Transport across cell membranes
The fluid-mosaic model of membranes
Cell-surface membranes and the membranes surrounding eukaryotic organelles share a basic structure: a double layer of phospholipids containing proteins. The fluid-mosaic model describes how these components are arranged and how some of them move.
The phospholipid bilayer
Each phospholipid has a hydrophilic head, which interacts with water, and hydrophobic tails, which avoid water. In a membrane, phospholipids form a bilayer: the heads face the watery environments on either side, while the tails point inwards, away from the water.
Proteins and carbohydrates in the membrane
Some membrane proteins extend across the whole bilayer. Others are associated with just one of its surfaces. The different proteins within the bilayer give the membrane its mosaic appearance.
Some proteins have carbohydrate chains attached and are called glycoproteins. Some lipids also have carbohydrate chains attached and are called glycolipids. In both cases, the carbohydrate chains project from the membrane surface.
Membrane fluidity
Phospholipids and some proteins can move sideways within the membrane. This lateral movement makes the membrane fluid: its components are not all fixed in place.
Where present, cholesterol fits between phospholipids. It restricts the movement of other membrane molecules, thereby affecting how fluid the membrane is.
Simple diffusion across membranes
Particles can cross a cell membrane directly from a region where they are more concentrated to one where they are less concentrated. This movement is called simple diffusion. It occurs through the phospholipid bilayer without a transport protein or an input of ATP.
Movement down a concentration gradient
A difference in concentration between two regions is a concentration gradient. Particles move randomly in both directions across the membrane, but while the concentration is higher on one side, more particles cross from that side than in the opposite direction. The result is a net movement down the concentration gradient.
Net movement continues until the concentrations are equal. Particles still move randomly across the membrane in both directions, but there is no net movement because the movements balance.
Which substances cross the bilayer?
The interior of the phospholipid bilayer is hydrophobic, meaning that it does not interact favourably with water. This limits which substances can pass directly through it.
Small non-polar or lipid-soluble molecules pass through relatively readily. Charged particles and large polar molecules cannot readily cross the hydrophobic interior, so they do not diffuse through the bilayer readily.
Facilitated diffusion
Some substances cannot readily pass through the hydrophobic interior of the phospholipid bilayer. They can cross the cell membrane through specific membrane proteins instead.
Facilitated diffusion is the passive net movement of substances through membrane proteins, from a region of higher concentration to a region of lower concentration. The net movement is down the concentration gradient and does not require ATP.
Membrane proteins in facilitated diffusion
Channel proteins form hydrophilic pathways across the membrane. Particular ions or other particles can pass through a channel that allows them through.
Carrier proteins bind particular substances. The carrier then changes shape and releases the bound substance on the other side of the membrane.
Both channel and carrier proteins are selective: each allows only particular substances to cross. Although they provide different ways through the membrane, neither type moves substances against their concentration gradient by facilitated diffusion.
Osmosis and water potential
Water can move across a membrane that allows water molecules through but restricts the movement of some dissolved substances. If the water on one side has a higher water potential than the water on the other, there is a net movement of water towards the side with the lower water potential. This net movement is called osmosis, and the membrane is described as partially permeable.
Water potential
Water potential describes the tendency of water to move from one region to another. Water moves down a water potential gradient: from higher water potential to lower water potential.
Water potential is affected by solute concentration and pressure. Adding solute lowers water potential. At atmospheric pressure, pure water has a water potential of zero, while solutions have water potentials below zero. Increasing pressure raises water potential, so water movement cannot always be predicted from solute concentration alone.
Osmosis and equilibrium
Water molecules move across a partially permeable membrane in both directions. When water potential is higher on one side, more water moves from that side to the lower-potential side than moves in the reverse direction. The difference between these opposing movements is the net movement.
Net movement continues down the water potential gradient until equilibrium is reached. At equilibrium, water potentials are equal and water molecules still move in both directions, but there is no net movement of water.
Active transport
Active transport moves a substance across a membrane from a region where it is less concentrated to a region where it is more concentrated. This is movement against the concentration gradient. It requires energy because passive diffusion alone would produce net movement in the opposite direction.
Carrier proteins and energy
A carrier protein in the membrane binds a specific substance on one side. The carrier changes shape, transferring the substance across the membrane and releasing it on the other side.
Energy supplied by the hydrolysis of ATP enables this carrier-mediated process. It allows the substance to be moved towards the side where its concentration is already higher.
Sodium-glucose co-transport in the ileum
Cells lining the ileum absorb glucose from the intestinal lumen and pass it towards the blood. Glucose can enter these cells even when its concentration is higher inside the cell than in the lumen. This uptake is coupled to the movement of sodium ions.
Co-transport is the movement of two substances together through the same carrier protein. In the membrane facing the lumen, a sodium-glucose co-transporter allows sodium ions ($$\ce{Na+}$$) and glucose to enter the cell together. Sodium moves down its electrochemical gradient: the combined difference in sodium concentration and electrical charge favours its entry. This movement provides the driving force for glucose to enter against its concentration gradient.
Maintaining the sodium gradient
Sodium-potassium pumps in the membrane facing the blood use energy from ATP hydrolysis to move sodium ions out of the cell. This keeps the sodium concentration inside the cell low, helping to maintain the gradient that drives sodium into the cell from the lumen.
