MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

LEARNING OBJECTIVE Analysing the movement of substances across the plasma membrane Understanding the movement of substances across the plasma membrane in everyday life Appreciating the movement of substances acrss the plasma membrane

Table content Movement across the plasma membrane Movement of substances across the plasma membrane in everyday life Appreciating the movement of substances across the plasama membrane

Movement across the plasma membrane Structure of plasma membrane The permeability of plasma membrane The movement of substances:passive transport The movement of substances :active transport

Structure of plasma membrane

The most accepted theory for plasma membrane structure is the fluid mosaic model. According to this model, the plasma membrane comprises of two lipid layers and associated protein molecules, which attach to the lipid layers directly or indirectly. The plasma membrane does the major function of regulating transportation of substances from inside the cell to the outside and vice versa. The specificity of plasma membrane structure plays a crucial role in the overall functioning of the cell. In simple terms, it acts in a similar manner to the skin of animals. Various scientific hypotheses have been proposed to explain the structure of the plasma membrane, out of which the most popularly accepted theory is the fluid mosaic model.

Plasma Membrane Structure: The Fluid Mosaic Model The fluid mosaic model was developed by S.J. Singer and Garth Nicolson in 1972. As per this theory, the plasma membrane consists of carbohydrates and different types of lipids and proteins. And the model is named in accordance to the structure of the plasma membrane; the membrane is not rigid, but more of a fluid type, containing various molecules like a mosaic pattern. These molecules are arranged in a proper manner, which helps in selective movement of the substances. Discussed below is a brief illustration of the fluid mosaic model:

Fluid-mosaic model

Lipid bilayer Lipid Bilayer: The fundamental part of the plasma membrane structure is the lipid bilayer. Types of lipids present in the plasma membrane are phospholipids, cholesterol and glycolipids. However, as majority of the molecules are of phospholipid type (containing a phosphate group), the two lipid layers are better known as phospholipid layers. The lipid tails are water repelling (hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside (facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the plasma membrane, one that faces the cytosol and the other facing the outside environment, are hydrophilic in nature.

The lipid tails are water repelling (hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside (facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the plasma membrane, one that faces the cytosol and the other facing the outside environment, are hydrophilic in nature.

Membrane protein Membrane Proteins: Another key component of the plasma membrane is proteins, which help in selective transport of the macromolecules like sucrose, amino acids and ions. Based on the location of proteins with reference to the phospholipid bilayers, there are two types of proteins:Integral membrane proteins attach to the lipids of the bilayered structure. And integral proteins that traverse the phospholipid bilayer are called transmembrane proteins.

Peripheral membrane proteins are indirectly or loosely attached to the membrane. They are non-covalently connected with the lipids or ends of the integral proteins.

carbohydrates Carbohydrates: In addition to phospholipids and proteins, the cell membrane also consists of carbohydrates, basically glycoproteins and glycolipids. These molecules are exclusively arranged in the outer side of the cell membrane, wherein the carbohydrate portions are exposed to the external surface of the cell.

Function of plasma membrane to act as an attachment to the non-living matter that is found outside the cell membrane. is the transportation of materials, that is crucial for the proper functioning of various cell organelles. This semi-permeable membrane of the cells helps in the transferring of those nutrients and chemicals that are required for the cell functioning. The other foreign materials are obstructed on their path thereby preventing the invasion of the plasma membrane.  it maintains a suitable 'cell potential'.

The permeability of plasma membrane

Plasma membrane Plasma membrane is selectively permeable or semi-permeables Its means that only some substances can move across the plasma membrane freely while the others cannot Two general factors that determine : size Polarity

The movement of substances

Passive transport Simple diffusion The simplest type of passive transport No energy need Small molecules can pass through :Oxygen,carbon dioxide,lipid soluble molecules. Simple diffusion is the net movement of molecules or ions from higher to lower concentration until equilibruim achieved. osmosis A passive process also but involves only water molecules Defined as the net movement of freely moving water molecules from a low to high solute concentration. Its important to cell because cell need adequate water to function well. Facilitated diffusion Water soluble which are not soluble in lipids cannot Pass throught the phospholipids bilayer but with the aids of carrier protein and pores that is called facilitated diffusion

Active transport Is the movement of molecules or ions against the concentration gradient across plasma membranes. Requires both carrier proteins and energy in it process. Energy is from ATP

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