Please enable JavaScript.
Coggle requires JavaScript to display documents.
2.5 Biological Membranes - Coggle Diagram
2.5 Biological Membranes
Function of Membranes
Cell membranes play different roles depending on whether they are on the outer surface of the cell (plasma membrane) or within cells:
-
-
Fluid Mosaic Model
The structure of the plasma membrane is made up of a bilayer of phospholipids with proteins and cholesterol interspersed throughout the structure. The fluid mosaic model is used to describe the arrangement of molecules in the membrane - ‘fluid’ because the phospholipids are constantly moving around and ‘mosaic’ because protein molecules are scattered throughout the phospholipids like tiles in a mosaic.
-
-
-
-
-
Simple Diffusion
Diffusion is the movement of molecules down their concentration gradients. When molecules move by simple diffusion, they pass directly through the phospholipid bilayer. It is a passive process which means that no energy is required. Oxygen and carbon dioxide move by simple diffusion when they pass from the alveoli into the bloodstream during gas exchange.
-
Facilitated Diffusion
Facilitated diffusion involves the movement of molecules down their concentration gradients. It differs from simple diffusion in the fact that a carrier protein or a channel protein within the cell membrane helps them get from one side to the other. This is also a passive process. An example of facilitated diffusion is the movement of glucose molecules into liver cells through glucose transporter proteins embedded in the plasma membrane.
-
Osmosis
Osmosis is the movement of water molecules down its concentration gradient across a partially permeable membrane. It is a passive process so does not require energy in the form of ATP. Osmosis is responsible for the movement of water molecules into the root hair cells of plants, for example.
-
Pure water has the highest water potential – all other solutions will have lower water potentials in comparison, with the more solute dissolved in it, the lower the water potential. Two solutions with the same water potential are isotonic.
Osmosis has different effects on plant cells and animal cells. When plant cells are placed in a hypertonic (concentrated) solution, water moves out of the cell by osmosis. The cell becomes flaccid and the cell membrane pulls away from the cell wall (plasmolysis). When they are placed in hypotonic (dilute) solution, water moves into the cell by osmosis. The vacuole swells and the cytoplasm pushes against the cell wall. The cell becomes turgid and the cell wall protects it from bursting.
Animal cells shrink when they are placed in a hypertonic solution and burst when they are placed in a hypotonic solution, since they don’t have a cell wall to maintain turgor pressure.
Active Transport
Active transport moves molecules against their gradient, from low to high concentration. This involves a carrier protein which carries the molecule from one side of the membrane to the other. It is an active process and uses ATP to release energy. An example is the transport of glucose from the villi of the intestine into the bloodstream. Cotransporters are a special type of carrier protein which can bind to two molecules at a time. The concentration gradient of one molecule is used to transport the other molecule against its gradient.
-
Endocytosis
If substances are too large to cross the membrane, they enter the cell by endocytosis. The cell surrounds the substance and folds its membrane around it. The membrane then pinches off to engulf the substance, which causes a vesicle to form inside the cell containing the ingested substance. This is an active process so will require energy in the form of ATP. An example of endocytosis is when phagocytes carry out phagocytosis, in which the phagocyte engulfs a whole bacterium in order to destroy it.
Exocytosis
When large substances need to leave the cell, such as hormones and digestive enzymes, they do so by exocytosis. These substances will be contained inside vesicles which move towards the plasma membrane and fuse with it. This causes the substances to either be released outside of the cell or they will be inserted straight into the membrane (for example, if the substance is a membrane protein). Exocytosis is an active process which requires ATP.