Eukaryotic Cell Structure and Function: Membranes, Organelles, and Cellular Coordination

Concept 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Features Common to All Cells

All cells, whether prokaryotic or eukaryotic, share several fundamental structural features that are essential for life.

• Plasma Membrane (Cell Membrane): A selective barrier that surrounds the cell, controlling the movement of substances in and out.

• Cytosol: The semifluid, jelly-like substance inside all cells where cellular components are suspended.

• Chromosomes: Structures that carry genetic information in the form of DNA.

• Ribosomes: Complexes that synthesize proteins according to instructions from the genes.

• Cell Size: Cells are limited in size by the surface area to volume ratio, which affects the efficiency of transport and communication.

Differences Between Prokaryotic and Eukaryotic Cells

The primary distinction between prokaryotic and eukaryotic cells is the location and organization of their DNA.

• Eukaryotic Cells: Most DNA is contained within a membrane-bound organelle called the nucleus.

• Prokaryotic Cells: DNA is located in a region called the nucleoid, which is not membrane-enclosed.

Features Unique to Specific Cell Types

• Prokaryotic Cells:

  1. Nucleoid

  2. Capsule

• Animal Cells:

  1. Lysosomes

  2. Centrosomes

• Plant Cells:

  1. Central Vacuole

  2. Chloroplasts

  3. Cell Wall

Concept 6.3: Eukaryotic Cell's Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

Nucleus

The nucleus is the control center of the eukaryotic cell, containing most of the cell's genetic material and coordinating activities such as growth, metabolism, and reproduction.

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

• Nuclear Pores: Protein-lined channels in the nuclear envelope that regulate the entry and exit of proteins, RNAs, and other macromolecules.

• Nuclear Lamina: A network of protein filaments that supports the shape of the nucleus and organizes genetic material.

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins. It also controls cell division and the life span of a cell.

• Chromosomes: DNA is organized into discrete units called chromosomes, which carry genetic information. Humans have 46 chromosomes in somatic cells and 23 in gametes.

Ribosomes

Ribosomes are the molecular machines responsible for protein synthesis, translating genetic instructions from mRNA into polypeptide chains.

• Structure: Composed of rRNA and proteins, assembled in the nucleolus.

• Types: Free ribosomes (suspended in cytosol) and bound ribosomes (attached to the endoplasmic reticulum or nuclear envelope).

• Function: Sites of protein synthesis.

Concept 6.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions in the Cell

Endomembrane System Components

The endomembrane system consists of various organelles that work together to modify, package, and transport lipids and proteins.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes glycoproteins and membrane proteins.

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

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Transport Vesicles: Shuttle proteins and other molecules between organelles.

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

• Vacuoles:

  • Food Vacuoles: Store nutrients.

  • Contractile Vacuoles: Pump excess water out of the cell (in protists).

  • Central Vacuole: Found in plant cells; stores water, ions, and other substances.

Concept 6.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Mitochondria

Mitochondria are the powerhouses of the cell, converting energy from nutrients into ATP, the cell's usable energy currency.

• Function: Extraction of energy from sugars, fats, and other fuels to generate ATP.

• Location: Found in nearly all eukaryotic cells, including plants, animals, fungi, and most protists.

• Evolutionary Origin: Thought to have originated from endosymbiotic bacteria.

Chloroplasts

Chloroplasts are organelles found in plants and algae that carry out photosynthesis, converting solar energy into chemical energy.

• Function: Site of photosynthesis.

• Location: Found in plants and algae.

Concept 6.6: The Cytoskeleton Is a Network of Fibers That Organizes Structures and Activities in the Cell

Cytoskeleton Components

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

• Microtubules: Thickest fibers; involved in cell shape, organelle movement, and separation of chromosomes during cell division.

  • Centrosomes/Centrioles (not in plants)

  • Cilia/Flagella

• Microfilaments (Actin Filaments): Thinnest fibers; involved in cell movement and muscle contraction.

• Intermediate Filaments: Provide mechanical strength and help maintain cell integrity.

Concept 6.7: Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Extracellular Matrix (Animal Cells)

The extracellular matrix (ECM) is a network of proteins and carbohydrates outside animal cells that provides structural support and mediates cell signaling.

• Integrins: Proteins that connect the ECM to the cell's cytoskeleton.

Cell Walls (Plant Cells)

Plant cells are surrounded by a rigid cell wall that provides structural support and protection.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells, facilitating communication and adhesion.

• Plasmodesmata (Plant): Channels that connect plant cells, allowing transport of materials.

• Tight Junctions (Animal): Prevent leakage of extracellular fluid between cells.

• Desmosomes (Animal): Anchor cells together, providing mechanical strength.

• Gap Junctions (Animal): Allow direct communication between cells through cytoplasmic channels.

Summary Table: Unique Features of Cell Types

Cell Type	Unique Features
Prokaryotic	Nucleoid, Capsule
Animal	Lysosomes, Centrosomes
Plant	Central Vacuole, Chloroplasts, Cell Wall

Key Equations

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

• ATP Generation (Simplified): $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis (Simplified): $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$