Cell Structure, Membrane Transport, and Water Potential: Study Guide

Cell Structure and Function

Prokaryotes vs. Eukaryotes

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

• Prokaryotes: Cells without a nucleus or membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotes: Cells with a true nucleus and membrane-bound organelles. Examples include Plants, Animals, Fungi, and Protists.

Key Differences:

• Prokaryotes: No nucleus, DNA in nucleoid region, generally smaller.

• Eukaryotes: Nucleus present, DNA enclosed, larger and more complex.

Universal Features of All Cells

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

• Plasma membrane

• Cytoplasm/cytosol

• Ribosomes

• Genetic material (DNA)

Cell Organelles and Structures

Major Organelles and Their Functions

• Cytoplasm/Cytosol: The fluid matrix inside the cell where organelles are suspended and metabolic reactions occur.

• Plasma Membrane: A selectively permeable barrier that controls the movement of substances in and out of the cell.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

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

• Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes and processes proteins.

• 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 to break down waste and cellular debris (mainly in animal cells).

• Central Vacuole: Large storage organelle in plant cells; stores water, nutrients, and waste products.

• Mitochondria: Sites of cellular respiration; produce ATP (energy currency of the cell).

• Chloroplasts: Sites of photosynthesis in plant cells; contain chlorophyll.

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

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

• Centrioles: Involved in cell division in animal cells.

• Cell Wall: Rigid outer layer found in plants, fungi, and some prokaryotes; provides support and protection.

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

• Desmosomes: Cell junctions that anchor cells together in animal tissues.

Note: Some organelles/structures are unique to plant cells (e.g., chloroplasts, cell wall, central vacuole) or animal cells (e.g., lysosomes, centrioles).

Cell Membrane Structure and Function

Selective Permeability and Amphipathic Nature

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. It is composed of a phospholipid bilayer with embedded proteins, making it amphipathic (having both hydrophilic and hydrophobic regions).

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

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

Integral vs. Peripheral Proteins

• Integral Proteins: Penetrate the hydrophobic core of the lipid bilayer; often function as channels or transporters.

• Peripheral Proteins: Bound to the surface of the membrane; do not penetrate the lipid bilayer.

Membrane Transport

Types of Transport Proteins

Transport proteins facilitate the movement of substances across the cell membrane. The two main types are:

• Channels: Provide corridors for specific molecules or ions to cross the membrane.

• Carriers: Bind to molecules and change shape to shuttle them across the membrane.

Passive vs. Active Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input.

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

Types of Membrane Transport

Type	Active/Passive	Description
Osmosis	Passive	Diffusion of water across a selectively permeable membrane.
Diffusion	Passive	Movement of molecules from high to low concentration.
Facilitated Diffusion	Passive	Transport of substances via membrane proteins down their concentration gradient.
Active Transport	Active	Transport of substances against their concentration gradient using energy.
Cotransport	Active	Coupled transport of two substances across a membrane, one down and one against its gradient.

Osmosis: Tonicity and Cell Response

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration outside the cell; water moves in, cell swells (and may burst in animal cells).

Bulk Transport Processes

Endocytosis and Exocytosis

• Exocytosis: Process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Endocytosis: Process by which cells take in materials by engulfing them in vesicles.

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

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

• Receptor-mediated Endocytosis: Uptake of specific molecules based on receptor binding.

Water Potential

Definition and Calculation

Water potential ($\Psi$) predicts the direction water will move in or out of cells. It is affected by solute concentration and pressure.

• Solute Potential ($\Psi_s$): The effect of solute concentration on water potential; always negative or zero.

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

Formula:

$\Psi = \Psi_s + \Psi_p$

Solute Potential Calculation:

$\Psi_s = -iCRT$

• i: Ionization constant (number of particles the solute dissociates into)

• C: Molar concentration (mol/L)

• R: Pressure constant ($0.0831$ L·bar/mol·K)

• T: Temperature in Kelvin (K = °C + 273)

Example: If a plant cell is placed in a solution with lower water potential, water will leave the cell, causing it to shrink (plasmolysis).

Application

• Understanding water potential is essential for predicting water movement in plant and animal cells.

• Helps explain phenomena such as wilting in plants and osmoregulation in animals.

Summary Table: Key Cell Structures and Their Presence in Plants and Animals

Structure	Plant Cells	Animal Cells
Cell Wall	Yes	No
Chloroplasts	Yes	No
Central Vacuole	Yes	No
Lysosomes	No	Yes
Centrioles	No	Yes
Plasmodesmata	Yes	No
Desmosomes	No	Yes

Additional info: This guide expands on brief notes to provide context and examples for each topic, ensuring a comprehensive review for exam preparation.