Introduction to Cell Structure

Overview of Eukaryotic Cell Organization

Eukaryotic cells are highly organized structures that perform the essential functions of life through compartmentalization. Their internal organization allows for specialized chemical reactions, efficient energy transformation, and precise genetic information storage and transmission.

• Compartmentalization: Internal membranes divide the cell into compartments, enabling specialized functions.

• Energy and Matter Transformations: Organelles such as mitochondria and chloroplasts are responsible for energy conversion and biosynthesis.

• Genetic Information Storage and Transmission: The nucleus stores DNA, and ribosomes synthesize proteins based on genetic instructions.

• Interactions with the Environment: The plasma membrane regulates the movement of substances into and out of the cell.

Microscopy and the Study of Cells

Tools for Visualizing Cells

Cells are typically too small to be seen with the naked eye. Biologists use microscopes and biochemical techniques to study their structure and function.

• Magnification: The ratio of an object's image size to its real size.

• Resolution: The clarity of the image; the minimum distance between two distinguishable points.

• Contrast: Visible differences in brightness between parts of the sample.

Cell Types: Prokaryotic vs. Eukaryotic

Basic Features of All Cells

• Plasma membrane: Selective barrier surrounding the cell.

• Cytosol: Semi-fluid substance within the cell.

• Chromosomes: Carry genetic information.

• Ribosomes: Sites of protein synthesis.

Prokaryotic Cells

• No nucleus: DNA is located in an unbound region called the nucleoid.

• No membrane-bound organelles.

• Cytoplasm: Bound by the plasma membrane.

Eukaryotic Cells

• Nucleus: DNA is enclosed within a double membrane.

• Membrane-bound organelles: Specialized structures for various functions.

• Cytoplasm: Region between the plasma membrane and nucleus.

• Generally larger than prokaryotic cells.

Plasma Membrane and Cell Size

Surface Area to Volume Ratio

The plasma membrane acts as a selective barrier, allowing the passage of oxygen, nutrients, and waste. The surface area-to-volume ratio is critical for cell function; as a cell increases in size, its volume grows faster than its surface area, limiting efficient exchange with the environment.

Internal Membranes and Organelles

Compartmentalization in Eukaryotic Cells

Eukaryotic cells contain internal membranes that divide the cell into organelles, each with specialized functions. The basic structure of biological membranes is a double layer of phospholipids and other lipids.

Major Organelles and Their Functions

• Nucleus: Contains most of the cell's DNA and is the site of genetic information storage and processing.

• Ribosomes: Complexes of ribosomal RNA and protein; sites of protein synthesis. Found free in the cytosol or bound to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER):

  • Smooth ER: Synthesizes lipids.

  • Rough ER: Studded with ribosomes; distributes transport vesicles and is a membrane factory for the cell.

• Golgi Apparatus: Modifies, sorts, and packages products of the ER into transport vesicles.

• Lysosomes: Membranous sacs of hydrolytic enzymes that digest macromolecules and recycle cellular components (autophagy).

• Vacuoles: Large vesicles with diverse functions, such as storage, waste disposal, and maintaining cell rigidity in plants.

• Mitochondria: Sites of cellular respiration and ATP production.

• Chloroplasts: Sites of photosynthesis in plants and algae.

Genetic Information Storage and Transmission

Nucleus and Chromosomes

• Chromosomes: Discrete units of DNA associated with proteins (chromatin).

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes

• Free ribosomes: Function in the cytosol.

• Bound ribosomes: Attached to the ER or nuclear envelope; synthesize proteins for membranes or export.

The Endomembrane System

Components and Functions

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

These components are either continuous or connected via vesicle transfer, coordinating protein traffic and metabolic functions.

Energy-Transforming Organelles

Mitochondria

• Structure: Smooth outer membrane and highly folded inner membrane (cristae).

• Function: Site of cellular respiration, generating ATP from organic molecules and oxygen.

• Compartments: Intermembrane space and mitochondrial matrix (where most respiration occurs).

Chloroplasts

• Structure: Contain thylakoids (membranous sacs) stacked into grana, and stroma (internal fluid).

• Function: Site of photosynthesis, converting solar energy into chemical energy.

• Contain: Chlorophyll, enzymes, and other molecules for photosynthesis.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that supports cell shape, anchors organelles, and facilitates movement.

• Microtubules: Shape the cell, guide organelle movement, and separate chromosomes during cell division.

• Microfilaments: Support cell shape and are involved in cell movement.

• Intermediate filaments: Maintain cell shape and fix organelles in place.

Centrosomes and Centrioles

• Centrosome: Microtubule-organizing center near the nucleus in animal cells.

• Centrioles: Paired structures within the centrosome, each with nine triplets of microtubules.

Cilia and Flagella

• Structure: Microtubule-containing extensions projecting from some cells.

• Function: Propel cells or move fluid over cell surfaces.

Extracellular Structures and Cell Interactions

Cell Walls

• Present in: Plants, prokaryotes, fungi, and some protists.

• Functions: Protects the cell, maintains shape, and prevents excessive water uptake.

Extracellular Matrix (ECM) in Animal Cells

• Composition: Glycoproteins such as collagen, proteoglycans, and fibronectin.

• Function: Provides structural support and mediates cell signaling.

• Integrins: Receptor proteins in the plasma membrane that bind ECM proteins.

Summary Table: Major Differences Between Prokaryotic and Eukaryotic Cells

Key Equations

• Surface Area of a Sphere:

• Volume of a Sphere:

• Surface Area to Volume Ratio:

Example: Why Are Cells Small?

As a cell grows, its volume increases faster than its surface area. This limits the rate at which materials can enter or leave the cell, making efficient exchange difficult in larger cells. Therefore, most cells remain small to maintain a high surface area-to-volume ratio.