5 - Plasma Membranes🫙 Flashcards
Active Transport
Define active transport
The movement of molecules or ions into or out of a cell from a region of a lower concentration to a region of higher concentration
Active Transport
What does this process require?
Carrier proteins and metabolic energy (supplied by ATP)
Active Transport
Where / what do carrier proteins do?
Span the membranes and act as ‘pumps’
Active Transport
Outside to inside a cell
Step 1 = the molecule or ion to…
be transported binds to receptors in the channel of the carrier protein on the outside of the cell
Active Transport
Outside to inside a cell
Step 2 = on the inside of the cell, ATP…
binds to the carrier protein and is hydrolysed into ADP and phosphate
Active Transport
Outside to inside a cell
Step 3 = binding of the phosphate molecule to…
The carrier protein causes the protein to change shape, opening up the inside of the cell
Active Transport
Outside to inside of cell
Step 4 = the molecule or ion is…
released to the inside of the cell
Active Transport
Outside to inside of a cell
Step 5 = the phosphate molecule is released from…
the carrier protein and recombines with ADP to form ATP
Active Transport
Outside to inside of a cell
Step 6 = the carrier protein…
returns to its original shape
Active Transport
What does it mean if the process is selective?
Specific substances are transported by specific carrier proteins
Active Transport
What is bulk transport?
Large molecules too large to move through channel or carrier proteins are moved into and out of the cell by bulk transport
Active Transport
What large molecules are transported by bulk transport?
Enzymes, hormones, bacteria
Bulk Transport
What are the two types of endocytosis?
Phagocytosis for solids
Pinocytosis for liquids
Bulk Transport
Endocytosis
What happens first in both types as the membranes comes into contact with the material being transported?
Cell-surface membrane invaginates (bends inwards), enfolding the material until the membrane fuses forming a vesicles
Bulk Transport
Endocytosis
What happens once the vesicle is formed?
It pinches off and moves into the cytoplasm to transfer the material for further processing within the cell
Bulk Transport
Endocytosis
Give an example of where/why the vesicles are further processed in the cell
Vesicles containing bacteria are moved towards lysosomes where bacteria can be digested by enzymes
Bulk transport
Exocytosis
How does exocytosis work?
Vesicles usually formed by the Golgi move towards and fuse with the cell surface membrane. The contents of the vesicles are then released
Bulk Transport
Exocytosis
What is it?
Movement to the OUTSIDE of a vell
Why is energy (ATP) required for vesicles?
- movement along cytoskeleton
- changing shape = engulfing
- fusion to membranes
Factors affecting membrane structure
How do phospholipids get more KE so they can move more than they already do?
Temperature
Factors affecting membrane structure
Why does more movement harm a membrane?
Makes it more fluid = begins to lose its structure = breaks down completely
Factors affecting membrane structure
What can the loss of structure from high temp increase?
The permeability of the membrane, making it easier for particles to cross it
Factors affecting membrane structure
What happens to carrier and channel proteins in high temp?
Denature, affecting membrane permeability
Factors affecting membrane structure
What is essential in the formation of a phospholipid bilayer?
Water, a polar solvent
Factors affecting membrane structure
Are organic solvents more or less polar than water?
Less, such as alcohol
Factors affecting membrane structure
What do organic solvents do to membranes?
Dissolve membranes, disrupting cells
Factors affecting membrane structure
Why are alcohols used in antiseptic wipes?
The alcohols dissolve the membranes of bacteria in a wound, killing them and reducing the risk of infection
Factors affecting membrane structure
Why are pure or very strong alcohol solutions toxic?
They destroy cells in the body
Factors affecting membrane structure
What do alcoholic drinks do to membranes?
Can enter the cell membrane and the presence of these molecules between the phospholipids disrupts the membranes
Factors affecting membrane structure
What happens when the membrane is disrupted?
Becomes more fluid = more permeable
Factors affecting membrane structure
Give an example of cells that need intact cell membranes for specific functions
The transmission of nerve impulses by neurones (nerve cells)
When these are disrupted, nerve impulses are no longer transmitted as normal
Factors affecting membrane structure
Why do peoples’ behaviour change after consuming alcoholic drinks?
