The Human Cell Study Guide

General Characteristics of the Cell

  • Definition of the Cell: The cell (Zelle) is defined as the smallest unit of life and the fundamental building block (Grundbaustein) of all living things.
  • Universal Features of Every Cell:
    • External Boundary: Every cell is delimited from its surroundings by a cell membrane (Zellmembran).
    • Internal Filling: The interior is filled with cytoplasm (Zytoplasma).
    • Genetic Information: Each cell contains hereditary information in the form of DNA.
    • Metabolism: Cells actively engage in metabolism (Stoffwechsel).
    • Reproduction: Cells possess the ability to reproduce through the process of cell division (Zellteilung).
    • Stimulus Response: Cells perceive external stimuli and have the capacity to react to them.

Cell Types and Differentiation

  • Prokaryotes:
    • Characterized by having no cell nucleus (Kein Zellkern).
    • DNA is found floating freely within the interior of the cell.
    • Example: Bacteria.
  • Eukaryotes:
    • Contains a defined cell nucleus where the DNA is housed.
    • Features a complex internal structure with various organelles.
    • Examples: Animal cells (Tierzellen), plant cells (Pflanzenzellen), and fungal cells (Pilzzellen).
  • Differentiation in the Body: All cells in the human body belong to the category of animal cells (Eukaryotes). However, they undergo differentiation to fulfill specific functions despite sharing the same basic classification.

Terminology and Definitions for Cell Membranes

  • Philos: Loving.
  • Phobos: Fear.
  • Lipos: Fat.
  • Hydor: Water.
  • Amphi: On both sides.
  • Hydrophobic: Water-fearing (Wasser meidend).
  • Hydrophilic: Water-loving (Wasser liebend).
  • Lipophobic: Fat-fearing (Fett meidend).
  • Lipophilic: Fat-loving (Fett liebend).
  • Amphiphilic: Describes a molecule where one part is water-loving (hydrophilic) and another part is fat-loving (lipophilic).
  • Chemical Properties Correlation:
    • Hydrophilic = Lipophobic = Polar.
    • Hydrophobic = Lipophilic = Apolar.

Structure of the Cell Membrane

  • General Composition: The membrane consists primarily of lipids and proteins.
  • Phospholipid Bilayer (Phospholipiddoppelschicht):
    • The basic structure is a double layer of phospholipids.
    • Phospholipid Structure:
      • Hydrophilic Head: Composed of Phosphate + Glycerol.
      • Hydrophobic Tail: Composed of 2 fatty acids (Fettsuren).
    • Amphiphilic Nature: Because the environment inside and outside the cell is watery, the hydrophilic heads point outward toward the water, while the hydrophobic tails point toward each other in the center of the membrane.
  • Membrane Proteins:
    • Peripheral Proteins: Located on the surface of the cell membrane.
    • Integral Proteins: Extend into or through the cell membrane.
    • Transmembrane Proteins: These are integral proteins that pass entirely through the membrane, protruding on both the intracellular and extracellular sides.
  • Glycocalyx (The "Business Card" of the Cell):
    • Some membrane proteins and lipids carry sugar residues (carbohydrates) on the extracellular side, forming Glycoproteins and Glycolipids.
    • The Glycocalyx is the collective term for all these sugar residues.
    • It forms a layer approximately 10−20 nm10 - 20\,nm thick.
    • Function: It serves as a recognition site for the immune system and is cell-specific.

Fluid-Mosaic Model and Membrane Functions

  • Definition: All membrane components are freely movable, a property known as membrane fluidity (Membranfluiditt).
  • Factors Affecting Fluidity:
    • Cholesterol Content: Cholesterol consists of 4 rigid rings providing stability. A decrease in cholesterol content (↓\downarrow) leads to an increase in fluidity (↑\uparrow).
    • Other Lipids: An increase in other lipid types (↑\uparrow) leads to an increase in fluidity (↑\uparrow).
    • Unsaturated Fatty Acids: An increase in unsaturated fatty acids (↑\uparrow) leads to an increase in fluidity (↑\uparrow).
    • Temperature: An increase in temperature (↑\uparrow) leads to an increase in fluidity (↑\uparrow).
  • Core Functions of the Membrane:
    • Boundary: Provides separation from the external environment.
    • Compartmentalization: Creates distinct spaces (compartments) within the cell.
    • Stability: Maintains a constant internal millieu.
    • Transport: Facilitates the movement of substances in and out of the cell.
    • Communication: Contains receptors for receiving external signals.

