Eukaryotic Cell- Structure and Functions

 

Eukaryotic Cell Structure and Function

1. Introduction:

• Eukaryotic cells are complex, characterized by membrane-bound organelles and a true nucleus.
• They are found in Protists, Fungi, Plants, and Animals.

 

2. Cell Envelope:

• Cell Wall:
• Present in Fungi, Algae (Protists), and Plants. Most animal eukaryotic cells lack a cell wall.
• Composition varies:
• Fungi: Primarily chitin, glucan.
• Algae/Plants: Primarily cellulose, pectin; may contain silica or calcium carbonate.
• Protist cysts: Often chitin.
• Generally simpler than bacterial peptidoglycan.
• Provides structural support and shape.
• Cell Membrane (Plasma Membrane):
• A lipid bilayer defining the cell boundary.
• Unique Lipid Composition (vs. Bacteria): High proportion of sphingolipids and sterols (e.g., cholesterol, ergosterol) in addition to phospholipids (Figure 5.4). These affect membrane fluidity and permeability.
• Asymmetric Distribution: Lipids differ between inner and outer leaflets. Contains lipid rafts (microdomains) involved in signaling, virus assembly, endocytosis.
• Glycocalyx: Carbohydrate-rich layer on the surface (often seen in protists like Figure 5.3b), involved in protection, recognition, adhesion.
• Functions: Regulates entry/exit of materials (facilitated diffusion, active transport, passive diffusion, endocytosis - unique to eukaryotes).

 3. Nucleus :

• Structure: Membrane-bound organelle containing the cell's DNA. Surrounded by a nuclear envelope (double membrane, continuous with ER) containing nuclear pore complexes for transport (Figure 5.11). Contains the nucleolus (site of rRNA synthesis).
• Genetic Material Organization:
• DNA organized into linear chromosomes composed of chromatin (DNA + histone proteins).
• Nucleosomes: DNA wrapped around histone octamers (H2A, H2B, H3, H4) with linker histone H1, forming the "beads-on-a-string" appearance (Figure 5.10).
• Site of DNA replication, transcription (RNA synthesis).
• RNA is processed and exported to the cytoplasm for translation.

   

Mitochondria:

Ø  Often called the "powerhouses" of the cell; site of the TCA cycle and ATP generation (via oxidative phosphorylation) using oxygen.

Ø  Size similar to bacteria (0.3-1.0 μm by 5-10 μm); number varies (1 to 1000+ per cell).

Ø  Double-membrane structure (outer and inner membranes).

Ø  Inner membrane has cristae (infoldings) increasing surface area; contains enzymes/electron carriers for ETC and ATP synthesis.

Ø  Inner membrane encloses the matrix (contains ribosomes, DNA, enzymes for TCA cycle and fatty acid breakdown).

Ø  Possess their own DNA (often circular) and ribosomes, reflecting bacterial ancestry, but synthesize only some of their own proteins.

Ø  Reproduce by binary fission.

Ø  Related organelles include hydrogenosomes (anaerobic, produce ATP via fermentation, produce H2) and mitosomes (non-ATP producing).

Endoplasmic Reticulum (ER):

Ø  Network of membranous tubules and flattened sacs (cisternae).

Ø  Two types:

·         Rough ER (RER): Studded with ribosomes on the cytoplasmic surface; involved in synthesizing proteins destined for secretion, membrane insertion, or transport to other organelles. Also adds sugars to proteins (glycosylation).

·         Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis.

·         Major site of cell membrane synthesis.

 Ribosomes:

Ø  Sites of protein synthesis.

Ø  80S ribosomes in eukaryotes (larger than bacterial 70S), composed of 60S and 40S subunits.

Ø  Free ribosomes (in cytoplasm) synthesize proteins for use within the cell (e.g., cytosol, nucleus, mitochondria).

Ø  Bound ribosomes (attached to RER via the 60S subunit) synthesize proteins for secretion or membrane insertion.

Golgi Apparatus:

Ø  Composed of stacked, flattened cisternae (forming a dictyosome).

Ø  Has distinct cis face (forming, near ER) and trans face (maturing, far from ER) with different enzyme compositions.

Ø  Functions: Packages, modifies, and prepares materials (especially proteins from the ER) for secretion or delivery to other organelles/locations.

Ø  Involved in constructing surface scales (some protists), developing cell membranes, and packaging cell products (e.g., for hyphal tip growth in fungi).

Lysosomes:

Ø  Membrane-bound, spherical organelles found primarily in animal cells.

Ø  Contain hydrolytic enzymes (hydrolases) optimal at acidic pH (3.5-5.0), maintained by proton pumping.

Ø  Function in intracellular digestion (e.g., nutrients via endocytosis).

Ø  Similar degradative organelles exist in fungi/protists (sometimes called vacuoles/phagocytic vacuoles/food vacuoles).

Ø  Also involved in storage (ions, amino acids) and autophagy (recycling cellular components via autophagosomes fusing with lysosomes).

Extracellular Matrix (ECM):

Ø  The document mentions the glycocalyx (carbohydrate-rich layer on cell surface) but does not detail a broader ECM like that found in animal tissues. Fungal cell walls and algal cell walls serve somewhat analogous structural functions outside the plasma membrane.

Chloroplasts:

Ø  Type of plastid found in algae and plants; site of photosynthesis (using light energy to convert CO2 and water to carbohydrates and O2).

Ø  Double-membrane structure (envelope).

Ø  Inner membrane encloses the stroma (contains DNA, ribosomes, enzymes for the "dark reactions"/Calvin cycle, starch granules, lipid droplets).

Ø  Internal membrane system of thylakoids (flattened sacs); site of the "light reactions" (containing chlorophyll and ETC components).

Ø  Thylakoids may be stacked into grana.

Ø  Evolved from a cyanobacterial endosymbiont.

Cytoskeleton:

·         Network organizing the cytoplasm and facilitating movement.

·         Composed of three main filament types:

·         Actin Filaments (Microfilaments): ~4-7 nm diameter; involved in cell shape changes (amoeboid movement), endocytosis, cytokinesis, and intracellular transport (via actin cables/patches/rings).

·         Intermediate Filaments (IFs): ~10 nm diameter; strong, flexible structural elements (e.g., nuclear lamina in animals), help position organelles; absent in fungi and plants.

·         Microtubules: ~25 nm diameter; hollow cylinders made of α- and β-tubulin dimers; form the spindle apparatus for chromosome separation, act as tracks for organelle/vesicle movement (via motor proteins kinesin/dynein), and are components of cilia and flagella. Also provide structural support (e.g., axopodia).

Flagella:

·         Long, whiplash (100-200 μm) structures for motility.

·         May be tinsel (with lateral hairs/mastigonemes/flimmer filaments) or whiplash (naked).

·         Structurally identical to cilia (axoneme with 9+2 microtubule pattern).

·         Move via undulating waves, pushing or pulling the cell.

Cilia:

·         Short, hair-like (5-20 μm) structures for motility.

·         Structurally identical to flagella (axoneme with 9+2 microtubule pattern).

·         Move via coordinated beating: effective stroke (stiff, propels) and recovery stroke (bent, returns).

·         Beat frequency ~10-40 Hz; can achieve high speeds (e.g., Paramecium).

Reference         

Madigan, M. T., Martinko, J. M., Bender, K. S., Buckley, D. H., & Stahl, D. A. (2017). Brock Biology of Microorganisms (15th ed.). Pearson.

Harvey E. Prescot et al. Text book of Microbiology