Robert Hooke discovered cells! He published his observations in "Micrographia" (1665). However, Hooke observed dead cells (cork cells are the cell walls of dead plant cells). Antonie van Leeuwenhoek, a Dutch scientist, was the first to observe living cells (bacteria, protozoa, red blood cells) in the 1670s. The cell theory was developed later by Matthias Schleiden (plants are made of cells, 1838), Theodor Schwann (animals are made of cells, 1839), and Rudolf Virchow (cells come from pre-existing cells, 1855). Cells are incredibly diverse – there are over 200 different types of cells in the human body alone. The smallest cells (bacteria) are about 0.2 micrometers (0.0002 mm). The largest cell in the human body is the egg cell (ovum), which is about the size of a grain of sand (0.12 mm). The longest cells are nerve cells (neurons), which can be over 1 meter long.

Eukaryotic cells have a nucleus! The nucleus stores DNA (chromosomes) and controls the cell's activities. Eukaryotes are generally larger and more complex than prokaryotes. Eukaryotic cells also contain membrane-bound organelles like mitochondria (energy production), endoplasmic reticulum (protein synthesis), Golgi apparatus (packaging), and lysosomes (digestion). Prokaryotes (bacteria) are much simpler; they have no nucleus or membrane-bound organelles. However, they can still carry out all life processes. Some prokaryotes have plasmids (small rings of extra DNA) that can be exchanged between bacteria (how antibiotic resistance spreads). The evolution of the nucleus and organelles was a major step in the history of life. The oldest fossils of eukaryotic cells date to about 1.7 billion years ago. All multicellular organisms (including humans) are eukaryotes. Prokaryotes are far more numerous – there are more bacteria in your gut than there are stars in the Milky Way galaxy (about 100 trillion bacteria).

The nucleus is the control center! It directs all cellular activities, including growth, metabolism, and reproduction (cell division). DNA contains the instructions for making proteins (which do most of the work in cells). RNA (ribonucleic acid) is a copy of the DNA instructions that leaves the nucleus to direct protein synthesis. The nucleus also stores the genetic information that is passed from parent to offspring. In eukaryotic cells, the nucleus is the most prominent organelle. Some cells (like red blood cells) lose their nucleus during maturation (mature human red blood cells have no nucleus). In plant cells, the nucleus is usually pushed to the side by the large central vacuole. The study of the nucleus is called nuclear biology. The nucleus was first observed by Antonie van Leeuwenhoek in the 1700s but was not fully understood until the 19th century. Nuclear pores regulate what enters and exits the nucleus (proteins, RNA, signaling molecules).

Mitochondria produce ATP! ATP is the "energy currency" of the cell. Cellular respiration: C₆H₁₂O₆ (glucose) + 6O₂ (oxygen) → 6CO₂ (carbon dioxide) + 6H₂O (water) + ATP (energy). Mitochondria have an inner membrane folded into cristae (to increase surface area) and a matrix (inner space). The number of mitochondria varies by cell type: liver cells have about 1,000-2,000; heart muscle cells have about 5,000; egg cells have hundreds of thousands. Mitochondria are passed from mother to offspring (maternal inheritance) because the sperm's mitochondria are usually destroyed after fertilization. This allows scientists to trace maternal lineage through mitochondrial DNA. Diseases caused by mitochondrial DNA mutations are inherited from the mother. The endosymbiotic theory (Lynn Margulis) proposes that mitochondria originated from free-living bacteria that were engulfed by a larger cell and formed a symbiotic relationship. Evidence: mitochondria have their own DNA (circular, like bacteria), their own ribosomes (similar to bacterial ribosomes), and reproduce independently of the cell.

Chloroplasts are unique to plant cells! Chloroplasts contain chlorophyll (green pigment) that captures light energy. Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. The cell wall (cellulose) gives plant cells their rigid shape (you can see the rectangular shape of plant cells under a microscope). Animal cells lack cell walls and are more flexible (allowing them to change shape). The large central vacuole in plant cells can take up to 90% of the cell's volume; it stores water, nutrients, and waste. When a plant is wilted (needs water), the vacuoles shrink, reducing turgor pressure. When well-watered, vacuoles swell, making the plant stiff. Animal cells have smaller vacuoles (if any) for storing materials. Plant cells also have plasmodesmata (channels through cell walls for communication between adjacent plant cells). Animal cells have gap junctions for similar purposes. Both plant and animal cells have mitochondria (unlike the common myth that plant cells don't need mitochondria – they do for cellular respiration at night and in non-green tissues).