Transmembrane Transport: Permeability and Passive Mechanisms

  • Membrane Permeability:
    • Permeable for: Small hydrophobic molecules (O2O_2, CO2CO_2), very small molecules (Urea, NH3NH_3), and small, uncharged, polar molecules (H2OH_2O, EtOHEtOH).
    • Impermeable for: Proteins, nucleic acids, charged molecules (Ions), and large, uncharged, polar molecules (Glucose). These require carriers, channels, or active systems to cross.
  • Passive Transport (No energy required):
    • Driven by diffusion and concentration gradients.
    • Diffusion: Movement of molecules from an area of higher concentration to lower concentration.
    • Simple Diffusion: Uncharged particles flow through the membrane without the aid of transport proteins.
    • Facilitated Diffusion: Diffusion aided by transport molecules (Carriers) or protein tunnels.
    • Osmosis: The diffusion of H2OH_2O through a semipermeable membrane.
    • Carriers: Transport specific substrates. The substance binds to the carrier, triggering a conformational change. Direction is determined by the electrochemical gradient. Types include Uniport, Symport, and Antiport.
    • Ion Channels: Proteins forming a "tunnel." Particles flow along a concentration gradient. They can be permanently open or gated (controlled by voltage, ligands, or mechanical forces).

Active Transmembrane Transport

  • Active Transport (Energy required):
    • Primary Active: Directly consumes ATP to provide energy for transport.
    • Secondary Active: ATP consumption is used to create an ion gradient, which then powers the transport of another substance.
  • The Na+−K+−ATPaseNa^+-K^+-ATPase (Primary Active):
    • Pumps against the concentration gradient.
    • Process:
      1. The pump opens to the intracellular space, and 3 Na+Na^+ bind.
      2. ATP binds; one phosphate remains attached to the pump while ADP is released.
      3. This phosphorylation causes the pump to open to the extracellular space.
      4. 3 Na+Na^+ are released, and 2 K+K^+ bind.
      5. The phosphate bond is released, returning the pump to its original form, opening intracellularly to release the 2 K+K^+.
    • Mnemonic: "3 Nazis raus, 2 Kumpel rein" (3 Nazis out, 2 buddies in).
  • Co-Transport (Secondary Active):
    • A substance uses an existing concentration gradient of another substance.
    • Example: Sodium-Glucose-Symport:
      1. The Na+−K+−ATPaseNa^+-K^+-ATPase uses ATP to pump 3 Na+Na^+ out.
      2. Na+Na^+ naturally wants to flow back into the cell; 2 Na+Na^+ bind to the symport along with 1 Glucose.
      3. The symport opens intracellularly, releasing the 2 Na+Na^+ and the Glucose.

Vesicular Transport: Exocytosis and Endocytosis

  • Exocytosis: Substances are transported out of the cell.
    • Substances like neurotransmitters or waste products are packaged in vesicles that fuse with the cell membrane.
    • Constitutive Exocytosis: Happens permanently.
    • Regulated Exocytosis: Occurs only as a reaction to a specific stimulus.
  • Endocytosis: Substances are taken into the cell via bubble-like invaginations of the membrane.
    • Pinocytosis: Intake of liquid substances.
    • Phagocytosis: Intake of solid substances.
    • Receptor-mediated Endocytosis: Specific receptors on the membrane bind to substances, triggering the invagination process.

Cytoplasma and Cytosol

  • Components: The cytoplasm includes the Cytosol, Cell Organelles, and the Cytoskeleton, but it does NOT include the cell nucleus.
  • Function: It is the localized site for many enzymatically catalyzed reactions, such as protein biosynthesis.
  • Regional Variation:
    • Ectoplasma: The thick (viscous) cytoplasm at the edge of the cell.
    • Endoplasma: The thin (fluid) cytoplasm in the interior of the cell.
  • Cytosol (Cell Fluid):
    • A gelatinous liquid.
    • Composition: 80−85% H2O80 - 85\%\,H_2O, 10−15% Proteins10 - 15\%\,\text{Proteins}, and the remainder consists of lipids, RNA, polysaccharides, and organic molecules.

