Chapter 5 Notes: Structure and Function of Plasma Membranes
5.1 Components and Structure
The plasma membrane defines the cell, outlines borders, and mediates interactions with the environment.
It is selectively permeable: allows some substances to cross freely, others require special structures or energy investment.
Fluid mosaic model: membrane is a fluid, dynamic mosaic of components including phospholipids, cholesterol, proteins, and carbohydrates, which are constantly moving and interacting within the plane of the membrane.
Principal components and their roles:
Lipids: phospholipids form the bilayer; cholesterol lies among phospholipids to regulate fluidity and organize membrane proteins into clusters (lipid rafts). At warm temperatures, cholesterol reduces phospholipid movement, making the membrane less fluid. At cold temperatures, it prevents phospholipids from packing too closely, thereby maintaining fluidity by disrupting tight packing.
Proteins: integral (transmembrane) proteins span the membrane and can function as transporters, channels, receptors, enzymes, or structural anchors; peripheral proteins attach to surfaces or to integral proteins.
Carbohydrates: attached to lipids (glycolipids) or proteins (glycoproteins) on the exterior surface; form the glycocalyx, which is highly hydrophilic and aids in water interactions and cell recognition. It also provides a protective barrier against chemical and mechanical damage and plays a crucial role in cell-cell adhesion and communication. It distinguishes between self and non-self cells, a process vital for immune responses, tissue formation, and embryonic development.
Membrane thickness: ~5–10 nm. For scale, human RBCs are ~8 µm wide (about 1000× the membrane thickness).
Glycocalyx: a sugar coat formed by glycoproteins and glycolipids; important for:
cell recognition and self/non-self discrimination (immune recognition, tissue formation, embryonic development)
interactions with the aqueous environment; attracts water; supports adhesion to other cells
viral interactions: glycoprotein/glycolipid patterns can determine virus binding to cells (e.g., HIV uses CD4/glycoprotein receptors to enter host cells)
Historical models:
Davson–Danielli (1935): a “sandwich” model with proteins as the bread and lipids as the filling; guided by early electron micrographs (railroad-track appearance).
TEM advances in the 1950s revealed a double rather than a single layer.
Singer–Nicolson (1972): fluid mosaic model, explaining membrane function with components that move laterally within a bilayer.
Principal components and approximate mass composition (typical human cell):
Protein ~50%
Lipids ~40%
Carbohydrates ~10%
Variation by cell type:
Myelin membranes: ~18% protein, 76% lipid.
Mitochondrial inner membrane: ~76% protein, 24% lipid.
RBC membrane: ~30% lipid.
Phospholipids:
Amphiphilic: hydrophilic head (phosphate-containing group) and hydrophobic tails (fatty acids).
In water, phospholipids assemble into a bilayer with heads facing aqueous environments and tails inward.
Head is polar; tail is nonpolar. Molecules with both hydrophilic and hydrophobic regions are called amphiphilic.
A phospholipid molecule has a glycerol backbone with two fatty acids (tails) and a phosphate head group.
Lipid bilayer formation details:
Hydrophilic heads contact water on both sides; hydrophobic tails face inward away from water.
In aqueous solutions, phospholipids can spontaneously form structures like micelles or liposomes with the hydrophilic heads outward and hydrophobic tails inward.
Proteins:
Integral proteins (e.g., integrins) embed fully into the membrane; hydrophobic regions interact with the lipid core.
Transmembrane segments commonly consist of 20–25 amino acids per hydrophobic span.
Some proteins span the membrane with multiple segments (e.g., up to 12 single segments forming larger proteins) and have both hydrophilic (extracellular/cytosolic) and hydrophobic regions.
Orientation: hydrophobic regions align with phospholipid tails; hydrophilic regions protrude into cytosol or extracellular fluid.
Peripheral proteins:
Located on the exterior or interior surfaces; attach to integral proteins or phospholipids.
Roles include enzymatic activity, anchoring cytoskeleton fibers, and participating in cell recognition.
Carbohydrates:
Always on the cell’s exterior surface.
Bound to proteins (glycoproteins) or lipids (glycolipids).
Carbohydrate chains range from 2–60 monosaccharide units; can be linear or branched.
Together with peripheral proteins, carbohydrates create cell-surface recognition sites that help cells recognize one another and contribute to self/non-self discrimination.
