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Endoplasm

The endoplasm is the inner, fluid, and granular region of the cytoplasm in eukaryotic cells, particularly in motile protozoans such as amoebas, where it is distinguished from the outer, clear, and gel-like ectoplasm that forms a thin peripheral layer.^[1]^[2] This division of the cytoplasm into endoplasm and ectoplasm is characteristic of sarcodine protozoans and other cells exhibiting amoeboid locomotion, with the endoplasm serving as a dynamic sol phase that facilitates internal transport and motility.^[3]^[4] In structure, the endoplasm is less viscous and more fluid-like (sol state) compared to the rigid, gel-like ectoplasm, containing the majority of the cell's organelles, including nuclei, mitochondria, granules, and food vacuoles, which support metabolic processes such as digestion and nutrient distribution.^[2]^[1] This inner layer's granular appearance arises from suspended cellular components, enabling cytoplasmic streaming—a process where the endoplasm flows forward into extending pseudopodia to propel the cell across substrates.^[4] In contrast, the ectoplasm, being hyaline and semi-rigid, provides structural integrity and aids in pseudopod extension for locomotion and prey capture.^[1]^[2] The functional significance of the endoplasm extends beyond protozoans to other motile eukaryotic cells, such as fibroblasts and certain algae, where it contributes to phenomena like cyclosis (cytoplasmic circulation) and overall cellular dynamics.^[5] In amoeboid movement specifically, the endoplasm converts from posterior ectoplasm through liquefaction, streams anteriorly within an ectoplasmic sheath, and reconverts to ectoplasm at the leading edge, creating a fountain-like flow that drives progression.^[4] This mechanism underscores the endoplasm's role in integrating cytoskeletal elements, like microfilaments, to regulate phase transitions between sol and gel states for efficient cellular function.^[4]^[5]

Definition and Overview

Definition

The endoplasm refers to the inner, granular, and fluid portion of the cytoplasm in certain eukaryotic cells, particularly protozoans such as amoebas, where it houses organelles, vesicles, and various metabolic components essential for cellular activities.^[6] This region is distinct from the ectoplasm, which forms the outer, clear, and more viscous layer of the cytoplasm adjacent to the cell membrane, and from the cytosol, the soluble aqueous phase that excludes suspended organelles and particles.^[6]^[2] Endoplasm is primarily observed in cells exhibiting dynamic shapes, like those of amoeboid protozoans that undergo shape changes for locomotion, though analogous inner cytoplasmic zones with similar granular characteristics appear in other eukaryotic cell types.^[6]^[2] The term originates from the Greek prefix "endo-" (inner or within) combined with "plasma" (formed or molded substance), with its first documented use dating to the late 19th century in biological literature.^[6]

Historical Discovery

The initial observations of what would later be identified as endoplasm began with early microscopic examinations of protozoan organisms, building on foundational work in microscopy. Robert Hooke, in his 1665 publication Micrographia, pioneered the use of compound microscopes to observe cellular structures in cork and other materials, laying the groundwork for later studies on more complex living substances in amoeboid organisms like Rhizopoda. These early tools enabled detailed investigations into the internal components of single-celled organisms. In 1835, French cytologist Félix Dujardin provided the first specific description of the living substance within amoebas, terming it "sarcode" after observing its viscous, contractile properties as it exuded from the cells of infusorians and other protozoans; he viewed it as the fundamental material of life in lower animals.^[7] This observation marked a key milestone in recognizing the dynamic inner content of cells, though Dujardin did not yet distinguish layered structures. The concept was refined in 1854 by German zoologist Max Schultze through his studies on Rhizopoda, where he differentiated the clear, outer ectoplasm from the inner, granular endoplasm in amoebae, establishing the dual-layered model of protoplasm in these organisms. The term "endoplasm" emerged in the 1870s amid advances in light microscopy by German cytologists, including Otto Bütschli, who described the inner protoplasmic region as a fluid, granular matrix distinct from the peripheral layer, emphasizing its role in cellular fluidity and inclusions. By the early 20th century, understanding shifted from viewing endoplasm as a static structure to a dynamic component, particularly with the advent of electron microscopy in the 1940s and 1950s; studies on Amoeba proteus, for instance, revealed intricate ultrastructures within the endoplasm, such as organelles and streaming patterns, highlighting its active involvement in cellular processes.^[8]

