Table of Contents
The Cell Landscape: An Overview
All living organisms are built from cells. In this chapter, we outline the main kinds of cells and the key structures (cell components) that appear in them. Detailed comparisons between prokaryotic and eukaryotic cells, and specialized topics like viruses, will be treated in their own chapters; here we build the common baseline.
What Is a Cell (in Practice)?
A cell is the smallest unit that:
- Is separated from its surroundings by a membrane.
- Contains genetic material (usually DNA).
- Has its own metabolism (chemical reactions).
- Can grow, respond, and, under suitable conditions, reproduce.
Cells are not just “bags” of fluid. They are highly organized, compartmentalized spaces where specific tasks happen in specific places.
Two broad categories:
- Prokaryotic cells (procytes) – structurally simpler, no membrane‑bound nucleus.
- Eukaryotic cells (eucytes) – more complex, with a true nucleus and many membrane‑bound organelles.
Later chapters will explore these two categories. Here we examine the basic components that most cells share, and the major organelles found especially in eukaryotes.
Fundamental Cell Components
1. Plasma (Cell) Membrane
The plasma membrane surrounds every cell and forms its outer boundary (unless there is also a cell wall).
Core features:
- Structure: A phospholipid bilayer with embedded proteins, often described by the “fluid mosaic model.”
- Selective barrier: It separates internal from external environments, allowing some substances to pass while blocking others.
- Communication: Contains receptor proteins that receive signals (hormones, neurotransmitters, etc.).
- Attachment: Links cells to each other and to their surroundings (e.g., extracellular matrix).
Key functions:
- Regulates transport of ions, nutrients, and wastes.
- Maintains internal conditions (homeostasis).
- Participates in cell recognition (e.g., immune system identifying “self” vs “non-self”).
2. Cytoplasm and Cytosol
The cytoplasm is everything inside the cell membrane except the nucleus (in eukaryotes). It includes:
- A fluid component, the cytosol (an aqueous solution of ions, small molecules, and macromolecules).
- Suspended structures (organelles in eukaryotes, ribosomes in all cells).
- A supporting protein framework (cytoskeleton, see below).
Roles of cytoplasm/cytosol:
- Medium for chemical reactions (metabolism).
- Distribution space for molecules and organelles.
- Helps maintain cell shape and consistency.
3. Genetic Material
All known cells use DNA as long-term information storage, organized differently in prokaryotes and eukaryotes (detailed later). At this level, note:
- DNA contains instructions for:
- Building proteins.
- Regulating metabolism and cell division.
- DNA is replicated before cell division to pass information to daughter cells.
- DNA is transcribed into RNA, which participates in protein synthesis.
Genetic material is often associated with special regions or organelles:
- Nucleoid region in prokaryotes.
- Nucleus in eukaryotes.
- Additional DNA may be found in mitochondria and chloroplasts.
Eukaryotic Cell Organelles
Eukaryotic cells are divided into many compartments by membranes. Each compartment (organelle) specializes in particular tasks, making cellular processes more efficient and better regulated.
1. Nucleus
The nucleus is the control center of eukaryotic cells, housing most of the cell’s DNA.
Key features:
- Nuclear envelope: A double membrane surrounding the nucleus, continuous with the endoplasmic reticulum.
- Nuclear pores: Gateways that regulate transport of RNA and proteins in and out of the nucleus.
- Nucleolus: A dense region where ribosomal RNA (rRNA) is made and ribosome subunits begin to assemble.
- Chromatin: DNA packaged with proteins (histones). Its degree of packing changes during the cell cycle.
Functions:
- Protects genetic information.
- Coordinates gene expression (which genes are active).
- Oversees DNA replication prior to cell division.
2. Ribosomes
Ribosomes are the sites of protein synthesis in all cells (prokaryotic and eukaryotic).
Characteristics:
- Not membrane-bound; made of rRNA and proteins.
- Two subunits (large and small) form an active ribosome.
- Can be:
- Free in the cytosol (making proteins used in the cytoplasm, nucleus, mitochondria).
- Bound to the endoplasmic reticulum (making proteins for membranes, secretion, or specific organelles).
Basic function:
- Translate the genetic information in mRNA into the amino acid sequence of proteins.
3. Endoplasmic Reticulum (ER)
The endoplasmic reticulum is a network of membranes continuous with the nuclear envelope.
Two main forms:
- Rough ER (RER):
- Studded with ribosomes.
- Involved in synthesis and initial folding of proteins that:
- Are secreted out of the cell.
- Become part of membranes.
- Are sent to lysosomes or other organelles.
- Smooth ER (SER):
- Lacks ribosomes.
- Involved in:
- Lipid and steroid synthesis.
- Detoxification (e.g., in liver cells).
- Storage and regulation of calcium ions (important in muscle cells).
4. Golgi Apparatus
The Golgi apparatus (Golgi complex) is a stack of flattened membrane sacs.
