1 Be able to describe the relationship between proteins and lipids in a cellular membrane and how the membrane changes with time; be able to give the correct name for this model of membrane structure. 2 Be able to describe the plasma membrane, including the role of the phospholipid heads and tails. 3 Be able to predict the direction in which most molecules will move when they are more concentrated in one region than another and to name this process.

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1.  The lipids in the cellular membrane are amphiphilic i.e., they have both the hydrophobic and hydrophilic ends. The cell membrane is made up of a lipid bilayer (two layers) with proteins attached to it. The membrane is selectively permeable, i.e. it allows the entry of only selected molecules, while rejecting others. The embedded proteins can be receptors and identifiers and enable the transport of substances across the membrane. These membrane protein also help in cell-cell contact, enzymatic activity, etc. This model of lipid bilayer with embedded proteins in it is known as Fluid Mosaic model. The membrane allows motion of proteins through it and hence is fluidic and the presence of protein throughout the membrane results in a mosaic.

2. Cell membrane is also known as plasma membrane and consists of lipids called phospholipids. These phospholipids are amphiphilic, i.e. they have a hydrophilic head and a hydrophobic tail. The membrane is self-assembled in such a way that the hydrophilic heads point towards the interior and exterior of the cell, while the hydrophobic tails point towards each other. It is this arrangement of lipid molecules that gives selective permeability to the plasma membrane.

3. When there are more molecules in a region as compared to another, the molecules will move from region of higher concentration to region of lower concentration. This movement across a concentration gradient, from higher to lower concentration, is known as diffusion. This process is mathematically described by Fick's law of diffusion. 

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