Structure and Composition

Physical Properties

The endoplasm exhibits a fluid, viscous nature characterized by sol-gel transitions that facilitate cytoplasmic streaming and flow within the cell. These transitions allow the endoplasm to shift from a more gel-like state to a sol-like state, enabling dynamic movement, with viscosity ranging from approximately 0.1 to 3 poise—roughly 10 to 300 times that of water—and varying based on metabolic activity and shear rates.^[9]^[10] Under light microscopy, the endoplasm displays a granular appearance due to suspended particles such as organelles and inclusions, in contrast to the clear, hyaline ectoplasm.^[11] The endoplasm maintains a pH typically between 7.0 and 7.4, with ionic compositions including high potassium and low sodium concentrations, alongside calcium ions that modulate fluidity by influencing sol-gel states.^[12]^[13] In amoebas, the endoplasm comprises the majority of the total cell volume, a proportion that changes dynamically during locomotion as ectoplasm expands at the leading edge.^[9]

Cytosolic Components

The endoplasm, as the fluid inner region of the cytoplasm in amoeboid protozoans such as Amoeba proteus, primarily consists of the cytosol, a soluble matrix that suspends various biochemical components excluding organelles. This cytosol is predominantly water, comprising 70-80% of its volume, which provides a low-viscosity environment conducive to molecular diffusion and transport processes.^[14] Dissolved within this aqueous medium are inorganic ions, including potassium (K⁺ at approximately 100-140 mM), sodium (Na⁺ at 10-20 mM), and calcium (Ca²⁺ at resting levels below 10⁻⁷ M), which contribute to osmotic balance, signaling, and contractility regulation.^[15] These ions are maintained at concentrations higher for K⁺ and lower for Na⁺ and Ca²⁺ compared to the extracellular fluid, establishing electrochemical gradients essential for cellular homeostasis.^[16] In addition to ions, the cytosol contains a diverse array of proteins and small molecules that support metabolic and structural functions. Proteins, accounting for 20-25% of the dry mass, include enzymes such as glycolytic and hydrolytic types that catalyze essential reactions, as well as molecular chaperones like heat shock proteins that assist in protein folding and prevent aggregation under stress.^[17] Small molecules, including ATP (at millimolar concentrations for energy transfer) and glucose (as a key metabolite for glycolysis), are present in soluble forms that enable rapid diffusion throughout the endoplasm.^[18] The cytosol's role as a liquid matrix facilitates the passive diffusion of these components, allowing efficient exchange and preventing sedimentation in the dynamic sol phase./05%3A_Cells/5.05%3A_Cytoplasm_and_Cytoskeleton) Among the cytosolic proteins, cytoskeletal elements such as actin and tubulin are notable, though they occur at lower concentrations in the endoplasm compared to the ectoplasm. Actin exists primarily in monomeric (G-actin) form within the endoplasm, with filament (F-actin) polymerization limited to short, sparsely cross-linked structures, contrasting with the dense, bundled F-actin networks in the gel-like ectoplasm that drive motility.^[11] Tubulin similarly maintains low levels of microtubules in the sol phase, supporting intracellular transport without the rigidity seen peripherally. These concentration differences contribute to the endoplasm's fluidity, enabling cytoplasmic streaming.^[19] Overall, the endoplasm's cytosolic components exhibit concentration gradients for metabolites and ions that are steeper than in the extracellular environment, with intracellular levels of ATP, glucose, and K⁺ elevated to sustain energy demands and osmotic pressure. These gradients are actively maintained by plasma membrane pumps, such as the Na⁺/K⁺-ATPase, which hydrolyze ATP to counteract passive leaks and ensure directional ion flow.^[15] This dynamic equilibrium supports the endoplasm's involvement in broader cellular respiration processes.^[16]

