Cell Structure, Function, and Membrane Transport: General Biology Study Guide

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Cell Structure and Function

Microscopes

Microscopes are essential tools for studying cells and their components. They vary in their mechanisms and capabilities.

Scanning Electron Microscope (SEM): Produces detailed images of cell surfaces by scanning with a focused beam of electrons.
Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens to reveal internal structures.
Resolution: The ability to distinguish two close objects as separate; higher in electron microscopes than light microscopes.
Magnification: The process of enlarging the appearance of an object.
Example: SEM is used to view the surface of pollen grains at high magnification.

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components for study.

Purpose: To isolate organelles and macromolecules for biochemical analysis.
How it works: Cells are broken apart, and components are separated by centrifugation based on size and density.
Example: Isolation of mitochondria to study ATP production.

Cell Size and Surface Area

The size and shape of cells are limited by the relationship between surface area and volume.

Surface Area: Determines the rate of exchange of materials with the environment.
Volume: Determines the amount of metabolic activity a cell can support.
Limitation: As cells grow, volume increases faster than surface area, limiting cell size.
Formula: ;

Prokaryotes vs. Eukaryotes

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

Prokaryotes: Lack a nucleus; DNA is in a region called the nucleoid. Examples: Bacteria, Archaea.
Eukaryotes: Have a true nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.
Comparison Table:

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Membrane-bound organelles	No	Yes
Size	Smaller	Larger
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Chromatin

Chromatin is the complex of DNA and proteins found in the nucleus of eukaryotic cells.

Function: Packages DNA into a compact form and regulates gene expression.
Example: Chromatin condenses to form chromosomes during cell division.

Common Components of All Cells

Despite differences, all cells share certain features.

Plasma membrane
Cytoplasm
Ribosomes
DNA

Organelles: Structure and Function

Eukaryotic cells contain specialized organelles with distinct functions.

Nucleus and Nucleolus: Stores genetic material; nucleolus synthesizes ribosomal RNA.
Mitochondria: Site of cellular respiration and ATP production.
Chloroplasts: Site of photosynthesis in plant cells.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosome: Contains hydrolytic enzymes for intracellular digestion.

Information Flow in Cells

Cells synthesize proteins using ribosomes, which may be free in the cytoplasm or bound to the ER.

Free Ribosomes: Produce proteins for use within the cell.
Bound Ribosomes: Produce proteins for export or for use in membranes.

Autophagy

Autophagy is the process by which cells degrade and recycle their own components.

Function: Maintains cellular homeostasis and removes damaged organelles.

Endosymbiosis Theory

This theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

Key Point: These organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement within cells.

Components: Microfilaments, microtubules, intermediate filaments.
Functions: Maintains cell shape, enables movement, assists in cell division.

Cell Junctions

Cell junctions connect cells and facilitate communication.

Plasmodesmata: Channels between plant cells for transport and communication.
Desmosomes: Anchor cells together in animal tissues.
Tight Junctions: Prevent leakage of extracellular fluid.
Gap Junctions: Allow ions and small molecules to pass between animal cells.

Cell Membranes and Transport

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts a dynamic structure of lipids and proteins.

Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.
Proteins: Embedded or attached; serve as channels, receptors, or enzymes.

Cholesterol

Cholesterol is interspersed within the membrane, affecting fluidity and stability.

Function: Maintains membrane fluidity at varying temperatures.
Example: Antarctic fish have membranes with more unsaturated fatty acids for fluidity in cold environments.

Saturated vs. Unsaturated Phospholipids

The type of fatty acids in phospholipids influences membrane properties.

Saturated: No double bonds; pack tightly, less fluid.
Unsaturated: Double bonds; kinked tails, more fluid.

Membrane Proteins

Proteins in the membrane are classified by their association with the lipid bilayer.

Integral/Transmembrane Proteins: Span the membrane; involved in transport and signaling.
Peripheral Proteins: Attached to the surface; involved in cell signaling and structure.
Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

Membrane Transport Mechanisms

Cells exchange materials with their environment through various transport mechanisms.

Diffusion/Passive Transport: Movement of molecules from high to low concentration without energy input.
Facilitated Diffusion: Passive transport via membrane proteins.
Osmosis: Diffusion of water across a selectively permeable membrane.
Active Transport: Movement against a concentration gradient using energy (ATP).
Example Equation:

Types of Molecules Crossing the Membrane

Membrane permeability depends on molecule size, polarity, and charge.

Can Pass: Small, nonpolar molecules (e.g., O2, CO2).
Cannot Pass: Large, polar molecules and ions (e.g., glucose, Na+).

Membrane Potential

Membrane potential is the voltage difference across the plasma membrane due to ion distribution.

Maintained by: Ion pumps (e.g., sodium-potassium pump).
Equation:

Bulk Transport

Cells transport large molecules via vesicles in processes called endocytosis and exocytosis.

Pinocytosis: Uptake of fluids and dissolved substances.
Phagocytosis: Engulfment of large particles or cells.
Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor proteins.

Organelle Functions and Cellular Processes

Chloroplasts

Chloroplasts are organelles in plant cells responsible for photosynthesis.

Function: Convert light energy into chemical energy (glucose).
Components: Thylakoids, stroma, DNA, ribosomes.

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration.

Structure: Double membrane, cristae (folds), matrix.
Function: ATP synthesis.
Labeling Example:

Label	Structure
A	Outer membrane
B	Inner membrane
C	Cristae
D	Matrix

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis.

Rough ER: Studded with ribosomes; synthesizes proteins.
Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Function: Forms lysosomes, buds off vesicles.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

Function: Breakdown of macromolecules, autophagy, and defense against pathogens.
Example: Tay-Sachs disease results from lysosomal enzyme deficiency.

Vacuoles

Vacuoles are large vesicles in plant and some animal cells for storage and waste disposal.

Central Vacuole: Maintains cell turgor and stores nutrients in plant cells.

Cellular Transport and Membrane Dynamics

Osmosis and Tonicity

Osmosis affects cell volume and shape depending on the surrounding solution's tonicity.

Hypotonic: Lower solute concentration outside; cell swells.
Hypertonic: Higher solute concentration outside; cell shrinks.
Isotonic: Equal solute concentration; no net water movement.

Sodium-Potassium Pump

The sodium-potassium pump is an electrogenic pump that maintains ion gradients across the membrane.

Function: Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed.
Equation:

Phospholipids and Membrane Components

Phospholipids are amphipathic molecules forming the bilayer; other components include cholesterol and glycolipids.

Phospholipids: Hydrophilic head, hydrophobic tail.
Cholesterol: Modulates fluidity.
Glycolipids: Lipids with carbohydrate chains; involved in cell recognition.

Carrier Proteins

Carrier proteins facilitate the transport of specific molecules across the membrane.

Specificity: Each carrier protein transports a particular molecule.
Energy: Some require energy (active transport), others do not (facilitated diffusion).

Bulk Flow and Vesicular Transport

Cells use vesicles for bulk transport of materials.

Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.
Endocytosis: Cell engulfs material by forming vesicles from the membrane.
Phagocytosis: Engulfment of large particles; used by white blood cells.
Pinocytosis: Uptake of fluids and small molecules.
Receptor-Mediated Endocytosis: Specific uptake via receptor proteins.

Additional info:

Some content inferred from standard General Biology curriculum and context of questions and outline.
Tables and diagrams described in text for clarity and completeness.