Theme B: B2 Cells - B2.1 Membranes and Membrane Transport Flashcards
phospholipid
- Each phospholipid is composed of a three-carbon compound called glycerol
- 2 of the glycerol carbons are combined with fatty acids. The 3rd carbon is attached to highly polar organic alcohol including a bond to a phosphate group
- Fatty acids are water insoluble, however, due to organic alcohol with phosphate group being highly polar, the phospholipid is soluble
- Therefore, phospholipids have 2 distinct areas in terms of polarity and water solubility
- Hydrophilic (water soluble and polar) = the phosphorylated alcohol side
- Hydrophobic (water insoluble and non polar) = fatty acid tails
hydrophobic
Hydrophobic (water insoluble and non polar) e.g. fatty acid tails
hydrophilic
Hydrophilic (water soluble and polar) e.g. the phosphorylated alcohol side
amphipathic
any molecule with hydrophobic and hydrophilic regaions is said to be amphipathic
how is the membrane strcture maintained?
- If there is water present, the hydrophilic and hydrophobic regions naturally align as a bilayer
- Hydrophobic regions attach to each other
- Hydrophilic regions attracted to the water (in cytoplasm and extracellular region)
Overall membrane structure maintained by relationship between the membrane’s chemical makeup and the chemical properties of water (hydrogen bonding is that main force holding it together)
important features of the membrane
1) the membrane is fluid as the fatty acid tails don’t attract each other strongly. Allows the membrane to have variable shape and allows endocytosis. It is fluid in the sense that they are not covalently bonded together, it is held together by relatively weak intermolecular forces (hydrogen bonding).
2) the bilayer forms an effective barrier The tightly packed bilayer forms a barrier to most molecules.
Large molecules and hydrophilic molecules (e.g., ions like iron) cannot easily pass due to the hydrophobic core.
3) selective permeability. The bilayer allows the cell to control what enters and exits, enabling the uptake of necessary molecules and exclusion of unwanted ones.
diffusion
a type of transport that can take place through the membrane. in diffusion particles move from a region of high concentration to lower concentration.
e.g. 1: oxygen used by cells in respiration. often a lower concentration of oxygen inside the cell compared to outside. ocxygen diffuses into the cell as a result.
e.g 2: CO2 diffuses into opposite direction. CO2 produced by mitochondrial respiration and present in high concentrations within the cell compared to outside.
both pf these exemplar molecules are small and uncharged, meaning they can move between the membrane’s phospholipid molecules and iffusion can easily occur.
membrane proetins
these membrane proteins create the extreme diversity in membrane function. the various types of proteins embedded in the fluid matrix of the phospholipid bilayer is what creates the mosaic like characteristic of the membrane.
two major types:
1) integral proteins
2) peripheral proteins
integral proteins
Proteins that are permanently embedded in the phospholipid bilayer of a cell membrane.
* show an amphipathic character where these proetins have their hydrophobic region in the mid-section of the phospholipid bilayer and the hydrophilic region is exposed to the water molecules on either side of the membrane.
peripheral proteins
Proteins that are temporarily attached to the surface of the cell membrane, either on the inner or outer side, without embedding into the hydrophobic core.
* Bind to the membrane indirectly via integral proteins or directly through interactions with the polar heads of phospholipids.
two general types of cellular transport
1) active transport
2) passive transport
passive transport
The movement of molecules across a membrane without requiring energy (ATP), following a concentration gradient (from high to low concentration)
* the soruce of energy for this movement comes from the kinetic energy of molecules.
* if uninterrupted, this movement will continue until equilibrium is reached (equal concenteations of the substance are found in both areas)
Types:
1) Simple Diffusion:
Movement of small, non-polar molecules (e.g., oxygen, carbon dioxide) directly through the phospholipid bilayer.
2) Facilitated Diffusion:
Movement of larger or charged molecules (e.g., glucose, ions) via channel or carrier proteins.
