Source: Penn State BIO 101 lecture notes
Tags: cell membrane, plasma membrane, phospholipid bilayer, fluid-mosaic model, selective permeability, passive transport, active transport, facilitated diffusion, aquaporins, amphipathic
Difficulty: Introductory | Prerequisites: Basic understanding of macromolecules (lipids, proteins, carbohydrates) and chemical polarity (hydrophilic vs hydrophobic).
Cell membranes sit at the heart of every topic you will meet in introductory biology, from metabolism to cell signalling to genetics. They are the boundary that separates a living cell from everything around it, and they decide what gets in and what stays out. This topic builds directly on your earlier work with macromolecules (particularly lipids) and connects forward to cellular respiration, photosynthesis, and homeostasis. If you missed the section on phospholipids, go back to that first, because the membrane's structure only makes sense once you understand amphipathic molecules.
Cell membranes are made mainly of phospholipids arranged in a bilayer, with proteins and carbohydrates embedded throughout (the fluid-mosaic model). The bilayer is selectively permeable: small nonpolar molecules slip through on their own, but polar molecules and ions need transport proteins. Movement across the membrane is either passive (no energy, down the concentration gradient) or active (requires energy, against the gradient).
Cell membrane (plasma membrane)
The semi-permeable lipid bilayer that surrounds the cell, controlling what enters and exits. Think of it as a security gate that checks every molecule's ID before letting it through.
Phospholipid
An amphipathic lipid molecule with a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails. These are the main building blocks of the membrane.
Amphipathic
Having both a hydrophilic region and a hydrophobic region within the same molecule. In simple terms, one end of the molecule is comfortable in water and the other end avoids it.
Phospholipid bilayer
The two-layered sheet of phospholipids that forms the structural core of the membrane. The hydrophilic heads face outward (toward water) and the hydrophobic tails face inward (away from water). Think of it as two rows of lollipops with their sticks pointing toward each other.
Fluid-mosaic model
The current model of membrane structure describing the membrane as a fluid (moving) sheet of lipids with a mosaic (patchwork) of proteins and carbohydrates embedded in or attached to it.
Selective permeability
The property of cell membranes that allows some substances to pass freely while blocking others. In simple terms, the membrane is choosy about what it lets through.
Passive transport
Movement of substances across a membrane without the cell spending energy. This includes simple diffusion (molecules move down their concentration gradient through the bilayer) and facilitated diffusion (molecules move down their gradient through a transport protein).
Active transport
Movement of substances across a membrane that requires metabolic energy (typically ATP), because the substance is moving against its concentration gradient, from low concentration to high.
Facilitated diffusion
A form of passive transport in which polar or charged molecules cross the membrane through specific transport proteins (channels or carriers) rather than passing through the lipid bilayer directly.
Aquaporins
Specialised channel proteins that allow water molecules to cross the membrane rapidly. Think of them as dedicated water lanes built into the membrane.
Integral (transmembrane) proteins
Proteins that span the entire lipid bilayer, often serving as channels, carriers, or receptors.
Glycoprotein / glycolipid
Proteins or lipids with carbohydrate chains attached, found on the outer surface of the membrane. They play roles in cell recognition, signalling, and immune response. Think of them as the cell's name tags.
Concentration gradient
The difference in concentration of a substance between two areas (e.g. inside vs outside the cell). Molecules naturally move from high concentration to low concentration.
All biological membranes share these roles, regardless of their specific composition:
Defining cellular boundaries: the plasma membrane acts as a semi-permeable barrier, selectively restricting what enters and exits the cell
Compartmentalising organelles: internal membranes enclose organelles such as the nucleus and mitochondria, giving each its own distinct chemical environment
Regulating molecular transport: membranes control the movement of molecules within the cell and between its organelles
Facilitating signal reception: receptor proteins embedded in the membrane detect extracellular signals (hormones, neurotransmitters) and trigger cellular responses
For this course, the main focus is on the transport function.
The fluid-mosaic model describes the membrane as a dynamic, flexible structure rather than a rigid wall. Three categories of molecule make up the membrane:
Lipids (primarily phospholipids): form the bilayer foundation
Proteins (integral and peripheral): embedded in or attached to the bilayer, handling transport, signalling, and structural support
Carbohydrates (as glycoproteins and glycolipids): attached to the outer surface, involved in cell recognition and immune function
The "fluid" part of the name refers to the fact that individual phospholipids and proteins can move laterally within the plane of the membrane. The "mosaic" part refers to the patchwork of different proteins and other molecules dotted across the bilayer. This lateral movement is essential for the membrane to adapt and respond to changes in the cell's environment.
Phospholipids are the most abundant lipid in cell membranes. Because each molecule is amphipathic, phospholipids spontaneously arrange themselves into a bilayer when placed in water:
Hydrophilic heads face outward, interacting with the watery environment on both sides of the membrane (extracellular fluid outside, cytoplasm inside)
Hydrophobic tails face inward, clustering together to avoid water
This arrangement is energetically favourable and happens without any external input. The result is a stable, self-assembling barrier.
The hydrophobic interior of the bilayer is the gatekeeper. It determines which molecules pass freely and which are blocked:
Can cross freely: small nonpolar molecules such as O₂, CO₂, and lipids dissolve in the hydrophobic core and diffuse straight through
Cannot cross freely: polar molecules, ions (Na⁺, K⁺, Cl⁻), and large molecules (glucose, amino acids) are repelled by the hydrophobic core and need help from transport proteins
Water is a special case. It is polar, but it is small enough to cross the bilayer slowly on its own. For efficient water transport, cells use aquaporins, specialised channel proteins with hydrophilic interiors that let water molecules pass through rapidly.
