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Flashcards in The Cytoplasm Deck (76):
1

First zygotic cellular division produces cells called

Blastomeres

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Blastomeres of the inner cell mass that are transfered to tissue culture

Embryonic stem cells

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Differentiation

Cells undergo a specialization process in which they differentially express sets of genes that mediate specific cytoplasmic activities becoming efficient in specialized functions

4

limiting membranes that envelop all eukaryotic cells

Plasma membrane Also called cell membrane/ plasmalemma

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Function of cell membrane

1.Selective barrier regulating the passage of material into and out of the cell and facilitating transport of molecules
2.Keep constant ion content of cytoplasm
3.Carry no. Of specific recognition and signaling functions (role in interactions of the cells with its environment)

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Intergins

Are Plasma membrane protein linked to both the cytoskeleton and ECM components.

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Under which microscope can the cell membrane can be seen and why

Since thickness is between 7.5-10 nm can only be seen under the electron microscope

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What part of the cell can be seen under a light microscope

The line between adjacent cells consist of plasma membrane protein plus ECM materials

9

Membrane phospholipids

Are amphipathic consisting of 2 non polar (hydrophobic) long chain fatty acids linked to a charged polar (hydrophilic) head that bears a phosphate group. They are stable when in bilayer with fatty acids chains located in the middle away from the water

10

Cholesterol

Sterollipids inserted at varying densities among the closely packed phospholipids fat acids. They are amphipathic

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Gylcolipids

Phospholipids on the outer layer are mostly glycolipids which are lipids with attached oligosaccharide chains that extend outward from the cell surface

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Cytoplasmic membrane under electron microscope

Exhibit Trilaminar appearance after fixation with osmium tetroxide, osmium binds to the polar head of phospholipid and the oligosaccharide chains producing two dark lines that enclose the light band of osmium free fatty acids.

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Integral protein

Incorporated directly within the lipid layer

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Peripheral protein

Bound to one of the two membrane surfaces

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How can peripheral Proteins be extracted

With salt solution

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Integral protein extraction

Only my using detergents to disrupt the lipids

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Multipass protein

The polypetide chains of many integral Proteins span the membrane from one side to the other several times

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How are Proteins integrated into the bilayer

Due to hydrophobic interaction between the lipids and non polar amino acids of Proteins

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Gylcocalyx

Delicate cell surface coating formed by Carbohydrate moieties, oligosaccharide of glycolipids and glycoproteins.

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Function of Carbohydrate moieties of the glycocalyx

1.Act as receptors
2.Cell adhesion
3.Cell recognition
4.Response to protein hormones

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Proteins movement

Protein are able to move laterally. Membrane protein comprise a movable mosaic within the fluid lipid bilayer - fluid mosaic model

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How is lateral diffusion restricted

1.by their cytoskeletal attachments
2.in epithelial cells tight junction between cells
3.Proteins that are components of large enzymes are less mobile

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Lipid rafts

Involved in transduction of signals from outside the cell. Specialized membrane patches with high concentration of cholesterol and saturated fatty acids which reduce fluidity

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Function of cholesterol

Affect the packing of fatty acids chains
Effect on membrane fluidity

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Diffusion

Transport of small non polar molecules through the lipid layer lipophilic molecules

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Channels

Multipass Proteins forming transmembrane pores through which ions or small molecules pass selectively

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Carriers

Transmembrane protein that binds small molecule and translocate then via confirmational change

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Membrane pumps

Enzymes engaged in active transport utilizing energy from the hydrolysis of ATP against steep concentration gradient. Because the consume ATP pumps they are referred to as ATPase

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Endocytosis

Macromolecules enter the cell by being enclosed within folds of plasma membrane which due and pinch of internally to form vesicles

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3 types of endocytosis

1. Phagocytosis
2. Pinocytosis
3. Receptor-mediated endocytosis

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Phagocytosis

Ingestion of particles such as bacteria or dead cell remnants.