3) Osmosis:
Diffusion of water molecules through a selectively permeable membrane, often via aquaporins.
active transport
The movement of molecules or ions across a membrane against their concentration gradient (from low to high concentration), requiring energy (ATP).
* equilibrium is not reached
* Involves specific carrier proteins or pumps
* it allows the cell to maintain its interior concentrations that are different from the exterior concentrations.
Examples:
1) Sodium-Potassium Pump (Na⁺/K⁺ Pump): Moves 3 sodium ions out and 2 potassium ions into the cell.
osmosis
a type of passive transport that moves water across a partially permemable membrane (selectively permeable membrane) from a region of lower solute concentration (hypotonic solution) to a region of higher solute concentration (hypertonic solution).
* Requires a concentration gradient of water molecules.
* Does not require energy (passive transport).
* Water can move directly through the membrane or via aquaporins (specialized channel proteins).
hypotonic
A solution with a lower solute concentration compared to another solution.
hypertonic
A solution with a higher solute concentration compared to another solution.
isotonic
A solution with the same solute concentration as another solution. there is no net movement of water evident as equilibrium has been achieved.
aquaporins
specialised channel proteins allow wtaer through a membrane.
facilitated diffusion (passive transport)
The passive transport of molecules or ions across a membrane with the help of transport proteins (form of integral proteins), down their concentration gradient (from high to low concentration).
1) carrier porteins
2) channel proteins
the rate of facilitated diffusion depends on:
1) concentration difference existing across the membrane
2) number of carrier proetains actively involved in transport
3) and/or number of channnel proetins open
carrier proteins
A type of transport protein that facilitates the passive or active transport of specific molecules across the membrane by binding to and changing shape to move the molecule. (invokved in facilitated diffusion: passive transport).
- they change shape to carry specific substances (usually an ion) from one side of membrane to the other
- can carry both insoluble and soluble molecules.
channel proteins
A type of transport protein that forms hydrophilic pores in the cell membrane, allowing the passive movement of specific ions or molecules across the membrane.
* most have gates that open/close in response to chemical or mechanical signals (as opposed to changing shape like carrier proteins)
* Only carries water soluble molecules specific for the ion they carry
what characteristic of the cell membrane do transport proteins create?
the presence of carrier and channel proteins makes the cell membrane selectively permeable.
how easily a suibstance can move passively across a membrane depends on…?
1) size
2) charge
small, nonpoalr subtsances will move easily across membrane. e.g. ions (chloride, potassium, and sodium ions) have great diffuclty as do large molecules like glucose and sucrose.
the diffusion of small simple molecules is not selective.
the cell can be selectively permeable to large charged molecules because they must travel through integral proteins.
haemodialysis
A medical procedure used to remove waste products and excess substances (e.g., urea, potassium) from the blood when the kidneys are no longer functioning properly.
glycolipids
when cell membrane phosphlipids have carbohydrates attacthed to them
glycoproteins
cell membrane peripheral porteins with chains of carbohydrates attacthed to them
what are glycolipids and glycoproteins important for?
they’re impirtant for cell adhesion and cell recgonition.
points about the carbohydrate chains attacthed to glycoproteins and glycolipids.
- the carbohydrte chains attatched are only found on the extracellular side of the membrane. they’re quite diverse based on seuqences of sugar types and branching structures.
- the characteristics of human blood types (A, B, O) are result of carbohydrate chains.
- they can workout which cells belong to itself and which are from outside the body (non-self)
glycocalyx
thin sugar layer made of carbohydrate chains attatched to proteins that can cover a cell. (common in animal cells)
function include:
* cell-to-cell adhesion, cell to cell reocgnition and recpetion of various signalling chemicals
* also present on surface of many bacterial and fungal cells where may have both protective and adhesion functions
* when glycocalyx occurs in plant cells, often helps anchor plant cell membrane to cell wall.
cholesterol
A lipid molecule that is an essential component of the cell membrane, embedded within the phospholipid bilayer. cholesterol found at various locations in the hydrophobic region (fatty acid taisl) of animal cells.
