Cell Structure, Function, and Transport: Study Notes for Anatomy & Physiology

Cellular Structure and Function

What Do Cells Do?

Cells are the basic units of life and perform essential functions necessary for the survival of organisms.

• Cell Metabolism: Chemical reactions that provide energy and build cellular components.

• Transport: Movement of substances into, out of, and within the cell.

• Communication: Cells send and receive signals to coordinate activities.

• Reproduction: Cells divide to produce new cells.

Major Parts of a Typical Cell

Most eukaryotic cells share three main structural components:

• Plasma Membrane: Outer boundary, regulates entry and exit of substances.

• Cytoplasm: Gel-like substance containing organelles.

• Nucleus: Contains genetic material (DNA).

Cell Differentiation and Specialization

Cells differentiate to perform specialized functions, such as:

• Red Blood Cells: Transport oxygen.

• Neurons: Transmit electrical signals.

• Muscle Cells: Enable movement.

The Plasma Membrane

Functions of the Plasma Membrane

The plasma membrane acts as a selective barrier and communication interface.

• Gatekeeper: Controls what enters and leaves the cell.

• Protection: Maintains the integrity of the cell's interior.

• Recognition: Contains receptors for signaling and cell identification.

• Junctions: Connects cells to each other.

The Phospholipid Bilayer

The plasma membrane is primarily composed of a double layer of phospholipids.

• Phospholipids: Molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer Structure: Hydrophobic tails face inward, hydrophilic heads face outward.

The Fluid Mosaic Model

This model describes the dynamic and flexible nature of the plasma membrane.

• Proteins: Embedded within the bilayer, serve as channels, carriers, and receptors.

• Carbohydrates: Attached to proteins and lipids, involved in cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

Membrane Receptors in Medicine

Membrane receptors are targets for many drugs.

• Drugs may mimic or block natural ligands (signaling molecules).

• Example: Beta-blockers block adrenaline receptors to lower blood pressure.

Movement Across the Cellular Membrane

Concentration Gradient

Substances move from areas of higher concentration to lower concentration.

Passive Transport

• Simple Diffusion: Movement of small, nonpolar molecules directly through the membrane.

• Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Diffusion

• Simple Diffusion: No energy required; molecules move down their concentration gradient.

• Facilitated Diffusion: Uses channel or carrier proteins for transport.

Osmosis

Water moves through specialized channels called aquaporins.

• Driven by differences in solute concentration.

• Water moves to balance solute concentrations across the membrane.

Tonicity

Tonicity describes how a solution affects cell volume via osmosis.

• Isotonic: No net water movement; cell shape remains unchanged.

• Hypertonic: Water leaves the cell; cell shrinks.

• Hypotonic: Water enters the cell; cell swells.

Summary of Diffusion and Osmosis

• Diffusion: Movement of solute from high to low concentration.

• Osmosis: Movement of water from low to high solute concentration.

• Isotonic Solution: No net water movement.

• Hypertonic Solution: Cell loses water.

• Hypotonic Solution: Cell gains water.

Active Transport

• Pumps: Use energy (ATP) to move substances against their concentration gradient.

• Vesicular Transport: Movement of large particles via vesicles (endocytosis, exocytosis).

• Pinocytosis: "Cell drinking"; uptake of fluid.

• Phagocytosis: "Cell eating"; uptake of large particles.

Sodium-Potassium Pump

Maintains cellular ion balance by moving sodium and potassium ions across the membrane.

• Uses ATP to transport 3 Na+ out and 2 K+ in.

$3 ext{Na}^+_{(in)} + 2 ext{K}^+_{(out)} + ext{ATP} \rightarrow 3 ext{Na}^+_{(out)} + 2 ext{K}^+_{(in)} + ext{ADP} + ext{P}_i$

Vesicular Transport

• Endocytosis: Uptake of substances into the cell via vesicles.

• Exocytosis: Release of substances from the cell via vesicles.

Cellular Organelles

Major Organelles and Their Functions

• Ribosomes: Protein synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Protein synthesis (contains ribosomes).

  • Smooth ER: Lipid synthesis, detoxification.

• Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.

• Lysosomes: Breakdown of cellular waste and debris.

• Peroxisomes: Breakdown of fatty acids and detoxification.

• Mitochondria: ATP production (cellular energy).

• Nucleus: Contains genetic information (DNA).

Table: Major Cytoplasmic Organelles

Organelle	Function
Mitochondria	ATP production; "powerhouse" of the cell
Ribosomes	Protein synthesis
Endoplasmic Reticulum (Rough)	Protein synthesis and processing
Endoplasmic Reticulum (Smooth)	Lipid synthesis, detoxification
Golgi Apparatus	Modification and shipping of proteins/lipids
Lysosomes	Breakdown of waste and cellular debris
Peroxisomes	Detoxification, breakdown of fatty acids
Nucleus	Contains genetic material (DNA)

Cytoskeleton

Components of the Cytoskeleton

The cytoskeleton provides structural support and enables cell movement.

• Microfilaments: Actin filaments; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Tubulin proteins; involved in transport and cell division.

• Centrioles: Organize microtubules during cell division.

Centrioles and Cell Division

Centrioles play a key role in organizing the mitotic spindle during cell division.

• Microtubules radiate from centrioles to separate chromosomes.

• Centrosome: Region containing centrioles.

Summary Table: Cytoskeletal Filaments

Filament Type	Main Protein	Function
Microfilaments	Actin	Cell movement, shape
Intermediate Filaments	Various (e.g., keratin)	Mechanical strength
Microtubules	Tubulin	Transport, cell division

Additional info: These notes expand on the original slides by providing definitions, examples, and context for each topic, ensuring a comprehensive review suitable for Anatomy & Physiology students.