1.3- Cell membranes and Transport Flashcards
What is the function of a cell membrane?
controls which substances can enter and exit the cells
controls the uptake of nutrients
allows waste products to pass out of the cell
cell recognition and signalling
What is the structure of a cell membrane?
selectively permeable membrane
composed of phospholipids and proteins
What are dietary lipids?
triglyceride
one glycerol 3 fatty acids
Phospholipids in the cell membrane
form bilayers, with one sheet of phospholipid forming over another
phosphate head is polar= hydrophilic, so its attracted to other polar molecules like water
fatty acid tails are non-polar= hydrophobic, so it repels water
one layer has its phosphate head pointing inwards, interacting with the water inside the cytoplasm
other layer has its phosphate head pointing outwards, interacting with the water that surrounds the cell
the bilayer forms the basis of the membrane structure
it allows lipid soluble molecules to enter and leave the cell but prevents water soluble molecules
What is the fluid mosaic model?
phospholipid bilayer is not a fixed structure
although the bilayer will always remain arranged in this configuration, the individual phospholipids are able to move (fluid) and the proteins embedded in the bilayer vary in shape and size, arranged randomly (mosaic)
phospholipids rotate on their axis
the movement increase the fluidity of the membrane
What are the types of proteins?
Intrinsic and extrinsic
How are the proteins arranged in the membrane?
randomly in contrast to the more regular patterns of phospholipids
What are extrinsic proteins?
occur on the surface of the bilayer, or partly embedded in it
they are polar and associate with hydrophilic heads of the phospholipids
provide structural support
also form recognition sites by identifying cells
many are receptor sites and binds with proteins such as hormones and neurotransmitters
What are intrinsic proteins?
span the phospholipid bilayer
they have polar and non-polar regions, which correspond with hydrophilic and hydrophobic regions of the bilayer
some act as channels or carriers to facilitate the diffusion of polar molecules, e.g ions, across the cell membrane
others form pumps and carry out active transport against a concentration gradient
What are glycolipids?
lipids which have combined with polysaccharide , they are found in the outer layer of the membrane and are involved in cell recognition
What are glycoproteins?
proteins combines with a polysaccharide
stick out of some membranes
What is cholesterol?
found in animal cells
fits between the phospholipid molecules
increasing the rigidity and stability of the membrane
membrane as a barrier
selectively permeable membrane
lipid soluble (non-polar) substances move through the membrane via the phospholipids
water soluble (polar) substances have to be transported via special protein molecules, which form water filled channels across the membrane
the hydrophobic core of the membrane impedes the transport of ions or polar molecules
ions and large molecules cannot diffuse across the non-polar centre of the phospholipid bilayer, because they are relatively insoluble in lipid
What are the passive processes?
diffusion
facilitated diffusion
osmosis
Which transport uses energy?
active transport
What does passive transport/diffusion mean?
the net movement of molecules or ions from a region of their high concentration to a region of their low concentration until they are equally distributed
What affects the rate of diffusion?
concentration gradient - the greater the difference in concentration of molecules in two areas, the greater the rate
distance of travel over which diffusion takes place- the shorter the distance between two areas, the greater the rate
surface area of membrane- larger the area the quicker the rate
thickness of surface- thinner the membrane the quicker the rate
an increase in temp- increase in rate due to kinetic energy
Particle size- smaller the particles, the quicker the rate
What is diffusion proportional to?
surface area x difference in conc / length of the diffusion path
What is facilitated diffusion?
ions and large molecules like glucose cannot pass through the cell membrane because they are insoluble in lipids
in the cell membrane intrinsic protein molecules span the membrane from one side to the other and help such particles to diffuse in or out of the cells
there are 2 types of proteins which facilitate diffusion- carrier and channel
What are channel proteins?
consist of pores lined with polar groups
this allows charged ions to pass though
each channel protein is specific for one type of ion
they can open and close depending on the needs of the cell, which are known as gated channels
What are carrier proteins?
allow facilitated diffusion across the membrane of larger polar molecules such as sugars and amino acids
a particular molecule attaches to a carrier protein at its binding site and causes the carrier protein to change shape or rotate within the membrane
this action releases the molecule on the other side of the membrane
What factors affect cell membrane permeability?
increasing temperature
increasing ethanol concentration
increasing sodium chloride concentration
increasing detergent concentration
How does increasing the temperature affect cell membrane permeability?
cell membrane and tonoplast are stable up to 40 degrees
temperatures above 40 degrees the cell membrane and tonoplast become increasingly unstable
increased heat energy leads to increased kinetic energy
the phospholipids vibrate more and move further apart, which increases the permeability of the membrane
the proteins within the membrane denature at high temperatures, this allows betalains to diffuse out of the cells more readily
therefore as temperature increases cell membrane and tonoplast permeability increases due to increased disruption of the membranes
How does increasing ethanol concentration affect cell membrane permeability?
