Membrane Transport and Cellular Organelles

Membrane Transport Mechanisms

Carrier Saturation and Transport Maximum ( $T_m$ )
- Carrier proteins have a finite number of binding sites.
- As the concentration of solute increases, the rate of solute transport across the plasma membrane also increases.
- However, once all carrier proteins are occupied and working at their maximum capacity, further increases in solute concentration will not increase the transport rate.
- This plateau defines the Transport Maximum ( $T_m$ ), representing the maximum rate at which a solute can be transported.
- This concept is illustrated by a graph where the rate of solute transport (molecules/s) eventually levels off despite increasing solute concentration.
Facilitated Diffusion
- A type of passive transport that utilizes carrier proteins to move solutes across the membrane.
- It does not directly require ATP.
- Mechanism:
  1. A solute particle enters the channel of a specific membrane protein (carrier).
  2. The solute binds to a receptor site on the carrier, triggering a conformational change in the carrier protein.
  3. The carrier protein then releases the solute on the opposite side of the membrane, usually moving it from a region of higher concentration to lower concentration (down its concentration gradient), such as from the Extracellular Fluid (ECF) to the Intracellular Fluid (ICF).
Primary Active Transport
- A type of active transport that directly uses energy from ATP hydrolysis to move solutes against their concentration gradient.
- A prime example is the Sodium-Potassium ( $ext{Na}^+- ext{K}^+$ ) Pump:
  - This pump expels three sodium ions ( $3 ext{ Na}^+$ ) from the cell into the ECF.
  - Simultaneously, it imports two potassium ions ( $2 ext{ K}^+$ ) into the cell from the ECF to the ICF.
  - The energy for this process is derived from the breakdown of ATP into ADP and inorganic phosphate ( $ext{P}_i$ ).
Secondary Active Transport
- An active transport mechanism that indirectly uses ATP, relying on an electrochemical gradient established by primary active transport.
- A common example is the Sodium-Glucose Linked Transporter (SGLT), often found on the apical surface of cells (e.g., in the intestines or kidneys):
  - Sodium ions ( $ext{Na}^+$ ) move down their concentration gradient into the cell (a gradient typically maintained by the Na+-K+ pump on the basal surface of the cell).
  - The energy released by $Na^+$ moving down its gradient is used to co-transport glucose into the cell, even against its own concentration gradient. This is a symport mechanism, meaning both substances move in the same direction.
  - On the basal surface, the Na+-K+ pump actively transports $Na^+$ out of the cell and $K^+$ into the cell, consuming ATP ( $ext{ATP} <br>ightarrow ext{ADP} + ext{P}_i$ ) to maintain the necessary $Na^+$ electrochemical gradient.

Vesicular Transport

Phagocytosis
- A form of endocytosis where a cell engulfs large particles, such as foreign matter, bacteria, or cellular debris. Often termed "cellular eating."
- Process includes:
  1. A phagocytic cell detects and encounters a particle of foreign matter (e.g., bacteria).
  2. The cell extends arm-like projections called pseudopods to surround the particle.
  3. The pseudopods fuse, engulfing the particle and enclosing it within a membrane-bound sac called a phagosome within the cytoplasm.
  4. The phagosome then fuses with a lysosome, forming a phagolysosome.
  5. Enzymes from the lysosome (hydrolytic enzymes) within the phagolysosome digest the foreign matter.
  6. After digestion, the phagolysosome fuses with the plasma membrane.
  7. Any indigestible residue is expelled from the cell via exocytosis.
Receptor-Mediated Endocytosis
- A selective form of endocytosis that allows cells to take up specific extracellular molecules.
- Process includes:
  1. Extracellular molecules (ligands) bind to specific receptors located on the plasma membrane.
  2. These receptor-ligand complexes then cluster together in specific regions of the membrane, which begin to sink inward, forming a clathrin-coated pit. Clathrin is a protein that helps to stabilize and shape the pit.
  3. The clathrin-coated pit eventually pinches off from the plasma membrane, forming a clathrin-coated vesicle that contains a high concentration of the specific extracellular molecules. This allows for efficient and selective uptake.
Exocytosis
- The process by which cells release substances (e.g., hormones, neurotransmitters, waste products) into the extracellular fluid.
- Mechanism:
  1. A secretory vesicle (containing the substance to be released) within the cytoplasm approaches the plasma membrane.
  2. The vesicle then "docks" on the plasma membrane, often facilitated by linking proteins, and the plasma membrane simultaneously caves inward to meet the vesicle.
  3. The membrane of the secretory vesicle fuses with the plasma membrane, forming a fusion pore.
  4. Through this fusion pore, the contents of the vesicle are released (secreted) into the extracellular space. This process is involved in secretion and the disposal of indigestible residues from phagocytosis.

