2.5 Flashcards
Fluid mosaic model
Theory of cell membrane structure with proteins embedded in a sea of phospholipids
Glycolipid
Lipid/phospholipid with a chain of carbohydrate molecules attached
Glycoproteins
Protein with a chain of carbohydrate molecules attached
Plasma membrane
Cell surface membrane
Diffusion
Movement of molecules from area of high concentration to low, may or may not be across a membrane and doesn’t involve metabolic energy (ATP)
Facilitated diffusion
Movement of molecules from an area of high to low concentration, across partially permeable membrane via protein channels or carriers, doesn’t involve metabolic energy (ATP)
Osmosis
Passage of water molecules down their water potential gradient across partially permeable membrane
Water potential
Measure of the tendency of water molecules to diffuse from one region to another
What is purpose of cell membrane
To form a barrier and separate cell content from cell exterior environment, or to separate organelles from cytoplasm
What do cell membranes need to do
Need to allow some molecules through, into or out of the cell, some organelles also have membranes within them which form barriers too
What does permeability refer to
The ability to let substances pass through
How do small molecules pass a membrane
Simply diffuse through cell membrane, in between its structural molecules
How apart from simply diffusing how do other larger molecules pass a membrane
Some substances dissolve in the lipid bilayer and pass through and others pass through special protein channels or are carried by carrier proteins
Why are plasma membranes described as partially permeable
As they don’t let all types of molecules pass through them
What determines a membranes permeability
Properties of the component molecules if the cell membrane, such as which molecules it allows through
What is the role of the plasma membrane
Separates cell content from external environment and regulates transport of materials in and out of the cell
What enzymes may plasma membrane contain
Enzymes involved in specific metabolic pathway, which has antigens
Why may plasma membrane contain enzymes
So that organisms immune system can recognise the cell as being ‘itself’ and not attacking it, they may release chemicals that signal to other cells
What else does the plasma membrane surface contain and for what
Contains receptors for such chemical signals and so its a site for cell communication or signalling, hormones and drugs may bind to membrane bound receptors and may be site of chemical reactions
What is the purpose of organelle membranes
Helps each organelle to have a discrete entity and able to perform its function
In some organelles where do metabolic processes occur
On membranes
What is the purpose of mitochondria folded inner membrane called Cristae
They give a large SA for reactions and some enzymes
What is purpose of chloroplasts inner membrane called thylakoid
Have chlorophyll for photosynthesis to occur
What is role of digestive enzymes in epithelial cells which line small intestines
They catalyse breakdown of some sugars
What did singer and Nicklson produce in 1972
Fluid mosaic model
What is the fluid mosaic model
A model which allows the passage of molecules through a membrane, it explains how cell membranes could be more dynamic and interact more with cell environment
What did the fluid mosaic mode propose
Proposed fabric of membranes has phospholipid bilayer with proteins floating in it making a mosaic pattern
What gives membrane fluidity
Lipid molecules change places with each other and some proteins move
What is a lipid bilayer made of
2 layers of phospholipid molecules
What is the structure of phospholipid in a phospholipid bilayer
Hydrophilic heads are in contact with watery exterior of interior (cytoplasm) and hydrophobic tails are in the centre of the membrane away from water
What is a glycoprotein on phospholipid bilayer
A carb chain attached to a protein molecule
On which side of the phospholipid bilayer is the protein receptor side
Exterior site-extrinsic protein
What is a glycolipid on phospholipid bilayer
A carb chain attached to a lipid
What is structure of a protein channel
Water filled channel with hydrophilic amino acids
What is the structure of carrier protein in bilayer
Long channel throughout the membrane with a peripheral lipid on the interior side of it
Where is cholesterol found in a membrane and what’s its function
In the middle of the bilayer, gives mechanical stability and flexibility/controls membrane fluidity
What is the carbohydrates structure on the outside of a membrane
Hydrophilic and attracts water with dissolved solutes, helps cells interact with its watery environment and obtain dissolved substances
What are channel/carrier protein function and structure in membrane
