Cell Biology and Membrane Transport Flashcards

Somatic Cells and Gametes

Somatic Cells
- Also known as body cells.
- Contain a diploid ( $2N$ ) number of chromosomes, which totals $46$ chromosomes ( $23$ pairs).
- Comprise the vast majority of body tissues and organs.
- Genetic information in these cells is not passed on to children.
Gametes
- Also known as sex cells (specifically sperm and egg cells).
- Contain a haploid ( $1N$ ) number of chromosomes, which totals $23$ individual chromosomes.
- Located within the reproductive organs (ovaries in females and testes in males).
- Function: Their nuclei fuse together during the process of fertilization.
- Genetic information in these cells is passed on to children.

General Cell Structure and Characteristic Functions

General Parts of a Cell
- Plasma (Cell) Membrane: The outer boundary of the cell; it allows the cell to interact with its external environment.
- Nucleus: The control center that directs cellular activities.
- Cytoplasm: The region located between the plasma membrane and the nucleus; it contains organelles, which are specialized structures that perform specific functions.
Characteristic Functions of the Cell
- Cell Metabolism and Energy Use: Involves energy transfer and the production of heat.
- Synthesis of Molecules: The creation of various biological molecules required for function.
- Communication: Interaction with other cells and the environment.
- Reproduction and Inheritance:
  - Genetic information ( $DNA$ ) determines the structural and functional characteristics of the cell.
  - Gametes are used for the transmission of genetic information to the next generation.

The Plasma Membrane: Functions and Potential

Plasma Membrane Functions
- Acts as a boundary separating cytoplasmic (intracellular) substances from the extracellular environment.
- Encloses and supports cell contents.
- Provides attachment sites to the extracellular environment or other cells.
- Enables the ability to recognize and communicate with other cells.
- Determines the selective movement of substances into and out of the cells.
Production of a Membrane Potential
- Definition: An electrical charge difference across the plasma membrane resulting from the cell's regulation of ion movement.
- Outside of Membrane: More positively ( $+$ ) charged ions are gathered along the outside, giving it a positive charge.
- Inside of Membrane: More negatively ( $-$ ) charged ions and proteins are located on the inside, giving it a negative charge.

Composition of the Plasma Membrane

Primary Composition: Mostly lipids and proteins, with a very small amount of carbohydrates.
Glycocalyx: Combinations of carbohydrates with lipids (glycolipids) and proteins (glycoproteins) located on the outer face of the membrane.
Membrane Lipids (The Predominant Component)
- Phospholipids: Arranged in a bilayer.
  - Polar Heads: Hydrophilic (water-loving); they face the water in both the interior and exterior of the cell.
  - Non-polar Tails: Hydrophobic (water-fearing); they face each other in the interior of the membrane.
- Cholesterol: Interspersed among the phospholipids. The amount of cholesterol determines the fluid nature of the membrane and provides stability.
Fluid-Mosaic Model: This fluid nature allows for:
- The distribution of molecules within the membrane.
- The automatic reassembly of phospholipids if the membrane is damaged.
- The ability of membranes to fuse with one another.

Membrane Proteins and Their Roles

Integral Membrane Proteins: These extend deeply into the membrane, often spanning from one surface to the other. They can form channels through the membrane.
Peripheral Membrane Proteins: These are attached to integral proteins at either the inner or outer surfaces or to the polar heads of phospholipids.
Functional Categories (Depend on 3-D Shape and Chemical Characteristics)
- Marker Proteins: Primarily glycoproteins or glycolipids; they allow cells to identify one another or other molecules.
  - Distinguish between "self" cells and foreign cells.
  - Recognition of the oocyte (egg) by the sperm cell.
  - Facilitate intercellular communication.
- Attachment Sites: For adhering to other cells or the extracellular matrix.
- Transport Proteins: Involve carrier proteins or channels.
  - Specificity: Specific for a single type of molecule based on shape.
  - Competition: Occurs among molecules of similar shape.
  - Saturation: The rate of transport is limited by the number of available carrier proteins.

