Cell biology

Form and Function of Cells and Organelles

• Organelle Design: Every organelle within a cell is specifically designed to perform a distinct function, with its structure optimized to facilitate that role.

• Mitochondria Example: These organelles are responsible for converting sugar into energy through cellular respiration.

  • Internal Membrane Structure: The internal membrane is the primary site for respiration. It possesses a compact zigzag shape.

  • Purpose of Shape: This structure increases the total surface area available for chemical reactions while keeping the overall volume of the organelle constant, which allows for more efficient production of $ATP$ (Adenosine Triphosphate) energy molecules.

• Large Central Vacuole (Plants): This organelle stores water and waste products items.

  • Turgor Pressure: When a plant has an adequate supply of water, the vacuole fills and exerts outward pressure against the cell wall.

  • Mechanical Support: This force, known as turgor pressure, allows the plant to remain upright and ensures the leaves stay firm.

  • Wilting: If the vacuole is not full, the cells shrink, resulting in a wilted appearance.

Specialized Human Body Cells

• Muscle Cells: These are used for skeletal movement, food digestion, and blood circulation.

  • Structural Variation: The structure depends on the location and specific demand. Muscle cells in the thigh are significantly longer than those in the heart.

  • Functional Difference: Thigh muscles require slow expansion and contraction to move bones. Heart muscles require less time but greater force to pump blood throughout the body.

• Connective Cells: Found in bones, blood, and adipose (fat) tissue.

  • Bone Cells: These cells contain fibers made of collagen that connect and strengthen them.

  • Adipose Cells: These lack a fibrous structure because they serve as protection and insulation, requiring the ability to move freely within their specific area.

• Nerve Cells: Specialized for communication.

  • Messaging System: Some cells are specialized to receive messages, while others send them. If an individual touches a hot pan, specific cells send a message of "HOT!" to the brain. Other cells decipher this and respond with a "Move!" message back to the muscles.

• Epithelial Cells: Found in the skin, intestines, and blood vessels.

  • Skin Cells: These are thin, flat, and tightly knit to create an essential watertight seal.

  • Intestinal Cells: Some contain microvilli, which are tiny hair-like protrusions that increase the surface area to maximize the absorption of nutrients.

Levels of Biological Organization and Myocytes

• Hierarchy of Organization: Specialized cells group into specialized tissues, which group into specialized organs. These organs form organ systems, and a collection of organ systems forms the total organism.

• Cardiac Example: Cardiac cells have unique organelles permitting expansion and contraction. They are joined and synchronized to provide a steady heartbeat. This thick, strong cardiac muscle tissue allows the heart (the organ) to force blood through the circulatory system.

• Myocytes: These are tubular-shaped cells that make up muscle tissue. Their components include:

  • Contractile protein filaments.

  • Mitochondria for generating $ATP$.

  • Strong cell-to-cell signaling mechanisms.

  • Proximity to nerves and blood vessels for nutrient intake.

Comparative Human Muscle Tissues

• Smooth / Visceral Muscle:

  • Structure: Non-striated fibers, capable of multi-directional contraction, high elasticity, and single-nucleus cells.

  • Location: Found in the lining of the bladder, digestive system, and contractile blood vessels.

  • Control: Involuntary control by the autonomic nervous system.

• Cardiac Muscle:

  • Structure: Striated fibers, coordinated contraction between right and left ventricles, and cells with one or two nuclei.

  • Location: The primary tissue of the heart.

  • Control: Responds to electrochemical signals originating from the $SA$ (Sinoatrial) node in the right atrium.

• Skeletal Muscle:

  • Structure: Striated fibers organized into fascicles (bundles), interspersed with connective tissue, and multi-nucleated cells.

  • Location: Muscles attached directly to the skeletal system.

  • Control: Voluntary control by the somatic nervous system.

Epithelial Tissue Classification and Surfaces

• General Functions: Makes up organ outer surfaces and skin; regulates moisture; acts as a primary barrier against pathogens.

• Tight Junctions: Adjacent cells are connected by tight junctions, allowing nutrients and waste to diffuse through layers since they lack a direct blood supply from capillaries.

• Cell Surfaces:

  • Basal Side: Faces inward toward internal tissue layers.

  • Apical Side: Faces outward toward the environment or an internal cavity.

• Classification by Shape:

  • Squamous: Flat, thin cells with high permeability, suited for rapid diffusion. Found in alveoli, glomeruli, and capillary linings.

  • Cuboidal: Cube-shaped with large nuclei; attached to the basal surface. Functions in protection, secretion, and absorption. Found in ovaries, kidneys, bronchi, and the thyroid.

  • Columnar: Vertical column shapes, sometimes ciliated. Functions in protection and secretion. Found in the digestive tract, bladder, uterus, and upper respiratory tract.

• Classification by Stratification:

  • Simple: A single layer of cells in direct contact with the basement membrane. Specialized for diffusion and filtration.

  • Stratified: Multiple layers of stacked cells. Specialized for protection.

  • Pseudostratified: Appears stratified due to varied cell sizes/shapes but is actually a single layer. Often secretes and moves mucus (e.g., upper respiratory system).

