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细胞生物.FINAL | Cellular Bio Review
0x00A0
  08/Sep/2025   08/Sep/2025   

PM

Properties:

  • Semipermeable

  • Fluid (Fluid Mosaic Model)

    • Rotation, Lateral diffusion, Flip-flop, Flexion

  • Asymmetric

    • Lipids:

      Outer: SM,PC

      Inner: PS,PE,PI

    • Proteins: Some face only outwards or inwards (Glycolipids face outside)

    • Carbohydrates: only on the outer layer

Fluidity

  • fatty acid tail length
  • saturation(saturated = a twist on tail)
  • temperature
  • cholesterol(like glue)
  • protein content

Glycocalyx

  • Proteins + Sugars on outer layer

  • Glycolipids and Glycoproteins

  • Functions:

    Protection (mechanical and chemical)

    Cell to cell interaction or repulsion

Proteins

Functions:

  • Receptors
  • Channel
  • Photosynthesis
  • Election transport (see mitochondria and ETC)
  • Structural support
  • Enzymes

Classes:

  • Tranmembrane (integral): alpha-helix or beta-barrel
  • Lipid-linked proteins (covalently bonded)
  • Peripheral proteins(non-covalently bonded)

Membrane potential

mainly determined by **K+ leak channels **(always open), also by Na+/K+ pump

Why Bilayer?

Gorter and Grender’s experiment:

Measure the surface area of a erythrocyte, then expand it to a single lipid layer, measure the surface area again, found the ratio is 1:2

Transport across the membrane

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Transport
├── 1. Passive Transport (Diffusion)   ← No ATP
│   ├── a) Simple diffusion            ← nonpolar, no protein
│   ├── b) Channel-mediated diffusion  ← ions, aquaporins (no binding)
│   └── c) Carrier-mediated diffusion  ← e.g., GLUT (binds to carrier)
│
└── 2. Active Transport                ← Requires ATP or ion gradient
    ├── a) ATP-driven pumps (Transport ATPases)
    │   ├── i. P-type pump              ← Na⁺/K⁺, Ca²⁺, self-phosphorylation
    │   ├── ii. V-type proton pump     ← vesicles, acidify
    │   └── iii. ABC transporter       ← pumps small molecules
    │
    ├── b) Co-transport (Secondary Active)
    │   ├── i. Symport                 ← Na⁺/glucose (same direction)
    │   └── ii. Antiport               ← Na⁺/H⁺ exchanger (opposite direction)
    │
    └── c) Secondary Active Transport
        └── Uses gradient from ATP pumps (e.g., Na⁺ gradient from Na⁺/K⁺ pump)

Endocytosis

Endocytosis is a process where the plasma membrane invaginates (forms a pocket) to bring substances into the cell. There are three main types:

  1. Phagocytosis (/ˌfæɡ.oʊ.saɪˈtoʊ.sɪs/) – “cell eating,” where large particles or even microorganisms are engulfed into phagosomes.
  2. Pinocytosis (/ˌpaɪ.nə.saɪˈtoʊ.sɪs/) – “cell drinking,” where the cell takes up extracellular fluid and dissolved solutes.
  3. Receptor-mediated endocytosis – highly specific; receptors on the membrane bind ligands, then coated pits with clathrin form vesicles.

This process is important for nutrient uptake, receptor regulation, and defense against pathogens.

Nucleus

Nuclear pores

Nuclear pores are large protein complexes embedded in the nuclear envelope. They allow selective transport of RNA and proteins between the nucleus and the cytoplasm.

