Cell Transport In & Out
For nutrition, communication, and defense, the swapping of substances is essential to all cells. Trafficking materials in and out of cells is controlled by a variety of proteins found in cell membranes, termed transporters. This commerce comprises letting vital compounds in, provisioning data and cellular products, as well as disposing of wastes by trucking them out.
Prokaryotes and eukaryotes differ in their ingestion and secretion. As prokaryotes lack membrane-bound organelles, their transport mechanics are simpler.
Once a meal is found, an archaeon or bacterium internally produces enzymes, which it then exudes. The enzymes fracture the food down into digestible bits.
The prokaryote then sucks the substance in, either by osmosis or active transport. Via osmosis, food simply flows in through the cell membrane. If osmosis fails, a prokaryote practices active transport: actively gobbling by pulling savory juices through the cell wall.
Osmosis takes advantage of an auspicious concentration gradient, owing to an unequal distribution of ions across the cell membrane. When diffusion disfavors a hungry prokaryote, active transport must be employed.
By contrast, eukaryotic cells employ their cell membranes for both intake and output. In endocytosis, a eukaryotic cell internalizes material from outside the cell. The opposite process, exocytosis, secretes select contents, often proteins, out of the cell membrane, onto the cell surface, or into extracellular space.
Both endocytosis and exocytosis employ vesicles as containers. A vesicle is formed from the cell membrane for endocytosis and absorbed back into the cell membrane after exocytosis. Thus, these complementary processes continuously recycle the plasma membrane.
Endocytosis
In endocytosis, a eukaryotic cell engulfs material to absorb it. There are 2 main types of endocytosis, distinguished by vesicle size and cellular machinery involved.
Cells drink via pinocytosis. The vesicles are small: convex pits for fluid collection and modest molecular uptake.
Pinocytosis is indiscriminate. Whatever chemicals are in the water are taken in.
Via phagocytosis, cells gobble large particles, be they cell debris or entire microorganisms.
While all cells continually ingest via pinocytosis, phagocytosis is a specialty act. In multicellular organisms, only specialized cells wolf down large chunks. For instance, immune system macrophages defend their organisms by swallowing microbial invaders.
Single-celled eukaryotes eat by engulfing their prey. For amoebas and protozoa, phagocytosis is a fancy term for dining.
In multicellular eukaryotes, endocytosis can play a crucial role in conforming a cell; driving shape changes by remodeling the cell membrane. This is one of many ways that the environment can influence cell development.
Exocytosis
In exocytosis, a cell exposes cellular products to select organelles or outside the cell. Proteins sequestered within an intracellular vesicle are let loose as the vesicle membrane fuses with a cell’s plasma membrane. The inner surface of the vesicle becomes the outer surface of the membrane.
Membrane fusion is an energetic exercise, requiring the coordinated interaction of adaptor molecules on both the vesicle and plasma membrane. The adaptors are highly selective, only allowing vesicles to fuse with membranes of certain organelles or the cell’s plasma membrane.
Once the appropriate adaptors bind with each other in an elaborate docking maneuver, stored ATP energy is released, forming a fusion pore between the vesicle membrane and plasma membrane.
Vesicle contents are released as the pore widens. Ultimately, the vesicle is either absorbed in the plasma membrane or recycled to the cytoplasm.
Exocytosis can be constitutive or regulated. Constitutive exocytosis occurs all the time; placing proteins, such as receptors, on the outside of a plasma membrane.
Regulated exocytosis is triggered when a cell receives a signal from outside. Calcium ions are typically involved in triggering.
The received signal is a substance, such as a hormone or neurotransmitter, which binds to a specific receptor on the cell surface. This activates the synthesis or release of a 2nd messenger within the cell.
Many secreted cellular products are for the tissue type in which the cell resides. Otherwise, outputs are transmitted to a more distant part of the organism. Cholesterol and hormones are exemplary secreted products.
Most exocytic products are enzymes or other proteins which have undergone rigorous quality control in the endoplasmic reticulum and Golgi complex. At the downstream end of the Golgi network, cellular products are sorted and accumulated in exocytic vesicles.
Exocytosis is a common vehicle for intercellular communication. Immune systems are extensive employers of exocytosis.
A cell harboring a virus displays viral by-products on its surface; a danger sign that attracts immune cells. Upon arrival, an immune cell tells an infected cell to self-destruct, to save its neighbors. Failing that, a good cell gone bad may be engulfed via phagocytosis.