Granules and Inclusions

The endoplasm of protozoan cells, such as amoebas and foraminifera, contains various granules and inclusions that contribute to its granular appearance and serve as storage structures for nutrients and waste products.^[20] Key types of these inclusions include food vacuoles, which enclose ingested particles like bacteria or debris; glycogen granules, which store carbohydrate reserves; lipid droplets, which accumulate neutral fats as energy sources; and pigment inclusions, such as those observed in foraminifera that may contain colored compounds for metabolic or protective roles.^[21]^[22]^[23]^[24] These structures primarily function as storage compartments for nutrient reserves, including carbohydrates and lipids, and for sequestering waste materials post-digestion, thereby maintaining cellular homeostasis in the dynamic endoplasm. Sizes of these granules and inclusions typically range from 0.1 to 10 μm, allowing them to be dispersed throughout the fluid endoplasm without impeding flow./04:_Cell_Structure_of_Bacteria_Archaea_and_Eukaryotes/4.06:_Specialized_Internal_Structures_of_Prokaryotes/4.6B:_Cell_Inclusions_and_Storage_Granules)^[25] Granules and inclusions exhibit dynamic movement within the endoplasm via cytoplasmic streaming, a process driven by actomyosin interactions that transports them at speeds up to 10 μm/s in amoebas, facilitating nutrient distribution and waste removal.^[26]^[27] Food vacuoles originate through endocytic processes, such as phagocytosis, where plasma membrane invaginations engulf external material, while metabolic granules like glycogen and lipid droplets form biosynthetically within the cytoplasm through enzymatic assembly of precursor molecules.^[28]^[29]^[30]

Functions in Cellular Processes

Metabolic Activities

The endoplasm serves as the primary site for numerous catabolic and anabolic processes in eukaryotic cells, particularly in amoeboid protozoa where it constitutes the granular, organelle-rich inner cytoplasm. Among these, glycolysis represents a fundamental anaerobic pathway that breaks down glucose into pyruvate, generating ATP and NADH without oxygen dependence. In protozoan endoplasm, such as that of Entamoeba histolytica, glycolysis proceeds via the Embden-Meyerhof-Parnas pathway, localized primarily in the cytosol, and is essential for energy production under varying oxygen conditions.^[31] Under anaerobic environments, pyruvate is further metabolized through fermentation, yielding end products such as ethanol, acetate, or lactate in various protozoans, allowing continued ATP synthesis via substrate-level phosphorylation.^[32] These processes highlight the endoplasm's adaptability to fluctuating microenvironments, such as those encountered by free-living or parasitic amoebae.^[31] Lipid metabolism in the endoplasm prominently features β-oxidation of fatty acids within peroxisomes, which are membrane-bound organelles embedded in the cytoplasmic matrix. This pathway initiates the breakdown of very long-chain fatty acids (exceeding 22 carbons) and branched-chain lipids, cleaving them into shorter acetyl-CoA units that can be transferred to mitochondria for further oxidation, while generating hydrogen peroxide as a byproduct detoxified by catalase.^[33] In amoeboid cells, peroxisomal β-oxidation supports lipid homeostasis by mobilizing stored neutral lipids from inclusions, contributing to membrane synthesis and energy reserves during nutrient scarcity.^[34] This compartmentalized activity underscores the endoplasm's role in integrating lipid catabolism with broader cellular lipid dynamics.^[35] Amino acid catabolism occurs diffusely in the endoplasm, where transamination and deamination reactions convert excess amino acids into keto acids and ammonia, providing carbon skeletons for gluconeogenesis or entry into the tricarboxylic acid cycle. In protozoans like Amoeba proteus, these processes generate nitrogenous wastes, including ammonia as the primary excretory product, with minor urea formation.^[36] This catabolic flux helps maintain amino acid balance and supports biosynthetic needs, such as supplying nitrogen for nucleotide and protein precursors.^[37] The endoplasm integrates these metabolic activities with the endomembrane system, where vesicle trafficking facilitates the transport of catabolic intermediates and secretion precursors between the endoplasmic reticulum, Golgi apparatus, and plasma membrane. COP-coated vesicles mediate anterograde flow from the ER to the Golgi, packaging lipids and amino acid-derived metabolites for export or lysosomal delivery, ensuring coordinated cellular responses to metabolic demands.^[38] This trafficking supports the provision of substrates for protein synthesis in adjacent ribosomal compartments.^[39]

