MoCell- Transport Across Cell Membranes

Functions of the cell membrane

1. Physical separation

2. Regulation of exchange

3. Secretion

4. Communication

5. Structural support

Lipid Bilayer

Two-dimensional fluid that provides basic structure for all cell membranes

Passive transport

Movement of substances across a cell membrane without the use of cellular energy (ATP), driven by the concentration gradient.

Active transport

Movement of substances against the concentration gradient, requiring ATP.

Bulk transport

Transport of large quantities of materials into or out of the cell.

Exocytosis

Bulk transport process that moves substances out of the cell (secretion)

Endocytosis

Bulk transport process that moves substances into the cell.

Phagocytosis

Type of endocytosis where the cell engulfs large particles or cells. Plasma membrane invaginates to engulf cell or material

Receptor-mediated endocytosis

Process by which a cell internalizes specific molecules (ligands) from the ECF.

Pinocytosis

Process by which a cell engulfs small volumes of extracellular fluid along with dissolved solutes; "Cell drinking"; occurs in most euks to sample envmt or acquire fluids.

Lateral mobility

The ability of lipids and proteins within the lipid bilayer to move freely side-to-side; provides fluid character

Restricted mobility

The phenomenon where molecules rarely 'flip-flop' between the outer and inner layers of the lipid bilayer

Fluidity of the lipid bilayer

Fundamental property allowing movement of proteins and membrane remodeling during bulk transport.

Electrochemical gradient

The combined difference in concentration and charge across a membrane that influences movement of ions.

Simple diffusion

Passive movement of molecules across a cell membrane without the need for energy (ATP) or transport proteins.

Facilitated diffusion

Passive movement of molecules across a cell membrane with the help of transport proteins, no need for ATP.

Permeability of cell membrane to material

Hydrophobic molecule > polar molecule > ions (most permeable to least permeable).

Examples of simple diffusion

Major gases: O2, CO2, N2; Hydrophobic materials: steroidal hormones, cholesterol.

Examples of facilitated diffusion

Ions, glucose, amino acids, water.

Channels

Forms a pore across the membrane; conformational change = open/close (gating); passive, fast

Transporters

Bind substrate → conformational change → release on other side; passive or active, slower, highly specific

Example of channels

Ion channels (Na⁺, K⁺, Cl⁻), aquaporins (H₂O).

Example of active transport

Na⁺/K⁺ pump, glucose transporter (GLUT4)

Types of endocytosis

Phagocytosis and receptor-mediated endocytosis.

Sodium-potassium pump

Moves 3Na+ out and 2K+ in against their concentration gradients; requires energy expenditure, associated with an ATPase

Constitutive exocytosis

Continuous process in all cells that does not require an external signal; delivers lipids and proteins to the plasma membrane. Also releases ECM proteins and housekeeping mcules

Regulated exocytosis

Occurs in specialized secretory cells; vesicles are stored until the cell receives a specific signal. (ex. insulin)

Lysosome-mediated exocytosis

Involves fusion of lysosomes with plasma membrane to release lysosomal contents into extracellular space. Important for membrane repair, waste disposal, and immune defense

Phagosome

A large vesicle or vacuole that forms around the ingested cell or material in the process of phagocytosis.

Clathrin-coated pits

Regions of the plasma membrane that invaginate to form vesicles during receptor-mediated endocytosis.

Pinosome

Small vesicle formed during pinocytosis.

Non-specific process

Refers to pinocytosis, which does not target any particular molecules.

Why is flip-flopping energetically unfavorable?

Hydrophilic head groups would need to cross the hydrophobic core

Hemoglobin

Protein found in red blood cells (RBCs) that enables oxygen to be acquired from the environment and transported to cells

What is hemoglobin composed of?

Two alpha globin subunits, two beta globin subunits, and four heme groups

Heme Group

Binds oxygen in a reversible manner; enables acquisition of oxygen from environment; transport of oxygen to cells

Sickle Cell Disease

Recessive trait that is expressed when a human has two copies of mutant hemoglobin beta gene

How does HBB gene mutation cause sickle cell?

Mutations cause change in hemoglobin structure which alters shape of RBCs; reduced ability of hemoglobin to bind to oxygen

What does a reduced lifespan of RBCs cause?

Anemia

Ischemia

Reduced blood flow to organs and tissues; clumping of RBCs occludes blood vessels

What are the current treatments of sickle cell?

Metal chelators and bone marrow transplants

What are the functions of the cytoskeleton?

1. maintains and changes cell shape

2. facilitates cell motility and division

3. serves as tracks for movement of organelles and vesicles

4. provides structural support and resisatnce to mechanical stress

Cytoskeleton

Complex, dynamic network of interlinking protein filaments on the cytoplasm of all cells

What are the three major types of protein filaments that form the cytoskeleton?

1. Actin filaments

2. Microtubules

3. Intermediate filaments

Actin filaments

Helical polymers of the protein actin; aka microfilaments

What are microfilaments composed of?

