Chapter 6: Tour of the Cell – Structure and Function of Eukaryotic Cells

Tour of the Cell

Introduction to Cell Structure and Function

Cells are the fundamental units of life, and their internal organization enables them to perform essential biological functions. Eukaryotic cells, in particular, possess complex internal structures that compartmentalize various processes, allowing for efficient energy transformation, genetic information storage, and interaction with the environment.

• Cell Theory: All living organisms are composed of cells, and all cells arise from pre-existing cells.

• Types of Cells: There are two main types: prokaryotic (Bacteria and Archaea) and eukaryotic (protists, fungi, plants, animals).

Microscopy and the Study of Cells

Microscopy Techniques

Because cells are typically too small to be seen with the naked eye, microscopes are essential tools for studying their structure.

• Magnification: The ratio of an object's image size to its real size.

• Resolution: The clarity of the image; the minimum distance between two distinguishable points.

• Contrast: Visible differences in brightness between parts of the sample.

Basic Features of All Cells

Universal Cell Structures

• Plasma Membrane: A selective barrier that controls the passage of oxygen, nutrients, and waste.

• Cytosol: A semifluid substance within the cell.

• Chromosomes: Structures that carry genetic information (DNA).

• Ribosomes: Complexes that synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

Key Differences

• Prokaryotic Cells:

  • No nucleus; DNA is in an unbound region called the nucleoid.

  • Lack membrane-bound organelles.

  • Cytoplasm is bound by the plasma membrane.

• Eukaryotic Cells:

  • DNA is contained within a nucleus, which is bounded by a double membrane.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Cytoplasm is the region between the plasma membrane and nucleus.

  • Generally larger than prokaryotic cells.

Cell Size and Surface Area

Surface Area-to-Volume Ratio

The plasma membrane must provide sufficient surface area to service the cell's volume. As a cell increases in size, its volume grows faster than its surface area, limiting the size a cell can attain.

• Formula: For a sphere, $\text{Surface Area} = 4\pi r^2$ $\text{Volume} = \frac{4}{3}\pi r^3$

• Cells remain small to maintain a high surface area-to-volume ratio for efficient exchange of materials.

Internal Membranes and Compartmentalization

Organelles and Their Functions

Eukaryotic cells have internal membranes that divide the cell into compartments, each with specialized functions. This compartmentalization allows for distinct microenvironments and efficient metabolic processes.

• Basic Fabric: Biological membranes are primarily composed of a double layer of phospholipids and proteins.

• Plant and animal cells share most organelles, but some are unique to each.

The Nucleus and Ribosomes

Genetic Information Storage and Transmission

• Nucleus: Contains most of the cell's DNA and is surrounded by a double membrane called the nuclear envelope.

• Chromosomes: Each consists of a DNA molecule associated with proteins (chromatin). Chromatin condenses to form discrete chromosomes during cell division.

• Nucleolus: Located within the nucleus; site of ribosomal RNA (rRNA) synthesis.

• Ribosomes: Complexes made of rRNA and protein; sites of protein synthesis. Found free in the cytosol or bound to the endoplasmic reticulum (ER).

The Endomembrane System

Components and Functions

The endomembrane system regulates protein traffic and performs metabolic functions in the cell.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• These components are either continuous or connected via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes; synthesizes lipids.

• Rough ER: Studded with ribosomes; distributes transport vesicles and is a membrane factory for the cell.

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Modifies products of the ER, manufactures certain macromolecules, and sorts/packages materials into transport vesicles.

Lysosomes

• Membranous sacs of hydrolytic enzymes that digest macromolecules.

• Work best in acidic environments.

• Engulf other cells or food particles by phagocytosis, forming food vacuoles.

• Fuse with food vacuoles to digest contents; also recycle the cell’s own organelles (autophagy).

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Types:

  • Food vacuoles: Formed by phagocytosis.

  • Contractile vacuoles: Pump excess water out of cells (common in freshwater protists).

  • Central vacuoles: Found in mature plant cells; contain cell sap.

Energy-Transforming Organelles

Mitochondria

• Sites of cellular respiration, a metabolic process that uses oxygen to generate ATP.

• Found in nearly all eukaryotic cells.

• Structure: Smooth outer membrane and a highly folded inner membrane (cristae), creating the intermembrane space and mitochondrial matrix.

• Most steps of cellular respiration occur in the mitochondrial matrix.

Chloroplasts

• Found in plants and algae; sites of photosynthesis.

• Contain the green pigment chlorophyll, enzymes, and other molecules for photosynthesis.

• Structure: Thylakoids (membranous sacs) stacked to form a granum; stroma (internal fluid).

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes structures and activities in the cell, providing support, shape, and facilitating movement.

• Interacts with motor proteins to produce cell motility.

• Vesicles and organelles can move along cytoskeletal tracks.

Components of the Cytoskeleton

• Microtubules: Shape the cell, guide movement of organelles, and separate chromosomes during cell division.

• Microfilaments: Help support the cell's shape.

• Intermediate Filaments: Support cell shape and fix organelles in place.

Centrosomes and Centrioles

• In animal cells, microtubules grow out from a centrosome near the nucleus.

• The centrosome contains a pair of centrioles, each with nine triplets of microtubules arranged in a ring.

Flagella and Cilia

• Microtubule-containing extensions that project from some cells.

• Unicellular protists are propelled through water by cilia or flagella.

Extracellular Components and Cell Interactions

Cell Walls of Plants

• Distinguish plant cells from animal cells.

• Also found in prokaryotes, fungi, and some protists.

• Functions: Protects the cell, maintains shape, and prevents excessive water uptake.

Extracellular Matrix (ECM) in Animal Cells

• Animal cells lack cell walls but are covered by an elaborate ECM.

• ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin.

• ECM proteins bind to receptor proteins in the plasma membrane called integrins.

Summary Table: Major Eukaryotic Cell Organelles and Their Functions

Organelle	Main Function	Present in
Nucleus	Stores genetic information; site of transcription	All eukaryotes
Ribosome	Protein synthesis	All cells
Endoplasmic Reticulum (ER)	Protein and lipid synthesis	All eukaryotes
Golgi Apparatus	Modification, sorting, and packaging of proteins and lipids	All eukaryotes
Lysosome	Digestion and recycling	Animal cells, some protists
Vacuole	Storage, waste disposal, water balance	Plant cells, some protists
Mitochondrion	ATP production (cellular respiration)	All eukaryotes
Chloroplast	Photosynthesis	Plants, algae
Cytoskeleton	Structural support, movement	All eukaryotes