뒤로The Cell: Structure, Function, and Membrane Transport (Anatomy & Physiology Study Guide)
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The Cell
Basic Processes of Cells
Cells are the fundamental units of life, performing essential processes to maintain homeostasis and support bodily functions.
Cell Metabolism: Chemical reactions within the cell, including anabolic (building up), catabolic (breaking down), and oxidation-reduction reactions.
Substance Transport: Movement of compounds into, out of, or within the cell.
Communication: Cells interact with their environment and other cells via signaling mechanisms.
Cell Reproduction: Most cells undergo division to produce new cells.
Overview of Cell Structure
Most animal cells share three basic structural components:
Plasma Membrane: The boundary separating the cell from its environment.
Cytoplasm: Includes cytosol (intracellular fluid), organelles (specialized structures), and cytoskeleton (structural framework).
Nucleus: Contains genetic material (DNA) and is the site of RNA production.
Cell Size and Diversity
Cells vary greatly in size and appearance, allowing for specialized functions throughout the human body.
Examples: Red blood cells, nerve cells, epithelial cells, and skeletal muscle cells each have unique shapes and roles.
The Plasma Membrane
The Phospholipid Bilayer
The plasma membrane is primarily composed of a phospholipid bilayer, forming a selective barrier between the extracellular fluid (ECF) and cytosol.
Hydrophilic (polar) heads: Face water.
Hydrophobic (nonpolar) tails: Repel water.
Phospholipids arrange themselves into a bilayer in aqueous environments, with heads facing outward and tails inward.
The Fluid Mosaic Model
The plasma membrane is a dynamic structure, with proteins, lipids, and carbohydrates moving within the bilayer, giving rise to the "fluid mosaic model." This fluidity is essential for membrane function.
Membrane Proteins
Membrane proteins are critical for cell function and are classified by their location and function:
Integral proteins: Span the membrane; transmembrane if they reach both sides.
Peripheral proteins: Located on one side; may be anchored or float freely.
Functional types include:
Channels: Allow passage of substances.
Carriers: Transport substances across the membrane.
Receptors: Bind ligands to trigger cellular changes.
Enzymes: Catalyze reactions.
Structural support proteins: Maintain cell shape.
Linker proteins: Connect adjacent cells.
Other Membrane Components
Cholesterol: Stabilizes membrane structure during temperature changes.
Glycolipids and Glycoproteins: Carbohydrate chains attached to lipids/proteins; function in cell recognition.
Transport Across the Plasma Membrane
Passive Transport
Passive transport does not require energy and relies on concentration gradients.
Diffusion: Movement of solute from high to low concentration.
Simple Diffusion: Nonpolar solutes and gases pass directly through the bilayer.
Facilitated Diffusion: Polar or charged solutes cross via channel or carrier proteins.
Osmosis
Osmosis is the movement of water across a selectively permeable membrane from lower to higher solute concentration.
Water moves through aquaporins or between phospholipids.
Results in changes in fluid volume in compartments.
Tonicity
Tonicity compares solute concentrations and their effect on cell volume:
Isotonic: No net water movement; cell volume unchanged.
Hypertonic: Cell loses water and shrivels (crenates).
Hypotonic: Cell gains water, swells, and may rupture (lyse).
Active Transport
Active transport requires ATP to move solutes against their concentration gradient via carrier proteins (pumps).
Primary Active Transport: Direct use of ATP; e.g., sodium-potassium pump moves 3 Na+ out and 2 K+ in.
Secondary Active Transport: Uses gradient created by primary transport to move another substance.
Vesicular Transport
Large particles are transported via vesicles, requiring ATP:
Endocytosis: Bringing substances into the cell.
Phagocytosis: "Cell eating"; ingestion of large particles.
Pinocytosis: "Cell drinking"; ingestion of fluid.
Receptor-mediated endocytosis: Specific uptake of substances.
Exocytosis: Release of substances from the cell.
Transcytosis: Transport across the cell.
Cytoplasmic Organelles
Membrane-Enclosed Organelles
Mitochondria: ATP production; double membrane with inner folds (cristae).
Peroxisomes: Oxidize toxic substances, break down fatty acids, synthesize phospholipids.
Endoplasmic Reticulum (ER): Rough ER (protein folding/modification), Smooth ER (lipid synthesis, detoxification, calcium storage).
Golgi Apparatus: Modifies, sorts, and packages proteins/lipids.
Lysosomes: Digest macromolecules, recycle organelles, function in immunity.
The Cytoskeleton
Types of Filaments
The cytoskeleton provides structural support, movement, and specialized functions.
Actin Filaments (Microfilaments): Support plasma membrane, involved in cell motion and division.
