Permeable selective barers and Transport of Macromolecules

Permeable selective barers

Cell membranes prevent the exchange of material freely from one side to the other at the same time. The plasma membrane must guarantee the exchange of molecules between the inner and outer parts at the right time.

Molecular transport

The plasma membrane contains a molecular transport machine from one side to the other which prevents molecules with low concentrations from entering the regional cells that have a high concentration. This machine allows cells to accumulate certain molecules in a higher concentration compared to the outside.
Signal delivery

Plasma membranes play an important role in cell response to signals. The process is called signal delivery. Cell membranes have respors that combine with certain molecules (ligands). Each different cell has a different receptor, which is able to recognize and respond to ligands in different environments.

Intercellular interaction

Cell membranes mediate cell-to-cell interactions in multicellular organisms. Cell membranes allow cells to know each other, bond and exchange material and information
Mechanism of Transport through Cell Membranes

Small molecules and ions move across the plasma membrane in two directions like sugar, amino acids and other nutrients enter the cell, and waste metabolism products leave the cell. Cells absorb oxygen for cellular respiration and release dioxidal carbon. The cell also regulates the concentration of inorganic ions such as Na +, K +, Ca2 + and Ca- by reversing their direction from one direction to another across the plasma membrane.

Although the traffic through this membrane is dense the cell membrane is selectively permeable (the membrane can only be traversed by certain polar ions and molecules), semipermeable (easily passed by water molecules) and substances cannot cross the barrier arbitrarily. These cells can take a variety of small molecules and ions and reject the other besides that the substances move across the membrane at different rates.

Cell membranes have a function in the movement of ions or molecules from inside or outside the cell. According to Campbell, the central part of the hydrophobic membrane blocks the transport of polat ions and molecules which are both hydrophilic. The structure of lipid bilayers is the cause of permeable selective properties in membranes. The molecular or ionic movements that occur in cell membranes and other organelles are:

Diffusion

Simple diffusion
Ulustrasi proses difusi pada larutan
Illustration of the diffusion process in the solution
Diffusion is a spontaneous process in which molecules move from areas of high concentration to areas with low concentrations. Membranes are selectively permeable which affects the diffusion rate of several types of molecules. One type of molecule that diffuses freely through many types of membranes is water.

Diffusion depends on the random movement of a solute. Molecules can pass through the plasma membrane by a simple diffusion path that is very limited in number and for this reason the plasma membrane still has a barrier.

Micromolecules especially hydrophobic types can easily pass through the plasma membrane. The ability of cells to be able to sort hydrophilically with a small molecular weight (BM) of compounds that have a high BM is often caused by the presence of porous in the plasma membrane. There are two types of porous.

The first type that can penetrate integral proteins or between groups of transmembrane protein molecules. The second type of porus is called statistical porus that forms randomly on the plasma membrane and penetrates the lipid bilayer.

Facilitated Diffusion

The diffusion of a compound or molecule across the membrane always occurs from areas with high concentrations to areas with low concentrations, but diffusion does not always occur through lipid bilayers or an open channel.

A number of substances are known to diffuse by first binding to a mebran protein called facilitative transporter which facilitates the diffusion process. Binding of molecules or compounds in the facilitator's transportitives on one side will trigger a transformational change in the protein and cause the solute to diffuse into low concentrated areas.

Compounds that pass through the plasma membrane by diffusion are facilitated also do not require ATP involvement, as is simple diffusion. But the movement of compounds from outside to inside or vice versa is faster than simple diffusion.

This is caused by the presence of carrier proteins that accelerate transport. The carrier protein molecule after binding to the compound or molecule to be carried, immediately moves the compound / molecule from the outside in or vice versa.

Osmosis

Osmosis is the event of the transfer of water molecules (solvents) through a semipermeable membrane from a low concentrated solution to a high concentration solution. This osmosis event occurs in cells.

