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 surroundings. This compartmentalization allows for specialized functions and regulation of cellular processes.

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

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

Plasma Membrane

The plasma membrane is a selectively permeable barrier composed of phospholipids and proteins. It is essential for maintaining cellular integrity and regulating interactions with the 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

The cytoplasm encompasses all cellular contents between the plasma membrane and the nucleus (in eukaryotes). The cytosol is the fluid portion of the cytoplasm.

• Cytoplasm: Includes cytosol and organelles (except the nucleus in eukaryotes).

• Cytosol: The aqueous solution within the cytoplasm, containing dissolved ions, molecules, and proteins.

Cytoskeleton

The cytoskeleton is a network of protein filaments and tubules that provides structural support, facilitates movement, and maintains cell shape.

• Functions:

  • Movement of organelles and vesicles

  • Cell shape maintenance

  • Cell division

Genetic Material

DNA and Chromatin

All cells contain genetic material in the form of deoxyribonucleic acid (DNA), which encodes hereditary instructions. In eukaryotes, DNA is organized with proteins into chromatin.

• Chromatin: Complex of DNA and histone proteins, forming chromosomes in eukaryotic cells.

• Chromosomes: Long threadlike associations of genes, visible during cell division.

Prokaryotic vs. Eukaryotic Cells

Key Differences

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

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

• Nucleoid: Dense region of DNA in prokaryotic cells.

• Plasmids: Small, circular DNA molecules in prokaryotes, separate from chromosomal DNA.

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

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

Structural Features of Prokaryotes

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

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

• Flagella: Long, whip-like structures for motility in liquid environments.

Cellular Organelles

Compartmentalization in Eukaryotes

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

• Organelles include:

  1. Nucleus

  2. Mitochondrion

  3. Chloroplast (in plants and algae)

  4. Endoplasmic Reticulum

  5. Golgi Apparatus

  6. Vesicles

• Benefits of Compartmentalization:

  • Localized conditions for specific metabolic processes

  • Sequestration of reactions to prevent interference

Nucleus and Nucleolus

The nucleus is the control center of the cell, housing genetic material. The nucleolus is a region within the nucleus where ribosomes are assembled.

• Nucleolus: Spherical region; may be multiple per cell; site of ribosome production.

Ribosomes

Ribosomes are complexes of rRNA and proteins responsible for protein synthesis. They are found in both prokaryotic and eukaryotic cells.

• Composed of two subunits

• In prokaryotes: free in cytoplasm

• In eukaryotes: free in cytoplasm or attached to membranes

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-transforming organelles. Mitochondria produce ATP via cellular respiration; chloroplasts conduct photosynthesis in plants and algae.

• General Features:

  • Double membrane

  • Contain their own DNA and ribosomes

  • Semi-autonomous: can grow and reproduce

Mitochondria

• 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 membranes: intermembrane compartment

• Space inside inner membrane: matrix

• Function: Use energy from organic molecules to produce ATP

• Equation for 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} $

Chloroplasts

• 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

• Grana: Stacks of thylakoids

• Function: Site of photosynthesis

• Equation for photosynthesis:

$ 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 $

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 products.

• Components:

  • Nuclear envelope

  • Endoplasmic reticulum (rough and smooth)

  • Golgi apparatus

  • Lysosomes

  • Vacuoles

  • Vesicles

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

Vesicles

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

Endoplasmic Reticulum (ER)

The ER is an extensive network of tubules and sacs, forming the largest part of the cell's membrane system. It exists 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. Protein moves through the ER membrane into the ER

3. Vesicle pinched off from ER carries protein to Golgi apparatus

4. Golgi apparatus modifies, sorts, and ships protein to plasma membrane for export

Golgi Apparatus

The Golgi apparatus consists of stacked, flattened membranous sacs. It modifies, stores, sorts, and routes products from the ER.

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

• Trans-face: Shipping side for vesicles leaving Golgi

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes. They break down 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 (apoptosis)

Vacuoles

Vacuoles are large membrane-bound sacs for storage and transport. In plant cells, the central vacuole maintains turgor pressure and stores nutrients.

Summary Table: Key Organelles and Their Functions

Organelle	Main Function
Nucleus	Stores genetic material; controls cell activities
Ribosome	Protein synthesis
Mitochondrion	ATP production via cellular respiration
Chloroplast	Photosynthesis (in plants and algae)
Endoplasmic Reticulum	Lipid and protein synthesis; membrane production
Golgi Apparatus	Modification, sorting, and shipping of cell products
Lysosome	Digestion and recycling of cellular materials
Vacuole	Storage and maintenance of cell turgor (plants)

Example: In a plant cell, the chloroplasts convert sunlight into chemical energy, mitochondria use this energy to produce ATP, and the vacuole stores water and nutrients.

Additional info: These notes expand on the original content by providing definitions, examples, and equations for key cellular processes, ensuring a comprehensive and self-contained study guide for General Biology students.