Biology · Cell membranes and transport
This chapter explores the structure and function of cell membranes, focusing on the fluid mosaic model and the roles of its components. It details various transport mechanisms across membranes, including passive and active processes, and introduces cell signalling as a crucial communication method.
fluid mosaic model — The currently accepted model of membrane structure, proposed by Singer and Nicolson in 1972, in which protein molecules are free to move about in a fluid bilayer of phospholipid molecules.
This model describes the cell membrane as a dynamic structure where phospholipids form a fluid bilayer, allowing proteins to move laterally within it, creating a mosaic-like pattern. This fluidity is crucial for membrane function, including cell growth, movement, and signalling. Imagine a sea of olive oil (phospholipids) with icebergs (proteins) floating and moving within it, some anchored, others drifting freely.
Students often think the membrane is a rigid, static structure, but actually it is fluid and dynamic, allowing components to move.
When describing the fluid mosaic model, ensure you mention both 'fluid' (phospholipid and protein movement) and 'mosaic' (scattered protein pattern) aspects for full marks.
cholesterol — A small, lipid-related molecule with a hydrophilic head and a hydrophobic tail which is an essential constituent of membranes; it is particularly common in animal cells and gives flexibility and stability to the membrane as well as reducing fluidity.
Cholesterol molecules insert between phospholipids, reducing membrane fluidity at higher temperatures and preventing close packing at lower temperatures, thus maintaining optimal membrane consistency. Its hydrophobic regions also help prevent the passage of ions and polar molecules. Think of cholesterol as the 'temperature regulator' and 'stabilizer' of the membrane, like adding a thickener to a sauce to control its consistency.
Students often think cholesterol only makes membranes more rigid, but actually it also prevents phospholipids from packing too closely at low temperatures, maintaining fluidity.
Remember to state both the 'stability' and 'fluidity regulation' roles of cholesterol, especially its importance in animal cells and its absence in prokaryotes.
Cell surface membranes are composed of phospholipids, cholesterol, and proteins, along with glycolipids and glycoproteins. Phospholipids form the basic bilayer structure, providing a barrier. Cholesterol regulates membrane fluidity and stability. Proteins embedded within or associated with the membrane perform diverse functions, including transport, cell signalling, and enzymatic activity. Glycolipids and glycoproteins are involved in cell-to-cell recognition.
cell signalling — The molecular mechanisms by which cells detect and respond to external stimuli, including communication between cells.
Cell signalling involves a ligand binding to a specific receptor, triggering a cascade of events inside the cell, often involving second messengers, to bring about a cellular response. This process allows cells to coordinate activities and respond to their environment. It's like a secret message system: a messenger (ligand) delivers a coded note to a specific mailbox (receptor) on a house (cell), which then triggers a series of actions inside the house.
Students often think signalling only involves direct contact, but actually it frequently involves chemical messengers transported over distances.
ligand — A biological molecule which binds specifically to another molecule, such as a cell surface membrane receptor, during cell signalling.
Ligands are the 'messenger molecules' in cell signalling, initiating a response by binding to a complementary receptor protein. This binding causes a change in the receptor's shape, transmitting the signal into the cell. A ligand is like a specific key that fits into a particular lock (the receptor) to open a door (initiate a cellular response).
Emphasize the 'specificity' of ligand-receptor binding when explaining its role in cell signalling.
transduction — Occurs during cell signalling and is the process of converting a signal from one method of transmission to another.
After a ligand binds to a receptor, the external signal is converted into an intracellular message, often involving a series of molecular changes or the production of second messengers. This allows the signal to be relayed and amplified within the cell. It's like a translator converting a message from one language (external signal) into another (internal cellular signal) so the cell can understand and act on it.
When outlining cell signalling, ensure you include the stages: stimulus, ligand secretion, transport, binding to receptor, transduction, and cellular response, mentioning amplification.
