The Cell: The Basic Unit of Life

Introduction to Cells

Cells are the fundamental units of life, responsible for carrying out all biological processes necessary for survival and function. Understanding cell structure and function is essential for grasping the principles of biology.

• Cell: The smallest unit capable of independent life and reproduction.

• Homeostasis: The ability of cells to regulate their internal environment to maintain stable, life-supporting conditions.

• Cells operate through the coordinated action of their internal structures (organelles).

Macromolecules in Cells

Major Classes of Biological Macromolecules

All cells contain four major types of macromolecules, each with distinct roles:

• Proteins: Perform most cellular functions, including catalysis, structure, and signaling.

• Nucleic acids: Store, transmit, and process genetic information (DNA and RNA).

• Carbohydrates: Provide chemical energy, structural support, and cellular identity.

• Phospholipids: Form the selectively permeable membrane barrier that defines the cell boundary.

Discovery and Theory of Cells

Historical Perspective

The discovery of cells was a pivotal moment in biology, leading to the development of cell theory.

• Cells were first described in 1665 by Robert Hooke using a compound microscope.

• Microscopy enabled the observation of cells and their internal structures.

Cell Theory

• The cell is the basic unit of life.

• All living things are composed of one or more cells.

• Cells arise only from the division of pre-existing cells.

• Each cell can regulate its internal conditions (homeostasis).

• Homeostasis at higher levels (tissues, organs, organisms) reflects the coordinated actions of many cells.

Cell Size and Surface Area-to-Volume Ratio

Why Are Cells Small?

Cells are small to maximize their surface area-to-volume ratio, which is critical for efficient exchange of materials with the environment.

• As a cell grows, its volume increases faster than its surface area.

• A high surface area-to-volume ratio allows for efficient nutrient uptake and waste removal.

Cell Morphology: Prokaryotic vs. Eukaryotic Cells

Major Differences

Cells are classified into two main types based on their structural features:

The Tree of Life: Three Domains

Classification of Life

All life is classified into three domains based on genetic and cellular differences:

• Bacteria (prokaryotes)

• Archaea (prokaryotes)

• Eukaryota (eukaryotes)

Prokaryotic Cell Structure

Major Characteristics

• No nucleus; DNA is located in a region called the nucleoid.

• DNA is typically circular and may include plasmids (small, extra-chromosomal DNA).

• Cell wall made of peptidoglycan (in bacteria).

• Simple cytoskeleton.

• Some have flagella (for movement) and fimbriae (for attachment).

• Simple organelles for photosynthesis or storage.

Eukaryotic Cell Structure

Major Characteristics

• Nucleus stores DNA.

• Many compartmentalized organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Complex cytoskeleton for structure and movement.

• Large diversity among kingdoms (animals, plants, fungi, protists).

Nucleus and DNA Packaging

Nucleus

The nucleus is the control center of the cell, containing most of the genetic material.

• Enclosed by a double membrane (nuclear envelope) with pores for transport.

• Contains chromosomes (DNA + proteins).

• Nucleolus: Site of ribosome biogenesis.

DNA Packaging

DNA is highly organized and compacted within the nucleus.

• DNA wraps around histone proteins to form nucleosomes.

• Further coiling and folding produce chromosomes.

Ribosomes

Structure and Function

Ribosomes are the molecular machines that synthesize proteins by translating mRNA.

• Composed of large and small subunits (protein and rRNA).

• Can be free in the cytoplasm or attached to the rough endoplasmic reticulum (ER).

• Follow the central dogma: DNA → RNA → Protein.

Mitochondria

Structure and Function

Mitochondria are the powerhouses of the cell, producing ATP through cellular respiration.

• Double membrane structure.

• Contain their own DNA and ribosomes.

• Divide independently of the cell.

The Endomembrane System

Components and Functions

The endomembrane system is a network of membranes involved in protein and lipid synthesis, modification, and transport.

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis)

• Golgi apparatus: Receives, modifies, and ships proteins and lipids

• Vesicles: Transport materials between organelles

• Lysosomes: Digestive organelles (animal cells)

• Peroxisomes: Break down toxins

Endoplasmic Reticulum (ER)

Rough and Smooth ER

• Rough ER: Studded with ribosomes; site of protein folding, processing, and packaging for export.

• Smooth ER: Lacks ribosomes; site of lipid synthesis and degradation.

Golgi Apparatus

Structure and Function

The Golgi apparatus is responsible for receiving, modifying, and shipping proteins and lipids.

• cis-side: Receives vesicles from the ER.

• Proteins and lipids are processed as they move through the cisternae to the trans-side, where they are shipped to their final destinations.

Lysosomes and Peroxisomes

Lysosomes (Animal Cells)

• "Recycling centers" of the cell.

• Hydrolyze macromolecules at acidic pH (~5).

• Contain many hydrolytic enzymes.

Peroxisomes

• Originate as vesicles from the ER.

• Break down toxins and their byproducts (e.g., hydrogen peroxide).

• Contain a large protein crystal core.

Cytoskeleton

Structure and Function

The cytoskeleton provides shape, support, and movement for the cell. It acts as both a skeleton and a system of tracks for intracellular transport.

• Microfilaments (actin): Cell movement, muscle contraction, cytokinesis, cytoplasmic streaming.

• Intermediate filaments: Structural support, cell shape, cell-cell anchoring.

• Microtubules: Support, vesicle transport, chromosome movement during cell division, cilia/flagella movement.

Plant Cell Characteristics

Unique Features of Plant Cells

• Chloroplasts: Site of photosynthesis; contain their own DNA and double membrane.

• Central vacuole: Stores water, enzymes, pigments, and toxins; can occupy up to 80% of cell volume.

• Cell wall: Provides structure and prevents dehydration; contains cellulose.

• Plasmodesmata: Channels between plant cells for communication.

Endosymbiotic Theory

Origin of Mitochondria and Chloroplasts

Mitochondria and chloroplasts are believed to have originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Both have double membranes, their own DNA, and divide by binary fission.

Summary Table: Key Differences Between Prokaryotic and Eukaryotic Cells

Practice Questions for Review

• Why are cells small? How does surface area-to-volume ratio affect cell function?

• What are the main differences between prokaryotic and eukaryotic cells?

• Where is DNA located in each cell type, and what form does it take?

• What is the endomembrane system, and what are the main functions of its organelles?

• What are the differences between plant and animal cells?

• What are the three main types of cytoskeletal fibers, and what are their functions?

• Which organelles are most abundant in cells with specific functions (e.g., energy production, protein synthesis)?