Section 2 - Transport across cell membranes Flashcards
What is the function of glycoproteins in the cell-surface membrane?
(5 things)
- Receptors
- Antigens
- Cell signalling
- Receptor for hormone
- Cell adhesion
How can small, non polar substances move through membrane?
Pass through phospholipid bilayer
By simple diffusion
How can large substances move through membrane?
Transport
Carrier proteins
Endocytosis
How can polar substances move through membrane?
Facilitated diffusion
Channel and carrier proteins
Active transport - use of ATP
What is the role of membranes within cells?
Compartmentalisation
Reactions take place on membranes
Enzymes attached to membrane
Nuclear pore permits RNA to leave
Increases surface area of organelle
Why are phospholipids in the cell-surface orientated in a specific way?
Hydrophilic heads attracted to water
Hydrophobic tail repelled by water so they point into centre.
What does cholesterol within the cell-surface membrane do?
Add strength to the membrane
Reduce movement
Reduces fluidity
What is the process of co-transport?
- Sodium ions actively transported out of epithelial cells by sodium-potassium pump
- Low concentration of sodium ions in epithelial cell, so sodium ions diffuses down concentration gradient bringing glucose/ amino acid with it
- Glucose passes into blood plasma by facilitated diffusion
How do bacteria divide?
Binary fission
Replication of DNA
Division of cytoplasm
How does the phospholipid bilayer affect entry and exit of substances into and out of a cell?
Allows movement of lipid soluble/ non-polar molecules
Prevents movement of water soluble molecules
Large molecules do not move through
What are the functions of the cell-surface membrane?
Providing partially permeable barrier Compartmentalisation Transport of solutes Signal transduction Cell to cell recognition
What are the 2 organelles without a membrane?
Ribosome
Centriole
How many nanometres wide are membranes?
7 nm
Why is the cell-surface membrane also known as the fluid mosaic model?
Fluid: individual phospholipids and proteins can move around freely within their monolayer
Mosaic: pattern produced by scattered protein molecules when the membrane is viewed from above
What is the difference between intrinsic and extrinsic proteins in the membrane?
Intrinsic proteins: span the bilayer and form channel/ carrier proteins
Extrinsic proteins: Found on one side of bilayer, involved in cell recognition or act to stabilise membrane
Why are phospholipids important in the membrane?
Form sheets (bilayer)
Hydrophilic heads attract water
Hydrophobic tails repel water
What is the function of the phospholipid in the membrane?
Allow lipid-soluble substances to enter and leave the cell
Prevent water-soluble substances entering and leaving the cell
Make the membrane flexible and self-sealing
What is the function of proteins in the membrane?
Provide structural support
Act as a channels transporting water-soluble substances across
Allow active transport through carrier proteins
Form cell-surface receptors for identifying cells
Acts as receptors
What are the functions of cholesterol in the membrane?
Reduce lateral movement of other molecules including phospholipids
Make membrane less fluid at high temperatures
Prevent leakage of water and dissolved ions from the cell
Why is cholesterol important in the membrane?
Add strength
Limit movement of fatty acid tails by pulling them together
What is the function of glycolipids in the membrane?
Act as a recognition site
Help to maintain stability of the membrane
Help cells to attach forming tissues
What is the function of glycoproteins in the membrane?
Help cells to attach forming tissues
Allows cells to recognise one another
Acts as a recognition site
What are the glycolipids in the membrane made of?
Carbohydrate covalently bonded to a lipid
What are the glycoproteins in the membrane made of?
Carbohydrate attached to extrinsic protein
What is fluidity of the membrane controlled by?
Amount of cholesterol
Length of fatty acid
(shorter increases fluidity)
Proportion of saturated and unsaturated fatty acids
(Saturated decreases fluidity)
Why do most molecules not freely diffuse across the membrane?
Not soluble in lipids
Too large to pass through
Of same charge as protein channel
Electrically charged (polar)
What is the function of the membrane in lysosmes?
Isolated harmful substances to the cell
What is the function of the membrane in mitochondria/ chloroplast?
Controls reaction rate
Increases surface area
Controls rate substances enter
What is the function of the membrane in nucleus?
Forms separate compartment for cells
Allows RNA out in the pores
What is the function of the membrane in vesicles?
Allows substances in and out of cell
What is the function of the membrane in cell surface?
Take up fluid and form membrane-bound vesicles
What is the overall function of the cell-surface membrane?