Substances move across cell membranes through various mechanisms, categorized as passive or active. Passive transport, including diffusion, facilitated diffusion, and osmosis, does not require metabolic energy and moves substances down their concentration or water potential gradients. Active transport and bulk transport, however, require energy (ATP) to move substances against gradients or to transport large quantities of material.
diffusion — The net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of the random movements of particles.
Diffusion is a passive process driven by the kinetic energy of molecules, leading to an even distribution of substances over time. It is effective over short distances and is how small, non-polar molecules like oxygen and carbon dioxide cross cell membranes. Imagine opening a bottle of perfume in a room; the scent molecules will gradually spread out until they are evenly distributed throughout the room.
Always include 'net movement' and 'down a concentration gradient' in your definition of diffusion for accuracy.
facilitated diffusion — The diffusion of a substance through a transport protein (channel protein or carrier protein) in a cell membrane; the protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane, which would otherwise be less permeable to it.
This is a passive process that allows larger polar molecules and ions, which cannot easily cross the hydrophobic lipid bilayer, to move down their concentration gradient with the help of specific membrane proteins. It does not require metabolic energy. It's like having a special gate or a revolving door (transport protein) in a wall (membrane) that only allows specific people (molecules) to pass through, but they still move from a crowded side to a less crowded side.
Students often confuse facilitated diffusion with active transport, but actually facilitated diffusion is passive and moves substances down a concentration gradient, while active transport is active and moves against it.
channel protein — A membrane protein of fixed shape which has a water-filled pore through which selected hydrophilic ions or molecules can pass by facilitating diffusion or active transport.
Channel proteins provide a hydrophilic pathway for charged or polar substances to cross the membrane. Many are 'gated', meaning they can open or close to control the passage of ions, playing a crucial role in nerve impulses. Think of a channel protein as a tunnel through a mountain (the membrane) that only allows specific types of vehicles (ions/molecules) to pass through, sometimes with a gatekeeper controlling access.
carrier protein — A membrane protein which changes shape to allow the passage into or out of the cell of specific ions or molecules by facilitated diffusion or active transport.
Carrier proteins bind to specific molecules or ions and then undergo a conformational change to transport them across the membrane. They can be involved in both passive (facilitated diffusion) and active transport (pumps). Imagine a revolving door or a shuttle bus (carrier protein) that picks up a specific passenger (molecule) on one side of a building (membrane) and drops them off on the other side by changing its orientation.
Emphasize the 'change in shape' as a key characteristic distinguishing carrier proteins from channel proteins.
osmosis — The net diffusion of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Osmosis is a specific type of diffusion for water, crucial for maintaining cell volume and turgor in living organisms. It is a passive process that does not require metabolic energy. It's like water trying to 'dilute' a concentrated solution by moving across a barrier that only lets water through, until the concentration is equal on both sides.
Students often think osmosis is the movement of solute, but actually it is specifically the net movement of water molecules.
Always include 'net diffusion of water molecules', 'partially permeable membrane', and 'down a water potential gradient' in your definition of osmosis.
water potential — A measure of the tendency of water to move from one place to another; water moves from a solution with higher water potential to one with lower water potential; water potential is decreased by the addition of solute, and increased by the application of pressure; the symbol for water potential is ψ or ψw.
Water potential quantifies the 'free energy' of water, indicating its tendency to move. Pure water has the highest water potential (0 kPa), and adding solutes lowers it (makes it more negative). Pressure increases water potential. Think of water potential as the 'pressure' or 'desire' of water to move. Water always 'wants' to move from where it has more 'desire' (higher potential) to where it has less 'desire' (lower potential).
Students often confuse higher water potential with a more negative value, but actually a higher water potential means a less negative value (closer to 0 kPa).
Remember that pure water has a water potential of 0 kPa, and all solutions have negative water potentials. A less negative value indicates a higher water potential.
The effects of osmosis differ between animal and plant cells due to the presence of a cell wall in plants. Animal cells, lacking a cell wall, can swell and burst (lyse) in hypotonic solutions or shrink (crenate) in hypertonic solutions. Plant cells, however, become turgid in hypotonic solutions as the cell wall prevents bursting, and undergo plasmolysis in hypertonic solutions where the protoplast shrinks away from the cell wall.
protoplast — The living contents of a plant cell, including the cell surface membrane but excluding the cell wall.