The glycocalyx (carbohydrate layer) is highly hydrophilic and attracts water, aiding interactions with the environment and solute uptake.
Evolution/virology connections:
Viral glycoprotein/glycolipid patterns enable viruses to recognize and bind host cells; e.g., HIV specifically targets receptors on T-helper cells.
Viral patterns can mutate rapidly, altering recognition sites and challenging immune recognition and vaccine development.
HIV infection can disrupt immune function by targeting immune cells and evolving surface patterns that evade antibodies.
Table 5.1 (overview of components and functions):
Phospholipid: main membrane fabric
Cholesterol: modulates fluidity; buffers temperature effects; contributes to lipid raft organization
Integral proteins: embedded, may span bilayer
Peripheral proteins: on surfaces; act as enzymes or cytoskeletal anchors
Carbohydrates: exterior surface; glycocalyx; cell recognition
CAREER CONNECTION: Immunologists study peripheral proteins and carbohydrates to understand immune recognition, vaccine development, autoimmune diseases, transplantation, and the influence of the environment on immunity.
5.2 Passive Transport
Passive transport basics:
Substances move down their concentration gradient (from high to low concentration) without cellular energy expenditure.
Plasma membranes are asymmetric: interior vs exterior differ in composition; membrane proteins anchor to cytoskeleton; extracellular matrix components bind outside membrane; carbohydrates on exterior influence selectivity and binding.
Diffusion:
Non-polar and small, lipid-soluble molecules diffuse readily through the lipid core.
Polar molecules and ions require channels or carriers; diffusion through the lipid core is hindered by polarity/charge.
Diffusion is energy-free; concentration gradients store potential energy that dissipates as gradients flatten.
Each solute diffuses according to its own gradient; different substances diffuse at different rates.
Channel vs. carrier diffusion:
Channel proteins: form hydrophilic passages; some channels are always open, others are gated.
Carrier proteins (facilitated diffusion): bind a substance and change shape to shuttle it across; often specific for a single substance; can be saturated when all carriers are bound.
Channel diffusion is typically much faster (tens of millions of molecules per second) than carrier diffusion (roughly 10^3–10^6 molecules per second).
Osmosis (water diffusion):
Special case of diffusion: diffusion of water across a semipermeable membrane in response to solute concentration differences.
Aquaporins facilitate rapid water movement, especially in red blood cells and kidney tubules.
Osmosis continues until the water gradient is balanced by hydrostatic/osmotic forces.
Diffusion factors:
Extent of concentration gradient (larger gradient
→ faster diffusion; as gradients approach equilibrium, diffusion slows).Molecular mass: heavier molecules diffuse more slowly; lighter molecules diffuse faster.
Temperature: higher temperatures increase molecular energy and diffusion rate.
Solvent density: denser solvents slow diffusion.
Tonicity and osmolarity:
Osmolarity = total solute concentration; tonicity describes how extracellular solutions affect cell volume via osmosis.
Hypotonic: extracellular solution has lower osmolarity than cell interior; water enters cell.
Hypertonic: extracellular solution has higher osmolarity than intracellular fluid; water exits cell.
Isotonic: extracellular solution has the same osmolarity as intracellular fluid; no net water movement. This is critically important for medical applications, such as intravenous fluids, to prevent damage to patients' cells.
Note: osmolarity concerns particle count; a solution containing many solutes can have high osmolarity even if it appears visually clear.
Cellular and organismal responses to tonicity:
Red blood cells in hypotonic solutions swell and can lyse; in hypertonic solutions, they crenate.
Plant cells rely on cell walls to resist lysis in hypotonic environments; turgor pressure stiffens plant walls and supports nonwoody plants.
Plasmolysis occurs when plant cell membranes detach from the cell wall in hypertonic conditions.
Some organisms employ osmoregulation (e.g., contractile vacuoles in paramecia) to expel excess water.
Osmoregulation strategies vary among vertebrates and invertebrates; kidneys regulate body water; osmoreceptors monitor blood solute levels; albumin helps maintain osmotic pressure in blood.
Visuals and connections:
Figure references: diffusion across membranes (Figure 5.8); osmosis diagrams (Figure 5.11); hyper/hypotonic effects on RBCs (Figure 5.12); plant turgor/plasmolysis (Figures 5.13–5.14); contractile vacuoles in protists (Figure 5.15).
Special notes:
The term tonicity relates to the cell interior, while osmolarity is a property of the extracellular solution; both influence water movement and cell volume.
