Cell membranes/Transport Flashcards
describe the role of phospholipids in membranes
1 glycerol
2 fatty acid
1 phosphate group - highly charged - polar - hydrophilic region
hydrophobic region consisting of fatty acids and glycerol
because phospholipids contain both a hydrophilic region and hydrophobic region, if they are placed in water they arrange themselves in a certain way
The hydrophilic part of the phospholipid molecules (the phosphate groups) can interact with water
However, the hydrophobic parts (the fatty acids) are buried in the centre away from the water
This structure is called a phospholipid bilayer
The phospholipid bilayer is the basis of all cell membranes - includes the cell surface membrane and the membranes that surround organelles such as lysosomes and mitochondria
functions of membranes in cells
they act as barriers e.g. between the internal contents of the cell/cytoplasm and the external environment OR between the contents of an organelle and the cytoplasm
Sometimes membranes separate one part of an organelle from another part e.g. in mitochondria
membranes can also be a location for chemical reactions e.g. some of the stages in respiration
membranes are involved in cell signalling
because the cell-surface membrane contains a hydrophobic centre, what does it mean for hydrophobic molecules trying to pass through
because the cell-surface membrane contains a hydrophobic centre, Hydrophobic molecules such as steroid hormones can easily pass through the membrane
because the cell-surface membrane contains a hydrophobic centre, what does it mean for hydrophilic molecules trying to pass through
because the cell-surface membrane contains a hydrophobic centre,
the hydrophobic centre prevents hydrophilic, water soluble molecules from easily passing through
This is because hydrophilic substances are polar - they have a charge
These substances cannot easily pass through the non-polar region of the membrane
can water molecules pass through the cell membrane
Water molecules can pass through the cell membrane, even though water molecules are polar.
This is because water molecules are extremely small
However, this does take place at a slow rate
what is the cell-surface membrane based on
the cell-surface membrane is based on the phospholipid bilayer
Other parts which make up the cell-surface membrane
Phospholipid bilayer
within bilayer there are :
large no. of Membrane protein molecules - some of these proteins are only on one side of the membrane, whereas other proteins span the membrane from one side to the other
cholesterol - contain a hydrophobic region and hydrophilic region (at one end)
The polar hydrophilic group can attract the polar head groups on the phospholipid molecules
Rest of cholesterol molecule is non-polar and hydrophobic - this part of the cholesterol molecule can attract the non-polar fatty acids in the phospholipids
Because cholesterol interacts with phospholipids, it increases the strength of the cell-surface membrane. This makes the membrane more stable and less likely to get damaged
Cholesterol reduces the sideways movement of phospholipids and other molecules in the membrane
This helps to control the fluidity of the membrane
This prevents the membrane from becoming too fluid under warm conditions and too rigid under cool conditions
By packing the spaces between phospholipids, cholesterol helps to reduce the movement of water-soluble chemicals across the cell-surface membrane.
describe the fluid-mosaic model of the cell surface membrane
The structure of the cell-surface membrane is called the fluid mosaic model
The word fluid is used because the phospholipid molecules can move around within each layer. This means that the membrane is flexible and can change shape
The word mosaic is used because the membrane is studded with protein molecules. The arrangement of these proteins varies - like tiles in a mosaic
describe structure of triglyceride and phospholipids
1 glycerol
3 fatty acid
non polar molecule
hydrophobic
non-soluble in water
1 glycerol
2 fatty acid
1 phosphate group - highly charged - polar - hydrophilic region
hydrophobic region consisting of fatty acids and glycerol
state the different types of proteins found in cell-surface membranes
intrinsic (integral) proteins
extrinsic (peripheral) proteins
describe intrinsic proteins
intrinsic proteins are fully embedded in the membrane from one side to the other
because they pass right through the lipid bilayer, intrinsic proteins have hydrophobic amino acids outside on the surface of the protein
These hydrophobic amino acids can interact with the hydrophobic fatty acid tails in the phospholipid bilayer
state and describe the two types of intrinsic proteins
Protein channels are intrinsic proteins - these contain a channel running through the centre. This channel is lined with hydrophilic amino acids and is filled with water molecules.