The protoplast is the functional, living part of a plant cell that undergoes osmotic changes. In plasmolysis, it is the protoplast that shrinks away from the cell wall. If a plant cell is a house with a strong brick wall, the protoplast is everything inside the house, including the inner skin (cell surface membrane) and all its contents.
plasmolysis — The loss of water from a plant or prokaryote cell to the point where the protoplast shrinks away from the cell wall.
Plasmolysis occurs when a plant cell is placed in a solution with a lower water potential, causing water to leave the cell by osmosis. The shrinking protoplast pulls away from the rigid cell wall, leading to loss of turgor. Imagine a deflated balloon (protoplast) inside a rigid box (cell wall); as the balloon loses air, it pulls away from the box's sides.
Students often think plasmolysis means the cell bursts, but actually it means the protoplast shrinks away from the cell wall due to water loss.
incipient plasmolysis — The point at which plasmolysis is about to occur when a plant cell or a prokaryote cell is losing water; at this point the protoplast is exerting no pressure on the cell wall.
This is the critical stage where the cell has lost enough water that its protoplast is no longer pressing against the cell wall, but has not yet visibly pulled away. It represents the transition from turgid to plasmolysed. It's like the moment a balloon inside a box has just enough air that it's touching all sides, but if you let out even a tiny bit more air, it will start to pull away.
active transport — The movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.
Active transport allows cells to accumulate substances or remove waste products, maintaining specific internal concentrations different from the external environment. It is an energy-consuming process, typically powered by ATP hydrolysis. It's like pushing a ball uphill (against the concentration gradient) which requires energy, unlike letting it roll downhill (diffusion).
Key elements for active transport are 'against concentration gradient', 'uses carrier proteins (pumps)', and 'requires ATP/energy'.
sodium–potassium pump (Na+–K+ pump) — A membrane protein (or proteins) that moves sodium ions out of a cell and potassium ions into it, using ATP.
This specific carrier protein is vital in animal cells for maintaining ion gradients across the cell membrane, crucial for nerve impulse transmission and osmotic balance. It pumps three Na+ out and two K+ in for each ATP molecule used, creating a potential difference. Imagine a bouncer at a club who lets two specific people (K+) in for every three specific people (Na+) he pushes out, and this requires constant effort (ATP).
endocytosis — The bulk movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP.
This process allows cells to take in large molecules, particles, or even other cells that are too big to pass through membrane proteins. It involves the formation of a vesicle from the cell surface membrane and requires energy. It's like a cell 'eating' or 'drinking' by wrapping its membrane around a large item and pulling it inside in a bubble.
exocytosis — The bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is an active process requiring ATP.
Exocytosis is the reverse of endocytosis, used by cells to secrete substances like hormones or enzymes, or to remove waste products. Vesicles containing the material fuse with the cell surface membrane, releasing their contents outside. It's like a cell 'spitting out' or 'secreting' substances by having a bubble containing the material merge with its outer skin and release the contents.
phagocyte — A type of cell that ingests (eats) and destroys pathogens or damaged body cells by the process of phagocytosis; some phagocytes are white blood cells.
Phagocytes are specialized cells, part of the immune system, that protect the body by engulfing harmful foreign particles, bacteria, and dead or dying cells. This process is a form of endocytosis called phagocytosis. Think of phagocytes as the 'clean-up crew' or 'pac-men' of the body, constantly engulfing and digesting unwanted invaders or debris.
Be ready to compare transport mechanisms. Use a table to revise the differences between simple diffusion, facilitated diffusion, and active transport (Gradient? Protein needed? ATP needed? Specificity?).