Cellular Organelles and Structures

The Cytoskeleton
- A dynamic network of protein filaments and tubules in the cytoplasm of many living cells, giving them shape and coherence.
- Provides structural support, facilitates cell movement, and aids in intracellular transport.
- Composed of three main types of filaments:
  - Microfilaments (Actin filaments):
    - Solid rods, typically about $7\ \text{nm}$ in diameter.
    - Involved in cell shape changes, muscle contraction, and forming the core of microvilli and the terminal web (a fibrous mat on the cytoplasmic side of the plasma membrane).
  - Intermediate Filaments:
    - Fibers with a diameter of about $8-12\ \text{nm}$ , intermediate in size between microfilaments and microtubules.
    - Provide mechanical strength and resist stretching.
    - Found in permanent structures like desmosomes (cell-to-cell junctions) and hemidesmosomes (cell-to-extracellular matrix junctions).
  - Microtubules:
    - Hollow cylinders, typically about $25\ \text{nm}$ in diameter.
    - Grow from the centrosome and radiate throughout the cytoplasm.
    - Maintain cell shape, act as tracks for motor proteins (like kinesin) to transport organelles (e.g., secretory vesicles, lysosomes, mitochondria) and other cellular components.
    - Crucial for cell division (forming the spindle fibers) and the movement of cilia and flagella. They undergo constant assembly and disassembly.
- An image often shows a cell with a nucleus, mitochondria, lysosomes, and various cytoskeletal elements. For instance, a cell might be $30\ \text{µm}$ across.
The Nucleus
- The largest organelle, typically spherical or ovoid, containing the cell's genetic material.
- Structure:
  - Nuclear Envelope: A double membrane perforated by nuclear pores that regulate the passage of molecules between the nucleus and the cytoplasm.
  - Nucleoplasm: The jelly-like substance filling the nucleus, analogous to cytoplasm.
  - Nucleolus: A dense, spherical body within the nucleus involved in ribosome synthesis.
- The interior of the nucleus might appear about $2\ \text{µm}$ in diameter, while the surface might show details like nuclear pores at a scale of $1.5\ \text{µm}$ .
Endoplasmic Reticulum (ER)
- An extensive network of interconnected membranes forming sacs (cisternae) and tubules throughout the cytoplasm.
- Two distinct types:
  - Rough Endoplasmic Reticulum (RER):
    - Characterized by the presence of ribosomes on its cytoplasmic surface, giving it a "rough" appearance.
    - Primarily involved in the synthesis of proteins destined for secretion, insertion into membranes, or delivery to other organelles (e.g., lysosomes, Golgi).
    - Ribosomes attached to the RER synthesize proteins directly into its cisternae for folding and modification.
    - Often seen with a $1\ \text{µm}$ scale bar.
  - Smooth Endoplasmic Reticulum (SER):
    - Lacks ribosomes, giving it a "smooth" appearance.
    - Involved in lipid synthesis (e.g., steroids, phospholipids), detoxification of drugs and poisons (especially abundant in liver cells), and storage and release of calcium ions ( $\text{Ca}^{2+}$ ).
    - May be associated with inclusions like oil droplets.
    - Often seen with a $1\ \text{µm}$ scale bar.
Golgi Complex (Golgi Apparatus)
- Also known as the Golgi apparatus.
- Consists of a stack of flattened, membrane-bound sacs called cisterns (or cisternae), typically $0.3\ \text{µm}$ in width.
- Function: Modifies, sorts, and packages proteins and lipids synthesized in the ER.
- Receives transport vesicles from the ER, processes their contents, and then buds off Golgi vesicles (secretory vesicles, lysosomes, etc.) to their final destinations.
Lysosomes
- Spherical organelles enclosing a bag of about $50$ different types of potent hydrolytic (digestive) enzymes.
- Typically seen with a scale bar of $0.2\ \text{µm}$ or $0.3\ \text{µm}$ in images.
- Function:
  - Intracellular digestion: Breaks down ingested foreign matter (phagolysosomes), worn-out organelles (autophagy), and cellular debris.
  - Autolysis: Programmed cell death, where lysosomes release their enzymes to degrade the cell itself.
Peroxisomes
- Similar in appearance to lysosomes but contain different enzymes.
- Typically seen with a scale bar of $0.3\ \text{µm}$ or $1\ \text{µm}$ in images.
- Function:
  - Break down fatty acids and amino acids.
  - Detoxify harmful substances (e.g., alcohol) by converting them into hydrogen peroxide ( $\text{H}<em>2\text{O}</em>2$ ) and then into water and oxygen. They are particularly abundant in liver and kidney cells.
Proteasome
- A large protein complex responsible for degrading misfolded, damaged, or unwanted proteins in the cell's cytosol.
- Mechanism:
  1. Proteins targeted for degradation are typically tagged with a small protein called ubiquitin (implied by "Tag").
  2. The tagged, often unfolded, protein enters the proteasome.
  3. Inside the proteasome, the protein is broken down into small peptide fragments.
- This process is vital for protein quality control and cellular regulation.
Mitochondria
- Often called the "powerhouses of the cell" because they are the primary sites of ATP synthesis (aerobic respiration).
- Structure:
  - Outer Membrane: Smooth and permeable to small molecules.
  - Inner Membrane: Highly folded into structures called cristae, which increase the surface area for ATP production.
  - Intermembrane Space: The region between the outer and inner membranes.
  - Matrix: The innermost compartment, containing enzymes for the citric acid cycle, mitochondrial DNA, and mitochondrial ribosomes.
- Typically shown with a scale bar of $1\ \text{µm}$ .

Vocabulary Words from Lecture 6

Cytoskeleton: Network of protein filaments and tubules in the cytoplasm giving cells shape and coherence.
Microfilaments: Solid rods composed of actin, involved in cell movement and shape.
Intermediate Filaments: Filaments providing mechanical strength to cells.
Microtubule: Hollow cylinders involved in cell shape, organelle transport, and cell division.
Organelle: A specialized subunit within a cell that has a specific function.
Nucleus: Membrane-bound organelle containing cell's genetic material.
Endoplasmic Reticulum: Network of membranes involved in protein and lipid synthesis.
Ribosome: Complexes of rRNA and protein that catalyze protein synthesis.
Golgi Complex: Organelle that modifies, sorts, and packages proteins and lipids.
Lysosome: Organelle containing digestive enzymes for cellular waste breakdown.
Autophagy: Process by which a cell degrades and recycles its own components.
Autolysis: Self-digestion of a cell by its own enzymes, often in programmed cell death.
Peroxisome: Organelle involved in metabolic processes, breaking down fatty acids, and detoxifying.
Proteasome: Protein complex that degrades unneeded or damaged proteins.
Mitochondria: Organelle responsible for cellular respiration and ATP production.