Transport across cell membranes
AQA
A-Level
Every membrane in a cell is built to the same basic plan: a phospholipid bilayer about 7 nanometres thick with proteins embedded in it. The reason lies in the phospholipid molecule itself, which has a hydrophilic phosphate head and two hydrophobic fatty acid tails. In an aqueous environment the molecules arrange themselves automatically so that the heads face the water on each side of the membrane while the tails point inwards away from it, producing a stable double layer. Because that arrangement follows from the molecule, it is the same whether you are looking at the cell-surface membrane, the double membrane of the nuclear envelope, mitochondrion or chloroplast, the endoplasmic reticulum, the Golgi apparatus, a lysosome or a vesicle. Membranes differ in the proteins and lipids they contain, which gives each membrane its particular functions.
32
Objectives
20
Exam Tips
35
Pitfalls
38
Key Terms
40
Mark Points
Subtopics in this area
The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same.
The arrangement and any movement of phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure.
Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane.
Movement across membranes occurs by: simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer) facilitated diffusion (involving the roles of carrier proteins and channel proteins) osmosis (explained in terms of water potential) active transport (involving the role of carrier proteins and the importance of the hydrolysis of ATP) co-transport (illustrated by the absorption of sodium ions and glucose by cells lining the mammalian ileum).
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.
Students should be able to: explain the adaptations of specialised cells in relation to the rate of transport across their internal and external membranes explain how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes.
Required practical 3: Production of a dilution series of a solute to produce a calibration curve with which to identify the water potential of plant tissue.
Required practical 4: Investigation into the effect of a named variable on the permeability of cell-surface membranes.
Transport across cell membranes Revision Guide
Learning Objectives
What you need to know and understand
- Explain why phospholipids form a bilayer when surrounded by water, referring to the two ends of the molecule.
- State what all cell membranes have in common and identify what makes the inner mitochondrial membrane different.
- Name four membranes inside a eukaryotic cell that share the same basic structure.
- Explain how the shared structure of membranes allows a vesicle to fuse with the cell-surface membrane.
- Explain what the words fluid and mosaic each describe in the fluid-mosaic model.
- Describe the arrangement and movement of phospholipids, proteins, glycoproteins and glycolipids.
- Explain how the hydrophobic and hydrophilic amino acids of a protein determine its position in the bilayer.
- State where glycoproteins and glycolipids are found in the membrane and give their functions.
- Describe where cholesterol sits in the bilayer and explain how it restricts phospholipid movement.
- Explain how cholesterol regulates membrane fluidity at different temperatures.
- Assess the importance of cholesterol for the mechanical stability of animal cells.
- Predict how the fluidity of a membrane would change if its cholesterol were removed.
- Explain why oxygen crosses a membrane by simple diffusion but a sodium ion cannot.
- Describe the roles of channel proteins and carrier proteins in facilitated diffusion.
- Describe the sequence by which glucose is absorbed from the ileum by co-transport with sodium ions.
- Explain, in terms of water potential, the direction in which water moves between a cell and its surrounding solution.
- Explain how microvilli increase the rate at which glucose is absorbed in the ileum.
- Name an internal membrane that is folded to increase surface area and state what the extra area carries.
- Explain how increasing the number of carrier proteins increases the rate of active transport.
- Suggest why damage to the epithelium of the ileum leads to water remaining in the lumen.
- Explain how surface area, number of transport proteins and gradient steepness each affect the rate of movement across a membrane.
- Describe adaptations of both internal and external membranes that increase the rate of transport.
- Explain why the rate of facilitated diffusion levels off as the external concentration keeps rising.
- Explain how a cell's blood supply helps maintain a steep concentration gradient for absorption.
- Calculate the volumes of stock solution and distilled water needed to produce each concentration in a dilution series of equal final volume.
- Explain why percentage change in mass, rather than change in mass, must be plotted.
- Determine the water potential of a plant tissue from the point where a calibration curve crosses zero change in mass.
- Describe an experiment to decide whether a named plant tissue has a lower water potential than a given solution.
- Explain why beetroot cylinders must be washed before the investigation begins.
- Explain how increasing temperature increases membrane permeability, distinguishing the effect on phospholipids from the effect on proteins.
- Describe how a colorimeter is set up and used to compare pigment leakage between samples.
- Identify the controlled variables in a membrane permeability investigation and explain why each matters.
