ch 6Introduction to the Cell and Cell Transport Mechanisms

Introduction to the Cell

The Cell: Fundamental Unit of Life

Cells are the basic structural and functional units of all living organisms. They interact with their environment and are highly specialized to perform diverse biological functions.

• Cell Theory: All living things are composed of cells, and all cells arise from pre-existing cells.

• Cells are highly diverse in structure and function across different organisms.

Size Range of Cells

• Prokaryotic cells (e.g., bacteria): 1–10 μm

• Eukaryotic cells: 10–100 μm

• Light microscopes resolve structures down to 0.2 μm; electron microscopes are required for smaller organelles.

Example: Human egg cells are among the largest single cells visible to the naked eye, while most bacteria require electron microscopy for visualization.

Techniques to Visualize Cells

• Light Microscopy: Used for general cell structure; staining enhances contrast.

• Fluorescence Microscopy: Uses fluorescent dyes to label specific cell components.

• Electron Microscopy: Includes Scanning (SEM) and Transmission (TEM) for high-resolution imaging of cell ultrastructure.

Cell Size and Surface Area

Why Are Cells Small?

Cells must efficiently exchange materials (O2, nutrients, wastes, CO2) with their environment, primarily through the plasma membrane.

• As cells grow, their surface area-to-volume ratio decreases, limiting the rate of exchange.

• Larger organisms are composed of more cells, not larger cells.

Total Surface Area	Total Volume	Surface-to-Volume Ratio
6	1	6
150	125	1.2
750	125	6

Additional info: As the number of smaller cells increases, the total surface area increases while the total volume remains constant, improving exchange efficiency.

Cell Structure Overview

Major Cell Components

• Nucleus: Contains genetic material (DNA).

• Endoplasmic Reticulum (ER): Rough (with ribosomes) and Smooth (lipid synthesis, detoxification).

• Golgi Apparatus: Protein modification and sorting.

• Lysosomes: Digestion of macromolecules.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis (plants and algae).

• Cytoskeleton: Microtubules, microfilaments, and intermediate filaments for structure and movement.

The Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, glycoproteins, glycolipids, and cholesterol.

• Regulates entry and exit of substances.

• Contains aquaporins for water transport.

Fluid-Mosaic Model

• The membrane is "fluid" with components floating freely.

• Asymmetry: Outer and inner halves have different compositions.

Membrane Proteins

• Integral (Transmembrane) Proteins: Span the membrane, involved in transport and signaling.

• Peripheral Proteins: Loosely attached to the membrane surface.

• Proteins may be anchored to the cytoskeleton (inside) or extracellular matrix (outside).

The Cell Wall and Extracellular Matrix

The Cell Wall

• Found in plants, fungi, prokaryotes, and some protists; never in animal cells.

• Plant cell walls are made of cellulose, providing structural support and resistance to osmotic pressure.

• Primary cell wall: Thin and flexible; Secondary cell wall: Thicker and stronger (e.g., wood).

The Extracellular Matrix (ECM)

• Animal cells secrete ECM, mainly glycoproteins (e.g., collagen, fibronectin).

• Integrins connect ECM to the cytoskeleton, facilitating cell signaling and attachment.

Cell Attachments

Types of Cell Junctions

• Gap Junctions: Channels for cytoplasmic sharing and rapid communication (especially in muscle cells).

• Tight Junctions: Seal neighboring cells to prevent leakage between compartments.

• Desmosomes: Strong attachments using intermediate filaments (keratins).

Membrane Permeability and Transport

Selective Permeability

Membranes allow some substances to cross more easily than others, based on:

• Inner lipid composition

• Polarity of solutes

• Concentration of solutes

• Size of solutes

Permeability of the Phospholipid Bilayer

• Small, nonpolar molecules (O2, CO2, N2) cross rapidly.

• Small, uncharged polar molecules (H2O, glycerol) cross slowly.

• Large, uncharged polar molecules (glucose, sucrose) and ions (Na+, K+, Cl-) cross very slowly, if at all.

Example: The permeability of a membrane increases with shorter, unsaturated hydrocarbon tails in phospholipids.

Transport Proteins

• Channels: Hydrophilic passageways for ions and small molecules.

• Carriers: Change shape to transport specific molecules across the membrane.

• Aquaporins: Specialized channels for water transport.

Types of Transport

• Diffusion: Passive movement down a concentration gradient (no energy required).

• Facilitated Diffusion: Passive transport via proteins for polar/charged molecules.

• Active Transport: Movement against a concentration gradient, requiring energy (ATP).

Additional info: Active transport is essential for maintaining concentration gradients of ions across membranes, such as the Na+/K+ pump in animal cells.