Difficulty: Introductory | Prerequisites: Basic chemistry concepts (polarity, hydrogen bonds, hydrophobic/hydrophilic interactions). Familiarity with Chapter 6 cell structure is helpful but not strictly required.
Every cell is wrapped in a plasma membrane that decides what gets in, what stays out, and how the cell communicates with its neighbours. This chapter covers what the membrane is made of and why its physical properties matter. You need to understand polarity and basic lipid chemistry from earlier chapters. If you can picture why oil and water do not mix, you already have the central intuition for how the phospholipid bilayer assembles itself.
The plasma membrane is a phospholipid bilayer studded with proteins, cholesterol, and carbohydrates. It behaves as a fluid mosaic, meaning its components move laterally and are unevenly distributed. Membrane fluidity depends on temperature, cholesterol content, and the ratio of saturated to unsaturated fatty acid tails. Different types of membrane proteins handle transport, signalling, enzymatic activity, and cell-cell recognition.
Phospholipid bilayer
Two back-to-back layers of phospholipids that form the core structure of every cell membrane. Hydrophilic heads face the watery environments on both sides; hydrophobic tails face inward, away from water. Think of it as a sandwich where the bread likes water and the filling hates it.
Fluid mosaic model
The accepted description of membrane structure: a fluid layer of lipids with proteins, cholesterol, and carbohydrates embedded in or attached to it, all capable of lateral movement. In simple terms, the membrane is not a rigid wall; it is more like a crowded dance floor where everything drifts around.
Selective permeability (semi-permeability)
The membrane's ability to allow some substances through while blocking others, based on size, polarity, and charge. In simple terms, the membrane is a bouncer that lets some molecules pass freely and turns others away.
Hydrophilic
Water-loving. Refers to polar or charged molecules and regions, such as the phospholipid head group. These dissolve in or are attracted to water.
Hydrophobic
Water-fearing. Refers to nonpolar molecules and regions, such as the fatty acid tails of phospholipids. These are repelled by water and cluster together to avoid it.
Cholesterol
A steroid molecule wedged between phospholipids in animal cell membranes. It acts as a fluidity buffer: at warm temperatures it restrains phospholipid movement, and at cool temperatures it prevents the membrane from packing too tightly and solidifying.
Integral protein
A protein partially or wholly embedded in the lipid bilayer. In simple terms, it sits inside the membrane rather than resting on the surface.
Transmembrane protein
An integral protein that spans the entire bilayer, protruding on both sides. These are the workhorses of membrane transport.
Peripheral protein
A protein attached to the membrane surface (inner or outer) but not embedded in the bilayer. It is anchored by interactions with other membrane components.
Glycoprotein
A protein with short carbohydrate chains covalently bonded to it, found on the extracellular face of the membrane. Important for cell-cell recognition.
Glycolipid
A lipid with a short carbohydrate chain attached, also found on the extracellular surface. Works alongside glycoproteins in cell recognition.
Unsaturated fatty acid
A fatty acid tail containing one or more double bonds, which introduce kinks. The kinks prevent tight packing and increase membrane fluidity.
Saturated fatty acid
A fatty acid tail with no double bonds. Straight chains pack tightly together, making the membrane more viscous and less fluid.
Prokaryotes have a nucleoid region (folded chromosomes, no membrane-bound nucleus).
Eukaryotes have a true nucleus with a nucleolus and chromatin, plus membrane-bound organelles.
Both cell types are bounded by a plasma membrane with the same fundamental bilayer structure.
The bilayer is composed of phospholipids, cholesterol, membrane carbohydrates (glycoproteins, glycolipids), and membrane proteins.
Each phospholipid has a hydrophilic head (glycerol linked to a phosphorus-containing group, attracted to water) and two hydrophobic tails (carbon-hydrogen chains, repelled by water).
Heads face the aqueous extracellular fluid and intracellular fluid; tails face each other in the membrane interior.
The bilayer is held together mainly by weak hydrophobic interactions, not covalent bonds. This is why the membrane is flexible rather than rigid.
The membrane is described as a fluid mosaic of lipids and proteins.
Components are not fixed in place; they move laterally within each leaflet of the bilayer.
At physiological temperature, the membrane has roughly the consistency of salad oil.
Different cell types have different membrane compositions and therefore different fluidity.
Temperature: higher temperatures increase fluidity; lower temperatures decrease it.
Unsaturated vs. saturated fatty acid tails:
Unsaturated tails have kinks (from double bonds) that prevent tight packing, keeping the membrane more fluid.
Saturated tails are straight, pack tightly, and make the membrane more viscous.
Cholesterol acts as a fluidity buffer in animal cells:
At warm temperatures, cholesterol restrains phospholipid movement (reduces fluidity).
At cool temperatures, cholesterol prevents tight packing (maintains fluidity and prevents solidification).
