Chap 5 - Biological membranes Flashcards
What are the functions of membranes? (5)
- keep cellular components inside the cell
- allow selected molecules to move in and out (partially permeable)
- isolate organelles from rest of cytoplasm - allow cellular processes to occur seperately
- site for biochem reactions
- allow cell to change shape
Define compartmentalisation
formation of separate membrane-bound areas in a cell
Explain why compartmentalisation is useful for cells
- allows chemical gradients to be formed/maintained
- isolates organisms from rest of cytoplasm – seperates cellular processes (many of which are incompatible)
Define partially permeable
membrane that allows some substances to cross, but not others
Define cell signalling
complex system of intercellular communication
Define phospholipid bilayer
arrangement of phospholipids found in cell membranes
- hydrophilic phosphate heads on inner & outer surface sandwich fatty acid tails to form hydrophobic core
Explain what is meant by fluid and mosaic in the fluid mosaic model of membrane structure
fluid - membranes not solid (molecules are weakly held in place)
- phospholipids are weakly attracted and can change places
mosaic - phosphates and proteins embedded form a tile pattern
Define glycoprotein
membrane proteins with attached carbohydrate molecules of varying lengths and shapes - intrinsic
Define glycolipid
cell surface membrane lipid with attached carbohydrate molecules of varying lengths and shapes
Explain the structure of phospholipids
lipids consisting of two fatty acids, a glycerol unit and a phosphate group
What is cholesterol
lipid with a hydrophilic end and a hydrophobic end
Define channel protein
hydrophillic channel that allows passive movement of polar molecules and ions down concentration gradient through membranes
Define carrier protein
membrane proteins that play part in transportation of large substances (passive & active) through a membrane
Define intrinsic protein
transmembrane proteins that are embedded through both layers of a membrane
- AA with hydrophobic R groups on external surfaces which interact with hydrophobic core of membrane
Define extrinsic protein
peripheral proteins that are present in one side of bilayer
- AA with hydrophilic R groups on external surfaces and interact with polar heads of membrane or intrinsic proteins
Define receptor proteins
proteins that bind to a specific molecule, triggering chemical reaction
What is the role of phospholipids in the membrane?
- main component of cell surface membranes (membrane formation)
- act as barrier between contents of a cell and its exterior (hydrophobic barrier)
What is the role of cholesterol in the membrane?
- maintaining membrane fluidity
- stabilising the bilayer
State the role of glycoproteins in the membrane
- cell adhesion
- cell communication/signalling (as receptors for chemical signals)
What is the role of glycolipids in the membrane?
cell recognition - act as antigens, often related to immune system
State the roles of different proteins in the membrane
- glycoproteins: receptors in cell signalling
- glycoproteins: cell adhesion
- enzymes kept in place by being attached to membrane
- channel/carrier: transport proteins
Define cell adhesion
when cells join together to form tight junctions in certain tissues
Explain how cholesterol affects the fluidity and stability of membranes
- chol molecules are positioned between phospholipids in the bilayer preventing them from aligning too closely together - preventing solidification - maintains fluidity
- hydrophilic end interacts with the heads and hydrophobic end interacts with tails, pulling them together - stabilises bilayer as it is attracted to the phospholipids
Explain the importance of membrane bound proteins in chemical reactions
- enzymes can be held in correct location by attachment to membrane
- kept in place where optimum conditions are
- positioned so they can access substrates
Explain the binding of a molecule to a membrane-bound glycoprotein can cause effects within a cell
- chemical messenger molecule moves around body, if able to fit the protein receptor binding site, it will bind (complementary fit)
- chemical reaction is caused - signal which causes a chemical reaction within a cell, altering its activity
Describe the effect of changing temp on permeability of cell membranes
increasing temperature will increase membrane permeability
Why does temp have an effect on permeability of cell membranes?
- kinetic energy of phospholipids increases - they vibrate more
- this ixtncreases average phospholipid separation
- this increases permeability of bilayer as there are more gaps
- phospholipid bilayer may melt completely
- proteins may also denature leaving gaps in membrane
- therefore membrane permeability will increase
Describe the effect of solvent concentration on permeability of cell membranes
- water, a polar solvent is essential to maintain the phospholipid bilayer structure
- organic solvents (less polar or non-polar) will increase its permeability as you increase concentration
Explain the effect of water on the permeability of cell membranes.
