Cell Structure and Function

Introduction

Cells are the fundamental units of structure and function in all living organisms. Understanding the differences between cell types and the roles of various organelles is essential for studying biology at the college level.

Prokaryotic vs. Eukaryotic Cells

Overview

Cells are classified into two major types: prokaryotic and eukaryotic. These types differ in their structural organization and complexity.

• Prokaryotic cells: Lack a membrane-bound nucleus and organelles. DNA is located in the nucleoid region.

• Eukaryotic cells: Possess a membrane-bound nucleus and various membrane-bound organelles.

Key Differences

Structure of Animal and Plant Cells

Animal Cells

Animal cells are eukaryotic and contain a variety of organelles that perform specialized functions.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Ribosomes: Sites of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and cellular debris.

• Mitochondria: Powerhouse of the cell; site of ATP production through cellular respiration.

• Cytoskeleton: Provides structural support and facilitates cell movement.

Plant Cells

Plant cells share many features with animal cells but also have unique structures.

• Cell Wall: Rigid outer layer providing structural support and protection.

• Chloroplasts: Organelles responsible for photosynthesis, converting light energy into chemical energy.

• Central Vacuole: Large, membrane-bound compartment for storage and maintaining cell turgor.

• Plasmodesmata: Channels between plant cells for communication and transport.

• Other organelles similar to animal cells (nucleus, ER, Golgi apparatus, etc.).

Comparison Table: Animal vs. Plant Cells

Cell Size and Surface Area-to-Volume Ratio

Explanation

Cells are small because a high surface area-to-volume ratio is necessary for efficient exchange of materials with the environment.

• As cell size increases, volume grows faster than surface area.

• Smaller cells can more efficiently transport nutrients and waste.

Formula:

Cell Organelles: Structure and Function

Nucleus

The nucleus is the command center of the cell, containing genetic material and directing cellular activities.

• Surrounded by a double membrane called the nuclear envelope.

• Contains nuclear pores for transport of molecules.

• Chromatin: DNA and associated proteins.

Ribosomes

Ribosomes are molecular machines that synthesize proteins by translating genetic instructions.

• Composed of RNA and proteins.

• Found free in cytosol or bound to rough ER.

Endoplasmic Reticulum (ER)

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

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

Golgi Apparatus

Golgi Apparatus

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Break down macromolecules, old organelles, and cellular debris.

• Important for cellular waste disposal and recycling.

Vesicles

Vesicles are small, membrane-bound sacs that transport materials within the cell.

Peroxisomes

Peroxisomes contain enzymes for breaking down fatty acids and detoxifying harmful substances.

• Produce hydrogen peroxide () as a byproduct, which is then broken down by catalase.

Mitochondria

Mitochondria are the power plants of the cell, generating ATP through cellular respiration.

• Double-membrane structure with inner folds called cristae.

• Matrix: Internal compartment where metabolic reactions occur.

• Site of aerobic respiration:

Chloroplasts (Plant Cells)

Chloroplasts are organelles in plant cells responsible for photosynthesis.

• Contain internal membrane structures called thylakoids, stacked into grana.

• Stroma: Fluid-filled space surrounding thylakoids.

• Site of photosynthesis:

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and facilitates movement.

• Microfilaments (actin filaments): Involved in cell movement and shape changes.

• Intermediate filaments: Provide mechanical strength.

• Microtubules: Involved in organelle movement and cell division.

Extracellular Components and Cell Junctions

Extracellular Matrix (ECM)

The extracellular matrix is a network of proteins and carbohydrates outside the cell that provides structural support and mediates cell signaling.

• Collagen fibers resist tension.

• Proteoglycans provide cushioning.

Cell Junctions

Cell junctions connect cells and coordinate activities in tissues.

• Tight Junctions: Seal cells together, preventing leakage of materials.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Channels that allow passage of ions and small molecules between cells.

Specialized Features and Organelles in Plant Cells

Central Vacuole

The central vacuole is a large, membrane-bound structure in plant cells that stores water, nutrients, and waste products.

• Maintains cell turgor and structural integrity.

Plasmodesmata

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

Diseases Related to Organelle Malfunction

Examples

• Mitochondrial diseases: Affect energy production, leading to cell degeneration.

• Lysosomal storage diseases: Defective lysosomes cause accumulation of waste, leading to nerve damage.

• Peroxisomal disorders: Defective peroxisomes result in toxin buildup and developmental issues.

• Cystic fibrosis: Misfolded CFTR protein stuck in ER, causing mucus buildup in lungs.

• Neurodegenerative diseases: Abnormal cytoskeletal proteins disrupt cell structure and transport.

Key Terms to Know

• Plasma membrane

• Nucleus

• Cytoplasm

• Cytosol

• Organelle

• Endoplasmic reticulum

• Golgi apparatus

• Lysosome

• Peroxisome

• Mitochondrion

• Chloroplast

• Cytoskeleton

• Extracellular matrix

• Cell junctions

Example: Muscle cells have more mitochondria than skin cells because they require more energy for contraction.

Additional info: Content adapted/modified from Professors Johnson, Webber & Mahrt Biology 189 course materials.