2.3 Transport across membranes Flashcards
What is the current model of the membrane called
And why is it called this
Fluid mosaic model
Fluid- bc the individual phospholipids can move relative to each other
Mosaic- bc the proteins vary in size and shape
What is the role of the cell surface membrane
It controls the movement of substances in and out of the cell
What is the name of the lipids in the membrane
Phospholipids
How are the phospholipids arranged in the membrane
As a bilayer
What are the characteristics of the phosphate head
- Polar
- Hydrophilic (attracted to water)
What are the characteristics of the fatty acids tails
- Non-polar
- Hydrophobic (repelled by water)
Function of phospholipids
- Allow lipid-soluble substances to enter and leave the cell
- Prevent water-soluble substances entering and leaving the cell
- Make the membrane flexible and self-sealing
What is the role of extrinsic proteins
- Give mechanical support to membrane
- Act as cell receptors to molecules like hormones (if connected to glycolipids)
What are the 2 intrinsic proteins
- Channel
- Carrier
What are channel proteins
Water filled tubes which allow water soluble ions to diffuse across the membrane
What do carrier proteins do
They bind to ions or molecules (like glucose and amino acids) then change shape to move these across the membrane
What are the functions of the intrinsic proteins
- Provide structural support
- Transport water-soluble substances
- Allow active transport to occur (carrier)
- Form cell surface receptors
- Help cells adhere together
- Act as receptors
Function of cholesterol (3 things)
- Reduce lateral movement of other molecules including phospholipids
- Make the membrane less fluid at high temperatures
- Prevent leakage of water and dissolved ions from the cell
Is cholesterol hydrophilic or hydrophobic
Hydrophobic
Why is cholesterol being hydrophobic a good thing
-It plays an important role in preventing loss of water and dissolved ions
- It also pulls the fatty acid tails together which limits their movement
What makes up a glycolipid
A carbohydrate covalently bonded with a lipid
Function of glycolipids
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- Act as recognition sites
- Help maintain the stability of the membrane
- Help cells to attach to one another and so form tissues
Function of glycoproteins
- Act as recognition sites
- Help cells to attach to one another and so form tissues
- Allows cells to recognise one another
Define diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until an equilibrium is reached
How does facilitated diffusion differ from simple diffusion
- F diffusion uses intrinsic proteins (both types)
What is transported during facilitated diffusion
Charged ions and polar molecules
How do the protein channels allow water soluble ions to pass through
They form water filled hydrophilic channels the stretch across the whole membrane
Are channel proteins selective
Yes
What happens if the particular ion isn’t present to a channel protein
It remains closed
What happens to a carrier protein when a molecule binds to it
It changes shape so the molecule is released on the other side of the protein
What 3 factors effect the rate of diffusion
- concentration gradient
- thickness of exchange surface
- temperature
- surface area
Define osmosis
The passage of water from a region where it has a higher water potential to a region where is has lower water potential through a selectively permeable membrane
What is the units that measure water potential
kPa
What is the water potential of pure water
0 kPa
The water potential of a solution must be …
negative
What is meant by isotonic solution
The water potential of the external solution is the same as the cell
What is meant by hypertonic solution
The water potential of the external solution is lower than the cell
What is meant by hypotonic solution
The water potential of the external solution is higher than the cell
What is the term used to describe a plant cell in a hypertonic solution, when water leaves the cell
Plasmolysed
What is the term used to describe a plant cell in a hypotonic solution, when water enters the cell
Turgid
What is the term used to describe an animal cell in a hypertonic solution, when water leaves the cell
Crenation
What is the term used to describe an animal cell in a hypotonic solution, when water enters the cell
Cell lysis (bursts)
Define active transport
The movement of molecules or ions into or out of a ell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
How does active transport differ from passive forms of transport
- Energy is needed in the form ATP
- Substances are moved against the concentration gradient
- Carrier proteins molecules act as a pump are involved
- It’s a very selective process
Where does the molecule or ion being transported in active transport bind to
The receptor sites on the carrier protein
Once ATP binds to the protein, what does it split into
ADP and a phosphate molecule
What does the splitting of ATP into ADP and a phosphate molecule cause the protein to do
Change shape and open on the opposite side of the protein
What happens to the protein when the phosphate molecule is released from the protein after active transport
The protein reverts back to it’s original shape
When does ADP and the phosphate molecule recombine to form ATP
During respiration
In the sodium potassium pump which ions are actively removed from the cell/organelle
The sodium ions
In the sodium potassium pump which ions are actively taken in from the surroundings
The potassium ions
State 1 similarity and 1 difference between facilitated diffusion and active transport
Similarity - Both use carrier proteins in the plasma membrane
Difference- Active transport requires energy (ATP) and occurs against the concentration gradient
State 3 ways in which the rate of movement across membranes can be increased
- By increasing the concentration gradient either side of it
- By increasing the surface area
- By increasing the density of protein channels (carrier proteins)
Explain why the term co-transport is used to describe the transport of glucose into cells
Because glucose molecules and sodium ions move into the cell coupled together
Is the movement of sodium ions out of the epithelial cell passive or active
Active
Is the movement of sodium ions into the epithelial cells passive or active
Passive
Is the movement of glucose molecules into the epithelial cells active or passive
Passive
Name and describe five ways substances can move across the cell-surface membrane into a cell. (5 marks)
- Simple diffusion of small/ non-polar molecules down a concentration gradient
- Facilitated diffusion down a concentration gradient via protein carrier/ channel
- Osmosis of water down a water potential gradient
- Active transport against a concentration gradient via protein carrier using ATP
- Co-transport of 2 different substances using a carrier protein
Describe and explain two features you would expect to find in a cell
specialised for absorption. (2 marks)
- Folded membrane so large surface area
- Large number of co-transport/ carrier/ channel proteins so fast rate
- Large number of mitochondria so make more ATP
- Membrane- bound enzymes so maintains concentration gradient
The movement of substances across cell membranes is affected by membrane structure. Describe how. (5 marks)
- Phospholipid bilayer allows movement of non-polar/ lipid-soluble substances
- Phospholipid bilayer prevents movement of polar/ lipid-soluble substances
- Carrier proteins allow active transport
- Channel/ carrier proteins allow faciliated diffusion
- Shape of channel/ carrier determines which substances move
- Number of channels/ carriers determines how much movement
- Membrane surface area determines how much diffusion
- Cholesterol affects fluidity
Give two similarities in the movement of substances by diffusion and by
osmosis. (2 marks)
- Movement down a gradient
- Passive processes
Compare and contrast the processes by which water and inorganic ions enter cells. ( 3 marks)
- Both move down concentration gradient
- Both move through channels in membrane
- Ions can move against a concentration gradient by active transport
Contrast the processes of facilitated diffusion and active transport. (3 marks)
- Faciliated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins
- Facilitated diffusion does not use ATP whereas active transport uses ATP
- Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient
Describe the processes of facilitated diffusion and active transport (3 marks)
- Movement of polar/ charged molecules
- Facilitated diffusion movement down a concentration gradient via carrier/ channel proteins
- Active transport movement against a concentration gradient via carrier protein using ATP
Sodium ions from salt are absorbed by the cells lining the gut. Some of these cells have membranes with a carrier protein called NHE3. NHE3 actively transports one sodium ion into the cell in exchange for one proton out of the cell. Use your knowledge of transport across cell membranes to suggest how NHE3 does this (3 marks)
- Co-transport;
- Uses (hydrolysis of) ATP;
- Sodium ion and proton bind to the protein;
- Protein changes shape (to move sodium ion
and/or proton across the membrane);
