Cell Structure and Function: Study Notes for General Biology

Unit 2: Cell Structure and Function

2.1 Cell Structure: Subcellular Components

Cells contain various subcellular components, each with specialized structures and functions that contribute to the overall activity and survival of the cell.

• Ribosomes: Composed of ribosomal RNA (rRNA) and proteins, ribosomes are the site of protein synthesis. They can be found free in the cytosol or bound to the endoplasmic reticulum (ER).

• Endoplasmic Reticulum (ER): The ER is a network of membranes involved in protein and lipid synthesis. The rough ER has ribosomes attached and synthesizes proteins, while the smooth ER is involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: The "powerhouse" of the cell, mitochondria generate ATP through cellular respiration.

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion.

• Chloroplasts: Found in plant cells, chloroplasts carry out photosynthesis.

• Vacuoles: Membrane-bound sacs for storage and transport; large central vacuole in plant cells maintains turgor pressure.

• Cytoskeleton: Network of protein filaments that provides structural support, cell movement, and intracellular transport.

Example: The rough ER in pancreatic cells produces digestive enzymes, which are then packaged by the Golgi apparatus and secreted into the digestive tract.

2.2 Plasma Membrane

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Its structure is described by the fluid mosaic model.

• Phospholipid Bilayer: Composed of amphipathic phospholipids with hydrophilic heads and hydrophobic tails.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Cholesterol: Maintains membrane fluidity and stability.

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

Example: The sodium-potassium pump is an integral membrane protein that actively transports Na+ and K+ ions across the membrane.

2.3 Membrane Transport

Cells use various mechanisms to move substances across the plasma membrane, maintaining homeostasis and allowing communication with the environment.

• Passive Transport: Movement of molecules from high to low concentration without energy input. Includes diffusion, osmosis, and facilitated diffusion.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (usually ATP).

• Bulk Transport: Endocytosis and exocytosis move large molecules or particles.

Example: Oxygen diffuses into cells by simple diffusion, while glucose enters via facilitated diffusion through a carrier protein.

2.4 Mechanisms of Transport

Transport across membranes can be passive or active, depending on the energy requirements and direction of movement.

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

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

• Facilitated Diffusion: Passive transport of molecules via membrane proteins.

• Active Transport: Uses ATP to move molecules against their gradient.

Equation:

$J = -D \frac{dC}{dx}$ Where: J = flux D = diffusion coefficient dC/dx = concentration gradient

Example: The Na+/K+ ATPase pump maintains electrochemical gradients in animal cells.

2.5 Facilitated Diffusion

Facilitated diffusion is a type of passive transport that uses membrane proteins to move substances across the membrane.

• Channel Proteins: Form pores for specific molecules to pass through.

• Carrier Proteins: Bind and transport molecules by changing shape.

Example: Aquaporins facilitate the rapid movement of water across cell membranes.

2.6 Tonicity and Osmoregulation

Tonicity describes the ability of a solution to cause a cell to gain or lose water. Osmoregulation is the control of water and solute concentrations within a cell or organism.

• Isotonic Solution: No net movement of water; cell volume remains constant.

• Hypertonic Solution: Water moves out of the cell; cell shrinks.

• Hypotonic Solution: Water moves into the cell; cell swells.

Example: Red blood cells placed in a hypotonic solution will swell and may burst (lyse).

2.7 Mechanisms of Transport: Endocytosis and Exocytosis

Cells use endocytosis and exocytosis to transport large molecules and particles across the membrane.

• Endocytosis: The cell engulfs material by forming a vesicle from the plasma membrane.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

Example: Neurotransmitters are released from neurons by exocytosis.

2.8 Compartmentalization

Compartmentalization refers to the presence of membrane-bound organelles in eukaryotic cells, allowing specialized functions to occur in distinct regions.

• Advantages: Increases efficiency by separating incompatible reactions and concentrating substrates and enzymes.

• Examples: Lysosomes digest cellular waste; mitochondria generate ATP; chloroplasts perform photosynthesis.

Example: The separation of glycolysis (cytosol) and the citric acid cycle (mitochondria) in eukaryotic cells.

2.9 Origins of Cell Compartmentalization

Endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, circular DNA, and ribosomes similar to prokaryotes.

• Significance: Mitochondria and chloroplasts have specialized functions and contribute to cellular energy metabolism.

Example: Chloroplasts in plant cells convert solar energy into chemical energy via photosynthesis.

Table: Comparison of Membrane Transport Mechanisms

Transport Type	Energy Required	Direction	Example
Simple Diffusion	No	High to Low	Oxygen entering cells
Facilitated Diffusion	No	High to Low	Glucose transport via carrier protein
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Osmosis	No	High to Low (water)	Water movement across membrane
Endocytosis/Exocytosis	Yes (ATP)	Bulk transport	Neurotransmitter release

Additional info:

• Some diagrams and figures referenced in the original material (e.g., membrane structure, transport proteins) are standard in biology textbooks and have been described in text.

• Equations for diffusion and osmosis have been included for academic completeness.

• Endosymbiotic theory is a key concept for understanding the evolution of eukaryotic cells.