Cell – Physiology · FCPS Part 1

Cell membrane: structure and composition

The plasma membrane is a fluid phospholipid bilayer that separates the intracellular and extracellular environments and selectively controls movement of substances.

Component	Approximate proportion in your notes	Function
Proteins	55%	Channels, carriers, receptors, enzymes
Lipids	40%	Structural barrier and membrane fluidity
Carbohydrates	5%	Cell recognition, protection and signalling

Phospholipid bilayer

Hydrophilic phosphate heads face the watery environments inside and outside the cell.
Hydrophobic fatty-acid tails face inward, away from water.
Phospholipids are amphipathic because they contain both hydrophilic and hydrophobic regions.

Factors affecting membrane fluidity

Component / factor	Effect
Unsaturated fatty acids	Kinks prevent tight packing → increased fluidity
Saturated fatty acids	Pack tightly → decreased fluidity
Cholesterol	Buffers membrane fluidity and stabilises the bilayer
Fluid mosaic model	Lipids and many proteins move laterally within the membrane

Membrane proteins

Type	Characteristics	Examples / functions
Integral proteins	Embedded in the lipid bilayer	Ion channels, carrier proteins, receptors
Peripheral proteins	Attached to the inner or outer membrane surface	Enzymes, structural support
Channel proteins	Form selective, water-filled pores	Ion movement down electrochemical gradients
Carrier proteins	Bind a substance and change conformation	Facilitated diffusion or active transport
Receptors	Bind specific ligands	Initiate cellular signalling
Glycoproteins	Proteins with attached carbohydrate chains	Recognition, protection and cell signalling

What crosses the lipid bilayer directly?

Substance	Direct passage	Reason
O₂ and CO₂	Yes	Small, nonpolar molecules
Steroid hormones and many lipids	Yes	Lipid-soluble
Alcohol	Generally yes	Relatively membrane-permeable
Water	Slowly	Small polar molecule; also passes through aquaporins
Glucose	No significant direct passage	Large, polar molecule
Na⁺, K⁺, Ca²⁺ and Cl⁻	No	Charged particles require transport proteins

Membrane transport mechanisms

Passive transport

Passive transport moves substances down their concentration or electrochemical gradient without direct ATP consumption.

Feature	Simple diffusion	Facilitated diffusion	Osmosis	Filtration
Driving force	Concentration gradient	Concentration / electrochemical gradient	Water potential / osmotic gradient	Hydrostatic pressure gradient
Transport protein	Not required	Required	Not always; aquaporins facilitate it	Depends on membrane
ATP required directly?	No	No	No	No
Saturable?	No carrier-mediated saturation	Yes, for carrier-mediated transport	Not typically described in basic physiology	Depends on the system
Example	O₂, CO₂	GLUT-mediated glucose transport	Water movement across membranes	Glomerular filtration

Simple vs facilitated diffusion

Feature	Simple diffusion	Facilitated diffusion
Route	Through lipid bilayer	Through channel or carrier proteins
Specificity	Generally low	Protein-dependent specificity
Competition	No carrier competition	Possible with shared carriers
Transport maximum	No carrier	Carrier-mediated transport can saturate
Examples	O₂, CO₂, steroid hormones	GLUT transporters, ion channels

Exam pearl: Facilitated diffusion is passive even though it uses a transport protein. A carrier does not automatically mean active transport.

Osmosis and tonicity

Osmosis: Net movement of water across a selectively permeable membrane toward the side with greater effective solute concentration.

External solution	Water movement	Effect on cell
Isotonic	No sustained net water movement	Maintains normal volume
Hypertonic	Water leaves cell	Cell shrinks
Hypotonic	Water enters cell	Cell swells; may lyse

Active transport

Active transport moves substances against their concentration or electrochemical gradients using energy directly or indirectly.

Primary vs secondary active transport

Feature	Primary active transport	Secondary active transport
Energy source	Direct ATP hydrolysis	Energy stored in an ion gradient
Transport direction	Can move a substance uphill	One substance's downhill movement drives another uphill
Typical example	Na⁺/K⁺-ATPase	Na⁺-glucose cotransport
Dependence on ATP	Direct	Indirect, because pumps maintain the gradient

Primary active transport pumps

Pump	Action	Clinical / FCPS pearl
Na⁺/K⁺-ATPase	3 Na⁺ out, 2 K⁺ in per ATP	Inhibited by digoxin; electrogenic
Ca²⁺-ATPase	Moves Ca²⁺ out of the cytosol	Maintains low intracellular Ca²⁺; SERCA stores Ca²⁺ in sarcoplasmic reticulum
H⁺/K⁺-ATPase	Exchanges H⁺ for K⁺	Gastric acid secretion; inhibited by proton pump inhibitors such as omeprazole

Secondary active transport

Mechanism	Direction	Example
Symport / cotransport	Both substances move in the same direction	Na⁺-glucose cotransporter (SGLT)
Antiport / countertransport	Substances move in opposite directions	Na⁺/H⁺ exchanger; Na⁺/Ca²⁺ exchanger

SGLT mechanism: Na⁺ moves down its electrochemical gradient and drives glucose uptake against its gradient.