They span the whole of the bilayer so are integral proteins and their membrane spanning regions interact with hydrophobic regions of phospholipid bilayer
What is the role of some integral proteins
Some have pores and act as channels to allow ions through, they can’t diffuse as have an electrical charge and surrounded by water molecules
What is the role of other integral proteins in the membrane
Some are carriers and by changing their shape carry specific molecules across the membrane
What is the final role of proteins in the membrane
Other proteins may be attached to carrier proteins and function as enzymes, antigens or receptor sites for complementary shaped signalling chemicals such as hormones
What lipid (not phospholipid) do membranes contain and what’s its role
Cholesterol- regulates membrane fluidity, maintains mechanical stability and resists the effect of temp change on the structure of the membrane
What is total thickness of a membrane
5-10nm
What is glycocalyx found on the outside of a membrane
Formed from carb chain attached to either a lipid or a protein (glycolipid/glycoprotein) in the membrane
Why do cells become specialised
To carry out their specific function based on the membranes protein distribution
What is protein channel/carriers use in neurone (nerve cells)
They cover the long axon and aloe entry and exit of ions to bring about conduction of electrical impulses along their length
What do neurones have which give them several layers of cell membrane
Neurones have myelin sheath formed by flattened cells wrapped around them several times, membrane forming myelin sheath is 20% protein and 76% lipid
What do white blood cells plasma membrane have to enable them to perform their function
It had special protein receptors that enable them to recognise antigens in foreign cells usually from invading pathogens but also from tissue/organ transplant
What do roots hair cells in plants have many carrier proteins
To actively transport nitrate ions from soil to cells
What is the lipid:protein ratio in inner membrane of mitochondria and why
76% protein and 24% lipid as their inner membrane contains electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes
Why do cells need to receive raw material to reactants
For biochemical processes in to sustain life
Why do cells respire
To make ATP which provides cellular energy to drive biochemical processes to sustain life
What do cells need for biochemical processes
Oxygen and glucose and to remove toxic metabolic waste products like CO2 and they need to export some molecules that they make, like enzymes, hormones or other signalling molecules
What are processes where substances can pass a cell membrane without the use of metabolic energy called
Passive processes as they only use kinetic energy if molecules and not ATP
What do all molecules have and what does this mean
All have kinetic energy and so can move freely and randomly within gas or a liquid medium, happens even if medium isn’t stored or shaken
What happens when there is a high concentration of molecules in one location
Then molecules will bump into each other as they randomly move and eventually they will spread further form each other, more will move to an area of lower concentration until they eventually are evenly dispersed
What happens when molecules move down their concentration gradient
They are still moving randomly but remain evenly dispersed so their is no net diffusion and they have reached equilibrium
How does small molecules like oxygen and CO2 pass a membrane
Simple diffusion
How do fat-soluble molecules like steroid hormones or even larger ones move through membrane
They can still diffuse as they dissolve in the lipid bilayer and still move down their concentration gradient
Why does water molecules a special case for being allowed through membrane
They are polar and insoluble in lipids so the barrier should be impenetrable to them
Why is water allowed through membranes
It is present in such great concentration that significant direct diffusion does happen, in membranes where very high rate of water movement is required there maybe specific water channel proteins know as aquaporins
What is role of aquaporins
To allow water molecules to cross the membrane without challenge of moving though a lipid environment
What happens once molecules pass into cell and what does this cause
Many pass into organelles and used for metabolic reactions which maintains concentration gradients to keep molecules entering the cell
What happens once oxygen diffuses into cytoplasm of respiring cells
It then diffuses into mitochondria and used for aerobic respiration maintaining a concentration gradient
What happen to carbon dioxide once it’s diffuses into palisade mesophyll cells
It then diffuses into chloroplast and is used for photosynthesis which maintains a concentration gradient
What does simple diffusion rely on and so what affects rate of simple diffusion