Protein Synthesis

Protein synthesis in the endoplasm occurs primarily through ribosomes, which are ribonucleoprotein complexes that translate messenger RNA (mRNA) into polypeptide chains. These ribosomes exist in two forms within the endoplasm: free-floating in the cytosol, which produce proteins destined for intracellular use such as enzymes, and membrane-bound to the rough endoplasmic reticulum (RER), which synthesize proteins for secretion or membrane insertion. In eukaryotic cells, ribosomes consist of small and large subunits that assemble on mRNA to form the functional unit, with eukaryotic cells containing millions of such ribosomes capable of adding amino acids at a rate of approximately 2 per second.^[40] The process of translation begins with initiation, where the small ribosomal subunit binds to the mRNA near the 5' cap and scans to the start codon (AUG), facilitated by eukaryotic initiation factors (eIFs) and the initiator tRNA carrying methionine. The large subunit then joins, forming the complete ribosome with A, P, and E sites for tRNA binding. During elongation, aminoacyl-tRNAs enter the A site, matching their anticodon to the mRNA codon; peptidyl transferase catalyzes peptide bond formation, transferring the growing chain to the new amino acid, followed by translocation to advance the mRNA. This cycle repeats, adding amino acids from N- to C-terminus according to the genetic code, until a stop codon (UAA, UAG, or UGA) is reached in termination, where release factors bind to hydrolyze the bond, freeing the completed polypeptide.^[40] For proteins synthesized on RER-bound ribosomes, translation is coupled with translocation into the ER lumen, enabling co-translational folding assisted by chaperones like BiP (an Hsp70 homolog) and initial glycosylation by oligosaccharyltransferase, which adds N-linked glycans to asparagine residues. This integration ensures proper folding and quality control for secreted or membrane proteins, preventing aggregation. In active eukaryotic cells, such as those with high secretory demands, ER-bound synthesis can account for a substantial portion of total translation, with rates often exceeding cytosolic synthesis by 2.5- to 4-fold, though steady-state protein levels balance due to differential turnover.^[41]^[42] The endoplasm thus produces a diverse array of proteins, including cytosolic enzymes for metabolic functions and secreted proteins like hormones or antibodies, with the process being energy-intensive and reliant on ATP and GTP hydrolysis throughout initiation, elongation, and translocation steps—linking it to broader cellular respiration mechanisms.^[40]

Cellular Respiration

In the endoplasm of eukaryotic cells, particularly in protozoans like amoebae, mitochondria are embedded and serve as the primary sites for aerobic cellular respiration, generating ATP through oxidative phosphorylation. Note that while many amoebae possess mitochondria, certain parasitic species such as Entamoeba histolytica lack them and rely exclusively on anaerobic glycolysis.^[43]^[31] These organelles house the enzymatic machinery for the Krebs cycle (also known as the citric acid cycle or tricarboxylic acid cycle) in their matrix, where acetyl-CoA from upstream metabolic pathways, such as glycolysis in the cytosol, is oxidized to produce high-energy electron carriers.^[44] The cycle begins with the condensation of acetyl-CoA and oxaloacetate to form citrate, followed by a series of dehydrogenation, decarboxylation, and substrate-level phosphorylation steps that yield reducing equivalents for the subsequent electron transport chain.^[44] The overall reaction for one turn of the Krebs cycle is given by:

\text{Acetyl-CoA} + 3 \text{NAD}^+ + \text{FAD} + \text{GDP} + \text{P}_\text{i} + 2 \text{H}_2\text{O} \rightarrow 2 \text{CO}_2 + 3 \text{NADH} + \text{FADH}_2 + \text{GTP} + 2 \text{H}^+ + \text{CoA}

This process produces three molecules of NADH, one FADH₂, and one GTP (equivalent to ATP) per acetyl-CoA, with the NADH and FADH₂ donating electrons to the electron transport chain.^[44] The electron transport chain, embedded in the inner mitochondrial membrane, transfers these electrons through a series of protein complexes (I-IV), pumping protons into the intermembrane space to establish an electrochemical proton gradient.^[45] This gradient drives ATP synthesis via ATP synthase, a rotary enzyme that harnesses proton flow back into the matrix to phosphorylate ADP to ATP.^[45] In eukaryotic cells, complete oxidation of one glucose molecule via the Krebs cycle and electron transport chain yields approximately 30-32 ATP molecules, with ~26-28 from oxidative phosphorylation.^[46] Regulation of these processes occurs primarily through the ADP/ATP ratio, where high ATP levels inhibit key enzymes like isocitrate dehydrogenase and α-ketoglutarate dehydrogenase in the Krebs cycle, slowing respiration when energy is abundant.^[47] Oxygen availability is also critical, as it acts as the final electron acceptor in complex IV of the electron transport chain; its absence halts the chain, preventing further ATP production.^[45]