- always must be some water present for bilayer to be maintained
- non-polar tails of phospholipids are oriented away from water, forming bilayer with hydrophobic core
- charged phosphate heads intereact with water helping to keep bilayer intact
- when phospholipids cant intereact with water the bilayer can no longer exist bc polar heads are not attracted to non-polar substances
Explain the effect of organic solvents on the permeability of cell membrane
as conc. increases around bilayer, its ability to interact with water decreases and bilayer starts to disperse
Describe a method using a colorimeter to investgate the effect of temperature on permeability of cell membranes.
- cut five small pieces of beetroot of equal size
- wash beetroot pieces then place in 100ml distilled water in a water bath
- increase temperature of water bath in 10*C intervals
- take samples of water containing beetroot five mins after each temperature was reached
- measure absorbance of each sample using a colorimeter with a blue filter
- repeat experiment three times, each time with fresh beetroot pieces and calculate mean absorbance for each temperature
Describe a method using a colorimeter to investgate the effect of ethanol conc on permeability of cell membranes.
- cut five small pieces of beetroot of equal size
- prepare 5 solutions of ethanol in test tubes with concentrations 0%, 0.13%, 0.25%, 0.5%, 1%
- place one piece of beetroot in each test tube at the same time and start a timer for 5min, shake each test tube 3 times
- once timer is done, remove the beetroot pieces from test tubes, shake each test tube 3 times and take sample of solution from each
- measure absorbance of each sample using a colorimeter with a blue filter
- repeat experiment each time with fresh beetroot pieces and calculate mean absorbance for each ethanol concentration
Define diffusion
net movement of particles from a region of higher concentration to lower concentration
Define simple diffusion
diffusion across a partially permeable membrane without the assistance of membrane proteins.
- passive process
Define facilitated diffusion
diffusion across a partially permeable membrane with the assistance of membrane proteins (carrier/channel proteins).
- passive process
Define osmosis
net movement of water from a solution of higher to lower water potential through a partially permeable membrane.
- passive transport
Define passive transport
- does not require energy
- does not use energy from cellular respiration.
Define active transport
movement of particles across a plasma membrane against a concentration gradient.
- energy is required.
Define endocytosis
bulk transport of materials into cells via invagination of the cell-surface membrane forming a vesicle.
Define exocytosis
bulk transport of materials out of cells where vesicles containing the material fuse with the cell-surface membrane and the contents are released outside of the cell.
Define pinocytosis
endocytosis of liquid materials
Define phagocytosis
endocytosis of solid materials
List 5 factors affecting the rate of diffusion of a molecule.
- surface area
- concentration difference
- distance
- temperature
- permeability
Explain how substances can move across a membrane by simple diffusion.
- particles on both sides are constantly moving in random directions
- over time, there will be a net movement of particles from area of high conc to the area of low conc (down a concentration gradient)
- particles are able to pass from one side to another directly through the phospholipid bilayer, by moving between the individual phospholipids
- this is carried out without the assistance of membrane proteins or ATP
State which types of molecule can move across a membrane by simple diffusion.
- small, non-polar, uncharged molecules (oxygen molecules)
- lipid-soluble molecules (steroids)
Explain how substances can move across a membane by facilitated diffusion.
- particles on both sides of membrane are moving constantly in random directions
- over time there will be a net movement of the particles from an area of high concentration to an area of low concentration
- particles can pass through channel proteins embedded in the membrane
State which types of molecules can move across a membrane by facilitated diffusion
- carrier proteins - ‘large’ molecules (eg. glucose)
- channel proteins - charged / polar particles (hydrophilic / water soluble) (eg. sodium ions)
Explain how substances can move across a membrane by active transport
- specific molecule or ion to be transported binds to receptors in the channel of the specific carrier protein on the outside of the cell
- on the inside of the cell ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
- binding of phosphate molecule to carrier protein causes protein to change shape - opens up inside of the cell
- molecule or ion is released inside of the cell
- phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- carrier protein returns to its normal shape
Explain how substances can move into a cell by endocytosis (phagocytosis)
- solid binds to cell receptors
- plasma membrane invaginates, surrounding the extracellular material
- phagosome vesicle forms and pinches off, bringing contents inside the cell
- phagosome can then fuse with a lysosome forming a phagolysosome to digest the contents
Explain how substances can move out of a cell by exocytosis (secretion)
- protein enters a secretory vesicle
- secretory vesicle containing the protein fuses with the cell membrane
- protein is secreted from the cell
Identify which transport mechanisms require an input of energy from ATP.
