Overview
This lecture covers the structure, functions, and unique features of eukaryotic cells, comparing them with prokaryotic cells and discussing differences between animal and plant cells.
Characteristics of Eukaryotic Cells
- Eukaryotic cells have DNA enclosed in a nucleus bounded by a nuclear envelope.
- They possess membrane-bound organelles for compartmentalized processes.
- Eukaryotic cells are generally larger and more complex than prokaryotic cells.
- Cytoplasm fills the region between the plasma membrane and the nucleus.
Cell Membrane Structure and Function
- The cell membrane protects the cell and maintains homeostasis (internal balance).
- It demonstrates selective permeability—only certain substances can enter or exit.
- Composed of a phospholipid bilayer: hydrophilic heads face outward, hydrophobic tails inward.
- Provides cell shape, flexibility (fluid mosaic model), and enables cell-to-cell communication.
Cytoplasm and Cytoskeleton
- Cytoplasm is a gel-like fluid holding organelles in place.
- Cytosol is the fluid portion of the cytoplasm.
- The cytoskeleton provides structural support and shape, consisting of microtubules (movement/division), microfilaments (shape/muscle movement), and intermediate filaments (stability).
Nucleus Structure and Function
- The nucleus functions as the cell’s control center, containing DNA.
- Nuclear envelope has inner/outer membranes and nuclear pores for exchange of molecules.
- Chromatin is DNA organized with histone proteins; nucleolus is the site of rRNA synthesis.
Ribosomes and Protein Synthesis
- Ribosomes synthesize proteins by translating mRNA instructions from DNA.
- Ribosomes can be free in the cytoplasm or attached to rough endoplasmic reticulum (ER).
- Protein synthesis follows the central dogma: DNA → mRNA → Protein.
Endoplasmic Reticulum (ER)
- Rough ER (with ribosomes): modifies and transports proteins; known as the membrane factory.
- Smooth ER (without ribosomes): synthesizes lipids, metabolizes carbohydrates, detoxifies poisons, and stores calcium.
Golgi Apparatus
- Sorts, modifies, and packages proteins and lipids from the ER.
- Has cis (receiving) and trans (shipping) faces for cargo flow.
- Forms transport vesicles (inside cell) and secretory vesicles (outside cell).
Endomembrane System
- Includes nucleus, rough ER, smooth ER, Golgi apparatus, and plasma membrane.
- Coordinates synthesis, modification, and transport of cellular products.
Other Organelles
- Mitochondria: powerhouses, generate ATP, contain their own DNA (maternal inheritance).
- Lysosomes: digest waste, foreign substances, and worn-out organelles; facilitate apoptosis.
- Peroxisomes: convert toxic hydrogen peroxide into water and oxygen; prevalent in liver and kidney.
- Vacuoles: storage of water and nutrients; large in plant cells, small in animal cells.
- Chloroplasts: site of photosynthesis in plant cells only.
Additional Structures
- Centrosomes: organize microtubules, essential for cell division (mitosis/meiosis).
- Cell wall: present in plants (cellulose), provides structure and protection; absent in animal cells.
- Plasmodesmata: channels between plant cells for communication.
- Cilia/Flagella: appendages for cell movement; flagella in sperm cells (animal).
- Glycocalyx: cell surface structure for signal reception.
Plant vs. Animal Cell Differences
- Plant cells: have cell wall (cellulose), chloroplasts, large central vacuole, plasmodesmata, and store starch.
- Animal cells: lack cell wall/chloroplasts, have small vacuoles, cholesterol in membrane, centrioles/centrosome, and store glycogen.
- Plant cells have regular shapes; animal cells have irregular (amorphous) shapes.
Key Terms & Definitions
- Phospholipid Bilayer — double-layered structure of the cell membrane with hydrophilic heads and hydrophobic tails.
- Cytoplasm — gel-like fluid inside the cell holding organelles.
- Cytoskeleton — protein framework providing structural support (microtubules, microfilaments, intermediate filaments).
- Nucleus — control center containing DNA.
- Ribosome — organelle for protein synthesis.
- Endoplasmic Reticulum (ER) — network for protein/lipid synthesis (rough: protein; smooth: lipid).
- Golgi Apparatus — organelle sorting and packaging proteins/lipids.
- Mitochondrion — generates cell energy (ATP).
- Lysosome — digests cellular waste and invaders.
- Peroxisome — breaks down harmful substances.
- Vacuole — stores substances (large in plants).
- Chloroplast — site of photosynthesis (plants only).
- Cell Wall — structural layer outside plant cell membrane.
- Plasmodesmata — channels for plant cell communication.
- Cilia/Flagella — structures for cell movement.
Action Items / Next Steps
- Watch the provided video differentiating plant and animal cells.
- Review the summary table comparing animal and plant cell structures.
- Prepare for the next lesson on cellular respiration and photosynthesis.