Cell Biology: Structure, Function, and Theory

Cell Biology Basics

• Cell biology studies cells in terms of structure, function, and chemistry.
• Structure refers to the molecular composition of cells.
• Function describes how cells interact to form an organism.
• Chemistry involves molecular interactions essential for life and cell processes.

Importance of Cells

• Cells form the basic unit of life.
• All organisms consist of one or more cells.
• Cells represent the smallest functional unit.
• All life functions occur at the cellular level, including breathing, moving, digestion, and reproduction.

Modern Cell Theory

• All known living organisms consist of cells.
• Unicellular organisms include prokaryotes (e.g., bacteria) and some eukaryotes (e.g., protozoa).
• Multicellular organisms primarily include eukaryotes like fungi, plants, and animals.
• Fungi can be either unicellular or multicellular.
• The cell is the basic structural and functional unit of living things.
• Body parts form from cells, organized as: Organism → organs → tissues → cells.
• Cell structure relates to organism function.
• Cells are fundamentally similar in chemical composition.
• Cells arise from pre-existing cells through division.
• Cells contain hereditary material passed during cell division.
• All energy pathways occur within cells.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells are single cells without a nucleus, while eukaryotic cells have an organized nucleus.
• Prokaryotic DNA is circular and not enclosed by a nuclear membrane ("Naked DNA").
• Eukaryotic DNA is linear.
• Prokaryotic cells lack membrane-bound organelles, while eukaryotic cells have them.
• Prokaryotic cells are smaller (0.1-5 μm), while eukaryotic cells are larger (10-100 μm).
• Examples of prokaryotes: bacteria, mycoplasma, and cyanobacteria.
• Examples of eukaryotes: protozoa, fungi, plant, and animal cells.

Cell Diversity and Function

• Cells vary in size, shape, and type, influenced by their roles within the organism.
• Nerve cells' long extensions facilitate impulse conduction.
• Epithelial cells, flat or plate-like, protect and cover body surfaces.
• Smooth muscle cells are spindle-shaped to enable contraction and relaxation.
• Sperm cells have tails for motility.

Cell Functions

• Metabolism: Cells convert nutrients into energy via metabolic processes.
• Growth and Repair: Cells divide and differentiate to replace damaged or dead cells.
• Homeostasis: Cells maintain stable internal conditions by regulating pH, temperature, and concentrations.
• Communication: Cells use chemical signals and electrical impulses to coordinate responses to stimuli.
• Immune Response: Specialized cells (e.g., white blood cells) defend against pathogens.
• Transport: Cells transport substances like oxygen and nutrients through blood and lymphatic systems.
• Synthesis: Cells synthesize biological molecules, including proteins, lipids, and nucleic acids.
• Reproduction: Some cells (gametes) contribute to the formation of new organisms.
• Storage: Cells store substances like fats and carbohydrates for energy reserves.
• Specialization: Cells (e.g., muscle, nerve, epithelial) perform specific functions.

Cell Composition

• Human cells have complex structures with specific functions.
• The cell membrane is a protective barrier controlling entry and exit.
• Cytoplasm contains organelles and various molecules within the cell.
• The nucleus houses DNA, controls gene expression, and regulates cell division.
• Mitochondria, the cell's "powerhouse," generate ATP (energy) through cellular respiration.
• Ribosomes synthesize proteins and are found in the cytoplasm or attached to the endoplasmic reticulum.
• Rough ER, studded with ribosomes, is involved in protein synthesis and processing.
• Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification.
• The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery.
• Lysosomes contain digestive enzymes to break down waste and cellular debris.
• Peroxisomes metabolize fatty acids and detoxify harmful substances.
• The cytoskeleton provides structural support, shape, and facilitates movement.
• Centrioles are involved in cell division and the formation of cilia and flagella.
• Vesicles transport materials within the cell.

Cell Membrane

• The cell membrane (plasma membrane) is the outer layer separating the cell interior from its environment.
• In animal cells, it is the main separation barrier, whereas plants, yeast, and bacteria have a cell wall for additional support and protection.
• The cell membrane acts as a barrier, regulates substance movement, and provides shape.
• It receives chemical messengers for cell communication and attaches cells to form tissues.

Cell Membrane Constituents

• Phospholipids form the membrane's fundamental building blocks arranged in a bilayer.
• Each phospholipid has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.
• Carbohydrates attach to proteins (glycoproteins) or lipids (glycolipids) for cell recognition, signaling, and adhesion.
• Cytoskeletal components maintain shape, facilitate movement, and create a selectively permeable membrane.
• Cholesterol stabilizes membrane fluidity, making it less permeable to small, water-soluble molecules.

Cholesterol and Membrane Fluidity

• Cholesterol immobilizes hydrocarbon tails of phospholipids.
• It decreases fluidity and increases rigidity.
• Plant cells lack cholesterol and need a cell wall for support.
• Lipids and proteins drift or move within the membrane.
• Fluidity allows the cell to change shape.
• Increased cholesterol decreases fluidity.
• Saturated fatty acids decrease fluidity.
• Unsaturated fatty acid chains increase fluidity.
• Integral proteins are embedded in the lipid bilayer and act as channels or transporters.
• Peripheral proteins attach loosely to the membrane's surfaces, aiding signaling and maintaining cell shape.