- active transport (by carrier proteins)
- cytosis (endocytosis & exocytosis)
Explain why it is easier for an oxygen molecule to diffuse across a membrane than a water molecule.
- diffusion across membranes involves particles passing thru the phospholipid bilayer
- because oxygen is small and non-polar it will not be repelled by the hydrophobic interior and will be able to pass through the bilayer easily
- water is also small but it is polar as it has partially positive and negative charges which will be repelled slightly by the hydrophobic interior of the bilayer making it harder for the molecules to pass through
Explain why steroid hormones can easily cross a membrane by simple diffusion
- steroid hormones are lipid soluble –> hydrophobic
- this means steroids can pass through the bilayer directly as they are not repelled by its hydrophobic interior and therefore do not need the aid of carrier proteins
Describe and explain the results you would expect to see in an investigation into how the rate of diffusion is affected by surface area.
- rate of diffusion will increase as you increase surface area
- bc there would be a greater surface for particles to collide with and enter/exit
Describe and explain the results you would expect to see in an investigation into how the rate of diffusion is affected by temperature.
- rate of diffusion will increase as you increase temperature
- bc at higher temp particles have greater avg kinetic energy which means higher avg speed of particles - they diffuse faster
Define solvent
liquid able to dissolve other substances
(eg. water, acetone)
Define solute
substance dissolved in the solvent
(eg. salt, sugar)
Define solution
liquid mixture in which the solute is distributed evenly within the solvent
Define water potential & its symbol
pressure exerted by water molecules as they collide with a membrane or container.
- measured in pascals (Pa) or kilopascals (kPa)
State the water potential of pure water and explain why water potential can’t have a positive value
- 0kPa (at standard temp and pressure)
- highest value because presence of solute in water lowers the water potential below 0
- more solute means more negative water potential
Define plasmolysis
when the plant cell membrane is pulled away from the cell wall (contraction of the protoplast)
- occurs when cell is in a solution that is too hypertonic
Define protoplast
contents of a cell within the cell membrane
Define crenation
contraction of a cell after exposure to a hypertonic solution (shriveling)
Define turgid
plant cell that is swollen from water uptake where cell wall prevents cell from bursting
Define cytolysis
when a cell bursts and releases contents due to great influx of water into a cell
- in hypotonic solution
Define haemolysis
rupture and destruction of red blood cells
Define solute potential
component of plant cell water potential that is caused by the dissolved substances in the cytoplasm
Define absolute uncertainty
number which when combined with the reported values gives the range of true values
Define relative certainty
ratio of the absolute uncertainty to the reported value (usually a percentage)
Define anomaly
values in a set of results which are judged not to be part of the variation caused by random uncertainty
- identified using standard deviation calculations
Define precision
the closeness of agreement between measurements , it refers to repeated results
Define accuracy
the closeness of agreement between an individual test result and the true value.
What are membranes
cover surface of every cell and surround most organelles
mostly eukaryotes - chloroplasts, mitochondria
Describe the structure of a phospholipid
- phosphate head: polar and hydrophilic
- fatty acid chain tails: non-polar and hydrophobic
Why do phospholipids form a bilayer?
due to polar nature and interactions with water
Explain why phospholipids form a bilayer in plasma membranes. 3 marks
phospholipids have polar phosphate group which are hydrophilic and face aqeuous solutons (1); fatty acid tails are non-polar and will move away from an aqeuous enviroment (1); tissue fluid and cytoplasm are aqueous, phospholipids form 2 layers with hydrophobic tails facing inwards and phosphate grops outwards interacting with aqeuous environment