To control the movement of substances in and out of the cell
Which end of the phospholipid lies towards the inside of the cell-surface membrane?
Hydrophobic tail
What 2 properties should a drug possess if it is to enter a cell rapidly?
Lipid soluble
Small in size
Have no electrical charge
Opposite charge of protein channel
What are the features of facilitated diffusion?
Passive process
Down concentration gradient
No energy is requires
Requires channel or carrier proteins to pass through membrane
What are channel protiens?
Form water-filled hydrophilic channels across membrane
Allow specific water-soluble ions to pass through
Channels are selective, each opening to specific ion
What are carrier proteins?
Molecule fits into proteins
Protein changes shape and molecule passes through
What is osmosis?
Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
What are 3 factors that affect the rate of diffusion?
Concentration gradient
Diffusion area
Thickness of exchange surface
Temperature
What is the difference between facilitated and simple diffusion?
Facilitated only occurs at channels on the membrane where there are specific protein carrier molecules
Why is facilitated diffusion a passive process?
No ATP from respiration used in the process
Why do glucose molecules not easily pass through the phospholipid bilayer?
Only lipid-soluble substances diffuse across the phospholipid bilayer
What are 2 structures of the membrane that would increase the rate at which glucose is transported into a cell?
Increase in surface area
More proteins with pores
What is osmosis?
Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
If you have a high water potential, you have a…
Low solute concentration
If you have a low water concentration, you have a…
High solute concentration
Where would water move with 3 water potentials of
- 400
- 600
- 200
- 200 into -400 & -600
- 400 into -600
What does hypotonic mean?
Region of higher water potential (lower solute)
What does hypertonic mean?
Region of lower water potential (higher solute)
What does isotonic mean?
Region where equal water potential on either side of membrane
What would happen to an animal cell if it was placed in a hypotonic cell?
Water potential of cell lower than external
Water enters cell by osmosis and bursts
What would happen to an animal cell if it was placed in a hypertonic cell?
Water potential of cell higher than external
Water leaves cell by osmosis and shrinks
What would happen to an animal cell if it was placed in a isotonic cell?
Water potential of cell same as external
Water enter/ leaves as same rate = dynamic equilibrium
What would happen to a plant cell if it was placed in a hypotonic cell?
As water enter turgor pressure increases, resists further entry
Cell is turgid
What would happen to a plant cell if it was placed in a hypertonic cell?
As water leaves, membrane pulls away from cell (plasmolysis)
Cell is flaccid
What would happen to a plant cell if it was placed in a isotonic cell?
No net movement of water
Cells stays the same
What would happen to a plant cell if it was placed in a isotonic cell?
No net movement of water
Cells stays the same
What is a selectively permeable membrane?
Membrane permeable to water molecules but not to larger molecules
Under standard conditions of pressure and temperature, what is the water potential of pure water?
Zero
What is active transport?
The movement of molecules into or out of a cell from a region of lower concentration to a region of higher concentration
What 2 things does active transport require?
ATP
Carrier proteins
What is the process of active transport?
- Molecule binds to complimentary site on carrier protein
- ATP is hydrolysed to ADP and carrier protein changes shape
- Molecule is transported across membrane
- One way flow as membrane can only fit into carrier protein on one side of membrane
What is the difference between endocytosis and exocytosis?
Endocytosis: moving large amounts in
Exocytosis: moving large amount out
How does active transport differ from passive forms of transport?
ATP is needed
Against concentration gradient
Carrier proteins act as ‘pumps’
Selective process
What is one difference and one similarity between active transport and facilitated diffusion?
Similarity: both use carrier protein in plasma membrane
Difference: active transport requires ATP/ occurs against concentration gradient
Why are many mitochondria present in cells that carry out active transport?
Mitochondria supply ATP in cells which is required for active transport to occur
What is the process of co-transport?
- Na ions actively transported out epithelial cell by sodium-potassium pump into blood
- Na ions diffuse into epithelial cell down conc. gradient through protein carrier and carries glucose with it
- Glucose passes into blood by facilitated diffusion using carrier protein
How is a low concentration of sodium ions in the cell ensured in active transport?
Sodium potassium ion ATPase pump, pumps out 3 sodium in exchange for 2 potassium ions
What are the 3 types of co-transporter proteins?
Uniport = single Symport = 2 in same direction Antiport = 2 in alternate direction
What are 3 ways the rate of movement across membrane can be increased?
