Plasma Membranes Flashcards
What is compartmentalisation?
Formation of separate membrane-bound areas.
What are the advantages of compartmentalisation?
Allows:
- metabolic reactions to be separated
- different environmental conditions in different parts of the cell
- chemical concentration gradients to be made
- protection of cellular components e.g. nucleus
What is the main property of membranes?
They are fluid
What is fluidity?
Flexible, able to break and fuse easily.
What are the environmental roles of a cell surface membrane?
Provides a fixed set of conditions inside the cell cytosol
What are the transport roles of the cell surface membrane?
Controls transport of of substances in and out of cell
What is cell to cell signalling in the plasma membrane?
Have proteins (glycoproteins and lipoproteins) on the plasma membrane to interact with other cells
What is the detection of changes in the environment in the cell surface membrane?
Signal transduction- proteins can act as receptors for hormones e.g. insulin receptors on liver cells.
What is pseudopodia in cell surface membrane?
Some plasma membrane may form temporary protrusions for movement or food e.g amoeba.
What is anchorage in the cell surface membrane?
Anchorage for the cytoskeleton on which membrane proteins are involved.
What is cell to cell joining in plasma membranes?
Similar cells join together for example in tissue formation.
What does the plasma membrane NOT do?
- Does NOT provide support for the cell shape
- The cytoskeleton does this in animals and the cell wall in plants
What are membranes mainly composed of?
Phospholipids
What is the structure of a phospholipid?
- A polar head, which is hydrophilic, composed of a glycerol and a phosphate molecule.
- Two non-polar tails, which are hydrophobic, composed of fatty acid (hydrocarbon) chains.
- Because they contain both hydrophilic and hydrophobic regions, phospholipids are referred to as amphipathic.
What does amphipathic mean?
“Both hating”
In phospholipids, there is a water hating and a fat hating region.
How are phospholipids arranged?
- Phospholipids arrange spontaneously (self-assemble) into a bilayer
- Hydrophobic tails face inwards, shielded from surrounding polar liquids
- Hydrophilic head associate with cytosolic (inside cell) and extra cellular (outside cell) fluids.
- Tails NEVER overlap!
What enzyme can move phospholipids from one mono layer to another?
Flipase
Who devised the Fluid Mosaic Model?
Singer and Nicholson in 1972
How does the fluid mosaic model describe the structure of plasma membranes?
- Phospholipid bilayer containing hydrophilic heads facing outwards and hydrophobic fatty acid tails facing inwards.
- With, proteins randomly arranged between phospholipids.
What can proteins do in the lipid bilayer?
- They are able to move freely through the lipid bilayer
- The ease at which they do this is dependent on the number of phospholipid with unsaturated fatty acids in the phospholipids.
How are phospholipids held together?
By hydrophobic interactions
What allows for membrane fluidity?
Phospholipids.
How do phospholipids determine fluidity?
Phospholipids with short or UNSATURATED fatty acids are MORE fluid.
What does fluidity allow for?
Breaking and remaking of the membranes. For example for exocytosis and endocytosis.
Explain why phospholipids form a bilayer in plasma membranes? (4 marks)
- Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions
- Fatty acid tails are non-polar and will be repelled from an aqueous environment
- Both tissue fluid and cytoplasm is aqueous thus phospholipids form two layers with the hydrophobic tails facing inwards
- Hydrophilic phosphate group faces outwards, interacting with the aqueous environment
What are intrinsic proteins?
- Proteins that extend across the membrane
- A.K.A Transmembrane proteins
What are the two large groups of transmembrane proteins?
CHANNEL proteins and CARRIER cell proteins
What are extrinsic membrane proteins?
- Proteins present on only one side of the membrane
- A.K.A Peripheral proteins.
What are glycoproteins?
- Proteins which contain oligosaccharide chains (10-20 monosaccharides)
- Chains covalently attached to amino acid R groups (glycosylation)
- Transmembrane proteins
What are the function of glycoproteins in plasma membranes?
- CELL ADHESION by forming tight junctions with the proteins claudin and occludin
- CELL SIGNALLING such as receptor proteins for neurotransmitters, hormones and drugs
What are glycolipids?
- Lipids with a monosaccharide or oligosaccharide bound
- Phosphate group in phospholipid replaced by sugar residue
What are the function of glycolipids in plasma membranes?
- CELL SURFACE MARKERS for cell to cell recognition (saccharide of glycolipid will bind to specific complementary carbohydrate or carbohydrate-binding protein of a neighbouring cell)
- IMMUNE RESPONSES by acting as antigens, notably the recognition of host cells by viruses
- ABO BLOOD GROUPS blood type is determined by the oligosaccharide attached to a specific glycolipid on the surface of red blood cells, which acts as an antigen
What is cholesterol?
