Structure of Eukaryotic Cells and Importance of Membranes

Structure of Eukaryotic Cells and Importance of Membranes
Structure of Eukaryotic Cells and Importance of Membranes

Structure of Eukaryotic Cells and Importance of Membranes

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Structure of Eukaryotic Cells and Importance of Membranes

Eukaryotic cells are the most structurally advanced of the major cell types. Describe the structure and function of each of the eukaryotic organelles. Distinguish between those that are and are not membranous. Most are membranous. Explain the importance of membrane structure and function in the organization of living processes within cells.

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Structure of Eukaryotic Cells and Importance of Membranes


Eukaryotic cells are present in plants, animals, protozoa, and fungi (Voet, 2012). This paper will explore the structure and function of the eukaryotic cell organelles. The paper will also discuss the structure and function of biological membranes including the cytoplasmic membrane. A special focus will be given to internal membranes that enclose cellular organelles such as the nucleus, the mitochondrion, the peroxisome, the lysosome, the chloroplast, and the endoplasmic reticulum.

Cell Structures and Functions

The Cell Wall and Glycocalyx

The cell wall is a rigid layer that surrounds some cells, is composed of one or more polysaccharides, and provides additional strength to the cell. Higher plants and algae have cell walls made up of cellulose, pectin, and hemicellulose. Chitin is the main polysaccharide of fungal cell walls, while yeast cells have cell walls composed of mannan and glucan. An external layer called glycocalyx that strengthens the cell and facilitates attachment to neighboring cells surrounds animal cells (Voet, 2012).

The Cytoplasm

The cytoplasm is bound by the plasma membrane and includes all the materials inside the cell with the exclusion of the nucleus. It comprises of a gel-like substance called cytosol and internal cell substructures called organelles. Most of the cell activities such as cell division and metabolism occur in the cytoplasm. It is approximately 80% water, has dissolved salts and biomolecules such as proteins and carbohydrates and suspended insoluble molecules such as lipids (Nelson & Cox, 2013; Voet, 2012).

The Cytoskeleton

The cytoskeleton is a lattice-like array of cell fibers and fine tubes. It has three components namely: microtubules, microfilaments, and intermediate filaments. Microtubules maintain the cell shape and play central roles in chromosome segregation during cell division, endocytosis, and cell differentiation. Other eukaryotic cell structures derived from microtubules include cilia, flagella, centrioles, and spindles. Microfilaments are involved in cell shape change, phagocytosis, cyclosis, and amoeboid movement while intermediate filaments anchor membrane-bound organelles in the cytoplasm (Berg, Tymoczko, & Stryer, 2012; Voet, 2012).

Membrane-Bound Organelles

The nucleus is arguably the largest cell organelle and is bound by a membrane called nuclear envelope, which is punctuated into pores. The nucleus contains the genetic material called DNA and controls all the activities of the cell. The Endoplasmic Reticulum (ER) is a network of tubules that act as the transport system of the cell. There are two types of ER: rough ER and smooth ER. The rough ER is coarse in appearance because it is lined with ribosomes and is involved in the transport of proteins, while the smooth ER has no ribosomes and is the lipid transport system.

The Ribosomes are small particles either scattered in the cytosol or lined on the surface of rough ER. They contain RNA and proteins in almost equal proportions. The ribosomes function as the sites of protein synthesis. The Golgi apparatus is a membrane-bound eukaryotic cell organelle made up of tubes called cisternae. The Golgi is supported by microtubules and is located in proximity to the nucleus and the ER. The Golgi performs post-translational modification of proteins, packages them into vesicles, and exports them into target cell compartments(Berg et al., 2012; Voet, 2012).

The lysosomes are roundish, vesicular structures of animal cells that have a lumen containing hydrolytic enzymes. The pH of the luminal contents is 4.5-5.0 which is optimal for lysosome enzymes. The lysosome digests unwanted materials from outside the cell as well as obsolete cell components. The centrosome is present in eukaryotic animal cells and is made up of two centrioles and surrounding pericentriolar materials. The centrioles are short cylinders arranged such that they are perpendicular to each other. The centrosomes are microtubule-organizing centres that contain gamma-tubulin. The microtubules grow out of this gamma-tubulin in the pericentriolar material. The Vacuole is the major acid-containing organelle of plant and fungal cells. It contains a fluid called cell sap and is surrounded by a membrane called tonoplast. The plant vacuole is the equivalent of the lysosome in animal cells as it has hydrolytic enzymes that digest waste materials. The vacuole is also involved in maintaining cell turgor pressure (Berg et al., 2012; Nelson & Cox, 2013).

The mitochondrion and the chloroplast are two organelles involved in energy production. The mitochondrion is sausage-shaped double membrane cell structure whose inner membrane is invaginated to form cristae. The mitochondrial matrix contains ribosomes and DNA and is therefore self-replicating and semi-autonomous. The main function of the mitochondrion is synthesis of ATP. The chloroplast also has a double membrane and is present in plant cells. It has internal structures such as thylakoids and stroma and its main function is to carry out the process of photosynthesis. The peroxisome is another self-replicating organelle that has enzymes for oxidative degradation of molecules such as uric acid, amino acids, purines, methanol, and fatty acids (Nelson & Cox, 2013).

