Thylakoid

Lore & Background

Thylakoids are membrane-bound structures embedded in the chloroplast stroma. A stack of thylakoids is called a granum, resembling a stack of coins. The thylakoid membrane is the site of the light-dependent reactions of photosynthesis, with photosynthetic pigments embedded directly in the membrane. It has an alternating pattern of dark and light bands measuring one nanometer each. The thylakoid lipid bilayer shares features with prokaryotic membranes and the inner chloroplast membrane, composed primarily of phospholipids and galactolipids, with galactolipids being more abundant. The thylakoid lumen is a continuous aqueous phase enclosed by the thylakoid membrane, playing an important role in photophosphorylation. During the light-dependent reaction, protons are pumped across the thylakoid membrane into the lumen, making it acidic down to pH 4. In higher plants, thylakoids are organized into a granum-stroma membrane assembly, with grana connected by stroma thylakoids. A recent electron tomography study revealed that stroma lamellae are organized in wide sheets perpendicular to the grana stack axis, forming multiple right-handed helical surfaces at the granal interface, with left-handed helical surfaces consolidating between them. Thylakoid formation requires light and the action of vesicle-inducing protein in plastids 1 (VIPP1). Plants cannot survive without this protein, and reduced VIPP1 levels lead to slower growth and paler plants with reduced ability to photosynthesize. VIPP1 is conserved in all organisms containing thylakoids, including cyanobacteria, green algae such as Chlamydomonas, and higher plants such as Arabidopsis thaliana.

Reader's Guide

Thylakoids are fundamental to life on Earth as the site of the light-dependent reactions of photosynthesis, the process by which light energy is converted into chemical energy. Their structure—membrane-bound compartments with a lumen—enables the generation of a proton gradient used by ATP synthase to produce ATP. The organization of thylakoids into grana and stroma lamellae maximizes surface area for light capture and electron transport. The thylakoid membrane houses four major protein complexes: Photosystems I and II, Cytochrome b6f complex, and ATP synthase, which together drive electron transport and chemiosmotic coupling. Understanding thylakoid structure and function is critical for insights into photosynthesis, bioenergy, and plant biology. The requirement of VIPP1 for thylakoid formation underscores the complexity of chloroplast development.

Did You Know?

• The word 'thylakoid' comes from the Greek word 'thylakos' meaning 'sac' or 'pouch'.
• During the light-dependent reaction, protons are pumped into the thylakoid lumen, making it acidic down to pH 4.
• Thylakoid formation requires the action of vesicle-inducing protein in plastids 1 (VIPP1), and plants cannot survive without this protein.