Histo: Cytoplasm, Membranous Organelles, Plasma Membrane Flashcards
Definition of cells
Basic structural, functional, and integral units of all multicellular organisms
(T/F) Some cells become specialized in certain functions.
True
2 major components of cells
Cytoplasm and nucleus
The cytoplasm contains…
Organelles and cytoplasmic matrix
The cytoplasmic matrix consists of…
Variety of solutes (including inorganic ions - Na+, K+, Ca2+), organic molecules (intermediate metabolites, carbohydrates, lipids, proteins, RNA), inclusions (crystals, pigment granules, lipids, glycogen, other stored waste products)
(T/F) Concentration of solutes within cytoplasmic matrix does not influence rate of metabolic activity within the cytoplasm.
False
The largest organelle in the cell with a distinct boundary is…
Nucleus
Nucleus contains…
Genome, enzymes necessary for DNA replication and RNA transcription
2 types of organelles
Membranous and nonmembranous
The spaces enclosed by organelle membranes constitute the…
Intracellular microcompartments
The membranous organelles include…
Plasma (cell) membrane), rough-surfaced endoplasmic reticulum (rER), smooth-surfaced endoplasmic reticulum (sER), Golgi apparatus, endosomes, lysosomes, transport vesicles, mitochondria, peroxisomes
The nonmembranous organelles include…
Microtubules (cytoskeleton in general), filaments, centrioles, ribosomes
What is the plasma membrane?
A lipid-bilayered structure that actively participates in many physiologic and biochemical activities essential to cell function and survival.
The plasma membrane consists of…
Amphiphatic phospholipid bilayer, cholesterol, protein molecules (integral and peripheral)
(T/F) Lipids are distributed symmetrically between inner and outer leaflets of the lipid bilayer.
False - Lipids are distributed asymmetrically.
In most plasma membranes, protein molecules constitute approximately what portion of the total membrane mass?
Half
Peripheral membrane proteins are associated with the membrane by…
Strong ionic interactions, mainly with integral proteins on the extracellular and intracellular surfaces.
Glycoproteins are formed when…
Carbohydrates attach to proteins, especially on extracellular membrane surface
Glycolipids are formed when…
Lipids attach to proteins, especially on extracellular membrane surface
What is the glycocalyx?
The “cell coat” formed by glycolipids and glycoproteins at the surface of a cell. It helps establish extracellular microenvironments and has specific functions in metabolism, cell recognition, and cell association.
What are lipid rafts?
Microdomains of the plasma membrane that control movement and distribution of proteins within the lipid bilayer. Acts as “signaling platforms.” Less fluidity/diffusion than surrounding membrane.
What do lipid rafts consist of?
High concentrations of cholesterol and glycosphingolipids. Integral and peripheral membrane proteins involved in cell signaling, enzymes, receptors, etc.
Integral membrane proteins can be visualized with what technique?
Freeze fracture - membranes cleaved along hydrophobic plane to expose E-face (backed by ECM) and P-face (backed by cytoplasm). Normally more integral proteins on P-face.
Integral membrane functions have important functions in…
Cell metabolism, regulation, and integration
6 functional categories of membrane proteins
Pumps, channels, enzymes, structural proteins, receptors, linkers
(T/F) A membrane protein can be both a receptor and a structural protein.
True
The fluidity of the membrane is a function of…
Types of phospholipids in membrane and variations in local concentrations
Lateral migrations of proteins in the membrane is limited by physical connections between membrane proteins and intracellular/extracellular structures such as…
Proteins associated with cytoskeletal elements, portions of proteins that extend into adjacent cytoplasm, cytoplasmic domains of membrane proteins, peripheral proteins associated with ECM, integral proteins that extend from cell surface
Transmembrane linkers are…
Proteins localized/restricted to specialized regions of the plasma membrane, link intracellular/extracellular filaments
Cell injury often manifests as…
Morphologic changes in the cell’s plasma membrane, which results in formation of plasma-membrane blebs
Plasma-membrane blebs are…
Protrusions of plasma membrane caused by detachments of plasma membrane from underlying actin filaments of the cell cytoskeleton
Simple diffusion allows what substances to pass through the plasma membrane?