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Cells specialized for Phagocytosis activity

Macrophages and neutrophils

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Vacuole formed by Phagocytosis

Phagosome

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Pinocytosis

Involves smaller invagination of cell membrane which fuse and entrap extra cellular fluid and is dissolved content the Pinocytotic vesicles pinch off inwardly. Either rise with lysosomes or move to opposite cell surface and fuse with the membrane and release their content outside the cell

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Cytoplasmic cytoskeleton consist of

1. microtubule
2. microfilaments
3. intermediate filaments

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Axonemes

Microtubules organised into larger more stable arrays called axonemes in the cytoplasmic extensions called cilia and flagella. two more microtubules linked side-by-side by protein arms or bridges.

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Protein subunits of microtubules

Heterodimer of alpha and beta tubulin each with the Molecular mass of 50kDa. Under appropriate conditions in the tubulin in heterodimers polymerize to form microtubules which have a slightly spiral organization

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structure of Microtubules

Tubulin subunit align length wise to form protofilaments, with 13 parallel protofilaments forming the circumference of a microtubule wall. Each microtubule is hollow with the outer diameter 25 nm and wall thickness 5 nm, length varies.

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What causes polymerization of tubulin

Directed by microtubule organizing center (MTOCs) which contain short assembly of tubulin that act as nucleating sites for further polymerization

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Polarity of microtubules

microtubules have a positive and negative end therefore are polarized structures with growth occurring more rapidly at the positive end

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Relative stability of microtubules

Microtubule show dynamic instability with continuous cycle so polymerization and depolarization at steady state conditions which depend on concentration of tubulin, calcium, magnesium and presence of various microtubule associated Proteins (MAPs). Stability varies greatly with cellular location

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DOMINANT MTOC in most cells is

Centrosones, which is organized around two cylindrical centrioles about 0.2 micrometers in diameter and 0.3-0.5 micrometers in length

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Centriole

Composed of 9 high organized microtubule triplets.

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Function of centrioles

1. organize nearby tubulin complexes and other protein as a Pericentriolar matrix found close to nucleus of non dividing cell
2. System of intracellular transport of membranous vesicles, Marco molecular complexes and organelles

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Centrioles during mitosis

Before cell division during dna replication each centrosome duplicates itself so that it has two pairs of centrioles. during mitosis the centrosome divides into half which move to opposite poles of the cell and become organizing centers for the microtubules of the mitotic spindle

46

Examples of microtubules as in just cellular transport system

1. Axoplasmic transport in neurons
2. transport in pigment cells
3. chromosome movement by mitotic spindle
4. movement among different cell compartments

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Mechanism of transport along microtubules

controlled by proteins call motor proteins, which use ATP in moving the large structures. Kinesins carry material away from the MTOC near the nucleus to the plus end of the microtubules ( anterograde transport), cytoplasmic dyneins carry material along microtubules in the opposite direction (retrograde transport) generally towards the nucleus.

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Dynamic instability

At stable tubulin concentration some microtubules grow while others shrink each existing in a condition called dynamic stability

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Mechanism of dynamic instability of Microtubules

Where tubulin conc. is high tubulin GTP is added at the microtubules faster than the incorporated GTP can be hydrolyzed. the resulting GTP cap stabilizes that end of the microtubule and promotes rapid growth. Free tubulin conc. decrease, rate of growth decreases allowing GTP hydrolysis to catch up the resulting GDP cap is unstable and favor rapid depolarization - termed catastrophe. This increases local conc of free monomeric tubulin that stop the microtubule from completely disappearing by producing another shirt period of Microtubule elongation

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function of microfilaments

Motility and contractile activities of the cell

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Structure of microfilaments

Thin, polarized polymers, shorter and more flexible. Composed of G-action monomers that assemble into double stranded helix of filamentous F-actin

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Formation of microfilaments

G-actin assembled in the presence of K+and Mg2+ into filamentous F-actin.
1. G-actin generally added to preexisting filaments
2. New filaments formed from pool of G-actin by action of nucleating protein - formin
3. Complex of polypeptides - Arp2/3 bind to the side of preexisting actin filament and induce new F-actin branch leads the formation of a microfilament network

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Plasma membrane consist of

phospholipids, cholesterol,
proteins, and oligosaccharide chains covalently linked to phospholipid and protein molecules.