Simple diffusion relies on kinetic energy of molecules so factors which effect kinetic energy effect rate of simple diffusion
How does temp affect rate of simple diffusion
As temp increases molecules have more kinetic energy so their rate of diffusion increases and if they lose heat it slows down t
How does diffusion distance affect rate of simple diffusion
Thicker the membrane across which molecules have to diffuse, the slower the rate of diffusion
How does SA affect rate of diffusion
More diffusion can take place across large SA, cells specialised for absorption have extensions to their cell surface membranes to increase SA
How does size of diffusing molecule affect rate of simple diffusion
Smaller ions or molecules diffuse more rapidly than larger molecules
How does concentration gradient affect rate of simple diffusion
Steeper the gradient the faster diffusion is down the gradient
Why do some molecules have to travel through water filled protein channels embedded in a membrane
Small molecules with polarity such as ions with an electrical charge are insoluble in lipids so they can’t interact with hydrophobic tails of the lipid bilayer
What do cholesterol molecules with membrane reduce
The permeability of membranes to small water soluble molecules
Why can’t glucose diffuse into cells and what is its solution
They are too large to diffuse through water filled protein channels in a membrane but instead they can bind to a transmembrane carrier protein which opens to allow glucose to pass to other side
Are there specific protein carrier for different types of molecules
Yes
What do different cell types have and what does this allow to do with carrier proteins
Have membranes with different proportions of proteins channels/carrier proteins which allows cell to control types of molecules that pass in or out
Why do neurones have a lot of channel proteins
Have many channels specific to either sodium or potassium ions, diffusion if these in and out of cells is vital for conduction of nerve impulses and at the synapses there were also calcium and chloride ion channels
What do plasma membrane of epithelial cells that line airway have in terms of protein channels
Have chloride ion channels and play a vital role in regulating the composition of mucus to trap particles and pathogens
What is a solvent in a solution
Liquid in which solute molecules are dissolved in
What is the solvent in aqueous solutions and why
Water as it can pass directly through phospholipid bilayer
What do some membrane have for water to be transported though then
Protein channels called aquaporins which allow water molecules to cross cel membrane more rapidly
Why is water found inside and outside of cells
The cells interior is water in the cytoplasm and the exterior is also a watery medium as cells are surrounded by extracellular tissue fluid
What energy do water molecules have and what does this cause
Kinetic energy so they move randomly and will spread out
What is osmosis
Net diffusion of water molecules from an area of high to low concentration through a partially permeable membrane
What happens when solute molecules added to water
Relative number of water molecules in resulting solution is changed
What happens if solute molecules in water dissociate into charged ions for example sodium chloride dissociating into sodium and chloride ions
They exert more effect on the relative number of water molecules than do larger but non-polar molecules like glucose because as sodium chloride molecules dissociate into chloride and sodium ions, the number of particles in a solution doubles
Why does water move by osmosis through the partially permeable membrane
There is a higher water potential as less solute molecules on one side and more solute molecules on the other side so a lower water potential, so water moves down a gradient
What is water potential
A measure of the tendency of water molecules to diffuse from one region to another
What go mmmmm has the highest possible water potential
Pure water (0kPa)
What happens to water potential of a solution when solute molecules added
Water potential lowers
If two aqueous solutions are separated by a partially permeable membrane which one will water move into by osmosis
From the solution with higher water potential to the solution with lower water potential
What happens when the water potential in 2 aqueous solutions separated by partially permeable membrane is equal
There is no net movement of osmosis but water molecules will continue to move randomly
What is water potential measures in
kPa
Why is water potential inside cells lower than pure water
As there are solutes in solution (cytoplasm and vacuole)
What happens to cells when there placed in a solution with higher water potential
Water molecules move by osmosis into the cell down a water potential gradient across partially permeable membrane
