Membranes

Lipids

Phospholipids:

• Amphipathic molecules with a polar head and two hydrophobic fatty acid tails

• Tails vary in length and saturation - affecting fluidity

• Saturated = straight, pack tightly, Unsaturated = kinked, pack poorly, more fluid

Cholestrol

• Regulates fluidity by preventing tight packing at low temperatures and excessive fluidity at high ones

Proteins

Integral (intrinsic) membrane proteins

• Form channels, transporters, receptors and enzy,es

Peripheral (extrinsic) membrane proteins

• Often interact with integral proteins of the cytoskeleton

• Easily removed by salt washes

Lipid-anchored proteins

Carbohydrates

Many membrane proteins are glycosylated

Carbohydrates form the glycocalyx which is important for:

• cell recognition

• protection

• adhesion

Micelles

Hydrophilic heads and hydrophobic tails - when placed in water geads face water, tails face away producing a spherical structure.

Only a single layer

Liposomes

Hydrophilic groups face the water, hydrophobic tails face inward forming a bilayer as a closed spherical vesicle.

How do detergents work?

Detergents can solubilise membrane proteins.

• They are amphipathic - hydrophobic tail, hydrophilic head

• They form micelles as they have 1 tail

1. Detergent inserts into the lipid bilayer of phospholipids, loosens and destabilises it

2. Detergent pulls lipids and proteins into mixed micelles

Freeze fracture electron microscopy experiment

1. Rapid freezing of cells or tissues with liquid nitrogen to immobilise molecules and prevent ice crystals forming

2. Fracturing the frozen sample using a sharp blade. The fracture splits between the two leaflets of the membrane along the hydrophobic interior

3. Exposing the membrane faces: P-face and E-face. Integral proteins remain embedded leaving bumbs or pits.

4. Creating a metal replica - a thin layer of platinum or carbon is evaporated on the surface to make a replica

5. View under electron microscope

Non selective uptake of small molecules

Does not discriminate between different solutes

Pinocytosis:

• Cell forms clathrin coated pits that invaginate and pinch off

• Whatever solbule molecules happen to be in extracellular fluid are taken up

Selective uptake of small molecules

Requires specific interaction between a molecule and a receptor.

1. Via membrane transport proteins e.g. channels (passive) or carriers (active or passive)

2. Receptor-mediated endocytosis: requires a specific receptor which binds to their ligand and cluster into clathrin coated pits

Types of membrane permeation mechanisms

• Simple diffusion – small non‑polar molecules cross directly.

• Channel‑mediated diffusion – ions move through hydrophilic pores.

• Carrier‑mediated transport – selective binding; passive or active.

• Primary active transport – ATP‑driven pumps (e.g., Na⁺/K⁺ ATPase).

• Secondary active transport – uses ion gradients (symport/antiport).

• Endocytosis – uptake of large molecules or particles (phagocytosis, pinocytosis, RME).

Passive transport

• No energy rquired

• Movement from a high to low concentraiton - down the conc/electrochemical gradient

• Channels (always)

• Carriers (usually unless coupled)

Active transport

• Requires energy

• Move against conc gradient - low to high

• Only carrier proteins (sometimes)

How are transporters organised in a polarised epithelial cell

• Apical membrane (lumen side): Na⁺/glucose symporter brings glucose into the cell

• Basal membrane (blood side): GLUT transporter releases glucose into the blood

• Basal Na⁺/K⁺ ATPase maintains the Na⁺ gradient

• Tight junctions prevent mixing of transporters between domains

Mechanisms for taking up large molecules

1. Phagocytosis: defence, clearance of apoptotic cells, tissue homeostasis - only performed by specialised cells (macrophages)

2. Pinocytosis: non selective, used for bulk uptake of nutrients and membrane turnover

3. Receptor mediated endocytosis (RME): selective

Role of clathrin in endocytosis

1. Ligands bind to receptors

2. Receptors cluster into coated pits - forming a clathrin-coated pit

3. Clathrin assembly bends the membrane - pulling it inward, creating a bud

4. Clathrin coat is shed

The uptake of LDL by RME

1. LDL circulates in the blood bound to LDL particles. Cholestrol is very insoluble so is carried in the blood in LDL particles which are the ligands for the LDL receptor

2. LDL binds to LDL receptors on the cell surface

3. Receptor-LDL complexes cluster into clathrin coated pits

4. The pit invaginates and pinches off to form a clathrim coated vesicle

5. Clathrin coat is shed

6. Vesicle fuses with an early endosome

7. LDL receptors and LDL particles are separated. receptors are recycled and particles are sent to lysosomes and broken down to release cholestrol

8. Cholestrol is released into the cytoplasm

Types of intercellular signalling

• Endocrine

• Paracrine

• Neuronal

• Contact - dependent

• Autocrine

Endocrine

Release hormones that have a long term, slow acting response

Paracrine

When a cell releases signalling molecules which interact with neighbouring cells

Neuronal

Electrical impulses travel across neurones

Contact dependent

Short range, direct physical contact is required between cells

Autocrine

Cell responds to signal produced by itself

Cell surface receptors

1. Ion-channel-coupled receptors

2. G-protein coupled receptors (GPCR)

3. Enzyme linked receptors

Ion-channel-coupled receptors

• A signalling molecule (often a neurotransmitter) binds to the receptor

• Binding causes the ion channel to open or close

• Ions flow down their electrochemical gradient

• This produces a rapid change in membrane potential

G-protein-coupled receptors (GPCRs)

• Ligand binds to the GPCR

• GPCR activates a trimeric G-protein

• The activated G-protein activates or inhibits an effector protein

• The effector generates a second messenger

• Second messengers diffuse and activate downstream targets

Enzyme linked receptors

• Ligand binds

• Receptors dimerise

• Tyrosine kinase domains autophosphorylate

• Phosphotyrosines recruit signalling proteins

• This triggers phosphorylation cascades

How do GPCRs detect an extracellular signal and convert it into a response?

1. GCPRs detect extracellular signals and a ligand binds to the extracellular domain of the GPCR, changind the receptor’s conformation

2. GPCR activates a trimeric G-protein

3. The acitvated G-protein dissociates

4. The G-protein activates an effector protein

5. Second messengers amplify the signal

6. PKA phosphorylates intracellular targets and transcription regulators

7. Signal termination

Describe a signallling pathway from the plasma membrane to the nucleus

1. Extracellular signal binds to GPCR at the plasma membrane converting it into an intracellular signal

2. GPCR activates a trimeric G-protein causing GDP→ GTP exchange on the α-subunit

3. Gαs activates adenylate cyclase (AC) (an enzyme in the plasma membrane). AC converts ATP → cAMP (second messenger)

4. cAMP diffuses through the cytosol and binds to Protein Kinase A (PKA) releasing its catalytic subunits

5. PKA enters the nucleus and phosphorylates transcription factors

6. Phosphorylated CREB activates transcription of specific target genes

The Secretory Pathway

1. Protein synthesis begins on ribosomes in the cytosol - an ER signal sequence directs the ribosome to the RER

2. CO-translational translocation into the RER: signal sequnce is recognised by SRP, pausing translation and bringing the ribsome ot the ER membrane. It docks on a translocon and translation resumes

3. ER → Golgi transport via vesicles: proteins are packaged into COPII coated vesicles

4. Golgi apparatus modifies and sorts proteins

5. Proteins are transported in vesicles to the plasma membrane, secretory vesicles or lysosomes

6. Exocytosis at the plasma membrane: vesicles fuse with the plasma memrbane through SNARE - mediated fusion