The Cell Nucleus (Nucleus/Karyon)

  • Contents: Contains DNA (Desoxyribonukleinsure) in the form of chromosomes and kernel bodies (Nucleoli).
  • Structure:
    • Double Membrane: Composed of 2 phospholipid bilayers (total of 4 phospholipid layers).
    • Nuclear Pores: Allow for exchange between the nucleus and cytoplasm.
    • Connections: The outer membrane transitions into the rough endoplasmic reticulum (rER).
    • Nuclear Lamina (Kernlamina): Located on the inside of the membrane.
    • Karyoplasm: The fluid inside where reactions occur and chromatin is held.
  • Tasks:
    • Protection of DNA.
    • Site of DNA replication.
    • Site of Transcription (RNARNA-Synthesis).
  • Chromatin Organization:
    • Euchromatin: Unraveled chromatin used for reading specific areas.
    • Heterochromatin: Densely packed, "untidy," and compact.
  • Nucleolus (Kernkrperchen): Responsible for producing the rRNArRNA of ribosomes (which consist of rRNArRNA and protein).
  • Cell Presence:
    • All cells have a nucleus EXCEPT Erythrocytes and Thrombocytes.
    • Some cells have multiple nuclei (Synzytium), such as in the heart or liver (formed by cell fusion).

Mitochondria: The Powerhouse of the Cell

  • Function: Site of ATP production (Energy) and storage for Calcium.
  • Distribution: Found in all cells except erythrocytes; cells with high energy requirements (nerve and muscle cells) have more mitochondria.
  • Genetic Material: Mitochondria have their own circular DNA (mtDNAmtDNA) and their own ribosomes. mtDNAmtDNA is inherited only maternally via the oocyte plasma.
  • Structure:
    • Double Membrane: Supported by the Endosymbiont Theory (formerly independent prokaryotes engulfed via phagocytosis).
    • Cristae: The inner membrane is heavily folded to increase surface area.
    • Intermembrane Space: The space between the two membranes.
    • Matrix: The space enclosed by the inner membrane.
  • Cellular Respiration (Dissimilation/Inner Respiration):
    • Process of producing ATP by oxidizing glucose in the presence of oxygen (aerobic).
    • Steps: Glycolysis, Citrate Cycle, Oxidative Phosphorylation.
    • ATP (Adenosintriphosphat) Components: Adenine (Base), Ribose (Sugar), and 3 Phosphates.
    • Energy Release: Splitting off 1 phosphate (ATP→ADPATP \rightarrow ADP) releases significant energy.
    • Localization: Enzymes of the respiratory chain (ATPATP-Synthase) are located in the inner membrane.

Endoplasmic Reticulum (ER)

  • General: A tubular membrane system connected to the outer nuclear membrane. Found in all cells except erythrocytes.
  • Rough ER (rER):
    • Studded with many ribosomes on the membrane.
    • Tasks: Protein synthesis and post-translational protein modifications.
  • Smooth ER (sER):
    • No ribosomes.
    • Tasks: Phospholipid synthesis (for membranes), steroid hormone synthesis, Ca2+Ca^{2+} storage (especially in muscle cells), and detoxification.
    • Products are packaged into vesicles and sent to the Golgi apparatus.

Golgi Apparatus: The Post Office of the Cell

  • Structure: Consists of several membrane-enclosed spaces called Cisternae, organized into stacks called Diktyosoms.
  • Orientation:
    • Cis-side: Faces the ER.
    • Medial part: The middle section.
    • Trans-side: Faces the cell membrane.
  • Process Flow:
    1. Receives vesicles from the rER at the cis-side.
    2. Further modifies products (post-translational modification).
    3. Releases Golgi-vesicles from the trans-side to their final destination.
  • Tasks:
    • Post-translational protein modifications.
    • Formation of secretory vesicles.
    • Production of Lysosomes.

Lysosomes, Endosomes, and Peroxisomes

  • Lysosome (The "Trash Can"):
    • Responsible for digesting foreign and cellular material; found predominately in leukocytes.
    • Stages:
      1. Primary Lysosome: "Empty" vesicle straight from the Golgi.
      2. Secondary Lysosome: "Full" after fusion with a substrate vesicle.
      3. Tertiary Lysosome (Residual Body): Contains non-degradable material.
    • Composition: Contains enzymes called acid hydrolases (e.g., Acid Phosphatase). Functions in an acidic milieu (pH≈4.5pH \approx 4.5) maintained by proton pumps (H+−ATPaseH^+-ATPase). Also plays a role in Apoptosis.
  • Endosome:
    • A vesicle formed via endocytosis that acts as a sorting station.
    • Material is either sent to the lysosome for degradation (forming an Endolysosome) or recycled back to the cell membrane (e.g., receptors).
    • Progresses from "early endosome" to "late endosome."
  • Peroxisome (Microbodies):
    • Membrane-bound vesicles containing peroxidases (e.g., Catalase).
    • Tasks: Metabolism of long-chain fatty acids (producing H2O2H_2O_2) and the breakdown of hydrogen peroxide (H2O2H_2O_2) into H2OH_2O and O2O_2 via Catalase.
    • Origin: Formed from the ER (no Golgi involvement) and can multiply through budding. Most frequent in liver cells.