Fat-soluble, small, uncharged molecules down their concentration gradient
2 classes of membrane transport proteins
Carrier proteins and channel proteins
Carrier proteins transfer what substances?
Specific small, water-soluble molecules by a conformational change of the protein. May be active or passive transport.
(T/F) Glucose carrier proteins do not require energy.
True
Channel proteins transfer what substances?
Specific small, water-soluble molecules through hydrophilic channels/a pore domain.
Channel protein transport can be regulated by…
Membrane potentials (voltage-gated ion channels in neurons), neurotransmitters (ligand-gated ion channels such as acetylcholine receptors in muscle cells), mechanical stress (mechanically gated ion channels in internal ear)
Endocytosis is…
Process of vesicular transport in which fluid and macromolecules enter the cell
Exocytosis is…
Process of vesicular transport in which fluid and macromolecules leave the cell
Clathrin is…
Best known protein that interacts with the plasma membrane in vesicle formation - endocytosis can be dependent on or independent
3 mechanisms of endocytosis
Pinocytosis, phagocytosis, receptor-mediated endocytosis
Pinocytosis is…
Nonspecific ingestion of fluid and small protein molecules via small vesicles; constitutive
Constitutive means…
Involves continuous dynamic formation of small vesicles at the cell surface
Dynamin is…
GTPase, a mechanoenzyme that helps with pinching off a pinocytotic vesicle scission
Characteristics of pinocytotic vesicles…
Visible with TEM, smooth surface, numerous in endothelium of blood vessels and smooth muscle cells, clathrin-independent
Phagocytosis is…
Nonselective ingestion of large particles such as cell debris, bacteria, nonbiologic materials such as carbon, inorganic dusts, asbestos fibers, etc.; involves pseudopodia engulfing particles in large vesicles called phagosomes; clathrin-independent, actin-dependent.
Phagocytosis is performed mainly by specialized group of cells belonging to the…
MPS
(T/F) Phagocytosis is generally a receptor-mediated process.
True
Phagocytosis is triggered by recognition of…
PAMPs that are commonly expressed on pathogen surfaces
Receptor-mediated endocytosis involves…
Cargo receptors accumulating in well-defined regions of cell membrane, forming coated pits. These receptors bind specific molecules. Clathrin mlcls, interacting with adaptin, form basketlike cage around formed “coated vesicle,” which moves to early endosomes and other parts of the cell.
Process of exocytosis
1) Molecules produced by the cell for export are delivered from site of formation to Golgi apparatus.
2) Sorting and packaging the secretory product into transport vesicles.
3) Fuses with plasma membrane, contents released.
2 pathways of exocytosis
Constitutive pathway and regulated secretory pathway
What is the constitutive pathway of exocytosis?
Substances are continuously and immediately delivered in transport vesicles to the plasma membrane - ex. immunoglobulins by plasma cells, procollagen by fibroblasts
What is the regulated secretory pathway of exocytosis?
Secretory proteins are concentrated and stored in secretory vesicles until regulatory event activated, then secretion occurs - ex. chief cells of gastric mucosa, acinar cells of pancreas. Signaling stimulus causes influx of Ca2+ into cytoplasm, which stimulates secretory vesicles to fuse with plasma membrane and release contents.
The precise targeting of vesicles to appropriate cellular compartments is initially controlled by…
Docking proteins - RAB-GTPase interacting with tethering proteins located on the target membrane
Specificity of targeting of vesicles to appropriate cellular compartment is ensured by…
Interactions between soluble SNARE proteins
t-SNARE interacts with v-SNARE to form…
cis-SNARE complex
SNARE complexes are dismantled wit help of…
NSF/alpha-SNAP protein complex
Early endosomes are…
Membrane-enclosed compartments restricted to portion of cytoplasm near cell membrane where vesicles originating from the cell membrane fuse
Early endosomes develop into…
Late endosomes and then lysosomes
2 models of the origin/formation of endosomal compartments in cell
Stable compartment model (they are stable cellular organelles), maturation models (early endosomes formed from endocytotic vesicles, developed into late ensomes, then lysosomes)
Hydrolases are…
Lysosomal enzymes
The precursor of a hydrolase is…
A prohydrolase - heavily glycosylated protein with signal patch that is modified by enzymes that attach M-6-P to the surface.
