Kloroplasts

kloroplasts are organelles specialized for photosynthesis found in the cells of plants and many algae, and in some protists. They capture light energy and convert it into chemical energy stored in ATP and NADPH, which fuels carbon fixation and other metabolic processes.

Structurally, kloroplasts have a double membrane enclosing a semifluid stroma. Embedded within are thylakoid membranes arranged in stacks called grana, where chlorophyll and other pigments reside. The kloroplast contains its own circular DNA and ribosomes, reflecting its evolutionary origin from cyanobacteria.

Photosynthesis occurs in two stages. The light-dependent reactions in the thylakoid membranes generate ATP and NADPH with oxygen as a byproduct. The Calvin cycle in the stroma uses these energy carriers to fix carbon dioxide into triose sugars, which are later converted to starches and other carbohydrates in the kloroplast or exported to the plant.

Kloroplasts are inherited and replicated within cells, typically by binary fission, and can vary in number per cell. They are part of a class of organelles called plastids; other plastids include chromoplasts, which synthesize pigments, and leucoplasts, which store starch, lipids, or proteins. In roots and non-photosynthetic tissues, kloroplasts can differentiate into amyloplasts or other forms.

Kloroplasts play a central role in plant ecology as primary producers, contributing to atmospheric oxygen and providing fixed carbon for growth. Their presence and function are widely studied in botany, physiology, and evolutionary biology.