Cell Structure and Membrane Transport: Study Notes for General Biology

Cell Structure and Function

Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack a membrane-bound nucleus and organelles. Examples include Bacteria and Archaea.

• Eukaryotes: Have a membrane-bound nucleus and various organelles. Examples include Plants, Animals, Fungi, and Protists.

Key Differences:

• Prokaryotes: No nucleus, generally smaller, single circular DNA molecule.

• Eukaryotes: Nucleus present, larger, multiple linear chromosomes, complex organelles.

Universal Features of All Cells

All cells, regardless of type, share four fundamental features:

• Plasma membrane

• Cytoplasm/cytosol

• Genetic material (DNA)

• Ribosomes

Cell Organelles and Their Functions

Cells contain specialized structures called organelles that perform distinct functions.

• Plasma Membrane: Semi-permeable barrier that controls entry and exit of substances.

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Smooth Endoplasmic Reticulum (ER): Synthesizes lipids and detoxifies toxins.

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes proteins for export or membrane insertion.

• Ribosomes: Sites of protein synthesis; found free in cytosol or attached to rough ER.

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

• Lysosomes: Contain digestive enzymes; break down waste materials and cellular debris.

• Central Vacuole (plants): Stores water, nutrients, and waste; maintains turgor pressure.

• Mitochondria: Site of cellular respiration; produces ATP (energy currency of the cell).

• Chloroplasts (plants and some protists): Site of photosynthesis; converts solar energy to chemical energy.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) that provide structural support, cell shape, and movement.

• Centrioles (animal cells): Involved in cell division; organize microtubules.

• Cell Wall (plants, fungi, some protists, and bacteria): Rigid outer layer that provides support and protection.

• Plasmodesmata (plants): Channels between plant cells for transport and communication.

• Desmosomes (animal cells): Junctions that hold adjacent cells together.

Organelles Not Found in Plants: Lysosomes and centrioles are typically absent in plant cells.

Organelles Not Found in Animals: Cell wall, chloroplasts, and central vacuole are absent in animal cells.

Cell Membranes and Transport

Membrane Structure

The plasma membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Selective Permeability: Allows some substances to cross more easily than others.

• Amphipathic Nature: Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

Membrane Proteins

• Integral Proteins: Span the membrane; involved in transport and signaling.

• Peripheral Proteins: Loosely attached to the membrane surface; often involved in signaling or maintaining cell shape.

Location: Peripheral proteins are on the surface; integral proteins are embedded within the bilayer.

Transport Across Membranes

Cells use various mechanisms to move substances across membranes.

• Passive Transport: Does not require energy; substances move down their concentration gradient.

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

Types of Transport

Type	Energy Required?	Description
Osmosis	No	Diffusion of water across a selectively permeable membrane.
Diffusion	No	Movement of molecules from high to low concentration.
Facilitated Diffusion	No	Diffusion of molecules via transport proteins.
Active Transport	Yes	Movement of molecules against concentration gradient using energy.
Cotransport	Yes	Coupled transport of two substances across a membrane.

Osmosis and Tonicity

Osmosis is the movement of water across a membrane. Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• Isotonic: No net water movement; cell size remains constant.

• Hypertonic: Higher solute concentration outside; cell loses water and shrinks.

• Hypotonic: Lower solute concentration outside; cell gains water and swells.

Application: Predicting cell behavior in different solutions is essential for understanding osmoregulation.

Bulk Transport Processes

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

• Endocytosis: Cell engulfs external substances by forming vesicles.

• Phagocytosis: "Cell eating"; cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid.

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

Water Potential

Water potential ($\Psi$) predicts the direction water will flow. It is determined by solute potential ($\Psi_s$) and pressure potential ($\Psi_p$):

$\Psi = \Psi_s + \Psi_p$

• Solute Potential ($\Psi_s$): Always negative; more solute lowers water potential.

• Pressure Potential ($\Psi_p$): Physical pressure on a solution; can be positive or negative.

Application: Water moves from areas of higher water potential to lower water potential.

Example Calculation: If a plant cell has $\Psi_s = -0.7$ MPa and $\Psi_p = 0.5$ MPa, then $\Psi = -0.2$ MPa.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen entering a cell
Facilitated Diffusion	No	High to Low	Glucose transport via carrier proteins
Osmosis	No	High to Low (water)	Water uptake by plant roots
Active Transport	Yes	Low to High	Sodium-potassium pump
Bulk Transport	Yes	Varies	Endocytosis, exocytosis

Additional info: The notes above expand on brief points from the original file, providing definitions, examples, and context for each concept to ensure a comprehensive understanding suitable for exam preparation.