cell structure exam 2

What is the plasma membrane?

a collection of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

What is important about the α-helix of transmembrane protein?

it’s typically long enough to cross the bilayer

How do the polar peptide bond atoms bond with each other?

H-bonds

What is one way of creating pores across the membrane?

One or more α-helices can be used

Describe the general structure of a potassium channel

Amphipathic α helices interacting to form a channel through the cell membrane

Ion Channels

These proteins create pathways for specific ions (like sodium, potassium, calcium, and chloride) to move across the membrane.

Ion channel examples

voltage-gated ion channels and ligand-gated ion channels.

Aquaporins

Transmembrane proteins that facilitate the transport of water molecules across the membrane, ensuring water balance within the cell

Gated channels

These channels open and close in response to specific signals, regulating the passage of molecules.

Gated channel examples

neurotransmitter-gated channels in nerve cells open in response to chemical signals.

Gap Junction Proteins (Connexins)

These proteins form gap junctions between adjacent cells, allowing direct communication and the exchange of ions, small molecules, and signaling molecules.

Porins

Found in the outer membranes of bacteria, mitochondria, and chloroplasts, these are proteins that create channels for the passage of ions and small molecules

Proton Pump (Proton ATPase)

This transmembrane protein actively transports protons across the membrane, contributing to the establishment of proton gradients essential for processes like ATP synthesis.

Sodium-Potassium Pump (Sodium-Potassium ATPase)

This pump actively transports sodium ions out of the cell and potassium ions into the cell, maintaining the cell's electrochemical gradient.

Calcium Channels

These channels regulate the influx of calcium ions, which play crucial roles in various cellular processes such as muscle contraction and neurotransmitter release.

Selective permeability of phospholipid bilayer molecule transfer ease (easiest to hardest)

1. small nonpolar molecules

2. small uncharged polar molecules

3. large uncharged polar molecules

4. ions

What does cell permeability depend on?

– Molecules size (better to be small)

– Polarity (electric dipole moment)

– Charge (anion vs. cation; negative vs positive)

– Water vs. lipid solubility

Passive transport

Simple diffusion and facilitated transport

Simple diffusion

diffusion of small, nonpolar, hydrophobic molecules

– lipids

– HIGH → LOW concentration gradient

Facilitated diffusion

diffusion of small, polar, hydrophilic molecules through a protein channel

– HIGH → LOW concentration gradient

Active transport

– diffusion against concentration gradient LOW → HIGH

– uses a protein pump

– requires ATP

Functions of plasma membranes

1. Selective barrier: surround cells to hold enzymes and metabolics inside. cellular compartmentation surrounds organelles

2. Contain enzyme systems: energy metabolism

3. Contain transport systems: bring food molecules inside and maintain ion concentrations

4. Contains specific recognition sites: for hormones, other cells, etc.

Functions of membrane proteins

transport, enzymatic activity, signal transduction, cell-to-cell recognition, intercellular joining, attachment to cytoskeleton and ECM

Osmosis

(type of simple diffusion) the net flow or movement of solvent molecules through a semipermeable membrane through which solute molecules cannot pass. The solvents move from less concentrated solution to more concentrated.

T/F: water passage through the pores in membrane integral proteins (porins) depends on temperature

False

How is aquaporin high specificity achieved?

a channel diameter of 0.2 nm (not even H+ or OH- can pass through)

What is the first law of thermodynamics?

Energy is not created or destroyed, but can change form

Entropy

A measure of the degree of disorder of a system

How does glucose compare to CO2 and H2O?

