4.3 Eukaryotic Cells

Eukaryotic cells are defined by their membrane-bound organelles, which compartmentalize different functions into specialized structures. Understanding how these organelles work together is central to the rest of cell biology, from energy metabolism to protein synthesis to cell division.

Animal and plant cells share most of their core machinery but differ in a few important ways. Plant cells have a cell wall, chloroplasts, and a large central vacuole. Animal cells lack those structures but have centrioles and lysosomes. Both cell types depend on the same basic organelles for energy production, protein synthesis, and transport.

Eukaryotic Cell Structure and Function

Key features of eukaryotic cells

What sets eukaryotic cells apart from prokaryotic cells is compartmentalization. By enclosing different processes inside membrane-bound organelles, eukaryotic cells can run multiple complex reactions at the same time without interference.

• Membrane-bound organelles divide the cell into functional compartments (nucleus, mitochondria, ER, Golgi apparatus), each with its own internal environment optimized for specific tasks.
• Nucleus houses the cell's DNA within a double-layered nuclear envelope. It controls DNA replication and gene expression, making it the command center of the cell.
• Cytoskeleton is a network of protein filaments that maintains cell shape, enables movement, and facilitates intracellular transport. It has three main components:
  • Microfilaments (actin) support cell shape and are involved in muscle contraction
  • Intermediate filaments provide mechanical strength
  • Microtubules guide organelle movement and form the mitotic spindle during cell division
• Larger size compared to prokaryotes. Eukaryotic cells typically range from 10–100 μm in diameter, which accommodates their more complex internal machinery.

Animal vs. plant cell structures

Both cell types share a nucleus, mitochondria, ER, Golgi apparatus, and ribosomes. The differences come down to how each cell type handles structural support, energy acquisition, and storage.

• Plant cells
  • Cell wall is a rigid layer of cellulose surrounding the plasma membrane. It provides structural support and protects against mechanical stress and osmotic pressure. This is why plant cells don't burst when they absorb water.
  • Chloroplasts contain chlorophyll and other pigments that capture light energy for photosynthesis, converting it into chemical energy stored in glucose.
  • Large central vacuole stores water, nutrients (sugars, ions), and waste products. When filled with water, it generates turgor pressure, which pushes the plasma membrane against the cell wall and keeps the plant rigid.
• Animal cells
  • Centrioles organize microtubules into the mitotic spindle during cell division, helping ensure chromosomes are distributed equally to daughter cells.
  • Lysosomes contain hydrolytic enzymes that break down cellular waste, damaged organelles, and foreign particles like bacteria. Think of them as the cell's recycling and defense system.
  • Animal cells lack a cell wall, chloroplasts, and a large central vacuole. They rely on the cytoskeleton for structural support and obtain energy by ingesting and breaking down organic molecules rather than performing photosynthesis.

Functions of the plasma membrane

The plasma membrane is the boundary between the cell and its environment. It doesn't just act as a wall; it actively controls what gets in and out.

• Phospholipid bilayer forms the membrane's core structure. Two layers of phospholipids arrange themselves with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward. This creates a semipermeable barrier.
• Selective permeability means the membrane allows some molecules through while blocking others, based on size, charge, and polarity. This is how the cell maintains the right concentrations of ions ($Na^+$, $K^+$, $Ca^{2+}$) and nutrients (glucose, amino acids).
• Membrane proteins carry out most of the membrane's active functions:
  • Receptors bind specific signaling molecules (hormones, neurotransmitters) to trigger cellular responses like changes in metabolism or gene expression.
  • Channels and transporters move ions and molecules across the membrane. Passive transport (diffusion, facilitated diffusion) moves molecules down their concentration gradient without energy. Active transport uses ATP to pump molecules against their concentration gradient.
• Fluid mosaic model describes the membrane as a flexible structure where proteins, cholesterol, and other molecules float within and move laterally through the phospholipid bilayer. This fluidity lets the membrane adapt to changing conditions.

Major organelles and their roles

• Endoplasmic reticulum (ER) is a network of membranes extending from the nuclear envelope. It comes in two forms:
  • Rough ER is studded with ribosomes on its surface. Those ribosomes synthesize proteins, which the rough ER then modifies (e.g., adding sugar chains in a process called glycosylation) and ships to the Golgi apparatus.
  • Smooth ER lacks ribosomes. It synthesizes lipids (phospholipids, steroids) and detoxifies harmful substances like drugs and alcohol.
• Golgi apparatus receives proteins and lipids from the ER, then modifies, sorts, and packages them. Finished products get shipped out in vesicles, either to the plasma membrane for secretion (exocytosis) or to other organelles like lysosomes.
• Mitochondria generate most of the cell's ATP through cellular respiration (specifically oxidative phosphorylation). They have their own double membrane and their own DNA, which is key evidence for the endosymbiotic theory.
• Ribosomes translate mRNA into proteins. They can be free-floating in the cytoplasm (making proteins used inside the cell) or attached to the rough ER (making proteins destined for membranes or export).
• Peroxisomes break down fatty acids and detoxify harmful compounds. They convert hydrogen peroxide, a toxic byproduct, into water using the enzyme catalase.
• Lysosomes (primarily in animal cells) contain digestive enzymes (hydrolases) that break down worn-out organelles, cellular debris, and engulfed pathogens.
• Vacuoles are storage compartments. In plant cells, the large central vacuole stores water and solutes and maintains turgor pressure. Animal cells have smaller, more numerous vacuoles (such as endosomes and phagosomes) involved in digestion and material transport.

Cellular transport and communication

Cells constantly move materials in and out, and the endomembrane system coordinates much of this traffic.

• Endocytosis brings material into the cell by wrapping it in a portion of the plasma membrane that pinches inward to form a vesicle. Two common types:
  • Phagocytosis ("cell eating") engulfs large particles like bacteria
  • Pinocytosis ("cell drinking") takes in small droplets of extracellular fluid
• Exocytosis is the reverse: vesicles inside the cell fuse with the plasma membrane and release their contents outside. This is how cells secrete hormones, neurotransmitters, and waste products.
• Endomembrane system is the interconnected network of organelles that work together to synthesize, modify, and transport proteins and lipids. It includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and the plasma membrane. Materials move between these compartments via transport vesicles.
• Endosymbiotic theory explains the evolutionary origin of mitochondria and chloroplasts. According to this theory, ancestral eukaryotic cells engulfed prokaryotic organisms. Instead of being digested, these prokaryotes became endosymbionts, eventually evolving into the organelles we see today. Supporting evidence includes the fact that both mitochondria and chloroplasts have their own double membranes, their own circular DNA, and replicate independently within the cell.