Plasma Membrane – Structure And Transport System

Key Concepts

• Plasma membrane
• Active transport
• Passive transport
• Endocytosis
• Exocytosis

Introduction: 

We know that cell membrane is also called as the plasma membrane or cytoplasmic membrane. It is a biological membrane that protects the internal organelles of a cell from its outside atmosphere. The main function of the plasma membrane is to protect the cell.  

Plasma membrane structure 

The cell membrane is a fluid mosaic of proteins, carbohydrates, and lipids. It is not permeable to ions and most water-soluble molecules.  The cell membrane can be crossed only through transmembrane channels, carriers, and pumps. These transmembrane proteins provide the cell with nutrients, control internal ion concentration, and create a transmembrane electrical potential. 

The cell membrane is mainly composed of a combination of proteins and lipids. The function of lipids is to give membranes their flexibility, proteins regulate and maintain the cell’s chemical environment and support during the transfer of molecules across the membrane. 

The basic structure of the plasma membrane is the phospholipid bilayer, which creates a stable barrier between two aqueous sections. Phospholipids are lipids with a phosphate group connected to it. The phospholipids have one polar and hydrophilic or water loving head and two tails that are non-polar and hydrophobic or water fearing. 

Lipid-soluble molecules and some small molecules can pass through the membrane, but the lipid bilayer prevents many large, water-soluble particles and electrically charged ions that the cell must take inside or push outside the membrane in order to survive. Transport of these substances is carried out by certain types of intrinsic proteins that form a variety of transport systems: some are open channels, which permit ions to diffuse directly into the cell; others are “facilitators,” which support solutes to diffuse past the lipid screen. There are other systems such as ‘pumps,’ which pushes solutes through the membrane when they are not concentrated enough to diffuse spontaneously. Particles too large to be diffused or pumped are often swallowed or ejected whole by an opening and closing of the membrane. 

Membrane transport system 

It is the transport system by which different molecules enter the cell and out of cell through cell membrane. Cells have several types of transport mechanism. Depending on whether the molecules pass directly through lipid bilayer or with the help of membrane channel; whether or not the molecules is changed as it goes through membrane. whether or not the process need energy, membrane transport system is grouped into two main groups or systems. 

1. Passive transport: In this system, cellular energy not required. 

• Diffusion 
• Facilitated diffusion 
• Osmosis 

2. Active transport: In this system, cellular energy is required. 

• Protein pumps 
• Endocytosis 
• Exocytosis 

Passive transport: 

Passive transport system does not need cellular energy to transport molecules across cell membrane. Hence it is a passive transport process. 

In this transport system, molecules are transferred from their higher concentration to their lower concentration till concentration gradient is diminished. This transport system does not work against concentration gradient. 

What is concentration gradient? 

Concentration gradient is the difference in concentration of a substance across a membrane or space. Molecules (or ions) will diffuse from more concentrated area to less concentrated area until they are evenly distributed in that space or membrane. (When molecules move in this way, it means that they are moving down their concentration gradient.) 

Passive transport system consists of – 

Simple diffusion: 

Simple diffusion is also called passive diffusion or diffusion. Simple diffusion is the movement of molecules from higher concentration to lower concentration without spending energy. In this diffusion process, molecules easily diffuse through the pore of cell membrane. 

Simple diffusion does not need transport protein. 

When the concentration of molecules is different inside and outside of the cell membrane, concentration gradient is created. Then the movement of molecules start from higher concentration to lower concentration until equilibrium is retained. 

When the concentration of molecules becomes same on both side of the membrane, transport of molecules stops. But in some cases, the molecules after entering the cell changes metabolically, and prevents to build up concentration of transported molecules, hence the concentration gradient remain created. 

The rate of diffusion is affected by concentration gradient and permeability of cytoplasmic membrane. If the concentration gradient and permeability of the cell membrane is higher, the rate of passive diffusion will be higher. 

Example of simple diffusion: Gases such as O₂ and CO₂ can easily disperse or diffuse through the lipid bilayer of the cell membrane. O₂ usually diffuses into cells because it is more concentrated outside the cell, and CO₂ usually diffuses out of cells because its concentration is more inside the cell. Neither of these examples needs any cellular energy, hence they use passive transport to move across the cell membrane. Cells rapidly use up oxygen in the course of metabolism, so there is a lower concentration of O₂ inside the cell than outside the cell. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the cell membrane into the cytoplasm of the cell. Cells produce CO₂ as a byproduct of metabolism, so CO₂ concentrations rise within the cytoplasm; thus, CO₂ will move from the cell through the lipid bilayer into the interstitial fluid, where its concentration is lower. This mechanism of molecules spreading from higher concentration to lower concentration is a form of passive transport called simple diffusion. 

Osmosis:

It is the diffusion of water through a semipermeable membrane. Water can move freely across the cell membrane of all cells, either through protein channels or by sliding between the lipid tails of the plasma membrane.  But concentration of solutes within the water decides whether or not water will be going into the cell or out of the cell, or both. 