Export:

  • mRNA, tRNA, Ribosome subunits(assembled in nucleolus)
  • with Nuclear Export Signal (a short amino acid seq)
  • carried by Exportins (Ran-GTP->Ran-GDP, hydrolysed)

Import:

  • RNA polymerase, DNA polymerase, Ribosomal Proteins (to nucleolus)
  • with Nuclear Localisation Signal (a short amino acid seq)
  • carried by Importins (Ran-GTP -> Ran-GDP, hydrolysed)

DNA metabolism proteins

  • DNA polymerases

    for DNA replication

  • DNA repair enzymes

    to repair DNA

  • Nucleases

  • Transcription factors

    Binds to DNA, initiates transcription

  • RNA polymerases

    Directly involved in RNA synthesis

Cell Cycle

  • Cell fate: Cells either divide (proliferation) or exit the cycle into terminal differentiation.

Stages

  • Interphase（G1, S, G2）

    • G1/G2: transcription, protein synthesis, organelle duplication.
    • S phase: DNA replication, chromosome duplication.

  • M phase (Mitosis): Chromosomes condense, nuclear envelope breaks down, cell divides.

    • Prophase

    ​ Chromosomes condense, nuclear envelope begins to break down

    • Prometaphase

      Nuclear envelop disappears, spindle fibres attach to chromosomes at kinetochores

    • Metaphase

      chromosomes align at the metaphase plate

    • Anaphase

      Sister chromatids separate, move toward opposite poles

    • Telophase

      Nuclear envelop reform around chromosomes, chromosomes decondense

    • Cytokinesis

Checkpoints:

  • G1 (restriction point): requires growth factors, nutrients, space
  • **G1/S: **checks DNA replication initiation, histone synthesis
  • G2/M: checks DNA damage and replication completion
  • M phase (spindle checkpoint): ensures correct chromosome alignment.

if DNA damages: Cell cycle arrest for repair or apoptosis

Apoptosis

Programmed cell death

Features

  • Cell shrinkage, membrane blebbing
  • Chromatin condensation and fragmentation
  • apoptotic bodies formation
  • No inflammation

Enzyme: Caspases

  • Exist as inactive procaspases
  • Activated by proteolysis
  • Cascade activation -> amplification of apoptosis signal
  • Executioner caspases

Cell Junctions

Morpho-functional polarity

  • Apical surface
  • Basal surface
  • Lateral surface

Types

  • Tight Junctions
    • at apical region
    • diffusion barrier
    • composed of claudins, occludins, linked to actin cytoskeleton
  • Adherens Junctions
    • Stronger
    • by e-Cadherin, need Ca
    • can resist pulling and shearing forces
    • can be found in epithelial tissues and cardiac muscle
  • Desmosomes
    • spot-like, most strong
  • Gap Junctions
    • cell communication (can start apoptosis)
    • by Connexins
  • Hemidesmosomes
    • connect cells to basal lamina

Cell Communication

Modes

  1. Secreted molecules
    • Autocrine
    • Paracrine
    • Endocrine (hormones)
    • Synaptic
    • Neuroendocrine
  2. Direct contact 直接接触
    • Cell–cell adhesion
    • Cell–ECM(Extracellular Matrix) adhesion
  3. Gap junctions
    • Direct exchange of ions/small molecules
  4. Extracellular vesicles
    • Carry proteins, RNAs, lipids
  5. Mechanotransduction (hair cells detect sound waves)

Mitochondria

Structure

  • Outer Membrane (Porins and TOM complex here)
  • Intermembrane space (Proton gradient happens here)
  • Inner Membrane (Cristae here)
  • Matrix (Contains their own DNAs, ribosomes, tRNAs, mRNAs and enzymes for the Krebs cycle)

ATP Synthesis

  1. Electrons from NADH and FADH2 are passed thro complexes in the inner membrane, releasing energy (Electron Transport Chain)
  2. This energy pumps protons into the intermembrane space, creating a proton gradient
  3. Protons can’t pass back freely to matrix, so they can only flow back through ATP synthase, ADP+Pi -> ATP (Oxidative phosphorylation)

Though mitochondria have their own DNAs, but most proteins are nuclear-encoded, made in the cytosol and imported

glucose –(glycolysis, in cytoplasm)–> pyruvate –(into matrix)–> acetyl-CoA -> Krebs cycle

