Question

A thin flexible semipermeable barrier around the cell which regulates what enters and leaves the cell

Answer 1

That is the cell membrane. Think of it as a gate around a city, that is how I remember it. The city is the cell, and the cell membrane is the gate that "protects" the city, and controls what can enter and exit.

Answer 2

Answer: The plasma membrane

Explanation:

The plasma membrane also known as the cell membrane, is a covering that surrounds and delimits the cell, separating it from the external environment. It functions as a barrier between the inside of the cell and its environment by allowing molecules to enter and exit through it. This passage of molecules is a phenomenon called permeability. But the membrane does not let all the molecules pass easily, so it is selectively permeable.   The cell membrane is characterized by a dynamic structure, with fluidity being one of its most important features.  This fluidity is temperature-dependent, as it increases with increasing temperature. It also depends on the nature of the lipids that it possesses, since the presence of unsaturated and short-chain lipids favors the increase of fluidity. The presence of cholesterol hardens the membranes, reducing their fluidity and permeability.  Other functions of the cell are related to transport, communication, recognition and cell adhesion.  

In the composition of the membrane 40% corresponds to lipids, 50% to proteins, and 10% to carbohydrates.

• The lipids that make up the membrane are phospholipids, glycolipids and cholesterol, and their main function is to act as a semi-permeable barrier.
• The proteins that form the membrane are integral or peripheral, and their functions are related to transport and communication.
• Carbohydrates are generally bound to lipids, forming glycolipids, and to proteins, generating glycoproteins. Their main function is to constitute the cellular cover or glucocalix. The different functions that the different cells exhibit are related to the type of carbohydrate that is in their cover.

The lipids that form the plasma membrane are mainly phospholipids. They form a double layer in which their hydrophilic heads are exposed and their hydrophobic tails are hidden. In each layer of the lipid bilayer the lipids are in continuous lateral movement, giving the membrane its fluid quality.  Membrane proteins are embedded in the lipid bilayer and their proportion varies from cell to cell.  Those on the outer surface or inside the lipid bilayer are peripheral proteins.  Those that penetrate the bilayer are integral proteins.  While the lipid bilayer determines the basic structure of the membrane, the proteins can perform multiple functions such as:

 

• Receptors, which pick up substances from the environment and trigger intracellular responses.
• Enzymes, accelerators of chemical reactions.
• Transporters, channels and pumps, in charge of allowing and regulating the passage of substances through the membrane.

In order to live and grow, cells need to constantly receive nutrients and eliminate waste substances before they accumulate in the cell and cause damage. The cells also secrete substances produced by themselves. All nutrients, waste and secretions must pass through the membrane.  There are three general ways in which substances pass through the plasma membrane:

• Passive transport (no energy required) , examples are osmosis, simple diffusion, facilitated diffusion, dialysis.
• Active transport (requires energy) , molecules cross the plasma membrane against a concentration gradient, i.e. from a low concentration zone to a high concentration zone with the consequent expenditure of energy.
• Vesicles-mediated transport , examples are endocytosis and exocytosis.