A Tour of the Cell

Introduction

This chapter provides an overview of the structure and function of eukaryotic cells, focusing on the membranes and organelles that compartmentalize and organize cellular activities. Understanding these components is fundamental to cell biology and helps explain how cells maintain homeostasis and perform specialized functions.

Membranes in Eukaryotic Cells

Structure and Function of Cellular Membranes

• Plasma Membrane: All cells are surrounded by a plasma membrane, which acts as a selective barrier between the cell and its environment.

• Internal Membranes: Eukaryotic cells contain several internal membranes that partition the cell into distinct compartments (organelles).

• Phospholipid Bilayer: Most cellular membranes consist of a double layer of phospholipids, with proteins embedded within or attached to the surface.

• Membrane Composition: Each membrane has a unique combination of lipids and proteins tailored to its specific function.

Key Functions of Membranes:

• Protection and structural support

• Regulation of the movement of substances into and out of the cell

• Compartmentalization of cellular processes

• Communication with the external environment

Example: The plasma membrane in animal cells forms the outer boundary, while in plant cells, it is located just inside the cell wall.

Cell Organelles

Overview

Organelles are specialized structures within eukaryotic cells, each performing distinct functions necessary for cell survival and activity.

Plasma Membrane (Cell Membrane)

• Location: Outermost boundary in animal cells; just inside the cell wall in plant cells.

• Functions:

  • Protection

  • Support

  • Controls what goes in and out of the cell

  • Acts as a barrier between the cell and its environment

Nucleus

• Most prominent organelle in eukaryotic cells (diameter about 5 μm).

• Enclosed by a nuclear envelope (double membrane).

• Contains chromatin fibers (DNA and proteins) and the nucleolus.

• Function: Acts as the control center or "brain" of the cell, directing cellular activities by regulating gene expression and protein synthesis.

Nuclear Envelope

• The nucleus is surrounded by a nuclear envelope, which consists of two phospholipid bilayer membranes with associated proteins.

• Function: Contains nuclear pores that regulate the entry and exit of large macromolecules and particles, maintaining the internal environment of the nucleus.

Chromatin and Chromosomes

• Chromatin: DNA molecules are attached to proteins, forming long fibers called chromatin.

• Chromatin appears as a diffuse network under a light or electron microscope.

• When a cell prepares to divide, chromatin condenses into distinct structures called chromosomes.

• Function: Chromatin contains the genetic material (DNA) that controls cell activities by directing protein synthesis.

Nucleolus

• Appears as a dense, darkly stained region within the nucleus.

• Cells may have one or more nucleoli, depending on species and activity level.

• Function: Responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosome subunits, using instructions from DNA.

Cytoplasm and Cytosol

• Cytoplasm: The region between the plasma membrane and the nucleus, containing organelles and other inclusions.

• Cytosol: The clear, thick, jelly-like fluid portion of the cytoplasm in which organelles are suspended.

• Function: Supports and allows movement of organelles within the cell.

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and the plasma membrane.

• Organelles are connected directly or via vesicles that transfer materials between them.

Endoplasmic Reticulum (ER)

• Structure: Extensive network of membranes continuous with the nuclear envelope.

• Two types:

  • Rough ER (RER): Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Ribosomes

• Small complexes of RNA and protein found free in the cytoplasm or attached to the rough ER.

• Function: Sites of protein synthesis (translation of mRNA into polypeptides).

Golgi Apparatus

• Stack of flattened membrane sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Acts as the cell's "shipping and receiving center."

Summary Table: Major Eukaryotic Cell Organelles and Their Functions

Additional info: Other organelles such as mitochondria, chloroplasts, lysosomes, and peroxisomes are also important in eukaryotic cells but are not detailed in this section.