The event depends on the ratio of the concentration of the solution inside and outside the cell. If the concentration of the solution outside the cell is lower than the solution in the cell, it means the cell is in a hypotonic solution. The concentration of the solution outside the cell is higher than the solution in the cell, meaning the cell is in a hypertonic solution.

Active Transport

Active transport is transport that uses energy to excrete and insert ions and molecules through a selectively permeable cell membrane. Active transport is affected by electrical charges inside the cell and outside the cell. This electric charge is determined by sodium ions (Na +), potassium ions (K +), and chlorine ions (C1-).

The entry of Na + and K + ions is regulated by the sodium-potassium pump. The sodium-potassium pump is responsible for the Na + and K + double active transport from inside the cell out. ATP provides energy for transport. The pump secretes three Na + ions from inside the cell for every two K + ions that are inserted into the cell. In the transport protein, there are Na + and K + called binding sites.


Active transport stages that occur in the cell membrane

• Three sodium ions (Na +) are taken in cells and occupy the binding sites (where the bonds of ions or molecules occur in the membrane).
•  Energy is needed to change the shape of an integral protein in the membrane to open to the outside of the cell.
• The integral protein in the membrane opens outward from the cell, then releases sodium ions out of the cell.    Two potassium ions (K +) from outside the cell occupy the binding sites on integral proteins.
•  The integral protein in the membrane returns to its original shape, which is opening towards the cell.
• The potassium ion is released into the cell.

Transport of Macromolecules through the Plasma Membrane

Macromolecules such as proteins or polysaccharides cannot pass through transmembrane proteins that act as carriers. But the cell can still enter and remove the macromolecules.

The transport of macromolecules is very different from the transport of micromolecules. The mechanism of transporting macromolecules from the external environment into a vesicle is carried out through a fold or invagination of the plasma membrane. Macromolecule removal from the extracellular matrix can be divided into two categories, phagocytosis, namely the taking of solid maromolecules and pinocytosis of taking liquid material.

Phagocytosis

Phagocystosis ("cell eating") is a common solid material taken by certain types of cells to be carried to lysosomes.

Single-celled organisms such as Amoeba and Ciliata take food by capturing food particles or small organisms by covering it with plasma merman. The fold then fuses to form sutu vacuoles (phagosomes) which will separate from the plasma membrane. Phagosomes will then join with lysosomes to reach intracellular food.

In some high-level animals, phagocytosis is more of a protective mechanism than the way food is taken. Mammalia has a variety of phagocyte cells such as macrophages and neutrophils that are found in the blood and other tissues that will "eat" organisms, cells that have been damaged, red blood cells that have been old or debris.

Endocytosis

In endocytosis, cells enter very small macro molecules and matter by forming new vesicles from the plasma membrane. The steps are basically the opposite of exocytosis.

A small portion of the plasma membrane is buried deepest into a pocket. As soon as this bag gets deeper, the bag is pinched, forming a vesicle containing material that is already outside the cell. There are three types of endocytosis, namely phagocytosis (cellular feeding) pinositosi (cellular drinking) and endocytosis which is receptor-bound.

Endocytosis can generally be divided into two groups: bulk-phase endocytosis and receptor-mediated endocytosis. Bulk-phase endocytosis takes extracellular fluid without an introduction process by the plasma membrane surface. Bulk-phase endocytosis can be observed by providing certain ingredients in culture medium such as the horseradish peroxidase enzyme which cells will take in general. Receptor-mediated endocytosis is the taking of certain macromolecules (ligands) that will bind to receptors on the outer surface of the membrane.

Exocytosis

Cells secrete macro molecules by combining vesicles with a plasma membrane called exocytosis. The transforic vesicles that are released from the golgi apparatus are transferred by the cytoskeleton to the plasma membrane. When the vesicular membrane and plasma membrane meet, the second bilayer lipid molecule rearranges itself so that the two membranes join. The vesicles are then spilled from the cell.