12/06/2022
⭐TRANSPORT ACROSS Cell MEMBRANE.
🌑The cell membrane is primarily made up of three things: 1. Phospholipids 2. Cholesterol 3. Proteins.
"FLUID MOSAIC MODEL" By "SINGER & NICOLSON(1972)
🌑The fluid mosaic model of the cell membrane is how scientists describe what the cell membrane looks and functions like, because it is made up of a bunch of different molecules that are distributed across the membrane. If you were to zoom in on the cell membrane, you would see a pattern of different types of molecules put together, also known as a "Mosaic". These molecules are constantly moving, in a fluid fashion, similar to icebergs floating in the ocean. The movement of the mosaic of molecules makes it impossible to form a completely impenetrable barrier.
🌑There are 3 main factors that influence cell membrane fluidity:
1)Temperature: The temperature will affect how the phospholipids move and how close together they are found. When it’s cold they are found closer together and when it’s hot they move farther apart.
_2) Cholesterol: The cholesterol molecules are randomly distributed across the phospholipid bilayer, helping the bilayer stay fluid in different environmental conditions. The cholesterol holds the phospholipids together so that they don’t separate too far, letting unwanted substances in, or compact too tightly, restricting movement across the membrane. Without cholesterol, the phospholipids in your cells will start to get closer together when exposed to cold, making it more difficult for small molecules, like gases to squeeze in between the phospholipids like they normally do. Without cholesterol, the phospholipids start to separate from each other, leaving large gaps.
3.)Saturated and unsaturated fatty acids : Fatty acids are what make up the phospholipid tails. Saturated fatty acids are chains of carbon atoms that have only single bonds between them. As a result, the chains are straight and easy to pack tightly. Unsaturated fats are chains of carbon atoms that have double bonds between some of the carbons. The double bonds create kinks in the chains, making it harder for the chains to pack tightly. These kinks play a role in membrane fluidity because they increase the space between the phospholipids, making the molecules harder to freeze at lower temperatures. In addition, the increased space allows certain small molecules, such as CO_2 & O2 to cross the membrane quickly and easily.
🌑CELL MEMBRANE
The detailed structure of the membrane was studied only after the advent
of the electron microscope in the 1950s. Meanwhile, chemical studies on
the cell membrane, especially in human red blood cells (RBCs), enabled
the scientists to deduce the possible structure of plasma membrane.
These studies showed that the cell membrane is mainly composed of
lipids and proteins. The major lipids are phospholipids that are arranged
in a bilayer. Also, the lipids are arranged within the membrane with the
polar head towards the outer sides and the hydrophobic tails towards
the inner part. This ensures that the nonpolar tail of saturated
hydrocarbons is protected from the aqueous environment.
In addition to phospholipids membrane also contains cholesterol.
Later, biochemical investigation clearly revealed that the cell membranes
also possess protein and carbohydrate. The ratio of protein and lipid varies
considerably in different cell types. In human beings, the membrane of the
erythrocyte has approximately 52 per cent protein and 40 per cent lipids.
Depending on the ease of extraction, membrane proteins can be
classified as integral and peripheral. Peripheral proteins lie on the surface
of membrane while the integral proteins are partially or totally buried in
the membrane.
🌑An improved model of the structure of cell membrane was proposed
by Singer and Nicolson (1972) widely accepted as fluid mosaic model. According to this, the quasi-fluid nature of lipid enables
lateral movement of proteins within the overall bilayer. This ability to move
within the membrane is measured as its fluidity.
The fluid nature of the membrane is also important from the point of
view of functions like cell growth, formation of intercellular junctions,
secretion, endocytosis, cell division etc.
🌑One of the most important functions of the plasma membrane is the
transport of the molecules across it. The membrane is selectively permeable
to some molecules present on either side of it. Many molecules can move
briefly across the membrane without any requirement of energy and this
is called the passive transport. Neutral solutes may move across the
membrane by the process of simple diffusion along the concentration
gradient, i.e., from higher concentration to the lower. Water may also move
across this membrane from higher to lower concentration. Movement of
water by diffusion is called osmosis.
🌑Large polar or ionic molecules, which are hydrophilic, cannot easily cross the phospholipid bilayer. Charged atoms or molecules of any size cannot cross the cell membrane (phospholipid hydrophobic nonpolar tails) via simple diffusion as the charges are repelled by the hydrophobic tails in the interior of the phospholipid bilayer.
SO, they require a carrier protein of the
membrane to facilitate their transport across the membrane.
In other words, we can Say Polar/Charged molecules Loves to Dissolves in Polar & Nonpolar Loves to dissolves in Nonpolar.
For Ex~ Water(polar) dissolves in Water(polar) But Water(Polar) Hates/Don't Dissolves in Oil (Nonpolar).
🌑A few ions
or molecules are transported across the membrane against their
concentration gradient, i.e., from lower to the higher concentration. Such
a transport is an energy dependent process, in which ATP is utilised and
is called active transport, e.g., Na+/K+
Pump.
🌑⭐CHANNEL & CARRIER PROTEINS
CHANNEL Proteins or TUNNEL Proteins are transport proteins that have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane (Ex: aquaporins for water).
Channel proteins create holes/pores that pe*****te the membrane making tunnel, enabling target molecules or ions to flow through via diffusion without interfering with one another.
The channel protein definition is a transmembrane protein that moves substances without binding to them and without spending energy.
There are different types of channel proteins depending on their function in the cell. Some channel proteins are always open to transporting solute, and thus are called non-gated. Other channel protein types only open upon certain stimuli and are called gated.
a) Non gated channels/Leaky Channels.
b) Mechanical Gated Channels
c) Voltage Gated Channels
d) Ligand Gated Channels
Example ~ AQUAPORINS, Na+ Leaky Channels, K+ Leaky Channels, Voltage Gated Na+ channel, Voltage Gated K+ Channel.
CARRIER proteins are transport proteins that bind to molecules and change shape to shuttle them across the membrane. Carrier proteins are proteins that bind to molecules or ions on one side of the membrane and release them on the other Side. carrier proteins do not form channels. Rather, they have binding sites from where molecules can bind to. Then, they shuttle the molecules towards their destination, i.e. the membrane’s interior or exterior. Having binding sites indicate that carrier proteins are more selective to the molecules that they transport. Carrier proteins are of 2 types:-
1)Carrier proteins can work with a concentration gradient (during passive transport i.e.,transport molecules from high to Low Conc.).
Ex~ GLUT, Carrier proteins involved in the facilitated diffusion of sugars, amino acids, and nucleosides across cell membranes of most cells.
2)Carrier proteins can move solutes against the concentration gradient (from low concentration to high concentration), with an input of energy(these type of Carrier proteins are known as PUMPS). Ex-Na/K Pump, Ca++ Pump, H+ Pump, H+/K+ Pump, Glucose-sodium transport proteins.
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