Chloroplasts are membrane-bound organelles present in green plants and algae. They contain chlorophyll, the biochemical used by plants for photosynthesis, which converts the energy from light into chemical energy that powers the plant’s activities. In addition, chloroplasts contain DNA and help an organism synthesize proteins and fatty acids. They contain disk-like structures, which are membranes called thylakoids.
Chloroplasts measure about 4 to 6 microns in length. The chlorophyll within chloroplasts makes plants and algae green. In addition to the thylakoid membranes, each chloroplast has an outer and inner membrane, and some species have chloroplasts with additional membranes. The gel-like liquid inside a chloroplast is known as stroma. Some species of algae have a cell wall between the inner and outer membranes composed of molecules containing sugars and amino acids. The chloroplast’s interior contains various structures, including DNA plasmids and ribosomes, which are tiny protein factories.
Thylakoids float freely within the chloroplast’s stroma. In higher plants, they form a structure called a granum that resembles a stack of coins 10 to 20 high. Membranes connect different grana to each other in a helical pattern, though some species have free-floating grana. The thylakoid membrane is composed of two layers of lipids that might contain molecules of phosphorus or sugar. Chlorophyll is embedded directly in the thylakoid membrane, which encloses the watery material known as the thylakoid lumen.
A thylakoid’s chlorophyll component makes photosynthesis possible. The process begins with the splitting of water to create a source of hydrogen atoms for energy production, while oxygen is released as a waste product. This is the source of the atmospheric oxygen we breathe. The subsequent steps use the liberated hydrogen ions, or protons, along with atmospheric carbon dioxide to synthesize sugar. A process called electron transport makes energy-storage molecules such as ATP and NADPH. These molecules power many of the organism’s biochemical reactions.
Another thylakoid function is chemiosmosis, which helps maintain an acidic pH in the thylakoid lumen. In chemiosmosis, the thylakoid uses some of the energy provided by electron transport to move protons from the membrane to the lumen. This process concentrates the proton count in the lumen by a factor of about 10,000. These protons contain energy that is used to convert ADP to ATP. The enzyme ATP synthase helps this conversion. The combination of positive charges and proton concentration in the thylakoid lumen creates an electrochemical gradient that provides the physical energy necessary for ATP production.