5.3 Active Transport
Active transport requires cellular energy (ATP) to move substances against their concentration or electrochemical gradients.
Electrochemical gradient:
Ions experience both concentration and electrical gradients across the membrane.
Example: Na+ tends to move into the cell down its concentration gradient and is driven inward by the cell’s negative interior; K+ tends to move inward by its concentration gradient but outward by its electrical gradient in many contexts.
The net driving force on an ion is its electrochemical gradient.
Moving against a gradient requires energy; active transport uses ATP and is sensitive to metabolic poisons.
Carrier proteins/pumps in active transport:
Uniporter: carries one type of ion or molecule.
Symporter: carries two different ions/molecules in the same direction.
Antiporter: carries two different ions/molecules in opposite directions.
Some of these transporters can also move small uncharged organic molecules like glucose during facilitated diffusion.
Primary active transport:
Directly uses ATP to transport substances and often to create an electrochemical gradient. These gradients are vital for maintaining cell volume, generating nerve impulses, and driving secondary active transport processes.
A classic example is the Na+/K+ ATPase (sodium–potassium pump).
Key steps for Na+/K+ ATPase (illustrated by six steps):
1) Orientation toward the interior; binding of three Na+ ions.
2) ATP hydrolysis; the enzyme is phosphorylated (phosphate group attached).
3) Conformational change; pump exposes Na+ to exterior and releases Na+.
4) Conformational change; pump binds two K+ ions on the exterior; phosphate group is released.
5) Return to original conformation; K+ ions are released into cytoplasm.
6) Pump returns to initial configuration, ready to begin again. - Result: more Na+ outside the cell than inside and more K+ inside than outside; creates a negative interior relative to exterior.
The Na+/K+ pump is an electrogenic pump, contributing to the membrane potential.
Net stoichiometry: for every three Na+ ions pumped out, two K+ ions are pumped in.
Equation form (conceptual): 3\ Na^+\ (\text{out}) \rightarrow 3\ Na^+\ (\text{in});\ 2\ K^+\ (\text{in}) \rightarrow 2\ K^+\ (\text{out}) when considering opposite directions; overall results in a negative interior.
Secondary active transport (co-transport):
Indirectly uses ATP by exploiting the electrochemical gradient established by primary active transport.
Sodium gradients power transport of other substances against their own gradients.
Common examples: amino acids and glucose use Na+-coupled transporters to enter cells.
Example mechanism: as Na+ moves back into the cell through a channel or transporter, other molecules bind to the same transporter and are carried with Na+ into the cell.
This process can also store energy in mitochondria (e.g., protons pumped to create proton motive force), driving ATP synthesis via ATP synthase.
Transport rates:
Channel/facilitated diffusion: much faster than carrier-mediated transport; channels ~$10^7$–$10^8$ molecules/s; carriers ~$10^3$–$10^6$ molecules/s.
Other pumps and transporters:
Ca2+ ATPase, H+-ATPase, H+/K+-ATPase are ATP-driven pumps that transport Ca2+ or H+ across membranes.
These pumps contribute to ion homeostasis and membrane potential, and their dysfunction can disrupt cellular processes.
Practical implications:
Primary active transport establishes the gradients used by secondary active transport.
The Na+/K+ ATPase underpins resting membrane potential and cellular excitability, especially in neurons and muscle cells.
Changes in extracellular pH can influence transporter activity and ion transport (illustrated by critical thinking prompts in the chapter).
5.4 Bulk Transport
Bulk transport handles large particles and whole cells, requiring energy.
Endocytosis (into the cell): a family of processes that invaginate the plasma membrane to form vesicles:
Phagocytosis (cell eating): engulfment of large particles or microbes by specialized cells (e.g., neutrophils).
The membrane associates with clathrin on the interior surface to stabilize a coated section.
The membrane pocket encloses the particle, forming a vesicle (phagosome).
The phagosome fuses with a lysosome; contents are degraded; remaining materials may be released back to extracellular space via exocytosis of endosome contents.
Pinocytosis (cell drinking): uptake of extracellular fluid and solutes via smaller vesicles; usually does not require lysosome fusion.
Potocytosis: a caveolae- or caveolin-dependent form of pinocytosis; vesicles are smaller and internalized via caveolae; can involve transcytosis (movement through cell to opposite side).
