Cell Structure and Function: General Biology Study Notes

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, possessing an internal environment distinct from their external surroundings. This compartmentalization is essential for maintaining homeostasis and supporting cellular functions.

• Cell: The smallest living unit, capable of independent life.

• Plasma Membrane: A selectively permeable bilayer of phospholipids with embedded proteins, separating the cell's interior from the external environment.

• Functions of 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

The cytoplasm is the region inside the cell membrane and outside the nucleus (in eukaryotes). It contains the cytosol and all cellular structures except the nucleus.

• Cytosol: The fluid portion of the cytoplasm.

• Cytoskeleton: A network of protein fibers and filaments that provides structural support and facilitates movement within the cell.

Genetic Material in Cells

DNA and Its Storage

Most cells contain genetic material in the form of deoxyribonucleic acid (DNA), which carries hereditary instructions for cellular activities.

• DNA: The molecule that stores genetic information.

Prokaryotic vs. Eukaryotic Cells

Key Differences

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

Feature	Prokaryotes	Eukaryotes
Nucleus	No nucleus or nuclear envelope	Nucleus with a nuclear envelope
Location of DNA	DNA in a "nucleoid region"	DNA within the nucleus

• Nucleoid: Dense region of DNA in prokaryotic cells.

• Plasmids: Small, circular DNA molecules found in prokaryotes, separate from the main chromosome.

• Nucleus: Membrane-bound organelle in eukaryotes containing most genetic material.

• Nuclear Envelope: Double membrane enclosing the nucleus in eukaryotes.

Cell Wall, Pili, and Flagella

Some cells possess additional structures for support, attachment, and movement.

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

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

• Flagella: Long, whip-like structures used for cell movement in liquid environments.

Eukaryotic Cell Complexity

Organelles and Compartmentalization

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions, allowing specialized environments for metabolic processes.

• Organelles: Specialized compartments within the cytoplasm, including:

  1. Nucleus

  2. Mitochondrion

  3. Chloroplast

  4. Endoplasmic Reticulum

  5. Golgi Apparatus

  6. Vesicles

• Compartmental Organization:

  • Provides localized conditions for specific metabolic reactions

  • Sequesters reactions to prevent interference between incompatible processes

Nucleus, Chromatin, and Chromosomes

The nucleus serves as the cell's genetic library, containing chromatin and chromosomes.

• Chromatin: Complex of DNA and histone proteins; appears as diffuse material in non-dividing cells.

• Chromosomes: Long, threadlike associations of genes; each species has a specific number (e.g., 46 in humans).

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

Ribosomes

Ribosomes are the sites of protein synthesis, present in both prokaryotic and eukaryotic cells.

• Structure: Composed of rRNA and proteins; made up of two subunits.

• Location:

  • Prokaryotes: Free in cytoplasm

  • Eukaryotes: Free in cytoplasm or attached to membranes (e.g., rough ER)

Energy-Transforming Organelles

Mitochondria

Mitochondria are organelles that use energy from organic molecules to produce ATP, the cell's energy currency.

• Features:

  • Size: 1–10 μm

  • Double membrane structure

  • Inner membrane highly folded into cristae to increase surface area

  • Space between membranes: intermembrane compartment

  • Space inside inner membrane: matrix

  • Contain their own DNA and ribosomes

  • Semi-autonomous: Can grow and reproduce

Chloroplasts

Chloroplasts are organelles containing chlorophyll, responsible for photosynthesis in plant cells.

• Features:

  • Size: 2.0–5.0 μm

  • Double membrane structure

  • Space inside inner membrane: stroma (viscous fluid)

  • Third membrane system: thylakoids (flattened sacs)

  • Stacks of thylakoids: grana

  • Can move and change shape

Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells, involved in synthesis, transport, and processing of biomolecules.

• Components:

  • Nuclear envelope

  • Endoplasmic reticulum (rough and smooth)

  • Golgi apparatus

  • Lysosomes

  • Vesicles

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

Vesicles

Vesicles are membrane-enclosed sacs that transport materials between organelles or to the cell membrane.

Endoplasmic Reticulum (ER)

The ER is the largest part of the cell's membrane system, consisting of an extensive network of tubules and sacs. It is found in two forms:

• Smooth ER: Lacks ribosomes; synthesizes lipids (especially phospholipids and 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. Newly synthesized protein moves through the ER membrane into the ER

3. Vesicle carrying the protein is pinched off the ER and travels to the Golgi apparatus

Golgi Apparatus

The Golgi apparatus is an organelle made of stacked, flattened membranous sacs that modifies, stores, sorts, and routes products of the ER.

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

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

Lysosomes

Lysosomes are membrane-bound organelles that digest macromolecules and recycle cellular material.

• Functions:

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

  2. Recycling of cellular material

  3. Programmed cell destruction

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The primary energy carrier in cells, produced mainly by mitochondria.

• Phagocytosis: The process by which cells engulf large particles or other cells, forming food vacuoles.

Summary Table: Major Eukaryotic Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material (DNA); controls cell activities
Mitochondria	Produces ATP via cellular respiration
Chloroplast	Performs photosynthesis (in plants)
Endoplasmic Reticulum	Synthesizes proteins (rough ER) and lipids (smooth ER)
Golgi Apparatus	Modifies, sorts, and ships proteins and lipids
Lysosome	Digests and recycles cellular waste
Vesicle	Transports materials within the cell

Important Equations

• ATP Production (Cellular Respiration):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis (in Chloroplasts):

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Additional info: These notes expand on the original content by providing definitions, examples, and context for key cell biology concepts, suitable for college-level General Biology students.