BIO 201 (chapter 3)

Chapter 3 - Cellular Level

INTRODUCTION

  • Cell: Basic, living, structural, and functional unit of the body.

  • Cytology: The study of cell structure.

  • Cell Physiology: The study of cell function.

PARTS of a CELL

  1. Plasma Membrane

  2. Cytoplasm

  3. Nucleus

PLASMA MEMBRANE

Structure

  • Basic Structure: Made of two layers of fat molecules called phospholipids.

  • Significance of Lipids: Align in a specific formation because of van der Waals forces.

Components

  • Other Molecules in Membranes:

    • Proteins

    • Sugar chains

    • Cholesterol

Protein Structure

  • Proteins are long chains of molecules called amino acids.

  • They can fold into complex shapes.

  • Amino Acids: There are 20 different amino acids, leading to unique protein sequences and functions.

Protein Structures

  • Secondary Structures:

    • Some proteins spiral to form an alpha helix, causing R-groups to point outward.

  • Anchoring Proteins:

    • Proteins can anchor to the membrane using one or more greased alpha helixes.

Functions of Proteins in Membranes

  • 1) Transporters

  • 2) Enzymes

  • 3) Receptors

  • 4) Cell identity

  • 5) Cell adhesion

  • 6) Cytoskeleton attachment

Membrane Permeability

  • What can get across:

    • Small, nonpolar molecules (including some gases).

    • Small polar molecules like water can also cross.

Aquaporins

  • A type of channel protein that facilitates water transport across the membrane.

  • Water molecules align single-file to pass through very tiny openings.

CONCENTRATION GRADIENTS

Definition

  • Concentration Gradient: A difference in the amount of a substance across space.

  • Graphical Representation: Concentration is plotted against distance, indicating the slope and direction of motion.

Importance

  • Gradients exist across cell membranes, influencing the direction of net movement.

  • Position of Molecules: Higher concentration areas move towards lower concentration areas, establishing gradients across the plasma membrane.

TRANSPORT ACROSS THE PLASMA MEMBRANE

Types of Transport

  • Passive Transport:

    • Simple Diffusion

    • Osmosis

    • Facilitated Diffusion

    • Channel-mediated facilitated diffusion

    • Carrier-mediated facilitated diffusion

  • Active Transport:

    • Primary Active Transport

    • Secondary Active Transport

  • Passive transport allows molecules to move down their concentration gradients, while active transport moves them up a gradient and requires energy.

Principles of Diffusion

  • Definition: Random mixing of particles in a solution due to their kinetic energy.

  • Movement: Particles move down their concentration gradient until achieving a steady state.

Factors Influencing Rate of Diffusion

  • Fick’s Law of Diffusion: Influenced by:

    • The magnitude of concentration gradient

    • The permeability of the membrane to a substance

    • The surface area of the membrane

    • The molecular weight of the substance

    • The distance through which diffusion occurs.

Applications of Diffusion in the Human Body

  • Examples:

    • Gas exchange in the lungs (O2 and CO2) with approximate concentrations of O2 (20.95%) and CO2 (0.04%).

    • Movement of ions (e.g., Sodium and Potassium ions) through membrane channels.

OSMOSIS

Definition

  • Osmosis: The movement of water across a membrane caused by differences in solute concentrations.

  • Osmotic Pressure: The force exerted by pure water across a semi-permeable membrane.

Tonicity

  • Tonicity: The effect a solution has on cell volume.

    • Isotonic Solution: Equal concentration of nonpenetrating solutes as normal cells.

    • Hypotonic Solution: Lower concentration of nonpenetrating solutes compared to cells.

    • Hypertonic Solution: Higher concentration of nonpenetrating solutes.

Examples and Effects on Cells

  • Normal RBC shape in isotonic solutions, hemolysis in hypotonic solutions, and crenation in hypertonic solutions.

FACILITATED DIFFUSION

Mechanisms

  • Channel Mediated:

    • Voltage-gated Channels

    • Ligand-gated Channels

  • Carrier Mediated: Requires a protein that changes shape to move molecules across the membrane.

