Chloroplast
Chloroplasts are double-membrane-bound organelles found exclusively in plant cells and algae that capture light energy and convert it into chemical energy through photosynthesis, producing glucose and oxygen from carbon dioxide and water. They contain a system of flattened membrane sacs called thylakoids, arranged in stacks called grana, which house the light-absorbing pigment chlorophyll and the protein complexes of the light-dependent reactions. Like mitochondria, chloroplasts contain their own circular DNA and ribosomes, supporting the endosymbiotic theory that they evolved from ancient photosynthetic cyanobacteria.
Chloroplast Structures and Their Roles in Photosynthesis
| Structure | Description | Role in Photosynthesis |
|---|---|---|
| Outer membrane | Smooth permeable membrane | Regulates molecule entry into chloroplast |
| Inner membrane | Less permeable membrane | Controls transport of metabolites |
| Thylakoid membrane | Flattened internal membrane sacs | Site of light-dependent reactions; contains chlorophyll |
| Granum (plural: grana) | Stack of thylakoids | Increases surface area for light capture |
| Stroma | Fluid surrounding thylakoids | Site of light-independent reactions (Calvin cycle) |
| Chlorophyll | Green pigment in thylakoid membrane | Absorbs light (mainly red and blue wavelengths) |
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Related Terms
Mitochondria
Mitochondria are double-membrane-bound organelles found in the cytoplasm of eukaryotic cells that serve as the primary site of cellular respiration, generating adenosine triphosphate (ATP) through oxidative phosphorylation on the inner membrane's folds called cristae. They contain their own circular DNA and ribosomes, supporting the endosymbiotic theory that they evolved from ancient free-living bacteria engulfed by ancestral eukaryotic cells. Mitochondria regulate cell metabolism, calcium signaling, and apoptosis, and their number in a cell varies from one to several thousand depending on the cell's energy demands.
Eukaryote
A eukaryote is an organism whose cells contain a membrane-bound nucleus housing the DNA, along with other membrane-enclosed organelles such as mitochondria, endoplasmic reticulum, and the Golgi apparatus. Eukaryotes include all plants, animals, fungi, and protists, and their cells are generally much larger and more structurally complex than prokaryotic cells. The compartmentalization of cellular functions within organelles allows eukaryotes to perform more specialized and regulated biochemical processes.
Cell Membrane
The cell membrane, also called the plasma membrane, is a thin, flexible bilayer of phospholipid molecules embedded with proteins that surrounds every cell and forms its outermost boundary. It is selectively permeable, meaning it regulates the passage of ions, nutrients, and waste products into and out of the cell through passive diffusion, facilitated diffusion, and active transport. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes its structure as a dynamic, two-dimensional fluid in which proteins float within or are attached to the phospholipid bilayer.
From Greek "chloros" meaning green or pale green and "plastos" meaning formed or molded. The term was first used in the 19th century by Swiss botanist Carl Wilhelm von Nägeli around 1839 to describe the green, plastid-like bodies in plant cells.