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Cell Structure
& Organelles

Explore the fundamental building blocks of life: from the plasma membrane to the nucleus, discover how each organelle contributes to cellular function.

Anatomy & Physiology

What Is a Cell?

The cell is the smallest structural and functional unit of all living organisms. Every tissue, organ, and organ system in the human body is built from trillions of cells working in coordination.

Understanding cell structure is essential for anatomy and physiology because the function of every organ depends on the specialized cells it contains.

Two Major Cell Types

Prokaryotic
No membrane-bound nucleus. DNA floats freely in the cytoplasm. Found in bacteria and archaea. Smaller and simpler in structure.
Eukaryotic
Contains a true nucleus enclosed by a double membrane. Possesses specialized organelles. Found in animals, plants, fungi, and protists.

The Animal Cell at a Glance

\( \text{Plasma Membrane} \)
\( \text{Nucleolus} \)
\( \text{Nucleus} \)
\( \text{Rough ER} \)
\( \text{Smooth ER} \)
\( \text{Mitochondrion} \)
\( \text{Golgi Apparatus} \)
\( \text{Lysosome} \)
\( \text{Ribosomes} \)
\( \text{Centrioles} \)
\( \text{Cytoplasm} \)

The Plasma Membrane

\( \text{Phospholipid Bilayer} \)
\( \text{Extracellular Fluid} \)
(outside cell)
\( \text{Hydrophobic} \)
Fatty Acid Tails
\( \text{Hydrophilic} \)
Phosphate Heads

The plasma membrane is a selectively permeable phospholipid bilayer. Its hydrophilic heads face the aqueous environment, while hydrophobic tails face inward, creating a barrier that controls what enters and exits the cell.

The Nucleus: Control Center

\( \text{Nucleolus} \)
\( \text{Chromatin} \)
\( \text{Nuclear Envelope} \)
\( \text{Nuclear Pore} \)
1
Nuclear Envelope: double membrane with pores that regulate transport of mRNA and proteins.
2
Chromatin: loosely coiled DNA and histone proteins; condenses into chromosomes during cell division.
3
Nucleolus: dense region within the nucleus responsible for ribosomal RNA (rRNA) synthesis.

Mitochondria: The Powerhouse

\( \text{Outer Membrane} \)
\( \text{Cristae (inner folds)} \)
\( \text{Matrix} \)

The Endomembrane System

\( \text{Rough ER} \)
Protein synthesis
\( \text{Smooth ER} \)
Lipid synthesis
\( \text{Golgi} \)
\( \text{Vesicle} \)
\( \text{Endomembrane System Pathway} \)

Proteins are synthesized on the Rough ER, lipids are assembled by the Smooth ER, then both are shipped to the Golgi apparatus for modification, packaging, and sorting into vesicles or lysosomes.

Supporting Structures

Ribosomes
Tiny molecular machines that translate mRNA into proteins. Found free in the cytoplasm or bound to the rough ER. Not membrane-bound.
Cytoskeleton
Network of microfilaments, intermediate filaments, and microtubules that provides structural support, shape, and intracellular transport.
Centrioles
Paired cylindrical structures composed of microtubules. Organize the mitotic spindle during cell division. Located near the nucleus in the centrosome.

Rough ER vs Smooth ER

Rough ER Smooth ER
Surface Studded with ribosomes No ribosomes (smooth)
Primary Function Protein synthesis and folding Lipid synthesis, detoxification
Location Continuous with nuclear envelope Extends from the Rough ER
Abundant In Cells that secrete proteins (e.g., pancreas) Cells that produce steroids (e.g., liver)

How to Identify Organelles

1
Observe the membrane: Is there a double membrane (nucleus, mitochondria), a single membrane (ER, Golgi, lysosome), or no membrane (ribosomes)?
2
Check for internal structures: Cristae suggest mitochondria. Stacked cisternae suggest Golgi. Ribosomes on the surface suggest Rough ER.
3
Consider the function clue: ATP production points to mitochondria. Protein synthesis points to ribosomes/Rough ER. Digestion points to lysosomes.
4
Note the cell type: Plant cells have chloroplasts and a cell wall; animal cells have centrioles and lysosomes but lack chloroplasts.

Example: Tracing a Protein

A digestive enzyme is produced inside a pancreatic cell. Trace its journey from gene to secretion:

1
Nucleus: DNA is transcribed into mRNA, which exits through nuclear pores.
2
Ribosomes on Rough ER: mRNA is translated into a polypeptide chain, which enters the ER lumen for folding.
3
Golgi Apparatus: the protein is modified (e.g., glycosylation), sorted, and packaged into a transport vesicle.
4
Secretory Vesicle: fuses with the plasma membrane via exocytosis, releasing the enzyme outside the cell.

Example: Lysosome in Action

\( \text{Phagocytosis} \)
\( \text{Bacteria} \)
\( \text{Phagosome} \)
+
\( \text{Lysosome} \)
\( \text{Phago-} \)
\( \text{lysosome} \)
\( \text{Intracellular Digestion} \)
Enzymes digest the pathogen into reusable molecules

Example: ATP Production

Why do muscle cells contain far more mitochondria than skin cells? Consider energy demand:

Muscle Cell
Contracts repeatedly, requiring enormous amounts of ATP. Contains thousands of mitochondria packed between myofibrils.
Skin Cell
Primarily serves as a barrier. Lower metabolic demand, so it has fewer mitochondria and relies less on oxidative phosphorylation.

Common Mistakes

✕ Confusing ER types
Students often swap Rough ER (protein synthesis) with Smooth ER (lipid synthesis). Remember: Rough = Ribosomes = pRotein.
✕ Cell wall in animal cells
Animal cells do not have a cell wall. Only plant, fungal, and bacterial cells do. Animal cells have only a plasma membrane.
✕ Lysosome vs Ribosome
Lysosomes digest materials (hydrolytic enzymes). Ribosomes build proteins (translation). Completely different structures and functions.

Organelle Summary

Organelle Function Membrane
Nucleus Stores DNA, controls gene expression Double
Mitochondrion ATP production (cellular respiration) Double
Rough ER Protein synthesis and folding Single
Smooth ER Lipid synthesis, detoxification Single
Golgi Apparatus Modifies, packages, sorts proteins Single
Lysosome Intracellular digestion Single
Ribosome Translates mRNA into protein None
Centriole Organizes mitotic spindle None

Question 1 of 5

Which organelle is the primary site of ATP production in a eukaryotic cell?

Question 2 of 5

A cell produces large quantities of secretory proteins. Which organelles would you expect to be most abundant in this cell?

Question 3 of 5

Which of the following structures is NOT surrounded by a membrane?

Question 4 of 5

A drug destroys all lysosomes in a white blood cell. What is the most likely consequence?

Question 5 of 5

The correct sequence for the secretory pathway is: