Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. The more solute a solution contains, the greater the osmotic pressure that solution will have. A solution that has a higher concentration of solutes than another solution is said to be hypertonic.
Water molecules tend to diffuse into a hypertonic solution because the higher osmotic pressure pulls water Figure 3. If a cell is placed in a hypertonic solution, the cells will shrivel or crenate as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting, a process called lysis. When cells and their extracellular environments are isotonic , the concentration of water molecules is the same outside and inside the cells, so water flows both in and out and the cells maintain their normal shape and function.
Various organ systems, particularly the kidneys, work to maintain this homeostasis. A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. 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. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell.
For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient. One of the most common types of active transport involves proteins that serve as pumps.
Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients from an area of low concentration to an area of high concentration.
These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes.
In living systems, diffusion of substances in and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered; their passage is made possible by specialized proteins, such as channels and transporters.
The chemistry of living things occurs in aqueous solutions; balancing the concentrations of those solutions is an ongoing problem. In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport. Key Points Plasma membranes are selectively permeable; if they were to lose this selectivity, the cell would no longer be able to sustain itself. In passive transport, substances simply move from an area of higher concentration to an area of lower concentration, which does not require the input of energy.
Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3.
Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Because of this, the process generally uses ATP as the driving force. In moving substances across a biological membrane , a passive transport may or may not need the assistance of a membrane protein.
There are four major types of passive transport are 1 simple diffusion , 2 facilitated diffusion , 3 filtration , and 4 osmosis. Simple and facilitated diffusions refer to the net movement of molecules from higher to lower concentrations.
Osmosis refers to the diffusion of a solvent usually water molecules through a semipermeable membrane from lower to higher solute concentrations.
Filtration is the movement of water and solute molecules across the cell membrane driven by hydrostatic pressure that is generated by the cardiovascular system. Passive transport is important for the proper functioning of plants and animals.
In plants, for instance, gases like carbon dioxide and oxygen diffuse into and out of a plant cell via stomatal openings according to their respective concentration gradients. Carbon dioxide is essential to plants as it is one of the main reactants of photosynthesis. One of the sources of carbon dioxide is the animals that release the gas through expiration. This release of carbon dioxide to the environment is mediated by passive transport.
In particular, the diffusion of carbon dioxide takes place at the capillary beds between the blood and the tissue fluid. As it diffuses from the tissues to the blood, it is then brought to the lungs where it again diffuses from the blood into the alveoli to be breathed out. As carbon dioxide moves out, oxygen, in turn, moves into the lungs and transported into the tissues of the body also by simple diffusion.
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