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Cell-surface membrane
Surround cells; act as barrier between cell and environment
Controls which substances enter/leave cell
Partially permeable → let some molecules through but not others
Organelle membranes
Membranes around organelles divide cell into different compartments
Act as barrier between organelle and cytoplasm
Partially permeable - control what substances enter/leave organelle
Fluid mosaic structure
Phospholipid molecules form continuous, double layer (bilayer)
Bilayer is ‘fluid’ because phospholipids are constantly moving
Cholesterol molecules are in bilayer
Proteins scattered through bilayer, like tiles in mosaic
Contains some glycoproteins
Some lipids have polysaccharide chain attached - glycolipids
Proteins in cell membranes
Channel + carrier proteins, allowing large molecules/ions to pass through membrane
Receptor proteins on cell-surface membrane allow cell to detect chemicals released from other cells
Chemicals signal to cell to respond in some way
Some proteins can move sideways through bilayer, some are fixed in position
Some proteins have polysaccharide (carbohydrate) chain attached - glycoproteins
Phospholipids in cell membranes
Phospholipid molecules have ‘head’ and ‘tail’
Head is hydrophilic - attracts water
Tail is hydrophobic - repels water
Molecules automatically arrange themselves into bilayer
Heads face out towards water on either side of membrane
Centre of bilayer hydrophobic → membrane doesn’t allow water-soluble substances (like ions) through it - acts as barrier to these substances
Cholesterol in cell membranes
Cholesterol is type of lipid, present in all cell membranes (except in bacteria)
Cholesterol molecules fit between phospholipids
They bind to hydrophobic tails, causing them to pack more closely together
→ restricts movement of phospholipids, making membrane less fluid + more rigid
Cholesterol helps maintain shape of animal cells (no cell walls)
Important for cells that aren’t supported by other cells, e.g. red blood cells (float free in blood)
RP4 method: Effect of temperature on membrane permeability of beetroot
Use scalpel to cut five equal-sized pieces of beetroot. Rinse pieces to remove any pigment.
Add five pieces to five test tubes, each containing 5cm³ water (measured using measuring cylinder / pipette)
Place each tube in water bath at different temp, e.g. 10ᵒC-50ᵒC, for same length of time
Remove beetroot pieces from tubes, leaving coloured liquid
Use colorimeter - machine that measures absorbance of light
Higher absorbance = more pigment released = higher permeability of membrane
Effect of temperature on membrane permeability graph
Effect of temperature on membrane permeability (temp below 0ᵒC)
Phospholipids have little energy, so can’t move much
They’re packed closely together + membrane is rigid
Channel + carrier proteins in membrane deform, increasing permeability of membrane
Ice crystals may form and pierce membrane, making it highly permeable when it thaws
Effect of temperature on membrane permeability (temp between 0 and 45ᵒC)
Phospholipids can move around
They aren’t packed as tightly together - partially permeable
As temp increases, phospholipids move more because they have more energy → increases permeability of membrane
Effect of temperature on membrane permeability (temp above 45ᵒC)
Phospholipid bilayer starts to melt → membrane more permeable
Water in cell expands, putting pressure on membrane
Channel + carrier proteins deform, so can’t control what enters/leaves cell → increases permeability of membrane
Diffusion definition
Net movement of particles (molecules/ions) from area of higher conc. to area of lower conc.
Concentration gradient
Path from area of higher conc. to area of lower conc.
Particles diffuse down conc. gradient
Diffusion is a ____ process
passive
→ no energy required
Simple diffusion
When molecules diffuse directly through cell membrane
Why is facilitated diffusion needed?
Some larger molecules (e.g. amino acids, glucose) would diffuse very slowly through phospholipid bilayer because they’re big
Charged particles, e.g. ions + polar molecules, would diffuse slowly because they’re water soluble, and centre of bilayer is hydrophobic
Facilitated diffusion
To speed things up, large/charged particles diffuse through carrier/channel proteins in membrane
Particles move down conc. gradient
Facilitated diffusion is a ____ process
passive - no energy
How do carrier proteins work?
Move large molecules across membranes
Diff carrier proteins facilitate diffusion of diff molecules
Large molecule attaches to carrier protein in membrane
Protein changes shape
This releases molecule on opposite side of membrane
How do channel proteins work?
Form pores in membrane for charged particles to diffuse through
Diff channel proteins facilitate diffusion of diff charged particles
How does concentration gradient affect rate of diffusion?
