Chapter 5 - Plasma Membranes Flashcards
What is the plasma membrane?
Cell-surface membrane which separates the cell from its external environment
What is the phospholipid bilayer?
Arrangement of phospholipids found
in cell membranes; the hydrophilic
phosphate heads form both the
inner and outer surface of a
membrane, sandwiching the fatty
acid tails to form a hydrophobic core
Why are phospholipid
bilayers suited as
membranes? (3)
- Cells normally exist in aqueous environments
- The inside of cells and organelles are also usually aqueous environments
- Phospholipid bilayers are suited because the outer surfaces of the hydrophilic phosphate heads can
interact with water
What is the fluid-mosaic model?
Model of the structure of a cell
membrane in which phospholipids
within the phospholipid bilayer are
free to move and proteins of various
shapes and sizes are embedded in
various positions
Who proposed the fluid-mosaic model?
American scientists Singer and Nicolson in 1972
What is the function of glycoprotein in the cell membrane?
Acts as a recognition site for chemicals e.g. hormones
What is a glycoprotein?
Branching carbohydrate portion of a protein which acts as a recognition site for chemicals e.g. hormones
What is the function of glycolipids in the cell membrane?
Acts as a recognition site e.g. for cholera toxins
What is the function of cholesterol in the cell membrane?
Stability/flexibility
What are the components of a cell membrane? (7)
*Glycoprotein
*Glycolipid
*Cholesterol
*Phospholipids
*Extrinsic proteins
*Intrinsic proteins
*Pores
What are membrane proteins?
Protein components of cell-surface membranes
What are intrinsic proteins also known as?
Integral proteins
What are intrinsic proteins?
*Transmembrane proteins that are embedded through both layers of a membrane
Describe intrinsic proteins (2)
*Have amino acids with hydrophobic R-groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place
*Channel and carrier proteins (both involved in transport across the membrane)
What are channel proteins?
Membrane proteins that provide a hydrophilic channel through a membrane, which allows passive movement of polar molecules and ions down a concentration gradient through membranes
How are channel proteins held together?
Held in position by interactions between the hydrophobic core of the membrane, and the hydrophobic R-groups on the outside of the proteins
What are carrier proteins?
Membrane proteins that have in important role in both passive transport and active transport into cells
- This often involves the shape the protein changing
What are glycoproteins?
Extrinsic membrane proteins with attach carbohydrate molecules of varying lengths and shapes that are embedded in the cell-surface membrane
What is the role of glycoproteins?
Play a role in cell adhesions and as receptors for chemical signals
What is cell signalling?
A complex system of intercellular communication
- When the chemical binds to the receptor, it elicits a response from the cell
- This may cause a direct response or set off a cascade of events inside the cell
Give examples of cell signalling (3)
- Receptors for neurotransmitters e.g. acetylcholine at nerve cell synapses. Binding of the neurotransmitters triggers or prevents an impulse in the next neurone
-
Receptors for peptide hormones inc. insulin and glucagon, which affect the uptake and storage of glucose by cells
* ß-blockers are used to reduce the response of the heart to stress
What are glycolipids?
Cell-surface membrane lipids with attached carbohydrate molecules of varying lengths and shapes
Which part of the membrane acts as cell markers/antigens?
Glycolipids
* They can be recognised by cells of the immune system as self or non-self
What are extrinsic proteins also called?
Peripheral proteins
Describe extrinsic proteins (3)
- Present in one side of the bilayer
- Normally have hydrophilic R-groups on their outer surface and interact with the polar heads of the phospholipids or with intrinsic proteins
- Can be present in either layer, and some move between layers
What is cholesterol?
A lipid with a hydrophilic end and a hydrophobic end, like a phospholipid
What is the role of cholesterol molecules in a membrane? (4)
- Regulates the fluidity of membranes
- Positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads, and the hydrophobic end interacting with the tails, pulling them together
- Adds stability to membranes without making them too rigid
- Prevent the membranes becoming too solid by stopping the phospholipid molecules rom grouping too closely and crystallising (at low temps)
What are the roles of membranes within cells? (3)
- Cristae in mitochondria give a large SA for the some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur
- Thylakoid membranes in chloroplasts have chlorophyll, and one these membranes some of the reaction of photosynthesis occur
- Digestive enzymes on the plasma membranes of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
What factors affect membrane structure? (2)
- Temperature
- Presence of solvents
What happens to a membrane when the temperature drops?
