TRANSPORT ACROSS THE CELL MEMBRANE
Three types of transport process occur across the membrane.
1. Non-mediated transport
2. Mediated transport
3. Transport in Vesicles
1. Non-mediated transport occurs through the simple diffusion process and the driving force for the transport of a substance through a medium depends on its chemical potential gradient. Thus, the substance diffuses in the direction that eliminates its concentration gradient; at a rate proportional to the magnitude of this gradient and also depends on its solubility in the membrane’s non-polar core.
2. Mediated transport is classified into two categories depending on the thermodynamics of the system:
- Passive-mediated transport or facilitated diffusion: In this type of process a specific molecule flows from high concentration to low concentration.
- Active transport: In this type of process a specific molecule is transported from low concentration to high concentration, that is, against its concentration gradient.
- Passive mediated transport:
Substances that are too large or polar diffuse across the lipid bilayer on their own through membrane proteins called carriers or permeases or channels or transporters. Unlike active transport, this process does not involve chemical energy. So the passive mediated transport is totally dependent upon the permeability nature of cell membrane.
Types of passive transport:
- Movement of molecules or ions from an area of high concentration to an area of lower concentration (down the concentration gradient).
- Until the concentrations of the two regions are equal (Dynamic equilibrium established).
- Passive process (No energy required)
- The differences of concentration between the two regions are termed as concentration gradient.
- The rate of diffusion is proportional to the concentration gradient the distance over which the diffusion must occur.
- Substances capable of diffusion across the cell surface membrane include
- carbon dioxide
- the fat-soluble vitamins (A,D,E,K)
- alcohols and
- Facilitated diffusion
- The process of the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane is known as facilitated diffusion or called carrier-mediated diffusion.
- Many large molecules, such as glucose, are insoluble in lipids and too large to fit into the porins, therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane.
- Used to transport molecules such as
- non fat-soluble vitamins
- urea and
- many ions
Figure 7- Facilitated Diffusion
- Filtration is the process of the movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the system.
- Depending on the size of the membrane pores, only solutes of a certain size may pass through it.
- Example – The membrane pores of the Bowman’s capsule in the kidneys are very small, and only albumins (smallest of the proteins) can filter through. On the other hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass through and be metabolized.
- Specialised form of diffusion.
- Diffusion of water molecules from an area of high water potential (high concentration) to an area of low water potential (low concentration), through a
partially permeable membrane
- It is a Passive process.
- A cell with a less negative water potential will draw in water but this depends on other factors as well such as solute potential (pressure in the cell e.g. solute molecules) and pressure potential (external pressure e.g. cell wall).
- Depending on the condition of the extracellular environment, different things can happen to the cell. If the cell is exposed to an isotonic environment (same concentration inside and outside the cell), the movement of water into and out of the cell occur at the same rate. If the cell is exposed to a hypertonic environment (outside of the cell has higher solute concentration than the inside), the cell will shrivel because of loss of water. If the cell is exposed to a hypotonic environment (inside of the cell has higher concentration than outside), the cell take up more water and becomes bloated and will eventually burst.
Figure 8 – Osmosis in Red Blood Cells
- Active transport:
Active transport is the movement of a substance against its concentration gradient (i.e. from low to high concentration). In this process energy is required.
Types of active transport
- Primary active transport: Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.
Example: Sodium-potassium pump, which helps to maintain the cell potential.
- Secondary active transport: Secondary active transport or co-transport, also uses energy to transport molecules across a membrane; however, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is used. The two main forms of active transport are antiport and symport.
- Antiport: In antiport two species of ion or solutes are pumped in opposite directions across a membrane. One of these species is allowed to flow from high to low concentration which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one.
Example: The sodium-calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out.
- Symport: In antiport two species of ion or solutes are pumped in same directions across a membrane. Symport uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient).
Example: Glucose symporter, which co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell.
Figure 9- Types of Secondary active transport
- Types of Transport in Vesicles
- Endocytosis: Endocytosis is the process by which cells absorb larger molecules and particles from the surrounding by engulfing them. It is used by most of the cells because large and polar molecules cannot cross the plasma membrane. The material to be internalized is surrounded by plasma membrane, which then buds off inside the cell to form vesicles containing ingested material.