Vesicular transport

Process	Direction	Mechanism	Example
Phagocytosis	Into cell	Engulfs large particles	Macrophages engulf bacteria
Pinocytosis	Into cell	Uptake of extracellular fluid and dissolved substances	Fluid uptake
Receptor-mediated endocytosis	Into cell	Specific ligands bind receptors before internalisation	LDL cholesterol uptake
Exocytosis	Out of cell	Vesicle fuses with plasma membrane	Hormone, enzyme and neurotransmitter secretion

Memory aid: Phagocytosis = cell eating; pinocytosis = cell drinking.

Drug transport note: Your source gives a rule of thumb of molecular weight below 1,000 Da for diffusion and above 1,000 Da for pinocytosis.

Cytoplasm and cell organelles

Cytoplasm = cytosol + organelles, excluding the nucleus.

Structure	Main function	FCPS pearl
Cytosol	Fluid portion of cytoplasm; site of many metabolic reactions	Contains enzymes, ions and metabolites
Nucleus	Stores DNA and regulates gene expression	Control centre of the cell
Nucleolus	Ribosomal RNA synthesis and ribosome subunit assembly	Not membrane-bound
Rough ER (RER)	Synthesis and early processing of secreted and membrane proteins	Nissl bodies in neurons are RER with ribosomes
Smooth ER (SER)	Lipid and steroid synthesis, detoxification, Ca²⁺ storage	Prominent in steroid-producing cells
Golgi apparatus	Modifies, sorts and packages proteins and lipids	Important for lysosomal enzyme targeting
Lysosomes	Intracellular digestion and autophagy	Contain acid hydrolases
Peroxisomes	Fatty-acid oxidation and H₂O₂ metabolism	Contain catalase
Mitochondria	ATP production, apoptosis regulation and metabolic reactions	Have their own DNA and double membrane

Rough ER vs smooth ER

Feature	Rough ER	Smooth ER
Ribosomes	Present	Absent
Main function	Protein synthesis	Lipid synthesis and detoxification
Other function	Initial processing of proteins	Intracellular Ca²⁺ storage
Typical association	Protein-secreting cells	Steroid-producing cells and hepatocytes

Golgi apparatus vs lysosomes

Feature	Golgi apparatus	Lysosomes
Main role	Modifies, sorts and packages molecules	Digests cellular material
Key contents	Processing and sorting enzymes	Acid hydrolases
FCPS pearl	Helps target lysosomal enzymes	Involved in autophagy and intracellular digestion

Clinical pearl: Lysosomal enzymes participate in tissue remodelling and autophagy. Postpartum uterine involution involves several processes, including autophagy and protein degradation.

Mitochondria, peroxisomes and reactive oxygen species

Feature	Mitochondria	Peroxisomes
Membrane	Double membrane	Single membrane
Main function	ATP production through oxidative phosphorylation	Fatty-acid oxidation and oxidative metabolism
Important enzymes / structures	Electron transport chain on inner membrane; ATP synthase	Oxidases and catalase
Reactive oxygen species	Important source of cellular ROS	Generate and detoxify H₂O₂
DNA	Own DNA	No independent genome
Other role	Apoptosis regulation	Metabolism of selected lipids and other substrates

Cytoskeleton

The cytoskeleton maintains cell shape, supports intracellular transport, and enables movement and cell division.

Component	Main protein	Main functions	FCPS association
Microtubules	Tubulin	Intracellular transport, cilia, flagella, centrioles, mitotic spindle	Resist compression
Microfilaments	Actin	Muscle contraction, cell movement, cytokinesis	Work with myosin
Intermediate filaments	Keratins and other structural proteins	Mechanical strength and structural stability	Resist tensile stress

Structure	Cytoskeletal basis
Cilia and flagella	Microtubules
Mitotic spindle	Microtubules
Muscle contraction	Actin and myosin
Cleavage furrow during cytokinesis	Actin and myosin
Epithelial mechanical strength	Keratin intermediate filaments

Cell adhesion molecules (CAMs) and junctions

Molecule / junction	Main function	FCPS clinical pearl
Integrins	Cell-to-extracellular matrix adhesion	Connect extracellular matrix to the cytoskeleton
Cadherins	Calcium-dependent cell-to-cell adhesion	Loss of E-cadherin promotes tumour invasion and metastasis
Selectins	Initial tethering and rolling of leukocytes	Important during inflammation
LFA-1 integrin	Firm leukocyte adhesion through ICAM binding	Deficiency can cause leukocyte adhesion deficiency
Tight junctions	Seal spaces between adjacent cells	Maintain epithelial barriers
Desmosomes	Strong cell-to-cell attachment	Connect intermediate filaments
Gap junctions	Permit ions and small molecules to pass between cells	Electrical coupling in cardiac muscle

Leukocyte migration: sequence to remember

Step	Main molecule / event
1. Rolling	Selectins
2. Firm adhesion	Leukocyte integrins bind endothelial ICAMs
3. Transmigration	Leukocytes pass between or through endothelial cells
4. Chemotaxis	Cells move toward chemical signals at the inflammatory site

FCPS pearl: Leukocyte adhesion deficiency classically involves impaired firm adhesion and recurrent bacterial infections with defective pus formation. LFA-1 is a β₂ integrin.