Receptor-mediated endocytosis: highly specific uptake via receptors that bind specific ligands (Figure 5.22).
Clathrin-coated pits help form vesicles during receptor-mediated uptake.
LDL receptor–mediated endocytosis clears LDL from blood; defects in LDL receptors cause familial hypercholesterolemia, elevating blood cholesterol levels.
Viruses and toxins may exploit this pathway by binding receptors that trigger endocytosis.
Exocytosis (out of the cell): reverse process; vesicles fuse with the plasma membrane and release their contents to the extracellular space.
Important for secretion of extracellular matrix proteins and neurotransmitter release into synapses (e.g., neurotransmitter-containing vesicles).
Vesicle membranes become part of the plasma membrane after fusion.
Transport method summary (Table 5.2):
Diffusion: Passive; small molecules; down concentration gradient.
Osmosis: Passive; diffusion of water via aquaporins.
Facilitated diffusion: Passive; channel or carrier proteins move substances down their gradients.
Primary active transport: Active; direct ATP use; creates/maintains gradients (e.g., Na+/K+ ATPase).
Secondary active transport (co-transport): Active; uses gradients created by primary transport to move additional substances.
Phagocytosis: Active; bulk intake of large particles or whole cells.
Pinocytosis and potocytosis: Active; uptake of fluids/small molecules through vesicles; potocytosis uses caveolin-coated pits.
Receptor-mediated endocytosis: Active; highly specific uptake via receptor binding and clathrin-coated pits.
Exocytosis: Active; bulk release of materials to exterior.
Key terms (quick reference)
Amphiphilic: molecule with both polar (hydrophilic) and nonpolar (hydrophobic) regions.
Antiporter: transporter moving two different ions/molecules in opposite directions.
Aquaporin: channel protein that facilitates water transport at high rates.
Carrier protein: membrane protein that moves substances by changing its shape.
Cavole: small flask-shaped invagination involved in potocytosis.
Clathrin: protein coating pits/vesicles during endocytosis.
Conformational change: shape change in a protein (e.g., carrier) that enables transport.
Electrochemical gradient: combined influence of chemical (concentration) and electrical (charge) gradients.
Electrogenic pump: pump creating a charge difference across the membrane.
Endocytosis: uptake of material into a cell via vesicle formation.
Exocytosis: release of material from a cell via vesicle fusion with the plasma membrane.
Glycocalyx: carbohydrate layer on the cell surface involved in protection and recognition.
Glycoprotein: carbohydrate attached to a protein.
Glycolipid: carbohydrate attached to a lipid.
Glycoprotein/glycolipid patterns: recognition sites for cells and pathogens; variability can influence immune responses.
Membrane potential: electrical potential difference across the plasma membrane.
Myelin: membrane with high lipid content in nerve insulation; example of membrane composition variation.
Phagosome: vesicle formed during phagocytosis.
Phospholipid bilayer: the core structure of membranes formed by two layers of phospholipids.
Receptor-mediated endocytosis: uptake of specific ligands via receptor binding.
Reticular/glycocalyx interactions: see above for role in recognition and hydration.
TAM (transmembrane) proteins: generic term for proteins crossing the membrane.
Vesicle: membrane-bound compartment transporting cargo.
Visual connections and examples (referenced in the text)
Figure 5.2: Fluid mosaic description; components include phospholipids, cholesterol, proteins, and carbohydrates; glycoproteins and glycolipids extend from membrane surface.
Figure 5.3: Phospholipid structure—glycerol backbone, two fatty acid tails, phosphate head; hydrophilic heads face water, hydrophobic tails face inward.
Figure 5.4: Phospholipids in aqueous solution form micelles or liposomes with heads outward and tails inward.
Figure 5.5: Integral membrane proteins may have one or more alpha-helices spanning the membrane or beta-sheets forming transmembrane domains.
Figure 5.6: HIV binding to CD4 receptor on T cells; illustrates receptor-mediated interactions.
Figure 5.9: Facilitated transport; channels allow diffusion down gradients.
Figure 5.10: Carrier proteins change shape to move molecules across membranes.
Figure 5.16: Electrochemical gradients across membranes for ions like Na+ and K+.
Figure 5.17–5.19: Carrier types (uniporter, symporter, antiporter); primary and secondary active transport concepts; Na+/K+ pump mechanism and its role in establishing gradients and membrane potential.