ethanol dissolves phospholipids
greater the concentration of ethanol the more permeable the membranes become
How does increasing sodium chloride concentration affect cell membrane permeability?
sodium ions attach to the oxygen atoms on the hydrophilic phosphate heads of the phospholipid bilayer
this reduces mobility of the phospholipid molecules so less beletain is released
as sodium chloride concentration increases the permeability will decrease
How does increasing detergent concentration affect cell membrane permeability?
detergents reduce surface tension of phospholipids and disperse the membrane
As the concentration of detergent increases the permeability of the membrane increases
What does a simple diffusion graph look like? explain
linear (straight line)
as the concentration gradient increases the rate of diffusion will also increase
the rate of uptake is directly proportional to the concentration difference across the membrane
What does a facilitated diffusion graph look like? explain
initial increased rate of diffusion as the concentration gradient becomes steeper, due to the channel and carrier proteins facilitating the process
then rate of diffusion levels off at higher concentration differences, due to the channel and carrier proteins being occupied, which limits the rate of diffusion
Does respiratory inhibitors affect simple/facilitated diffusion?
no as ATP is not required for simple/facilitated diffusion
What are respiratory inhibitors?
stop ATP production
e.g cyanide
What is active transport?
ATP requiring process in which ions and molecules are moved across membranes against a concentration gradient
How does active transport take place?
the molecule or ion which needs to be transported combines with a specific intrinsic protein called a pump
ATP transfers a phosphate group to the pump on the inside of the membrane
this causes the pump to change shape or rotate within the membrane and transports the molecule or ion across the membrane against its concentration gradient
the molecule or ion is released into the cell
What processes involve active transport?
protein synthesis
muscle contraction
nerve impulse transmission
absorption of minerals e.g nitrates by plant root hair cells
What does an active transport graph look like? explain
initial increased rate of uptake as the concentration gradient increases, due to the pumps actively pumping ions and molecules across the cell membrane
rate of active transport levels off at higher concentration differences, due to the pumps being full, which limits the rate of active transport
Active transport is affected by respiratory inhibitors as ATP is required for this process
rate of transport sharply drops after addition of respiratory inhibitor
What is osmosis?
movement of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane
What is the highest water potential?
0
What has the highest water potential?
pure water
What does water potential of the cell equal?
solute potential + pressure potential
What is the unit for water potential?
kPa
What is water potential?
the measure of pressure exerted by freely moving water molecules in a system
Are all water potentials positive or negative? and except from what?
negative
pure water
What does higher water potential mean?
less negative water potential value
What does lower water potential mean?
more negative water potential value
The more concentrated the solution…
the more negative the water potential as there’s fewer free water molecules
What lowers a plant cells water potential?
the presence of solute molecules in the vacuole
What does solute potential mean?
the concentration of dissolved substances inside the cell vacuole
Is solute potential positive or negative?
negative
What does pressure potential mean?
the hydrostatic pressure generated when the cell contents push against the cell wall, due to the water entering the cell by osmosis
What does isotonic solution mean?
the water potential inside the cell is equal to the water potential outside the cell
What does hypertonic solution mean?
the solution has a lower water potential than the cell
What does hypotonic solution mean?
the solution has a higher water potential than the cell
What happens to plant cells in hypotonic solutions?
water has moved into the cell until the vacuole is full, and the cell contents are pushed against the cell wall
become turgid
turgid plant cells support the plant tissues and structures
What happens to animal cells in hypotonic solutions?
when water moves into the cell, it swells and may burst as it does not have a cell wall to prevent it bursting, called lysis
(red blood cells- haemolysis)
What happens to plant cells in hypertonic solutions?
water will move out of the cell
the vacuole shrinks causing the cell membrane to pull away from the cell wall
the cell is flaccid because the contents no longer push against the cell wall
a cell in this condition is said to be plasmolysed
What happens to animal cells in hypertonic solutions?
water will move out of the cell causing it to shrivel
What does plasmolysis mean?
when water leaves a plant cell by osmosis, the cytoplasm and vacuole shrinks and the cell membrane pulls away from the cell
What happens to plant cells in isotonic solutions?
cells become flaccid
this is the point of incipient plasmolysis
What is endocytosis?
a cell surrounds a substance with a section of its cell membrane
the membrane riches off to form a vesicle inside the cell
e.g phagocytes use this to ingest and digest bacteria and dead cells
What is exocytosis?
substances leave the cell after being transported through the cytoplasm in transport vesicles to the Golgi body and then to the cell membrane via secretory vesicles
secretory vesicles fuse with the cell membrane and the contents are secreted outside the cell
What are the types of endocytosis?
phagocytosis and pinocytosis
What is phagocytosis?
the process of ingesting solid substances
What is pinocytosis?
the process of ingesting liquid substances