Role in Cell Motility

Amoeboid Locomotion

Amoeboid locomotion in cells such as Amoeba proteus relies on the dynamic flow of endoplasm, the fluid inner cytoplasm, which enables the cell to undergo continuous shape changes and directed movement across substrates. This process involves the protrusion of temporary extensions called pseudopodia, where the endoplasm streams forward to fill and expand these structures, converting at the leading edge into a more rigid ectoplasm that provides structural support.^[48] Pseudopod formation occurs as endoplasm flows into nascent ectoplasmic extensions, propelled primarily by actin polymerization at the advancing front. Actin monomers assemble into filaments under the influence of polymerization factors, generating pushing forces that drive the extension while the endoplasm supplies the cytoplasmic volume for expansion. This sol-to-gel transition maintains the pseudopod's integrity, allowing the cell to adhere and pull itself forward.^[49] The fountain zone model describes the characteristic circular flow pattern of endoplasm during locomotion in amoebas, where the fluid endoplasm advances toward the front of the cell and rises against the plasma membrane in a fountain-like manner before transforming into ectoplasm. At the rear, the ectoplasm solates back into endoplasm, retreating centrally to complete the cycle and propel the cell body forward. This model, emphasizing contraction at the fountain zone, accounts for the coordinated streaming observed in motile amoebae.^[50] In Amoeba proteus, the speed of this locomotion typically reaches up to 1-2 μm/s, reflecting the rate of endoplasm flow into pseudopodia under optimal conditions.^[51] The energy cost of amoeboid locomotion is substantial, driven by high ATP consumption in cytoskeletal dynamics, including actin polymerization and myosin-mediated contractions that sustain endoplasm flow and pseudopod cycling.^[52]

Endoplasmic Flow Mechanisms

Endoplasmic flow in amoeboid cells is primarily driven by cytoplasmic streaming, a process mediated by interactions between myosin motors and actin filaments that generate contractile forces. Myosin molecules, such as myosin II in amoebae, bind to actin filaments and undergo ATP-dependent conformational changes, producing sliding forces that propel the fluid endoplasm forward. These interactions enable the bulk movement of cytoplasmic components at rates typically ranging from 1 to 10 μm/s in motile amoebae.^[15]^[26] A key regulatory mechanism for this flow involves sol-gel transitions within the endoplasmic matrix, where the cytoplasm alternates between a fluid sol state and a more viscous gel state. These transitions are facilitated by dynamic remodeling of the actin cytoskeleton, including polymerization, depolymerization, and cross-linking by proteins such as actin-binding proteins. Polymer network theories model this behavior as a viscoelastic network where gelation increases rigidity through entanglement and branching, while solation reduces viscosity to allow streaming; for instance, in amoeboid extracts, ATP and calcium modulate these shifts to convert ectoplasmic gel into flowing endoplasm.^[53]^[54] Calcium ions play a central role in triggering contractions that sustain endoplasmic circulation, acting as a second messenger to activate contractile elements. Elevations in free Ca²⁺ concentrations above approximately 7 × 10⁻⁷ M bind to calmodulin and other effectors, stimulating myosin light-chain kinase to phosphorylate myosin, thereby enhancing actin-myosin interactions and initiating gel contraction. This calcium-dependent cycling ensures periodic relaxation and recontraction, maintaining flow directionality without permanent stiffening of the endoplasm.^[55] Observation of these mechanisms relies on advanced in vivo imaging techniques, particularly videomicroscopy, which captures real-time dynamics of flow rates and patterns. Differential interference contrast (DIC) or phase-contrast videomicroscopy, often combined with particle tracking of injected fluorescent beads, reveals rotational or fountain-like streaming patterns in the endoplasm, with velocities varying by cell region and motility state; such methods have quantified heterogeneous flows, showing faster streams in central endoplasm compared to peripheral zones. These observations confirm the coordinated interplay of molecular drivers in generating coherent circulation.^[26]^[56]

