Cell Structure and Function: A Study Guide

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They maintain an internal environment distinct from the external environment, which is essential for proper cellular function.

• Cell: The smallest living unit, capable of carrying out all life processes.

• Internal Environment: Maintained by the plasma membrane, which separates the cell from its surroundings.

Plasma Membrane

The plasma membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins.

• 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 is the region inside the plasma membrane and outside the nucleus (in eukaryotic cells). It contains the cytosol and all cellular structures except the nucleus.

• Cytosol: The fluid component of the cytoplasm.

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

Genetic Material and Its Storage

DNA and Its Location

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

• In prokaryotic cells, DNA is located in a region called the nucleoid.

• In eukaryotic cells, DNA is stored within the nucleus, a membrane-bound organelle.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on the presence or absence of a nucleus and other membrane-bound 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 found in prokaryotes, separate from chromosomal DNA.

• Nucleus: Prominent, membrane-bound organelle in eukaryotes containing most genetic material.

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

Other Prokaryotic Features

• Cell Wall: Provides shape and protection; permeable to small molecules.

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

• Flagella (singular: flagellum): Long, whip-like structures used for movement in liquid environments.

Cellular Organelles

Compartmentalization in Eukaryotic Cells

Eukaryotic cells are structurally more complex, with internal compartments called organelles that perform specialized functions.

• Organelles include:

  1. Nucleus

  2. Mitochondrion

  3. Chloroplast (in plants and algae)

  4. Endoplasmic reticulum

  5. Golgi apparatus

  6. Vesicles

Compartmentalization allows for localized environmental conditions and the segregation of incompatible metabolic processes.

Nucleus and Genetic Material

• Nucleus: Contains the cell's genetic library.

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

• Chromosomes: Long, threadlike associations of genes; humans have 46 chromosomes.

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

Ribosomes

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

• Composed of rRNA and proteins.

• Consist of two subunits.

• In prokaryotes: free in cytoplasm.

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

Mitochondria and Chloroplasts

These organelles are the main energy transformers of the cell.

• General Features:

  • Double membrane

  • Not part of the endomembrane system

  • Contain their own ribosomes and DNA

  • Semi-autonomous (can grow and reproduce independently)

Mitochondria

• Use energy from organic molecules to produce ATP.

• Size: ~1-10 μm

• Number per cell varies with metabolic activity.

• Can move and change shape.

• Inner membrane is highly folded into cristae to increase surface area.

• Space between membranes: intermembrane compartment.

• Space inside inner membrane: matrix.

Chloroplasts

• Contain chlorophyll and are the site of photosynthesis (in plants and algae).

• 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.

The Endomembrane System

The endomembrane system is a group of interconnected organelles in eukaryotic cells that work together in the synthesis, modification, and transport of cellular materials.

• Includes:

  • Nuclear envelope

  • Endoplasmic reticulum (rough and smooth)

  • Golgi apparatus

  • Lysosomes

  • Vesicles

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

Vesicles

• Membrane-enclosed sacs that transport materials between organelles.

Endoplasmic Reticulum (ER)

The ER is an extensive network of tubules and sacs, and is the largest part of the endomembrane system. It comes 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 is synthesized by a bound ribosome.

2. The protein moves through the ER membrane into the ER.

3. A vesicle carrying the protein buds off from the ER and travels to the Golgi apparatus.

Golgi Apparatus

The Golgi apparatus is made of stacked, flattened membranous sacs. It modifies, sorts, and ships products from the ER.

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

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

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes. They break down macromolecules and recycle cellular components.

• Functions of Lysosomes:

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

  2. Recycling of cellular material.

  3. Programmed cell destruction (apoptosis).

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
DNA location	Nucleoid region	Nucleus
Cell wall	Present (most)	Present (plants, fungi); absent in animals
Ribosomes	Present (smaller)	Present (larger)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Additional info: The notes above have been expanded for clarity and completeness, with definitions, examples, and logical groupings based on standard biology curriculum.