Intermediate Filaments: Provide mechanical strength, support nucleus, unite cells.
Microtubules: Maintain cell architecture, move organelles, form cilia and flagella.
The Nucleus
Structure and Function
The nucleus directs cellular activities and houses DNA, which contains genetic instructions for protein synthesis.
Nuclear Envelope: Double membrane with nuclear pores.
Nucleoplasm: Gel-like substance containing DNA and proteins.
Nucleolus: Site of ribosome assembly.
Chromatin and Chromosomes
Chromatin: DNA and histone proteins; compact form for storage.
Chromosomes: Condensed chromatin during cell division; humans have 46 chromosomes.
Protein Synthesis
Gene Expression
Protein synthesis involves transcription (DNA to mRNA) and translation (mRNA to protein).
Transcription: RNA polymerase copies DNA into mRNA.
Translation: Ribosomes read mRNA and assemble amino acids into polypeptides.
Posttranslational Modification: Folding and alteration of polypeptides to functional proteins.
The Cell Cycle
Phases of the Cell Cycle
The cell cycle is the series of events from cell formation to division.
Interphase: Growth and preparation (G1, S, G2 phases).
M Phase: Mitosis (division of genetic material) and cytokinesis (division of cytoplasm).
Mitosis Stages
Prophase: Chromatin condenses, spindle forms.
Metaphase: Chromosomes align at cell equator.
Anaphase: Sister chromatids separate.
Telophase and Cytokinesis: Nuclear envelope reforms, cell splits.
Summary Table: Plasma Membrane Transport
Type of Transport | Definition | Example(s) |
|---|---|---|
Simple Diffusion | Movement of solute with its concentration gradient through the plasma membrane unaided by a transport protein; energy source is the solute’s own kinetic energy. | Oxygen, Carbon dioxide, Lipids |
Facilitated Diffusion | Movement of solute with its concentration gradient with the help of a carrier or channel protein; energy source is the solute’s own kinetic energy. | Sodium ions, Potassium ions, Calcium ions, Glucose, Amino acids |
Osmosis | Movement of solvent (water) from a solution of lower solute concentration to one of higher solute concentration through a selectively permeable membrane. | Water absorption from intestines, Water reabsorption from kidneys |
Primary Active Transport | Movement of solute against its concentration gradient using ATP. | Na+/K+ ATPase pump |
Secondary Active Transport | ATPase pump drives a solute out/in against its gradient; movement of this solute with its gradient powers transport of another solute against its gradient. | Symporters use sodium ion gradient to bring glucose, chloride ions, bicarbonate ions into the cell |
Phagocytosis | "Cell eating"; bringing large substances or particles into the cell via a phagosome; ATP required. | Ingestion of bacteria and cell debris by phagocytes |
Pinocytosis | "Cell drinking"; bringing substances in the ECF into the cell via a transport vesicle formed from a protein-coated pit; ATP required. | Nutrient transport |
Receptor-Mediated Endocytosis | Bringing a specific substance into a transport vesicle using receptors on the plasma membrane; ATP required. | Cholesterol, iron, hormone transport |
Exocytosis | Release of a substance from the cell via an exocytic transport vesicle; ATP required. | Secretion of hormones, neurotransmitters, enzymes |
Summary Table: Cytoplasmic Organelles
Organelle | Structure | Function |
|---|---|---|
Mitochondrion | Double membrane; inner membrane folded into cristae; has own DNA and ribosomes | Synthesizes the majority of the cell’s ATP |
Peroxisome | Membrane-enclosed; similar to large vesicle | Detoxifies chemicals, metabolizes fatty acids, synthesizes certain phospholipids |
Ribosome | Two subunits made of proteins and rRNA; not membrane-enclosed | Synthesizes proteins |
Rough ER | Series of saclike membranes; surface studded with ribosomes | Modifies and folds proteins; manufactures and assembles membrane components |
Smooth ER | Series of tubular membranes; surface does not contain ribosomes | Stores calcium ions, synthesizes lipids, detoxifies substances |
Golgi Apparatus | Stack of flattened, membrane-enclosed sacs | Sorts, modifies, and packages proteins and other products |
Lysosome | Membrane-enclosed structure with digestive enzymes | Digests damaged organelles and products brought into the cell by endocytosis; recycles organelles |
Key Equations
Diffusion Law: Where J is the flux, D is the diffusion coefficient, and \frac{dC}{dx} is the concentration gradient.
Osmotic Pressure: Where \Pi is osmotic pressure, i is the van't Hoff factor, M is molarity, R is the gas constant, and T is temperature.
Additional info: This guide expands on brief points from lecture slides and textbook images, providing definitions, examples, and academic context for self-contained study notes.
