Cell Structure, Function, and Membrane Transport: Study Guide

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are the fundamental units of life, and they are classified into two main types: prokaryotic and eukaryotic cells. Understanding their differences is essential for grasping cell biology.

• Prokaryotes: Cells lacking a nucleus and membrane-bound organelles. Domains: Bacteria and Archaea.

• Eukaryotes: Cells with a nucleus and membrane-bound organelles. Groups: Protists, Fungi, Plantae, Animalia.

• Basic features shared by all cells: cell membrane, cytoplasm, DNA, ribosomes.

Surface Area to Volume Ratio: Cells rely on their surface for material exchange. As cell size increases, the surface area to volume ratio decreases, making exchange less efficient. Cells use modifications to increase surface area:

• Microvilli: Finger-like projections (intestinal cells) for nutrient absorption.

• Membrane folding: Folds increase surface area (seen in active transport/energy production).

• Elongation: Long, thin cells (e.g., nerve cells) increase surface area.

• Flattening: Flat cells (e.g., lung cells) maximize diffusion surface.

Eukaryotic cells can be larger due to internal transport systems (cytoskeleton, organelles) that reduce reliance on surface area.

Eukaryotic Cell Components

Eukaryotic cells belong to the domain Eukarya and contain specialized organelles. Each organelle has distinct functions:

• Nucleus: Stores genetic material (DNA).

• Ribosomes: Protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis/modification), Smooth ER (lipid synthesis).

• Golgi Apparatus: Packages and sorts proteins/lipids.

• Vacuoles: Storage; large and permanent in plants (water, turgor, waste), small and temporary in animals (transport, storage).

Endomembrane System

The endomembrane system coordinates the production, packaging, and transport of proteins and lipids:

• Proteins: Nucleus → Ribosomes (on Rough ER) → ER (folding/modification) → Transport vesicles → Golgi Apparatus → Vesicles → Final destination (membrane, lysosome, secretion).

• Lipids: Synthesized in Smooth ER → Golgi Apparatus → Final destination.

• Free ribosomes: Proteins remain in cytoplasm.

• Bound ribosomes: Proteins targeted to membrane, lysosomes, or export.

Energy Organelles

Eukaryotic cells contain organelles specialized for energy conversion:

• Mitochondria: Site of cellular respiration, produces ATP.

• Chloroplasts: Site of photosynthesis, produces glucose using light (in plants).

• Number of mitochondria: Varies by cell energy demand.

Endosymbiont Theory: Mitochondria and chloroplasts originated from free-living prokaryotes. Evidence includes:

• Own DNA

• Double membrane

• Independent reproduction

• Similar size to bacteria

Cytoskeleton

The cytoskeleton provides structural support, movement, and organization:

• Microfilaments (Actin): Shape and movement.

• Intermediate filaments: Strength and stability.

• Microtubules: Transport tracks, spindle fibers in cell division.

• Cilia: Move fluid across cell surface.

• Flagella: Propel the cell.

Extracellular Components

Cells have structures outside the membrane for support and communication:

• Plant cell wall: Composed of cellulose; provides protection, shape, prevents over-expansion.

• Extracellular matrix (animal cells): Composed of glycoproteins and proteoglycans; supports, anchors, communicates, regulates behavior.

• Plasmodesmata (plants): Channels for direct transport and communication.

• Animal cell junctions:

  • Tight junctions: Seal between cells, prevent leakage.

  • Desmosomes: Anchor cells, provide strength.

  • Gap junctions: Channels for ions/small molecules.

Membrane Structure and Transport

Fluid Mosaic Model of the Plasma Membrane

The plasma membrane is described by the fluid mosaic model:

• Proteins embedded and scattered throughout the membrane.

• Phospholipids form a bilayer; lipid composition varies.

• Hydrophobic tails inside, hydrophilic heads outside.

• Membrane proteins differ by function: receptors, enzymes, identification tags, transporters.