Passive transport moves substances down their concentration gradient, from high concentration to low. The cell does not spend any energy (ATP). Two main forms:
Simple diffusion: nonpolar molecules pass directly through the lipid bilayer. No proteins involved. Examples: O₂ and CO₂ crossing the membrane during gas exchange.
Facilitated diffusion: polar or charged molecules cross through transport proteins (channels or carriers) embedded in the membrane. The molecules still move down their gradient, so no energy is needed. Example: glucose entering a cell through a glucose transporter.
Active transport moves substances against their concentration gradient, from low concentration to high. This requires metabolic energy, usually from ATP. Cells use active transport to maintain internal concentrations that differ from the external environment (e.g. keeping intracellular K⁺ high and Na⁺ low).
Note from the course material: you do not need to know the detailed subtypes of transport for this course. The key distinction is passive (no energy, down the gradient) vs active (energy, against the gradient).
Membrane transport is not just about getting nutrients in and waste out. It is also central to how cells produce energy:
In cellular respiration, the inner mitochondrial membrane uses controlled ion movement (a proton gradient) to drive ATP synthesis
In photosynthesis, the thylakoid membrane in chloroplasts uses a similar proton gradient to produce ATP
Both processes depend on the membrane being selectively permeable. Understanding membrane dynamics here lays the groundwork for those upcoming topics.
Selective permeability is the reason anaesthetics work: many anaesthetic drugs are small, nonpolar molecules designed to cross cell membranes freely and reach nerve cells without needing a transporter. Dialysis machines are an engineered version of a selectively permeable membrane, filtering waste from blood the way a cell membrane filters molecules. And in food science, osmosis (water moving across a semi-permeable membrane) is why salting meat draws out moisture for preservation.
Students often think the cell membrane is a rigid wall. It is not. The fluid-mosaic model describes a flexible, moving structure where lipids and proteins drift laterally.
Students often confuse "semi-permeable" with "impermeable." Semi-permeable means some things pass through and others do not. The membrane is selective, not sealed.
Students sometimes believe that all transport across the membrane requires energy. Passive transport (simple diffusion, facilitated diffusion) requires no energy at all. Only active transport uses ATP.
Students often think water cannot cross the lipid bilayer because it is polar. Water can cross slowly on its own, but aquaporins speed the process up dramatically.
⚠️ Be able to explain why the phospholipid bilayer forms spontaneously (amphipathic nature, hydrophilic heads out, hydrophobic tails in).
⚠️ Know the difference between passive and active transport and be able to give an example of each.
⚠️ Understand which molecules can cross the membrane freely (small, nonpolar) and which cannot (polar, charged, large).
⚠️ Be ready to explain the fluid-mosaic model in your own words: what makes it "fluid" (lateral movement of components) and what makes it a "mosaic" (variety of embedded proteins and other molecules).
⚠️ The course notes explicitly state you do not need the detailed subtypes of transport, but you do need the passive vs active distinction.
True or false: The cell membrane is a rigid, fixed structure. (False. The fluid-mosaic model describes it as flexible, with components that move laterally.)
Fill in the blank: Phospholipids are described as ______ because they have both hydrophilic and hydrophobic regions. (Amphipathic.)
True or false: O₂ can cross the cell membrane without a transport protein. (True. O₂ is small and nonpolar.)
Fill in the blank: Movement of molecules against their concentration gradient requires ______ and is called ______ transport. (Energy / ATP; active.)
True or false: Facilitated diffusion requires ATP. (False. Facilitated diffusion is a form of passive transport.)
Q: What does it mean for the cell membrane to be "selectively permeable"?
A: It means the membrane allows some substances to pass through while blocking others. Small, nonpolar molecules can cross freely, while polar molecules, ions, and large molecules need transport proteins to get through.
Q: Describe the arrangement of phospholipids in the cell membrane and explain why they organise this way.
A: Phospholipids form a bilayer with hydrophilic heads facing the watery environments on both sides and hydrophobic tails pointing inward, away from water. This arrangement occurs because it is the most energetically stable configuration for amphipathic molecules in an aqueous environment.
Q: What are the three main components of the fluid-mosaic model?
A: Lipids (primarily phospholipids) forming the bilayer, proteins (integral and peripheral) embedded in or attached to the membrane, and carbohydrates (as glycoproteins and glycolipids) on the outer surface.
Q: A student says, "Facilitated diffusion is active transport because it uses proteins." Is this correct? Explain.
A: This is incorrect. Facilitated diffusion is passive transport. Although it uses transport proteins, the molecules still move down their concentration gradient without any energy input from the cell. Active transport is defined by the requirement for energy (ATP), not by the involvement of proteins.
Q: Why is membrane transport important for cellular energy production?
A: Both cellular respiration and photosynthesis rely on proton gradients across membranes (the inner mitochondrial membrane and the thylakoid membrane, respectively) to drive ATP synthesis. Without selective permeability, cells could not maintain these gradients.
This material connects directly to cellular respiration (Chapter on metabolism) because the electron transport chain depends on the inner mitochondrial membrane maintaining a proton gradient, which is a form of active transport in reverse: energy stored as a gradient, then released to make ATP.
It also connects to osmosis and tonicity, which extend the idea of selective permeability to water movement specifically. Understanding why water moves across membranes is essential for topics on plant cell turgor, kidney function, and IV fluid balance in medicine.
More broadly, membrane structure underpins cell signalling: receptor proteins in the membrane are the first step in nearly every signalling pathway you will study later in the course.
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