Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, possessing an internal environment distinct from their surroundings. This compartmentalization is essential for maintaining the conditions necessary for life and for carrying out specialized functions.

• Cell: The smallest living unit, capable of independent existence and reproduction.

• Internal environment: Maintained by the plasma membrane, separating the cell from the external environment.

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins. It is essential for maintaining the cell's internal environment.

• Functions of the plasma membrane:

  • Isolates the cell’s internal contents from the external environment

  • Regulates the flow of materials into and out of the cell

  • Allows communication with other cells

Cytoplasm and Cytosol

Definitions and Components

The cytoplasm is the region inside the plasma membrane and outside the nucleus (in eukaryotic cells). It includes the cytosol (the fluid component) and all organelles and cellular structures within the cell.

• Cytosol: The fluid matrix of the cytoplasm.

Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers and filaments in the cytoplasm. It provides structural support, maintains cell shape, and facilitates movement of organelles and the cell itself.

• Composed of microfilaments, intermediate filaments, and microtubules.

• Involved in intracellular transport and cell division.

Genetic Material

DNA and Its Role

All cells contain genetic material in the form of deoxyribonucleic acid (DNA), which carries hereditary instructions for cell function and reproduction.

Prokaryotic versus Eukaryotic Cells

Key Differences

Cells are classified as prokaryotic or eukaryotic based on the presence or absence of a nucleus and other membrane-bound organelles.

Prokaryotic Cell Structures

• Nucleoid: Dense region of DNA in prokaryotic cells (not membrane-bound).

• Plasmids: Small, circular DNA molecules separate from the main chromosome, often carrying additional genes.

• Cell wall: Rigid structure providing shape and protection; permeable to small molecules.

• Pili (singular: pilus): Short, hair-like protein projections for attachment to surfaces or other cells.

• Flagella (singular: flagellum): Long, whip-like structures used for cell movement by rotation.

Eukaryotic Cell Structures

• Nucleus: Membrane-bound organelle containing most of the cell’s genetic material.

• Nuclear envelope: Double membrane enclosing the nucleus.

• Organelles: Specialized compartments within the cytoplasm, including mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, and vesicles.

Compartmentalization in Eukaryotic Cells

Purpose and Advantages

• Provides localized environments for specific metabolic processes.

• Sequesters reactions to prevent interference between incompatible processes.

Nucleus and Genetic Material

Structure and Function

• Nucleus: Contains the cell’s genetic library; surrounded by the nuclear envelope.

• Chromatin: Complex of DNA and histone proteins; forms chromosomes during cell division.

• Chromosomes: Long, threadlike associations of genes composed of chromatin.

• Nucleolus: Spherical region within the nucleus where ribosomes are assembled; may be two or more per cell.

Ribosomes

Structure and Function

• Cytoplasmic complexes responsible for protein synthesis.

• Composed of ribosomal RNA (rRNA) and proteins; consist of two subunits.

• Present in both prokaryotic and eukaryotic cells.

• In prokaryotes: free in cytoplasm; in eukaryotes: free or bound to membranes (e.g., rough ER).

Mitochondria and Chloroplasts

Energy-Transforming Organelles

• Mitochondria: Organelles that extract energy from organic molecules to produce ATP (cellular respiration).

• Chloroplasts: Organelles containing chlorophyll; sites of photosynthesis in plants and algae.

• Both have double membranes, contain their own DNA and ribosomes, and are semi-autonomous (can grow and reproduce independently).

Mitochondrial Features

• Size: 1–10 μm

• Number per cell varies with metabolic activity

• Can move and change shape

• Inner membrane highly folded into cristae to increase surface area

• Space between outer and inner membranes: intermembrane compartment

• Space inside inner membrane: matrix

Chloroplast Features

• Size: 2.0–5.0 μm

• Number per cell varies with metabolic activity

• Can move and change shape

• Space inside inner membrane: stroma (viscous fluid)

• Thylakoids: Third membrane system, shaped like flattened sacs, found in the stroma

• Grana: Stacks of thylakoids

Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells that coordinates the synthesis, modification, and transport of cellular materials.

• Nuclear envelope

• Endoplasmic reticulum (rough and smooth types)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Vesicles

Components are connected directly (physical contact) or indirectly (via vesicles).

Vesicles

• Membrane-enclosed sacs that transport materials between organelles or to the plasma membrane.

Endoplasmic Reticulum (ER)

• Largest part of the cell’s membrane system; network of tubules and sacs.

• Two types:

  • Smooth ER: Lacks ribosomes; synthesizes lipids (phospholipids, steroids).

  • Rough ER: Studded with ribosomes; manufactures membranes and synthesizes secretory proteins.

Pathway of a Secretory Protein

1. Protein synthesized by a bound ribosome

2. Protein moves through the ER membrane into the ER

3. Vesicle carries the protein from the ER to the Golgi apparatus

4. Vesicle carries the protein to the plasma membrane for export

Golgi Apparatus

• Stacked, flattened membranous sacs that modify, store, and route products from the ER.

• Has two faces:

  • Cis-face: Receiving side for vesicles from the ER

  • Trans-face: Shipping side for vesicles leaving the Golgi

Lysosomes

• Membrane-bound organelles containing digestive enzymes.

• Functions:

  1. Intracellular digestion (e.g., food vacuoles formed by phagocytosis)

  2. Recycling of cellular material

  3. Programmed cell destruction (apoptosis)

Additional info: The notes above are expanded and clarified for academic completeness. Some terminology and explanations (e.g., apoptosis, types of cytoskeletal elements) are inferred from standard biology curricula.