Amoeba is unicellular! An amoeba is a single cell that can move, eat, reproduce, and respond to its environment. Some unicellular organisms form colonies (groups of similar cells), but each cell remains independent. Multicellular organisms have cells that are interdependent; they cannot survive alone. The advantage of multicellularity is specialization: cells can perform specific tasks (e.g., nerve cells conduct signals, red blood cells carry oxygen, muscle cells contract). This allows multicellular organisms to grow much larger than any unicellular organism. The largest unicellular organisms are certain algae (e.g., Caulerpa taxifolia, which can be several feet long but is a single cell with many nuclei). The bacterium Escherichia coli (E. coli) is unicellular; about 1,000 of them could fit in the period at the end of this sentence. Unicellular organisms reproduce by binary fission (splitting in two). Some unicellular organisms can form cysts to survive harsh conditions. The first life on Earth (about 3.5 billion years ago) was unicellular. Multicellular life did not appear until about 600 million years ago.

Organs come after tissues! Tissues combine to form organs (e.g., the stomach is an organ made of epithelial tissue, muscle tissue, and connective tissue). Multiple organs work together in organ systems (e.g., the digestive system includes the mouth, esophagus, stomach, small intestine, large intestine, liver, pancreas). The organism is the highest level. Plants also have organization: cells (plant cells) → tissues (dermal, vascular, ground) → organs (leaves, stems, roots) → organ systems (shoot system, root system) → organism. The human body has 11 major organ systems: circulatory, respiratory, digestive, excretory, nervous, endocrine, immune, skeletal, muscular, integumentary (skin), and reproductive. Each organ system depends on others; for example, the circulatory system transports oxygen from the respiratory system and nutrients from the digestive system to all cells. The failure of one organ system can cause a cascade of failures, leading to illness or death. Understanding levels of organization is fundamental to anatomy and physiology.

Selective permeability means the membrane controls what passes through! Small nonpolar molecules (O₂, CO₂) diffuse freely. Water (polar) diffuses slowly through the lipid bilayer but passes more quickly through aquaporins (water channels). Larger molecules (glucose) and ions (Na⁺, K⁺, Ca²⁺) require transport proteins or active transport (using ATP). This selectivity allows the cell to maintain a stable internal environment (homeostasis). For example, nerve cells maintain a voltage (resting potential) by pumping sodium ions out and potassium ions in. The cell membrane also contains receptor proteins that detect chemical signals (hormones, neurotransmitters). The fluid mosaic model (Singer and Nicolson, 1972) describes the membrane as a fluid (phospholipids can move laterally) with proteins embedded (like tiles in a mosaic). The cell membrane is also involved in cell communication, cell adhesion, and endocytosis/exocytosis (bringing materials in or out via vesicles). In plant cells, the cell wall (outside the membrane) provides structural support but is fully permeable (allows all molecules to pass).

Chlorophyll is the green pigment! There are two main types: chlorophyll a (blue-green) and chlorophyll b (yellow-green). They absorb light in the blue (430 nm) and red (660 nm) wavelengths and reflect green light (making plants appear green). Plants also contain accessory pigments (carotenoids, xanthophylls) that capture other wavelengths and give fall leaves their orange and yellow colors. Photosynthesis has two stages: light-dependent reactions (light energy converted to chemical energy, producing ATP and NADPH) and the Calvin cycle (carbon fixation, using CO₂ to make glucose). The oxygen released comes from water (H₂O), not CO₂. Photosynthesis produces about 100 billion tons of glucose per year globally, and the oxygen we breathe comes from photosynthetic organisms. Without photosynthesis, there would be no oxygen in the atmosphere and no food for most life on Earth. Photosynthesis evolved about 2.4 billion years ago in cyanobacteria, leading to the Great Oxygenation Event that transformed Earth's atmosphere.

Stem cells are unspecialized and can become specialized! In embryos, stem cells give rise to all the different cell types (skin, nerve, muscle, blood, etc.). In adults, stem cells are found in bone marrow, skin, and other tissues; they replace damaged or worn-out cells. For example, hematopoietic stem cells in bone marrow produce all the types of blood cells (red blood cells, white blood cells, platelets). The first stem cell therapy was a bone marrow transplant (performed in 1956). Induced pluripotent stem cells (iPSCs) are adult cells that have been genetically reprogrammed to act like embryonic stem cells (Shinya Yamanaka, Nobel Prize 2012). This avoids the ethical issues of using embryonic stem cells. Stem cell research is controversial because some methods involve destroying human embryos. Scientists are exploring alternative sources. Stem cells hold promise for regenerative medicine: growing replacement organs, repairing spinal cords, and treating degenerative diseases. However, many challenges remain (uncontrolled growth could lead to tumors).