Marking Points
Key points examiners look for in your answers
- one mark for every membrane consisting of a phospholipid bilayer with proteins embedded in it
- one mark for explaining the bilayer from hydrophilic phosphate heads facing the aqueous surroundings and hydrophobic fatty acid tails facing inwards
- one mark for naming organelle membranes built on the same plan, such as the nuclear envelope, mitochondrial or chloroplast membranes, endoplasmic reticulum, Golgi apparatus, lysosomes and vesicles
- one mark for the difference between membranes lying in the proteins and lipids present rather than in the basic bilayer structure
- one mark for the shared structure allowing membranes to fuse, for example when vesicles join the cell-surface membrane
- one mark for phospholipids, proteins, glycoproteins and glycolipids being able to move laterally, making the membrane fluid
- one mark for proteins scattered through the bilayer in a mosaic, with some spanning it and some on one surface
- one mark for hydrophobic regions of an intrinsic protein lying next to the fatty acid tails
- one mark for hydrophilic regions forming a channel for ions or polar molecules
- one mark for glycoproteins and glycolipids projecting from the outer surface for cell recognition
- Cholesterol is located between the phospholipids in the bilayer, binding to their fatty acid tails
- It restricts the movement of phospholipids and other molecules making up the membrane
- This regulates fluidity, preventing the membrane from becoming too fluid at high temperatures
- It provides mechanical stability to membranes, especially in animal cells lacking a cell wall
- Simple diffusion is passive net movement down a concentration gradient, restricted by the bilayer to small non-polar molecules
- Facilitated diffusion uses channel proteins or carrier proteins to move substances down a gradient without ATP
- Osmosis is the movement of water across a partially permeable membrane from a higher to a lower water potential
- Active transport requires carrier proteins and the hydrolysis of ATP to move substances against a concentration gradient
- Co-transport in the ileum involves sodium ions moving down their gradient, driving the absorption of glucose against its gradient via a co-transporter
- one mark for an increased surface area of membrane, produced by microvilli or by folding such as cristae
- one mark for an increased number of channel or carrier proteins in the membrane
- one mark for linking either adaptation to a faster rate of movement across the membrane
- one mark for the converse: a reduced surface area, or fewer co-transport, carrier or channel proteins, reduces the rate of absorption
- one mark for relating reduced absorption to a change in water potential and so to water moving by osmosis in the opposite direction
- A greater surface area of internal or external membranes increases the rate of transport, because more membrane is available for movement.
- More channel proteins increase the rate of facilitated diffusion, and more carrier proteins increase the rate of facilitated diffusion or active transport.
- The rate plateaus once all carrier proteins are saturated, because they are occupied and cannot work faster.
- A steeper concentration or water potential gradient increases the rate of net movement.
- Folded internal membranes, such as cristae, increase membrane surface area; their main role is to support the electron transport chain and ATP synthase, so use them as a transport example only with that qualification.
- one mark for a correctly calculated dilution series in which each tube has the same final volume
- one mark for recording the initial and final mass of each cylinder while controlling time, temperature and volume of solution
- one mark for blotting the surface dry before reweighing so that surface liquid is not included in the mass
- one mark for calculating percentage change in mass rather than change in mass, so cylinders of different starting mass can be compared
- one mark for identifying the concentration at which the percentage change is zero as the point where the external water potential equals that of the tissue
- one mark for controlling cylinder diameter by cutting with a cork borer, so that diameter is a controlled variable rather than a measured dependent variable
- one mark for washing the cut cylinders to remove pigment released by cells damaged during cutting
- one mark for controlling the volume of solution, the exposure time, and the size and number of beetroot pieces
- one mark for using a colorimeter calibrated with distilled water, with a higher absorbance meaning more pigment and so greater permeability
- one mark for explaining increased permeability above about 45 degrees Celsius by denaturation of membrane proteins, with bonds broken and tertiary structure changed
- one mark for higher temperature also increasing phospholipid movement so that gaps appear in the bilayer
Examiner Tips
Expert advice for maximising your marks
- 💡Explain the bilayer from the properties of the phospholipid; simply stating that there are two layers is a description, not an explanation.
- 💡Name a specific organelle membrane when asked for an example; organelles is too vague.
- 💡Remember the number: about 7 nanometres, which is why membranes cannot be resolved with a light microscope.
- 💡Explain fluid and mosaic separately; each word usually carries its own mark.
- 💡When describing intrinsic proteins, mention both hydrophobic regions facing tails and hydrophilic regions lining channels.
- 💡Use the phrase 'restricts movement of molecules' when describing cholesterol's function in the membrane.
