Living Cell : Overview

I. Prokaryotic Cells:

A. Cell Envelope:

1. Cell Wall Composition: Prokaryotic cell walls vary in composition. Gram-positive bacteria have a thick layer of peptidoglycan, while Gram-negative bacteria have a thinner peptidoglycan layer surrounded by an outer membrane.
2. Capsule: Some prokaryotic cells possess a capsule, a protective layer external to the cell wall. The capsule aids in adherence to surfaces, protection against host defenses, and resistance to desiccation.
3. Pili and Flagella: Pili are short, hair-like structures that facilitate bacterial adhesion, while flagella are longer whip-like appendages responsible for bacterial motility.

B. Cytoplasmic Structures:

1. Nucleoid: The nucleoid is the region within the cytoplasm where the bacterial chromosome (single circular DNA molecule) is located. It lacks a membrane-bound nucleus.
2. Plasmids: Prokaryotic cells may contain extrachromosomal DNA elements called plasmids, which often carry genes for antibiotic resistance or other advantageous traits.
3. Ribosomes: Prokaryotic ribosomes (70S) play a crucial role in protein synthesis. They consist of a small (30S) and large (50S) subunit.

II. Eukaryotic Cells:

A. Nucleus and Genetic Material:

1. Nuclear Pores: Nuclear pores are protein complexes that regulate the passage of molecules between the nucleus and cytoplasm. They play a role in RNA export and protein import/export.
2. Chromatin Structure: Within the nucleus, DNA is organized into chromatin, a complex of DNA and proteins. During cell division, chromatin condenses into visible chromosomes.

B. Endomembrane System:

1. Endoplasmic Reticulum (ER):
   • Rough ER is studded with ribosomes and is involved in protein synthesis and modification.
   • Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.
2. Golgi Apparatus: The Golgi apparatus processes, modifies, and sorts proteins and lipids received from the ER. It plays a crucial role in the secretion of cellular products.
3. Lysosomes: Lysosomes are membrane-bound vesicles containing enzymes for intracellular digestion. They break down cellular waste and damaged organelles.

C. Energy-Related Organelles:

1. Mitochondria:
   • The outer membrane surrounds the organelle, and the inner membrane forms cristae, increasing surface area for energy production.
   • The matrix contains mitochondrial DNA and enzymes for the Krebs cycle.
2. Chloroplasts:
   • Thylakoids inside chloroplasts contain chlorophyll for photosynthesis.
   • Stroma, the fluid-filled space, houses enzymes for the Calvin cycle.

D. Cytoskeleton:

1. Microtubules: Microtubules provide structural support, act as tracks for intracellular transport, and form the mitotic spindle during cell division.
2. Microfilaments (Actin Filaments): These are involved in cell motility, support, and the formation of pseudopodia during cell movement.
3. Intermediate Filaments: Intermediate filaments contribute to cell shape and provide mechanical strength. They are involved in maintaining cell integrity.

E. Extracellular Structures:

1. Extracellular Matrix (ECM): Animal cells are embedded in the ECM, consisting of proteins like collagen and glycoproteins. It provides structural support, regulates cell behavior, and is crucial in tissue development.
2. Cell Junctions:
   • Tight junctions, desmosomes, and gap junctions in animal cells facilitate communication, adhesion, and exchange of materials between adjacent cells.

F. Specialized Structures:

1. Centrioles: Centrioles are involved in organizing microtubules during cell division in animal cells.
2. Cilia and Flagella: Cilia and flagella are hair-like structures extending from the cell surface, facilitating cell movement. Cilia are typically numerous and short, while flagella are longer.