A

The Cell - Vocabulary Flashcards

The Cell

  • All body processes depend on cellular activity.
  • Cells are the functional units of the body.
  • Understanding cell structure and function is crucial for understanding anatomy and physiology.

Objectives

  • Describe the main structural features of a cell.
  • Identify the membrane-bound and non-membrane-bound organelles.
  • Distinguish between organelles and cell inclusions.
  • Explain the general functions that cells must perform.

The Variety of Cell Shapes

  • Irregular-shaped: Nerve cells
  • Biconcave disc: Red blood cells
  • Cube-shaped: Kidney tubule cells
  • Column-shaped: Intestinal lining cells
  • Spherical: Cartilage cells
  • Cylindrical: Skeletal muscle cells

The Structure of a Cell

  • Key components:
    • Plasma membrane (cell membrane)
    • Cytoplasm (between the plasma membrane and nucleus)
    • Nucleus
    • Cytosol (intracellular fluid)
    • Cytoskeleton
    • Organelles (membrane-bound and non-membrane-bound)
  • Cytoplasm includes organelles suspended in cytosol, cytoskeleton, and other chemicals; many metabolic reactions occur here.
  • The plasma membrane forms the boundary of the cell and participates in cell signaling, transport, and interactions with the environment.

The Plasma Membrane

  • A phospholipid bilayer with embedded proteins; also contains cholesterol and carbohydrates.
  • Functions:
    1) Physical barrier and flexible boundary; protects cellular contents and supports structure.
    2) Selective permeability: regulates entry/exit of ions, nutrients, and wastes (active or passive).
    3) Electrochemical gradients: establishes and maintains membrane potential.
    4) Communication: contains receptors for molecular signals.
  • Key components:
    • Phospholipid bilayer
    • Glycoproteins (protein with carbohydrate attached)
    • Glycolipids (lipid with carbohydrate attached)
    • Integral membrane proteins
    • Peripheral membrane proteins
    • Cholesterol
    • Filaments of the cytoskeleton (internal support)
  • Structural layout:
    • Polar head of phospholipid (hydrophilic) faces outward
    • Nonpolar tails (hydrophobic) face inward
  • Diagrammatic notes (conceptual):
    • Glycolipid and glycoprotein extend carbohydrate groups to the exterior.
    • The plasma membrane is a dynamic, fluid mosaic with proteins embedded or attached to the bilayer.

Lipid Components of the Plasma Membrane

  • Phospholipids: Balloons with two tails
    • Polar, hydrophilic head; two nonpolar, hydrophobic tails
    • Form two parallel sheets tail-to-tail; hydrophobic tails face inward; heads face outward
  • Phospholipid bilayer is the basic structure of the framework:
    • Ensures cytosol remains inside the cell
    • Ensures interstitial fluid remains outside
  • Other components:
    • Glycolipids
    • Cholesterol
    • Integral proteins
    • Peripheral proteins
    • Filaments of cytoskeleton

Structure and Functions of the Plasma Membrane

  • Polar head facing outward; inner layer of nonpolar tails
  • Components and their functions:
    • Phospholipid bilayer
    • Glycolipid
    • Glycoprotein
    • Integral protein
    • Peripheral protein
    • Cholesterol
    • Cytosolic interior with many proteins
  • Primary functions (summary):
    1) Physical barrier
    2) Selective permeability
    3) Electrochemical gradients
    4) Communication via receptors
  • Structural context (high magnification example): TEM at high magnification shows the membrane in detail; cytosol and interstitial fluid in relation to the membrane

Membrane Proteins

  • Membrane proteins account for roughly half of the plasma membrane by weight.
  • They float and move within the fluid bilayer and perform most membrane functions.
  • Two structural types:
    • Integral membrane proteins (embedded and spanning the bilayer)
    • Peripheral membrane proteins (not embedded, attached to surfaces of the membrane)
  • Many integral proteins are glycoproteins with carbohydrate portions.
  • The membrane functions are largely mediated by these proteins (channels, receptors, transporters, enzymes, anchors).

Integral vs Peripheral Membrane Proteins

  • Integral proteins:
    • Embedded within and extend across the phospholipid bilayer
    • Hydrophobic regions interact with the interior of the bilayer; hydrophilic regions exposed to aqueous environments on either side
    • Many are glycoproteins with carbohydrate portions
  • Peripheral proteins:
    • Not embedded in the lipid bilayer
    • Loosely attached to external or interior surfaces of the membrane

Transport Across the Plasma Membrane: Passive vs Active

  • Transport mechanisms (conceptual map):
    • Passive diffusion
    • Facilitated diffusion
    • Osmosis
    • Active pumping (active transport)
    • Endocytosis
    • Exocytosis
  • Key example: Na+/K+ pump (ATP-powered pumping)
  • Directionality and energy considerations:
    • Passive processes do not require ATP and move down electrochemical gradients.
    • Active processes require energy (ATP or another energy source) and can move substances against gradients.
  • Typical flows (summary):
    • Gases like O2, CO2 diffuse across membranes depending on gradients.
    • Na+,K+ gradients are maintained by pumps; water movement via osmosis depends on solute gradients.
  • Visual cues:
    • Channels and transporters facilitate selective movement; some are always open, others gate according to signals or voltage

Cytoplasm

  • The region between the plasma membrane and the nucleus.
  • Contains cytosol (gel-like fluid), organelles, cytoskeleton, and various chemicals.
  • Many metabolic reactions occur in the cytoplasm.

