3.1.4 Transport into and out of Cells Flashcards
What is a cell-surface membrane?
(Partially permeable) barrier between cell and its environment, controlling which substances enter and leave the cell
What is the functions of membranes around organelles?
- Act as barriers between organelle and cytoplasm
- Also control what comes in and out + partially permeable
Describe the phospholipids arrangement in a plasma membrane
- Bilayer
- Hydrophobic tails to inside
- Hydrophilic heads to outside
Describe how the plasma membrane acts as a barrier to water-souble substances
Centre of bilayer is hydrophobic ∴ membrane doesn’t allow water-souble substances (like ions) through it
Name 3 functions of phospholipids in plasma membranes
- Allow lipid-soluble substances to enter & leave the cell
- Prevent water-soluble substances entering and leaving the cell
- Makes membrane flexible
Cholesterol is present in all cell membranes expect _____ ____ ________
bacterial cell membranes
Where are cholesterol molecules found in cell membranes?
Between phospholipids
What is the role of cholesterol in cell membranes?
Controls the fluidity of the membrane
(provides stability e.g. helps to maintain shape of animal cells)
Describe how cholesterol gives cell membranes stability
- Cholesterol molecules bind to hydrophobic tails of phospholipids = causes them to pack more closely together
- Restricts movement of phospholipids
- Makes membrane less fluid and more rigid
What model is used to describe the arrangement of molecules in membrane?
Fluid mosaic model
Why is the bilayer is ‘fluid’?
∵ phospholipids are constantly moving
How are proteins arranged in the cell membrane?
They’re scattered through bilayer (like tiles in mosaic)
What are extrinsic proteins?
Proteins are partly embedded in bilayer (occur either on surface of it)
What are intrinsic proteins?
Proteins that span entire bilayer from one side to the other
What do channel proteins and carrier proteins allow?
Allow large molecules and ions to pass through membrane
What do receptor proteins on the cell-surface membrane do?
Detect chemicals released from other cells
(chemicals signal to cell to respond in some way)
Some proteins can ___ _______ through bilayer & others are _____ __ _____
Some proteins can move sideways through bilayer & others are fixed in position
What are glycoproteins?
Proteins that have a polysaccharide chain attached
What are glycolipids?
Lipids that have a polysaccharide chain attached
Increasing the temperature ____ membrane permeability
INCREASES
Temperatures below 0°C: Describe and explain the membrane permeability
- Phospholipids don’t have much energy = can’t move much
- Packed closely together & membrane is rigid
- BUT channel proteins and carrier proteins deform = increasing permeability of membrane
- Ice crystals may form and pierce the membrane = makes highly permeable when it thaws
Temperatures between 0 and 45°C: Describe and explain the membrane permeability
- Phospholipids can move around & aren’t packed as tightly together = membrane is partially permeable
- As temperature increases = phospholipids move more ∵ have more energy = increases permeability of membrane
Describe the effect of high temperature on cell membranes
- Phospholipids have more kinetic energy
- Movement makes membranes more fluid and leaky
- Increases no. of gaps between phospholipids
- Glycoproteins/channel & carrier proteins denature
All increases permeability of membrane as substances can enter and leave that wouldn’t normally
Sketch a graph showing how increasing the temperature affects membrane permeability
Surrounding cells in an increasing concentration of solvent (e.g. acetone) will _______ membrane permeability
INCREASE
Why does surrounding cells in an increasing concentration of solvent (e.g. acetone) increase membrane permeability?
∵ the solvent dissolves lipids in the cell membrane, causing it to lose its structure
What could you use to investigate how different conditions (e.g. temperature and solvent concentration) affects membrane permeability?
Beetroot → higher permeability of membrane = more pigment leaks out of cell
Define diffusion
Net movement of particles from an area of higher concentration to area of lower concentration
Diffusion is a ____ process
passive
Particles diffuse ___ a concentration gradient
DOWN
What type of particles can diffuse through cell membranes?
Small and non-polar particles
Polar molecules have… & non-polar molecules don’t
partial postive and negative charges
Why are small particles able to diffuse through the cell membrane?
∵ they can pass through spaces between phospholipids
Why are non-polar particles able to diffuse through the cell membrane?
∵ they can dissolve in the hydrophobic bilayer as being non-polar makes them soluble in lipids
What is simple diffusion? (in membrane terms)
When molecules diffuse directly through cell membrane
Why don’t large/charged particles diffuse through the phospholipid bilayer?