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Function of integrin

Exchange of influences in both directions between the cytoplasm and the material in the ECM

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Fiction of cholesterol

restricting the movement of the phospholipid fatty acids in the cell membrane and modulating their fluidity

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General mechanisms that allow the movement of most small molecules cross the membrane

1. Diffusion
2. Channels
3. Carriers

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Process of Phagocytosis

When a bacterium bound to the surface of a cell it becomes surrounded by extension of plasmalemma and cytoplasm in a process dependent on cytoskeletal changes. Fusion if the membrane encloses the bacterium in an intracellular Phagosome which merges with lysosomes for degradation.

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Process where pinocytotic vesicles may
move to the opposite cell surface where they fuse with the membrane and release their contents outside the cell.

transcytosis.

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Function of transcytosis

This accomplishes bulk transfer of material across the cell

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Receptor-mediated endocytosis:

Receptors are integral membrane proteins at the cell surface. High affinity binding of ligands such as low-density lipoproteins and protein hormones, to their receptors causes these proteins to aggregate in special membrane regions that then invaginate and pinch off internally as vesicles.

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are regions of the cell membrane specialized in receptor-mediated endocytosis occupied by receptors associate with other proteins on the cytoplasmic membrane surface and begin invagination

Coated pits

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electrondense coating on the cytoplasmic surface of coated pits contains several polypeptides, a major one being:

clathrin

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Function of clathrin molecules

interact like the struts in a geodesic dome, forming that region of cell membrane into a cage-like invagination that is pinched off into the
cytoplasm as a coated vesicle with receptor ligands inside.

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caveolae

Another type of receptor-mediated endocytosis very prominent in endothelial cells uses invaginations called caveolae that involve the membrane protein caveolin.

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endosomal compartment

a dynamic system of membranous tubules and vacuoles of various sizes and shapes located in the cytoplasm near the cell surface (early endosomes) or deeper in the cytoplasm (late endosomes).

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common path of the vesicles after endocytotic processes

1.vesicles produced quickly enter and fuse with the endosomal compartment,
2.The clathrin molecules separated from the coated vesicles recycle back to the cell membrane to participate in the formation of new coated pits.
3.vesicle trafficking through the endosomal compartment is directed through peripheral membrane G proteins called Rab protein.

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Rab protein


Small GTPases that bind guanine nucleotides and associated Proteins.

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Ligands may have different fates within the endosomal compartment:

1. Receptors and ligands may be carried to late endosomes and then to lysosomes for
degradation.
2. Ligands may be released internally and the receptors recycled to the cell surface.
3. Vesicles may move to and fuse with another cell surface, where the ligands are released
again outside the cell (transcytosis).

69

How do endosomes activate lysosomes

The menace of many late endosomes have ATP driven H+ pumps that acidify the interior and thus activate the hydrolytic enzymes of lysosomes

70

exocytosis

In this process a membrane-limited cytoplasmic vesicle fuses with the plasma membrane, resulting in the release of its contents into the extracellular space without compromising the integrity of the plasma membrane.

71

Exocytosis is triggered in many
cells by

transient increase in cytosolic Ca2+

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Protein secretion involving exocytosis may follow two pathways:

1.Constitutive secretion: for products that are released from cells continuously, as soon as
synthesis is complete, such as procollagen for the ECM.
2. Regulated secretion occurs in response to signals coming to the cells, such as the release of digestive enzymes from pancreatic cells in response to specific stimuli.

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membrane trafficking

A process of membrane movement and recycling. Portions of the cell membrane become part of the endocytotic vesicles or vacuoles during endocytosis; during exocytosis, membrane is returned to the cell surface.

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Importance of membrane trafficking

1. crucial for maintaining the cell
2.and for physiologically important processes such as reducing blood lipid levels.

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multivesicular bodies.

Subpopulations of vacuoles among the early and late endosomes in many cells accumulate small
vesicles and tubules within their lumens by further invaginations of their limiting membranes becoming multivesicular bodies.

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Fate of multi vesicular bodies

1. multi-vesicular bodies may merge with lysosomes for selective degradation of their contents,
2. may also fuse with the plasma membrane release the intralumenal vesicles outside the cell.