Cell Structure & Function: Study Notes

Cell Structure & Function

Introduction

This chapter explores the fundamental unit of life: the cell. It covers the major types of cells, their structures, and the functions of various organelles, providing a foundation for understanding biological processes at the cellular level.

The Four Big Ideas in Cell Biology

Overview

• Evolution: All cells arise from pre-existing cells, forming an unbroken lineage that traces back nearly 4 billion years. This principle underpins the continuity of life.

• Energy & Homeostasis: Eukaryotic cells contain specialized organelles that cooperate to maintain internal stability and perform essential life functions.

• Information & Signaling: Every cell contains DNA, which encodes the information necessary for all cellular structures and functions. This information may be housed within a nucleus or free in the cytoplasm.

• Interactions & Systems: Cellular systems metabolize, interact, and adapt to changing environmental conditions, ensuring survival and function.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural features and complexity.

• Prokaryotic Cells:

  • Include bacteria and archaea.

  • Lack membrane-bound organelles.

  • Genetic material is located in a nucleoid region, not enclosed by a membrane.

  • May contain plasmids (small, circular DNA molecules).

  • Have structures such as pili (for conjugation), flagella, and sometimes cilia for locomotion.

  • Possess a cell wall (except some archaea) and ribosomes.

• Eukaryotic Cells:

  • Include animal and plant cells, as well as fungi and protists.

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

  • Genetic material is enclosed within a nuclear envelope.

  • Plant cells have additional structures such as a cell wall, chloroplasts, and a large central vacuole.

Cell Size and Surface Area

• Cells are generally small to maximize the surface-area-to-volume ratio, which facilitates efficient exchange of nutrients and waste.

• Typical sizes:

  • Bacterial cells: 1–10 micrometers (μm)

  • Eukaryotic cells: 10–100 micrometers (μm)

  • For comparison, the diameter of a human hair is about 20 μm.

Cell Theory

Principles

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in organisms.

• All cells arise from pre-existing cells through cell division.

Studying Cells

Microscopy

• Compound Light Microscope: Uses visible light and glass lenses to magnify specimens up to 1000x. Suitable for viewing cells but not most organelles.

• Transmission Electron Microscope (TEM): Provides detailed images of internal cell structures (ultrastructure).

• Scanning Electron Microscope (SEM): Produces 3D images of cell surfaces.

Cell Fractionation

• Technique to separate cellular components for study.

• Uses a centrifuge to spin samples at various speeds, isolating organelles based on size and density.

Comparing Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
Membrane-bound organelles	No	Yes
Nucleus	No (nucleoid region)	Yes
DNA form	Circular chromosome, plasmids	Linear chromosomes
Cell wall	Yes (most)	Yes (plants, fungi); No (animals)
Ribosomes	Yes (smaller)	Yes (larger)

Cellular Organelles and Their Functions

Nucleus

• Contains the cell's DNA and controls gene expression.

• Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for molecular transport.

• Contains the nucleolus, where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled.

Ribosomes

• Sites of protein synthesis.

• Composed of two subunits (large and small) made of rRNA and proteins.

• Types:

  • Free ribosomes: Suspended in cytosol; synthesize proteins for use within the cell.

  • Bound ribosomes: Attached to the rough endoplasmic reticulum (ER); synthesize proteins for export or for membranes.

Endoplasmic Reticulum (ER)

• Network of membranes connected to the nuclear envelope.

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

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and carbohydrate metabolism.

Golgi Apparatus

• Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Consists of flattened membrane sacs (cisternae).

• Products are shipped in vesicles.

Lysosomes

• Contain hydrolytic enzymes for digesting macromolecules, old organelles, and foreign substances.

• Play a role in programmed cell death (apoptosis).

• Malfunction can lead to lysosomal storage diseases (e.g., Tay-Sachs disease).

Vacuoles and Vesicles

• Vacuoles: Large membrane-bound sacs for storage and transport. Plant cells have a large central vacuole for water and nutrient storage.

• Vesicles: Small membrane-bound sacs that transport materials within the cell.

• Specialized vacuoles include food vacuoles (formed by phagocytosis) and contractile vacuoles (pump excess water out of protist cells).

Peroxisomes

• Break down fatty acids and detoxify harmful substances.

• Produce hydrogen peroxide ($H_2O_2$) as a byproduct, which is then converted to water.

Energy-Transforming Organelles

Mitochondria

• Site of cellular respiration; converts glucose and oxygen into ATP (energy currency of the cell).

• Structure: Double membrane, with a highly folded inner membrane (cristae) and a fluid-filled matrix containing DNA and ribosomes.

• Can divide independently of the cell; number correlates with cell's energy needs.

Chloroplasts

• Found in plant cells and some protists; site of photosynthesis.

• Structure: Double membrane, internal stacks of thylakoids (grana), stroma (fluid), DNA, and ribosomes.

• Convert solar energy into chemical energy (glucose).

• Other plastids include amyloplasts (store starch) and chromoplasts (store pigments).

Comparison of Mitochondria and Chloroplasts

Feature	Mitochondria	Chloroplasts
Function	Cellular respiration (ATP production)	Photosynthesis (glucose production)
Membranes	Double	Double
DNA & Ribosomes	Yes	Yes
Found in	All eukaryotes	Plants, algae

Endosymbiosis Theory

• Proposes that mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells.

• Evidence: Both have their own DNA (circular), ribosomes similar to bacteria, and double membranes; they can divide independently.

Cellular Equations

Photosynthesis

$6CO_2 + 6H_2O + ext{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Cellular Respiration

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Cytoskeleton

Structure and Function

• Network of protein fibers that provide structural support, maintain cell shape, and anchor organelles.

• Involved in cell movement (motility) and intracellular transport.

• Components include microfilaments, intermediate filaments, and microtubules.

Centrioles

• Found in animal cells; involved in organizing microtubules during cell division.

• Usually exist as a pair near the nucleus (centrosome).

Programmed Cell Death (Apoptosis)

Role of Lysosomes

• Lysosomes can initiate apoptosis, a process of programmed cell death essential for development and tissue homeostasis.

• Examples: Removal of tadpole tails during metamorphosis; elimination of webbing between human fingers during fetal development.

• Failure of apoptosis regulation can contribute to diseases such as cancer.