Chapter 4: Cell Membranes And Transport Flashcards
(41 cards)
What is a micelle?
A micelle is a phospholipid molecule that arranges itself in a spherical form in aqueous solutions.
How wide is the phospholipid bilayer?
7nm
What is the phospholipid bilayer?
The phospholipid bilayer makes up the cell surface membrane that is made up of hydrophilic phosphate heads facing outwards and hydrophobic tails facing inwards. It is partially permeable.
Why is the membrane structure described as a fluid mosaic model?
It is described as fluid because the phospholipids and proteins can move about by diffusion. The word ‘mosaic’ describes the pattern produced by the scattered protein molecules when the cell membrane is viewed from above.
Explain the factors that affect the fluidity of a membrane. (3)
- The more unsaturated they are, the more fluid the membrane. This is because the unsaturated fatty acid tails are bent(branched) and therefore fit together more loosely.
- The longer the tail, the less fluid the membrane.
- As temperature decreases, membranes become less fluid.
What are intrinsic proteins? What are its features?
Proteins that are found embedded within the membrane are called intrinsic proteins. They may be found in the inner layer, outer layer or most commonly spanning the whole membrane, in whihc case they are known as transmembrane proteins.
Intrinsic proteins have hydrophobic and hydrophilic regions. They stay in the membrane because the hydrophobic regions, made from hydrophobic amino acids are next to the hydrophobic fatty acid tails are repelled by the watery environment on either side of the membrane. The hydrophilic regions, made of hydrophilic amino acids are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment on either side of the membrane.
What are extrinsic proteins?
Extrinsic proteins are found on the inner and outer surface of the membrane. Many are bound to intrinsic proteins or to phospholipids.
What is one of the functions of phospholipids in cell membranes?
Because the tails of phospholipids are hydrophobic/non-polar, they act as a barrier to most water soluble molecules.
Describe the features of a cholesterol molecule.
They are relatively small molecules that, like phospholipids, have hydrophobic tails and hydrophilic heads, so they fit neatly between phospholipids with their heads at the membrane surface.
In which organisms can cholesterol be found?
Cell membranes in animal cells contain almost as much cholesterol as phospholipid.
Cholesterol is much less common in plant cell membranes.
It is absent from prokaryotes.
Explain the functions of cholesterol in cell membranes.
At low temperatures, cholesterol increases the fluidity of the membranes by preventing the close packing of phospholipid tails, allowing the cell to survive at lower temperatures. The interaction between the cholesterol molecules and phospholipid tails helps to stabilise cells at higher temperatures when the membrane would otherwise become too fluid.
Cholesterol also helps with preventing water soluble molecules from entering the cell due to its hydrophobic tail.
Helps with mechanical stability.
What is the glycocalyx? What are its features?
Carbohydrate chains attached to membrane proteins (glycoproteins) and lipids (glycolipids) form hydrogen bonds with the water molecules to stabilise the membrane structure, forming a sugary coating to the cell known as the glycocalyx.
What does the glycocalyx mainly comprise of in animal and plant cells?
In animal cells, the glycocalyx is formed mainly from glycoproteins; in plant cells, the glycocalyx is formed mainly from glycolipids.
What are the functions of the carbohydrate chains of the glycocalyx? (4)
They act as receptors:
- Some can act as ‘signalling receptors’, because they form a part of the signalling system that coordinates the activities of the cell. The receptors recognise messages like hormones and neurotransmitters, binding with them and in turn, triggering a series of chemical reactions inside the cell.
- Some are involved in endocytosis wherein the receptors bind to the molecules that are to be engulfed by the cell surface membrane.
- A third group of receptors are involved in binding cells to other cells(cell adhesion) in tissues and organs of animals.
Some glycolipids and glycoproteins act as antigens or cell-markers, allowing cell-to-cell recognition.
What are some of the functions of proteins in the cell surface membrane?
- Many proteins act as transport proteins, providing hydrophilic channels for ions and polar molecules to pass through the membrane. There are two types of transport protein: carrier and channel.
- Other membrane proteins may be enzymes e.g. Can catalyse the hydrolysis of molecules.
- Some proteins on the inside of the cell are attached to a system of protein filaments that make up the cytoskeleton. This helps to maintain the shape of the cell.
How does cell signalling work with hydrophobic signalling molecules?
The hydrophobic signalling molecules can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm and nucleus.
Describe the process of cell signalling with a hydrophilic signalling molecule?
The signalling pathway starts with the signal arriving at a protein receptor on the surface of the cell surface membrane. The receptor is a specific shape which recognises the signal and only cells with this receptor can recognise the signal. The signal brings about a change in the shape of the receptor, and since this spans the membrane, this change in shape allows the message to be transmitted to the inside of the cell is known as signal tranduction. The change in shape allows it to interact with the next component of the pathway known as the ‘G protein’ which in turn transmits the message. The G protein acts as a switch to bring about the release of the second messenger, a small molecule which diffuses through the cell relaying the message. Many second messenger molecules can be made in response to one receptor molecule being stimulated causing an amplification of the original signal. The second messenger typically activates an enzyme, which in turn activates further enzymes, until an enzyme is produced which brings about the required change. The sequence of events triggered by the G protein is known as a signalling cascade.
What is diffusion?
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down the concentration gradient, as a result of the random movement of particles.
Explain the factors that affect the rate of diffusion across a membrane. (5)
- The steepness of the concentration gradient: The greater the difference in gradient, the greater number of particles moving in two directions, the faster the rate of diffusion.
- Temperature: At high temperatures, molecules have more kinetic energy than at lower temperatures and hence move around faster, increasing the rate.
- Surface area: The greater the surface area, the more molecules can cross it at any one moment.
- Distance: The time it takes a molecule to reach a certain destination by diffusion increases rapidly with distance travelled. The rate falls in proportion to the square of the distance.
- The nature of the molecules or ions: Large molecules require more energy to get them moving than small molecules. Non-polar molecules including oxygen, can cross the phospholipid bilayer easily. Water molecules, despite being polar, can diffuse rapidly across the cell surface membrane because they are small enough.
What is facilitated diffusion?
Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane. The proteins provide hydrophilic areas that allow the molecules or ions to pass through them from a region of higher concentration to lower concentration, down the concentration gradient.
What are the two types of proteins involved in facilitated diffusion?
Channel and carrier
What are channel proteins?
They are water-filled pores which act as a gate to allow charged substances to diffuse through the membrane. Each is highly specific, allowing certain substances to pass. They have a fixed shape and can be gated to control ion exchange. This process does not use ATP and is in facilitated diffusion.
What are carrier proteins?
Carrier proteins, unlike channel proteins, do not have a fixed shape and instead flip between two shapes. It is involved in active transport where it uses ATP to move substances up a concentration gradient. It is also used in facilitated diffusion down the concentration gradient without the use of energy. The binding site of a carrier protein is alternately open to one side of the membrane, then the other.
What is osmosis?
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random movement.
