Plasma Membrane Protection and Nutrient Acquisition

Slide 1 · Plasma membrane: protection and nutrient acquisition

Architecture, asymmetry, protein anchoring and selective permeability

Robbins, Cotran & Kumar · Pathologic Basis of Disease

11th edition · Chapter 1

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

By the end of the lecture, you should be able to:

Explain the amphipathic bilayer and its permeability barrier

Map major lipids to their preferred membrane leaflets

Relate signaling lipids, membrane domains and protein anchors to function

Describe the glycocalyx, epithelial polarity and passive diffusion

Slide 3 · The membrane is an organized biological interface

Three linked functions

Protection

A lipid core separates aqueous compartments

Selective exchange

Transport proteins and vesicular pathways regulate entry and exit

Communication

Membrane proteins and lipids organize signaling and adhesion

One physical boundary integrates three cellular tasks.

Slide 4 · The bilayer is fluid rather than a static sheath

The foundational architecture

Amphipathic phospholipids self-organize into a bilayer

Hydrophilic groups face extracellular fluid or cytoplasm

Hydrophobic acyl tails face one another

EXTRACELLULAR

CYTOPLASM

Phospholipid bilayer

Slide 5 · Amphipathic phospholipids explain membrane orientation

Two chemically different regions

Polar head groups interact with surrounding water

Hydrophobic tails pack in the interior

The molecular geometry generates a nonpolar core

EXTRACELLULAR

CYTOPLASM

Polar heads face aqueous compartments

Hydrophilic heads

Slide 6 · The hydrophobic core creates a diffusion barrier

Why a membrane is selectively permeable

Large or charged solutes cannot diffuse freely

Lipid solubility helps determine direct passage

Protein-dependent routes are needed for many solutes

EXTRACELLULAR

CYTOPLASM

Hydrophobic interior limits passage

Nonpolar interior

Slide 7 · Membrane constituents move laterally

Fluidity within the plane of the membrane

Many molecules diffuse sideways in each leaflet

Lateral movement permits local redistribution of components

Specialized domains remain possible despite this mobility

EXTRACELLULAR

CYTOPLASM

Lateral mobility within each leaflet

Motion within each leaflet

Slide 8 · Bilayer composition is heterogeneous

Not every membrane region has the same lipids

Different phospholipids occupy different positions

Local lipid composition influences membrane-protein behavior

Some domains have distinctive lipid enrichment

EXTRACELLULAR

CYTOPLASM

Distinct outer and inner leaflets

Outer: PC + SM | Inner: PS + PE

Slide 9 · The two membrane leaflets are not equivalent

Membrane asymmetry

Inner and outer faces have different lipid distributions

This asymmetry is maintained by specific transport proteins

Distribution contributes directly to membrane function

EXTRACELLULAR

CYTOPLASM

Distinct outer and inner leaflets

Outer: PC + SM | Inner: PS + PE

Slide 10 · Outer-leaflet lipids have a characteristic pattern

Predominantly extracellular face

Phosphatidylcholine is enriched on the outer face

Sphingomyelin preferentially occupies this leaflet

Glycolipids expose their carbohydrate groups outward

EXTRACELLULAR

CYTOPLASM

Outer leaflet enriched in PC and SM

PC · SM · glycolipids

Slide 11 · Inner-leaflet lipids support cytoplasmic functions

Predominantly cytoplasmic face

Phosphatidylserine (PS) enriches the inner face

Phosphatidylethanolamine (PE) is inner-leaflet predominant

Signaling phosphatidylinositol pools interact with cytosolic proteins

EXTRACELLULAR

CYTOPLASM

Inner leaflet enriched in PS and PE

PS · PE · signaling PI

Slide 12 · Flippases help maintain the transbilayer pattern

Asymmetry is actively organized

Flippases transfer particular phospholipids between faces

They contribute to the distribution shown Fig. 1.7

Loss of normal distribution can alter biological signals

EXTRACELLULAR

CYTOPLASM

Flippases preserve asymmetric distribution

Flippase

Slide 13 · Lipid localization is functionally significant

Beyond a compositional difference

Specific phospholipids interact with membrane proteins

Leaflet position changes electrostatic interactions

Redistribution can generate recognition signals

