General Biology: Cell Structure and Function

Cells are the basic units of life, but they are also active, organized systems that solve problems every second: how to separate an internal environment from the outside world, how to capture and use energy, how to copy information accurately, and how to communicate with neighboring cells. Understanding cell structure and function is foundational to every branch of biology because physiology, development, genetics, and disease all trace back to what cells do and how they are built.

Cellular Organization: What All Cells Have in Common

Despite the diversity of life, most cells share a core set of features:

• A plasma membrane that defines the boundary of the cell
• Cytoplasm (cytosol plus structures suspended in it)
• Genetic material (DNA, and RNA as the working copy for many processes)
• Ribosomes to synthesize proteins

Cells also maintain internal order through controlled chemistry and compartmentalization. This organization shows up differently in the two broad categories of cells.

Prokaryotic and Eukaryotic Cells

Prokaryotes (bacteria and archaea) generally lack membrane-bound organelles. Their DNA is typically found in a region called the nucleoid, and many species have additional DNA in plasmids. Prokaryotes are often small and efficient, relying on the plasma membrane and cytosolic machinery for most functions.

Eukaryotes (animals, plants, fungi, protists) contain membrane-bound organelles. This compartmentalization allows specialized environments for processes like energy conversion and protein processing. Eukaryotic DNA is housed in a nucleus, packaged with proteins into chromatin.

Organelles and Their Roles

Organelles are not just “parts” of the cell; they are functional modules that manage distinct tasks while coordinating with the rest of the system.

Nucleus: Information Storage and Control

The nucleus stores the cell’s DNA and regulates gene expression. DNA is transcribed into RNA in the nucleus, and many RNA molecules are processed before exiting to the cytoplasm. The nuclear envelope contains pores that control what enters and leaves, balancing protection with access.

Endomembrane System: Making, Modifying, and Shipping

Many proteins and lipids move through an interconnected set of membranes:

• Rough endoplasmic reticulum (RER) is studded with ribosomes and is central to synthesizing proteins destined for secretion, membranes, or certain organelles.
• Smooth endoplasmic reticulum (SER) contributes to lipid synthesis, detoxification, and calcium storage.
• The Golgi apparatus modifies, sorts, and packages molecules into vesicles. A useful way to think about the Golgi is as a processing and distribution center, where proteins can be glycosylated and tagged for delivery.

Lysosomes (common in animal cells) contain hydrolytic enzymes that break down macromolecules and worn-out organelles. They are essential for recycling cellular components, a process closely tied to cell health.

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria generate ATP through cellular respiration. They have inner membrane folds called cristae that increase surface area for energy-producing reactions. Mitochondria contain their own DNA and ribosomes, reflecting their evolutionary history and the semi-autonomous nature of this organelle.

In plants and algae, chloroplasts capture light energy and convert it into chemical energy through photosynthesis. Like mitochondria, chloroplasts have internal membranes and their own genetic material.

Energy transfer in cells often centers on ATP hydrolysis. The free energy change is commonly discussed as $\Delta G$, and reactions proceed spontaneously when $\Delta G<0$. Cells couple unfavorable reactions to favorable ones (like ATP hydrolysis) to drive work.

Cytoskeleton: Structure, Transport, and Motion

The cytoskeleton is a dynamic network that gives cells shape, organizes internal components, and enables movement:

• Microtubules help separate chromosomes during cell division and serve as tracks for motor proteins.
• Actin filaments support cell shape and are crucial for muscle contraction and cell crawling.
• Intermediate filaments provide tensile strength in many cell types.

Motor proteins such as kinesins and dyneins move cargo along microtubules, allowing directed transport that is far more efficient than diffusion alone.

Membranes: The Selective Boundary

Cell membranes are built primarily from phospholipids arranged in a bilayer. This structure forms spontaneously because phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails. The fluid mosaic model captures the idea that proteins and lipids move laterally within the membrane, creating a flexible but organized barrier.

Membrane Proteins and Their Functions

Membrane proteins carry out key tasks:

• Transport proteins move substances across the membrane
• Receptors detect signals such as hormones or neurotransmitters
• Enzymes catalyze reactions at the membrane surface
• Adhesion molecules help cells bind to each other and to the extracellular matrix

Membrane composition affects function. Cholesterol in animal cell membranes, for example, modulates fluidity and permeability.

Transport Across Membranes: Getting Materials Where They Need to Go

Cells must import nutrients, export waste, and maintain ion gradients. Transport falls into a few main categories.

Passive Transport: Moving With the Gradient

Diffusion moves molecules from high to low concentration. Small nonpolar molecules (like oxygen and carbon dioxide) can cross the lipid bilayer directly.

Facilitated diffusion uses channels or carriers to move polar molecules or ions down their gradients. This is still passive because it does not require added energy.

Osmosis is the diffusion of water across a selectively permeable membrane. Differences in solute concentration create osmotic pressure that can change cell volume, an especially important concept in red blood cells and plant cells (where the cell wall contributes to turgor pressure).

Active Transport: Moving Against the Gradient

Active transport uses energy, often from ATP, to move substances against their concentration or electrochemical gradients. A classic example is ion pumping that helps establish membrane potential. Maintaining gradients is costly but essential because gradients power many processes, including nutrient uptake and signaling.

Vesicular Transport: Bulk Movement

Large molecules and particles move via vesicles:

• Endocytosis brings material into the cell (including phagocytosis for large particles).
• Exocytosis exports material or inserts membrane proteins and lipids into the plasma membrane.

These processes are fundamental to secretion, neurotransmission, and immune responses.

The Cell Cycle: Growth, DNA Replication, and Division

Cell division is not a single event but a regulated cycle. The cell cycle is commonly divided into:

• Interphase (G1, S, G2): growth and DNA replication
• M phase: mitosis and cytokinesis

During S phase, DNA is replicated so that each daughter cell receives a complete genome. Mitosis ensures accurate chromosome segregation, while cytokinesis splits the cytoplasm.

Control of the cell cycle depends on checkpoints and regulatory proteins that assess DNA integrity and cell readiness. When these controls fail, cells may divide inappropriately, a key feature of cancer biology.

Cell Signaling: Communication and Coordination

Cells interpret their environment and coordinate behavior through signaling pathways. The basics of signaling can be summarized as:

1. Reception: a ligand binds a receptor (often at the membrane, sometimes inside the cell)
2. Transduction: intracellular relay steps amplify and route the message
3. Response: changes in gene expression, enzyme activity, cytoskeletal arrangement, or membrane properties

Common Signaling Mechanisms

• G protein-coupled receptors (GPCRs) activate intracellular proteins and second messengers.
• Receptor tyrosine kinases (RTKs) often trigger phosphorylation cascades tied to growth and differentiation.
• Ion channel receptors change membrane permeability rapidly, critical in neurons and muscle cells.

Second messengers such as calcium ions can spread signals quickly within the cell, translating an external cue into coordinated internal action.

Connecting Structure to Function

A central theme in biology is that cell structure enables cell function. Membrane organization supports selective transport and signaling. Organelles create specialized compartments for chemistry that would be inefficient or harmful if mixed. The cytoskeleton supports both stability and movement. The cell cycle ensures continuity of life through controlled division, and signaling pathways integrate information so cells act in context rather than isolation.

Learning cell structure and function is not memorizing parts. It is learning how a living system stays organized, adapts, and persists. Once these cellular principles are clear, topics like metabolism, immunology, development, and disease become variations on the same core logic: what cells can do, and how they do it.