Increasing concentration gradient
Increasing surface area
Increasing density of protein channel
Why is the term co-transport use to describe the transport of glucose into cells?
As glucose molecules and sodium ions move into the cells coupled together
What are 2 similarities between the movement of substances in diffusion and osmosis?
Both move down concentration gradient
Passive process
Don’t use energy/ ATP
Where in plants can cells undergoing mitosis be found?
Shoot and root tips
What is the mitotic index?
Ratio of cells undergoing mitosis to the total number of cells in sample
How do you work out mitotic index?
Mitotic index=
number of cells with visible chromosomes / number of cells in sample
What are the hazards and precautions needed to be taken in the mitotic index practical?
HCl = corrosive, avoid contact with skin, eye protection
Blue stain = irritant, avoid contact with skin
Scalpel = cut away from fingers
What is the purpose of calibration curves?
Used to determine concentration of unknown sample, comparing it to set of standard values with known concentrations
How is a calibration curve used to find concentration of plant tissue?
Plot calibration curve of percentage change in mass against concentration
Find x-intercept where plant tissue is isotonic to sucrose solution
How would you prepare a root tip squash?
- Use mounted needle to transfer root tips to clean microscope slide
- Add drop of water and spread root tips with mounted needle
=no overlap - Place another microscope slide on top, at right angles, minimising air bubbles
- Press slides together to squash root tips and spread them out
What occurs when plant tissue is placed in a hypotonic solution?
Water moves into plant tissue by osmosis, plant tissue increases in mass
What occurs when plant tissue is placed in hypertonic soltuion?
Water moves out of plant tissue by osmosis, plant tissue decreases in mass
Why are potato discs left in solution for 20 minutes?
To allow time for osmosis until plant tissue reaches equilibrium with its surrounding solution
What is water potential determined by?
The concentration of solutes
Why is percentage change in mass of potato used rather than actual change in mass?
Potato chips may not all have same starting mass
Percentage change allows comparison
What is the procedure of investigating osmosis using potato tissue?
- Make simple dilution of 1M sucrose to produce 5 concentration. Add 5cm to 5 different test tubes.
- Cut potato into equal sized chips and weigh
- Place chip in each test tube and leave for 20 minutes
- Take out and dab excess water and weigh again
- Calculate percentage change in mass
What is indicated by the x-intercept of the calibration curve?
The concentration that is isotonic to solution tested
What does the mass of potato change after 20 minutes?
Potato chips with concentration lower than sucrose solution lose mass as there is not movement of water out of cell
Potato chips with concentration higher than sucrose solution gain mass as there is net movement of water into cells
Why are potato chips dabbed dry after removing from the sucrose solution?
To remove any excess water clinging to its surface
What are the controlled variables of the potato practical?
Volume of sucrose solution
Size of potato chips
Length of time left in solution
What are 2 factors that affect permeability of cell membranes?
Temperature
Concentration of solvents
How is beetroot used to measure permeability of cell membranes?
Higher permeability, more red pigment leaks into solution.
Colorimeter used to determine absorbance
What is the process of investigating the effect of temperature on permeability of cell membrane?
- Cut beetroot into 6 identical cubes
- Place each cube in different test tube with equal volume of distilled water
- Place each test tube into water baths at various temperatures
- Filter each solution into cuvette and measure absorbance using colorimeter
What is the effect on temperature on membrane permeability?
Increasing temperature results in increase membrane permeability
What is the effect of ethanol concentration on membrane permeability?
Increasing ethanol concentration leads to increased membrane permeability
Why would placing potato in ethanol stop osmosis?
Ethanol dissolved the phospholipids
Why would boiling a potato piece stop osmosis?
Boiling disrupts membrane by denaturing the proteins
What is the sequence of events following the production of extracellular proteins that leads to their release from the cell?
Moved to golgi apparatus where they’re processed
Into vesicles which fuse with membrane
Exocytosis
What are some of the roles of membranes within cells?
Compartmentalisation
Nuclear pore permits RNA to leave nucleus
Attachment of ribosomes
How can a carrier protein move glucose across a membrane by facilitated diffusion?
Glucose molecule attaches to carrier protein
Protein changes shape and is released on the opposite side
Describe a method a student could use to prepare colour standards without using a colorimeter?
Use known concentration of blueberry juice
Prepare dilution series
Use colour standards for comparisons