EXTRACELLULAR

CYTOPLASM

Distinct outer and inner leaflets

Outer: PC + SM | Inner: PS + PE

Slide 14 · Phosphatidylserine confers negative surface charge

The normal inner-leaflet state

PS is normally restricted predominantly to the inner face

Its anionic character promotes electrostatic protein interactions

Its outward translocation changes what other cells detect

EXTRACELLULAR

CYTOPLASM

PS localized to inner leaflet

Inner-leaflet PS: Negative charge

Slide 15 · Externalized phosphatidylserine signals apoptosis

An “eat me” surface cue

Programmed cell death is accompanied by PS exposure

External PS promotes recognition of dying cells

The same lipid has different consequences depending on leaflet

EXTRACELLULAR

CYTOPLASM

Externalized PS: Altered surface signal

Surface PS: Recognition cue

Slide 16 · Platelet phosphatidylserine supports coagulation

A second context for PS exposure

Exposed PS acts as a cofactor in blood clotting

Its biologic role depends on cell state and surface accessibility

Apoptotic recognition and hemostasis are distinct outcomes

EXTRACELLULAR

CYTOPLASM

Externalized PS: Altered surface signal

Surface PS: Recognition cue

Slide 17 · Anionic lipids recruit cytoplasmic proteins

Electrostatic membrane organization

Negative inner-leaflet charge helps position proteins

Protein–lipid interactions can change protein distribution

Localization influences signaling and membrane function

EXTRACELLULAR

CYTOPLASM

PS localized to inner leaflet

Inner-leaflet PS: Negative charge

Slide 18 · Phosphatidylinositol is a signaling platform

Functions of the cytosol-facing PI pool

Phosphatidylinositol (PI) can be phosphorylated

Phosphorylated PI species provide electrostatic docking sites

Their modified head groups can recruit intracellular proteins

EXTRACELLULAR

CYTOPLASM

PS localized to inner leaflet

Inner-leaflet PS: Negative charge

Slide 19 · Phosphorylation changes phosphoinositide function

From membrane lipid to binding site

Membrane-associated phosphatidylinositol

Head-group phosphorylation

Electrostatic scaffold

Recruitment of intracellular proteins

Head-group phosphorylation turns a lipid into a protein-binding platform.

Slide 20 · Phospholipase C cleaves polyphosphoinositides

An alternative outcome for signaling PI lipids

Membrane polyphosphoinositide

Phospholipase C activity

DAG + IP₃

Intracellular second-messenger signaling

One membrane lipid can yield two intracellular second messengers.

Slide 21 · DAG and IP₃ are lipid-derived second messengers

Two products of phosphoinositide hydrolysis

Diacylglycerol (DAG)

Lipid-derived signal

Associated with the membrane

Inositol trisphosphate (IP₃)

Inositol phosphate signal

Acts inside the cell

Phosphoinositide cleavage produces two distinct signals.

Slide 22 · Phosphatidylethanolamine favors the inner leaflet

A major feature of leaflet asymmetry

PE is predominantly cytoplasmic in the illustrated membrane

PE differs in localization from sphingomyelin

The collective lipid pattern defines membrane organization

EXTRACELLULAR

CYTOPLASM

Inner leaflet enriched in PS and PE

PS · PE · signaling PI

Slide 23 · Glycolipids project carbohydrates externally

Extracellular surface chemistry

Glycolipids occur on the outward-facing surface

Their carbohydrate moieties add chemical complexity

Together with glycoproteins, they contribute to the glycocalyx

EXTRACELLULAR CARBOHYDRATES

Glycoprotein · glycolipid · proteoglycan

Carbohydrate chains shown schematically

Slide 24 · Gangliosides bear complex sugar and sialic acid groups

Surface carbohydrate specialization

Gangliosides are glycolipids with complex sugar chains

Terminal sialic acids contribute negative charge

This surface chemistry supports charge-dependent interactions

EXTRACELLULAR CARBOHYDRATES

Glycoprotein · glycolipid · proteoglycan

Carbohydrate chains shown schematically

Slide 25 · Glycolipids contribute to intercellular interactions

Two examples given

Inflammatory cell recruitment

Extracellular carbohydrate and charge-based interactions

Sperm–egg fusion

Surface glycolipid interactions contribute to recognition

Membrane organization

Glycolipids can enrich specific lipid domains

Surface carbohydrate chemistry participates in cell recognition.