Comparison to Ectoplasm

Structural Differences

The endoplasm constitutes the inner core of the cytoplasm in protozoans such as amoebas, characterized by its granular texture and abundance of organelles, including mitochondria, food vacuoles, and the nucleus, which contribute to its fluid, sol-like consistency. In contrast, the ectoplasm forms a peripheral layer immediately adjacent to the cell membrane, appearing clear and nongranular with a gel-like structure that lacks most organelles, providing structural support at the cell's periphery. This differentiation allows the endoplasm to serve as the dynamic interior region, while the ectoplasm maintains a more rigid outer boundary. The boundary between the endoplasm and ectoplasm is dynamic, marked by a zone of structural transition where the fluid endoplasm (plasmasol) converts to the more viscous ectoplasm (plasmagel) through actin polymerization and cross-linking, facilitating cytoplasmic streaming during movement.^[19] The plasmalemma, or plasma membrane, distinctly separates the ectoplasm from the external environment, enclosing the entire cytoplasmic structure and regulating exchanges with the surroundings.^[57] The ectoplasm typically forms a thin peripheral layer, often described as a few micrometers in thickness, while the endoplasm occupies the bulk of the cytoplasmic volume, enabling efficient internal transport and organization. Under light microscopy, the endoplasm exhibits a dense, granular appearance due to its organelle content and inclusions, which take up vital dyes such as neutral red more readily, resulting in stronger staining compared to the translucent ectoplasm.^[58] This contrast highlights the ectoplasm's hyaline quality, making the interface between the two regions clearly visible even in living cells.^[19]

Functional Distinctions

The endoplasm serves as the primary site for metabolic processes within amoeboid protozoans, housing organelles such as the nucleus, mitochondria, and food vacuoles. In contrast, the ectoplasm functions mainly as a structural support layer, providing rigidity to maintain cell shape and enabling pseudopodial extensions for locomotion and attachment to substrates or prey.^[59] This division allows the endoplasm to focus on internal processes while the ectoplasm interacts with the external environment.^[60] In response to external stimuli, such as mechanical disturbance or adverse chemical conditions, the endoplasm undergoes internal contractions driven by calcium-mediated actin-myosin interactions, enabling rapid cytoplasmic streaming and withdrawal from unfavorable areas.^[61] Meanwhile, the ectoplasm expands at the cell periphery to form adhesive pseudopodia, promoting attachment to substrates or prey and facilitating directed movement away from the stimulus.^[59] These complementary responses ensure cellular homeostasis by coordinating avoidance behaviors with structural integrity. From an evolutionary perspective, the endoplasm's fluid nature enables internal reorganization in various protozoans, contributing to adaptability across motile and sessile lifestyles. The functional interdependence of endoplasm and ectoplasm is evident during stress or locomotion, where the ectoplasm acts as a conduit for endoplasm flow: pressure gradients generated by endoplasm contractions propel fluid cytoplasm forward, which then gels into ectoplasm at the leading edge, recycling material to sustain continuous movement.^[59] This dynamic interplay underscores their coordinated contribution to cellular function.

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Amoeboid movement refers to a type of cell locomotion characterized by the constant change of shape through the protrusion and retraction of extensions ...
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We present a hypothesis in which actin polymerization is one of the major driving forces for pseudopod extension during chemotaxis.Missing: endoplasm | Show results with:endoplasm
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Aug 5, 2013 · Our results suggest that the inherent sol–gel transition property plays an essential role in the self-regulation of autonomous translational ...
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This also suggests that biological regulation may play a role controlling the sol to gel transition aside from the physical properties of the cytoplasm. Our ...
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endoplasm the granular portion of the cell cytoplasm between the ectoplasm and nuclear membrane. Page 182. NOAA Coral Reef Information System - Glossary of ...
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To start, center an amoeba so that you know it is in the field of view. In ... The endoplasm is quite fluid and contains the multiple nuclei, granules ...<|control11|><|separator|>
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In addition to the endoplasm is the ectoplasm that surrounds the test; this ectoplasm has protrusions called pseudopodia that can join and split dynamically.
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The protoplasm on the outside of the test makes long filaments, which are used for locomotion and capturing food particles.