Ribosomes and Proteasomes

  • Ribosomes (Protein Factories):
    • Translate mRNAmRNA into proteins (Translation).
    • Located on the rER or free in the cytoplasm; absent in erythrocytes.
    • Structure: Consist of proteins and rRNArRNA; have no membrane.
    • Subunits:
      • Eukaryotes: Small (40S40S) + Large (60S60S) = Total 80S80S. (S=SvedbergS = Svedberg unit).
      • Prokaryotes: Small (30S30S) + Large (50S50S) = Total 70S70S.
    • Subunits remain dissociated in the cytoplasm and join only for translation. The 60S60S subunit has 3 binding sites: A, P, and E.
  • Proteasomes:
    • Large protein complexes containing proteases; no membrane.
    • Function: Degradation of incorrect or unneeded proteins.
    • Mechanism: Proteins are marked with Ubiquitin, which the proteasome recognizes. Proteins are then split into short peptides or amino acids for reuse.

The Cytoskeleton

  • Definition: A network of fibers providing stability and enabling motility.
  • Three Fiber Types (Ordered by Size):
    1. Microtubules / Makrofilamente (25 nm25\,nm):
      • Cylinder-shaped proteins built from Tubulin (alpha and beta heterodimers forming protofilaments; 13 protofilaments per tube).
      • Functions: Cell stability, formation of the spindle apparatus (mitosis/meiosis), motility (cilia/flagella), and intracellular transport.
      • Motor Proteins: Kinesin (travels toward the ++/beta-end) and Dynein (travels toward the −-/alpha-end).
    2. Intermediate Filaments (10 nm10\,nm):
      • Rope-like filaments made of various protein polypeptide chains.
      • Structure: Monomer →\rightarrow Dimer →\rightarrow Tetramer →\rightarrow Protofilament →\rightarrow Intermediary Filament (comprised of 8 protofilaments).
      • Functions: High tensile strength and stability, signal transmission, anchoring to desmosomes, and surrounding the cell nucleus.
    3. Microfilaments / Aktinfilamente (5 nm5\,nm):
      • Built from G-Actin; two strands wound helically.
      • Located directly under the plasma membrane; can be rapidly assembled/disassembled.
      • Functions: Cell mobility, microvilli structure, muscle contraction (with Myosin), and cell division.

Centrioles and Centrosomes

  • Centriole Structure: Hollow cylinders composed of microtubule triplets in a 9×39 \times 3 (or 9×3+09 \times 3 + 0) arrangement. One microtubule has 13 protofilaments; triplets consist of 13+10+1013 + 10 + 10 protofilaments.
  • Centrosome: Consists of 2 centrioles plus their pericentriolar matrix. Also known as the Microtubule Organizing Center (MTOC).
  • Function: Forms the spindle apparatus during cell division.

Extracellular Matrix (EZM) and Cell Contacts

  • Extracellular Matrix (EZM): The space between cells.
    • Ground Substance: Water-rich gel containing Proteoglycans and Glycoproteins.
    • Fibers: Collagen, Reticular, and Elastic fibers.
    • Tissue Specificity: Tendons (high collagen = tear-resistant), Hyaline cartilage (high proteoglycan/water = cushioning), Elastic cartilage (high elastin = flexible).
  • Classification of Cell Contacts:
    • Impermeable Connections (Verschlusskontakte):
      • Tight Junctions (Zonula occludens): Membranes are interlocked so nothing can pass between cells. Found in the blood-brain barrier, bladder, and intestinal epithelium.
    • Communicating Connections (Kommunizierende Verbindungen):
      • Gap Junctions (Nexus): Channels formed by 2 Connexons (each Connexon consists of 6 Connexins). Allow exchange of ions and molecules for electrical, metabolic, and chemical coupling. Found in almost every tissue, especially the heart muscle.
    • Adhesion Contacts (Haftverbindungen):
      • Desmosomes: Spot-like "pressure plates" linked to intermediate filaments via Cadherin.
      • Zonula adhaerens: Belt-like bands linked to actin filaments via Cadherin.
      • Hemidesmosomes: Cell-matrix connections (half a desmosome) linking intermediate filaments to the EZM via Integrins.

