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Transport Across Membranes - Coggle Diagram
Transport Across Membranes
Plasma Membrane
All cells and organelles share the same structure for their plasma membrane
The membrane is arranged in a fluid-mosaic model which relates to how there are various components like:
cholesterol, glycolipids and glycoproteins
and their fluid movement
The collection of all the molecules form a partially permeable membrane; which is known as the cell surface membrane
Phospholipid Bilayer
Made up of a "Phosphate" group head
And a "Lipid" group tail (made up of fatty acids)
Two interconnected layers of phospholipids
Head is hydrophilic (polar/-ve) which means it attracts water whilst repelling lipids
The tail is hydrophobic (non polar/no charge) which means that it repels water whilst attracting lipids
Cholesterol
This is particularly useful in high temperatures as it prevents water and dissolved ions from leaving the cell
Cholesterol acts as a barrier preventing the lateral (sideways) movement of molecules in the plasma membrane
Proteins
Proteins are usually embeded across the membrane
This can be done either...
Peripherally (where it isn't extended completely across the membrane)
Or integrally (where they span from one side of the membrane to another)
Peripheral proteins
Combine with lipids/proteins to form GLYCOLIPIDS/PROTEINS
The primary function of glycolipids/proteins is cell recognition - they act as receptors
Provide mechanical support
Integral proteins
Their primary function is in the transport of molecules
This happens across the membrane
Protein carriers/channels
The two types of transport molecules are known as
carrier
and
channel
proteins
Protein channels: Form tubes that fill with water and this allows water soluble ions to diffuse across
Protein carriers: Bind with other proteins and larger molecules such as glucose and amino acids, the carrier protein changes shape to transport them to the other side of the molecule
Partially permeable membrane means that only certain molecules can diffuse through like lipid soluble substances and very small molecules, bigger molecules and water soluble substances fail to pass through
Simple Diffusion
Net movement of molecules
From an area of higher concentration to an area of lower concentration
This happens until the concentration is the same on either side (we would say the reaction is at an equilibrium)
The process doesn't require ATP (it's a passive process)
For the molecules to move, they do have energy in the form of kinetic energy
This allows them to move constantly in fluids (liquid and gas)
Molecules have to be lipid solube and small in order to diffuse through the phospholipid bilayer which means larger like proteins can't partake in diffusion
Facilitated Diffusion
Protein channels form tubes/vesicles that fill up with water
This enables the water soluble ions (which wouldn't be able to diffuse across) to enter the protein channel and pass through the membrane
Facilitated diffusion has the same fundamental principle as simple diffusion, it occurs from an area of high concentration to an area of lower concentration
This is still selective, as these channel proteins only open in the presence of certain ions when they bind to the protein
Because of this mechanism ions an polar molecules which wouldn't usually be able to diffuse across can be transported
Carrier proteins work in a similar way, they bind to a molecule like glucose which then causes the protein's tertiary structure to change shape
However transport proteins are used to move molecules
This shape then enables the molecule to be released on the other side of the membrane
Still a passive process like simple diffusion - diffusion require enegy like ATP
Osmosis
The movement of water from an area of higher water potential to an area of lower water potential
This happens across a partially permeable membrane
Water potential is the pressure created by water molecules and is measured in kilo pascals
It is impossible to get a positive value for water potential as pure water has a water potential of 0
Upon solutes dissolving in the water, the water potential will become more negative
The more negative the water potential becomes the more solutes have been dissolved in it
During Osmosis there are three types of solutions
Isotonic = Water potential is the same in solution and cell
Hypotonic = Water potential is more +ve in the solution than cell
Hypertonic = Water potential is more -ve in the solution than the cell
Because there is a lack of structual integratiy in animal cells (cell wall) in
Hypotonic
and
Hypertonic
solutions the cell will be damaged
Hypotonic, water will move into the cell causing it to swell and ultimately burst. In plants (because of the cell wall) the pressure formed is known as
Turgor
and the plant cell becomes turgid
Hypertonic, water will laeve the cell and cause it to shrivel. If this were to happen to a plant cell, the cell will shrivel but be supported by the cell wall (it will be
Flaccid
)
Active Transport
The movement of molecules from an area of lower concentration to an area of higher concentration
This method of transport goes agains the concentration gradient and is an active transport which is different from all other methods of transport
So ATP is used as an energy source and carrier proteins are used
The carrier proteins act as 'pumps' to move substances across the membrane
This process is still selective, as only certain molecules can biinds to the carrier proteins
Certain molecules can binds to the receptor site on carrier proteins
ATP will bind to the protein inside the membrane
ATP will be hydrolysed into ADP and Pi (Adenine Diphosphate + Phosphate)
Once ADP and Pi have bound to the protein it will change shape and this means it will open once it has passed the membrane
The pi molecules are then released from the protein, this results in the protein reverting to its original shape
Then ADP and Pi reformed into ATP