Slide 26 · Cholesterol is present in both leaflets

A key component of membrane domains

Cholesterol is distributed across the bilayer

It enriches particular lateral membrane regions

It occurs with glycosphingolipids in lipid rafts

EXTRACELLULAR

CYTOPLASM

Cholesterol occurs in both leaflets

Slide 27 · Lipid rafts are compositionally specialized domains

Nonrandom lateral organization

Some membrane regions are cholesterol-rich

Glycosphingolipids are also enriched in these areas

Rafts coexist with overall lateral membrane fluidity

EXTRACELLULAR

CYTOPLASM

Cholesterol- and glycosphingolipid-rich domain

Lipid raft

Slide 28 · Protein partitioning depends on local lipid composition

Not every protein enters every domain

Proteins differ in solubility across lipid environments

That difference generates nonrandom protein distributions

Lipid composition therefore helps spatially organize function

EXTRACELLULAR

CYTOPLASM

Cholesterol- and glycosphingolipid-rich domain

Lipid raft

Slide 29 · Membrane domains coordinate cellular operations

Consequences of geographic organization

Signaling

Local concentration of receptors and other effectors

Adhesion

Cell–cell and cell–matrix interaction sites

Membrane traffic

Specialized sites for vesicle budding or fusion

Spatial localization organizes signaling, adhesion and membrane traffic.

Slide 30 · Membrane proteins mediate essential cellular functions

Three major functional classes

Transport

Ions, metabolites and other solutes

Macromolecule uptake

Fluid-phase and receptor-mediated entry

Interfaces

Cell–ligand, cell–matrix and cell–cell interactions

Membrane proteins determine which tasks a cell surface can perform.

Slide 31 · Protein attachment often predicts functional role

A useful structural rule of thumb

Multi-pass proteins

Often pores

Often molecular transporters

Superficial attachments

More often participate in signaling

May be stabilized by lipid linkages

The way a protein contacts the bilayer helps predict its function.

Slide 32 · Integral proteins traverse the lipid bilayer

One of four attachment mechanisms

One or more hydrophobic α-helical segments cross the membrane

The segments provide a stable lipid-facing interface

Transmembrane proteins transmit signals across the bilayer

EXTRACELLULAR

CYTOPLASM

Hydrophobic α-helical region

Membrane-protein topology · schematic

Slide 33 · Multiple membrane-spanning regions support transport

A topology commonly associated with pores

Some transmembrane proteins cross the bilayer repeatedly

Multi-pass architecture often marks channels or carriers

Specific transport mechanisms are covered in the next topic

EXTRACELLULAR

CYTOPLASM

Multiple transmembrane segments

Membrane-protein topology · schematic

Slide 34 · Membrane proteins may be tethered by fatty acids

Posttranslational lipid anchors

Fatty acids are added after protein synthesis

Examples include palmitic and myristic acids

These groups insert into the cytoplasmic face

EXTRACELLULAR

CYTOPLASM

Cytosolic fatty-acyl anchor

Membrane-protein topology · schematic

Slide 35 · Prenylation is another cytosolic membrane anchor

Lipid modification of protein

Prenyl groups provide hydrophobic membrane attachment

Farnesyl groups are an example

These modified proteins associate with the inner face

EXTRACELLULAR

CYTOPLASM

Cytosolic prenyl anchor

Membrane-protein topology · schematic

Slide 36 · GPI anchors attach proteins externally

A different membrane-facing orientation

Glycosylphosphatidylinositol (GPI) tails are added after synthesis

The linkage holds proteins at the extracellular face

GPI linkage is distinct from cytoplasmic lipid modifications

EXTRACELLULAR

CYTOPLASM

GPI linkage · extracellular face

Membrane-protein topology · schematic

Slide 37 · Peripheral proteins can bind indirectly

Noncovalent membrane association

Some proteins contact integral membrane proteins

Noncovalent interactions maintain their attachment

They need not themselves penetrate the lipid core

EXTRACELLULAR

CYTOPLASM

Peripheral partner binds integral protein

Membrane-protein topology · schematic

Slide 38 · Membrane association: Four major mechanisms

Topology and linkage determine location

Crosses or anchors the bilayer

Integral transmembrane

Cytosolic lipid modification

Association without spanning

External GPI linkage

Noncovalent association

Four structural solutions for linking protein to membrane

Slide 39 · Protein complexes can assemble in the rough ER

The assembly site need not be the cell surface

Newly synthesized membrane proteins

Chaperone-directed assembly in rough ER

Multiprotein complex

Functional membrane localization

Some membrane complexes assemble before reaching the cell surface.

