5 - BIOLOGICAL MEMBRANES Flashcards
Why is the plasma membrane described as a ‘fluid mosaic’?
Because the phospholipids in the bilayer make the membrane fluid, and the proteins floating in between them make up a mosaic pattern.
What are some functions of the cell membrane?
- Compartmentalising organelles
- Allowing cell signalling
- Controlling what passes in and out of the cell (partially permeable)
- Sites for enzyme attachment and chemical reactions
- Forms vesicles
- Creation of conc.gradients
What are the components which make up the plasma membrane?
- Phospholipid bilayer (phospholipids)
- Integral proteins: channels and carriers
- Peripheral proteins
- Glycocalyx: glycoproteins and glycolipids
- Carbohydrates
- Cholesterol
- Cytoskeleton filaments
How does the structure of individual phospholipids enable their function as as bilayer?
They act as a barrier to water-soluble substances, but allow fat-soluble substances through. This is because they have hydrophobic fatty acid tails which are not attracted to water like their hydrophilic phosphate heads.
How does the structure of proteins enable their function?
They span the whole bilayer by interacting with hydrophobic regions as they are integral.
Channels allow small charged particles through by creating pores, whereas carriers change shape and flip-flop to aid in processes such as active transport and diffusion.
How does the structure of the glycocalyx enable their function?
They interact with the cell’s external environment. They stabilise the membrane by forming hydrogen bonds with surrounding water molecules. They act as sites where drugs, hormones and antibodies bind. They also act as receptors for cell-signalling and antigens involved in immune respone.
How does the structure of carbohydrate molecules enable their function?
They help the cell interact with watery environment of cell. They are very hydrophobic so attract with dissolved solutes. They also act as antigens involved in immune response.
How does the structure of cholesterol enable its function?
It fits inbetween individual phospholipid molecules by binding to their tails; this causes them to pack more closely together. It regulates fluidity and rigidy of membrane. It maintains its mechanical stability and flexibility as well as buffers the effect of extreme temperatures on its structure.
Why is it important for cells to communicate with eachother?
To control processes inside the body and respond to changes in the environment
How do cell signal eachother?
Using messenger molecules which travel in the body and bind to receptors made of proteins. These have specific shapes complemetary to particular messengers, and are found on the surface of target cells.
How are drugs involved in this process?
They also work by binding to receptors in cell membranes, either triggering a response in the cell or blocking the recepotor from working.
What is the effect of high temperature on the phospholipid bilayer?
- Phospholipids acquirev more kinetic energy and move around more randomly
- Membrane fluidity and permeability increases
- Depositions membrane-embedded proteins such as enzymes
- Affects the rates of reactions catalysed by those enzymes
- Increase in fluidity affects membrane’s ability to signal other cells as well as phagocytosis and exocytosis
What is the effect of low temperature on the phospholipid bilayer?
- More unsaturated fatty acids become compressed and so the kinks in their tails push adjacent phospholipid molecules away
- Less saturated fatty acids become compressed
- This maintains membrane fluidity
How do organisms respond to changes in temperature?
They change their composition of fatty acids
How does cholesterol act as a buffer in the phospholipid bilayer?
- Prevents phospholipid molecules from packing too closely together to prevent a reduction in membrane fluidity at low temperatures
- Reduces increase in membrane fluidity to some extent in increasing heat
What is the effect of high temperatures on proteins?
- Cause the atoms within their large molecules to vibrate thus breaking hydrogen and ionic bonds that hold their structure together causing them to unfold
- This causes their tertiary shape to change causing them to denature and so they cease to function
- Those that act as enzymes will experience a chnage in their active site causing the rate of reactions they catalyse to be slowed
- Those that make up cytoskeleton threads to denature making the mebrane fall apart and become more permeable due to the formation of holes
What is diffusion?
It is the net movement of particles down a concentration-gradient, expending no metabollic energy as it is a passive process.
What factors affect diffusion?
Temperature, distance, concentration-gradient, surface area, and size of molecules
Describe sample diffusion.
It is the rapid dffuison of small non-charged particles such as oxygen and carbon dioxide, as well as the slow diffusion of small and polar molecules such as wter containing urea.
Describe facilitated diffusion.
Channel proteins form pores (gates) through which larger molecules can pass through one by one. Carrier proteins change shape to allow molecules to pass through in either direction.
What is osmosis?
It is the net diffusion of water down a water-potential gradient through the partially-permeable membrane; it is similarly a passive process requiring no energy.
What is water potential?
It is the tendeny of water molecules in a system to move. It is affected by the amount of solute it contains, because water molecules bind to solute molecules, reducing the number of them that are free to diffuse. Pure water has the highest water potential of 0kPa, so all solutions with lower water potentials have negative values.
Why is the water potential inside cells usually lower than that of pure water?
Because there are solutes in solution in cytoplasm and in vacuoles of plant cells.
What is cytolysis?
When animal cells swell up and burst as the plasma membrane breaks due to the diffusion of many water molecules into the cell.