- **Saturated fatty acids become compressed **
- Many unsaturated fatty acids in phospholipid bilayer, and as they become compressed, kinks in their tails push adjacent phospholipids molecules away, maintaining membrane fluidity
- Proportions of different types of fatty acids determine the membrane’s fluidity at low temps
- Cholesterol prevents reduction in the membrane’s fluidity
What happens to a membrane when temperature increases?
- Phospholipids get more kinetic energy, move around more, in a random way, increasing the membrane’s fluidity
- **Permeability increases **
- Affects the way membrane-embedded proteins are positioned and may function
- If some of the proteins that act as enzyme drift sideways, it could alter the rate of reaction they catalyse
- Increase in membrane fluidity may affect the infolding of the plasma membrane during phagocytosis
- Cholesterol molecules buffer to some extent the effects of increasing heat, as they reduce the increase in membrane fluidity
What happens to proteins when temperature increases?
- Atoms within their large molecule vibrate, breaking the hydrogen and ionic bonds that hold their structure together
- They unfold
- Tertiary structure changes and cannot change back again when they cool - they are denatured
- If both the membrane-embedded proteins and the cytoskeleton threads become denatured, the plasma membrane will begin to fall apart, and it will be more permeable there will be holes in it
What type of solvents affect membranes? Give examples of each (2)
- Organic solvents e.g. alcohols
- Non-polar solvents e.g. benzene
What effect do organic solvents have on membranes?
- They will dissolve membranes, disrupting cells
- Pure or very strong alcohol solutions are toxic as they destroy cells in the body
- Less concentrated solutions e.g. alcoholic drinks, will not dissolve membranes, but still cause damage
- Non-polar alcohol molecules enter the cell membrane, and the presence of these molecules between the phospholipids disrupts the membrane
- Disrupted membrane = more fluid and more permeable
Give an example of a cell that needs an intact cell membrane for a specific function
- Nerve cells
- Need intact cell membranes for the transmission of nerve impulses
- When neuronal membranes are disrupted, nerve impulses are no longer transmitted as normal
- Also happens in neurones in the brain, explaining by behaviour changes after the consumption of alcoholic drinks
What is diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration.
Describe diffusion
- Passive process i.e. only uses the kinetic energy of the molecules, NOT ATP
- Takes place down a concentration gradient
- Will continue until there is a concentration equilibrium between the two areas
What is simple diffusion?
Diffusion in the absence of a barrier or membrane
What are the factors that affect the rate of simple diffusion?
Temperature
* As temp increases, molecules have more KE, so rate of diffusion will increase
Concentration gradient
* The steeper the gradient, the faster the diffusion to the side where there are fewer molecules, down the gradient
What kind of molecules can diffuse easily across membranes?
Non-polar molecules
Why can ions not easily pass through membranes?
They are repelled by the hydrophobic interior of the membrane
Can polar molecules diffuse through membranes?
Yes but only at a very slow rate
* Small polar molecules pass through more easily than large ones
What are the factors that affect the rate of diffusion across a membrane? (2)
Surface area
* The larger the area of an exchange surface, the high the rate of diffusion
Thickness of membrane
* The thinner the exchange surface, the higher the rate of diffusion
What is facilitated diffusion?
Diffusion across a plasma membrane through protein channels
Describe facilitated diffusion
- Down a concentration gradient and doesn’t require external energy
- Membranes contain channel proteins through which polar molecules and ions can pass
- Membranes with protein channels are selectively permeable as most protein channels are specific to one molecule or ion
- Can also involve carrier proteins which change shape when a specific molecules binds
What is the rate of facilitated diffusion dependent on? (4)
- Temperature
- Concentration gradient
- Membrane surface area and thickness
- Number of channel proteins present
What is active transport?
