Plasma Membrane, cell wall and cell surface Course Title: Biochemistry and Molecular Biology Course No. PHR 202 Course Teacher: Shahana Sharmin (SHN)

The Cell 2

Cell membrane All living things are surrounded by a membrane. A cell membrane is also known as plasma membrane. A cell membrane is also known as plasma membrane. Controls exchange of materials such as nutrients and waste between cells and their environment. Controls exchange of materials such as nutrients and waste between cells and their environment. Has other important functions for example to enable cells to receive hormones. Has other important functions for example to enable cells to receive hormones.

Cell Membranes from Opposing Neurons } cell membrane 7nm wide Cell membrane { Nerve cell Gap between cells

HYDROPHILIC heads (water liking) -Attracted to the water called POLAR HYDROPHOBIC tails (water fearing) -Not attracted to the water called NON-POLAR Cell membranes are made of PHOSPHOLIPIDs A Phospholipid

Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO 4- ) replaces one of the three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and two nonpolar "tails".

A phospholipid HYDROPHILIC HEAD At the other end of the phospholipid is a phosphate group and several double bonded oxygen. The atoms at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR HYDROPHOBIC TAILS The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR. Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water. 3D model of a Phospholipid

A Phospholipid Bilayer Phospholipids can form: BILAYERS -2 layers of Phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes.

Structure of the cell membrane Phospholipids Cell membranes are made mainly of phospholipids. They have: HYDROPHILIC heads (water liking) -Attracted to the water POLAR HYDROPHOBIC tails (water fearing) -Not attracted to the water NON-POLAR Phospholipids can form BILAYERS -2 layers of phospholipids with hydrophobic tails protected inside by the hydrophilic heads. The PHOSPHOLIPID BILAYER is the basic structure of membranes.

Diagram representing the cell membrane Remember the membrane is 7nm wide

Fluid mosaic model Cell membranes also contain proteins within the phospholipid bilayer. This ‘model’ for the structure of the membrane is called the: FLUID MOSAIC MODEL FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

Diagram of a cell membrane

Transport Passive transport : It requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion.

Transport Active transport: It requires the cell to spend energy, usually in the form of ATP. Examples include transport of large molecules (non-lipid soluble) and the sodium-potassium pump.

Endocytosis & Exocytosis Endocytosis is the process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobicplasma or cell membrane. The process opposite to endocytosis is exocytosis. Exocytosis also known as 'reverse pino-cytosis', is the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane.

Semipermeability A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion". The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the thin film on the inside of an egg. An example of a semi-permeable membrane is the lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. A group of phospholipids (consisting of a phosphate head and two fatty acid tails) arranged into a double-layer, the phospholipid bilayer is a semipermeable membrane which is very specific in its permeability. The hydrophilic phosphate heads are in the outside layer and exposed to the water content outside and within the cell. The hydrophobic tails are the layer hidden in the inside of the membrane. The phospholipid bilayer is the most permeable to small, uncharged solutes. Protein channels float through the phospholipids, and, collectively, this model is known as the fluid mosaic model.

Semipermeability In the process of reverse osmosis, thin film composite membranes (TFC or TFM) are used. These are semipermeable membranes manufactured principally for use in water purification or desalination systems. They also have use in chemical applications such as batteries and fuel cells. In essence, a TFC material is a molecular sieve constructed in the form of a film from two or more layered materials. Membranes used in reverse osmosis are, in general, made out of polyimide, chosen primarily for its permeability to water and relative impermeability to various dissolved impurities including salt ions and other small molecules that cannot be filtered. Another example of a semipermeable membrane is dialysis tubing. Other types are cellulose ester membrane (CEM), charge mosaic membrane (CMM), bipolar membrane (BPM), anion exchange membrane (AEM) alkali anion exchange membrane (AAEM) and proton exchange membrane (PEM). The diffusion of water through a selectively permeable membrane is called osmosis.

Semi permeable membrane Fig : Scheme of semi permeable membrane during hemodialysis, where red is blood, blue is the dialysing fluid, and yellow is the membrane.