Membrane-Bound Organelles: Endomembrane System Overview

  • Major components include:
    • Endoplasmic reticulum (ER) — rough and smooth
    • Golgi apparatus
    • Lysosomes
    • Peroxisomes
    • Vesicles
    • Plasma membrane
    • Nuclear envelope
  • The endomembrane system is interconnected directly or through transport vesicles; it provides means of transporting substances within cells and coordinates synthesis, processing, and shipping of proteins and lipids.

Endoplasmic Reticulum (ER)

  • An extensive interconnected membrane network with a single continuous lumen.
  • Extends from the nuclear envelope to the plasma membrane and accounts for about half of the membrane within the cell.
  • Attachment site for ribosomes:
    • Rough ER (RER) with ribosomes
    • Smooth ER (SER) without ribosomes
  • Functions (RER):
    1) Synthesis of proteins destined for secretion, incorporation into the plasma membrane, and enzymes within lysosomes
    2) Processing of proteins (e.g., glycosylation) and storage
    3) Organelle formation (helps form peroxisomes)
    4) Vesicle formation for shipping proteins to the Golgi apparatus
  • Functions (SER):
    1) Synthesis of lipids (e.g., steroid synthesis)
    2) Carbohydrate metabolism (e.g., glycogen synthesis)
    3) Detoxification of drugs and poisons
    4) Vesicle formation for shipping to the Golgi apparatus
  • Structural note: RER is described in TEM images showing bound ribosomes; SER lacks ribosomes
  • The ER is a central hub of the synthesis and trafficking pathway in the cell

Nucleus and Ribosomes

  • Nucleus contains:
    • Nucleolus
    • Nuclear envelope
    • Nucleoplasm
    • Nucleoi (plural of nucleolus)
  • Ribosomes exist in two forms:
    • Bound ribosomes (attached to rough ER; synthesize proteins for secretion, membranes, or lysosomes)
    • Free ribosomes (synthesize proteins for use within the cell)
  • The nucleus functions as the repository of genetic information and the site of transcription; ribosomes are the site of translation

Golgi Apparatus

  • The Golgi apparatus sorts, packages, and tags lipids and proteins so they are delivered to correct destinations (the cell’s post office).
  • Structure includes cis face (receiving) and trans face (shipping) with cisternae; secretory vesicles bud from the trans face.
  • Roles:
    1) Synthesis: forms proteoglycans
    2) Processing: modifies and stores proteins produced by the RER
    3) Organelle formation: synthesizes digestive enzymes for lysosomes
    4) Vesicle formation: forms secretory vesicles for delivering plasma membrane components or exocytosis
  • Movement/flow through Golgi: RER → Golgi cis face → processing in cisternae → secretory vesicles from trans face
  • Vesicles from Golgi can become:
    • Lysosomes (containing digestive enzymes)
    • Secretory vesicles delivering components to the plasma membrane or releasing contents by exocytosis
  • Diagrammatic note: Golgi is part of the endomembrane system that coordinates protein/lipid trafficking

Lysosomes

  • Small, membrane-enclosed sacs containing digestive enzymes formed by the Golgi.
  • Participate in digestion of unneeded substances and in digestion of endocytosed vesicles contents
  • Functions:
    • Digestion of molecules within endocytosed vesicles
    • Autophagy: digestion of damaged organelles and cellular components
    • Autolysis: digestion of cellular components after cell death
  • Visual: lysosomes are acidic compartments that degrade macromolecules

Peroxisomes

  • Membrane-enclosed sacs smaller than lysosomes; formed by budding from the rough ER
  • Contain enzymes to perform metabolic functions including:
    • Chemical digestion and detoxification
    • Beta-oxidation of fatty acids
    • Lipid synthesis
  • Noted as performing both digestion and synthesis roles within the cell

Mitochondria

  • Powerhouse or energy factory of the cell; generate ATP via cellular respiration
  • Contain their own DNA
  • Abundant in energy-demanding cells (e.g., muscle cells)
  • Structure:
    • Outer membrane
    • Inner membrane with cristae increases surface area
    • Matrix within the inner membrane
  • Common descriptors: "Powerhouse of the cell"; mitochondria drive most ATP production

Ribosomes (Detailed View)