- 💡Link cholesterol to its consequence: restricted movement regulates fluidity and provides stability.
- 💡Specify the protein involved: channel or carrier for facilitated diffusion, but only carrier for active transport and co-transport.
- 💡For osmosis, always phrase the gradient in water potential terms and include 'partially permeable membrane'.
- 💡Match the adaptation to the transport process named in the stem; proteins matter for facilitated diffusion, active transport and co-transport, surface area for all of them.
- 💡Internal membranes count too, so cristae and grana are valid answers when the question says internal.
- 💡When damage to a gut lining is described, follow the chain through: fewer proteins or less surface area, then water potential, then osmosis.
- 💡If a graph plateaus for carrier-mediated transport, name the limiting factor: all carrier proteins are occupied.
- 💡Check what the stem has already ruled out, since questions often say 'other than surface area'.
- 💡State the variables you controlled, not just the one you changed; time, temperature, volume of solution and size of tissue all earn credit.
- 💡An increase in mass or length shows that water has entered by osmosis, so the cells had the lower water potential; write that reasoning out rather than assuming it.
- 💡The same method appears in unfamiliar contexts such as root tissue in seawater, so learn the steps as a transferable procedure.
- 💡Say which quantity your colorimeter measures and in which direction it changes, because examiners expect the link from reading to permeability to be explicit.
- 💡Give a full explanation of temperature effects in two parts: phospholipid movement below the denaturing point, protein denaturation above it.
- 💡List controlled variables specifically, naming volume, time and tissue size, rather than writing keep everything else the same.
Common Mistakes
Pitfalls to avoid in your exam answers
- describing organelle membranes as structurally different from the cell-surface membrane
- putting the fatty acid tails on the outside facing the aqueous surroundings and the heads inwards
- writing phospholipid layer rather than phospholipid bilayer
- calling the membrane a cell wall, or claiming an animal cell has both
- saying membranes differ only in their proteins, when lipid composition (cholesterol, glycolipids, phospholipid types) also differs
- placing glycoproteins and glycolipids on both faces of the membrane, or on the inner face
- describing proteins as floating on top of the bilayer rather than embedded in or spanning it
- stating all spanning proteins are transport proteins, ignoring that many are integral receptors
- saying the membrane is fluid because liquid passes through it
- Stating cholesterol makes the membrane stronger without explaining that it restricts molecular movement
- Placing cholesterol on the surface of the membrane rather than between the phospholipid tails
- Confusing membrane cholesterol with blood cholesterol and discussing atheroma and heart disease
- Describing a water potential as higher when it is more negative; pure water is 0 kPa and solutions are negative
- Stating that facilitated diffusion uses ATP, or that active transport can use channel proteins
- Describing glucose entering ileum cells directly by active transport, rather than by co-transport with sodium ions
- writing villi when the cell-level adaptation required is microvilli
- offering more mitochondria as the answer when the question specifies surface area or number of proteins
- saying more channel proteins speed up simple diffusion, which does not use proteins at all
- claiming that increasing surface area makes the concentration gradient steeper
- describing the adaptation without saying what it achieves, so the rate of transport is never mentioned
- Answering a contrast question with facts about only one process. Correction: both sides of the difference are needed.
- Saying the rate rises indefinitely with concentration in facilitated diffusion. Correction: it plateaus once carrier proteins are saturated.
- Describing an adaptation without connecting it to a rate. Correction: state the mechanism by which the adaptation changes the rate.
- Ignoring internal membranes when asked about cellular adaptations for transport. Correction: include cristae or other folded internal membranes, qualified by their main role.
- Stating that channel proteins carry out active transport. Correction: active transport requires carrier proteins (pumps); channel proteins only allow facilitated diffusion.
- plotting change in mass rather than percentage change, so cylinders of different starting sizes cannot be compared
- recording size as the dependent variable, which is often rejected; mass or length are required, while diameter is controlled by the cork borer
- assuming a gain in mass means the solution had the lower water potential, when it means the solution had the higher, less negative water potential
- quoting a water potential as a positive value, when the water potential of any solution is negative
- cutting cylinders of different sizes, or leaving them in the solutions for different times
- saying the membrane melts, or that the phospholipids denature; only proteins denature
- omitting the washing step, so pigment from cutting damage is counted as leakage caused by the variable
- reporting absorbance and transmission the wrong way round, so greater leakage appears as a lower reading
- leaving the beetroot in the tube when taking the colorimeter reading
- describing ethanol as denaturing the bilayer rather than dissolving the lipid out of it