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Cell Biology & Energy (BIO 101) · Module 1 of 8

Cell Structure & Compartmentalization

The membrane, organelle system, and cytoskeleton that organize a eukaryotic cell's chemistry.

In this module
  1. The Fluid Mosaic Membrane
  2. The Endomembrane System
  3. The Cytoskeleton
  4. Mitochondrial & Chloroplast Structure
  5. Check yourself

The Fluid Mosaic Membrane

  • The cell membrane is a phospholipid bilayer with embedded and peripheral proteins that can move laterally within it — the fluid mosaic model.
  • Cholesterol (in animal cells) moderates membrane fluidity, keeping it neither too rigid at low temperatures nor too fluid at high temperatures.
  • Integral membrane proteins that span the bilayer often function as channels, carriers, or receptors.
Worked example

Why is the membrane described as a "mosaic"?

Solution. Because it's composed of a patchwork of different proteins embedded within or attached to the phospholipid bilayer, rather than a uniform, unbroken sheet.

The Endomembrane System

  • The endomembrane system — nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane — works together to synthesize, modify, and transport proteins and lipids.
  • Proteins made on the rough ER are folded and tagged, then shipped to the Golgi via vesicles, where they are further modified and sorted to their final destination.
Worked example

Trace the general path of a protein destined for secretion, from synthesis to release.

Solution. It's synthesized by ribosomes on the rough ER, folded and processed there, transported via vesicle to the Golgi apparatus for further modification and sorting, then packaged into a secretory vesicle that fuses with the plasma membrane to release the protein outside the cell.

The Cytoskeleton

  • The cytoskeleton is a network of protein filaments — microtubules (largest), intermediate filaments, and microfilaments/actin (smallest) — that gives the cell shape, enables movement, and organizes organelles.
  • Microtubules also form the mitotic spindle during cell division and the core of cilia and flagella.
Worked example

Which cytoskeletal component is primarily responsible for separating chromosomes during mitosis?

Solution. Microtubules, which form the mitotic spindle that pulls sister chromatids apart.

Mitochondrial & Chloroplast Structure

  • Mitochondria have a smooth outer membrane and a highly folded inner membrane (cristae) that increases surface area for the electron transport chain; the enclosed space is the matrix.
  • Chloroplasts have an outer and inner membrane surrounding the stroma, which contains stacks of thylakoid membranes (grana) where the light reactions occur.
  • Both organelles have their own DNA and ribosomes, supporting the endosymbiotic theory that they originated from free-living prokaryotes.
Worked example

What structural feature do mitochondria and chloroplasts share that supports the endosymbiotic theory?

Solution. Both have their own circular DNA and ribosomes, similar to bacteria, suggesting they originated as free-living prokaryotic cells engulfed by an ancestral eukaryotic cell.

Check yourself

2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.

1. What does the "fluid mosaic model" describe?

2. What is the role of the Golgi apparatus?

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