A Tour of the Cell (Chapter 6): Structure and Function of Cells

Fundamental Units of Life

Introduction to Cells

Cells are the basic structural and functional units of all living organisms. Understanding their structure and function is essential to the study of biology.

• All organisms are made of cells: Every living thing, from bacteria to plants and animals, is composed of one or more cells.

• The cell is the simplest collection of matter that can be alive: Cells are the smallest units that exhibit all the characteristics of life.

• Cells are related by their descent from earlier cells: All cells arise from pre-existing cells through cell division, maintaining continuity of life.

• Cells can differ substantially from one another but share common features: Despite diversity in form and function, all cells have certain structural similarities.

Internal Organization of Eukaryotic Cells

Compartmentalization and Cellular Functions

Eukaryotic cells possess internal membranes that divide the cell into compartments, allowing specialized chemical reactions to occur efficiently.

• Energy and Matter Transformations:

  • The endomembrane system synthesizes and processes proteins, lipids, and carbohydrates.

  • Chloroplasts convert light energy to chemical energy via photosynthesis (in plants and algae).

  • Mitochondria break down molecules to generate ATP, the cell's main energy currency.

• Genetic Information Storage and Transmission:

  • DNA is stored in the nucleus, which contains instructions for making proteins.

  • Ribosomes are the sites of protein synthesis, translating genetic information into functional proteins.

• Interactions with the Environment:

  • The plasma membrane controls the movement of substances into and out of the cell, maintaining homeostasis.

  • Some cells have a cell wall that provides additional protection and structural support (e.g., plant cells, fungi, and some protists).

Comparing Prokaryotic and Eukaryotic Cells

Key Differences and Similarities

Cells are classified as either prokaryotic or eukaryotic based on their structural characteristics.

• Prokaryotic Cells:

  • Include Bacteria and Archaea.

  • Lack a nucleus; DNA is located in an unbound region called the nucleoid.

  • Do not have membrane-bound organelles.

  • Generally smaller in size than eukaryotic cells.

  • Key structures: plasma membrane, cytosol, chromosomes, ribosomes, cell wall, sometimes fimbriae and flagella.

• Eukaryotic Cells:

  • Include Protists, Plants, Animals, and Fungi.

  • Have a nucleus bounded by a double membrane.

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

  • Generally larger than prokaryotic cells.

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

• Common Features of All Cells:

  • Plasma membrane

  • Cytosol (semifluid substance)

  • Chromosomes (carry genes)

  • Ribosomes (make proteins)

Plasma Membrane Structure and Function

Selective Barrier and Surface Area

The plasma membrane is a critical structure that separates the cell from its environment and regulates the passage of materials.

• Structure: Composed of a phospholipid bilayer with embedded proteins. The bilayer has hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Function: Acts as a selective barrier, allowing sufficient passage of oxygen, nutrients, and waste to service the cell's volume.

• Surface Area to Volume Ratio: As a cell increases in size, its volume grows faster than its surface area, limiting the efficiency of material exchange. This sets an upper limit on cell size.

Formula for Surface Area and Volume of a Cube:

• Surface Area: $6 \times (\text{side length})^2$

• Volume: $(\text{side length})^3$

• Surface Area to Volume Ratio: $\frac{\text{Surface Area}}{\text{Volume}}$

Genetic Control: Nucleus and Ribosomes

Nucleus

The nucleus is the control center of the eukaryotic cell, housing most of its genetic material.

• Nuclear Envelope: A double membrane that encloses the nucleus, separating it from the cytoplasm.

• Chromosomes: DNA is organized into discrete units called chromosomes, each consisting of a single DNA molecule associated with proteins (chromatin).

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

Ribosomes

Ribosomes are molecular machines responsible for protein synthesis.

• Composed of rRNA and proteins.

• Carry out protein synthesis in two locations:

  • Free ribosomes: suspended in the cytosol.

  • Bound ribosomes: attached to the endoplasmic reticulum or nuclear envelope.

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

  • Plasma membrane

• These components are either continuous or connected via transfer by vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER:

  • Synthesizes lipids

  • Metabolizes carbohydrates

  • Detoxifies drugs and poisons

  • Stores calcium ions

• Rough ER:

  • Has bound ribosomes

  • Distributes transport vesicles, proteins surrounded by membranes

  • Is a membrane factory for the cell

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Modifies products of the ER.

• Manufactures certain macromolecules.

• Sorts and packages materials into transport vesicles.

Lysosomes

• Membranous sacs of hydrolytic enzymes that digest macromolecules.

• Carry out intracellular digestion by phagocytosis and autophagy.

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Perform a variety of functions in different kinds of cells (e.g., storage, waste disposal, water balance, cell growth, and protection).

• Plant cells have a large central vacuole that maintains cell shape and stores nutrients.

Energy Conversion: Mitochondria and Chloroplasts

Mitochondria

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

Chloroplasts

• Found in plants and algae; sites of photosynthesis, converting solar energy to chemical energy.

The Cytoskeleton

Structure and Function

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

• Microtubules:

  • Hollow rods made of tubulin dimers; about 25 nm in diameter.

  • Functions: shaping the cell, guiding movement of organelles, separating chromosomes during cell division.

• Microfilaments (Actin Filaments):

  • Solid rods about 7 nm in diameter, built as a twisted double chain of actin subunits.

  • Functions: support cell shape, involved in cell movement (e.g., muscle contraction, amoeboid movement).

• Intermediate Filaments:

  • Range from 8–12 nm in diameter; more permanent structures than microtubules or microfilaments.

  • Functions: support cell shape, fix organelles in place.

Extracellular Components and Cell Interactions

Plant Cell Walls

• Provide structural support, protection, and regulate water intake.

• Composed mainly of cellulose.

• Plasmodesmata are channels between plant cells that allow for transport and communication.

Extracellular Matrix (ECM) of Animal Cells

• Made up of glycoproteins such as collagen, proteoglycans, and fibronectin.

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

• Functions: support, adhesion, movement, and regulation.

Cell Junctions in Animal Cells

• Tight Junctions: Prevent leakage of extracellular fluid across a layer of epithelial cells.

• Desmosomes: Fasten cells together into strong sheets.

• Gap Junctions: Provide cytoplasmic channels between adjacent cells for communication.