∵ they would diffuse extremely slowly as they’re big/water soluble (& centre of bilayer is hydrophobic)
Facilitated diffusion moves particles _____ a concentration gradient & is _____
DOWN … PASSIVE
What do carrier proteins do?
Move large molecules across membranes = down their concentration gradient
______ carrier/channel proteins facilitate the diffusion of different molecules
Different
Describe how carrier proteins move large molecules across membranes
- Large molecule attaches to carrier protein
- Protein changes shape
- This releases the molecule on the opposite side of membrane
What do channel proteins do?
Form pores in membrane for charged particles to diffuse through (down their concentration gradient)
What is facilitated diffusion?
When large/charged particles diffuse through carrier proteins or channel proteins down a concentration gradient
Name 3 factors that affect the rate of simple diffusion
- Concentration gradient
- (Higher it is = faster rate of diffusion)
- Thickness of exchange surface
- Surface area
What are microvilli?
Projections formed by cell-surface membrane folding up on itself
How do microvilli increase the rate of diffusion?
- Give cell a larger surface area
- ∴ more particles can be exchanged in same amount of time = increasing rate of diffusion
Name 2 factors that affect the rate of facilitated diffusion
- Concentration gradient
- Number of channel/carrier proteins
Describe how the number of channel/carrier proteins affects the rate of facilitated diffusion
- Once all proteins in membrane are in use = facilitated diffusion can’t happen any faster (even if concentration gradient is increased)
- Greater no. of carrier/channel proteins = faster rate of facilitated diffusion
Why can water diffuse directly through the membrane even though it’s polar?
∵ it’s relatively small
Define osmosis
Diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential
What is water potential?
Tendency of water molecules to diffuse out of or into a solution
What has the highest water potential?
Pure water
All solutions have ____ water potential than pure water
lower
If 2 solutions have same water potential, they are said to be ____
isotonic
Name 3 factors that affect the rate of osmosis
- Water potential gradient
- (Higher water potential gradient = faster rate of osmosis)
- Thickness of exchange surface
- Surface area of exchange surface
Red Blood Cell
Fill in the blanks
Plant Cell
Fill in the blanks
Describe what happens when a plant cell is placed in a very concentrated solution (i.e. how it becomes plasmolysed)
- Water moves out of cell by osmosis
- Cell has greater water potential
- Water moves down water potential gradient
Make a 15 cm3 solution of 0.4 M sucrose solution using 1 M sucrose solution
What technique do you use to make several solutions of different, known concentrations?
Serial dilution technique
Investigating water potential using serial dilutions
Describe how you would make 5 serial dilutions of sucrose solutions, starting with intial sucrose solutions of 2 M
Dilute each solution by factor of e.g. 2
- Line up 5 test tubes in rack
- Add 10 cm3 of intial 2 M sucrose solution to test 1st test tube & 5 cm3 of distilled water to other 4 test tubes
- Using pipette, draw 5 cm3 of solution from 1st test tube & add it to distilled water in 2nd test tube
- Mix solution throughly
- Now have 10 cm3 of solution that’s half concentrated as solution in 1st test tube (it’s 1 M)
- Repeat process 3 more times to create solutions of 0.5 M, 0.25 M and 0.125 M
Describe how you can use serial dilutions to find the water potential of potato cells
- Use cork borer to cut potatoes into 5 indentically size chips (1 cm in diameter)
- Blot potato chips with paper towel
- Weigh mass of each potato chip on mass balance & record these values
- Place each potato chip into each sucrose solution
- Leave chips in solution for 20 minutes
- Remove chips and gently pat them dry with paper towel
- Weigh each chip again and record these masses
- Calculate % change in mass for each chip
- Use results to make calibration curve, showing % change in mass against sucrose solution
Potato chips will gain water (& thus mass) in solutions with ____ _____ ______ than the chips
higher water potential
Describe how you would find the water potential of potato cells from a calibration curve
- Point at which curve crosses x-axis = point where water potential of sucrose solution = water potential of potato cells
- Find concentration at this point & then look up water potential for that concentration of sucrose solution
Define Active Transport
Movement of molecules/ions into or out of a cell from an area of lower concentration to an area of higher concentration using energy from ATP and carrier molecules
Describe how carrier proteins move molecules in active transport
- Molecules/ions bind to receptors on carrier protein
- On inside of cell: ATP binds to carrier protein = causes it to split into ADP and phosphate molecule
- Causes carrier protein molecule to change shape, releasing molecules/ions to other side of membrane
- Phosphate molecule is released from carrier protein & recombines with ADP to from ATP during respiration
- Causes carrier protein to revert to its original shape
Name 2 differences between active transport and facilitated diffusion
- Active transport = moves solutes from low to high concentration
- Active transport requires energy
Describe how ATP enables active transport (as well as other biological processes)
- Undergoes hydrolysis reaction = splits ADP and P¡ (inorganic phosphate)
- = releases energy so solutes can be transported
Co-transporters are type of ___ _____
carrier protein
Describe how co-transporters enable molecules to move in active transport
- Bind 2 molecules at a time
- Concentration gradient of one of the molecules = used to move other molecule against its own concentration gradient
Name 3 factors that affect the rate of active transport
- Speed of individual carrier proteins
- Number of carrier proteins
- Rate of respiration in cell & availability of ATP
Glucose is absorbed by _______ in mammalian ileum
Co-Transport
Why is glucose absorbed from lumen (middle) of ileum by co-transport?