Detailed Transport Proteins: Channels and Carriers

Channel Proteins (Integral Proteins)
- Form tiny tunnels; the size, shape, and charge of the tunnel determine what can move through.
- Hydrophobic regions face the membrane lipids; hydrophilic regions line the internal tunnel.
- Leak Ion Channels (Nongated): Always open; responsible for permeability when the cell is at rest.
- Gated Ion Channels: Opened or closed by stimuli.
  - Ligand-gated: Open in response to small molecules (ligands) binding to proteins/glycoproteins.
  - Voltage-gated: Open when there is a change in the electrical charge across the membrane.
- Clinical Note: Cystic Fibrosis is a genetic disorder affecting chloride ion channels, resulting in thick, viscous secretions.
Carrier Proteins (Transporters)
- The specific molecule enters and attaches to a binding site; the protein changes shape to transport the molecule to the other side and resumes its original shape afterward.
- Uniporters: Move one specific ion or molecule.
- Symporters (Cotransport): Move two different ions/molecules in the same direction simultaneously.
- Antiporters (Countertransport): Move two different ions/molecules in opposite directions simultaneously.
ATP-Powered Pumps
- Contain binding sites for specific ions/molecules and $ATP$ .
- Hydrolysis of $ATP$ to $ADP$ releases energy to change the pump's shape to move substances.
- Once the ion and phosphate are released, the pump resumes its original shape.
Receptor Proteins
- Proteins/glycoproteins with exposed receptor sites on the outer surface.
- Act as an intercellular communication system (ligands only bind to cells with specific receptors).
- Receptors Linked to Channel Proteins: Attachment of a signal (e.g., acetylcholine) changes the channel's shape to open or close it, altering permeability. Toxins can block these sites, disrupting activity.

Movement through the Plasma Membrane

Selective Permeability: The membrane only allows certain substances to pass to maintain homeostasis.
- High Concentration Inside: Enzymes, proteins, glycogen, and potassium ( $K^+$ ).
- High Concentration Outside: Sodium ( $Na^+$ ), Calcium ( $Ca^{2+}$ ), and Chloride ( $Cl^-$ ).
- Lipid-soluble molecules (e.g., $O_2$ , $CO_2$ , steroids): Dissolve directly in the lipid bilayer.
- Non-lipid-soluble molecules/ions: Require transport proteins or vesicles.
Passive Membrane Transport (No $ATP$ required; moves from high to low concentration)
- Diffusion: Net movement of solutes down a concentration gradient due to random thermal motion. Rate is affected by:
  - Gradient: Steeper is faster.
  - Temperature: Higher is faster.
  - Particle Size: Larger is slower.
  - Solvent Viscosity: More viscous is slower.
- Osmosis: Diffusion of water (solvent) across a selectively permeable membrane. Water moves toward the higher solute concentration.
  - Aquaporins: Specific water channel proteins.
  - Osmotic Pressure: The force required to prevent water movement via osmosis.
- Facilitated Diffusion: Mediated transport using carrier/channel proteins for large or charged molecules (e.g., glucose) without using $ATP$ .
Osmotic Terms
- Isosmotic: Solutions with equal solute concentrations.
- Hyperosmotic: Solution with a higher solute concentration (greater osmotic pressure).
- Hyposmotic: Solution with a lower solute concentration (lesser osmotic pressure).
- Isotonic: Cell size remains constant.
- Hypertonic: Cell shrinks (crenation) as water moves out.
- Hypotonic: Cell swells and may rupture (lysis) as water moves in.

Active and Vesicular Transport

Active Transport: Requires $ATP$ to move substances against their concentration gradient (low to high).
- Example: Sodium-Potassium Pump ( $Na^+-K^+$ Pump): Moves $Na^+$ out and $K^+$ into the cell using $ATP$ .
Secondary Active Transport: Uses the potential energy of one substance's concentration gradient (usually $Na^+$ ) to move another substance (like glucose) against its gradient.
Vesicular Transport (Requires $ATP$ )
- Endocytosis: Movement into the cell.
  - Phagocytosis: Ingestion of solid particles; forms large vesicles.
  - Pinocytosis: Ingestion of dissolved molecules; forms small vesicles.
  - Receptor-Mediated Endocytosis: Molecules bind to specific receptors before being internalized in a vesicle.
- Exocytosis: Release of materials from a secretory vesicle out of the cell.
- Transcytosis: Movement through a cell (endocytosis on one side, exocytosis on the other); utilized by $HIV$ .

