4. Cell membranes and transport Flashcards
How can the fluid mosaic model be described?
- Fluid- because the individual phospholipid and protein molecules can move relative to one another and this gives the membrane a flexible structure that is constantly changing in shape
- Mosaic- because the proteins that occur in the phospholipid bilayer vary in shape, size and pattern (scattered) in the same way as the tiles in a mosaic
What makes up a phospholipid?
- A hydrophilic phosphate head – attracted to water but not lipid
- 2 fatty acid hydrophobic tails- repelled by water but mix readily with lipid
How do the hydrophobic and hydrophilic interactions account for the formation of the phospholipid bilayer and the arrangement of proteins?
- One layer of phospholipids with their hydrophilic heads pointing inwards (attracted to water in the cell cytoplasm)
- The other layer of phospholipids with their hydrophilic heads pointing outwards (attracted to watery environment which surrounds the cell)
- The hydrophobic tails of both layers pointing to the center of the membrane (repelled by water on both sides and forms hydrophobic core)
*phospholipids can move within their own layer
What are the components of the cell surface membrane?
- Phospholipids
- Glycoproteins
- Glycolipids
- Cholesterol
- Protein
What is the arrangement of proteins in the cell surface membrane?
- Proteins- arranged more randomly (scattered) in the bilayer and some move within the bilayer while others are anchored in one location, held by fibers in the cytoplasm. They are associated with the phospholipid bilayer in 2 ways: peripheral and integral proteins
Extrinsic (peripheral) proteins - Occur on both surfaces of the bilayer, but never extend completely across it
Intrinsic (integral) proteins - May have regions embedded in the bilayer from one side to the other
- Some are channel proteins (form open water- filled tubes to allow water-soluble ions to diffuse across the membrane), while others are carrier proteins (bind to molecules such as glucose and amino acids, then change shape to move these molecules across the membrane)
- Proteins have regions of amino acids with hydrophobic R-groups and these are repelled by the aqueous environment and face the internal hydrophobic core of the bilayer, regions that face the aqueous internal/external environment have amino acids with hydrophilic R-groups
What is the arrangement of glycoproteins in cell surface membrane?
carbohydrate chains are attached to many proteins on the outer surface of the cell surface membrane
What is arrangement of glycolipids in cell surface membrane?
made up of a carbohydrate portion, that is hydrophilic and covalently bonded with a lipid tail that is hydrophobic and located in hydrophobic core of the bilayer. The carbohydrate portion extends from the bilayer into the watery environment outside the cell
What is arrangement of cholesterol in cell surface membrane?
complex lipid that has a hydrophilic and hydrophobic portion (which is embedded in the hydrophobic region of the bilayer) and cholesterol molecules interact with the fatty acid tails of the phospholipid molecules
What is role of phospholipids in permeability?
the hydrophobic core that is created by the fatty acid tails of the bilayer which is permeable allows many substances (small, lipid soluble/non polar, hydrophobic, respiratory gases, water) to cross the membrane, however the bilayer is impermeable to small polar molecules and ions as the hydrophobic interactions in the hydrophobic core prevent water-soluble substances from crossing the bilayer directly.
What is role of phospholipids in fluidity?
- the saturated and unsaturated fatty acid tails of phospholipids produce kinks in the tails and these help to keep the membrane fluid as they prevent phospholipids form packing very closely to one another
- Hydrocarbon chains of fatty acids interact with each other. The shorter chains interact less which makes the phospholipids less closely packed, making the membrane more fluid. Hydrophobic interactions of the fatty acid tails contribute to membrane stability
- Membranes that high a proportion of phospholipids with short fatty acid chains and that have unsaturated fatty acids with a high degree of bending (due to kinks) will be very fluid and flexible
What parts of the phospholipid bilayer contribute to membrane fluidity?
- Cholesterol
- Phospholipoids (saturated and unsaturated fatty acid tails, hydrophobic interactions of fatty acid tails and composition of phospholipid)
What is role of cholesterol in permeability?
- Cholesterol contains a small hydrophilic portion located in the phosphate head region of the bilayer and a longer hydrophobic tail that interacts with the fatty acid tails of the phospholipid molecules
- Therefore, plays a role in preventing the movement of dissolved ions and polar molecules across the bilayer
What is role of cholesterol in fluidity?
- Regulates fluidity and adds stability to membrane
- Prevents close packing of phospholipids when temps decrease to help keep membrane fluid (increase fluidity in cold temps)
- When temps increases they can prevent excessive movement of phospholipids therefore acts to decrease fluidity
What is the role of cholesterol in temperature?
- An increase in the proportion of cholesterol molecules makes the membrane less fluid
- An increase in temp increases the fluidity of membranes
- The increase in kinetic energy will increase the movement of molecules
- A high temp will affect tertiary and quaternary protein stricture and if too high can denature them and stop them from functioning
- Microorganisms that live in high-temp environments have heat-stable proteins in their membranes
Why is temperature not a main factor when considering membrane fluidity in mammalian cells?
temp is not a main factor when considering membrane fluidity in mammalian cells, because they are in a constant temperature environment, however unicellular organisms can vary their membrane composition in response to a change in the external environment
What is role of glycolipids in cell surface membrane?
- Act as antigens for cell-to-cell recognition
- Helps maintain stability of membrane
- Helps with cell-to-cell adhesion and therefore forms tissues
What is role of proteins (in general) in cell surface membrane?
- Provides structural support
- Acts as carrier proteins (transporting water-soluble substances such as small polar substances and ions)
- Forms channel proteins and aquaporins (transports water-soluble substances such as small polar substances and ions)
- Functions as enzymes
- Helps with cell-to-adhesion and therefore forms tissues
- Acts as cell surface receptors for cell signaling molecules
What is role of glycoproteins in cell surface membrane?
- Acts as recognition sites for hormones and neurotransmitters
- Helps with cell-to-cell adhesion and therefore forms tissues
- Acts as antigens for cell-to-cell recognition (proteins can also act as antigens)
What is the process of cell signaling?
- Process by which cells interact with their environment and with the cells around them
- Synthesis and secretion of specific chemicals (ligands) from cells
- Particular cells synthesize and secrete specific cell signaling molecules (ligands which bind to other biological molecules, but can be glycoproteins/proteins
- Transport of ligands to target cells
- Cell signaling molecules are transported in the capillary network and pass into tissue fluid
- Only target cells respond to the presence of cell signaling molecules
- In plants, cell signaling molecules can be transported within the phloem sap/move form cell to cell via plasmodesmata/cell walls
- Binding of ligands to cell surface to cell surface receptors on target cells
- Target cell have membrane proteins that act as receptors for cell signaling molecules
- Each type of cell signaling molecule only binds to a receptor that has a complementary shape (cell signaling is specific)
- Response by target cells
- The binding of a ligand to its specific receptor triggers events to occur within the target cell that eventually leads to the desired response