Roles:
- Receives proteins and lipids from the ER in transport vesicles.
- Modifies them (e.g., adds sugar groups, sorts, and packages).
- Directs them to:
- The cell membrane (for secretion or insertion).
- Lysosomes.
- Other specific destinations.
The Golgi thus acts as a central sorting and processing station for cellular products.
5. Lysosomes and Related Organelles
Lysosomes are membrane-bound vesicles containing digestive enzymes, mainly found in animal cells and some protists.
Functions:
- Break down:
- Macromolecules taken up from outside the cell (e.g., food particles, pathogens).
- Damaged or obsolete cell components (autophagy).
- Maintain an acidic internal environment optimal for their enzymes.
Related structures:
- Vacuoles:
- In animal cells, small vacuoles often serve as temporary storage/transport vesicles.
- In plant cells, a large central vacuole provides storage (e.g., ions, pigments, waste) and helps maintain internal pressure (turgor).
6. Mitochondria
Mitochondria are often called the “powerhouses” of the cell.
Key traits:
- Surrounded by two membranes; the inner membrane is highly folded into cristae.
- The internal space (matrix) contains enzymes and mitochondrial DNA.
- Present in almost all eukaryotic cells; especially numerous in energy-demanding tissues (e.g., muscles, neurons).
Main functions:
- Carry out stages of cellular respiration to produce ATP from nutrients.
- Participate in regulation of cell death (apoptosis).
- Contribute to some metabolic pathways (e.g., parts of amino acid and lipid metabolism).
7. Chloroplasts (and Other Plastids)
Chloroplasts are specialized organelles in plants and some protists responsible for photosynthesis.
Features:
- Double membrane.
- Internal membrane system forming thylakoids, stacked into grana.
- The fluid surrounding thylakoids is the stroma, containing enzymes and chloroplast DNA.
- Contain pigments like chlorophyll that capture light energy.
Basic role:
- Convert light energy, water, and carbon dioxide into sugars and oxygen (photosynthesis).
Other plastids (also in plant cells) may:
- Store starch (amyloplasts).
- Contain pigments (chromoplasts) that color fruits and flowers.
8. Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments throughout the cytoplasm.
Main components:
- Microfilaments (actin filaments):
- Thin filaments involved in cell shape, movement, and muscle contraction.
- Intermediate filaments:
- Provide mechanical strength and help anchor organelles.
- Microtubules:
- Hollow tubes that serve as tracks for vesicle and organelle movement.
- Form the spindle during cell division.
- Build structures like cilia and flagella (see below).
Functions:
- Maintains and changes cell shape.
- Enables movement of the cell (e.g., amoeboid movement) and of structures within the cell.
- Organizes the placement and movement of organelles.
9. Cilia, Flagella, and Other Surface Structures
Many eukaryotic cells bear cilia or flagella, which are extensions of the cell membrane supported by microtubules.
- Cilia:
- Short, numerous.
- Beat in coordinated waves.
- Examples: move mucus in the human respiratory tract; move protists like paramecia.
- Flagella:
- Longer, usually fewer in number.
- Move in whip-like motions.
- Example: sperm cells.
In plants and fungi, cell walls provide additional support and shape; in animals, this function is largely replaced by cytoskeleton and extracellular matrix.
Comparing Cell Types (High-Level View)
While details are handled in later chapters, it is helpful to outline a few broad differences in cell components:
- Prokaryotic cells:
- No membrane-bound nucleus; DNA in a nucleoid region.
- Generally lack membrane-bound organelles like ER, Golgi, mitochondria, and chloroplasts.
- Often have a cell wall, sometimes surrounded by a capsule.
- May have flagella with a different structure than eukaryotic flagella.
- Have ribosomes and a cytoskeleton-like system, but simpler.
- Eukaryotic cells:
- DNA enclosed in a nucleus.
- Possess membrane-bound organelles (mitochondria, ER, Golgi, etc.).
- Some (plants, fungi) have cell walls of different compositions (cellulose in plants, chitin in fungi).
- Often larger and more complex internally.
Animal, plant, fungal, and protist cells vary in which specific components they have, and in how large or prominent those components are. Such specializations will be explored when we examine these groups in more depth.
Cells as Integrated Systems
It is crucial to see cell components not as isolated “parts,” but as interacting elements in a system:
- DNA in the nucleus (or nucleoid) is transcribed to RNA.
- Ribosomes translate RNA to proteins.
- Proteins enter the ER, are modified in the Golgi, and are sorted to their destinations.
- Mitochondria and (in plants) chloroplasts supply energy to fuel these processes.
- The cytoskeleton positions organelles, organizes transport, and supports cell division.
- Membranes form controlled interfaces between compartments and with the outside world.
This coordination allows the cell to maintain order, adapt to changes, and carry out the complex life processes of growth, reproduction, and response to the environment.