The neurones in the brain are no longer transmitted as normal
Investigating membrane permeability
What is the red pigment in beetroot called?
Betalain
Investigating membrane permeability
Why is betalain useful in this experiment?
The red pigment is released and the surrounding solution is coloured when membranes are disrupted
The amount of pigment released is related to the disruption
What is described as either active or passive?
The exchange of substances between cells and their environment
Is diffusion active or passive?
Passive transport
How does passive movement get energy?
Utilises energy from the natural motion of particles rather than from another soruce
Define diffusion
The net movement of particles from a region of higher concentration to a region of lower concentration
How long will diffusion go on for?
Until an equilibrium is reached
Why does diffusion happen?
Because particles have energy so the random and unequal distribution of particles will move to become equal
What slows down particles movements
The high speed collisions
When is diffusion faster?
Over short distances as fewer collisions occur
Factors affecting rate of diffusion
The higher the temperature =
more KE = higher rate of diffusion
Factors affecting rate of diffusion
Greater concentration difference =
faster rate of diffusion = larger overall movement
How can diffusion be calculated?
Two ways
- By distance travelled / time
- by volume filled / time
When is diffusion not as easy?
When the hydrophobic interior of the membrane repels substances with a positive or negative change (ions
Can polar molecules (such as water) diffuse?
Yes but at a slow rate
The rate at which molecules or ions diffuse across membranes is affected by:
SA = the larger the exchange surface, high rate
Thickness of membrane = the thinner the exchange surface, high rate
Phospholipid bilayer membranes are barriers to what?
Polar molecules and ions
How do polar molecules and ions pass a membrane?
Through channel proteins
What is diffusion across a membrane through protein channels called?
Facilitated diffusion
What quality do membranes with protein channels have?
They are selectively permeable as most protein channels are specific to one molecule or ion
What is the rate of facilitated diffusion dependent on?
- temperature
- conc gradient
- SA
- thickness
- number of channel proteins present
Which proteins does facilitated diffusion need?
Channel and carrier
Define osmosis
The net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential
Define a solute
A substance dissolved in a solvent, forming a solution
Define water potential
The pressure exerted by water molecules as they collide with a membrane or cobtainer
What is water potential measured in?
Pressure pascals (Pa) or kilopascal (kPa)
What is the symbol for water potential?
The Greek letter psi
What water potential does pure water have?
0 kPa
Why is 0kPa the highest possible value for water potential?
As the presence of a solute in water lowers the water potential below 0
Why happens to water potential in a more concentrated solution?
The more negative the water potentiak
Effects of Osmosis on Animal Cells
What happens if an animal cell is placed in a solution with a higher water potential than that of the cytoplasm?
Water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell
Define hydrostatic pressure
The diffusion of water into a solution leads to an increase in volume of this solution. It the solution is in a closed system (eg. A cell) this results in an increase in pressure
What are the units for hydrostatic pressure?
kPa
Effects of Osmosis on Animal Cells
Explain the event of cytolysis
- the cell-surface membrane cannot stretch much and can’t withstand the increased hydrostatic pressure
- it will break and the cell will burst
Effects of Osmosis on Animal Cells
What happens if an animal cell is placed in a solution that has a lower water potential than the cytoplasm?
It will lose water to the solution by osmosis down the water potential gradient
Effects of Osmosis on Animal Cells
What is caused by a loss of water to the solution by osmosis down the water potential gradient?
Causes a reduction in the volume of the cell and the cell-surface membrane to ‘pucker’ referred to as crenation
Effects of Osmosis on Animal Cells
What do multicellular animals usually have to prevent cytolysis or crenation?
Control mechanisms to make sure thei cells are continuously surrounded by aqueous solutions with an equal water potential = isotonic
In blood the aq solution is blood plasma
Effects of Osmosis on Plant Cells
How are animal and plant cells similar?
contain a variety of solutes, mainly dissolved in a large vacuole
Effects of Osmosis on Plant Cells
How are animal and plant cells unlike?
Plants are unable to control the water potential or the fluid around them
Eg roots are surrounded by almost pure water
Effects of Osmosis on Plant Cells
What does the increased hydrostatic pressure due to osmosis cause?