Endoplasmic reticulum

A group of interconnected membrane-bound organelles working together to modify, package and transport lipids/proteins

Continuous with outer membrane of nucleus

  • Smooth for Calcium storage/release(SR), detoxification(in liver cells), and Lipids production

    Fatty acids synthesised in cytoplasm, then transport to SER

    Cholesterol synthesised in SER

  • Rough with ribosomes for protein productions

Includes:

  • PM
  • ER
  • Golgi apparatus
  • Lysosomes
  • Vesicles/Endosomes
  • Vacuoles

Co-Translation

  • The free ribosome starts translating an mRNA with a signal peptide
  • This signal is recognized by the signal recognition particle (SRP)
  • SRP pauses translation and guides the ribosome–mRNA–peptide complex to the ER membrane
  • The ribosome binds to a translocon (a channel protein on ER)
  • Translation resumes, and the polypeptide is threaded directly into the ER lumen(soluble or secreted proteins) or inserted into the ER membrane(membrane protein)

Folding and Glycosylation

  1. Inside the ER lumen, enzymes attach a pre-made oligosaccharide to an asparagine in the protein
  2. After transfer, the glycoprotein enters the ER lumen, where trimming and initial folding occur
  3. Later, in the Golgi apparatus, the sugar chains are further modified into their mature forms.

Coat Protein Complexes

They carry proteins:

  • COP II: ER->Golgi
  • COP I: Golgi->ER

Golgi Apparatus

has 2 faces: Cis face and Trans face

  1. receives proteins from the ER (by COP II)
  2. modifies and tags them (via Glycosylation, Phosphorylation) on Medial cisternae
  3. packages them into vesicles
  4. sends them to destinations(PM, lysosomes, outside the cell, etc,.)

Cytoskeleton

Building blocks:

  • Actin filaments (microfilaments): G-actin monomers (ATP)
  • Microtubules: α/β tubulin dimers (GTP)

Structure & size:

  • Actin: 7 nm, flexible double helix, near membrane.
  • Microtubules: 25 nm, hollow tube, from centrosome (MTOC).

Dynamics:

  • Actin: treadmilling (add at +, lose at −).
  • Microtubules: dynamic instability (GTP cap).

Motors:

  • Actin → Myosin (short-range).
  • Microtubules → Kinesin (+), Dynein (−) (long-range).

Functions:

  • Actin: cell shape, motility, cytokinesis, microvilli support.
  • Microtubules: organelle transport, mitosis (spindle), cilia/flagella movement.

Regulation & drugs:

  • Actin: regulated by ABPs, disrupted by Cytochalasin, Latrunculin.
  • Microtubules: regulated by MAPs, targeted by Taxol, Colchicine, Vinblastine.

Nervous

CNS and PNS

Parts:

  • Dendrites
  • Cell body
  • Axon
  • Myelin sheath (from Schwann cells)
  • Axon terminal

Glial cells

Support and protect

  • in CNS:
    • Astrocytes
    • Microglia
    • Oligodendrocytes
    • Ependymal cells
  • in PNS:
    • Schwann cells
    • Satellite glial cells

Classes

  • Multipolar
    • One axon, many dendrites
    • most common
    • integrate signals
  • Bipolar
    • One axon and one dendrite
    • sensory organs(retina)
  • Unipolar
    • one single process splits into 2 branches
  • Pseudounipolar
    • start as bipolar, later fuse into a single process splits into 2 branches
    • different from unipolar

Muscles

Types

  • Skeletal muscle
    • attached to bones
    • voluntary
    • striated
    • long fibres, multinucleated
    • fast, strong
  • Smooth muscle
    • walls of hollow organs
    • involuntary
    • single nucleus
    • slow, weak
  • Cardiac muscle
    • heart walls
    • involuntary
    • striated
    • branched fibres, usually single nucleus, connected by gap junctions
    • medium, moderate