∵ In ileum (final part of small intestine) = concentration of glucose is too low for glucose to diffuse into blood
Describe how glucose is absorbed from lumen (middle) of ileum by co-transport
- Sodium ions = actively transported out of ileum epithelial cells into blood by sodium-potassium pump
- = creates concentration gradient
- Higher conc. of Na+ ions in lumen than inside cell
- Causes sodium ions to diffuse into epithelial cell by facilitated diffusion (down their concentration gradient)
- Via sodium-glucose co-transporter proteins
- Glucose absorbed with Na+ ions against their concentration gradient
- = concentration of glucose inside cell increases
- Glucose diffuses out of cell into blood, down its concentration gradient through protein channel by facilitated diffusion
ATP has _ ____ ___
3 phosphate groups
(Bonds between phosphate groups = unstable ∴ low activation energy = easily broken)
Describe how ATP is synthesised to produce immediate source of energy
- Conversion of ATP to ADP = reversible reaction
- ∴ energy can be used to add an inorganic phosphate to ADP to form ATP
ATP cannot be stored ∴ it ______ has to be ___
continuously has to be made
Describe how ATP is used in metabolic processes
ATP provides energy needed to build macromolecules from their basic units (e.g. making starch from glucose)
Describe how ATP is used in active transport
Provides energy to change shape of carrier proteins
What is the function of a glycolipid?
Acts as a recognition site
Describe what the terms hydrophobic and hydrophilic mean and explain why this property of membranes helps their structure (4)
- Hydrophobic = water hating
- Hydrophilic = water loving
- Membrane is stable because heads always move towards the water
- And tails always move away from the water, filling any gaps/holes that may form
Name the roles of glycoproteins on the cell membrane (6)
- Acts as a receptor/binding site for hormones
- Can be used for recognition
- Cell adhesion
- Cell signalling
- Act as antigens
- Can act as a switch to trigger changes in membrane
- Attaches to water molecules
- Receptor to transfer proteins
List some of the molecules that require channel proteins to cross the membrane and explain why (3)
- Glucose – too large to pass across
- Ions like sodium and potassium as they’re not lipid soluble
- Any molecules moving against a concentration gradient that requires ATP for active transport
Define Cell Signalling
Communication between cells using various signals, particularly hormones
What is meant by recognition in plasma membranes?
Receptors can be used to target certain cells and to distinguish between self and non-self
What is meant by receptors in plasma membranes?
A glycoprotein molecule on the surface of a cell that triggers a response to environmental changes, hormones and other cells
Using serial dilutions to find the water potential of potato cells:
Before weighing, why do you blot dry the outside of each potato cylinder? (2)
- ∵ water will affect the mass
- Amount of water on cylinders varies /ensures same amount of water on outside
Using serial dilutions to find the water potential of potato cells:
Why do you calculate the % change in mass? (2)
- ∵ allows comparison
- cylinders may have different starting masses
Using serial dilutions to find the water potential of potato cells:
Why should you carry out several repeats at each concentration of sucrose solution? (2)
- Allows anomalies to be identified
- Makes the mean / line of best fit more reliable / allows concordant results