Pushes the membranes against the rigid cell walls = turgor
Effects of Osmosis on Plant Cells
What happens as the turgor pressure increases?
It resists the entry of further water and the cell is said to be turgid
Effects of Osmosis on Plant Cells
What happens when plant cells are placed in a solution with a lower water potential than their own?
Water is lost from the cells by osmosis
Effects of Osmosis on Plant Cells
What does water lost from the cells by osmosis lead to?
Reduction in the volume of the cytoplasm
Effects of Osmosis on Plant Cells
What does reduction in the volume of the cytoplasm cause?
Pulls the cell-surface membrane away from the cell wall - the cell is said to be plasmolysed
What forms a membrane?
A phospholipid bilayer
Where do cells normally exist?
In aqueous environments
What is the head of a phospholipid?
Charged hydrophilic head
What is the tail of a phospholipid?
Hydrophobic fatty acids tails
Who created the fluid-mosaic model and when?
1972 Singer and Nicholson
Why is it a FLUID mosaic model?
Because the phospholipid said are free to move within the layer relative to each other
What does fluidity give a membrane?
Flexibility
Why is it a fluid MOSAIC model?
Because the proteins embedded in the bilayer vary in shape, size and position
What do plasma membranes contain?
Various proteins and lipids - the type and number of which are particular to each cell
What important role do membrane proteins have?
the various functions of membranes
What type of membrane proteins are there?
Intrinsic and extrinsic proteins
Intrinsic Proteins
Define integral proteins
Are transmembrane proteins that are embedded through both layers of a membrane
Intrinsic Proteins
What do intrinsic proteins have?
Amino acid with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane keeping them in place
Intrinsic proteins
What do channel proteins provide?
A hydrophilic channel that allows passive movement of polar molecules and ions down a conc gradient through membranes
Intrinsic Proteins
How are channel proteins held in position?
By interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins
Intrinsic Proteins
What do carrier proteins have a role in?
Both active and passive transport
Intrinsic Proteins
What do carrier proteins often have to do?
Change shape
Intrinsic Proteins
Where are glycoproteins found?
Embedded in the cell-surface membrane with attached carbohydrate chains
Intrinsic Proteins
What role do glycoproteins play?
In cell adhesion and as receptors for chemical signals
Define cell adhesion
When cells join together to form tight junctions in certain tissues
Intrinsic Proteins
What happens when chemicals bind to the receptor?
It elicits a response from the cell. This msg cause a direct response or set off a cascade of events inside the cell
This is known as cell signalling
Give examples of cell signalling
- receptors for neurotransmitters such as acetylcholine at nerve cell synapses
- receptors for peptide hormones which affect the uptake and storage of glucose by crlls
Intrinsic Proteins
How do Beta blockers work?
Act by binding to cell receptors and are used to reduce the response of the heart to stress
Intrinsic Proteins
What are glycolipids similar to?
Glycoproteins
Intrinsic Proteins
What are glycolipids?
Lipids with attached carbohydrate chains
Intrinsic Proteins
What can glycolipids be called?
Cell markers / antigens
Intrinsic Proteins
What is the role of glycolipids?
Be recognised by the cells of the immune system as self or non-self
Extrinsic proteins
Where are extrinsic/peripheral proteins found?
Present in one side of the bilayer and can move between layers
Extrinsic proteins
What do they normally have in their outer surface?
Hydrophilic R-groups
Extrinsic proteins
What do they interact with?
With the polar heads of the phospholipids or with intrinsic proteins
Extrinsic proteins
What is cholesterol?
A lipid with a hydrophilic and hydrophobic end that regulates the fluidity of a membrane
Extrinsic proteins
Where are cholesterol molecules positioned?
Between phospholipids in a membrane bilayer
Extrinsic proteins
What does cholesterol do to the bilayer?
Add stability without making then too rigid
Extrinsic proteins
What does cholesterol prevent?
Membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising
Membranes can be the site of…
Chemical reactions
Mark Scheme Points
How does cholesterol affect fluidity?
- binds to fatty acid tails
- increasing the packing of membrane
- therefore reducing the fluidity/permeability
- makes the membrane less permeable to very small water-soluble molecules