In this process, energy is not used. In osmosis, the only movement of water molecules takes place by diffusion. Here, the movement of solute does not occur. 

Osmotic pressure: It is the pressure that needs to be applied to a solution to stop the inward flow of water across a semipermeable membrane. 

Based on the tonicity of solutions, osmotic solutions are called as hypotonic, hypertonic, or isotonic solutions. Let us see, what happens when cell is kept in various solutions. 

In isotonic solution, water move uniformly in both directions, i.e., inside the cell and outside the cell. 

In hypertonic solution (concentrated solution where solute is more and solvent is less), water moves out of the cell as a result cell shrinks. This method is known as plasmolysis. 

In hypotonic solution (Dilute solution where solute is less and solvent is more), water moves inside the cell as a result the cell swells up. 

So, water moves both in and out of the cells and the cells maintain their normal shape (and function). Different organ systems, particularly the kidneys, work to maintain this homeostasis. 

Osmosis in animal cell 

Facilitated diffusion:

This process is used for those substances that cannot pass the lipid bilayer due to their size and/or polarity. An example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP (Adenosine tri phosphate). Even though glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer through simple diffusion because it is large and polar. So, it needs a carrier protein (also called as transport protein) called the glucose transporter. It will move glucose molecules into the cell to facilitate its inward diffusion.   

Facilitated diffusion of substances takes place with the help of proteins such as channel proteins and carrier proteins.  

Channel proteins are less selective than carrier proteins, and generally mildly differentiate between their load based on size and charge.  

Carrier proteins are very selective, allow only one particular type of molecule to cross the membrane. 

The transporter protein is specific, but some can transport multiple compounds. Solute molecule attaches with the transporter protein and changes the 3D structure of the transporter protein and this change in shape helps the solute to move across the membrane. 

Active transport: 

Active transport process needs transporter protein and constant supply of cellular energy for the transport of molecules across concentration gradient of the membrane. Active transport is very vital to transfer the molecules that are present in very low concentration in the medium. 

In active transport, permease or transporter protein takes the molecules across the membrane and the energy needed to transport the molecules is obtained by Ion gradient or ATP. The substances transferred by active transport are glucose, amino acids, organic acids and inorganic ions such as SO4–, PO4–, K+ etc. 

There are two types of active transport (Pump transport): 

Primary active transport:  

In this transport system, ATP provide energy required for transport of molecules from lower concentration to higher concentration across membrane. 

Secondary active transport: 

Here, one type of molecule transfers from higher concentration to lower concentration, releasing energy. This released energy is used to transfer other molecule from its lower concentration to higher concentration throughout cell membrane. 

Sodium-potassium pump: It is also called N+/K+. ATPase transfers sodium out of a cell while moving potassium into the cell. The Na+/K+ pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly plentiful in nerve cells, which are constantly pumping out sodium ions and dragging in potassium ions to retain an electrical gradient across their cell membranes. An electrical gradient is a change in electrical charge across a space. 

Active transport by vesicles 

Endocytosis: It means bringing “into the cell”. It is the process of a cell consuming material by enveloping it in a portion of its cell membrane, and then taking off that portion of membrane. Once pinched off, the portion of membrane and its material or contents grow into an independent, intracellular vesicle. A vesicle is a membranous bag—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings contents into the cell that must to be broken down or digested.       

Endocytosis is divided into three types: phagocytosis, pinocytosis, and receptor-mediated endocytosis. 

Phagocytosis: It involves the ingestion of large particles, like microorganisms or dead cells through large vesicles called phagosomes (generally >250 nm in diameter). Unicellular organisms such as amoeba use phagocytosis to obtain food. 

Pinocytosis: It is the consumption of surrounding fluid(s).  This type of endocytosis permits a cell to consume (engulf) dissolved substances that bind to the cell membrane prior to internalization.  Pinocytosis is a “drinking” mechanism in which a cell actively consumes external fluids over time.   

Receptor-mediated endocytosis: It is a specific type of pinocytosis. During receptor-mediated endocytosis, macromolecules attach to receptors along the surface of the cell’s plasma membrane. Cholesterol uptake is a good example of receptor-mediated endocytosis. 

Exocytosis: It is the process by which cells remove materials from the inside to the outside into the extracellular fluid. Exocytosis happens when a vesicle combines with the plasma membrane, allowing its contents to be released outside the cell. 

Summary:

• Cell membrane consists of phospholipid bilayer with proteins enclosed in it.
• The cell membrane is selectively permeable to ions and some organic molecules.
• The movement of small molecules across the plasma membrane is done by active and passive transport. Passive transport does not require cellular energy.
• Passive transport is of 3 types – Diffusion, facilitated diffusion, osmosis.
• Active transport requires cellular energy.
• Active transport is of three types – Protein pumps, endocytosis, exocytosis.