Bigger gradient = faster diffusion
→ as diffusion occurs, difference in conc. between two sides of membrane decreases until it reaches equilibrium
→ diffusion slows down over time
How does thickness of exchange surface affect rate of diffusion?
Thinner exchange surface = faster diffusion
→ shorter distance for particles to travel
How does surface area affect rate of diffusion?
Larger SA = faster diffusion
Example of adaptation to increase rate of simple diffusion
Some cells (e.g. epithelial cells in small intestine) have microvilli - projections formed by cell-surface membrane folding up on itself
Microvilli give cell larger SA
Larger SA → more particles exchanged in same time → faster diffusion
How does number of carrier/channel proteins affect rate of facilitated diffusion?
Once all proteins are in use, facilitated diffusion can’t happen any faster
→ greater number of channel/carrier proteins = faster diffusion
Example of adaptation to increase rate of facilitated diffusion
Aquaporins are special channel proteins that allow facilitated diffusion of water through cell membranes
Some kidney cells are adapted to have lots of aquaporins
Aquaporins allow cells to reabsorb a lot of water that would otherwise be excreted by body
Osmosis definition
Diffusion of water molecules across partially permeable membrane, from area of higher water potential to area of lower water potential
Water potential
Potential (likelihood) of water molecules to diffuse out of or into a solution
Pure water has the ____ water potential
highest (0), all other solutions are negative
Isotonic
Two solutions with same water potential
RP3 method: Production of dilution series of a solute to identify water potential of plant tissue
Line up five test tubes in a rack
Add 10cm³ of 2M sucrose solution to first test tube and 5cm³ of distilled water to the other four test tubes
Using pipette, draw 5cm³ of solution from first tube, add it to distilled water in second tube and mix thoroughly
You now have 10cm³ of solution that’s half the concentration of solution in first test tube (1M)
Repeat process three more times to create solutions of 0.5M, 0.25M, 0.125M
RP3 calibration curve: Production of dilution series of a solute to identify water potential of plant tissue
Can use solutions to find water potential of potato cells
Use cork borer to cut potatoes into identically sized chips, around 1cm diameter
Divide chips into groups of three and measure mass of each group using mass balance
Place one group into each sucrose solution
Leave chips in solutions for 20 mins
Remove chips and dry with paper towel
Weigh each group again and record results
Calculate % change in mass for each group
Use results to make calibration curve, showing % change in mass against sucrose conc.
RP3 analysis: Production of dilution series of a solute to identify water potential of plant tissue
Potato chips gain water (and therefore mass) in solutions with higher water potential than the chips, and lose water in solutions with lower water potential
The point where the curve crosses x-axis (% change in mass is 0) is the point where water potential of sucrose solution is same as water potential of potato cells
→ find the conc. at this point, then look up the water potential for that conc. of sucrose solution in a textbook
How does active transport work?
Molecule attaches to carrier protein
Protein changes shape → moves molecule across membrane
Molecule released on other side
Active transport involves ____ proteins
carrier
Differences between active transport and facilitated diffusion
Active transport usually moves solutes from low to high conc.
Facilitated diffusion always moves them from high to low conc.
Active transport requires energy - facilitated diffusion doesn’t
How is ATP used in active transport?
ATP undergoes hydrolysis reaction, splitting into ADP + Pᵢ
→ releases energy so solutes can be transported
Active transport of calcium
Co-transporters
Type of carrier protein
Bind two molecules a time
Conc. gradient of one of the molecules is used to move other molecule against its own conc. gradient
Diagram shows sodium ions moving into cell down conc. gradient
→ this moves glucose into cell, against its conc. gradient
How does speed of individual carrier proteins affect rate of active transport?
The faster they work, the faster the rate of active transport
How does number of carrier proteins affect rate of active transport?
More proteins = faster rate of active transport
How does rate of respiration affect rate of active transport?
If respiration is inhibited, won’t be any ATP → active transport can’t take place
Why is co-transport needed in the ileum?
In ileum, glucose conc. is too low for glucose to diffuse out of blood
→ glucose is absorbed from lumen of ileum by co-transport
Co-transport of glucose in ileum
Sodium ions are actively transported out of ileum epithelial cells, into blood, by Na-K pump
→ creates conc. gradient - now there’s higher conc. of Na⁺ in lumen of ileum than inside cell
Causes Na⁺ to diffuse from lumen of ileum into epithelial cell, down conc. gradient, via sodium-glucose co-transporter proteins
Co-transporter carries glucose into cell with sodium
→ glucose conc. inside cell increases
Glucose diffuses out of cell, into blood, down conc. gradient, through protein channel by facilitated diffusion
