2.1 Cell Structure and Function

2.1 Cell Structure and Function

Quick Answer: Animal cells are eukaryotic: they have a true nucleus and membrane-bound organelles. Know the job of each major organelle (nucleus, mitochondria, ribosomes, ER, Golgi, lysosomes, cytoskeleton, plasma membrane), how cells fit into levels of organization, and how plant cells differ (cell wall, chloroplasts, large central vacuole). On the NEX, cell structure is high-yield Biology content (~36% of scored Science items) and the base for all later anatomy and physiology.

Cells are the basic structural and functional units of life. Every tissue you will study in nursing—epithelium, muscle, nerve, blood—is built from cells that share a common toolkit of organelles and then specialize. Mastering this section pays off twice: directly on Biology questions, and again when A&P topics assume you already know what mitochondria, membranes, and nuclei do.

Prokaryotic vs Eukaryotic Cells

All living cells fall into two broad categories.

Human body cells are eukaryotic. Bacteria that cause infection are prokaryotic. That contrast matters clinically: many antibiotics target structures unique to prokaryotes (e.g., 70S ribosomes or peptidoglycan cell walls) so they can kill pathogens while sparing human cells.

Levels of Biological Organization

NEX questions often ask where a structure sits in the hierarchy. From simplest to most complex:

1. Atom — carbon, hydrogen, oxygen, nitrogen
2. Molecule — water, glucose, DNA, proteins
3. Organelle — mitochondrion, nucleus, ribosome
4. Cell — neuron, erythrocyte, hepatocyte
5. Tissue — group of similar cells (e.g., cardiac muscle)
6. Organ — heart, liver, kidney
7. Organ system — cardiovascular, nervous, urinary
8. Organism — the whole person

A cell is the smallest unit that can carry out all life processes. An organelle alone (say, an isolated mitochondrion) cannot independently sustain life the way a complete cell can.

The Plasma Membrane

The plasma membrane (cell membrane) forms the outer boundary of the animal cell. It is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates—the fluid mosaic model.

• Phospholipids arrange with hydrophilic (water-loving) heads outward and hydrophobic (water-fearing) fatty-acid tails inward, creating a barrier to most polar and charged substances.
• Proteins act as channels, carriers, receptors, and enzymes.
• Cholesterol modulates membrane fluidity.
• Carbohydrates (glycoproteins/glycolipids) support cell recognition and adhesion.

The membrane is selectively permeable: it lets some substances pass freely (small nonpolar molecules like O₂ and CO₂) while tightly controlling ions, large polar molecules, and macromolecules. Transport details are covered in Section 2.2; for structure, remember that the membrane both contains the cell and communicates with the extracellular environment.

Major Organelles of the Animal Cell

Nucleus

The nucleus houses most of the cell’s genetic material (DNA) as chromosomes. The nuclear envelope is a double membrane with nuclear pores that regulate RNA and protein traffic. Inside, the nucleolus assembles ribosomal subunits. The nucleus directs protein synthesis by controlling which genes are transcribed into mRNA.

Ribosomes

Ribosomes are the sites of protein synthesis (translation). They may float free in the cytosol (making proteins that stay in the cell) or attach to rough ER (making proteins for secretion or membranes). Ribosomes are not membrane-bound; they are ribonucleoprotein complexes present in both prokaryotes and eukaryotes.

Endoplasmic Reticulum (ER)

• Rough ER is studded with ribosomes. It synthesizes and folds proteins destined for membranes, lysosomes, or export, and begins glycosylation of many proteins.
• Smooth ER lacks ribosomes. It synthesizes lipids and steroids, detoxifies drugs and toxins (especially in hepatocytes), and stores calcium in muscle cells (sarcoplasmic reticulum).

Golgi Apparatus

The Golgi apparatus (Golgi complex) is a stack of flattened membrane sacs. It modifies, sorts, and packages proteins and lipids from the ER into vesicles for delivery to the plasma membrane, lysosomes, or secretion outside the cell. Think of it as the cell’s shipping and finishing center.

Mitochondria

Mitochondria produce most of the cell’s ATP through cellular respiration (glycolysis products enter the Krebs cycle and electron transport chain). They have a double membrane; the inner membrane folds into cristae that increase surface area for ATP synthesis. Mitochondria contain their own circular DNA and ribosomes—a clue to their evolutionary origin (endosymbiotic theory). Tissues with high energy demand (cardiac muscle, neurons) are rich in mitochondria.

Lysosomes

Lysosomes are membrane-bound vesicles filled with hydrolytic enzymes that digest worn-out organelles, engulfed pathogens, and macromolecules. They maintain an acidic interior so enzymes work optimally. In white blood cells, lysosomes help destroy phagocytosed bacteria—directly relevant to infection and immunity.

Cytoskeleton

The cytoskeleton is a network of protein filaments that gives the cell shape, anchors organelles, and enables movement:

• Microfilaments (actin) — support shape, enable cell crawling and muscle contraction
• Intermediate filaments — mechanical strength (e.g., keratin in skin)
• Microtubules — tracks for vesicle transport; form the spindle in cell division; build cilia and flagella

Other Structures Worth Knowing

• Centrosome / centrioles — organize microtubules; important for mitotic spindle formation in animal cells
• Peroxisomes — break down fatty acids and detoxify hydrogen peroxide (H₂O₂ → water + O₂)
• Cytosol / cytoplasm — cytosol is the aqueous fluid; cytoplasm is cytosol plus organelles (excluding nucleus in many textbook definitions)

Organelle Function Quick Reference

Plant vs Animal Cells (Brief Comparison)

Both are eukaryotic, but plant cells typically have:

• A rigid cell wall (cellulose) outside the plasma membrane
• Chloroplasts for photosynthesis
• A large central vacuole for storage and turgor pressure

Animal cells lack these three. Animal cells have centrioles (most plant cells do not form the same centrosome structure). On the NEX, if a question mentions chloroplasts or a cell wall of cellulose, the cell is plant (or a photosynthetic organism), not a typical human body cell.

Why This Matters for Nursing and the NEX

• Pharmacology: Many drugs target organelles or prokaryotic structures (e.g., antibiotics vs bacterial ribosomes).
• Pathology: Lysosomal storage diseases, mitochondrial disorders, and cancer (uncontrolled cell division—Section 2.3) all start at the cell level.
• IV therapy and fluid balance: Depend on membrane properties and osmosis (Section 2.2).
• A&P foundation: Every organ system is specialized cells doing specialized jobs with the same organelle toolkit.

Exam Traps

• Ribosomes vs mitochondria: Ribosomes make proteins; mitochondria make ATP. Do not reverse them.
• Rough vs smooth ER: Ribosomes on rough ER → protein work; smooth ER → lipids/detox/Ca²⁺.
• Prokaryote nucleus: Prokaryotes do not have a membrane-bound nucleus—DNA is in the nucleoid.
• Cell wall: Human cells do not have a cell wall; bacteria and plants do (different chemistry).
• Lysosome vs Golgi: Golgi packages; lysosomes digest.

Build a mental map: membrane encloses the cell → nucleus holds the instructions → ribosomes/ER/Golgi make and ship proteins → mitochondria power the work → lysosomes clean up. That map is enough to answer most NEX cell-structure items and prepares you for membrane transport and cell division next.