Always link a larger surface area to volume ratio with a faster rate of diffusion in application questions.
fluid mosaic model
The currently accepted model of membrane structure, proposed by Singer and Nicolson in 1972, in which protein molecules are free to move about in a fluid bilayer of phospholipid molecules.
cholesterol
A small, lipid-related molecule with a hydrophilic head and a hydrophobic tail which is an essential constituent of membranes; it is particularly common in animal cells and gives flexibility and stability to the membrane as well as reducing fluidity.
cell signalling
The molecular mechanisms by which cells detect and respond to external stimuli, including communication between cells.
ligand
A biological molecule which binds specifically to another molecule, such as a cell surface membrane receptor, during cell signalling.
transduction
Occurs during cell signalling and is the process of converting a signal from one method of transmission to another.
diffusion
The net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of the random movements of particles.
facilitated diffusion
The diffusion of a substance through a transport protein (channel protein or carrier protein) in a cell membrane; the protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane, which would otherwise be less permeable to it.
channel protein
A membrane protein of fixed shape which has a water-filled pore through which selected hydrophilic ions or molecules can pass by facilitating diffusion or active transport.
carrier protein
A membrane protein which changes shape to allow the passage into or out of the cell of specific ions or molecules by facilitated diffusion or active transport.
osmosis
The net diffusion of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
water potential
A measure of the tendency of water to move from one place to another; water moves from a solution with higher water potential to one with lower water potential; water potential is decreased by the addition of solute, and increased by the application of pressure; the symbol for water potential is ψ or ψw.
protoplast
The living contents of a plant cell, including the cell surface membrane but excluding the cell wall.
plasmolysis
The loss of water from a plant or prokaryote cell to the point where the protoplast shrinks away from the cell wall.
incipient plasmolysis
The point at which plasmolysis is about to occur when a plant cell or a prokaryote cell is losing water; at this point the protoplast is exerting no pressure on the cell wall.
active transport
The movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.
sodium–potassium pump (Na+–K+ pump)
A membrane protein (or proteins) that moves sodium ions out of a cell and potassium ions into it, using ATP.
endocytosis
The bulk movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP.
exocytosis
The bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is an active process requiring ATP.
phagocyte
A type of cell that ingests (eats) and destroys pathogens or damaged body cells by the process of phagocytosis; some phagocytes are white blood cells.
| Command word | What examiners expect |
|---|---|
| Describe | For the fluid mosaic model, describe both the 'fluid' nature (phospholipid and protein movement) and 'mosaic' pattern (scattered proteins). For transport mechanisms, describe the direction of movement, energy requirement, and any proteins involved. |
| Explain | For osmosis, explain the movement of water in terms of water potential gradients and the role of the partially permeable membrane. For cholesterol, explain how it regulates fluidity at both high and low temperatures. For cell signalling, explain the sequence of events from ligand binding to cellular response. |
| Compare | When comparing transport mechanisms (e.g., facilitated diffusion vs. active transport), clearly state similarities and differences regarding concentration gradient, energy requirement, and protein involvement. |
| Illustrate | When illustrating the principle of surface area to volume ratios, explain how increasing size leads to a decrease in the ratio and its implications for transport efficiency. |
Mistake
Thinking the cell membrane is a rigid, static structure.
Correction
The cell membrane is fluid and dynamic, with components able to move laterally.
Mistake
Believing cholesterol only makes membranes more rigid.
Correction
Cholesterol also prevents phospholipids from packing too closely at low temperatures, maintaining fluidity.
Mistake
Confusing facilitated diffusion with active transport.
Correction
Facilitated diffusion is passive (no ATP) and moves substances down a concentration gradient, while active transport is active (uses ATP) and moves against it.
Mistake
Mistaking osmosis for the movement of solutes.
Correction
Osmosis is exclusively the net movement of WATER molecules.
Mistake
Thinking a 'higher' water potential is a more negative value.
Correction
A higher water potential is less negative (closer to 0 kPa). Pure water has the highest water potential (0 kPa).
Mistake
Believing plasmolysis means the cell bursts.
Correction
Plasmolysis means the protoplast shrinks away from the cell wall due to water loss, but the cell wall prevents bursting.