Comparative Analysis of Eukaryotic and Prokaryotic Cells
11a. Plant, Animal, and Microbial Cells
Plant and animal cells are eukaryotic, featuring a true nucleus and membrane-bound organelles, whereas most microbial (bacterial) cells are prokaryotic, lacking a true nucleus and typical organelles.
Plant cells possess a cellulose cell wall, chloroplasts, a large central vacuole, and plasmodesmata. Animal cells lack a cell wall and chloroplasts but contain lysosomes, centrioles, and a prominent extracellular matrix. Bacterial cells have a peptidoglycan cell wall, 70S ribosomes, a nucleoid, plasmids, flagella or pili, and often a capsule.
Cellular Functions
Plant cells perform photosynthesis, storage, and mechanical support. Animal cells specialize in contraction, impulse conduction, secretion, and immune defense. Microbial cells perform rapid growth, metabolism, nitrogen fixation, or pathogenesis and act as decomposers in ecosystems.
11b. Eukaryotic Cell Diagram Guidelines
Draw a large circle for the plasma membrane and show: the nucleus (nuclear envelope, nucleolus, chromatin), mitochondria, endoplasmic reticulum (rough with ribosomes and smooth), Golgi apparatus, lysosomes, peroxisomes, ribosomes, and cytoplasm. If representing a plant cell, include the chloroplast, large vacuole, and cell wall.
Label each organelle and write one key function beside it, such as:
- Mitochondrion: Site of ATP production.
- Golgi body: Packaging and secretion.
- Rough ER: Protein synthesis.
- Lysosome: Intracellular digestion.
12a. Plasma Membrane Structure and Transport
The plasma membrane is a fluid mosaic: a phospholipid bilayer with embedded integral and peripheral proteins, cholesterol, and carbohydrate chains forming glycolipids and glycoproteins.
Phospholipids create a selectively permeable hydrophobic barrier. Proteins act as channels, carriers, receptors, and enzymes; cholesterol modulates fluidity, and carbohydrates mediate cell recognition and adhesion.
Transport Functions
Passive transport includes simple diffusion of small non-polar molecules, osmosis of water, and facilitated diffusion via specific channel or carrier proteins, all moving down concentration gradients without energy.
Active transport uses ATP-driven pumps or cotransporters to move ions and solutes against gradients. Bulk transport occurs by endocytosis and exocytosis, enabling the uptake of macromolecules and secretion.
12b. Chloroplast Structure and Functions
Chloroplasts are double-membrane organelles in plant cells containing an internal system of thylakoid membranes stacked into grana, embedded in fluid stroma with circular DNA and 70S ribosomes.
Thylakoid membranes contain chlorophyll and other pigments and are the site of light reactions, where light energy drives ATP and NADPH formation. The stroma houses enzymes of the Calvin cycle that fix carbon dioxide into carbohydrates.
Additional Roles
Chloroplasts store starch, synthesize some amino acids and lipids, and can divide independently, supporting their origin from endosymbiotic bacteria.
13a. Plasmodesmata and Gap Junctions
Plasmodesmata are microscopic channels crossing plant cell walls that connect the cytoplasm and often the endoplasmic reticulum of adjacent cells, forming a symplastic continuum.
These channels allow the movement of ions, sugars, hormones, RNAs, and some proteins between cells, coordinating development and responses to signals throughout plant tissues.
Gap Junctions
Gap junctions in animal cells are clusters of intercellular channels formed by connexon proteins that align to connect the cytoplasm of neighboring cells.
They permit the direct passage of small molecules and ions, enabling electrical and metabolic coupling—for example, in cardiac muscle, smooth muscle, and some epithelia—which is essential for coordinated activity.
13b. Extracellular Matrix of Animal Cells
The Extracellular Matrix (ECM) is a complex network of fibrous proteins such as collagens and elastin, adhesive glycoproteins like fibronectin and laminin, and hydrated proteoglycan gels filling intercellular spaces.
It includes the interstitial matrix between cells and specialized basement membranes underlying epithelia; its composition varies among tissues such as bone, cartilage, loose connective tissue, and blood.
ECM Functions
The ECM provides mechanical support, determines tissue elasticity and rigidity, and anchors cells via integrins, influencing their shape and polarity.
It also binds and presents growth factors, regulates cell adhesion, migration, proliferation, and differentiation, thereby playing a central role in development, wound healing, and disease.
14a. Stages of Mitosis
- Prophase: Chromatin condenses into visible chromosomes consisting of sister chromatids joined at centromeres; the nuclear envelope breaks down and the mitotic spindle forms as centrosomes move to opposite poles.
- Metaphase: Spindle fibers attach to kinetochores and align chromosomes along the metaphase plate, ensuring each sister chromatid is linked to opposite poles.
- Anaphase: Centromeres split and spindle fibers shorten, pulling sister chromatids (now daughter chromosomes) to opposite poles of the cell.
- Telophase: Chromosomes reach poles and decondense, nuclear envelopes re-form around each set, the spindle disassembles, and cytokinesis usually divides the cytoplasm to yield two genetically identical daughter cells.
14b. Cell-Cycle Checkpoints and Significance
The cell cycle has key checkpoints at G1, G2, and during mitosis (spindle checkpoint), where surveillance mechanisms monitor DNA integrity, cell size, nutrient status, and spindle attachment.
At the G1 checkpoint (restriction point), cells decide whether to enter S phase; damaged DNA or insufficient growth signals cause arrest or entry into G0, preventing the replication of faulty genomes.
The G2/M checkpoint blocks entry into mitosis if DNA replication is incomplete or damage persists, with regulators such as CDK1–cyclin B, Wee1, and Cdc25 controlling progression.
The spindle checkpoint during the metaphase–anaphase transition ensures all chromosomes are correctly attached to spindle microtubules before allowing chromatid separation, thereby avoiding aneuploidy and preserving genomic stability.
15a. Modes of Cell Locomotion
- Amoeboid movement: Occurs in many protozoa and leukocytes and involves the formation of pseudopodia by localized actin polymerization and cytoplasmic streaming; adhesion at the front and detachment at the rear propel the cell.
- Ciliary movement: Uses many short, hair-like cilia with a 9+2 arrangement of microtubules and dynein arms; rhythmic effective and recovery strokes of cilia move cells or fluids, as in Paramecium or human respiratory epithelium.
- Flagellar movement: Uses fewer, longer flagella with a similar 9+2 axoneme; bending waves generated by dynein-driven sliding of microtubules or rotary motion in bacteria propel sperm cells or unicellular algae through liquids.
15b. Cancer Cells: Features and Significance
Cancer cells show uncontrolled proliferation due to mutations that activate oncogenes and inactivate tumor-suppressor genes, allowing them to bypass normal cell-cycle checkpoints and apoptosis.
They exhibit genetic instability with chromosomal aberrations, altered metabolism, reduced dependence on growth factors, the ability to evade immune surveillance, and often changes in shape, adhesion, and stiffness.
Biological Significance
Cancer cells invade surrounding tissues by degrading the extracellular matrix and then enter the blood or lymph to establish secondary tumors (metastases) at distant sites.
Such uncontrolled growth and metastasis disrupt normal tissue architecture and organ function, ultimately leading to morbidity and mortality if not controlled by therapy.