cell membranes Flashcards
where can membranes be found
they cover the surface of every cell and also surround most organelles in the cell
what are the functions of a cell membrane
- Allow selected molecules to move in and out of the cell
- isolate cytoplasm from extracellular environment and keeps all the Cell components inside
- Isolate organelles from the rest of the cytoplasm allowing cellular processes to occur separately eg respiration
- site for biochemical reactions eg respiration
- Allows a cell to change shape
- allows communication and recognition From other cells
what are phospholipids
- the main component of cell membrane They act as a barrier to control what enters and exits the cell
- made of a polar hydrophilic phosphate group an glycerol heads and 2 non-polar hydrophobic fatty acid tails
when in water, what are the two structures the phospholipids form
micelle (enclosed circle)
bilayer (rectangle shape with a hydrophobic core)
how thick is the Phospholipid bilayer
7nm
What is the fluid mosaic model
Model describing the arrangement of molecules in the membrane,
It is fluid because it is constantly moving,
It contains protein molecules scattered throughout like tiles in a mosaic
what can Diffuse across a membrane
Non polar substances: Oxygen & Carbon dioxide
water & urea (are polar but can dissolve Slowly as they are very small)
Why can water soluble molecules not pass through the bilayer
Because the centre of the bilayer is hydrophobic and Repels the water soluble molecules
What can disrupt the hydrophobic bilayer
Fat soluble substances EG alcohol, nicotine and caffeine
why do phospholipids form a bilayer in water
The layer is formed as the hydrophilic phosphate heads are facing the water because they are polar and hydrophobic core don’t Because they are nonpolar and they try to avoid the water.
Both tissue fluid and cytoplasm has water in it so the phospholipids form 2 layers with the tails facing inwards and phosphate groups outwards interacting with the aqueous environment
What are other components that can be found making up a cell membrane
Glycolipid
protiens
glycoprotein
cholesterol
Channel proteins
carrier proteins
What is the difference between intrinsic and extrinsic proteins
Intrinsic proteins span the whole width of the membrane (both layers)
Extrinsic proteins are confined to the inner or outer surface of the membrane
What are and what is the function of glycoproteins
Are Intrinsic Proteins with attached carbohydrate chains,
- used in cell signalling to Indicate self cells
- Binds cells together to form tissues
- are Hormone receptors
what is the function of glycolipids in membranes
They are Hormone receptors and are used in Cell signalling (Indicate self-cell)
What is the general purpose of Intrinsic proteins
Involved in facilitated diffusion - transport substances like ions and amino acids that cannot diffuse across the membrane but are still vital to a cells functioning
how do intrinsic proteins stay in place?
amino acids with hydrophobic R groups interact with the hydrophobic core of the membrane keeping them in place
function of Channel proteins in membranes
It provides a hydrophilic channel that allows diffusion of water soluble polar substances and ions
function of Carrier proteins
Transport of specific large substances which can’t pass directly across the membrane eg glucose
what happens at Receptor sites on membranes
hormones bind with Specific shaped receptors
What are and What is the function of aquaporins
channel proteins which Allow water to pass through The membrane much faster
What are the three ways substances can move across the plasma membrane
Osmosis, active transport and diffusion
How does the membrane assist the processes of mitochondria
- The substances needed for respiration are kept together inside by the mitochondria’s outer membrane
- The inner membrane of mitochondria contains enzyme needed for respiration
- it has a large surface area which increases the number of enzymes present making the rate of respiration faster and more efficient
How is the plasma membrane used in protein synthesis
Membranes form vesicles to transport the proteins between different areas of the cell (EG from RER to Golgi Apparatus) and out of the cell
What is cholesterol and what is its function in the plasma membrane
- it is a lipid present in all cell membranes except bacterial cell membranes
- in hot temperatures, cholesterol molecules fit between the Phospholipids, they bind to the hydrophobic tails causing them to pack more closely together making the membrane less fluid and more rigid and stable
