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Cell membranes surround each cell and some organelles, such as the nucleus and the Golgi body. They are comprised of a phospholipid bilayer and this acts as a semipermeable barrier that regulates what enters and exits the cell or organelle. This is highly important as this is how cells can obtain molecules such as oxygen and remove waste products like urea.
There are two main ways in which molecules are transported across the cell membrane:
Passive transport
Active transport
The main difference between these modes of transport is that active transport requires energy in the form of ATP, but passive transport does not.
Passive transport refers to transport across the cell membrane that does not require energy from metabolic processes. Instead, this form of transport relies on the natural kinetic energy of molecules and their random movement . There are three modes of passive transport:
Simple diffusion is the movement of molecules from a region of high concentration to a region of low concentration until an equilibrium is reached.
Oxygen can freely diffuse through the cell membrane using this form of passive transport.
Simple diffusion
Facilitated diffusion is the same as simple diffusion but this form of transport uses membrane proteins, such as channel proteins and carrier proteins. Channel proteins provide a hydrophilic channel for the passage of charged and polar molecules, like ions. Meanwhile, carrier proteins change their conformational shape for the transport of molecules.
Glucose is an example of a molecule that is transported across the cell membrane through facilitated diffusion.
Facilitated diffusion
Osmosis is the movement of water molecules from a region of high water potential to a region of lower water potential through a semipermeable membrane. This form of transport looks very different between plant and animal cells as plant cells have cell walls surrounding each cell. Cell walls allow plant cells to withstand water flowing in. However, animal cells are more prone to cell bursting as they do not have this added support.
The rate of osmosis can be increased if aquaporins are present in the cell membrane. Aquaporins are membrane proteins that selectively transport water molecules.
The uptake of water by plant root hair cells is dependent on this form of passive transport.
The diagram shows the movement of molecules through the cell membrane during osmosis
Active transport is the transport of molecules across the cell membrane using carrier proteins and energy from metabolic processes in the form of ATP. Carrier proteins are membrane proteins that allow the passage of specific molecules across the cell membrane. They are used in both facilitated diffusion and active transport. Carrier proteins use ATP to change their conformational shape in active transport, allowing a bound molecule to pass through the membrane. In facilitated diffusion, however, ATP is not needed to change the shape of the carrier protein.
The diagram shows the movement of molecules in active transport
A process that relies on active transport is the uptake of mineral ions in plant root hair cells. The type of carrier proteins involved is specific for mineral ions.
There are different modes of active transport, such as cotransport and bulk transport.
There are two ways in which molecules are transported across the cell membrane: passive transport and active transport. The passive transport methods are simple diffusion, facilitated diffusion osmosis - these rely on the natural kinetic energy of molecules. Active transport requires energy in the form of ATP.
Amino acids are transported across the cell membrane via facilitated diffusion. Facilitated diffusion uses membrane proteins to transport molecules. Amino acids are charged molecules and therefore need membrane proteins, specifically channel proteins, to cross the cell membrane.
Membrane proteins such as channel proteins and carrier proteins facilitate transport across membranes. This type of transport is called facilitated diffusion.
Water molecules are transported across the cell membrane via osmosis which is defined as the movement of water from a region of high water potential to a region of lower water potential through a semipermeable membrane. The rate of osmosis is increased if aquaporins are present in the cell membrane.
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