Ch. 2 - Cytoplasm

cytoplasm

fluid containing a system of membranous organelles, nonmembranous macromolecular assemblies, and polymerized cytoskeletal proteins

Animal cells are eukaryotic with…

distinct, membrane-enclosed nucleus surrounded by cytoplasm

Prokaryotic cells lack

nuclei, membranous cytoplasmic structures, and cytoskeletons

blastomeres

the 1st zygotic cells resulting from fertilization that undergo cleavage and ultimately form the embryo, which give rise to all tissue types

differentiation

the process by which cells become specialized in structure and function through expressing different sets of genes

plasma membrane (aka cell membrane or plasmalemma)

envelops every eukaryotic cell consists of phospholipids, cholesterol, and proteins, with oligosaccharide chains covalently linked to many of the phospholipids and proteins

two important role of the plasma membrane

keep constant the ion content of cytoplasm, which differs from that of the extracellular fluid AND recognition and signaling functions

integrins

plasma membrane proteins that are linked to both the cytoskeleton and ECM components and allow continuous exchange of influences in both directions between the cytoplasm and material in the ECM

membranes are made of

phospholipids that are amphipathic, consisting of two nonpolar (hydrophobic or water-repelling) long-chain fatty acids linked to a charged polar (hydrophilic or water-attracting) head that bears a phosphate group

Phospholipids are most stable when

organized into a double layer (bilayer) with the hydrophobic fatty acid chains located in a middle region away from water and the hydrophilic polar head groups contacting the water

Molecules of cholesterol, a sterol lipid, insert at

varying densities among the closely packed phospholipid fatty acids, restricting their movements and modulating the fluidity of all membrane components.

Phospholipids in each half of the bilayer are different.

in the well-studied membranes of red blood cells, phosphatidylcholine and sphingomyelin are more abundant in the outer half, while phosphatidylserine and phosphatidylethanolamine are more concentrated in the inner layer.

lipid rafts

protein complexes that occur in specialized membrane patches with higher concentrations of cholesterol and saturated fatty acids reducing lipid fluidity. This together with the presence of scaffold proteins, that maintain spatial relationships between enzymes and signaling proteins, allows the proteins assembled within ___ to remain close and interact more efficiently.

Diffusion

transports small, nonpolar molecules directly through the lipid bilayer. Lipophilic (fat-soluble) molecules diffuse through membranes readily, water very slowly.

Channels

are multipass proteins forming transmembrane pores through which ions or small molecules pass selectively. Cells open and close specific channels for Na⁺, K⁺, Ca²⁺, and other ions in response to various physiological stimuli. Water molecules usually cross the plasma membrane through channel proteins called aquaporins.

Carriers

are transmembrane proteins that bind small molecules and translocate them across the membrane via conformational changes in the proteins

endocytosis

Macromolecules normally enter cells by being enclosed within folds of plasma membrane (often after binding specific membrane receptors) which fuse and pinch off internally as spherical cytoplasmic vesicles

Phagocytosis

“cell eating” is the ingestion of particles such as bacteria or dead cell remnants. Certain blood-derived cells, such as macrophages and neutrophils, are specialized for this activity. When a bacterium becomes bound to the surface of a neutrophil, it becomes surrounded by extensions of plasmalemma and cytoplasm which project from the cell in a process dependent on cytoskeletal changes. Fusion of the membranous folds encloses the bacterium in an intracellular vacuole called a phagosome, which then merges with a lysosome for degradation of its contents as discussed later in this chapter.

Pinocytosis

(“cell drinking”) involves smaller invaginations of the cell membrane which fuse and entrap extracellular fluid and its dissolved contents. The resulting pinocytotic vesicles (~80 nm in diameter) then pinch off inwardly from the cell surface and either fuse with lysosomes or move to the opposite cell surface where they fuse with the membrane and release their contents outside the cell. The latter process, called transcytosis, accomplishes bulk transfer of dissolved substances across the cell.

