Cell Structure and Function

Overview

This study guide covers the fundamental aspects of cell structure and function, focusing on the differences between prokaryotic and eukaryotic cells, organelle structure and function, and the roles of cellular components. Understanding these concepts is essential for grasping the organization and operation of living organisms at the cellular level.

Key Terms and Definitions

• Cell: The basic structural, functional, and biological unit of all living organisms.

• Prokaryote: Organisms whose cells lack a nucleus and membrane-bound organelles (e.g., bacteria, archaea).

• Eukaryote: Organisms whose cells contain a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

• Organelle: Specialized subunit within a cell that has a specific function, usually membrane-bound in eukaryotes.

• Plasma Membrane: The selectively permeable boundary of the cell, composed of a phospholipid bilayer with embedded proteins.

• Cell Wall: A rigid layer outside the plasma membrane found in plants, fungi, and some protists and bacteria, providing structural support.

• Flagella: Long, whip-like structures used for cell movement.

• Cytoplasm: The jelly-like substance within the cell, excluding the nucleus, containing organelles and cytosol.

• Ribosome: Molecular machines that synthesize proteins by translating mRNA.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis; rough ER has ribosomes, smooth ER does not.

• Golgi Apparatus: Stacks of flattened membranes that modify, sort, and package proteins and lipids for secretion or delivery to other organelles.

• Lysosome: Membrane-bound organelle containing digestive enzymes for breaking down waste.

• Peroxisome: Organelle that breaks down fatty acids and detoxifies harmful substances.

• Mitochondrion: Organelle responsible for cellular respiration and energy (ATP) production; contains its own DNA.

• Chloroplast: Organelle found in plants and some protists that carries out photosynthesis; contains its own DNA.

• Vacuole: Membrane-bound sac for storage and maintaining cell rigidity, especially large in plant cells.

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

• Microfilament: Thin protein fibers (actin) involved in cell movement and shape.

• Microtubule: Hollow tubes (tubulin) that maintain cell shape and serve as tracks for organelle movement.

• Intermediate Filament: Protein fibers that provide mechanical support for the cell.

Cell Size and Surface Area-to-Volume Ratio

The surface area-to-volume ratio is crucial in determining cell size. As a cell grows, its volume increases faster than its surface area, limiting the rate of material exchange with the environment. This is why most cells are small.

• Prokaryotic cells are generally much smaller than eukaryotic cells.

• Eukaryotic cells can be up to 10x larger than prokaryotic cells.

Prokaryotic vs. Eukaryotic Cells

Key Differences

• Nucleus: Eukaryotes have a true nucleus; prokaryotes do not.

• Membrane-bound organelles: Present in eukaryotes, absent in prokaryotes.

• DNA location: Eukaryotic DNA is in the nucleus; prokaryotic DNA is in the nucleoid region.

• Cell size: Eukaryotic cells are generally larger.

• Examples: Prokaryotes: Escherichia coli; Eukaryotes: Homo sapiens (humans), Zea mays (corn).

Chromosome Structure

• Prokaryotic chromosome: Typically a single, circular DNA molecule.

• Eukaryotic chromosome: Multiple, linear DNA molecules associated with histone proteins.

Cellular Organelles and Their Functions

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

• Ribosome: Site of protein synthesis; found free in cytoplasm or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery.

• Lysosome: Contains hydrolytic enzymes for intracellular digestion.

• Peroxisome: Breaks down fatty acids and neutralizes toxins.

• Mitochondrion: Site of cellular respiration; produces ATP.

• Chloroplast: Site of photosynthesis in plants and algae.

• Vacuole: Stores nutrients, waste products, and helps maintain turgor pressure in plant cells.

Membrane-Bound Organelles

• Organelles surrounded by a single membrane: lysosomes, peroxisomes, vacuoles, ER, Golgi apparatus.

• Organelles surrounded by a double membrane: nucleus, mitochondria, chloroplasts.

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes: nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, plasma membrane.

• Proteins synthesized in the rough ER are packaged into vesicles, processed in the Golgi, and secreted via exocytosis.

• Secretory pathway: The route taken by proteins destined for secretion.

Autophagy and Phagocytosis

• Autophagy: The process by which cells degrade and recycle their own components using lysosomes.

• Phagocytosis: The process by which cells engulf large particles or other cells.

• Both processes involve lysosomes for digestion.

Mitochondria and Chloroplasts

• Both have a double membrane and their own DNA.

• Mitochondria: Site of aerobic respiration; converts glucose and oxygen into ATP, water, and carbon dioxide.

• Chloroplasts: Site of photosynthesis; converts light energy, water, and carbon dioxide into glucose and oxygen.

Structure of Mitochondria

• Outer membrane

• Inner membrane (folded into cristae)

• Matrix: The innermost compartment

• Intermembrane space: Space between the inner and outer membranes

Cellular Respiration and Photosynthesis

• Cellular respiration: The process by which cells extract energy from glucose to produce ATP.

• Photosynthesis: The process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose).

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and facilitates movement.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, organelle movement, and chromosome separation during mitosis.

• Microfilaments: Thin filaments made of actin; involved in cell movement and muscle contraction.

• Intermediate filaments: Rope-like fibers; provide mechanical strength.

Functions of Cytoskeletal Elements

• Microtubules: Cell shape, intracellular transport, cilia and flagella structure, mitotic spindle formation.

• Microfilaments: Cell movement, cytokinesis, muscle contraction.

• Intermediate filaments: Structural support, anchoring organelles.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Sample Equations

• Cellular Respiration:

• Photosynthesis:

Additional Info

• Hydrolytic enzymes in lysosomes function optimally at acidic pH.

• Membrane-bound refers to organelles surrounded by at least one lipid bilayer.

• Some organelles, such as mitochondria and chloroplasts, are thought to have originated via endosymbiosis.