What happens as water moves down a water potential gradient into animal cells
If lots of water molecules enter the cell will swell and burst as plasma membrane breaks as there is no cells wall known as cytolysis
What happens in plant cells as water moves into them down a water potential gradient
Rigid and strong cellulose cell wall prevents bursting, cell will swell to certain size when context of cell is pushed against cell wall, which resist further swelling, the cel is then described as turgid
What happens when a cell is placed in a solution with lower water potential such as a salt solution
Water leaves cell by osmosis across a partially permeable membrane
What happens to animal cells as water moves out of them down a concentration gradient
Animal cells become shrivelled and described as crenated
What happens as water moves out of plant cell down a concentration gradient
Cytoplasm and vacuole shrink and membrane pulls away from cell wall, so cells are plasmolysed so described as flaccid
What happens to plant cells which are plasmolysed
They suffer a degree of dehydration and their metabolism can’t proceed as enzyme catalysed reactions need to be in solution
What is hypotonic solution
Water moves into the cell as it has a higher water potential than cells cytoplasm
What is hypertonic
Water moves out of cell as exterior has a lower water potential than cells cytoplasm
What is isotonic
No net movement of water molecules as concentration of water outside the cell is the same as inside the cell
What is cells moving in or out across a plasma membrane against their concentration gradient similar to and why
Swimming against a tide as it requires more energy than the kinetic energy of the molecules
What energy is used in active processes across a cell membrane
This energy is provided by hydrolysis of ATP
What is ATP often described as
A universal energy currency as all the cells make use of it to supply their energy needs
When does active processes occur across a cell membrane
Cells and organelles may need to accumulate more of a particular ion than they could do by simple diffusion or facilitated diffusion alone
What is an example of active transport
Root hair cells use active transport to absorb ions from the soil
How do sodium ions diffuse out a cell
Sodium ions bind to a specific site on sodium potassium protein pump, ATP bonds to its site and is hydrolysed to ADP+P, releasing energy causes potassium ions to bind to their specific site, energy released from hydrolysis of ATP enables sodium potassium protein pump to change its shape so sodium ions are now on the outside of the cell and potassium ions are on the inside
What do carrier proteins have to help movement of substances
These membrane proteins have specific regions or sites that combine reversibly with only certain molecules/ions and also have region that binds to allow hydrolysis of ATP, to release energy so act as enzymes
How does energy help carrier proteins
Helps them change their confirmation
Why do carrier proteins need to be able to change their conformation
So it can carry ions from one side of the cell membrane to the other
Give an example of when carrier proteins change their confirmation to help substances move through the membrane
In guard cells ATP made by chloroplasts provide energy to actively transport potassium ions from surrounding cells into guard cells, this influx of ions lowers water potential in guard cells, so water enters from surrounding cells by osmosis, as guard cells swell their tips bulge and their stomata open
What do membranes do when they need to transport molecules too large to pass in or out of their membrane
They use bulk transport which requires energy from ATP
What is endocytosis
How large molecules may be brought into a cell, as they don’t pass the plasma membrane
How does endocytosis work
A segment of plasma membrane surrounds and enclosed the particles and brings it into the cell enclosed in a vesicle
What is endocytosis by being enclosed in a vesicle called and what does it mean
Phagocytosis- means eating by cell and refers to the type of intake of solid matter
What is it called if cells ingest liquid by endocytosis
Pinocytosis
Is energy needed for endocytosis
Yes ATP is needed to provide energy to form the vesicles and move them using molecular proteins along the cytoskeleton threads into cells exterior
What is exocytosis
How large molecules are exported out of the cells without passing through the plasma membrane
How does exocytosis work
A vesicle containing the substance is moved towards and then fuses with the plasma membrane
Where is an example of where exocytosis happens
At synapses where chemicals in vesicles age moved, by motor proteins moving along cytoskeleton threads, to the presynaptic membrane, here they vesicle membrane and plasma membrane fuse and neurotransmitter chemicals are released into synaptic cleft