It is of two types i.e. Phagocytosis & Pinocytosis.
- Phagocytosis or “cell eating,” is a mechanism whereby the cell can ingest solid particles. Phagocytosis is the process by which certain living cells called phagocytes engulf larger solid particles such as bacteria, debris or intact cells.
When the solid particle binds to the receptor on the surface of the phagocytic cell, then the pseudopodia extends and later surrounds the particle. Then their membrane fuses to form a large intracellular vesicle called phagosome. These phagosomes fuse with the lysosome, forming phagolysosomes in which ingested material is digested by the action of lysosomal enzymes. During its maturation, some of the internalized membrane is recycled to plasma membrane by receptor mediated endocytosis.
- Pinocytosis, or “cell drinking,” allows the cell to consume solutions. An infant’s intestinal lining ingests breast milk by pinocytosis, allowing the mother’s protective antibodies to enter the baby’s bloodstream.
Figure 10- Endocytosis (Phagocytosis & Pinocytosis)
- Exocytosis: The process by which the cells direct the contents of secretory vesicles out of the cell membrane is known as exocytosis. These 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.
Some of the examples include secretion of proteins like enzymes, peptide hormones and antibodies from cells and release of neurotransmitter from presynaptic neurons.
Figure 11 – Exocytosis
Table 1- Transport of Materials into and out of Cells
|Transport process||Description||Substances transported|
|PASSIVE PROCESSES||Movement of substances down a concentration gradient until equilibrium is reached; do not require cellular energy in the form of ATP|
|Diffusion||Movement of molecules or ions down a concentration gradient due to their kinetic energy until they reach equilibrium.|
|Simple diffusion||Passive movement of a substance down its concentration gradient through the lipid bilayer of the plasma membrane without the help of membrane transport proteins.||Nonpolar, hydrophobic solutes: oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids; and fat-soluble vitamins. Polar molecules such as water, urea, and small alcohols|
|Facilitated diffusion||Passive movement of a substance down its concentration gradient through the lipid bilayer by transmembrane proteins that function as channels or carriers.||Polar or charged solutes: glucose; fructose; galactose; some vitamins; and ions such as K+, Cl, Na+, and Ca2+|
|Osmosis||Passive movement of water molecules across a selectively permeable membrane from an area of higher to lower water concentration until equilibrium is reached.||Solvent: water in living systems.|
|ACTIVE PROCESSES||Movement of substances against a concentration gradient; requires cellular energy in the form of ATP.|
|Active Transport||Active process in which a cell expends energy to move a substance across the membrane against its concentration gradient by transmembrane proteins that function as carriers.||Polar or charged solutes.|
|Primary active transport||Active process in which a substance moves across the membrane against its concentration gradient by pumps (carriers) that use energy supplied by hydrolysis of ATP||Na+, K+, Ca2+, H+, I–, Cl–, and other ions.|
|Secondary active transport||Coupled active transport of two substances across the membrane using energy supplied by a Na or H concentration gradient maintained by primary active transport pumps. Antiporters move Na+ (or H+) and another substance in opposite directions across the membrane; symporters move Na+ (or H+) and another substance in the same direction across the membrane.||Antiport: Ca2, H out of cells. Symport: glucose, amino acids into cells.|
|TRANSPORT IN VESICLES||Active process in which substances move into or out of cells in vesicles that bud from plasma membrane; requires energy supplied by ATP|
|Endocytosis||Movement of substances into a cell in vesicles.|
|Receptor-mediated endocytosis||Ligand–receptor complexes trigger infolding of a clathrin-coated pit that forms a vesicle containing ligands.||Ligands: transferrin, low-density lipoproteins (LDLs), some vitamins, certain hormones, and antibodies.|
|Phagocytosis||“Cell eating”; movement of a solid particle into a cell after pseudopods engulf it to form a phagosome.||Bacteria, viruses, and aged or dead cells.|
|Pincytosis||“Cell drinking” allows the cell to consume solutions.||An infant’s intestinal lining ingests breast milk by pinocytosis|
|Exocytosis||Movement of substances out of a cell in secretory vesicles that fuse with the plasma membrane and release their contents into the extracellular fl uid.||Neurotransmitters, hormones, and digestive enzymes.|