M-6-P acts as…
A target for proteins possessing M-6-P receptor (on early/late endosomes, lysosomes, Golgi aparatus). Acidic environment of late endosome causes release of prohydrolases destined for transport to endosomes.
Early and late endosomes differ in their…
Cellular localization, morphology, state of acidification and function.
What is the name of vesicles that transport substances between early and late endosomes?
MVBs
Late endosomes are also called…
Prelysosomes
The major function of early endosomes is…
To sort and recycle proteins internalized by endocytotic pathways.
What constitutes the basis of the sorting mechanism in early endosomes?
Localized changes in pH (causes dissociation of ligands from receptors), mechanical sorting of molecules by size (through different sized tubules)
What are the pathways for processing internalized ligand-receptor complexes in the cell?
1) Receptor is recycled and ligand is degraded.
2) Both receptor and ligand are recycled.
3) Both receptor and ligand are degraded.
4) Both receptor and ligand are transported through the cell.
What is transcytosis?
Transport of substances through a cell.
Examples where ligand is degraded and receptor is recycled?
Low-density lipoprotein (LDL)-receptor complex, insulin-glucose transporter (GLUT) receptor complex, peptide hormones and receptors
Examples where both receptor and ligand are recycled?
Low pH of endosome dissociates iron from iron-carrier protein transferrin, but transferrin remains with receptor. Also major histocompatibility complex (MHC) I and II mlcls.
Examples where both receptor and ligand are degraded?
Epidermal growth factor and its receptor
Example where both receptor and ligand are transported through the cell?
Secretion of immunoglobulins (secretory IgA) into saliva and human milk.
Function of rER?
Binds ribosomes for translation; chemical modifications of proteins; membrane lipid synthesis
Function of sER?
Lipid and steroid metabolism
Function of Golgi apparatus?
Chemical modification of proteins, sorting/packaging of molecules for secretion/transport
Function of mitochondria?
Aerobic energy supply
Function of peroxisomes?
Production and degradation of H2O2 and degradation/oxidation of fatty acids
Function of microtubules?
Form elements of cytoskeleton, provide dynamic instability (ability to elongate and shorten)
Function of filaments?
Form elements of cytoskeleton, actin filaments (flexible), intermediate filaments (ropelike fibers) provide tensile strength to withstand tension and confer resistance to shearing forces
Function of centrioles?
Short, paired cylindrical structures in center of MTOC/centrosome and whose derivatives give rise to basal bodies of cilia
Function of ribosomes?
Protein synthesis and synthesis of rRNA and ribosomal proteins
What do membranes do for the cell?
Divide it into compartments with unique functions, leading to chemical specificity. Also insure integrity of the cells (keep outside out and inside in).
(T/F) In cells, you can have membranes inside of membranes.
True
The space between 2 cells is called…
The intercellular space
What was the first experimental evidence demonstrating membrane fluidity?
1 human and 1 mouse cell, each with unique colored antibody, were fused together. After heat added and time, the resulting heterokaryon showed an intermixing of colors, demonstrating that molecules could diffuse within the membrane.
Endocytosis would not occur efficiently in the absence of…
Membrane fluidity - it allows receptors to diffuse in plasma membrane until they are stopped by binding to the adaptin molecules in the clathrin coated pits.
High blood cholesterol is a major risk factor for…
Cardiovascular disease (in the US)
What are the domains of an LDL receptor?
LDL binding domain (N terminus, upper tip), ligand dissociation domain, transmembrane domain, coated pit binding (C terminus)
How do receptors “know” to enter the cell via the coated pit?
- The receptors are transmembrane proteins
- They have a consensus sequence at the end of the cytoplasmic domain (ex. Tyr-X-Arg-Phe) that recognizes the coated pit
- This sequence binds to adaptin molecules in the clathrin coat
- This binding stops and concentrates the receptors
Membrane phospholipids are amphiphatic, which means…
They have both hydrophilic and hydrophobic portions
What are the normal components of a phospholipid?