Slide 40 · Membrane proteins can assemble after lateral diffusion

Complex formation at the plasma membrane

Individually distributed proteins

Lateral diffusion in the bilayer

Local encounter

In situ multiprotein complex

Other protein complexes form directly on the plasma membrane.

Slide 41 · Ligand binding can promote receptor oligomerization

Functional signaling assemblies

Ligand binds membrane receptor

Receptors dimerize or trimerize

Functional signaling unit

Intracellular response

Ligand binding promotes an active multi-subunit receptor unit.

Slide 42 · Fluid membranes still establish physical boundaries

Two localization strategies

Lipid-domain sorting

Distinct lipid compositions

Preferential protein partitioning

Intercellular junctions

Protein–protein junctional contact

Boundaries between membrane regions

Mobility coexists with spatial boundaries.

Slide 43 · Tight junctions can delimit membrane compartments

Membrane domains remain distinct

Intercellular protein interactions establish boundaries

Distinct domains maintain differing compositions

This strategy is important in epithelial polarity

LUMEN / APICAL ENVIRONMENT

Junctional boundary

Slide 44 · Epithelial cells maintain apical–basolateral polarity

Different membrane domains, different roles

Apical membrane faces the luminal environment

Basolateral membrane faces a different environment

Compartmentalization supports specialized cell functions

LUMEN / APICAL ENVIRONMENT

APICAL

BASOLATERAL

Slide 45 · Polarity depends on more than lipid rafts

Multiple interacting organizing systems

Cell–cell interactions help define boundaries

Membrane proteins interact with one another

Cytoskeletal attachment helps maintain membrane organization

LUMEN / APICAL ENVIRONMENT

Membrane–cytoskeleton interactions

Slide 46 · Surface carbohydrates form the glycocalyx

A diffuse extracellular carbohydrate coat

Oligosaccharides decorate glycoproteins and glycolipids

Polysaccharide chains attach to membrane proteoglycans

These components collectively build the glycocalyx

EXTRACELLULAR CARBOHYDRATES

Glycoprotein · glycolipid · proteoglycan

Carbohydrate chains shown schematically

Slide 47 · The glycocalyx provides two types of protection

An extracellular protective layer

Chemical barrier

Carbohydrate-rich surface chemistry

Mechanical barrier

Surface coating contributes a physical interface

Surface organization

Glycoproteins, glycolipids and proteoglycans coexist

Extracellular carbohydrate chains build a protective surface.

Slide 48 · Barrier properties govern modes of exchange

How material crosses a plasma membrane

Bilayer diffusion

For sufficiently small or lipid-soluble molecules

Transport proteins

For many ions, metabolites and water

Vesicular pathways

For uptake and movement of macromolecules

Solute chemistry determines the required route across membranes.