* Modulates membrane fluidity, helping to maintain the membrane’s integrity at different temperatures.
* At high temperatures, it stabilizes the membrane by preventing it from becoming too fluid.
* At low temperatures, it prevents the membrane from becoming too rigid by disrupting tight packing of phospholipids.
* Interacts with fatty acid tails of phospholipids, making the membrane less permeable to small water-soluble molecules.
Plant cells to NOT have cholesterol in cell membrane; they depend on saturated or unsaturated fatty acid to maintain proper membrane fluidity.
fatty acids in membranes in relation to temperature
Animal cells
Key Features:
* Unsaturated fatty acids (with double bonds) create kinks in the chain, preventing tight packing, which maintains fluidity at lower temperatures.
* Saturated fatty acids (with no double bonds) allow tight packing of molecules, which reduces fluidity and can make the membrane more rigid at lower temperatures.
* Membranes with more unsaturated fatty acids tend to stay fluid in cooler environments, while those with more saturated fatty acids become more solid.
Function:
* The composition of fatty acids in membranes helps cells maintain optimal fluidity and function across temperature changes, contributing to membrane stability.
Bacteria
* individual cells that are vulnerable to drastic changes in tmerpature evolve mechanisms to retain membrane fluidity, alowing them to always be effective. they’re known as fatty acid desatures
fatty acid desatures
Enzymes that introduce double bonds into the hydrocarbon chains of fatty acids, converting saturated fatty acids into unsaturated fatty acids.
(exterior) plasma membrane vs (interior) membrane (e.g. ER membrane)
plasma membrane: thicker with saturated fatty acids and more cholesterol molecules. becuase plasma membrane are subject to more extreme temperatures.
interior membrane: thinner with unsaturated fatty acids and less cholesterol molecules.
endocytosis
**allows macromolecules to enter the cell
**The process by which cells engulf external substances by folding the cell membrane inward to form a vesicle.
Key Features:
Energy-dependent process (requires ATP).
Can involve the uptake of large molecules, particles, or liquids.
Types include phagocytosis (cell eating) for large particles and pinocytosis (cell drinking) for liquids.
Function:
Allows cells to take in large molecules (e.g., nutrients, hormones) or particles (e.g., pathogens).
Plays a key role in immune response and nutrient uptake.
exocytosis
allows macromolecule to exit the cell.
The process by which cells expel substances from inside the cell to the extracellular environment by fusing vesicles with the cell membrane.
Key Features:
* Energy-dependent process (requires ATP).
* Involves the transport of large molecules like proteins, hormones, and waste out of the cell.
* The vesicle containing the substances fuses with the plasma membrane, releasing its contents outside the cell.
Function:
* Allows cells to secrete substances such as enzymes and signaling molecules.
* Plays a role in processes like neurotransmitter release, hormone secretion, and waste removal.
e.g. protein exocytosis. insulin produced in pancreas and secreted into bloodstream. nuerotransmitters released at synapses in the nervous system
what do endocytosis and exocytosis depend on?
membrane fluidity.
Neurotransmitter-Gated Ion Channels
Membrane proteins that open or close in response to the binding of a neurotransmitter, allowing ions to flow across the membrane. e.g. is a nictinic acetylcholine receptor
nictinic acetylcholine receptor
type of neurotransmitter-gated ion channel that is activated by binding to neurotransmitter acetylcholine (ACh). This is specifically found at neuromuscular junctions and certain synapses in the nervous system. when acetlycholine attatches to the receptor the channel through the membrane is opened an dpositive ions (like Na+ K+ Ca2+) can pass through. this causes teh mebrane potential to change so can impulse can be generated. nerve impulses can be carried alonh multiple, connected neurons so responses are possible. when the neurotransmitter is released at the junction between a nerve and a muscle, the opening of this receptor and the movement of positve ions can cause muscle movement.
voltage gated membrane proteins
Membrane proteins that open or close in response to changes in the membrane potential or membrane polarity, allowing ions to flow across the membrane.
e.g. Na+ and K+ channels are examples of voltage gated protein carriers:
* an electircal stimulus opens and closes the gates aon the proteins. though they only remain open for a very short periods of time, allowing specific ions to move rapidly through them.