The ratio of saturated to unsaturated fatty acids varies between cell types and even between the inner and outer leaflet of the same membrane.
Different species of phospholipid give different properties to the bilayer.
Cells can vary their phospholipid composition to tune membrane function and fluidity.
Different phospholipid types lead to different functions, which lead to different fluidity profiles.
Membranes have directionality: they are built in a specific order.
The distribution of proteins, lipids, and carbohydrates is established when the membrane is assembled in the endoplasmic reticulum (ER) and Golgi apparatus.
The two leaflets of the bilayer are not identical in composition.
Peripheral proteins: not embedded in the bilayer; anchored to the surface by interactions with other membrane components.
Integral proteins: partially or wholly embedded in the bilayer.
Transmembrane proteins: a subset of integral proteins that span the entire bilayer. These handle most transport work.
Transport: moving substances across the membrane (channels, carriers, pumps).
Enzymatic activity: catalysing reactions at the membrane surface.
Signal transduction: relaying chemical messages from outside to inside the cell (receptor binds a signalling molecule, triggers an intracellular response).
Cell-cell recognition: glycoproteins on the surface allow cells to identify one another. Example: HIV can only infect cells carrying the CCR5 co-receptor.
Intercellular joining: proteins link adjacent cells together.
Attachment to the cytoskeleton and extracellular matrix (ECM): anchoring the membrane to internal and external structural elements.
Carbohydrates may be covalently bonded to lipids (glycolipids) or, more commonly, to proteins (glycoproteins).
Found exclusively on the extracellular face of the membrane.
Critical for cell-cell recognition, surface signalling, and immune function.
Students often think cholesterol is always bad for membranes. It is not. Cholesterol is essential for maintaining proper fluidity across a range of temperatures.
Students sometimes assume the two leaflets of the bilayer are identical. They are not. The inner and outer leaflets differ in phospholipid composition, and carbohydrates appear only on the extracellular face.
Students frequently confuse integral proteins with transmembrane proteins. All transmembrane proteins are integral, but not all integral proteins span the entire membrane. An integral protein may be only partially embedded.
Students often think the membrane is a static, rigid barrier. The fluid mosaic model emphasises that components move laterally, and the membrane behaves more like a viscous liquid than a solid wall.
⚠️ Know the fluid mosaic model and be able to explain what "fluid" and "mosaic" each refer to.
⚠️ Be able to describe cholesterol's dual role at warm vs. cool temperatures.
⚠️ Know the difference between peripheral, integral, and transmembrane proteins.
⚠️ Be able to list all six functions of membrane proteins (transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, cytoskeleton/ECM attachment).
⚠️ Understand why unsaturated fatty acid tails increase fluidity (kinks prevent packing).
True or False: Cholesterol always decreases membrane fluidity. (False: it decreases fluidity at warm temps but prevents solidification at cool temps.)
Fill in the blank: The membrane is described by the _____ _____ model. (Fluid mosaic.)
True or False: Saturated fatty acid tails have kinks that prevent tight packing. (False: unsaturated tails have the kinks.)
Fill in the blank: Proteins that span the entire bilayer are called _____ proteins. (Transmembrane.)
True or False: Glycoproteins are found on the intracellular face of the membrane. (False: they are found on the extracellular face.)
Q: What are the four main components of the plasma membrane?
A: Phospholipids, cholesterol, membrane proteins, and membrane carbohydrates (glycoproteins and glycolipids).
Q: How does cholesterol affect membrane fluidity at high temperatures vs. low temperatures?
A: At high temperatures, cholesterol restrains phospholipid movement, reducing fluidity. At low temperatures, it prevents tight packing of phospholipids, maintaining fluidity and hindering solidification.
Q: Explain the difference between peripheral, integral, and transmembrane proteins.
A: Peripheral proteins are not embedded in the bilayer; they are anchored to the surface. Integral proteins are partially or wholly embedded in the bilayer. Transmembrane proteins are a type of integral protein that extend all the way through both leaflets and are most involved in transport.
Q: Why do unsaturated fatty acids increase membrane fluidity?
A: Unsaturated fatty acids contain double bonds that create kinks in the hydrocarbon tails. These kinks prevent the phospholipids from packing tightly together, keeping the membrane more fluid.
Q: Name the six functions of membrane proteins.
A: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).
Q: Why is cell-cell recognition important? Give one example.
A: Cell-cell recognition allows cells to identify one another, which is critical for immune function and tissue organisation. Example: HIV can only infect cells that carry the CCR5 co-receptor on their surface.
This material connects directly to Chapter 7's transport content (passive transport, active transport, osmosis), which builds on the selective permeability introduced here. Understanding membrane protein types is essential for signal transduction (Chapter 11) and the immune system's recognition of self vs. non-self. The role of the ER and Golgi in membrane assembly ties back to the endomembrane system covered in Chapter 6.
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