Cell Membrane Models and Transport Mechanisms
Membrane Models
1. Gorter and Grendel (1925): Extracted lipids from erythrocyte membranes. By calculating that spreading them on water resulted in twice the area that should be addressed by erythrocyte membranes, they concluded that the membrane is a bimolecular lipid layer.
2. Danielli and Davson (1935): Proposed a model of the plasma membrane structure in which proteins are located in the polar groups (hydrophilic) of the lipid bilayer. Their model included pores or channels in the membrane to explain the passage of substances.
3. Unit Membrane of Robertson (1950): Explained the trilaminar appearance of many membranes under the electron microscope.
4. Fluid Mosaic Model (Jonathan Singer and Garth L. Nicolson, 1972): This is the currently accepted model. The structure of the bilayer is rather fluid. Protein molecules can move laterally in the bilayer, taking some provisions (mosaic) that change over time and place.
Membrane Transport
A cell membrane is a selectively permeable barrier; some materials cross freely, while others cross at certain times, controlled by the phospholipid bilayer and the particular arrangement of proteins embedded in it.
Types of Membrane Transport
- Simple Diffusion: The movement of materials from an area of higher concentration to an area of lower concentration.
- Osmosis: The diffusion of water through a cell membrane. Water molecules are small enough to spread through holes in the phospholipid structure. The movement of water in or out of a cell depends on the dissolved concentrations inside and outside the cell.
- Isotonic: A cell immersed in a solution with the same concentration of dissolved materials as its internal cytoplasm.
- Hypertonic: When immersed in a very salty solution, the concentration of dissolved materials is higher outside than inside the cell (the water concentration is lower outside than inside).
- Hypotonic: A cell that is in a solution where the concentration is higher outside than inside the cell.
- Facilitated Diffusion: Large ions and molecules are disseminated through channels in membrane proteins. This process does not require cellular energy, as the materials move down a concentration gradient. Examples include glucose, potassium, and sodium. This occurs more rapidly when the temperature is higher and when there are more special proteins in the membrane. Two proteins are required: a channel protein and a receiver protein, which allows the passage of materials that cannot penetrate the phospholipid layer.
- Active Transport: The movement of materials from regions of low concentration to regions of high concentration. Cells accumulate materials such as potassium ions or amino acids. This requires a configuring protein and expending energy (ATP) by the cell. Transport through the protein is generated when one end of it creates a link to the material; the link changes the protein structure and opens the channel through which the material enters.
- Transport of Materials in Suspension: This involves moving relatively large materials through the formation of vesicles that do not allow mixing with the cytosol. There are two ways:
- Endocytosis: When materials are moved inside the cell. Endocytosis of a fluid is called pinocytosis, and of a solid, phagocytosis.
- Exocytosis: Materials are carried from the inside out. The membrane vesicles travel to fuse with the cell membrane, releasing their cargo from the cell (secretion).
Organelles
- Ribosomes: Protein synthesis.
- Endoplasmic Reticulum:
- Smooth Endoplasmic Reticulum: A network formed by smooth membranous sacs in the absence of ribosomes. Responsible for cellular lipid metabolism and detoxification.
- Rough Endoplasmic Reticulum: Synthesizes glycoproteins and enzymes. Carries substances and originates organelles such as dictyosomes and lysosomes.
- Golgi Apparatus: Regulates intracellular traffic via vesicles. Synthesis of complex molecules such as secreted mucopolysaccharides. Responsible for the repair of the cell membrane.
- Lysosomes: Digestive and degradative function.
- Mitochondria: Aerobic respiration and provides energy for cellular activity.
- Centrosomes: Cell division.
- Glyoxysomes: Convert carbohydrates to sugars.