1. sodium channels open first and the sodium ions move from outside to inside of neuron, depolarising the membrane. they then quickly close.
2. potassium channels ope more slowly, once open potassium ions move through from inside to outside of cell and the membrane potential returns to normal membrane potential (repolarisation)
membrane potential
the difference in eletrical potential between the interior and exterior of a biological cell.
indirect active transport
A form of active transport where the movement of one molecule or ion against its concentration gradient is powered by the downhill movement of another molecule or ion, rather than directly using ATP.
most common example of this involves transport of glucose into the cells lining intestines of animals.
repolarization
process of re-establishing membrane potential after depolarisation
sodium potassium pump process
1) pump proetin with an attacthed ATP molecule binds to 3 intracellular sodium ions
2) the bonding of sodium ions causes the pump to split the ATP, providing usable energy and leaves a phosphate group attatched the carrier. the addition of a phosphate is called phosphorylation. when ATP carries out phosphrylation of the pump it loses a phosphate and becomes adenosine diphosphate (ADP).
3) the phosporylation causes proetin to change shpae, thus expelling the sodium ions into extracellular fluid. at this point, the pump has a low affinity for sodium ions but the shape change results in a high affinity for potassium ions
4) 2 extracellular potassium ions bind to different regions of the protein, causing the release of the phosphate group.
5) the loss of phosphate group restores the protein’s original shape thus cuasing the release of the potassium ions into the intracellular space. carrier is now ready to repeat the process.
sodium potassium pump and role in neurons
Because the sodium-potassium pump moves more positive ions out than in, it creates an imbalance of ions across the membrane.
This imbalance of ions creates a difference in charge between the inside and outside of the cell, known as the membrane potential. this is especially important in neurons.
a nerve impulse takes place when sodium ions diffuse into the cell through specialised channels, creating a change in charge called depolarization. this depolarization spreads down the neuron generating a nerve impulse. potassium ions diffuse out the cell in 2nd part of the process and repolarises the cell. the cell maintains the correct concentrations of sodium and potassium using the pump.
the sodium potassium pump are exchange transporters (antiporters) and don’t directly form a nerve impulse.
sodium-potassium pump (chatgpt overview)
An active transport protein that moves sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell, against their concentration gradients, using energy from ATP.
Key Features:
* Transports 3 sodium ions out and 2 potassium ions in per cycle.
* Helps maintain the resting membrane potential and cell volume.
especially important in neurons
sodium-dependent glucose transporter (SGLT)
plays a crucial role in the active transport of glucose into cells, particularly in tissues like the intestinal epithelial cells and kidney cells.
* often higher concnetration of glucose inside the cell, therefore energy in form of ATP must be provided for this transport to occur.
* K+ and Na+ ions are also being transported by the same carriers, this system is also referred to as “coupled transport:” there are two pumps involved here where the ATP produced by the S-P pump is needed for the coupled glucose transport protein.
* SGLT can be used in the intestine or in the kidney (nephron). they are differentiated through this naming system:
SGLT1: intestinal transporter
SGLT2: nephron transporter
cell-adhesion molecule (CAM)
(context) multicellular organisms are dependent on adhesion. adhesive interaction can happen via plasma membrane and can be stable or temporary.
cell-adhesion molecues are usually involved in cell connections. cell connections generally allow coordinated behaviour and have important strcuctural functions
Desosomes
Specialized structures in the plasma membrane that provide strong adhesion between adjacent cells, forming a mechanical link to resist shear forces. they help form sturdy but flexible sheets of cells in organs like the heart, stomasch, bladder.
* tissue in these organs get stretched, but the desosomes hold the cells together
plasmodesmata
plant cells often produce plasmodesmata, whcih are are tubes connecting cytoplasm of adjacent cells. they allow exchange of materials: especially water and small sollutes between connected cells.