ATP therefore supports glucose uptake indirectly: the pump uses ATP to maintain the sodium gradient, while the co-transporter uses that gradient to drive glucose uptake. The co-transporter does not use ATP directly.
Glucose movement towards the blood
Once inside the cell, glucose can leave through carrier proteins in the membrane facing the blood. It moves down its concentration gradient by facilitated diffusion, passing from the cell towards the blood.
Factors affecting membrane transport rate
The rate at which a substance crosses a membrane depends on how much membrane is available, whether the substance needs a suitable transport protein, and the size of the gradient driving its movement. Increasing one factor raises the rate only if another factor does not limit it.
Membrane surface area
A larger surface area provides more membrane through which substances can cross. This can increase the rate of transport, provided the other conditions needed for transport are also present.
Some specialised cells have folded external or internal membranes. The folds increase the membrane surface area available for transport without requiring the cell to become much larger.
Gradients and passive movement
A concentration gradient is a difference in the concentration of a substance between two regions. During diffusion, particles move in both directions, but there is a net movement from higher to lower concentration. A steeper concentration gradient produces a greater net diffusion rate. As the concentration difference decreases, net diffusion slows.
In osmosis, water moves across a partially permeable membrane from higher to lower water potential. A steeper water potential gradient produces a greater net rate of osmosis. As the difference in water potential decreases, the net movement of water slows.
Channel and carrier proteins
Some substances cross membranes through specific channel proteins or by binding to specific carrier proteins. Having more suitable channels or carriers can increase the rate of protein-mediated transport, provided other factors, such as the driving gradient, are not limiting. Specialised cells may contain more of the proteins needed to transport particular substances.
Carrier-mediated transport is limited by both the number of available carriers and their turnover: how quickly each carrier completes a transport cycle. If all suitable carriers are already working at their maximum rate, increasing the supply of the transported substance cannot make those carriers transport it any faster.
Determining plant tissue water potential
Plant tissue gains mass when there is a net movement of water into its cells and loses mass when there is a net movement of water out. By placing comparable tissue pieces in solutions of different concentrations, we can estimate the solution in which there is no net movement of water. The water potential of that solution estimates the water potential of the tissue.
Preparing a dilution series
A dilution series is a set of solutions with known solute concentrations. Prepare it by mixing measured volumes of a stock solution with water. Use different proportions of stock solution and water while keeping the final volume the same.
For example, a solution made from $$2\ \text{cm}^3$$ of a $$1.0\ \text{mol dm}^{-3}$$ stock solution and $$8\ \text{cm}^3$$ of water has a concentration of $$0.20\ \text{mol dm}^{-3}$$. Other measured proportions give the remaining concentrations in the series.
Measuring changes in tissue mass
Cut comparable pieces of plant tissue. Blot each piece consistently and measure its initial mass, then place it in a solution. Use several pieces at each concentration as repeats. After the same immersion time, remove each piece, blot it consistently to remove surface solution, and measure its final mass.
Calculate the percentage change in mass for each piece:
A positive value indicates a gain in mass; a negative value indicates a loss. Calculate the mean percentage change for the repeats at each concentration. Repeats make the mean more reliable than a measurement from a single piece.
Keep tissue size, solution volume, temperature and immersion time controlled because differences in these conditions can affect the measured mass changes. Consistent blotting also matters: solution left on the surface would add to the measured mass.
Finding the tissue water potential
Plot the mean percentage change in mass on the vertical axis against solute concentration on the horizontal axis. Draw a calibration curve through the results and identify the concentration where it crosses zero percentage change in mass.
At this concentration, there is no net movement of water into or out of the tissue. Obtain the water potential corresponding to that solution concentration under the relevant conditions. This value estimates the water potential of the plant tissue.
Investigating cell membrane permeability
Cell membranes control the movement of substances into and out of cells. Their permeability can be investigated by changing one factor and measuring how much of a substance escapes from the cells.
Beetroot tissue contains a coloured pigment. If the membranes become more permeable, more pigment can escape from the tissue into the surrounding water. The amount of pigment in the water can therefore be used as a measure of leakage.
Testing the effect of temperature
In this investigation, temperature is the independent variable: it is the factor deliberately changed. Pigment release is the dependent measurement: it is measured to see how it changes with temperature.
- Cut comparable pieces of beetroot tissue. Rinse them to remove pigment released from cells damaged by cutting.
- Place the pieces in equal volumes of water at different controlled temperatures. Keep each sample at its chosen temperature for the same length of time.
- Remove the beetroot pieces and measure the absorbance of each solution using a colorimeter. Use water to set the colorimeter to zero and use the same colorimeter settings for all samples.
- Repeat the measurements at each temperature and compare the results.
Keep tissue size, water volume, treatment time and other conditions consistent. This makes temperature the main difference between treatments, so differences in pigment release can be compared fairly. Repeats help show whether the measurements are consistent.
Interpreting pigment leakage
A solution containing more beetroot pigment absorbs more light. A greater absorbance therefore indicates more pigment in the solution and greater leakage from the tissue. If absorbance increases with temperature, this indicates that permeability has increased.
The result is an indirect measure: pigment passes through the relevant cell membranes as it escapes from the tissue. The investigation does not directly observe the permeability of one membrane alone.