Cell Membrane And Transport - Session I

Have you ever wondered how the cells in our bodies are organized and how they are protected? Well, the answer lies in a fascinating model known as the fluid mosaic model. The fluid mosaic model describes the structure of the cell surface membrane, also known as the plasma membrane. We already know that plasma membrane acts as a barrier. It separates the internal environment of the cell from the external environment.

According to the fluid mosaic model, the plasma membrane is composed of a special structure called a lipid bilayer. This lipid bilayer consists of two layers of phospholipid molecules. Phospholipids are a type of lipid molecules that have hydrophilic head and hydrophobic tail. Hydrophilic means water attracting. Hydrophobic means water repelling. The head of the phospholipid is made up of a phosphate group. It has a strong attraction to water. On the other hand, the tail of the phospholipid is made up of fatty acid chains. It repels water strongly.

In the plasma membrane, the heads of the phospholipids in one layer face outward, towards the watery environment. Meanwhile, the heads of the phospholipids in the other layer face inward, towards the inside of the cell. The hydrophobic tails of the phospholipids are sandwiched in between the hydrophilic heads. This creates a hydrophobic interior region within the membrane.

In a plasma membrane, Phospholipid molecules can move around within their own layer. This makes the membrane flexible and fluid. It is like a constantly changing shape. This fluidity is really important for many things happening in the cell. It helps molecules move across the membrane. It allows the cell to change its shape when needed. So, the fluid nature of the lipid bilayer is useful and important for various processes that keep the cell functioning properly.

In addition to the phospholipid bilayer, the cell membrane also contains proteins. Integral proteins are a type of protein that is embedded within the lipid bilayer of the cell surface membrane. They have regions that span across the entire membrane. Portions of these integral proteins extend into both the inner and outer surfaces of the membrane. This unique structure allows integral proteins to interact with both the intracellular and extracellular environments. Intracellular means within the cell. Extracellular means outside the cell.

Peripheral proteins are attached to the inner or outer surface of the membrane. They are not embedded within the lipid bilayer. They do not span across the entire membrane. Peripheral proteins interact with the integral proteins and the lipid molecules in the membrane. Peripheral proteins help the membrane stay stable and take part in specific tasks inside the cell.

In the cell surface membrane, there are other important components like cholesterol, glycolipids, and glycoproteins. They are positioned between the layers of phospholipid. They interact with the hydrophobic tails. Cholesterol helps maintain an optimal level of fluidity. It prevents the membrane from becoming more rigid or more fluid. Cholesterol also makes the membrane less permeable to particular molecules by acting as a barrier.

Glycolipids are found on the outer part of the membrane. They have carbohydrates attached to them. These carbohydrates face the outside of the cell. Glycolipids are involved in recognizing other cells. They help cells stick together. They also have a role in the immune response.

Glycoproteins are proteins with attached carbohydrates. They are also on the outer part of the membrane. Glycoproteins help with cell recognition and communication. Some glycoproteins act as receptors. They receive signals from the environment and send them into the cell.

Cell signaling is the process by which cells communicate with each other to coordinate their activities. It involves sending and receiving signals through special proteins on the cell surface called receptors. When signaling molecules, like hormones bind to these receptors, they trigger a chain of events inside the cell. This leads to specific responses. These responses can include changes in gene expression, protein activity, and overall cell behavior.

Cells communicate with each other by releasing signaling-molecules. These signaling molecules are called ligands. These molecules can be hormones, growth factors, neurotransmitters, or cytokines. Signaling molecules are produced and released by cells into the extracellular space.

Cell surface receptors are proteins located on the plasma membrane of the target cell. These receptors have a specific structure that allows them to bind to a particular ligand. The binding between the ligand and the receptor is highly specific, similar to a lock and key mechanism.When a signaling molecule binds to its corresponding receptor, it induces a conformational change in the receptor protein. This change in shape activates the receptor.

Once the receptor is activated, it triggers a series of intracellular signaling events known as signal transduction. During signal transduction, proteins inside the cell can be turned on or off by adding or removing phosphate groups, which are small chemical tags. These changes in protein activity help transmit and amplify the signal. This allows the cell to respond appropriately to the initial signal it received.

A cellular-response is what happens inside a cell after receiving a signal. Some possible cellular responses include modifications in the functioning of proteins which alters their activity or interactions. The response might also include the release of specific molecules into the surrounding environment. In extreme cases, cell death might be triggered as a response to the signal received. The cellular response is specific to the type of signal and the receptors in the cell. It enables the cell to adapt and carry out the necessary actions based on the received signal.

Cell recognition is the mechanism by which cells identify and interact with each other through specific molecules on their outer surface. These molecules are known as antigens or cell surface markers. These antigens help cells recognize and stick to other cells or molecules. In organ transplantation, matching cell surface markers between the donor organ and immune system of recipient is crucial to reduce rejection risk. If the immune system sees the transplanted organ as foreign, it might trigger rejection. Compatibility in cell surface markers improves success in transplanting.