Cell Structure, Metabolism, and Division: Essential Concepts

Posted on Dec 2, 2024 in Biology

Cell: The Building Block of Life

The cell is the fundamental functional and morphological unit of all living things.

Eukaryotic Cell

All eukaryotic cells, including plant and animal cells, have membrane-enclosed compartments (organelles) within their cytoplasm where specific chemical reactions occur. Their hereditary material is contained within a nucleus.

Cell Morphology

Cells vary in shape, often spherical, depending on their function or lack thereof.

Plasma Membrane

The plasma membrane is a single, selective barrier that allows exchange of matter and energy with the cell’s surroundings. It’s composed of proteins and polar lipids. Proteins can be on the surface or embedded within the membrane (intrinsic proteins). The membrane may have adaptations and a typical distance between components of 75Å.

Cytoplasm

The cytoplasm contains various membrane-bound organelles with similar composition and structure to the plasma membrane. It also includes non-membranous structures like:

  • Cytoskeleton: Gives the cell shape and enables movement.
  • Centrosome: Organizes filaments.
Endoplasmic Reticulum (ER)

A network of interconnected tubules and flattened sacs throughout the cytoplasm.

  • Smooth ER (SER): Lacks ribosomes and manufactures membrane tubules.
  • Rough ER (RER): Studded with ribosomes and synthesizes proteins.
Golgi Apparatus

A series of membranous sacs and vesicles. It processes and packages macromolecules synthesized in the ER for secretion or transport to other organelles.

Lysosomes

Membranous vesicles containing digestive enzymes produced by the RER. They digest intracellular and extracellular materials, breaking down macromolecules into simpler organic molecules. They also destroy unnecessary cellular components.

Mitochondria

Cylindrical organelles responsible for cellular respiration. They combust organic molecules in the presence of O2 to produce energy (ATP) for cellular activities. The mitochondrial matrix contains ribosomes and DNA, enabling some protein synthesis.

Nucleus

Separated from the cytoplasm by a double membrane (nuclear envelope) continuous with the ER. The nuclear membrane has pores allowing molecular exchange with the cytoplasm.

  • Chromatin: Consists of DNA and associated proteins. During cell division, chromatin fibers condense into chromosomes.
  • Nucleoli: One or more granular regions where ribosomes are assembled.

Cell Metabolism

The sum of anabolic and catabolic processes.

  • Anabolism: Processes where cells synthesize their components from chemical substances, requiring energy (light or chemical).
    • Autotrophic cells: Manufacture organic nutrients from inorganic substances.
    • Heterotrophic cells: Incorporate organic matter from their environment.
  • Catabolism: Processes breaking down complex compounds into simpler ones, releasing energy stored in their bonds.

Photosynthesis

Anabolic process used by autotrophs to produce organic matter from inorganic matter.

  • Light-dependent reactions: Occur in the thylakoid membranes. Sunlight energy captured by chlorophyll synthesizes ATP. Water molecules are split, releasing O2 and providing H+ for the next phase.
  • Light-independent reactions (Dark reactions/Calvin Cycle): Occur in the stroma. CO2 is fixed and converted into organic molecules using the ATP and H+ from the light-dependent reactions. The cycle begins with ribulose-1,5-bisphosphate (RuBP) and involves the enzyme ribulose phosphate carboxylase (RuBisCO).

Chemosynthesis

Process of obtaining energy from the oxidation of inorganic compounds. This energy is used to synthesize ATP, which then drives CO2 fixation. Examples include iron bacteria (oxidize ferrous/ferric iron), sulfur bacteria (oxidize sulfur compounds), and nitrifying bacteria (oxidize ammonia/nitrite to nitrate).

Cellular Respiration

Catabolic process where cells use nutrients to produce energy.

  • Aerobic respiration: Uses O2 as the final electron acceptor.
  • Anaerobic respiration: Uses molecules other than O2 (e.g., sulfur or organic substances) as the final electron acceptor.
Glycolysis

Occurs in the cytoplasm. Glucose (6 carbons) is broken down into two molecules of pyruvic acid (3 carbons). This involves an initial energy investment phase (ATP is used) followed by an energy payoff phase (ATP is produced, and NAD+ is reduced to NADH2).

Krebs Cycle (Citric Acid Cycle)

Occurs in the mitochondrial matrix. Pyruvic acid is converted to acetyl-CoA, which enters the cycle. The cycle involves a series of reactions that oxidize acetyl-CoA, releasing CO2, producing ATP, and reducing electron carriers (NAD+ to NADH2 and FAD to FADH2).

Electron Transport Chain

Occurs in the inner mitochondrial membrane. Electrons from NADH2 and FADH2 are passed along a series of electron carriers, releasing energy that is used to pump protons (H+) across the membrane, creating a proton gradient. This gradient drives ATP synthesis through chemiosmosis. Oxygen is the final electron acceptor, forming water. Each NADH2 molecule generates approximately 3 ATP molecules.

Fermentation

Anaerobic process that occurs when oxygen is limited. Pyruvate is converted into other products (e.g., lactic acid or ethanol) to regenerate NAD+, allowing glycolysis to continue. Fermentation produces much less ATP than aerobic respiration.

  • Lactic acid fermentation: Occurs in muscle cells during strenuous activity. Pyruvate is reduced to lactic acid.
  • Alcoholic fermentation: Occurs in yeast. Pyruvate is decarboxylated to acetaldehyde, which is then reduced to ethanol.

Cell Division

Mitosis

Cell division that produces two identical daughter cells with the same number of chromosomes as the parent cell.

  • Prophase: Chromatin condenses into chromosomes, the nuclear envelope breaks down, and the spindle apparatus forms. Centrioles migrate to opposite poles of the cell.
  • Metaphase: Chromosomes align at the metaphase plate (equatorial plane) attached to spindle microtubules.
  • Anaphase: Sister chromatids separate and are pulled to opposite poles of the cell.
  • Telophase: Chromosomes decondense, nuclear envelopes reform around the separated chromosomes, and the spindle apparatus disappears. Two daughter nuclei are formed.

Meiosis

Cell division that produces four daughter cells (gametes or spores) with half the number of chromosomes as the parent cell. It involves two successive nuclear divisions.