Exocytosis (ek-soh-sy-TOH-sis; from Greek ἔξω "out" and English cyto- "cell" from Gk. κύτος "receptacle") is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.

Phagocytosis and pinocytosis

In endocytosis, membranes invaginate (like when you put a finger to an inflated balloon) to form a vesicle, leading the way materials within the cell. This process can take several forms, each involving its own specific cellular machinery.

In phagocytosis (cell eating the equivalent), the cell engulfs debris, bacteria, or other large objects. Phagocytosis is carried out in specialized cells called phagocytes, which includes macrophages, neutrophils and other white blood cells. The invagination produces a vesicle called phagosome, which are usually fused with one or more lysosomes containing hydrolytic enzymes. The materials in the phagosome are broken and degraded by these enzymes.

In pinocytosis (the equivalent of eating cell) cell engulfs extracellular fluid, including molecules such as sugar and protein. These materials enter the cell in a vesicle, but not mix with the cytoplasm. The epithelial cells in capillaries, pinocytosis used to make the liquid portion of blood in the capillary surface. The resulting vesicles travel through the hair cells and release their contents into the tissue around it, while blood cells remain in the blood.

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium.

Molecular diffusion is typically described mathematically using Fick's laws.

Osmosis


Osmosis is the diffusion of water through a semi-permeable membrane.[1] More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration). It is a physical process in which a solvent moves, without input of energy, across a semi-permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations.[2] Osmosis releases energy, and can be made to do work[3].


Shot of a computer simulation of the process of osmosisNet movement of solvent is from the less-concentrated (hypotonic) to the more-concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic. The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.

Passive Transport

A molecule or ion that crosses the membrane by moving down a concentration or electrochemical gradient and without expenditure of metabolic energy is said to be transported passively. Another name for this process is diffusion. All molecules and ions are in constant motion and it is the energy of motion - kinetic energy - that drives passive transport. Transport of uncharged species across a membrane is dictated by differences in concentration of that species across the membrane - that is, by the prevailing concentration gradient. For ions and charged molecules, the electrical potential across the membrane also becomes critically important. Together, gradients in concentration and electric potential across the cell membrane constitute the electrochemical gradient that governs passive transport mechanisms.

Active transport


The action of the sodium-potassium pump is an example of primary active transport.Active transport is the transportation of things from a region of lower concentration to a higher concentration. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses energy, unlike passive transport, which does not use any energy.

Cell membrane
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Illustration of a Eukaryotic cell membraneThe cell membrane (also called the plasma membrane or plasmalemma) is one biological membrane separating the interior of a cell from the outside environment.[1]

The cell membrane surrounds all cells and it is semi-permeable, controlling the movement of substances in and out of cells.[2] It contains a wide variety of biological molecules, primarily proteins and lipids, which are involved in a variety of cellular processes such as cell adhesion, ion channel conductance and cell signaling. The plasma membrane also serves as the attachment point for the intracellular cytoskeleton and, if present, the extracellular cell wall

Function
The cell membrane surrounds the protoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of skin. In fungi, some bacteria, and plants, an additional cell wall forms the outermost boundary; however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary[citation needed]. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues. The barrier is differentially permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.

Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are transduced and passed in a different form into the cell. For example, a hormone binding to a receptor could open an ion channel in the receptor and allow calcium ions to flow into the cell. Other proteins on the surface of the cell membrane serve as "markers" that identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system.