Cisternae are flattened, membrane-bound sacs or compartments within eukaryotic cells. These enclosed spaces are filled with fluid and play a role in various biochemical processes. They are involved in processing and transporting molecules throughout the cell.
Cellular Locations and Structure
Cisternae are found within several key organelles, each with a unique structural arrangement tailored to its function.
In the endoplasmic reticulum (ER), cisternae form a continuous, interconnected network of flattened sacs and branching tubules that extend throughout the cytoplasm. This interconnectedness facilitates the broad distribution of materials within the ER lumen. The ER’s structure varies, with some regions appearing as flat sheets and others as a network of tubules.
The Golgi apparatus is characterized by its stack of distinct, flattened membrane-bound sacs. These stacks display a clear polarity, with three primary compartments: the cis-Golgi network (CGN), the medial-Golgi, and the trans-Golgi network (TGN). The cis face is oriented towards the endoplasmic reticulum, serving as the entry point for molecules, while the trans face is the exit point. Unlike the interconnected network of the ER, the Golgi cisternae are generally separate compartments within their stack.
Within chloroplasts, the organelles responsible for photosynthesis in plant and algal cells, cisternae take the form of thylakoids. These are flattened, disc-like sacs suspended in the chloroplast’s stroma. Thylakoids are often arranged into stacks called grana, with individual grana interconnected by stromal lamellae. This arrangement provides a large surface area for the processes that occur within them.
Functional Roles in the Cell
Cisternae in each organelle perform specialized functions integral to cellular operations.
In the endoplasmic reticulum, rough ER (RER) cisternae are studded with ribosomes, making them the primary site for the synthesis and secretion of proteins destined for the cell exterior, other organelles, or insertion into membranes. Within the RER lumen, these newly synthesized proteins undergo folding and various modifications. Smooth ER (SER) cisternae, lacking ribosomes, are involved in lipid synthesis and the production of steroid hormones. Additionally, SER cisternae play a role in carbohydrate metabolism and the storage and regulation of calcium ions.
The Golgi apparatus cisternae sequentially process, sort, and package proteins and lipids received from the ER. Molecules enter at the cis-Golgi network, where initial modifications and sorting occur. As proteins and lipids move through the medial and trans cisternae, they undergo further biochemical modifications, such as the addition or removal of carbohydrate groups in glycoproteins. The specific enzymes present in each cisterna type dictate the modifications that occur. Finally, in the trans-Golgi network, modified proteins and lipids are sorted into vesicles and dispatched to their final destinations.
Chloroplast thylakoid cisternae are the site of the light-dependent reactions of photosynthesis. Their membranes house photosynthetic pigments, primarily chlorophyll, organized into photosystems, which absorb light energy and initiate reactions. The energy from these excited electrons is harnessed to pump protons into the thylakoid lumen, creating a concentration gradient. This proton gradient drives ATP synthase to produce ATP, while electrons reduce NADP+ to NADPH. Both ATP and NADPH power the subsequent light-independent reactions (Calvin cycle) in the chloroplast stroma, leading to sugar synthesis.