Movement of particles across a plasma membrane against a concentration gradient. Requires energy and carrier proteins
Describe active transport
- Metabolic energy is supplied by ATP
- Carrier proteins span the membranes and act as ‘pumps’
Describe the process of active transport (e.g. outside to inside a cell) (6)
- The molecule or ions to be transported binds to receptors in the channel of the carrier protein on the outside of the cell
- On the inside of the cell ATP binds to the carrier proteins is hydrolysed into ADP and phosphate
- Binding of the phosphate molecule to the carrier protein causes the protein to change shape - opening up to the inside of the cell
- The molecule or ion is released to inside of the cell
- The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- The carrier protein returns to its original shape
What is bulk transport?
A form of active transport where large molecules or whole bacterial cells are moved into or out of a cell by endocytosis or exocytosis
What is energy in the form of ATP required for? (3)
- Movement of vesicles along the cytoskeleton
- Changing the shape of cells to engulf materials
- The fusion of cell membranes as vesicles form or as the meet the cell-cell-surface membrane
What is endocytosis?
The bulk transport of materials into cells via invagination of the cell surface membrane forming a vesicle. Phagocytosis (solids) and Pinocytosis (liquids)
Describe the process of endocytosis (4)
- Cell membrane invaginates when it comes into contact with the material to be transported
- The membrane enfolds the material until eventually the membrane fuses, forming a vesicle
- Vesicle pinches off and moves into the cytoplasm to transfer the material for further processing
- e.g. vesicles containing bacteria are moved towards lysosomes, where the bacteria are digested by enzymes
What is exocytosis?
The bulk transport of materials out of cells. Vesicles (usually formed by the Golgi apparatus) containing the material fuse with the cell-surface membrane and the contents are released to the outside of the cell
What is osmosis?
Diffusion of water through a partially permeable membrane down a water potential gradient
*A passive process and energy is not required
What is water potential (Ψ)?
Measure of the quantity of water compared to solutes, measured as the pressure created by the water molecules in kilopascals (kPa)
What is the water potential of pure water?
- 0 kPa (at standard temperature and atmospheric pressure : 25˚C and 100kPa)
- Highest possible value of water potential
Describe the water potentials of solutions
- Presence of a solute in water lowers the water potential below zero
- All solution have negative water potentials
- More concentrated solution = more negative the water potential
What happens when solutions of different concentrations are separated by a partially permeable membrane?
There will be a net movement of water from the solution with higher water potential (less concentrated) to the solution with lower water potential (more concentrated).
*This will continue until the water potential is equal on both sides (equilibrium)
What is hydrostatic pressure?
The pressure created by water in an enclosed system e.g. by the diffusion of water into a solution leading to an increase in pressure
- Units: kPa
What happens if an animal cell is placed in a solution with a higher water potential than that of the cytoplasm? (4)
- Water will move into the cell by osmosis
- Hydrostatic pressure inside the cell will increase
- Cell surface membrane is thin (7nm), cannot stretch much and cannot withstand the increased pressure
- The cell membrane will break and the cell will burst in an event called cytolysis
What happens if an animal cell is placed in a solution with a lower water potential than that of the cytoplasm?
- The cell will lose water to the solution by osmosis down the water potential gradient
- This will cause a reduction in the volume of the cell
- The cell membrane will ‘pucker’ which is referred to as crenation
How do multicellular animals prevent cytolysis or crenation?
They usually have control mechanisms to make sure their cells are continuously surrounded by aqueous solutions with an equal water potential (isotonic)
- e.g. in blood the aqueous solution is blood plasma
What happens if a plant cell is placed in a solution with a higher water potential than their own?
- Water enters the cell by osmosis
- The increased hydrostatic pressure pushes the membrane against the rigid cell walls
- This pressure against the cel wall is called turgor
- As the turgor pressure increases, it resists the entry more water, and the cell is said to be turgid
What happens if a plant cell is placed in a solution with a lower water potential than their own?
- Water is lost from the cells by osmosis
- This leads to a reduction in the volume the cytoplasm, which eventually pulls the cell-surface membrane away from the cell wall
- The cell is said to be plasmolysed