Cell Appendages: Cilia, Flagella, and Microvilli

  • Kinozilia (Cilia):
    • Fascinated outpocketings of the membrane (2 - 20\,m long). Motile, with a uniform beat.
    • Structure: 9×2+29 \times 2 + 2 microtubule arrangement (9 circular doublets and 2 central tubules). Doublets connected by Nexin; movement powered by Dynein.
    • Occurrence: Respiratory tract (Flimmerepithel) and Fallopian tubes.
  • Flagella (Geieln):
    • Very long (200\,m) motile outpocketings, essentially a single long cilium. Propeller-like rotation used for sperm movement.
  • Microvilli:
    • Finger-like, non-motile outpocketings for increasing surface area. Collectively called the "Brush Border" (Brstensaum). Centered around 20−3020 - 30 actin filaments for stability. Found in the small intestine epithelium.
  • Pseudopodia:
    • Temporary foot-like outpocketings containing actin, myosin, and cytoplasm. Used for movement and phagocytosis in macrophages.

Cell Death: Apoptosis and Necrosis

  • Apoptosis:
    • Programmed cell death; "cell suicide." Controlled process to maintain cell number equilibrium.
    • Triggers: Internal (intrinsic), External (extrinsic, e.g., radiation/toxins), or Stress-induced (oxygen/glucose deficiency).
    • Process: Cytoskeleton collapses, cell shrinks, breaks into apoptotic bodies, and is eaten by macrophages.
  • Necrosis:
    • Accidental cell death caused by tissue damage; "death by accident."
    • Process: Cell bursts, content is released, triggering an uncontrolled inflammatory response.

Questions & Discussion

  • Q: What are the two components of the cell membrane?
    • A: Lipids and proteins.
  • Q: Which part of a phospholipid is hydrophobic and which is hydrophilic?
    • A: Head is hydrophilic, tail is hydrophobic.
  • Q: What is the term for a substance that is both hydrophobic and hydrophilic?
    • A: Amphiphilic.
  • Q: How can membrane proteins be positioned?
    • A: Peripheral or integral.
  • Q: What is the name of the total sugar residues on the cell membrane?
    • A: Glycocalyx.
  • Q: What does the Fluid-Mosaic Model state?
    • A: All membrane components are freely movable.
  • Q: Does fluidity increase or decrease with higher cholesterol?
    • A: Fluidity decreases.
  • Q: Is the membrane permeable to small uncharged molecules like H2OH_2O?
    • A: Yes.
  • Q: Is the membrane permeable to large uncharged molecules like glucose?
    • A: No.
  • Q: What are the two types of active transport?
    • A: Primary and Secondary.
  • Q: Is substance uptake achieved via exocytosis or endocytosis?
    • A: Endocytosis.
  • Q: What is found inside the cell nucleus?
    • A: DNA and kernel bodies (Nucleoli).
  • Q: What is the role of the Nucleoli?
    • A: Producing the rRNArRNA for ribosomes.
  • Q: Which cells have no nucleus?
    • A: Erythrocytes and thrombocytes.
  • Q: Which organelles have a double membrane?
    • A: Mitochondria and the cell nucleus.
  • Q: What is the main task of the rough ER?
    • A: Protein synthesis.
  • Q: How is the Golgi apparatus structured?
    • A: Cis-side (facing ER), medial part, trans-side (facing membrane).
  • Q: What is the main task of the Golgi apparatus?
    • A: Post-translational protein modifications.
  • Q: Which enzyme is in the lysosomes?
    • A: Acid phosphatase.
  • Q: Which enzyme is in the peroxisomes?
    • A: Catalase.
  • Q: What are ribosomes composed of?
    • A: rRNArRNA and proteins.
  • Q: Rank the 3 cytoskeleton types by size.
    • A: Microtubules > Intermediate filaments > Actin filaments.
  • Q: Which two motor proteins do microtubules use for intracellular transport?
    • A: Kinesin and Dynein.
  • Q: How are actin filaments constructed?
    • A: Two actin strands are helically wound.
  • Q: What is the structure of centrioles?
    • A: 9×3+09 \times 3 + 0.
  • Q: What are the types of impermeable junctions?
    • A: Tight junctions.
  • Q: How are Gap Junctions constructed?
    • A: 2 Connexons with 6 Connexins each.
  • Q: Which adhesion contacts are connected to actin filaments?
    • A: Zonula adhaerens.
  • Q: Which cell appendages are not motile?
    • A: Microvilli.
  • Q: What is the basic structure of Kinozilia?
    • A: Microtubules.
  • Q: What is the basic structure of Microvilli?
    • A: Actin filaments.
  • Q: What are the two types of cell death?
    • A: Apoptosis and Necrosis.