- In low temperatures, phospholipids are freezing and move less And cholesterol doesn’t fit as well, tight packing is inhibited so can increase fluidity
- cholesterol also has hydrophobic regions so it is able to create a further barrier to polar substances
Where are extrinsic proteins found in
They may be free on the cell membrane or bound to an intrinsic protein
what are the functions of extrinsic protein in cell membrane and how are they bound to the membrane
- They have amino acids with hydrophilic r groups and interact with the polar hydrophilic heads of the phospholipids or intrinsic proteins
- On the exterior side they act as receptors/neurotransmitters for hormones or are involved in cell recognition (Many of these are Glycoproteins)
- On the interior side they are involved in cell signalling, they can dissociate from the membrane and move into the cytoplasm
What is the name and function of the membrane surrounding chloroplast
Thylakoid membrane keeps the components needed for photosynthesis together
what is the function of the membrane surrounding a bacterial cell
To control what goes in and out of the cell
To allow cell communication, cell recognition
What does compartmentalization mean
Isolating each area from its external environment eg membranes form cells and separate areas within cells
Why do alcohol, caffeine and nicotine (lipid soluble molecules) have an instant and widespread effect on the body
lipid soluble molecules can pass through membranes by simple diffusion so they go through the whole body
Suggest the advantage to a highly folded membrane eg in Mitochondria
Processes on membranes are enzyme controlled, folding increases surface area, which means there are more enzymes, which increases the rate of respiration and therefore increases rate of atp produce Which is needed for cellular activities to survive
How do ions pass through the membrane
Channel or carrier proteins
Why are there more proteins in energy releasing organelles like mitochondria
Energy releasing organelles need lots of substances (like nutrients, enzymes, ATP) to travel across their membranes, some of these substances will require help from proteins to get through the membrane, so they will have a higher protein content
Why is it important that a cell membrane maintains its fluidity
It is necessary for the diffusion of substances across the membrane and
For cells to move and change shape eg like phagocytes when they engulf foreign objects
State two features of a gas exchange surface
Large surface area and thin lining
What is the effect of really High temperature on the cell membrane
- the kinetic energy of the phospholipid increases,
- creating gaps between the bilayer Making the layer more loosely packed,
- which molecules can pass through.
- Channel protein and carrier proteins have denatured So they can’t control what enters and exits
- Water inside the cell puts pressure on the membranes
- therefore the permeability of the membrane has increased
What happens to the Cell membrane at temperatures below freezing
- The phospholipids don’t have much kinetic energy so they don’t move very much.
- they are packed closely to form a Rigid layer.
- channel proteins and carrier proteins denature.
- ice crystals that have formed from the water inside the cell pierce the membranes making large holes
- increasing permeability significantly when the ice crystals melt
What is the effect of water on the cell membrane
- Water is polar, so it is important in creating membrane stability with the phospholipids when forming the general bilayer structure,
- phospholipids only dissolve in non-polar solvents so water doesn’t disrupt the cell membrane but can actually diffuse through slowly
What molecules disrupt the bilayer
Alcohols caffeine nicotine
what is the effect of alcohol on the body
- the water soluble ethanol it contains helps it pass freely through your body, it rides in your bloodstream, passes through the cell membranes
- It mainly goes to the brain where it becomes a central nervous system depressant, it causes feel good dopamine to be released
How does alcohol slow down the brain
- Binds to glutamate (a neurotransmitter) and inactivates it so responses are slower
- Binds to gamma aminobutyric acid (GABA) and activates them to release calm and sleepy receptors so brain function slows further
What does the effect of ethanol have on the permeability of the membrane
- Ethanol is a non polar solvent so dissolves non polar substances like lipids (Phospholipids).
- The membrane becomes permeable and allows substances to leak in and out of the cell.