Plant Tissue Systems

• Dermal Tissue: The outer plant covering.

  • Cuticle: A waxy layer on stems and leaves used for protection and water retention.

  • Stomata: Openings on the underside of leaves for gas exchange ($CO_2$ and $O_2$). They facilitate transpiration (release of water vapor). Stomata close at night to prevent water loss.

  • Root Hairs: Small porous protrusions on roots that maximize surface area for water absorption.

• Ground Tissue: Located between dermal and vascular layers.

  • Mesophyll: Found in leaves, contains chloroplast-rich cells and a spongy layer for gas diffusion.

  • Meristem: Regions or nodes in stems and roots containing undifferentiated cells for new growth and injury repair.

• Vascular Tissue: The transport system.

  • Xylem: Transports water and dissolved nutrients from roots upward. Made of non-living cells called tracheids and located toward the center of the bundle.

  • Phloem: Transports photosynthesis products (sugars) to other plant areas. Made of living cells called sieve elements and located toward the outside of the bundle.

Cell Growth Constraints: Surface Area and Volume

• Dimensional Growth Differential: Surface area is a function of two dimensions ($d^2$), while volume is a function of three dimensions ($d^3$).

• Exponential Growth: When a cell grows, its volume increases exponentially faster than its surface area.

• Diffusion Efficiency: A high surface-area-to-volume ratio is necessary for rapid diffusion. If a cell grows too large, it can no longer transport nutrients and waste efficiently.

• Division Requirement: Once a cell exceeds its optimum surface-area-to-volume ratio, it must undergo division.

The Cell Cycle and Mitosis (PMAT)

• Interphase (Preparation Phase):

  • $G1$ Stage: Primary growth and organelle replication.

  • $S$ Stage: DNA synthesis; a complete copy of the DNA is made.

  • $G2$ Stage: Additional growth and final replication of organelles.

• Mitotic Phase (Division Phase):

  • Mitosis: Division of DNA into four steps.

    1. Prophase: Chromosomes condense; the spindle forms; nucleolus disappears; centrioles move to opposite sides.

    2. Prometaphase: The nuclear membrane disappears.

    3. Metaphase: Chromosomes line up in the "Middle" (remember $M$ for Middle) captured by the spindle.

    4. Anaphase: Spindles pull chromosomes "Apart" (remember $A$ for Apart) into two groups of chromatids.

    5. Telophase: Two nuclear envelopes form around the chromosome sets (remember $T$ for "Two" nuclei); chromosomes relax into chromatin.

  • Cytokinesis: The division of the cytoplasm.

    • Animals: The cell pinches via a cleavage furrow.

    • Plants: A cell plate forms in the middle to create a new cell wall.

• Post-Mitosis: Resulting daughter cells are genetically identical to the parent. Cells enter $G0$ phase to work (e.g., liver cell) or restart the cycle.

Cell Cycle Regulation, Checkpoints, and Cancer

• Regulators: Cyclins and kinase proteins control the cycle.

• Checkpoints:

  • $G1$ Checkpoint / Restriction Point: Initiates the first growth phase.

  • Spindle / $M$ Checkpoint: Ensures each chromosome is secured to a spindle fiber before anaphase to prevent separation errors.

  • $G2$ Checkpoint: Final check of DNA replication and repair of DNA damage before entering mitosis.

• Cancer: A disease where growth control malfunctions, leading to mutations and DNA damage.

  • Cancer cells do not respond to the $G1$ checkpoint regarding space/nutrients.

  • Cancer cells may bypass the $G2$ checkpoint, avoiding apoptosis (cell death) despite severe DNA damage.

Stem Cells and Cellular Differentiation

• Cellular Differentiation: Process where cells become specialized. Although all somatic cells share the exact same DNA sequence, their diversity arises from gene expression (using only a subset of genes).

• Stem Cells: Undifferentiated cells capable of becoming any cell type.

• Plant Stem Cells: Located in apical and lateral meristems (within the ground layer). These cells do not age and allow indefinite growth.

• Animal Stem Cells:

  • Hematopoietic Stem Cells: Found in bone marrow; limited to forming blood components (platelets, white blood cells, red blood cells).

  • Adult Stem Cells: Found in brain, skin, teeth, muscles, etc. These are multipotent (limited differentiation capabilities).

  • Embryonic Stem Cells: Formed $4-5$ days after fertilization. These are pluripotent (can become any cell type in the embryo).

• Controversy: Scientific potential for treating diseases vs. moral considerations of using human embryos.

Reproduction and Somatic vs. Sex Cells

• Asexual Reproduction: Offspring are produced by a single parent via mitosis. They are genetically identical clones. It is fast and requires no partner (e.g., bacteria, hydras budding).

• Somatic Cells: General body cells (nerve, blood, skin, muscle). They contain a full set of DNA. Mutations here are not passed to offspring.

• Sex Cells (Gametes): Sperm (male) and egg (female) cells.

  • DNA Content: Contain half ($\frac{1}{2}$) the amount of DNA found in somatic cells.

  • Fertilization: Combining two sex cells restores a full set of DNA in the offspring.

  • Inheritance: Mutations in sex cells are passed down to offspring.