Specific Examples

  • Voltage-gated ion channels respond to electrical changes; ligand-gated channels respond to signaling molecules, like neurotransmitters (e.g., acetylcholine).

ACTIVE TRANSPORT

Definition

  • Active Transport: Movement against concentration gradient requiring a transporter protein and energy.

Types of Active Transport

  1. Primary Active Transport: Directly uses ATP for molecule movement.

  2. Secondary Active Transport: Utilizes a concentration gradient established by primary active transport.

  3. Vesicular Transport: Materials move in/out of cells wrapped in membranes (e.g., endocytosis and exocytosis).

Example: Na+ K+ Pump

  • Operation: Pumps 3 Na+ ions out and 2 K+ ions in per ATP used; vital for maintaining membrane potential and cellular homeostasis.

ORGANELLES

Overview

  • Cytosol: The semifluid part of cytoplasm containing inclusions, organelles, and dissolved solutes.

  • Organelles: Membrane-bound structures performing specific functions (e.g., growth, maintenance, reproduction).

Major Organelles

  • Endoplasmic Reticulum (ER):

    • Rough ER: Protein folding and modification.

    • Smooth ER: Lipid production and calcium release.

  • Golgi Complex: Sorts, processes, and delivers proteins and lipids.

  • Lysosomes: Contain digestive enzymes, involved in digestion and recycling orgnanelles.

  • Mitochondria: Sites of ATP production through cellular respiration.

  • Ribosomes: Assemble proteins; not membrane-bound.

NUCLEUS

  • Contains 46 DNA molecules (chromosomes) with instructions for protein synthesis.

  • Processes Involved:

    • Transcription: Temporary RNA copy is made.

    • Translation: Ribosomes read mRNA to synthesize proteins.

CELL DIVISION

  • Mitosis vs Meiosis:

    • Mitosis: Nuclear division with cytokinesis for body cells.

    • Meiosis: Reduces chromosome number to produce gametes.

What Can Go Wrong?

  • Cancer: Abnormal, uncontrolled cell division.

    • Types of Cancer:

    • Carcinomas: Arise from epithelial cells.

    • Melanomas: From melanocytes.

    • Sarcomas: From muscle/connective tissues.

    • Leukemia: Cancer of blood-forming organs.

    • Lymphoma: Cancer of lymphatic tissue.

  • Mechanism of Cancer Spread:

    • Angiogenesis occurs, allowing cancer cells to metastasize by entering blood or lymph.

Causes of Cancer

  • Carcinogens, exposure to radiation, and mutations in growth-regulating genes lead to hyperplasia and tumor formation.

INTRODUCTION

  • Cell: The fundamental, living, structural, and functional unit of all organisms. It is the smallest unit capable of performing all life processes.

  • Cytology: The branch of biology concerned with the structure and function of plant and animal cells.

  • Cell Physiology: The study of the chemical and physical processes that occur within cells and how they interact with their environment.

THE PLASMA MEMBRANE

Fluid Mosaic Model

  • Basic Structure: A mosaic of proteins floating in or on the fluid lipid bilayer.

  • Lipid Bilayer: Composed of two back-to-back layers of three types of lipid molecules: phospholipids, cholesterol, and glycolipids.

    • Phospholipids: Comprise approximately 75\% of membrane lipids. They are amphipathic, having a polar (hydrophilic) head and nonpolar (hydrophobic) tails.

    • Cholesterol: (20\% of lipids) Interspersed among phospholipids; it provides structural integrity and fluidity at different temperatures.

    • Glycolipids: (5\% of lipids) Found only on the layer facing the extracellular fluid; important for cell-to-cell recognition.

Membrane Proteins

  • Integral (Transmembrane) Proteins: Extend through the lipid bilayer; most are glycoproteins with carbohydrate groups attached to the ends facing the ECF.

  • Peripheral Proteins: Attached to the polar heads of membrane lipids or to integral proteins at the inner or outer surface of the membrane.

  • Functional Categories:

    • 1) Ion Channels: Pores through which specific ions (like Na^{+} or K^{+}) can flow.

    • 2) Carriers (Transporters): Move a polar substance or ion across the membrane by changing shape.

