Biology chapter 5 & 9

What is membrane made of? why?

• Phospholipid molecules.
  Hydrophilic head (polar)

Hydrophobic tails (non polar)

Head towards water, tail away from water

The phospholipid bilayer is fluid and flexible - lipids can move within the membrane, and membranes can bend and form vesicles

• proteins

• sterols

• carbohydrates

Sterols

Inside membrane with polar and nonpolar ends.

Cholesterol - major sterol in animal cells, inserted between phospholipids to regulate fluidity of membrane with temperature change.

Influences how tightly phospholipids pack

Proteins

• Transport proteins - move ions, water, solutes

• Receptor proteins - bind signals → responses

• Recognition proteins - identify self vs foreign

• Adhesion proteins - cells attach to one another

• Integral membrane proteins - around the bilayer, transport molecules and act as receptors:

  • Hydrophobic regions interact with lipid tails.

  • Hydrophilic regions face aqueous environments or create channels.

Peripheral proteins - attach to membrane/integral proteins (support signaling, structure, or enzyme activity)

Carbohydrates

• Found on the outer membrane surface

• Glycolipids = lipid + carbohydrate.

• Glycoproteins = protein + carbohydrate.

• In animal cells, these can form the glycocalyx

Main function: cell recognition, cell to cell communication, attachment aid

The Fluid Mosaic Model (Singer & Nicolson, 1972)

• membrane as a fluid phospholipid bilayer with molecules that can move

laterally

• Membrane proteins - within or attached to the bilayer, creating a "mosaic" appearance

• Cholesterol - regulates membrane fluidity and stability in animal cells.

• Carbohydrates - attached to lipids and proteins. Cell recognition, signaling, and adhesion.

• Membrane - dynamic, allowing transport, communication, membrane repair, and vesicle formation.

• The model explains how membranes are selectively permeable while remaining flexible and adaptable.

Misconceptions:

• Everything moves at the same rate

• Bags holding things

• Proteins are loose

Why fluidity is important?

• Proteins need correct lipid environment to function

• Fusion, vesicle formation, and signaling depend on fluidity

• Too rigid: transport and protein movement can be impaired.

• Too fluid: permeability and stability can be disrupted

Factors Affecting Fluidity

• Temperature

• Fatty acids

• Cholesterol

Selective Permeability

What crosses depends on the properties of the molecule and the presence of membrane transport proteins.

Pass:

Small Nonpolar Molecules

Limited Pass:

Small Uncharged Polar Molecules

Do not Pass:

Large Polar Molecules

Charged Ions

Simple diffusion

Movement Down a Concentration Gradient

• Higher concentration → lower concentration

• No transport protein

• No ATP

• Small, nonpolar, lipid-soluble molecules

• O₂, CO₂, N₂, steroid hormones, and other lipid-soluble molecules

Facilitated diffusion

Passive transport through proteins

• Down concentration gradient or electrochemical gradient

• No ATP

• Requires transport protein

• Used by water, ions, sugars, amino acids, and other polar molecules

Channel Proteins: Fast, Selective Passageways

• Hydrophilic pores through the bilayer

• Aquaporins - water channels

• Ion channels - specific ions to move down electrochemical gradients

• Many ion channels are gated: open, closed, or intermediate states.

Carrier Proteins

• Binds solute on one side

• binding → conformational change

• solute is released on opposite side

• Carriers can become saturated when all binding sites are occupied

Active transport

Primary: Moving Against a Gradient Requires Energy

• Lower concentration → Higher concentration

• Requires ATP (directly or indirectly)

• Maintains ion gradients and membrane potential

• Supports nutrient uptake, waste removal, pH regulation, and cell signaling

Secondary: Symport and Antiport

• ion moving down its gradient drives another solute uphill

• Symport: both substances move in the same direction

• Antiport: substances move in opposite directions

• Energy comes indirectly from ATP used to build the ion gradient

Active transport: Na⁺/K⁺ Pump

Builds an Electrochemical Gradient

• Found in the plasma membrane of animal cells

• ATP → pump 3 Na⁺ out and 2 K⁺ in per cycle

• creates ion gradient (contributes to membrane potential)

• Provides stored energy for secondary active transport

Membrane Potential: Electrical Energy Across a Membrane

▪ Unequal ion distributions create a voltage across the membrane.

▪ The inside of many cells is relatively negative compared with the outside.

▪ Membrane potential is essential for neurons, muscle cells, and transport.

▪ Electrochemical gradients combine concentration and electrical forces.