Protein channels allow water-soluble molecules and ions to diffuse through.
Carrier proteins are intrinsic proteins. These can change their shape or position to transfer molecules or ions from one side of the membrane to the other
what are extrinsic proteins
Extrinsic proteins do not span the membrane
They are found on one side of the membrane or the other
Sometimes extrinsic proteins are attached to intrinsic protiens
ontop/below phosphate group
or in one region of phospholipid bilayer
functions of extrinsic proteins
Some play a structural role within the membrane
Some can act as enzymes
Some are receptors for other molecules such as hormones
What do many membrane proteins (both extrinsic and intrinsic) also have attached to their surface
Many membrane proteins also contain a carbohydrate molecule attached.
These are called glycoproteins
Functions of glycoproteins within the cell
Some glycoproteins allow cells to attach to each other to form tissues such as nervous tissue
Other glycoproteins play a role in the immune system presenting antigens to T cells.
Other glycoproteins act as receptors for hormones
Where can carbohydrate molecules also be found attached to
Carbohydrates can also be found attached to phospholipid molecules - called glycolipids
what is meant by a glycolipid
Carbohydrates can also be found attached to phospholipid molecules - called glycolipids
describe the function of glycolipids
Glycolipids are often used when cells come in contact with each other
The glycolipids on the surface of one cell can be recognised by another cell
This can determine whether cell come into contact.
Glycolipids can also act as antigens for example in determining your blood group
particles have kinetic energy since they are moving randomly
colliding constantly with each other
no concentration gradient means
concentration of gas is same everywhere
what is diffusion
diffusion is the net (overall) movement of particles from a region of higher concentration to a region of lower concentration (down the concentration gradient)
net movement - overall movement
particles move randomly in all directions
however during diffusion, more particles are moving in one direction than the other
e.g. co2 in cell
conc. gradient for co2
co2 molecules will diffusion from region with a higher conc. (inside the cell) to a region with lower conc. (outside cell)
eventually co2 concentration is the same inside and outside
equilibrium has been reached - no net movement of particles therefore diffusion has stopped
what type of process is diffusion
diffusion is a passive process
doesn’t require metabolic energy to take place
metabolic energy means energy released by respiration
cell membrane = partially permeable
some chemicals can easily through the membrane while others cannot
factors affecting rate of diffusion
concentration gradient - the greater the concentration gradient, the greater the rate of diffusion.
particle charge/size
because cell-membrane contains a hydrophobic core, hydrophilic substances charged particles such as ions/polar molecules will not be able to diffuse through the membrane
however uncharged molecules e.g. oxygen can diffuse rapidly across the membrane
size - smaller particles will diffuse faster than larger ones
temp - particles have greater E.K. and diffuse faster in warmer conditions than colder ones
not an issue for mammals and birds which maintain a constant body temperature
SA of the membrane - diffusion takes place more rapidly if the membrane has a larger SA compared to a smaller one
distance that diffusion takes place over - greater distance, slower rate of diffusion - why membranes are so thin
Presence of carrier/channel proteins in the membrane
facilitated diffusion
explain how water can diffuse across membranes
Water can diffuse across membranes - despite it being polar
Because water molecules are very small
which type of substances cannot diffuse across cell membrane
Hydrophilic substances (polar molecules + ions) cannot diffuse across cell membrane# - since cell membrane contains a hydrophobic fatty acid core
problem - many of these substances required for processes inside the cell
describe facilitated diffusion
In facilitated diffusion, hydrophilic substances diffuse across the cell membrane via protein molecules (carrier proteins or protein channels)
These protein molecules allow the hydrophilic substances to cross the membrane without interacting with the hydrophobic centre of the phospholipid bilayer
TYPE OF DIFFUSION
CHEMICAL STILL MOVES FROM REGION OF HIGHER CONC. TO A REGION OF LOWER CONC.