Glucose is more organized, more potential energy, and less stable (less entropy)

Bulk transport

Movements of large solids (phagocytosis) or liquid (pinocytosis) molecules across the cell membrane using energy from respiration

Endocytosis

External large molecules enter the cell by forming an invagination in the cell membrane and eventually a vesicle surrounding these molecules; an active process, requires ATP

Exocytosis

Types of endocytosis

1. Phagocytosis

2. Pinocytosis

3. receptor-mediated endocytosis

Basic process of endocytosis

1. the cell membrane folds inward, forming a pocket around the target particle or particles

2. the pocket then pinches off with the help of specialized proteins, leaving the particle trapped in a newly created vesicle or vacuole inside the cell

Phagocytosis

a form of endocytosis in which large particles, such as cells or cellular debris, are transported into the cell. Cells ingest large particles into membrane-bound vesicles called phagosomes, which are then targeted by lysosomes for enzymatic degradation

Neutrophils

abundany in blood, quickly enter tissues, and phagocytize pathogens in acute inflammation

Macrophages

Closely related to monocytes in blood, longer-lived cells predominate in chronic inflammation. Also release some important inflammatory paracrine

Dendritic cells

phagocytosis in these cells is important for the elaboration of a specific immune response rather than for directly destroying the pathogens

B Lymphocytes

a small amount of phagocytosis in these cells is necessary…

Invasion mechanisms

some bacteria induce phagocytosis via zipper mechanism. Once phagocytosed, bacteria such as Listeria and shigella escape the phagosome and live within the host cytosol

Salmonella endocytosis

Salmonella destroys the microvilli of enterocytes and then invaginates into the cell, where they exist within a Salmonella-containing vacuole

How does induced phagocytosis occur?

• SipA and SipC interact directly with actin

• Host GEF mimetics SopE, SopE2, and SopB activate the host RhoGTP pathway, which induces actin polymerization

• Following the successful endocytosis, SptP induces actin depolymerization and facilitates the recovery of the host cell

How do the vesicles of pinocytosis compare to phagocytosis?

They are smaller

Spontaneous endocytosis

bulk-phase endocytosis or pinocytosis; doesn’t need specific signals or stimuli.

Receptor-mediated endocytosis

highly-specific and well-controlled. form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule. The receptors, which are transmembrane proteins, cluster in regions of the plasma membrane known as coated pits.

Describe the process of receptor-mediated endocytosis

Receptors bind to specific target molecules → endocytosis is triggered → receptors and attached molecules are taken into the cell in a vesicle

Aspects of clathrin-coated vesicles

1. clathrin

2. adaptor protein

3. cargo complex: membrane + cargo receptor + cargo

Stages of exocytosis in secretory cells.

(a) Formation of vesicles

(b) vesicle "docking”

(c) the transformation of vesicles into fusion-competent vesicles ("priming")

(d) fusion.

What allows exocytosis that is primed to occur?

Calcium

Types of chemical signals

receptor-ligand interactions, signaling molecules, pathogen entry

Receptor-Mediated Endocytosis

Specific molecules or ligands bind to cell surface receptors

What does receptor-mediated endocytosis binding trigger?

receptor clustering and signals the cell to internalize the ligand-receptor complex

Example of receptor-mediated endocytosis

low-density lipoprotein (LDL) particles binding to LDL receptors.

Types of signaling molecules

neurotransmitters, hormones, and growth factors

What do neurons release?

neurotransmitters

What do neurotransmitters bind to, and what does this trigger?

receptors on neighboring cells, triggering endocytosis of neurotransmitter-receptor complexes.

What can hormones initiate?

endocytosis upon binding to their cell surface receptors.

What do growth factors bind to?

receptor tyrosine kinases (RTKs) on the cell surface

What does internalization of growth factor-receptor complexes initiate?

signaling cascades and cellular responses.

Pathogen Uptake

Pathogens like bacteria or viruses can enter host cells through endocytosis

What do some pathogens exploit?

receptor-mediated endocytosis mechanisms, enabling their entry into host cells.

Examples of mechanical signals

membrane tension, shear stress, Filopodia and Lamellipodia, extracellular matrix interactions

Stretch-Activated Endocytosis

Mechanical stretching of the plasma membrane, caused by factors such as cell movement or external forces, can activate endocytosis.

What can mechanosensitive ion channels and other proteins can sense, and what does it trigger?

membrane tension. Triggers endocytosis to help regulate membrane surface area

Fluid-Phase Pinocytosis

Cells experiencing fluid shear stress, such as endothelial cells in blood vessels, can perform pinocytosis (cellular drinking) in response to the mechanical stimulus.

What does Fluid-Phase Pinocytosis help cells to do?

helps cells uptake nutrients or clear debris from the extracellular environment.

