Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Cell Structure and Function

Overview of Prokaryotic and Eukaryotic Cells

Cells are the fundamental units of life, and they are classified as either prokaryotic or eukaryotic based on their structural characteristics. Both types share some features but also have key differences that affect their function and complexity.

• Both Eukaryotic and Prokaryotic cells:

  • Are bounded by a selective barrier called the plasma membrane or cell membrane.

  • Contain a semifluid substance called cytosol in which cellular components are suspended.

  • Have chromosomes that carry genes in the form of DNA.

  • Contain ribosomes, which are complexes that make proteins according to genetic instructions.

• Differences:

  • Eukaryotic cells have DNA in the nucleus, which is bounded by a double membrane.

  • Prokaryotic cells have DNA concentrated in a non-membrane-enclosed region called the nucleoid.

Prokaryotic Cell Components

Prokaryotic cells, such as bacteria and archaea, have a simpler structure compared to eukaryotic cells.

• Fimbriae: Attachment structures on the surface of prokaryotes.

• Nucleoid: Region where the cell's DNA is located (not enclosed by a membrane).

• Ribosomes: Complexes that synthesize proteins.

• Plasma membrane: Membrane enclosing the cytoplasm.

• Cell wall: Rigid structure outside the plasma membrane, providing support and protection.

• Glycocalyx: Outer coating of many prokaryotes, consisting of a capsule or slime layer.

• Flagella: Locomotion organelles of some prokaryotes.

Cytoplasm

The cytoplasm is the region between the nucleus and the plasma membrane in eukaryotes, and the interior of the cell in prokaryotes.

• Eukaryotic cells: Contains organelles with specialized form and function.

• Prokaryotic cells: Membrane-bound structures are absent, but the cytoplasm contains all cellular components.

Plasma Membrane

The plasma membrane is a selective barrier that allows passage of oxygen, nutrients, and wastes to service the entire cell. It is composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Function: Maintains homeostasis by regulating the movement of substances in and out of the cell.

• Structure: Phospholipid bilayer with hydrophilic heads and hydrophobic tails.

Surface Area to Volume Ratio

The ratio of surface area to volume is critical for cell function, as it affects the ability of the cell to exchange materials with its environment.

• Smaller cells have a higher surface area to volume ratio, facilitating efficient exchange.

• Larger organisms have more cells rather than larger cells to maintain this ratio.

• Elongated or flattened cells increase surface area relative to volume.

Animal Cell Structures and Functions

• Flagellum: Motility structure composed of microtubules, allowing movement.

• Centrosome: Region where the cell's microtubules are initiated; contains a pair of centrioles.

• Cytoskeleton: Reinforces cell shape and aids in movement; includes microfilaments, intermediate filaments, and microtubules.

• Microvilli: Membrane projections that increase surface area for absorption.

• Peroxisome: Breaks down fatty acids and detoxifies harmful substances; produces hydrogen peroxide as a by-product and converts it to water.

• Mitochondria: Site of cellular respiration and ATP generation.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies, and stores calcium ions.

• Nucleus:

  • Nuclear Envelope: Double membrane enclosing the nucleus, perforated by pores.

  • Nucleolus: Site of ribosomal RNA synthesis.

  • Chromatin: Material consisting of DNA and proteins.

• Plasma Membrane: Encloses the cell, controls movement of substances.

• Ribosomes: Make proteins from mRNA.

• Golgi Apparatus: Synthesizes, modifies, sorts, and secretes cell products.

• Lysosome: Digestive organelle where macromolecules are hydrolyzed.

• Vacuole: Storage, waste disposal, and hydrolysis of macromolecules.

Plant Cell Structures and Functions

• Nucleus: Contains most of the cell's genes; includes nuclear envelope, nucleolus, and chromatin.

• Golgi Apparatus: Modifies, sorts, and packages products into vesicles.

• Mitochondria: Powerhouse of the cell, site of ATP production.

• Peroxisome: Breaks down fatty acids, detoxifies substances.

• Plasma Membrane: Regulates entry and exit of substances.

• Cell Wall: Rigid layer that maintains cell shape and protects against mechanical damage; made of cellulose.

• Chloroplast: Photosynthetic organelle that converts solar energy to chemical energy in sugar.

• Central Vacuole: Large organelle that stores water, waste products, and helps maintain turgor pressure.

• Plasmodesmata: Channels through cell walls that connect the cytoplasms of adjacent cells.

• Endoplasmic Reticulum: Both rough and smooth types present, similar to animal cells.

• Cytoskeleton: Includes microfilaments and microtubules.

Comparison of Animal and Plant Cells

Feature	Animal Cell	Plant Cell
Cell Wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Central Vacuole	Small or absent	Large, central
Lysosomes	Present	Rare
Plasmodesmata	Absent	Present
Centrioles	Present	Absent in most

Key Terms and Definitions

• Organelle: Specialized structure within a cell that performs a specific function.

• Chromatin: Complex of DNA and proteins that forms chromosomes within the nucleus.

• Ribosome: Molecular machine that synthesizes proteins from amino acids using mRNA as a template.

• Endomembrane System: Group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

Important Equations

• Surface Area of a Sphere: $A = 4\pi r^2$

• Volume of a Sphere: $V = \frac{4}{3}\pi r^3$

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

Summary Table: Major Eukaryotic Cell Organelles and Functions

Organelle	Structure	Function
Nucleus	Double membrane, contains DNA	Genetic control center
Ribosome	RNA and protein complex	Protein synthesis
Endoplasmic Reticulum (Rough/Smooth)	Network of membranes	Protein and lipid synthesis
Golgi Apparatus	Stack of flattened sacs	Modification, sorting, and packaging of proteins
Lysosome	Membrane-bound sac	Digestion of macromolecules
Mitochondrion	Double membrane, own DNA	ATP production (cellular respiration)
Chloroplast (plants)	Double membrane, own DNA	Photosynthesis
Vacuole	Large membrane-bound sac	Storage, waste disposal, turgor pressure (plants)