- A sterol or modified steroid or a type of lipid
- It is biosynthesised by all animal cells and is an essential component of animal cell membranes
- Also serves as a precursor for the biosynthesis of steroid hormones, bile acid and Vitamin D
What are the functions of cholesterol in plasma membranes?
- Required to build and maintain membranes
- Modulates membrane fluidity over the range of physiological temperatures
- Increases membrane packing, which alters membrane fluidity
- Membrane remains stable and durable without being rigid, allowing animal cells to change and shape and animals to move
How does cholesterol fit into the plasma membrane?
- The hydroxyl group of each cholesterol molecule interacts with the water molecules surround the membrane
- The bulky steroid and the hydrocarbon chain are embedded in the membrane a long side the non polar fatty-acid chains of phospholipids.
What biological molecules can intrinsic proteins be?
They can be enzymes for example many of the enzymes involved in aerobic respiration are located as transmembrane proteins in the inner mitochondrial membrane.
What experiment showed that cell-surface proteins are laterally mobile?
- Human and mouse cell fused
- Immediately after fusion, surface antigens of two cell types remained localised in their respective cells (detected by fluorescent dye)
- After several hours of incubation, the mouse and human proteins were evenly distributed throughout the membrane of the fused cell
This demonstrated that most of the surface proteins were not rigidly held in place in the membranes of the original mouse and human cells.
How many carbon atoms do fatty acid tails contain?
Usually an even number of carbon atoms
Typically between 16 and 18
How does the saturation of fatty acid tails affect membrane fluidity?
Unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus INCREASING fluidity of membrane.
What is the ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition?
HOMEOVISCOUS ADAPTION
What is an example of homeoviscous adaption and how do they change this?
Bacteria, fungi, fish, protists, etc.
They all tend to increase UNSATURATED fatty acids at lower temperatures and increase SATURATED at higher temperatures.
(temp also affects membrane fluidity)
What effect does the length of a fatty acid carbon chain have on membrane fluidity?
Longer chain means stronger Van der Waals forces, which causes the membrane to be LESS fluid.
How does temperature affect plasma membranes?
Increase in temperature cause the phospholipids to have more kinetic energy
Therefore the phospholipids more around more so there are more gaps in the bilayer
Initially this will increase membrane fluidity and permeability
HOWEVER membrane proteins will also denature at higher temperatures, affecting the plasma membrane’s ability to control movement of molecules and ions across it
What type of solvents will dissolve membranes?
Organic solvents, such as ethanol or benzene.
It affects the hydrophobic regions of the bilayer
Alcohol disrupts membranes to such an extent that water floods into the cell (why its used as a disinfectant)
Non polar alcohol molecules enter the cell membrane and the presence of these molecules between the phospholipids make the membrane more fluid and more permeable, affecting their normal function
What is diffusion?
Net movement of anything from a region of higher concentration to a region of lower concentration
What is diffusion driven by?
A concentration gradient
What is a concentration gradient?
A change in concentration over a distance
What does diffusion not require?
Diffusion does NOT require external energy, it is a PASSIVE process
The energy comes from the kinetic energy of the particles
What does Brownian Motion rely on?
Relies on random movement of particles suspended in a liquid or a gas
What is an ENTROPIC FORCE?
Phenomenon resulting from an entire system’s statistical tendency to increase its entropy or disorder.
What does FICK’S LAW state?
- Higher the temperature, higher the rate of diffusion
- Higher the concentration difference, higher the rate of diffusion
- Higher the surface area of the exchange surface, higher the rate of the diffusion
- Higher the membrane thickness, lower the rate of diffusion
What is an equilibrium in diffusion?
When the particles are moving equally in all directions
What is BULK FLOW?
The movement of the whole substance or all the particles in a direction.
What is simple diffusion?
Diffusion between the phospholipids
What kind of molecules can do simple diffusion?
-Small and non-polar molecules e.g oxygen and carbon dioxide
-Bigger the non-polar molecule lower the rate of diffusion
-Water(small and polar) can diffuse
through a cell membrane but much more slowly due to the partially charged particles being repelled by the hydrophobic core of membrane.
-Large and polar molecules diffuse through very slowly or not at all
-Fully charged molecules or ions cannot easily or not at all diffuse through a cell membrane as the charges are repelled.
What is facilitated diffusion?
Diffusion involving intrinsic or transmembrane proteins.
What does a channel protein do?
It has a permanently open pore, which is specific to a molecule or ion
What is the rate of diffusion like for channel proteins?
High rate of diffusion but is saturable due to the limited pores available.
What do carrier proteins do?
- Have a pore that is NEVER open across the membrane.
- Protein changes shape when a specific molecule or ion binds to the protein and causes a conformational change to allow molecule or ion to cross cell membrane.
