and leaves they support, allowing them to remain alive at maturity. Sclerenchymal cells lack this ability.

As described in the background to the question, sclerenchyma cells are specialized to support the plant as it grows. These cells have thick secondary walls that are further strengthened by the hardening agent called lignin.  As a result, these cells are highly rigid and inflexible.  

At maturity, these cells cannot elongate and are found in regions of the plant that have stopped growing. In some parts of the plant, the sclerenchyma cells may even be dead; however, the rigid walls remain and act like a skeleteon that supports the remainder of the plaint over its lifetime.  

Sclerenchyma cells can also further differentiate into two types called sclereids and fibers. Sclerids can provide hardness to nut shells. Fibers, as their name suggests, are usually arranged in long threads and have commercial uses, such as being made into rope.

The vascular system in plants is designed to transport materials (water, nutrients, food) between the roots and shoots. There are two primary types of tissue dedicated to these processes. Xylem transports water and dissolved minerals upward from the roots; phloem transports sugars—the products of photosynthesis—from where they are synthesized to where they are needed, such as roots and new growth areas of leaves and fruits.

Both xylem and phloem are comprised of a variety of cell types that are specialized for transport and support.