Cell Structure

Introduction

Understanding cell structure is fundamental to biology. This topic covers the importance of the surface area to volume ratio, differences between cell types, and the functions of organelles within eukaryotic cells.

Key Vocabulary

• Membrane: The phospholipid bilayer that surrounds a cell, also called the plasmalemma.

• Cytoplasm: The substance inside the cell, consisting of cytosol and dissolved solutes.

• Prokaryote: A cell with no nucleus or internal membranes.

• Eukaryote: A cell with a nucleus and internal membranes.

• Organelle: A structure inside the cell with a specific function (e.g., mitochondria, nucleus).

Cell Theory

Foundational Principles

• All organisms are composed of cells.

• Cells are the smallest living things.

• Cells arise only from pre-existing cells.

• All cells alive today descended from the same ancestral cell(s).

Domains of Life

Cells are classified into three domains based on evolutionary relationships:

Cell Size and Diffusion

Limitations on Cell Size

• Cells rely on diffusion to move materials across membranes and within the cell.

• Diffusion is quick over short distances but inefficient over long distances.

• Most cells are small to maximize efficiency of diffusion.

Fick's Law of Diffusion

The rate of diffusion is determined by several factors:

• Distance

• Concentration gradient

• Surface area

• Temperature (kinetic energy & Brownian motion)

Gradients represent potential energy for transport processes.

Fick's Law equation:

Where is the rate of diffusion, is the diffusion coefficient, is the concentration gradient.

Importance of Surface Area to Volume Ratio

Transport and Metabolism

• Cells must exchange reactants and products with their environment.

• Supply (surface area) must meet demand (volume) for efficient metabolism.

• As cells grow, volume increases faster than surface area, limiting transport efficiency.

Surface Area:Volume Calculations

For a spherical cell:

• Surface area:

• Volume:

As radius increases, volume grows more rapidly than surface area.

Simple diffusion only works if there is enough surface area to supply the cell volume.

Adaptations to Surface Area Limitations

• Cells may adopt shapes that increase surface area (e.g., microvilli in intestinal cells).

• Many small cells are more efficient than fewer large cells.

Types of Cells

Prokaryotic Cells

• No nucleus or internal membrane-bound organelles.

• DNA is located in a region called the nucleoid.

• Domains: Bacteria and Archaea.

• Most have a cell wall made of peptidoglycan (carbohydrates & proteins).

• Cell wall functions: protection, shape maintenance, prevents excessive water uptake.

• Antibiotics (e.g., penicillin, vancomycin) target peptidoglycan synthesis.

Eukaryotic Cells

• Membrane-bound nucleus containing DNA.

• Internal membranes compartmentalize functions into organelles.

• Possess a cytoskeleton for support and movement.

• Examples: Protists, Plants, Animals, Fungi.

Cell Structures and Organelles

Basic Cell Structures (All Cells)

• Cell membrane: phospholipid bilayer controlling entry/exit of substances.

• Cytoplasm: semifluid matrix containing solutes and cytosol.

• DNA: genetic material (nucleoid in prokaryotes, nucleus in eukaryotes).

• Ribosomes: sites of protein synthesis (translation), found in all cell types.

Ribosomes

• Composed of two subunits (protein + rRNA).

• May be free in cytoplasm or attached to internal membranes (in eukaryotes).

Endomembrane System (Eukaryotes)

• Network of membranes within the cytoplasm.

• Divides cell into compartments for specialized functions.

Nucleus

• Double membrane-bound organelle containing genetic information.

• Most eukaryotic cells have a single nucleus.

• Nucleolus: region for rRNA synthesis and ribosome assembly.

• DNA organized into multiple linear chromosomes.

Endoplasmic Reticulum (ER)

• Rough ER (RER): studded with ribosomes; site of protein synthesis.

• Smooth ER (SER): involved in synthesis of carbohydrates, lipids, steroids; storage and detoxification.

Golgi Apparatus

• Stack of internal membranes for processing and distribution of molecules.

• Involved in vesicular transport.

Vesicles

• Membrane-enclosed sacs for isolating materials from cytoplasm.

• Examples: Lysosomes (contain digestive enzymes), Vacuoles (storage, isolation).

Mitochondria & Chloroplasts

• Both have double membranes, their own DNA, and ribosomes.

• Involved in energy metabolism.

• Resemble prokaryotic cells (endosymbiotic theory).

Mitochondria

• Site of aerobic respiration and ATP production.

• Inner membrane contains enzymes for oxidative phosphorylation.

• Divide independently during cell division.

Chloroplasts

• Found in plants and some protists.

• Site of photosynthesis.

• Contain stacks of thylakoids within inner membrane.

• Divide independently during cell division.

Cytoskeleton

• Network of protein fibers in eukaryotes.

• Supports cell shape, anchors organelles, and facilitates movement of materials.

• Dynamic and constantly changing.

Cell Walls & Extracellular Matrix

• Cell walls in plants, fungi, and some protists provide structural support.

• Plant cell walls: cellulose and lignin.

• Fungal cell walls: chitin.

• Animal cells lack cell walls but secrete an extracellular matrix (ECM) of glycoproteins for protection and signaling.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Conclusion

Cell structure is closely linked to function. The surface area to volume ratio limits cell size and influences adaptations. Understanding the differences between prokaryotic and eukaryotic cells, and the roles of organelles, is essential for studying cell biology.