Filopodia and Lamellipodia Formation

During processes like phagocytosis and macropinocytosis, cells extend membrane protrusions called filopodia and lamellipodia, respectively.

What are filopodia and lamellipodia driven by, and what can they probe?

by actin polymerization and can probe the extracellular environment.

What happens if filopodia and lamellipodia encounter suitable ligands or mechanical cues?

endocytosis can be initiated.

Integrin Signaling

Integrins, cell adhesion receptors, can transmit mechanical signals from the extracellular matrix to the cell interior.

What can integrin clustering and activation induce, and what does that influence?

an induce endocytosis, influencing cell adhesion and migration.

Spontaneous endocytosis

the continuous internalization of small vesicles from the plasma membrane without the need for specific external chemical signals

What does spontaneous endocytosis help cells to do?

regulate their surface area, retrieve membrane components, and maintain cell membrane integrity.

When will spontaneous endocytosis occur?

when it Lowers the Free Energy of the system.

What cellular mechanisms does the initiation of spontaneous endocytosis involve?

1. Membrane Tension and Curvature

2. Lipid Composition

3. Actin Dynamics

4. Dynamin-Independent Endocytosis

Membrane tension

When the plasma membrane experiences decreased tension due to various factors such as cell movement or mechanical stress, it can trigger spontaneous endocytosis

What does lower membrane tension promote?

the formation of membrane invaginations and subsequent vesicle budding.

Membrane Curvature

can arise due to the insertion of proteins or lipids that induce bending of the membrane

What can curved membrane regions facilitate?

The formation of endocytic pits, which are the initial structures of vesicle formation.

Lipid Rafts

a type of lipid composition rich in cholesterol and sphingolipids that can create

microdomains in the plasma membrane that favor spontaneous endocytosis

What do lipid rafts serve as, and what do they promote?

serve as platforms for clustering membrane proteins, promoting the formation of endocytic vesicles.

Actin Polymerization

Actin filaments can push the plasma membrane inward, forming endocytic pits. an ATP-driven process where ATP is hydrolyzed to provide the energy for the assembly and disassembly of actin filaments.

What are actin dynamics regulated by?

by various proteins and signaling pathways,

Dynamin

a GTPase essential for vesicle fission. essential for pinching off vesicles from the plasma membrane.

What happens to vesicles after endocytosis?

vesicles are transported along the cytoskeleton to reach their destination

What facilitates vesicle transport?

Motor proteins, such as dynein and kinesin

What do motor proteins require?

TP hydrolysis to generate the mechanical force necessary for vesicle movement.

What does membrane fusion require?

energy

SNARE proteins

essential for membrane fusion, undergo conformational changes that are ATP-dependent.

What gives the nuclear envelope shape?

• Nuclear lamin (Meshwork of intermediate filaments (type A and B))

• Sun-Linc Complex

• Nuclear Pore Complex

Sun-Linc Complex

• Holds the double membrane together

• Connects cytoplasm to the nucleus

• Required for intracellular force transmission (Is utilized for mechanotransduction; translates physical forces (compression, tension, shear stress) into biochemical signals)

Dilated cardiomyopathy

Impaired pumping of the left ventricle. Loss of muscle control causes the space to become dilated and impairs blood flow.

Arrhythmogenic cardiomyopathy

Heart muscles fail to contract properly, resulting in arrhythmia.

Emery-Dreifuss Muscular Dystrophy (EDMD)

Muscle wasting, cardiac weakness, and joint stiffness resulting from improper muscle formation and function

How many genes have been linked to EDMD, and how many of those genes encode components of the SUN-LINC complex?

6 genes, 4 linked to SUN-LINC complex

What makes EDMD a degenerative disease?

it causes a gradual, progressive breakdown and loss of muscle function and structure over time.

Satellite Cells

Specialized progenitor cells in muscle tissue

What does mechanical stress cause?

chromatin remodeling via the LINC complex

What does chromatin remodeling expose?

MRF promoter accessibility and causes production of MRFs

Nucleocytoplasmic Transport

A Gated Transport between cytoplasm and nucleus