Cell Structure and Membrane Function: Study Guide for General Biology

Tour of the Cell

Microscopy and Cell Observation

Understanding the tools and techniques used to study cells is fundamental in biology. Microscopes allow scientists to visualize cellular structures and proteins.

• Microscope Types: Light microscopes are used for general cell observation, while electron microscopes (such as transmission electron microscopes) are required to view proteins and other small organelles.

• Cell Size: The average size of an animal cell is typically 10–30 micrometers in diameter.

• Cell Number: The human body contains approximately 37 trillion cells.

Cell Structure and Function

Cells are the basic units of life, with distinct structures and functions. Prokaryotic and eukaryotic cells differ in complexity and organization.

• Prokaryotic Cell: Lacks a nucleus and membrane-bound organelles. Key parts include the cell wall, plasma membrane, cytoplasm, ribosomes, and nucleoid.

• Surface Area to Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting efficient exchange of materials.

• Cell Size Limitation: Cells are small to maximize surface area for nutrient uptake and waste removal.

Comparing Plant and Animal Cells

Plant and animal cells share many features but also have key differences.

• Major Differences: Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells do not.

• Mitochondria: Both plant and animal cells contain mitochondria, the site of cellular respiration.

Cellular Components

Cells contain various organelles, each with specialized functions.

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

• Nucleolus: Located within the nucleus; responsible for ribosome synthesis.

• Ribosomes: Sites of protein synthesis; free ribosomes produce cytosolic proteins, while bound ribosomes synthesize proteins for membranes or export.

• Endomembrane System: Includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles; involved in protein and lipid processing.

Origin and Function of Organelles

Some organelles have unique evolutionary origins and functions.

• Mitochondria and Chloroplasts: Believed to have originated from endosymbiotic bacteria; both have their own DNA and double membranes.

• Similarities with Bacteria: Both organelles share features with bacteria, such as circular DNA and ribosomes.

• Peroxisomes: Organelles involved in lipid metabolism and detoxification.

Cell Movement and Communication

Cells use specialized structures for movement and communication.

• Cilia and Flagella: Hair-like structures for movement; flagella are longer and whip-like, cilia are shorter and often numerous.

• Cell Junctions: Include tight junctions, desmosomes, and gap junctions; facilitate communication and adhesion between cells.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside cells; connects to the cytoskeleton and supports tissue structure.

Membrane Structure and Function

Phospholipid Bilayer and Membrane Properties

The plasma membrane is a dynamic structure composed mainly of a phospholipid bilayer, proteins, and carbohydrates.

• Phospholipid Bilayer: Forms the basic structure of cell membranes; hydrophilic heads face outward, hydrophobic tails face inward.

• Fluid Mosaic Model: Describes the membrane as a fluid combination of lipids and proteins.

• Membrane Fluidity: Influenced by lipid composition, temperature, and cholesterol content.

Membrane Proteins and Carbohydrates

Proteins and carbohydrates embedded in the membrane perform various functions.

• Membrane Proteins: Serve as channels, receptors, enzymes, and anchors.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition and signaling.

Transport Across Membranes

Cells regulate the movement of substances across membranes through various mechanisms.

• Passive Diffusion: Movement of molecules from high to low concentration without energy input.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of molecules against their concentration gradient using energy (ATP).

• Sodium-Potassium Pump: An example of active transport; moves Na+ out and K+ into the cell.

• Bulk Transport: Includes endocytosis and exocytosis for large molecules.

Key Terms and Definitions

• Concentration Gradient: Difference in concentration of a substance across a space.

• Ligand: A molecule that binds specifically to a receptor site of another molecule.

• Channel Proteins: Facilitate the movement of water and ions across the membrane.

Important Equations

• Surface Area to Volume Ratio:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Osmosis:

$\text{Osmosis Rate} \propto \text{Difference in Water Potential}$

• Diffusion:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Example Table: Comparison of Cell Types

Feature	Prokaryotic Cell	Animal Cell	Plant Cell
Nucleus	No	Yes	Yes
Cell Wall	Yes	No	Yes
Chloroplasts	No	No	Yes
Mitochondria	No	Yes	Yes
Ribosomes	Yes (smaller)	Yes (larger)	Yes (larger)

Additional info:

• Some explanations and definitions have been expanded for clarity and completeness.

• Equations and table content inferred from standard biology curriculum.