    • 3) Receptors: Serve as cellular recognition sites; each type recognizes and binds a specific type of molecule (ligand).

    • 4) Enzymes: Catalyze specific chemical reactions at the inside or outside surface of the cell.

    • 5) Linkers: Anchor proteins in the plasma membranes of neighboring cells or to protein filaments inside and outside the cell.

    • 6) Cell-Identity Markers: (e.g., ABO blood type markers) Allow a cell to recognize other cells.

TRANSPORT PROCESSES

Passive Transport

  • Simple Diffusion: Movement of substances through the lipid bilayer without the help of membrane transport proteins (e.g., O{2}, CO{2}, nitrogen, fatty acids, steroids, and fat-soluble vitamins A, D, E, and K).

  • Facilitated Diffusion:

    • Channel-mediated: Movement of ions through specific water-filled cytosol-filled channels.

    • Carrier-mediated: Movement of glucose, fructose, and galactose.

  • Osmosis: The net movement of a solvent (water) through a selectively permeable membrane from an area of higher water concentration to lower water concentration.

    • Hydrostatic Pressure: The pressure exerted by a liquid at rest.

    • Osmotic Pressure: The pressure required to stop osmosis; proportional to the concentration of solute particles that cannot cross the membrane.

Active Transport

  • Primary Active Transport: Energy derived from hydrolysis of ATP changes the shape of a carrier protein, which "pumps" a substance across a plasma membrane against its concentration gradient.

    • Sodium-Potassium Pump: Expels 3 Na^{+} ions and brings in 2 K^{+} ions. This maintains a low concentration of Na^{+} and a high concentration of K^{+} in the cytosol.

  • Secondary Active Transport: Uses energy stored in a Na^{+} or H^{+} concentration gradient to drive other substances across the membrane against their own gradients.

    • Symporters: Move two substances in the same direction.

    • Antiporters: Move two substances in opposite directions.

  • Vesicular Transport:

    • Endocytosis: Receptor-mediated endocytosis, phagocytosis ("cell eating"), and bulk-phase endocytosis (pinocytosis, "cell drinking").

    • Exocytosis: Release of materials from a cell by fusion of vesicles with the plasma membrane.

CYTOPLASM AND ORGANELLES

  • Cytosol: Intracellular fluid; the site of many chemical reactions required for existence.

  • Cytoskeleton: A network of protein filaments that extends throughout the cytosol.

    • Microfilaments: Involved in muscle contraction and cell division.

    • Intermediate Filaments: Stabilize the position of organelles.

    • Microtubules: Determine cell shape and help in the movement of organelles.

  • Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes; synthesizes secretory proteins and membrane molecules.

    • Smooth ER: No ribosomes; synthesizes fatty acids and steroids (estrogen, testosterone) and detoxifies chemicals.

  • Golgi Complex: Consists of 3 to 20 flattened, membranous sacs called cisternae. It modifies, sorts, and packages proteins into vesicles for transport.

  • Mitochondria: The "powerhouses" of the cell. They generate most of the ATP through aerobic respiration. They contain their own DNA and can self-replicate.

NUCLEUS AND CELL DIVISION

  • Nucleus: Shielded by a double membrane (nuclear envelope). Contains nucleoli for ribosome assembly and chromatin (DNA, proteins, and some RNA).

  • Protein Synthesis:

    • Transcription: Occurs in the nucleus; DNA is used as a template to create mRNA.

    • Translation: Occurs in the cytoplasm; ribosomes read the mRNA sequence to build a polypeptide chain of amino acids.

  • The Cell Cycle:

    • Interphase: The state of high metabolic activity in which the cell does most of its growing (G{1}, S, G{2} phases). DNA replication occurs during the S phase.

    • Mitotic Phase: Consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division).

    • Phases of Mitosis: Prophase, Metaphase, Anaphase, and Telophase.

CANCER PATHOLOGY

  • Hyperplasia: Rapid and uncontrolled local cell division.

  • Metastasis: The spread of cancerous cells to other parts of the body via blood or lymph.

  • Angiogenesis: The growth of new networks of blood vessels to feed a tumor.