Cytoplasm and Cytoskeleton

Cytoplasm: Cellular material outside the nucleus. Consists of:
- Cytosol: The fluid portion containing dissolved ions and molecules (especially enzymes).
- Cytoplasmic Inclusions: Aggregates of chemicals like lipid droplets, melanin, glycogen, and lipochromes.
Cytoskeleton: Provides support and allows movement.
- Microtubules: Hollow tubes of tubulin; provide scaffold, used in cell division, and form centrioles, cilia, and flagella.
- Actin Filaments (Microfilaments): Provide structure, support for microvilli, and contractility.
- Intermediate Filaments: Provide mechanical strength (e.g., in nerve cell extensions).

The Nucleus and Organelles

Nucleus: Membrane-bound structure containing DNA.
- Nuclear Envelope: Double membrane.
- Nuclear Pores: Fused areas regulating movement in/out.
- Nucleolus: Site where ribosomes are manufactured.
- DNA Organization: Associated with histone proteins.
  - Nucleosomes: Structural units of chromosomes.
  - Chromatin: Dispersed form of chromosomes during most of the cell cycle.
Ribosomes: Sites of protein synthesis.
- Composed of a large and small subunit (rRNA + proteins).
- Free Subunits: Synthesize proteins for internal use.
- Attached Subunits: Found on the Rough ER; produce secreted or integral proteins.
Endoplasmic Reticulum (ER)
- Network of membranes forming cisternae.
- Rough ER: Studded with ribosomes; site of protein production and modification.
- Smooth ER: No ribosomes; site of lipid manufacture, detoxification, and $Ca^{2+}$ storage.
Golgi Apparatus: Flattened sacs that modify, package, and distribute proteins and lipids.
- Uses transport vesicles.
- Has a cis face (receiving) and a trans face (shipping).
Lysosomes and Peroxisomes
- Lysosomes: Form at the Golgi; contain hydrolytic enzymes for digestion of nutrients, bacteria, or old organelles (autophagy).
  - Tay-Sachs Disease: Genetic inability to break down gangliosides (membrane lipids of neurons).
- Peroxisomes: Smaller; break down fatty acids and amino acids. Produce hydrogen peroxide ( $H_2O_2$ ) as a byproduct, which is then broken down into water and oxygen by the enzyme catalase.
Mitochondria: Major site of $ATP$ synthesis.
- Cristae: Inner membrane infoldings (electron transport chain enzymes).
- Matrix: Fluid space (citric acid/Krebs cycle enzymes).
- Contain their own $DNA$ .

Surface Extensions and Specialized Structures

Centrioles: Located in the centrosome; organize spindle fibers for cell division.
Cilia: Project from surfaces; move materials (e.g., mucus) over the cell surface.
Flagella: Used for cell movement (e.g., sperm).
Microvilli: Extensions of the plasma membrane that increase surface area; supported by actin; do not move.

Gene Expression and Apoptosis

Genes: Functional units of heredity ( $DNA$ segments).
Gene Expression Steps:
1. Transcription (Nucleus): $RNA$ polymerase binds to a promoter region. It uses $DNA$ as a template to create $mRNA$ . Stops at a terminator sequence.
  - Basing pairing: Adenine-Uracil ( $A-U$ ), Cytosine-Guanine ( $C-G$ ).
2. Translation (Cytoplasm/Ribosomes): $mRNA$ nucleotide sequence determines polypeptide chain composition.
  - Codons: 3-nucleotide sequences on $mRNA$ ( $64$ total).
  - Start Codon: $AUG$ (Methionine).
  - Stop Codons: $UAA$ , $UGA$ , $UAG$ .
  - tRNA: Carries amino acids to ribosomes; has anticodons complementary to codons.
  - Peptide Bonds: Formed between amino acids.
Apoptosis: Programmed cell death.
- Used to remove excess tissue, damaged cells, virus-infected cells, or potential cancer cells.
- Cell fragments are ingested by macrophages.