- As ethanol concentration increases membrane permeability increases,
- If ethanol is high enough it can disintegrate the membrane and completely kill the cell
Effect of cholesterol on membrane fluidity
In high temperatures cholesterol decreases the fluidity of the membrane
in low temperatures cholesterol increases membrane fluidity
What increases fluidity of membrane
shorter phospholipid tails
Unsaturated tails/more double bonds
Less cholesterol
Less packed together
High temperatures
What increases fluidity of membrane
shorter phospholipid tails
Unsaturated tails/more double bonds
Less cholesterol
Less packed together
High temperatures
Why is liquid seen to come out of frozen fruit after defrosted
The membrane (which would have holes from the piercing of the ice crystals Making it highly permeable) Allows the pigment to leak through
How can water get through the cell membrane
Facilitated diffusion through channel protein aquaporin or simple diffusion through phospholipid bilayer
What is cell signalling
- Processes that lead to communication and coordination between cells
- One cell releases a messenger molecule eg a hormone, this travels to another cell in the blood and is detected as it binds to a receptor (protein) on its membrane. the binding triggers a change in the cell ag a chemical signal is set of
why might sell signalling be important
To control processes inside the body,
When responding to environmental changes
What are membrane-bound receptors
Sensors on the outer cell surface membrane
often Protein molecules or glycoproteins
complementary in shape to signal molecule
What are examples of signal/ Messenger molecules
hormones EG adrenaline insulin glucagon
Neurotransmitters eg noradrenaline
Cytokines - role in immune response
What are drugs
Drugs have been developed that mimic the shape of Messenger molecules, they can trigger a response in the cell or block the Receptor and prevent it from working
What is the function of the messenger molecule glucagon
glucagon is released when there isn’t enough glucose in the blood, it binds to receptors on the liver cells causing the liver to break down stores of glucagon to glucose
What is the function of the messenger molecule FSH
Fsh is released by the pituitary gland during the menstrual cycle, it binds to the receptors on the cells in the ovaries causing an egg to mature ready for ovulation
What is the function of the drug morphine
The body produces chemicals called Endorphins to relieve pain. endorphins bind to opioid receptors (Which stop vehicles containing neurotransmitters moving to and fusing with signups membrane therefore less neurotransmitters are released in this case pain signals) in the brain and reduce the transmission of pain signals. morphine is a drug used to relieve pain, it works by binding to the same opioid receptors as endorphins and also triggering a reduction in pain signals
What is the function of the drug antihistamines
Cell damage causes the release of a chemical called histamines, these bind to receptors on the surface of other cells and cause Inflammation. antihistamines work by blocking histamine receptors on cell surfaces. this prevents histamines from binding to the cell and stops inflammation
Function of clostridium botulinum (botox)
This is a poison which binds to receptors on muscle cells causing paralysis
What does partially permeable mean
Allow some molecules through but not others
What is the difference between hydrophobic and hydrophilic
Hydrophobic repels water hydrophilic attracts water
How do drugs work
the drug is a complementary shape to the receptor so this means it will bind to the receptor blocking the messenger molecule from doing so preventing the messenger molecule from triggering a response in the cell
Why can mutated versions of the receptor not respond to messenger molecules
The complementary shape of the site has changed meaning the messenger molecules can no longer bind and are unable to trigger a response in the target cells
Why can this specific messenger molecule only produce a response in liver cells
The messenger molecule can only bind to receptors with a complementary shape, different cells have different membrane bound receptors, liver cells have the correct receptor for this specific messenger so only liver cells can respond
Give an example of what the process of exocytosis Transports
Digestive enzymes, hormone, lipid
Protein production process
- The instructions to make the protein are in the DNA in the nucleus.
- The specific instruction to make the protein is known as the gene which is on a chromosome.
- The nucleus copies the instructions in the DNA into a molecule called mRNA.
- The mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome possibly on the RER
- The ribosome reads the instructions and uses the code to assemble amino acids.
- The amino acid is folded into protein in the RER
- The protein is pinched off in a transport vesicle and transported to the Golgi which packs the protein and may modify it so it’s ready for release.
- then the protein is packaged into a secretory vesicle and fuses into the cell surface membrane where it’s secreted.
Why must glucose be transported through facilitated diffusion
Glucose is a polar molecule phosphor lipid bilayer is hydrophobic which forms a barrier against diffusion of polar water soluble substances