Receptor-mediated endocytosis

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

endosomal compartment

a dynamic collection of membranous tubules and vacuoles in the peripheral cytoplasm

exocytosis

a cytoplasmic vesicle containing the molecules to be secreted fuses with the plasma membrane, resulting in the release of its contents into the extracellular space without compromising the integrity of the plasma membrane

Constitutive secretion

is used for products that are released from cells continuously, as soon as synthesis is complete, such as collagen subunits for the ECM

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. Regulated exocytosis of stored products from epithelial cells usually occurs specifically at the apical domains of cells, constituting a major mechanism of glandular secretion

membrane trafficking

Portions of the cell membrane become part of the endocytotic vesicles or vacuoles during endocytosis; during exocytosis membrane returns to the cell surface

gap junctions

Adjacent cells often form communicating ___ that couple the two cells and allow exchange of ions and small molecules

target cells

cells bearing receptors for a specific ligand

endocrine signaling

the signal molecules (here called hormones) move in the blood from their sources to target cells throughout the body

paracrine signaling

the chemical ligand diffuses in extracellular fluid but is rapidly metabolized, limiting its effect only local to target cells near its source

synaptic signaling

a specialized type of paracrine interaction, neurotransmitters act on adjacent cells through special contact areas called synapses

autocrine signaling

signals bind receptors on the same cells that produced the signal molecule.

juxtacrine signaling

important in early embryonic tissue interactions, the signaling molecules are cell membrane-bound proteins, which bind surface receptors of the target cell when the two cells make direct physical contact.

Channel-linked receptors

open associated channels upon ligand binding to promote transfer of molecules or ions across the membrane.

Enzymatic receptors

usually protein kinases in which ligand binding induces activation and phosphorylation of other proteins.

G-protein–coupled receptors

upon ligand binding stimulate associated G-proteins to bind the guanine nucleotide GTP and be released to activate other cytoplasmic proteins.

Ribosomes

macromolecular machines, about 20 × 30 nm in size, which assemble polypeptides from amino acids on molecules of transfer RNA (tRNA) in a sequence specified by mRNA

endoplasmic reticulum (ER)

The cytoplasm of most cells contains this convoluted membranous network called the ___. The network extends from the surface of the nucleus throughout the cytoplasm and encloses a series of intercommunicating channels called cisternae. With a membrane surface up to 30 times that of the plasma membrane, it is a major site for vital cellular activities, including biosynthesis of proteins and lipids. Polyribosomes attached to the membrane in different regions of it allow two types of it to be distinguished.

Rough Endoplasmic Reticulum

is prominent in cells specialized for protein secretion, such as pancreatic acinar cells (making digestive enzymes), fibroblasts (collagen), and plasma cells (immunoglobulins). The RER consists of saclike as well as parallel stacks of flattened cisternae, each limited by membranes that are continuous with the outer membrane of the cell’s nucleus. Polyribosomes on the cytosolic surface of the RER confer basophilic staining properties on this organelle.

Smooth Endoplasmic Reticulum

Regions of ER that lack bound polyribosomes. perform synthesis of phospholipids and steroids. allow detoxification of potentially harmful exogenous molecules such as alcohol, barbiturates, and other drugs. vesicles are also responsible for sequestration and controlled release of Ca²⁺, which is part of the rapid response of cells to various stimuli. This function is well developed in striated muscle cells, where the SER has a key role in the contraction process and assumes a specialized form called the sarcoplasmic reticulum

Golgi Apparatus or Golgi complex

completes posttranslational modifications of proteins produced in the RER, while packaging and addressing these proteins to their proper destinations.

Secretory Granules

Originating as condensing vesicles in the Golgi apparatus

Lysosomes

sites of intracellular digestion and turnover of cellular components; are membrane-limited vesicles that contain about 40 different hydrolytic enzymes and are particularly abundant in cells with great phagocytic activity; The membrane surrounding each shields cytoplasmic components from the degradative enzymes

Proteasomes

abundant, membrane-free cytoplasmic and nuclear protein complexes, each approximately the size of the small ribosomal subunits. They function to degrade denatured or otherwise nonfunctional polypeptides. They also remove proteins no longer needed by the cell and provide an important mechanism for restricting the activity of a specific protein to a certain window of time. Whereas lysosomes digest organelles or membranes by autophagy, they deal primarily with free proteins as individual macromolecules.

ubiquitin

an abundant cytosolic 76-amino acid protein found in all cells; acts as a molecular "tag" attached to other proteins inside eukaryotic cells; aka kiss of death bc if too many of these connect and form a chain it is marked for destruction by a proteasome

Mitochondria

are membrane-enclosed organelles with arrays of enzymes specialized for aerobic respiration and production of adenosine triphosphate (ATP), with high-energy phosphate bonds, which supplies energy for most cellular activities. They are highly plastic, rapidly changing shape, fusing with one another and dividing, while being moved through the cytoplasm along microtubules