Vesicle Transport

• Large molecules move by membrane vesicles.

• Exocytosis - exports material and adds membrane to the plasma membrane.

• Endocytosis - imports material and removes membrane from the plasma membrane.

• Both processes require energy and involve membrane remodeling.

Exocytosis

Export by Vesicle Fusion

• Vesicles from Golgi complex

• Vesicles from inside fuses with the membrane

• Cargo released outside

• Vesicle membrane becomes part of the plasma membrane

Endocytosis

Import by Vesicle Formation

• Bulk - non specific import

• Receptor Mediated - selective import

  • Target molecule binds specific receptors

  • receptors cluster in pits reinforced by clathrin

  • The pit pinches off to become a vesicle

  • Cargo may be digested, receptors can be recycled

• Phagocytosis - Engulfing Large Particles

  • Cells extend membrane lobes around a large particle or cell.

  • The enclosed particle forms a large vesicle called a phagosome.

  • Vesicles can fuse with lysosomes for digestion.

  • Important in immune defense and feeding by some protists.

Signal transduction

• Many signals cannot cross the membrane directly

• Receptor receives the signal outside or inside

• Transduction relays and amplifies the signal inside the cell

• Response changes cell activity ( cell activity, such as secretion, transport, gene expression, or movement)

Reception

Receptors Recognize Specific Signals

• Ligands - signal molecules that bind receptors

• Membrane receptors bind hydrophilic signals outside the cell

• Binding changes receptor shape/activity

• Specific receptors allow cells to respond only to certain signals

Surface Receptors

Transmembrane Proteins

• Polar signals can’t cross the hydrophobic core

• Surface receptors bind signals outside

• Binding changes receptor shape

• Cytoplasmic side initiates an internal response pathway

G-Protein-Coupled Receptors

▪ Seven transmembrane segments.

▪ Signal binding activates a G protein associated with GPCR

▪ G proteins GDP-bound (inactive) → GTP-bound (active) states.

▪ Activated G proteins regulate activate a signal relay through second messenger.

Receptor Tyrosine Kinases

• Ligand binding promotes receptor dimerization.

• Kinase domains phosphorylate tyrosine’s on the partner receptor.

• Phosphorylated sites recruit signaling proteins.

• RTKs often regulate cell growth, division, and differentiation

Ligand-Gated Ion Channels

• Ligand binding changes channel conformation.

• The channel opens or closes.

• Ion flow changes membrane potential or cellular activity.

• These receptors are important in synaptic signaling

Internal Receptors

• Nonpolar signals such as steroid hormones can cross the membrane

• They bind intracellular receptors

• Activated receptor complexes regulate gene expression

• Responses are often slower but longer-lasting than ion channel responses

Transduction: Relay, Amplify, and Integrate

Signals are often amplified: one activated receptor can activate many intracellular molecules.

Signal Amplification

• One receptor can activate many molecules downstream

• Enzyme cascades amplify the response

• More catalytic steps → greater amplification

• Amplification helps cells respond to very low signal concentrations

Signal amplification second messenger

• cAMP is a 2nd messenger

• Adenylyl cyclase converts ATP to cAMP.

• cAMP diffuses through the cytoplasm, activates protein kinases.

• Phosphodiesterase breaks cAMP down to help turn the signal off.

Signal transduction Pathway: GPCR Pathway

▪ Ligand binds to GPCR, which activates G protein

▪ G protein activates an effector enzyme → produces second messengers → activate protein kinases to phosphorylate target proteins

Signal transduction Pathway: Ras/MAP Kinase Pathway

▪ Activated RTKs → activate Ras (a small G protein)

▪ Ras initiates a kinase cascade, activating multiple kinases that lead to changes in gene expression and cell proliferation

▪ Linking external signal to membrane signaling to cell proliferation decisions

Response: Changing Cell Behavior

▪ Responses may be rapid, such as opening ion channels or triggering secretion.

▪ Responses may be slow, such as changing gene expression.

▪ Signals can regulate membrane traffic, including exocytosis and endocytosis.

▪ Cells integrate multiple signals before committing to a response.

Off Switches

• Signals must be turned off to avoid excessive responses.

• Ligands can be degraded or removed.

• Receptors can be endocytosed and degraded or recycled.

• Second messengers can be broken down.

• G proteins inactivate themselves by hydrolyzing GTP to GDP

Cross-Talk and Integration

• Cells often receive many signals at the same time and pathway can converge on shared targets.

• Integration allows flexible, context-dependent responses