Cell Structure and Function

Cell Structure and Function

• Cell Membrane: Outermost boundary of the cell, acts as a barrier, separating the internal environment from external.
• Nucleus: Dense membrane-bound structure within the cell, containing chromosomes.
• Chromosomes: Structures within the nucleus, composed of DNA and protein. DNA (Deoxyribonucleic acid) carries genetic information.
• Eukaryotic Cells: Cells that have membrane-bound nuclei and other organelles.
• Prokaryotic Cells: Cells lacking a membrane-bound nucleus and other organelles.
• Cytoplasm: Semifluid matrix filling the space between the cell membrane and the nucleus, where cellular activities take place.
• Organelles: Membrane-bound structures within eukaryotic cells, performing specific functions.
  • Examples: Endoplasmic Reticulum (ER), Golgi complex, Lysosomes, Mitochondria, Vacuoles.
• Ribosomes: Non-membrane bound organelles found in both eukaryotic and prokaryotic cells, responsible for protein synthesis.
• Centrosome: Non-membrane bound organelle present in animal cells, involved in cell division.
• Cell Size and Shape: Cells vary greatly in size and shape. Mycoplasmas (smallest cells) are 0.3 µm inlength. Bacteria range from 3 to 5 µm.
  • Red blood cells: Round and biconcave.
  • White blood cells: Amoeboid (irregular, changing shape).
  • Columnar epithelial cells: Long and narrow.
  • Nerve cells: Branched and long.
  • Mesophyll cells (plants): Round and oval.
  • Tracheids (plants): Elongated.
• Mesosomes (prokaryotes): Infoldings of the plasma membrane, forming vesicles, tubules, and lamellae.
  • Functions: Cell wall formation, DNA replication, distribution to daughter cells, respiration, secretion, increasing plasma membrane surface area, enzymatic content.

Prokaryotic Cell Features

• Chromatophores (some prokaryotes): Membranous extensions into the cytoplasm, containing pigments.
• Flagella: Thin filamentous extensions from the cell wall, used for movement.
  • Structure: Filament (longest), hook, basal body.
• Pili and Fimbriae: Surface structures that are NOT involved in motility.
  • Pili: Elongated, tubular structures.
  • Fimbriae: Small, bristle-like fibers.
  • Functions: Attachment to surfaces (rocks, host tissues).

Prokaryotic Cell Structures

• Ribosomes: About 15 nm by 20 nm in size, composed of two subunits (50S and 30S).
  • Together, they form 70S prokaryotic ribosomes.
• Inclusion Bodies: Reserve material stored in the cytoplasm, not enclosed by any membrane.
  • Examples: Phosphate granules, cyanophycean granules (in cyanobacteria), glycogen granules.
• Gas vacuoles: Found in photosynthetic bacteria (e.g., blue-green bacteria).

Eukaryotic Cell Features

• Compartmentalization: Cytoplasm is divided into compartments by membrane-bound organelles.
  • This increases efficiency and specialization of cellular functions.

Eukaryotic Cell Organelles

• Endoplasmic Reticulum (ER): Network of membrane-bound tubules and sacs.
  • Rough ER: Has ribosomes attached, responsible for protein synthesis and transport.
  • Smooth ER: Lacks ribosomes, involved in lipid synthesis, detoxification, and steroid hormone production.
• Golgi Complex: Stack of flattened, membrane-bound sacs called cisternae.
  • Cis face: Forming face, receives vesicles from ER.
  • Trans face: Maturing face, packages and modifies proteins.
  • Function: Modifies, sorts, and packages proteins and lipids for secretion, and for use within the cell.
• Lysosomes: Membranous vesicles containing hydrolytic enzymes (lipases, proteases, carbohydrases).
  • Functions: Digesting cellular waste, damaged organelles, and foreign materials.
• Vacuoles: Membrane-bound sacs containing water, sap, excretory products, and other materials.
  • In plants: Can occupy up to 90% of cell volume, important for turgor pressure and storage.
  • In Amoeba: Contractile vacuole is involved in osmoregulation (regulating water balance) and excretion.
  • Food vacuoles: Formed by engulfing food particles.
• Mitochondria: "Powerhouses of the cell," responsible for ATP production through cellular respiration.
  • Have their own DNA and ribosomes.
• Plastids: Double-membrane bound organelles found in plants and some protists.
  • Chloroplasts: Contain chlorophyll for photosynthesis.
• Microtubules: Hollow cylinders made of tubulin protein.
  • Functions: Cell shape, cell division, movement of organelles, and cilia/flagella structure.
• Microfilaments: Solid rods made of actin protein.
  • Functions: Cell shape, muscle contraction, cell division, and movement of organelles.
• Cilia and Flagella: Hair-like outgrowths from the cell membrane.
  • Cilia: Short, numerous, used for movement of fluids or the cell.
  • Flagella: Long, whip-like, used for cell movement.
  • Both have a 9+2 arrangement of microtubules.

Nucleus

• Nuclear Envelope: Double membrane surrounding the nucleus.
  • Outer membrane is continuous with the endoplasmic reticulum.
  • Nuclear pores: Openings in the nuclear envelope that allow for the passage of RNA and proteins.
• Nucleoplasm: The fluid inside the nucleus, containing the nucleolus and chromatin.
• Nucleolus: Dense, spherical region within the nucleus, responsible for ribosome synthesis (rRNA).
• Chromatin: A network of DNA and proteins.
  • During cell division, chromatin condenses into chromosomes.
  • Carries genetic information.
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Test your knowledge on cell structure and function with this quiz. Explore key components such as the cell membrane, nucleus, and organelles that play essential roles in cellular activities. Understand the differences between eukaryotic and prokaryotic cells and the functions of various organelles.