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Transport across the Cell Membrane
One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell.
These substances include ions such as Ca++, Na+, K+, and Cl-
Nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell.
selective permeability
The membrane’s lipid bilayer structure provides the first level of control.
The phospholipids are tightly packed together, and the membrane has a hydrophobic interior.
This structure causes the membrane to be selectively permeable.
A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided.
In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar).
Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol.
water-soluble materials like glucose, amino acids, and electrolytes need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer.
All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required.
Passive transport
Passive transport is the movement of substances across the membrane without the expenditure of cellular energy.
Active transport active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).
In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion.
A concentration gradient
A concentration gradient is the difference in concentration of a substance across a space.
Molecules or ions will move/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space.
When molecules move in this way, they are said to move down their concentration gradient.
Diffusion
Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.
Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so.
Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so.
Substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2.
O2 diffuses into cells because it is more concentrated outside of cell membrane .
CO2 typically diffuses out of cells because it is more concentrated inside .
Neither of these requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane.
Facilitated diffusion
Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity .
A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP.
Facilitated diffusion - Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar.
To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.
(a) Facilitated diffusion of substances crossing the cell membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge.
(b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.
symporter
In some cases, facilitated diffusion might move two substances in the same direction across the membrane, called a symporter.
For example, in intestinal cells, sodium ions and glucose molecules are co-transported into the cells.
Uniporter
#Transport across the Cell Membrane#