Two parallel strands forming a helix, twisting every 37 nm; actin monomer has either ATP or ADP bound in a deep cleft in the center of the actin molecule

Where are actin filaments highly concentrated?

Cortex (just below plasma membrane)

Where can actin filaments be found?

Microvilli and striated muscle

Stress Fibers

Contractile bundles that play a role in cell adhesion, motility, and mechanosensing; made of actin filaments

Mechanosensing

Ability of a cell to perceive and respond to mechanical forces

Focal Adhesions

Connects cell's cytoskeleton to the extracellular matrix; made from actin filaments

Cytoskeletal filaments

Enables a cell to rapidly alter its shape in response to a change in its environment or a stimulus; example is cell division

Actin Monomer

Asymmetric, globular protein with an ATP/ADP-binding cleft

G-actin

Globular, monomer actin with a pointed end and barbed end

In the actin filament, orientation of the actin monomer is _______

Uniform

F-actin

Actin filament, has polarity

Plus end of actin:

Barbed end, fast-growing

Minus end of actin:

Pointed end, slow-growing

Actin is an ___ase

ATPase

ATPase

Bind and hydrolyze ATP; cleaves phosphate to generate ATP

Actin can exist as ATP-actin or ADP-actin

True

Which end is ATP-actin preferentially added to?

Plus end

Why does ATP-actin add to the plus end?

ATP-actin has a high affinity for filament ends vs ADP-actin

What happens after ATP-actin is incorporated into the filament?

ATP is hydrolyzed into ADP and Pi

Which end will have ADP-actin?

Minus end

Why is ADP-actin at the minus end?

ADP-actin has weaker binding, more likely to dissociate

What is the outcome of ATP hydrolysis on an actin filament?

Greater rate of polymerization in plus end, greater rate of depolymerization in minus end

What are the phases of actin polymerization?

1. Nucleation (rate limiting)

2. Elongation

3. Steady State

What happens during nucleation?

G-actin must form an aggregate to nucleate a filament (usually a stable trimer)

What happens during elongation?

Once a nucleus is formed, additional actin monomers are quickly added; plus end grows quickly, minus end grows slowly

What happens during steady state?

Filament maintains a roughly constant length, monomer contue to add to plus end and dissociate at minus end; results in treadmilling

Treadmilling

Dynamic flux of subunits through the filament

Capping Proteins

Caps the plus ends of F-actin, slowing down filament growth

Capped plus ends of F-actin are _____

Inactive (won't assemble or disassemble, only minus ends can at this point)

Distinct modes of cell migration rely on the actin cytoskeleton.

True

Microtubules

Long, hollow cylinders composed of the protein tubulin

Are microtubules or actin filaments more rigid?

Microtubules

Tubulin Subunit

Heterodimer formed from alpha tubulin and beta tubulin, held together by noncovalent bonds

What does each alpha or beta monomer have a binding site for one molecule of?

GTP (guanosine triphosphate)

What is the difference between alpha tubulin and beta tubulin binding?

GTP bound to alpha tubulin is physically trapped at the dimer interface, never hydrolyzed or exchanged; beta tubulin can be bound to either GTP or GDP

Protofilament

Alpha and beta tubulin heterodimers are stacked head to tail

How many protofilaments are there?

13 protofilaments, each has lateral contacts with the adjacent protofilament

Where do contacts occur between in protofilaments?

Between neighboring alpha-alpha and beta-beta subunits, slight stagger gives rise to helical microtubule lattice

Where are the minus ends of microtubules anchored?

MTOC

Dynamic Instability

Fundamental property of microtubules where they continuously and rapidly switch between phases of growth and shrinkage at their ends

Which phases are polymerization and depolymerization?

Polymerization is growth and depolymerization is shrinkage

Beta tubulin has ___ase activity

GTPase activity

What does GTP tubulin-dimer favor?

Polymerization

What does GDP tubulin-dimer favor

Depolymerization (due to weaker interactions with another tubulin subunit)

What does a GTP cap do?

Promotes growth

What happens if a GTP cap is removed and exposes a GDP tubulin?

Promotes shrinking

What are the four major phases of the cycle of dynamic instability?

1. Growth

2. Catastrophe

3. Shrinkage

4. Rescue

What happens in the growth phase?

Polymerization with GTP-tubulin

What happens in the catastrophe phase?

Sudden switch to shrinking

What happens in the shrinkage phase?

Depolymerization, GDP-tubulin peeling off

What happens in the rescue phase?

Switch back to growth when GTP-tubulin is incorporated and re-caps the end

Centrosome

Consists of a pair of centrioles surrounded by fibrous proteins and gamma tubulin ring complexes that nucleate microtubule growth; major MTOC in animal cells

What does the centrosome enable?

Rapid growth of spindle fibers during mitosis and rapid disassembly after cell division

Explain the plus and minus ends of microtubules in relation to centrosomes.

Minus end of each microtubule is embedded in the centrosome, plus end is free in the cytoplasm and can polymerize/depolymerize as needed

Microtubule organization varies widely among cell types

True