Structure of Biological Membranes

A biological membrane is composed of a phospholipid bilayer. The membrane is amphipathic, meaning that the polar phosphate lipid heads are on the surface while the hydrophobic tails point inwards. The lipid molecules diffuse rapidly in the plane of the biomembrane but not across. Also, the phospholipid molecules can move laterally from one side of the bilayer to the other, a process called the flip-flop. Moreover, biological membranes are asymmetric, meaning that the two phases are different from each other. In addition to the lipids, membranes also have proteins that move freely within the membrane, and this makes the membrane fluidic and mosaic. The proteins are categorized into either integral or peripheral proteins depending on their degree of association with the membrane. Integral proteins penetrate deep into the bilayer while peripheral proteins are superficially located. Some lipids are linked to carbohydrates to form glycolipids. Cholesterol is present in animal cells and is involved in maintaining membrane fluidity (Berg et al., 2012; Nelson & Cox, 2013).

General Functions of Biological Membranes

The plasma membrane plays a role in establishing a physical barrier between the cell contents and extracellular environment. Biomembranes also facilitate the formation of membrane-enclosed organelles a process called intracellular compartmentalization. Compartmentalization establishes microenvironments and biological barriers between biochemical processes, which allow the cell to carry out different processes simultaneously. Biomembranes are selective permeability barriers as they confine certain molecules within a specific region while restricting the entry of others (Voet, 2012). They contain molecular pumps, sinks, and gates or channels that regulate the molecular and ionic composition of the intracellular or intra-organelle medium. Membranes are the sites of biochemical processes such as oxidative phosphorylation (inner mitochondrial membrane) and photosynthesis (thylakoid membrane). Membranes also have receptors that trigger signal transduction (Berg et al., 2012).

The Plasma and Organelles Biomembranes

The Plasma Membrane

This is the biological barrier between the cell and the external environment. It has biomolecules for intercellular communication and transport. Based on the external environment, the cell membrane can either be an apical, sinusoidal or basolateral membrane. Contact between cells is either through tight junctions, gap junctions or desmosomes (Voet, 2012).

The Nuclear Membrane

The nucleus has a double membrane that is often continuous with the ER membrane. It houses and protects the genetic material and keeps the confines the DNA processing molecules closer to the DNA itself. The nuclear membrane also creates a barrier between transcription and translation and ensures that the two occur as separate processes. Nuclear membrane has nuclear pores, which allow passage of mRNA-protein complexes from the nucleus to the cytoplasm and passage of regulatory proteins from the cytoplasm into the nucleoplasm (Berg et al., 2012).

The Mitochondrial Membrane

The mitochondrion has inner and out membranes. The outer membrane has integral channels called porins that allow proteins less than 5KDa to diffuse through. A translocase is involved in the shipping of larger proteins. The outer membrane forms structures with the ER called mitochondria associated-ER membrane that are useful in calcium signaling and transfer of lipids between the two organelles. The inner membrane is impermeable to all molecules, and they require a transporter to pass through. The inner membrane is convoluted to many cristae to increase surface area for ATP synthesis (Berg et al., 2012; Nelson & Cox, 2013).

The ER and the Golgi Membranes

The ER membrane is an extension of the plasma membrane and is attached to the nuclear membrane. The ER membrane can form vesicles containing proteins that then fuse with the Golgi membrane. The Golgi membrane also facilitates the secretion of processed proteins via exocytosis (Berg et al., 2012).

The Chloroplast Membrane

This is a double membrane enclosing a third internal membrane called thylakoid membrane, which is a system of interconnecting compartments. The thylakoid membrane is the site of energy synthesis and contains a series of proteins collectively referred to as electron transport chain. The outer chloroplast membrane is highly permeable to small organic molecules, while the inner membrane is less permeable and has transport proteins as well as light harvesting pigments (Berg et al., 2012; Voet, 2012).

Lysosome Membrane

This membrane separates the cytoplasm from the acidic milieu of the lysosome. The lysosome membrane has glycosylated membrane proteins called lysosome-associated membrane protein (LAMP) which mediates contact to cytosolic proteins and with other cell organelles. Thus, the lysosome membrane and its proteins facilitate lysosome motility, exocytosis, phagocytosis, macroautophagy among other lysosome functions (Voet, 2012).

Peroxisome Membrane

This biological barrier surrounds the peroxisome and provides a compartment for oxidation reactions. It has membrane proteins called peroxins (PEX) that shuttle proteins between the peroxisome membrane and the cytosol. The peroxisome shuttling process is dependent on ATP and ubiquitylation (Voet, 2012).


Eukaryotic cells have subcellular structures called organelles that have specific functions. Both the plasma membrane and the organelle membrane are composed of lipid bilayers, proteins, and glycans. The plasma membrane is the biological barrier to the extracellular environment. The organelle membranes create microenvironments suitable for specific biochemical reactions.


Berg, J. M., Tymoczko, J. L., & Stryer, L. (2012). Biochemistry (7 ed.): W. H. Freeman.

Nelson, D. L., & Cox, M. M. (2013). Lehninger Principles of Biochemistry (6 ed.): W.H.Freeman.

Voet, D. (2012). Fundamentals of Biochemistry: Life at the molecular level: Wiley.

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