Organisation

  • Muscle
    • Fascicle
      • Muscle Fibre
        • Sarcomere
          • Myofilaments
            • Actin
            • Myosin

Smooth muscle doesn’t have sarcomeres so they are not striated

Contraction and Relaxation

  1. CNS sends a signal via a motor neuron to skeletal muscle
  2. the signal reaches the axon terminal at the neuromuscular junction, via the axon
  3. the signal opens Ca²⁺ channels(voltage-gated channels, passive transport) on the neuron membrane, Ca²⁺ enters from extracellular fluid, making vesicles to release acetylcholine(ACh) to the synaptic cleft
  4. ACh binds sarcolemma receptors, opens Na⁺ channels(ligand-gated, passive transport)
  5. The action potential spread along the sarcolemma, into T-tubules, making the signal deep inside the fibre
  6. this signal opens Ca²⁺ channels on SR(Sarcoplasmic Reticulum), releases Ca²⁺ into the cytoplasm
  7. those Ca²⁺ can bind to troponin, causing tropomyosin to move and to expose binding sites on actin
  8. myosin binds to actin, forming Cross-bridges, the myosin can pull actin inwards with the help of ATP, muscle contraction happens
  9. when the signal ends, Ca²⁺ are pumped back into the SR actively
  10. without Ca²⁺, tropomyosin covers the binding sites again, cross-bridges break, the muscle relaxes

Connectives

Has a lot of extracellular matrix and a few cells, it may be bone, blood, or fat tissue

  • Cartilage
  • Bone
  • Blood

Epithelias

细胞生物.FINAL | Cellular Bio Review
2025/09/08/Cell Bio/CELLULAR-BIO-REVIEW/
Author
0x00A0
Published
2025-09-08 15:20
License
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  1. PM
    1. Properties:
    2. Fluidity
    3. Glycocalyx
    4. Proteins
    5. Membrane potential
    6. Why Bilayer?
    7. Transport across the membrane
  2. Endocytosis
  3. Nucleus
    1. Nuclear pores
    2. DNA metabolism proteins
    3. Transcription related proteins
  4. Cell Cycle
    1. Stages
    2. Checkpoints:
  5. Apoptosis
    1. Features
    2. Enzyme: Caspases
  6. Cell Junctions
    1. Morpho-functional polarity
    2. Types
  7. Cell Communication
    1. Modes
  8. Mitochondria
    1. Structure
    2. ATP Synthesis
  9. Endoplasmic reticulum
    1. Co-Translation
    2. Folding and Glycosylation
    3. Coat Protein Complexes
  10. Golgi Apparatus
  11. Cytoskeleton
  12. Nervous
    1. Parts:
    2. Glial cells
    3. Classes
  13. Muscles
    1. Types
    2. Organisation
    3. Contraction and Relaxation
  14. Connectives
  15. Epithelias
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  1. PM
    1. Properties:
    2. Fluidity
    3. Glycocalyx
    4. Proteins
    5. Membrane potential
    6. Why Bilayer?
    7. Transport across the membrane
  2. Endocytosis
  3. Nucleus
    1. Nuclear pores
    2. DNA metabolism proteins
    3. Transcription related proteins
  4. Cell Cycle
    1. Stages
    2. Checkpoints:
  5. Apoptosis
    1. Features
    2. Enzyme: Caspases
  6. Cell Junctions
    1. Morpho-functional polarity
    2. Types
  7. Cell Communication
    1. Modes
  8. Mitochondria
    1. Structure
    2. ATP Synthesis
  9. Endoplasmic reticulum
    1. Co-Translation
    2. Folding and Glycosylation
    3. Coat Protein Complexes
  10. Golgi Apparatus
  11. Cytoskeleton
  12. Nervous
    1. Parts:
    2. Glial cells
    3. Classes
  13. Muscles
    1. Types
    2. Organisation
    3. Contraction and Relaxation
  14. Connectives
  15. Epithelias