• Phospholipids and proteins can move; membrane is fluid and dynamic.

• Membrane is selectively permeable.

Phospholipid Bilayer Formation: Hydrophilic heads face water, hydrophobic tails face inward, forming a bilayer in aqueous environments.

Cholesterol: Regulates membrane fluidity; reduces fluidity at high temperatures, increases at low temperatures.

Inner vs. Outer Surfaces: Not identical; outer surface has carbohydrates (cell recognition/signaling), inner surface lacks these.

Integral Membrane Proteins

Integral proteins serve various functions:

• Transport: Move substances across membrane.

• Enzyme activity: Catalyze reactions.

• Signal transduction: Relay messages.

• Cell-cell recognition: Identify other cells.

• Intercellular joining: Connect cells.

• Attachment to cytoskeleton: Anchor cell.

Glycolipids and Glycoproteins: Carbohydrate chains attached to lipids (glycolipids) or proteins (glycoproteins) function in cell-cell recognition (e.g., immune response, tissue development).

Permeability of the Plasma Membrane

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Substances that slip between phospholipids: Small, nonpolar molecules (O2, CO2).

• Substances assisted by carrier proteins: Glucose, amino acids, ions (Na+, K+, Ca2+).

• Substances needing transport proteins: Polar molecules, ions, large molecules.

• Substances crossing by vesicle formation: Large molecules, particles, whole cells (exocytosis/endocytosis).

Transport Mechanisms

Transport across membranes occurs via passive or active mechanisms:

• Passive transport: Moves substances down concentration gradient, no energy required.

• Active transport: Moves substances against gradient, requires energy (ATP).

Exocytosis: Substances exit cell via vesicles (active).

Endocytosis: Substances enter cell via vesicles (active).

Diffusion and Osmosis

Diffusion: Net movement from high to low concentration.

Osmosis: Movement of water across a selectively permeable membrane toward lower water potential (higher solute concentration).

Water Potential Equation:

$\Psi = \Psi_s + \Psi_p$ Where $\Psi$ is water potential, $\Psi_s$ is solute potential, $\Psi_p$ is pressure potential.

Solution Effects:

• Isotonic: No net water movement.

• Hypotonic: Water enters cell; animal cells may burst, plant cells remain turgid.

• Hypertonic: Water leaves cell; animal cells shrink, plant cells shrink.

Adaptations:

• Freshwater organisms: Contractile vacuoles pump excess water out.

• Marine organisms: Actively transport salts to increase internal solute concentration.

Facilitated Transport

Facilitated transport is passive, using membrane proteins to move molecules down their gradient without ATP.

Active Transport

Active transport moves molecules against their gradient, requiring ATP and protein pumps.

Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ in per cycle, using ATP.

$3 \text{Na}^+_{\text{out}} + 2 \text{K}^+_{\text{in}}$

Types of Endocytosis

• Phagocytosis: Cell engulfs large particles/cells ("cell eating").

• Pinocytosis: Cell takes in fluids/solutes ("cell drinking").

• Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

Receptor proteins are essential for receptor-mediated endocytosis, ensuring specificity.

Cotransport Proteins

Cotransport proteins move two solutes simultaneously; one moves down its gradient (passive), providing energy to move the other against its gradient (active).

Carrier, Channel, and Cotransport Proteins

Type	Mechanism	Example	Purpose
Carrier Protein	Bind solute, change shape to move across membrane	Glucose transporter	Uptake of glucose for energy
Channel Protein	Form pore for ions/molecules	Ion channels	Allow passage of ions
Cotransport Protein	Move two solutes simultaneously	Sodium-glucose transporter	Couple transport of Na+ and glucose

Example:

The sodium-glucose cotransporter uses the energy from Na+ moving down its gradient to transport glucose against its gradient.

Additional info: Water potential questions require calculation using the provided equation. Understanding the effects of different solutions on cells is critical for physiology and cell biology.