Fundamental Properties and Diversity of Cells

General Properties of Cells

Size

Cells are typically small, with diameters ranging from 2 to 200 μm. This small size allows for efficient exchange of materials with the environment and rapid cellular processes.

• Small size: Typically 2–200 μm

Composition

Cells are defined by their membrane-bound nature, which allows them to concentrate chemicals in aqueous solution and maintain distinct internal environments.

• Membrane bounded: Enclosed by a plasma membrane

• Concentrated chemicals: Internal environment contains dissolved molecules and ions

Capabilities (Functionalities)

Cells possess a range of capabilities that enable them to survive, grow, and reproduce.

• Store, process, and transmit information: Genetic information is stored in DNA and used to direct cellular activities

• Self-replication: Ability to reproduce and generate new cells

• Acquire and utilize energy: Cells obtain energy from their environment and use it for metabolism

• Carry out chemical reactions (metabolism): Enzymatic reactions transform molecules for growth and maintenance

• Respond to stimuli: Cells detect and react to changes in their environment

• Regulate: Maintain homeostasis and control internal conditions

• Adapt (evolve): Populations of cells can change over time through evolution

Organization

Cellular organization is highly complex, with different classes of cells exhibiting distinct levels of structural and functional organization.

• Complexity: Even the simplest cell is more complex than any man-made system

• Variability: Different cell classes have different organizational levels

Higher Order Organization

Cells can exist as single organisms or as part of multicellular communities, leading to higher levels of biological organization.

• Single cells: Can represent discrete organisms

• Multicellular organisms: Formed by association of communities of cells

Definition

A cell is a small, membrane-bound, living unit with the capacity for self-replication.

Cellular Classification

Major Groups

Cells are classified into two major groups based on their structural characteristics: prokaryotic and eukaryotic cells.

Prokaryotic Cells ("before nucleus")

• Represented by bacteria: Ancient and diverse class of organisms

• Major subgroups:

  • Eubacteria (true bacteria)

  • Archaebacteria (evolutionary link between prokaryotes and eukaryotes)

• Lack membrane-bound organelles

• Organization: Can exist as single cells, chains, clusters, or in functional communities (biofilms)

Eukaryotic Cells ("true nucleus")

• Contain nucleus and other membrane-bound organelles

• Comprise all complex, multicellular organisms:

  • Animals

  • Plants

  • Fungi

  • Protists

• Organization: More complex than prokaryotes

Organization of Eukaryotic Cells

Nucleus

The nucleus is the most prominent organelle in eukaryotic cells, housing the genetic material and controlling cellular activities.

• Identifiable by light microscopy

• Enclosed by a double membrane (nuclear envelope)

• Nuclear envelope: Double bilayer structure

• Nuclear lamina: Cytoskeletal network on the internal side of the membrane

• Genetic archive: Contains chromosomes (DNA associated with protein)

Mitochondria

Mitochondria are the site of aerobic energy manipulation and ATP production through cellular respiration.

• Cellular respiration: $O_2$, $CO_2$ exchange

• Energy harnessing: Oxidative breakdown of food molecules for ATP production

• Present in almost all eukaryotic cells (except some)

• Enclosed by two biochemically distinct membranes

• Contains organellar DNA

• Capable of self-replication independent of cell division

• Physiology similar to that of bacteria

Chloroplasts

Chloroplasts are specialized organelles found in plants and some protists, responsible for photosynthesis.

• Use energy from sunlight to manufacture energy-rich sugar molecules (photosynthesis)

• Found only in cells of plants, algae, and a few protists

• Green due to the pigment chlorophyll

• Contains organellar DNA

• Capable of self-replication independent of cell division

• Physiology similar to that of bacteria

Endoplasmic Reticulum (ER)

The ER is a network of single membrane-enclosed tubules involved in synthesis and transport of cellular components.

• Site of synthesis: Most cell membrane components and molecules destined for secretion

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages molecules for secretion or delivery to other organelles.

• Series of parallel, flattened, membrane-bound discs (cisternae)

• Site of modification: For materials synthesized in the ER

• Strategic location: For intracellular sorting

Lysosomes

Lysosomes are small, irregularly sized organelles responsible for intracellular digestion and recycling.

• Site of intracellular macromolecular digestion

• Breakdown of unwanted molecules for excretion or recycling

Peroxisomes

Peroxisomes are small, membrane-bound vesicular organelles involved in the breakdown of reactive oxygen species.

• Compartmentalize reactive oxygen species, particularly hydrogen peroxide ($H_2O_2$)

• $H_2O_2$ synthesis and degradation

Transport Vesicles

Transport vesicles mediate traffic of materials between membrane-bound organelles.

• Responsible for membrane recycling and intracellular trafficking of proteins

Cytoplasm

The cytoplasm is the aqueous gel containing concentrated molecules and organelles, where many cellular processes occur.

• Location of free ribosomes and protein synthesis

• Site of anaerobic energy manipulation (glycolysis and fermentation)

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides mechanical strength, determines cell shape, and enables motility.

• Three fiber types:

  • Microtubules

  • Intermediate filaments

  • Microfilaments

• Provides mechanical strength

• Determines cell shape

• Responsible for cell motility

Cellular Diversity

Cells demonstrate remarkable diversity in size, shape, and function. Some nerve ganglion cells can be several centimeters long. Chemical requirements and biochemical activities vary by cell type.

• Cell size and shape: Vary widely among cell types

• Chemical requirements: Vary by cell type

• Biochemical activities: Vary by cell type

Cellular Unity

Despite their diversity, all cells share fundamental characteristics and structures, suggesting a common evolutionary origin.

• Common ancestry: All cells appear to have evolved from a common ancestor

• Basic chemistry: All cells use the same molecular building blocks

• Common structures:

  • Cell or plasma membrane

  • DNA as genetic material

  • Ribosomes for protein synthesis