What kind of rate of diffusion do carrier proteins do?
Low rate of diffusion due to limited pores available and the need for protein to change conformation.
Where does the energy for carrier protein diffusion come from?
The binding of the molecule or ion to the protein
Why does facilitated diffusion require no metabolic energy?
Particles have their own kinetic energy for movement down the concentration gradient.
How do you maintain a concentration gradient?
Molecules or ions must be moved UP the concentration gradient at a rate that is faster than they diffuse DOWN.
What is active transport?
The movement of molecules or ions across a plasma membrane from a region of lower concentration to a region of higher concentration AGAINST the concentration gradient
What is a feature of active transport?
It is SELECTIVE
What is an example of active transport?
The Sodium (Na+) Potassium (K+) pump Sodium is pumped OUT of the cell and potassium is pumped INTO cell
What proteins is required for active transport?
Carrier proteins
What is required for active transport?
Energy, supplied from ATP
Where does a lot of active transport take place?
The internal lining of the small intestine e.g. active transport of glucose into the blood
Plants uptake Cl- and NO3- ions from very dilute solution in the soil
How does active transport occur?
- Molecule binds to receptor site on inner wall of the pore of the carrier protein
- ATP binds to carrier protein on the cytoplasm side of the carrier protein, then is hydrolysed into ADP + Pi and the Pi remains bound to carrier protein
- Carrier protein changes conformation to open the pore to the other side of the membrane and molecule is released to the inside of the cell
- Pi is released from carrier protein and carrier protein changes conformation again to adopt original conformation
What are the two types of bulk transport?
Endocytosis and Exocytosis
What are the three types of endocytosis?
Phagocytosis, Pinocytosis, Receptor-mediated endocytosis
What is the process of phagocytosis and pinocytosis?
Plasma membrane bends inwards when it comes into contact with the material to be transported
Membrane enfolds until eventually the membrane fuses, forming a vesicle
Vesicle pinches off and moves into cytoplasm to transfer the material for further processing
E.g. bacteria brought in in vesicles would be brought to the lysosomes where the bacteria are digested by enzymes.
What is endocytosis?
Bulk transport of material INTO cells
What is exocytosis?
Bulk transport of material OUT OF cells
What is the process of exocytosis?
Vesicles (usually formed by Golgi apparatus) move towards (via cytoskeleton) and fuse with the cell surface membrane
Contents of vesicles are then released outside of the cell
What is osmosis?
- Specific type of diffusion
- Water molecules moving across a partially permeable membrane from an area of higher water potential to an area of low water potential until an equilibrium is reached
What is water potential?
Is the pressure exerted by water molecules as they hit a container or membrane
What are the units for water potential?
kPa (kiloPascal)
What does water potential quantify?
The tendency of water to move from one area to another due to osmosis and hydrostatic pressure effects
What is pure water’s water potential?
0kPa
What is the diagram to help work out movement of water by osmosis?
0 Pure water | movement
| of
-100 Dilute solution | water
| by
-200 Concentrated solution | osmosis
|
-300 Very concentrated solution v
How does water move by osmosis?
From an area of high water potential to an area of low water potential
What is an equilibrium in osmosis?
Water is moving in and out of the cell at the same rate.
What causes an increase in HYDROSTATIC PRESSURE?
Movement of water into cells causes cell volume to increase, leading to an increase in hydrostatic pressure.
What is hydrostatic pressure measure in?
kPa (kiloPascals)
What is a hypertonic solution?
A solution outside the cell with a higher concentration, therefore a LOWER WATER POTENTIAL compared to the cell
What is an isotonic solution?
A solution with the same water potential than the cell
What is a hypotonic solution?
A solution with a lower concentration therefore a HIGHER WATER POTENTIAL compared to the cell
What happens to cells when they are put in a hypertonic solution?
Water leaves the red blood cell and the cell becomes CRENELATED
What happens to cells when they are put into an isotonic solution?
Water moves in equilibrium
What happens to red blood cells when they are put into a hypotonic solution?
Water enters the red blood cell causing it to swell and burst causing CYTOLYSIS
What do plasma membranes not do?
Stretch easily and have no mechanical strength to withstand increased hydrostatic pressure
Why don’t plant cells burst when hydrostatic pressure increases?
Because the strong cellulose cell wall can withstand the pressure and become TURGID
What happens when you put plant cells in a hypertonic solution?
Water leaves the cell and it becomes plasmolysed
What does plasmolysed mean?
The cytoplasm shrinks down
What happens the plant cells in an isotonic solution?
Water moves in equilibrium and the cell becomes FLACCID
What happens when you put plant cells in a hypotonic solution?
Water enters the cell and developed turgor pressure
Cell becomes TURGID
What do plants do to remain turgid?
Put salts into the tonoplast to create low water potential so water can move into cell