Hydrophilic head (choline-phosphate-glycerol) and hydrophobic tail (2 fatty acids, one with a cis-double bond). Phosphate has - charge.
What are the major types of phospholipids?
- Phosphatidyl-ethanolamine (PE)
- Phosphatidyl-serine (PS)
- Phosphatidyl-choline (PC)
- Sphingomyelin (SM)
What is phosphatidylinositol?
Minor type of phospholipid with overall (-) charge, primarily on inner (cytosolic) leaflet.
Involved in cell signaling - lipid kinases phosphorylate various portions, resulting in recruitment of signaling molecules to the cell membrane.
What are characteristics of PE?
+ charge on end (neutral overall), primarily on inner (cytosolic) leaflet, randomly distributed
What are characteristics of PS?
+ and - charges on end (- overall), primarily on inner (cytosolic) leaflet, randomly distributed
What are characteristics of PC?
+ charge on end (neutral overall), primarily on outer (extracellular) leaflet, randomly distributed
What are characteristics of SM?
+ charge on end (neutral overall), primarily on outer (extracellular) leaflet, tend to cluster in the membrane (along with cholesterol) to form lipid rafts
What are the types of movement/diffusion phospholipid molecules can undergo?
Lateral diffusion (rapid), flexion (swaying side to side), rotation (rapid), flip-flop (rarely occurs, may require enzymes)
What enzymes allow phospholipids to “flip-flop”?
Flippase and scramblase
What is the general structure and placement of cholesterol in the plasma membrane?
Rigid steroid ring - between polar head and hydrophobic tail of phospholipids.
Glycolipids consist of about what percent of the membrane phospholipids?
5%
What are the different functions of membrane proteins?
Transporters, anchors (between extracellular and intracellular proteins), receptors, enzymes
Proteins account for about what percent of the plasma membrane’s mass?
50%
What are the different structural types of membrane proteins?
Transmembrane, membrane-associated (integral, not necessarily transmembrane), lipid-linked, protein attached (peripheral, attached to transmembrane protein)
Integral proteins can be removed from the membrane only with…
Detergent extraction of the membrane
How are peripheral proteins connected to the membrane?
Only through non-covalent interactions with other proteins
Peripheral proteins can be removed from the membrane by…
Extremes of ionic strength or pH
What are characteristics of transmembrane proteins?
Integral, amphiphatic, may be single-pass or multi-pass. Hydrophobic regions are often made of alpha-helical structures.
(T/F) Alpha-helical membrane proteins are always transmembrane.
False, not necessarily.
What are characteristics of lipid-linked proteins?
Integral, hydrophilic protein covalently connected to phospholipid modification on membrane. Can be direct or through carbohydrate linkers.
(T/F) Membrane proteins are asymmetrical.
True
(T/F) Membrane proteins can function in large complexes.
True
What is the purpose of connections between membrane proteins and the cytoskeleton?
Reduce membrane protein diffusion/movement, aid in intracellular communication, reinforce the integrity of the plasma membrane.
What is the glycocalyx?
Carbohydrate coating on cell surface that protects cells in harsh conditions (ex. stomach) or can be involved in cell signaling and recognition.
What are membrane domains?
Specific regions of the membrane where membrane proteins can be sequestered into. Lateral diffusion is restricted.
Formation of membrane domains can be mediated by:
Cytoskeletal components, extracellular matrix, cell surface proteins (ex. attachment to membrane proteins of other cells), diffusion barriers (ex. tight junctions that “fence” proteins in)
Of the drugs currently in use, what percent of them target some aspect of the plasma membrane?
Over 50%
What is the major cell type in blood?
Red blood cells
What are the features of a mature red blood cell?
Lacks internal organelles or a nucleus, major function is to transport O2 and CO2 between lungs and tissues, biconcave shape (increasing surface area for gas exchange), flexible/deformable and strong because of the “membrane skeleton”
Why must RBCs be deformable?
Because they are subject to shear forces as they move around in the circulatory system. Also need to pass through tight spaces in blood capillaries.
What is the major protein in the RBC membrane skeleton?
Spectrin
How is the RBC membrane skeleton attached to the plasma membrane?
Mediated by protein ankyrin and protein 4.1
