2.1.5 Biological Membranes Flashcards
Role of membranes
Partially permeable barriers: between cell and environment; between organelle and cytoplasm; within organelles
Site of chemical reaction
Site of cell communication
Structure of membrane
Fluid-mosaic model
Phospholipid bilayer: 2 rows of phospholipids arranged tail to tail with hydrophilic heads facing aqueous environments
Glycoproteins and glycolipids
Cholesterol
Proteins
Examples of intrinsic proteins
Channel and carrier proteins
Glycoproteins
Role of channel proteins
Provide hydrophilic channel that allows passive movement of polar molecules and ions down conc gradient
Held in place by interactions between hydrophobic core of membrane and hydrophobic R groups
Role of carrier proteins
Passive transport and active transport into cells by shape of protein changing
Role of glycoproteins
Cell adhesion and receptors for chemical signals
Role of glycolipids
Cell markers/antigens, recognised by cells of immune system as self or non-self
What are extrinsic proteins
Present in one side of bilayer, usually have hydrophilic R groups on outer surfaces and interact with polar heads of phospholipids
Role of cholesterol
Regulates fluidity of membranes
Has hydrophilic and hydrophobic end that interact with phospholipids, pulls them together
Adds stability without making it rigid, stops phospholipids getting too close and crystallising
How does temperature effect membrane structure
Phospholipids have more kinetic energy and move more, membrane is more fluid and loses structure
Increases permeability
Denatures proteins, affects permeability
Effect of solvents on membrane structure
Solvents less polar than water, eg organic solvents, they’ll dissolve membranes and disrupt cells
Strong alcohol solutions cause damage
Makes membranes more fluid and permeable
Experiment to investigate effect of temperature on membrane permeability
Cut 5 small pieces of beetroot equally using a cork borer
Wash then place in distilled water in boiling tubes
Place boiling tubes in water baths of different temperatures
Sample after 5 minutes
Measure absorbance of each sample with colorimeter with blue filter, repeat 3 times and create mean
Definition of diffusion
Net movement of particles from a region of higher concentration to a region of lower concentration
Passive process
How does temperature affect rate of diffusion
Higher temp = higher rate of diffusion
Due to more kinetic energy in particles
How does concentration gradient affect rate of diffusion
Greater conc gradient = greater rate of diffusion
Overall movement from high to low conc is greater
How does diffusion distance affect rate of diffusion
As distance increases, rate of diffusion slows down
What does membranes being described as partially permeable mean
Non-polar molecules diffuse through freely down conc gradient
Ions can’t easily pass due to hydrophobic interior
Polar molecules can diffuse at slow rate (small more easily than larger)
Factors affecting rate at which molecules or ions diffuse across membranes
Surface area: larger=faster rate
Thickness of membrane: thinner=faster rate
What is facilitated diffusion
Diffusion through a membrane through protein channels/ carrier proteins
Down a conc gradient
Selectively permeable membrane
Factors affecting rate of facilitated diffusion
Temperature
Concentration gradient
Membrane SA
Membrane thickness
Number of channel proteins
How to carry out an investigation into factors affecting diffusion rates in model cells
Use dialysis tubing to create a model cell, tie one end and fill with solution then tie other end
Place cell in another solution which contains different conc of solute
Measure change in conc in both solutions then calculate rate of diffusion
As glucose can cross the tubing, Benedict’s solution can be used to test for presence of glucose
Water can pass, starch and sucrose can’t
Definition of active transport
Movement of molecules or ions into/out of a cell from region of lower concentration to region of higher concentration
Requires energy and carrier proteins as process goes against concentration gradient
Describe the process of active transport
Molecule binds to receptors in channel of carrier protein on outside of cell
Inside of cell, ATP binds to carrier proteins, hydrolysed to ADP and phosphate
Binding of phosphate to carrier protein causes protein to change shape, opening up inside of cell
Molecule released inside of cell
Phosphate molecule released and recombines to form ATP
Carrier protein returns to OG shape
Describe the process of endocytosis
Bulk transport of material into cells
Cell-surface membrane bends inwards when in contact with material to be transported
Membrane enfolds material until it fuses and forms vesicle
Vesicle moves into cytoplasm
Energy in the form of ATP required
Describe the process of exocytosis
Vesicles (formed by golgi) move to cell-surface membrane and fuse
Contents released outside of cell
Energy in form of ATP required
Definition of osmosis
Net movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane
Effect of osmosis on animal cells: cytolysis
Cell placed in solution with higher water potential than cytoplasm
Water moves into cell, hydrostatic pressure increases
Cell-surface membrane can’t stretch and withstand increased pressure
Cell bursts (cytolysis)
Effect of osmosis on animal cells: crenation
Cell placed in solution with lower water potential than cytoplasm
Water lost to solution down water potential gradient
Reduces volume of cell, cell-surface membrane shrinks (crenation)
Effect of osmosis on plant cells: turgid
Water enters cell, hydrostatic pressure pushes membrane against cell walls, turgor
Effect of osmosis on plant cells: plasmolysed
Cells placed in solution with lower water potential, water lost from cells, volume reduced of cytoplasm
Cell-surface membrane pulled away from cell wall, plasmolysed
How to investigate effect of osmosis on plant cells
Place pieces of potato into sugar solutions with different concentrations (diff water potential)
Water moves in/out of cells and mass of plant tissue will change as this happens