types of protein molecules involved in facilitated diffusion and uses
Carrier protein - have a binding site for a specific chemical
when a chemical binds it causes the tertiary structure of the carrier protein to change
This change in the tertiary structure brings the chemical across the membrane, where the chemical is now released
Protein channels - protein with central pore
pore is lined with hydrophilic amino acids and contains water
hydrophilic substances can pass through the channel from one side of the membrane to the other
Protein channels are selective for the chemical that can pass through - means only certain chemicals can pass through - each type of protein channel
while some types of protein channels are always open, other protein channels only open in response to a certain trigger - e.g. chemical binding to protein channel or change in the voltage across the membrane
describe what is meant by active transport
in active transport, carrier molecules in the membrane transport a chemical from a region of lower concentration to a region of higher concentration (against the concentration gradient)
Active transport requires metabolic energy which is provided by the molecule ATP
ACTIVE PROCESS REQUIRING METABOLIC ENERGY
describe role of ATP in active transport
during active transport, the molecule/ion to be transported attaches to a receptor site on the carrier protein
Takes place on the side of the membrane where the chemical is at a lower concentration
A molecule of ATP then binds to the carrier protein
Next, the ATP molecule undergoes hydrolysis producing phosphate and a molecule of ADP
The phosphate attaches to the carrier protein and causes it to change shape
This shape change causes the carrier protein to transport the molecule or ion to the other side of the membrane where it is released
Phosphate now leaves the carrier protein, causes it to return to its previous shape
ADP and phosphate will later reform ATP during respiration
key points about active transport
Uses a lot of ATP
So we find lots of mitochondria in cells which carry out a lot of active transport
carrier proteins used in active transport are specific
Each carrier protein will only transport one type of molecule/ion
carrier protein different in facilitated diffusion and active transport
two types of active transport
direct active transport - protein molecule directly moves the molecule/ion against conc. gradient
cotransport
what is meant by cotransport
describe the role of cotransport in the ileum
role of ileum - to absorb the molecules produced by digestion includes the sugar glucose
glucose is absorbed from lumen of ileum into epithelial cells lining the lumen
glucose molecules then move from epithelial cell to the bloodstream
After Digestion, there is a high conc. of glucose in the lumen of the ileum
glucose molecules move down the concentration gradient by facilitated diffusion into the epithelial cells lining the ileum
Takes place via carrier proteins
these glucose then move into the bloodstream and are carried away - by same process
PROBLEM: rate of facilitated diffusion depends on the size of concentration gradient for glucose
As glucose moves into the epithelial cells, this concentration gradient falls
Means that facilitated diffusion cannot absorb all of the available glucose
So as well as facilitated diffusion, glucose is also absorbed by active transport
Two stages to this process:
1st stage involves a carrier protein called the sodium-potassium pump
Using ATP, the sodium potassium pump actively transport sodium ions out of the epithelial cells into the bloodstream
At the same time, it actively transports potassium ions into the epithelial cell
Now there is a very low conc. of sodium ions inside the epithelial cells
The lumen of the ileum contains a relatively high conc. of sodium ions
Now there is a concentration gradient for sodium ions between the lumen of the ileum and the interior of the epithelial cells
In the membrane of the epithelial cells there is a protein called the sodium-glucose cotransporter
Sodium ions can diffuse through this protein down the conc. gradient into the epithelial cell
As sodium ions diffuse down their concentation gradient, the sodium glucose cotransporter also transports glucose molecules into the cell
this means that the glucose molecules are being transported against their conc. gradient
Glucose is being active transported into the cell
ensures glucose is transported at a fast rate
energy for this glucose transport comes indirectly from the concentration gradient of the sodium ion
Epithelial cells - lots of mitochdonria
provide ATP needed for the sodium-potassium pump
The membrane of epithelial cells is folded into a large number of microvilli
increases SA of the membrane
provides more space/greater SA for membrane proteins needed for facilitated diffusion and active transport
Glucose molecules which are absorbed, are rapidly carried away by the bloodstream
this means that there is a steep conc. gradient between the epithelial cells and the blood
So facilitated of glucose into the bloodstream takes place rapidly
