Thylakoid Definition and Function

What Thylakoids Are and How They Work

Cutaway illustration of plant chloroplast showing the sheet-like thylakoid or lamellae supporting the stacks of grana, which resemble coins.
Cutaway illustration of plant chloroplast showing the sheet-like thylakoid or lamellae supporting the stacks of grana, which resemble coins. FRANCIS LEROY, BIOCOSMOS/SCIENCE PHOTO LIBRARY. / Getty Images

Thylakoid Definition

A thylakoid is a sheet-like membrane-bound structure that is the site of the light-dependent photosynthesis reactions in chloroplasts and cyanobacteria. It is the site that contains the chlorophyll used to absorb light and use it for biochemical reactions. The word thylakoid is from the Green word thylakos, which means pouch or sac. With the -oid ending, "thylakoid" means "pouch-like".

Also Known As: Thylakoids may also be called lamellae, although this term may be used to refer to the portion of a thylakoid that connects grana.

Thylakoid Structure

In chloroplasts, thylakoids are embedded in the stroma (interior portion of a chloroplast). The stroma contains ribosomes, enzymes, and chloroplast DNA. The thylakoid consists of the thylakoid membrane and the enclosed region called the thylakoid lumen. A stack of thylakoids forms a group of coin-like structures called a granum. A chloroplast contains several of these structures, collectively known as grana.

Higher plants have specially organized thylakoids in which each chloroplast has 10-100 grana that are connected to each other by stroma thylakoids. The stroma thylakoids may be thought of as tunnels that connect the grana. The grana thylakoids and stroma thylakoids contain different proteins.

Role of the Thylakoid in Photosynthesis

Reactions performed in the thylakoid include water photolysis, the electron transport chain, and ATP synthesis.

Photosynthetic pigments (e.g., chlorophyll) are embedded into the thylakoid membrane, making it the site of the light-dependent reactions in photosynthesis. The stacked coil shape of the grana gives the chloroplast a high surface area to volume ratio, aiding the efficiency of photosynthesis.

The thylakoid lumen is used for photophosphorylation during photosynthesis.

The light-dependent reactions in the membrane pump protons into the lumen, lowering its pH to 4. In contrast, the pH of the stroma is 8. 

The first step is water photolysis, which occurs on the lumen site of the thylakoid membrane. Energy from light is used to reduce or split water. This reaction produces electrons that are needed for the electron transport chains, protons that are pumped into the lumen to produce a proton gradient, and oxygen. Although oxygen is needed for cellular respiration, the gas produced by this reaction is returned to the atmosphere.

The electrons from photolysis go to the photosystems of the electron transport chains. The photosystems contain an antenna complex that uses chlorophyll and related pigments to collect light at various wavelengths. Photosystem I uses light to reduce NADP+ to produce NADPH and H+. Photosystem II uses light to oxidize water to produce molecular oxygen (O2), electrons (e-), and protons (H+). The electrons reduce NADP+ to NADPH. In both systems.

ATP is produced from both Photosystem I and Photosystem II. Thylakoids synthesize ATP using an ATP synthase enzyme that is similar to mitochondrial ATPase. The enzyme is integrated into the thylakoid membrane.

The CF1-portion of the synthase molecule extended into the stroma, where ATP supports the light-independent photosynthesis reactions.

The lumen of the thylakoid contains proteins used for protein processing, photosynthesis, metabolism, redox reactions, and defense. The protein plastocyanin is an electron transport protein that transports electrons from the cytochrome proteins to Photosystem I. Cytochrome b6f complex is a portion of the electron transport chain that couples proton pumping into the thylakoid lumen with electron transfer. The cytochrome complex is located between Photosystem I and Photosystem II.

Thylakoids in Algae and Cyanobacteria

While thylakoids in plant cells form stacks of grana in plants, they may be unstacked in some types of algae.

While algae and plants are eukaryotes, cyanobacteria are photosynthetic prokaryotes.

They do not contain chloroplasts. Instead, the entire cell acts as a sort of thylakoid. The cyanobacterium has an outer cell wall, cell membrane, and thylakoid membrane. Inside this membrane is the bacterial DNA, cytoplasm, and carboxysomes. The thylakoid membrane has functional electron transfer chains that support photosynthesis and cellular respiration. Cyanobacteria thylakoid membranes don't form grana and stroma. Instead, the membrane forms parallel sheets near the cytoplasmic membrane, with enough space between each sheet for phcobilisomes, the light harvesting structures.