Cell Structure and Function: A Study Guide

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, providing a distinct internal environment that is separate from the external surroundings. Understanding cell structure and function is essential for comprehending all biological processes.

• Cell: The smallest living unit, capable of maintaining an internal environment distinct from the outside.

Plasma Membrane

The plasma membrane is a selectively permeable bilayer of phospholipids with embedded proteins, crucial for maintaining cellular integrity.

• Functions:

  • 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 (the fluid component) and all cellular structures.

• Cytoskeleton: A network of protein fibers and filaments in the cytoplasm, providing structural support and enabling cell movement and shape changes.

Genetic Material

• DNA (Deoxyribonucleic acid): The hereditary material that contains instructions for cell function and development.

Prokaryotic versus 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 in prokaryotes, separate from chromosomal DNA.

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

• Nuclear envelope: Double membrane enclosing the eukaryotic nucleus.

Other Prokaryotic Cell Features

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

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

• Flagella (singular: flagellum): Structures for motility, rotating to propel the cell in liquid environments.

Cellular Organelles

Eukaryotic cells contain specialized structures called organelles, each with distinct functions. Compartmentalization allows for efficient and regulated metabolic processes.

• Examples of Organelles:

  1. Nucleus

  2. Mitochondrion

  3. Chloroplasts

  4. Endoplasmic reticulum

  5. Golgi apparatus

  6. Vesicles

Why compartmentalization?

• Provides localized environments for specific metabolic processes

• Sequesters reactions to prevent interference between incompatible processes

Nucleus

The nucleus is the control center of the cell, containing the genetic library in the form of DNA.

• Chromatin: Complex of DNA and histone proteins; condenses to form chromosomes during cell division.

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

• Nucleolus: Spherical region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins. There may be multiple nucleoli per cell.

Ribosomes

Ribosomes are the sites of protein synthesis, 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 endoplasmic reticulum

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are the main energy-transforming organelles in cells.

• Both have a double membrane

• Contain their own DNA and ribosomes

• Semi-autonomous: can grow and reproduce independently

• Not part of the endomembrane system

Mitochondria

• 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 outer and inner membranes: intermembrane compartment

• Space inside inner membrane: matrix

• Function: Use energy from organic molecules to produce ATP

Chloroplasts

• Size: 2–5 μm

• Number per cell varies with metabolic activity

• Can move and change shape

• Space inside inner membrane: stroma (viscous fluid)

• Contain thylakoids: third membrane system, shaped like flattened sacs

• Grana: stacks of thylakoids

• Function: Sites of photosynthesis (contain chlorophyll)

Endomembrane System

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

• Includes:

  • Nuclear envelope

  • Endoplasmic reticulum (rough and smooth)

  • Golgi apparatus

  • Lysosomes

  • Vacuoles

  • Vesicles

• Components are connected directly or via vesicles

Vesicles

Vesicles are small, membrane-enclosed sacs that transport materials within the cell.

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is an extensive network of membranes, the largest part of the endomembrane system. It comes in two forms:

• Smooth ER: Lacks ribosomes; synthesizes lipids (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 carries the protein to the Golgi apparatus

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

Golgi Apparatus

The Golgi apparatus is a stack of flattened membranes that modifies, sorts, and ships products from the ER.

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

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

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes, responsible for breaking down macromolecules.

• Functions:

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

  2. Recycling of cellular material

  3. Programmed cell destruction

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent (nucleoid region)	Present (with nuclear envelope)
Organelles	Few or none	Many, membrane-bound
DNA Location	Cytoplasm (nucleoid, plasmids)	Nucleus
Cell Wall	Usually present	Present in plants, fungi; absent in animals
Ribosomes	Present (smaller)	Present (larger)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Example: Escherichia coli is a prokaryotic bacterium, while Homo sapiens (humans) are composed of eukaryotic cells.