Membrane Proteins + Cell Communication 3A+3B

What can cross the cell membrane easily?

Small hydrophobic (nonpolar) molecules (e.g. gases like O2, CO2)

What cannot cross the membrane easily?

Large, polar, or charged molecules (e.g. ions, glucose, amino acids)

What is a concentration gradient?

Difference in concentration of a substance across a space or membrane

What is diffusion?

Movement of molecules from high to low concentration down a concentration gradient

What is equilibrium?

State where concentration is equal and no net movement occurs

What is osmosis?

Movement of water across a semi-permeable membrane

How does osmosis balance solutes?

Water moves to balance solute concentration when solutes cannot cross membrane

What is tonicity?

Relative concentration of solutes outside vs inside a cell

What is isotonic?

Same solute concentration; no net water movement

What is hypertonic?

Higher solute outside; water leaves cell → cell shrinks

What is hypotonic?

Lower solute outside; water enters cell → cell swells

What are integral membrane proteins?

Proteins embedded in lipid bilayer; often transmembrane; penetrate hydrophobic core; often glycosylated

What are peripheral membrane proteins?

Attached indirectly via protein interactions; not embedded; can dissociate easily; soluble in aqueous environments

What are the six major functions of membrane proteins?

Transport; enzymatic activity; signal transduction; cell-cell recognition; intercellular joining; attachment to cytoskeleton and ECM

What are transport proteins?

Proteins that move substances across membrane

What are channel proteins?

Provide corridors for specific ions/molecules to cross membrane

What are carrier proteins?

Bind solute and change shape to move it across membrane

What is facilitated diffusion?

Passive transport using proteins; increases speed; moves down gradient

What is active transport?

Movement against concentration gradient; requires energy (ATP)

What are pumps?

Proteins that actively transport substances using ATP

What is uniport?

Carrier transports one substance

What is symport?

Carrier transports two substances in same direction

What is antiport?

Carrier transports substances in opposite directions

What is co-transport?

Coupled transport where movement of one solute drives movement of another

How does co-transport work?

The first solute is actively transported from low to high concentration, creating a gradient. The second solute then moves from high to low concentration using the energy stored in that gradient.

How are membrane enzymes regulated?

pH; temperature; salt; co-factors; co-enzymes; inhibitors

What is the role of membrane enzymes?

Catalyse biochemical reactions at membrane or within organelles

What is signal transduction?

Process where receptor receives signal and relays it into the cell

What are the three stages of signalling?

Reception; transduction; response

What happens in reception?

Ligand binds receptor → receptor activated

What happens in transduction?

Signal relayed through cascade of molecules (often phosphorylation)

What happens in response?

Cell changes (gene expression, shape, division, death)

What is cell-cell recognition?

Cells identify each other via glycoproteins (carbohydrate tags)

Why is cell-cell recognition important?

Tissue formation; immune response; blood groups; distinguishing self vs non-self

What is intercellular joining?

Proteins connect neighbouring cells (tight junctions, adherens junctions)

What are tight junctions?

Cells tightly packed; form barriers

What are adherens junctions?

Cells connected but with more flexibility/space

How are cells attached to ECM?

Membrane proteins link ECM outside to cytoskeleton inside

What structures connect inside and outside of cell?

Transmembrane proteins linking cytoskeleton (actin) to ECM (collagen, laminin, fibronectin, proteoglycans)

What is the extracellular matrix (ECM)?

Network outside cells providing support and signalling

What problems exist for cell communication in multicellular organisms?

Long distances; ECM barriers; signal dilution

What are ligands?

Signalling molecules that bind receptors to trigger responses :contentReference[oaicite:1]{index=1}

What types of molecules can be ligands?

Proteins; peptides; amino acids; nucleotides; steroids; retinoids; fatty acids; gases

What is long-range signalling?

Communication over long distances via bloodstream or neurons

What is endocrine signalling?

long-distance cell communication in which specialised endocrine glands release hormones directly into the bloodstream. These hormones travel at low concentrations to distant target tissues, producing slower but widespread effects throughout the body.

What is neuronal signalling?