Is energy needed for exocytosis
Yes in all cases ATP is needed to fuse the membranes together as well as for moving the vesicles
How many molecules of ATP is required for every step the motor protein takes
One molecule of ATP is needed for every step it takes along the cytoskeleton threads as it stages its cargo(the vesicle)
What are the steps of exocytosis
1.membrane bound vesicle containing substances to be secreted is moved towards cell surface membrane 2. Cell surface membrane and membrane of vesicle fuse together 3.fuse site opens releasing the content of its secretory vesicle
How does temperature affect kinetic energy of molecules
Increasing temp gives molecules more kinetic energy resulting in these molecules moving faster, decreasing temp lowers kinetic energy of the molecules causing them to move more slowly
Why do many organisms heat vary with their environment
Many organisms don’t generate heat to maintain their body temp
What happens to phospholipids in a bilayer when inside temperature drops
Saturated fatty acids become compressed but there are many unsaturated fatty acids making up cell membrane phospholipid bilayer and as they become compressed the kinks in their tails push adjacent phospholipids away so membrane fluidity is maintained
What does kinks in phospholipids tail in cold temp do to membranes permeability
Makes it more permeable
What does proportion of saturated and unsaturated fatty acids determine
Membranes fluidity at cold temps
What in a cell membrane buffers the effect of cold temps and why
Cholesterol, to prevent a reduction in membranes fluidity, it does this by preventing the phospholipid molecules from packing to closely together as cholesterol are in between groups of phospholipid molecules
What can some organisms like fish and microorganisms do to their fatty acids in membrane bilayer and some plants
They can change their composition of fatty acids in their cell membrane in response to lower temps
What happens to phospholipids when temperature increases
Phospholipids acquire more kinetic energy and move around more in a random way which increases membrane fluidity so permeability increases
What is also affected by temperature increases in a membrane
Affects the way membrane embedded proteins are positioned and may function, if some of the proteins that act as enzymes in a membrane soft sideways, it could alter rate of reactions they catalyse
What does an increase in membrane fluidity/permeability mean
May change the ability of cells to signal to other cells by releasing chemicals, often by exocytosis
How does the presence of cholesterol molecules acting as buffers to some extent help the membrane as temp is increased
The effect of increasing temp is reduced by cholesterol so also reducing membrane fluidity
Although increase temp alters movement of phospholipid what doesn’t it do
Doesn’t drastically alter their integral molecular structure, but proteins aren’t always as stable as lipids
What does high temp cause atoms inside their large molecule to do
To vibrate and this breaks the hydrogen bonds and ionic bonds that hold their structure together, so they unfold
What happens once high temp unfolds a protein
Their tertiary structure changes and cannot change back again when they cool so described as denatured
What is just under the plasma membrane
Cytoskeleton threads made of protein
What happens to both membrane embedded proteins and cytoskeleton when temp becomes to high
They denature, then plasma membrane will begin to fall apart (more permeable and holes will begin to appear in it) and membrane-embedded enzymes will not function again if they become denatured
What happens to membrane embedded enzyme when temp increase changes the shape of their active site
The rate of reaction they catalyse at will be slower
Why do organic solvents like ethanol and acetone damage cell membranes
As they dissolve lipids so they would dissolve the membrane
Describe the graph showing membrane permeability at different temps (cold)
Membrane permeability is high as lots of unsaturated fatty acids in cold temp, their kinks create gaps in the bilayer and protein carriers will freeze so they remain open increasing permeability
Describe the graph showing membrane permeability at different temps (20degrees)
Membrane permeability is lower as the temp is high enough that there are fewer unsaturated fatty acids and not and low enough that phospholipids don’t have too much kinetic energy
Describe the graph showing membrane permeability at different temps (hot)
Permeability high as temp increases kinetic energy creating gaps
What happens to membranes if a solute like alcohol is added
Increase the permeability as hydrophobic tails don’t repel alcohol like they repel water, so it distorts the bilayer because it messes up the hydrophilic and hydrophobic attractions