Inner and outer membranes of mitochondria

Both mitochondrial membranes contain a higher density of protein molecules than other membranes in the cell and have reduced fluidity. The outer membrane is sieve-like, containing many transmembrane proteins called porins that form channels through which small molecules such as pyruvate and other metabolites readily pass from the cytoplasm to the intermembrane space.

apoptosis

rapid cell death; Another role for mitochondria occurs at times of cell stress when the protein cytochrome c dislodges from the inner membrane’s electron-transport chain and leaves the organelle. In the cytoplasm this protein activates sets of enzymes that degrade all cellular components in a regulated process called ___

Peroxisomes

spherical organelles enclosed by a single membrane and named for their enzymes which produce and degrade hydrogen peroxide, H₂O_2; Oxidases located here oxidize substrates by removing hydrogen atoms that bind molecular oxygen (O₂), producing H₂O₂. Peroxidases such as catalase immediately break down H₂O₂, which can potentially damage cells. These enzymes also inactivate various potentially toxic molecules, including certain prescription drugs, particularly in the large and abundant peroxisomes of liver and kidney cells.

cytoskeleton

a complex array of (1) microtubules, (2) microfilaments (also called actin filaments), and (3) intermediate filaments. These protein polymers determine the shapes of cells, play key roles in the movements of organelles and cytoplasmic vesicles, as well as allowing movements of entire cells.

Microtubules

most of which are highly dynamic in length; also form key elements of larger, more stable arrays termed axonemes in the cytoplasmic extensions called cilia and flagella. Each microtubule is hollow, with an outer diameter of 25 nm and a 5-nm-thick wall, a structure that confers significant rigidity to help maintain cell shape; they vary in length but can become many micrometers long. Two or more of them often become linked side-by-side by protein arms or bridges, which are particularly important in the axonemes of cilia and flagella

α and β tubulin

the protein subunit of a microtubule is a heterodimer of ___

motor proteins

Transport occurs along the outer microtubule surface and requires proteins called ___ which use ATP

Kinesins

carry material away from the MTOC near the nucleus toward the plus end of microtubules (anterograde transport)

cytoplasmic dyneins

carry material along microtubules in the opposite direction (retrograde transport), toward the nucleus. Important roles for this system include extending the ER from the nuclear envelope to the plasmalemma and moving vesicles to and through the Golgi apparatus.

Microfilaments (Actin Filaments)

are composed of actin subunits and allow motility and most contractile activity in cells, using reversible assembly of the actin filaments as well as interactions between these filaments and associated myosin family proteins; are thin and polarized polymers, shorter and more flexible than microtubules

Actin-binding proteins

such as formin and others just mentioned, change the dynamic physical properties of microfilaments, particularly their lengths and interactions with other structures, and this determines the viscosity and other mechanical properties of the local cytoplasm. Cross-linking within networks of F-actin increases cytoplasmic viscosity, while severing (and capping) the filaments tends to decrease viscosity. The lengths and other physical properties of actin filaments depend on activities of various other actin-binding proteins

Intermediate Filaments

are stable, confer increased mechanical stability to cell structure, and consist of different protein subunits in different cell types. More than a dozen proteins, ranging in size from 40 to 230 kDa, serve as subunits of various intermediate filaments and can be localized by immunohistochemistry in various cells.

Vimentin

is the most common class III intermediate filament protein and occurs in most cells derived from embryonic mesenchyme. Important vimentin-like proteins include desmin found in most muscle cells and glial fibrillar acidic protein (GFAP) found especially in astrocytes, supporting cells of central nervous system tissue

Neurofilament

proteins of three distinct sizes make heterodimers that form the subunits of the major intermediate filaments of neurons.

Lamins

make up a family of seven isoforms present in the cell nucleus, where they form a structural framework called the nuclear lamina just inside the nuclear envelope

Cytoplasmic inclusions

contain accumulated metabolites or other substances, but unlike organelles have little or no metabolic activity themselves. Most inclusions are transitory structures not enclosed by membrane. ie, lipid droplets, glycogen granules, melanin (pigment granules), and residual bodies (also called lipofuscin).

Primary lysosomes

emerge from the Golgi apparatus containing inactive acid hydrolases specific for degrading a wide variety of cellular macromolecules.

Secondary lysosomes

are more heterogeneous, having fused with vesicles produced by endocytosis that contain material to be digested by the hydrolytic enzymes.