Chapter 3 Notes: Cells, Membranes, and Homeostasis

Learning outcomes

• Review chapter 3 learning outcome slide to ensure understanding of all contents by the end of the chapter; if there are questions, contact the instructor for clarification.
• Introduction to chapter themes: cells as fundamental units of life, cell types (prokaryotes and eukaryotes), membranes, homeostasis, and organelles.

Cell theory

• Core concepts that apply across all cells, whether unicellular or multicellular:
  • All organisms are made up of cells.
  • The cell is the fundamental (smallest) unit of life.
  • New cells arise from preexisting cells (cell division).
• Examples of organisms illustrating these concepts:
  • Protist (unicellular)
  • Algae (multicellular)
  • Fungus (multicellular)
• Historical note: discovery of cells began with light microscopy and the work of Robert Hooke around 1665, when he studied cork tissue and observed arrays of cavities under the microscope.
  • Hooke called these cavities "cells"; the structures he observed were cell walls of plant cork, not living cells.
• Transition to modern cell biology: we now recognize two main cell classes—prokaryotes and eukaryotes—and will compare them in terms of nucleus presence, membrane-bound organelles, and other features.

Prokaryotes vs. Eukaryotes

• Key visual and structural differences:
  • Prokaryotic cells are smallest and lack a nucleus and membrane-bound organelles.
  • Eukaryotic cells are larger, contain a nucleus, and have multiple membrane-bound organelles.
• Nucleus:
  • Eukaryotes have a nucleus (a membrane-bound structure that houses DNA).
  • Prokaryotes lack a true nucleus; DNA is not enclosed by a nuclear membrane.
• Presence of membrane-bound organelles:
  • Eukaryotes contain many membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).
  • Prokaryotes do not have these membrane-bound organelles.
• Size differences:
  • Prokaryotic cells: typically around 1{-}2\, ext{μm} in size.
  • Eukaryotic cells: typically around 10{-}100\,\text{μm} in size.
• Taxonomic examples:
  • Prokaryotes: bacteria (e.g., Salmonella) and archaea (single-celled organisms).
  • Eukaryotes: plants, animals, and many fungi; examples shown include plant/animal cells and specialized cells like a frog egg and a nerve cell.
• Transcription and translation locations (differences highlighted):
  • In prokaryotes: transcription and translation occur in the cytoplasm (no nucleus).
  • In eukaryotes: transcription occurs in the nucleus and translation occurs in the cytoplasm.
  • These location differences will be discussed in more detail later (chapter on transcription/translation).
• Summary of differences:
  • Prokaryotes: no nucleus, no membrane-bound organelles, smaller size, cytoplasmic transcription/translation.
  • Eukaryotes: nucleus, membrane-bound organelles, larger size, transcription in nucleus and translation in cytoplasm.

Cell size and shape relate to function

• Shape and size are diverse across cell types and reflect function:
  • Shape can define how a cell interacts with its environment and carries out its role.
• Examples of function-linked morphology:
  • Nerve cells: very long with projections (axons) to facilitate communication between different parts of the body; projections enable signaling over distances.
  • Red blood cells (RBCs): biconcave shape increases surface area, enhancing gas exchange (O2 and CO2) with the environment.
• Concept: cells are adapted in shape and size to support their specific functions; thus, morphology is tied to cellular roles.

Membranes, transport, and homeostasis

• The cell membrane is a surrounding layer that regulates the movement of molecules:
  • Transport of molecules from outside to inside the cell.
  • Transport of wastes from inside to outside the cell.
  • Movement of other molecules in and out of the cell as needed.
• Function of membranes:
  • Maintain a stable internal environment (homeostasis) by controlling exchange with the external environment.
• Homeostasis: the chapter’s focus on how membranes contribute to maintaining internal stability essential for cellular function.

Organelles and compartments

• Cells contain internal compartments called organelles.
  • The chapters will cover the structure and function of these organelles in more detail.
• Conceptual point: these organelles compartmentalize cellular processes and enable specialization of functions within the cell.

Transcription and translation (brief definitions)

• Transcription: the synthesis of RNA from DNA; described as RNA synthesis or mRNA synthesis.
• Translation: the synthesis of protein from RNA; described as protein synthesis.
• These definitions will be revisited with more detail later in the course.

Connections to broader themes and real-world relevance

• Understanding cell theory underpins modern biology and medicine because it explains why all living organisms share common cellular foundations.
• Membranes and homeostasis are foundational to physiology, pharmacology, and disease (e.g., how drugs cross membranes, how cells regulate internal conditions).
• The distinction between prokaryotic and eukaryotic cells underpins microbiology, genetics, and biotechnology, including how transcription and translation are organized in different organisms.
• The morphologies of cells illustrate how structure determines function, a principle that informs fields from neurobiology (nerve cell signaling) to hematology (gas transport by RBCs) and beyond.

Next steps in the chapter

• The next part of the chapter will introduce cell networks and further details about organelles and cellular compartments.
• Students are encouraged to review the chapter’s learning outcomes and come prepared with questions for follow-up discussion.