Fast; precise; neurotransmitters released at synapse

short distance

What is synaptic signalling?

Electrical signal travels along axon; neurotransmitters released at synapse (~100 nm gap)

a precise form of cell-to-cell communication where neurons transmit chemical signals (neurotransmitters) across a narrow gap, known as a synapse, to another neuron or muscle cell

What is short-range signalling?

Local signalling between nearby cells

What is paracrine signalling?

Cell releases signal affecting nearby cells

a form of local cellular communication where a cell produces signals (paracrine factors) to induce changes in nearby cells, altering their behavior.

What is autocrine signalling?

Cell releases signal affecting itself

a form of cell communication where a cell secretes hormones or chemical messengers (e.g., growth factors, cytokines) that bind to receptors on the same cell, triggering a self-generated response

What is contact-dependent signalling?

Cells directly interact via membrane-bound proteins

two cells must be in direct physical contact to communicate.

• the signalling molecule remains attached to the membrane of one cell, and

• it binds to a receptor on the membrane of a neighboring cell.

So, the cells communicate through membrane-bound proteins rather than released chemical signals.

How is paracrine/autocrine signalling regulated?

• limited diffusion of signalling molecules

• uptake by cells

• enzymatic degradation

• immobilisation by the extracellular matrix (ECM)

• antagonists that block signalling.

What are cell surface receptors?

Proteins on membrane that bind ligands and trigger intracellular signals

What are ion-channel-linked receptors?

cell surface receptors that open or close an ion channel when a ligand binds, allowing ions to pass across the membrane and producing rapid cellular signalling.

What are G-protein coupled receptors (GPCRs)?

seven-pass transmembrane receptors that activate a G protein when a ligand binds, triggering enzymes and intracellular signalling cascades.

What are enzyme-linked receptors?

Single-pass transmembrane receptors that activate an enzyme directly or through interactions with other proteins when a ligand binds.

What are receptor tyrosine kinases?

enzyme-linked receptors that dimerise when a ligand binds and activate tyrosine kinase activity, leading to phosphorylation and intracellular signalling cascades.

What do kinases do?

Transfer phosphate groups to proteins (phosphorylation)

What is phosphorylation?

the addition of a phosphate group to a molecule, usually a protein, by a kinase enzyme to alter the protein’s activity or function.

What are intracellular receptors?

Receptors inside cell; require small/hydrophobic ligands

What types of ligands bind intracellular receptors?

Gases (NO, CO); steroids; thyroid hormones; retinoids; vitamin D

What do intracellular receptors do?

Control gene transcription

What are fast intracellular responses?

when a ligand rapidly activates an existing intracellular protein, producing an immediate cellular effect without changing gene transcription.
Example: nitric oxide signalling.

What are slow intracellular responses?

when intracellular receptors regulate gene transcription, leading to the synthesis of new proteins and slower but longer-lasting cellular effects.

What determines speed of response?

Type of signalling and type of cellular response

Why can the same ligand cause different responses?

Different receptors in different cells → different signalling pathways

What is signal amplification?

One signal activates many molecules in cascade → amplified response

What are second messengers?

Small molecules that relay signals inside cell (e.g. cAMP)

What is cAMP?

Second messenger made from ATP by adenylyl cyclase

What does cAMP do?

Relays signal and activates protein kinase A

How is cAMP inactivated?

Converted to AMP by phosphodiesterase

What is the role of G-proteins?

G-proteins transmit signals from activated receptors to target enzymes, such as adenylyl cyclase, to initiate intracellular signalling pathways.

How can cells regulate sensitivity to signals?

Sequester receptor; degrade receptor; inactivate receptor; inactivate signalling proteins; produce inhibitors

“Sequester receptors” means the cell temporarily hides or removes receptors from the cell surface so signalling molecules cannot bind to them as easily.

What are gap junctions?

Channels connecting cytoplasm of adjacent animal cells

What are plasmodesmata?

Channels connecting cytoplasm of adjacent plant cells

What are possible cellular responses to signals?

Grow; divide; differentiate; survive; die (apoptosis)

What is differentiation?

Process where cells become specialised with specific functions in tissues