Slide 49 · Small nonpolar gases readily diffuse

Direct movement through lipid

Oxygen (O₂) dissolves in the bilayer

Carbon dioxide (CO₂) also crosses readily

A dedicated channel is not required for this form of passage

EXTRACELLULAR

CYTOPLASM

Lipid-soluble solute

Simplified passive transbilayer movement

Slide 50 · Lipid-soluble steroid molecules can cross readily

Hydrophobicity favors passive passage

Estradiol is a steroid-based example

Vitamin D is another lipid-soluble example

Larger hydrophobic molecules may still penetrate bilayers

EXTRACELLULAR

CYTOPLASM

Lipid-soluble solute

Simplified passive transbilayer movement

Slide 51 · Water shows both diffusion and channel-mediated movement

A special case among small polar solutes

Water is small (18 Da) and can cross slowly

Aquaporins provide transmembrane water channels

Renal tubular epithelium is a tissue where water control matters

EXTRACELLULAR

CYTOPLASM

Transport pathway

Membrane-protein topology · schematic

Slide 52 · Glucose and ions are excluded by the lipid core

Size, polarity and charge all matter

Glucose (180 Da) is effectively excluded from direct passage

The source describes strong exclusion of larger polar molecules

Ions cannot pass freely because of charge and hydration

EXTRACELLULAR

CYTOPLASM

Hydrophobic interior limits passage

Nonpolar interior

Slide 53 · Permeability differs strongly across solute classes

Conceptual comparison

Nonpolar gases: Readily permeable

Water: Limited bilayer diffusion, channels important

Glucose and ions: Require non-bilayer transport routes

O₂ / CO₂

Diffusion through bilayer

Estradiol / vitamin D

Lipophilic passage

Water

Slow diffusion; Aquaporins

Glucose

Bilayer excludes

Hydrated ions

Bilayer impermeable

Slide 54 · Application: A dying cell exposes an inner-leaflet lipid

Apply membrane organization to a short problem

Which component provides the “eat me” recognition cue?

A. Phosphatidylserine

B. Sphingomyelin

C. Glycosylphosphatidylinositol

Select one answer and explain why the alternatives are less appropriate.

Slide 55 · Application: Exposed phosphatidylserine marks dying cells

Correct answer: Phosphatidylserine

PS is normally located predominantly on the cytosolic face

External exposure creates a recognition signal during apoptosis

The biological interpretation depends on the context of exposure

EXTRACELLULAR

CYTOPLASM

Externalized PS: Altered surface signal

Surface PS: Recognition cue

Slide 56 · Application: An epithelium loses membrane polarity

Apply membrane organization to a short problem

Which structure helps establish apical versus basolateral domains?

A. Tight-junction boundary

B. Unrestricted lateral mixing

C. Uniform lipid composition

Select one answer and explain why the alternatives are less appropriate.

Slide 57 · Application: Tight junctions maintain membrane domains

Correct answer: Tight-junction boundary

Cell–cell protein interactions delimit membrane regions

These boundaries contribute to epithelial polarity

Membrane proteins and cytoskeleton also support organization

LUMEN / APICAL ENVIRONMENT

Junctional boundary

Slide 58 · Application: A protein faces the extracellular space

Apply membrane organization to a short problem

Which anchor is specifically described for an extracellular-face protein?

A. GPI linkage

B. Cytosolic farnesyl anchor

C. Cytosolic myristoyl anchor

Select one answer and explain why the alternatives are less appropriate.

Slide 59 · Application: GPI linkage fixes extracellular orientation

Correct answer: Glycosylphosphatidylinositol

GPI tails are added posttranslationally

They link proteins to the extracellular face

Prenyl and fatty-acyl attachments localize to the cytosolic face

EXTRACELLULAR

CYTOPLASM

GPI linkage · extracellular face

Membrane-protein topology · schematic

Slide 60 · Integrated model: Structure determines function

The membrane as an organized interface

Amphipathic bilayer → diffusion barrier

Asymmetric lipids → signaling and recognition

Domains + anchored proteins → spatial specialization

Glycocalyx + polarity → tailored external interfaces

Architecture governs permeability, recognition and communication.

Slide 61 · Core principles to retain

Five organizing principles

The bilayer is fluid, asymmetric and selectively permeable.

PS exposure, phosphoinositide cleavage and surface glycolipids have distinct functions.

Membrane proteins use four major association mechanisms.

Rafts, junctions and cytoskeleton organize spatially specialized membranes.

The glycocalyx provides a carbohydrate-rich protective surface.

Slide 62 · Primary source and figure provenance

Textbook source

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th ed. (2026).

Chapter 1, The Cell as a Unit of Health and Disease, pp. 7–9.

Sections: Plasma Membrane; Membrane Transport; Passive Diffusion.

Membrane diagrams: original editable schematics based on concepts in Fig. 1.7.

Schematic provenance

All diagrams were constructed with editable PowerPoint text, lines and shapes. Robbins Fig. 1.7 is used as an information source, not reproduced.

No authentic microscopy or patient-derived images appear in this foundational cell-biology lecture.