Figure 5.20–5.23: Bulk transport processes—phagocytosis, pinocytosis, receptor-mediated endocytosis, and exocytosis.
Connections to broader themes
The fluid mosaic model explains how membranes remain both stable and dynamic, enabling regulation of material transport, signal transduction, and intercellular communication.
The balance between membrane fluidity and rigidity is essential for maintaining integrity under varying temperatures; cholesterol helps buffer fluidity extremes and organizes membrane domains.
Energy-dependent transport creates and maintains ion gradients that power numerous cellular processes (nerve impulses, muscle contraction, nutrient uptake).
Receptor-mediated endocytosis illustrates how cells specifically recognize and internalize molecules, with direct implications for nutrition, immune defense, and disease.
Osmoregulation and tonicity are fundamental for cell viability across diverse environments (e.g., freshwater vs. marine organisms, plant cells with rigid walls).
Practical clinical ties: intravenous solutions must be isotonic; improper tonicity can cause hemolysis or cell damage; disorders like familial hypercholesterolemia illustrate the role of membrane receptors in disease.
Critical thinking prompts (from the chapter)
Why is membrane fluidity advantageous for cellular function?
How do phospholipids spontaneously organize into a bilayer with hydrophobic cores?
How can extracellular peripheral proteins serve as receptors to transduce signals intracellularly?
How do molecular size, temperature, solvent density, and diffusion distance affect diffusion rates?
Why does water move through a membrane, and how do aquaporins influence this process?
Why are certain intravenous solutions designed to be isotonic, and what happens if tonicity is off?
How would decreasing temperature affect diffusion rates, and why?
If a mutation prevents potassium ions from leaving a cell but aquaporins remain functional, what happens to tonicity and osmolarity?
Where does the cell obtain energy for active transport processes, and how does this energy usage affect cell health?
How does the Na+/K+ pump contribute to the net negative charge inside the cell, and why is this important for cellular physiology?
Why might intestinal epithelial cells use active transport for glucose uptake when many body cells use facilitated diffusion instead?
Why is secondary active transport considered a linkage to primary transport?
Why is it important to have multiple protein types in membranes for transport?
Why do ions struggle to cross lipid bilayers despite their small size?
Chapter summary (condensed)
The plasma membrane is best described by the fluid mosaic model: a phospholipid bilayer with embedded proteins and attached carbohydrates forming the glycocalyx.
Membrane fluidity is influenced by temperature, phospholipid tail saturation, cholesterol content, and protein distribution (mosaic nature).
Passive transport (diffusion and osmosis) moves substances down their gradients without energy; diffusion rates depend on gradient, mass, temperature, and solvent density. Osmosis specifically moves water via aquaporins toward higher solute concentration.
Tonicity and osmolarity describe how extracellular solutions affect cell volume; hypotonic solutions swell cells, hypertonic solutions shrink them, and isotonic solutions maintain cell size.
Active transport uses ATP to move substances against gradients; primary active transport directly uses ATP (Na+/K+ ATPase as a key example) to create electrochemical gradients; secondary active transport uses these gradients to move other substances (co-transport).
Bulk transport includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis, potocytosis) and exocytosis, enabling uptake and release of large particles, fluids, and signaling molecules.
Membrane components play roles in recognition, signaling, and immune responses; pattern variability in glycocalyx affects pathogen interactions and vaccine design.
The chapter links structure to function, illustrating how physical chemistry and protein dynamics enable the myriad tasks cells perform.
Practice/Review prompts (student study aids)
Explain how the fluid mosaic model accounts for membrane flexibility and functionality.
Differentiate integral and peripheral proteins and describe how they contribute to membrane function.
Describe how the Na+/K+ ATPase generates an electrochemical gradient and why this is essential for secondary active transport.
Compare and contrast diffusion, facilitated diffusion (channel vs. carrier), and active transport in terms of energy usage and rate.
Define tonicity vs osmolarity and give examples of hypotonic, isotonic, and hypertonic conditions for animal and plant cells.
Outline the steps of phagocytosis and receptor-mediated endocytosis and explain how defects in these processes can lead to disease.
Discuss how the glycocalyx can influence viral infection and immune recognition, using HIV as an example.
Explain why cholesterol is important for membrane fluidity across temperature ranges and how lipid rafts influence membrane organization.
Given a hypothetical mutation that eliminates aquaporins in a cell, predict the effects on osmosis and cell volume in a hypotonic environment.