  • Functional ribosomes consist of a large subunit and a small subunit
  • Bound ribosomes: associated with RER; synthesize proteins intended for membranes, secretion, or lysosomes
  • Free ribosomes: synthesize proteins for use inside the cell
  • TEM images commonly show ribosomes as ribosomal subunits bound to ER or free in cytosol

Non-Membrane-Bound Organelles: Cytoskeleton

  • The cytoskeleton provides intracellular support, organizes organelles, aids in cell division, and facilitates movement of materials inside the cell
  • Three types of cytoskeletal elements:
    • Intermediate filaments
    • Microfilaments
    • Microtubules
  • Functions include maintaining cell shape, securing organelles, enabling vesicle transport, and supporting movement of cilia/flagella and microvilli

Cytoskeleton: Detailed Elements

  • Intermediate filaments: mid-sized; provide structural support and stabilize cell junctions; protein composition varies by cell type (e.g., keratin in skin, hair, nails; neurofilaments in nerve cells)
  • Microfilaments: smallest; actin-based; support cell shape and enable movement and contraction in some contexts
  • Microtubules: largest; hollow tubes of tubulin; organize intracellular transport and form the mitotic spindle during cell division

Centrosome and Centrioles

  • Centrosome is near the nucleus and contains a pair of centrioles arranged perpendicularly; surrounded by amorphous protein
  • Primary function: organizes microtubules within the cytoskeleton; directs formation of the mitotic spindle during cell division in dividing cells
  • TEM images show centrioles as cylindrical arrays with microtubule triplets

Proteasomes

  • Large, barrel-shaped protein complexes located in the cytosol and nucleus
  • Function: proteolysis of proteins via an ATP-dependent pathway; degrade damaged, misfolded, or no-longer-needed proteins
  • Proteins to be degraded are tagged with ubiquitin prior to degradation
  • Roles: quality control and recycling of amino acids

Extracellular Matrix (ECM)

  • Space outside the cell containing glycoproteins and collagen
  • Components include:
    • Collagen fibers
    • Proteoglycan complexes
    • Carbohydrates (sugars)
    • Proteins (e.g., fibronectin)
  • ECM interacts with cells via integrins and links to the cytoskeleton through various proteins, providing structural support and signaling cues

Membrane Junctions

  • Junctions connect and support adjacent cells; three main types:
    • Tight junctions (zonula occludens)
    • Desmosomes
    • Gap junctions

Tight Junctions

  • Composed of integral membrane proteins forming strands or rows around apical surfaces of adjacent cells
  • Function: prevent substances from passing between cells (materials must pass through cells, not between them); also prevent leakage of urine in epithelia

Desmosomes

  • Protein plaques connected by intermediate filaments; provide strong adhesion between adjacent cells
  • Help resist mechanical stress especially in tissues that stretch

Gap Junctions

  • Composed of six transmembrane proteins called connexons
  • Form tiny, fluid-filled channels between cells; permit direct passage of ions and small molecules
  • Example: rapid ion flow between cardiac myocytes enables synchronized contraction

Structures of the Cell’s External Surface

  • Extensions of the plasma membrane:
    • Cilia
    • Flagella
    • Microvilli
  • Cilia and flagella: extensions involved in cell movement; coordinated beating (flagella in few cells; cilia in many)
  • Microvilli: microscopic extensions that increase surface area; lack powered movement; supported by microfilaments; common in cells of the small intestine
  • Microvilli are smaller and more densely packed than cilia/flagella and are supported by microfilaments

Review Prompts and Quick Checks

  • What organelle is involved in detoxification and beta oxidation? (Peroxisomes)
  • Which organelle is chiefly involved in energy production? (Mitochondria)
  • Which non-membrane-bound organelle functions primarily in digestion within the cell? (Lysosomes)
  • What structure of the cell’s external surface is found in parts of the respiratory tract and participates in moving mucus toward the throat? (Cilia)
  • What type of membrane junction provides a direct passage for substances to travel between neighboring cells? (Gap junctions)

Additional Notes and Context

  • The content reflects foundational topics in cellular anatomy and physiology, aligning with the general cell theory that:
    • All living things are composed of cells
    • The cell is the basic unit of structure and function
    • New cells arise from existing cells
  • The organization of the organelles reflects a functional compartmentalization: synthesis and trafficking (ER, Golgi, vesicles), energy production (mitochondria), breakdown and recycling (lysosomes, proteasomes), structural support and movement (cytoskeleton), communication and boundary with the environment (plasma membrane and ECM).

Quick Reference: Notable Numerical References

  • TEM magnifications and imaging notes:
    • Rough ER with ribosomes commonly depicted in TEM images at high magnification (e.g., TEM around 12,000x to 96,000x ranges noted in the slides)
    • TEM examples: 12{,}510x; 50{,}000x; 96{,}000x; 120{,}000x
  • Size references:
    • Typical cell and organelle dimensions include approx. 1 \, ext{μm} scale bars for mitochondria and other organelles
  • Size and scale notes are used across diagrams to illustrate relative dimensions and structures