Cell organization and function

A cell is the smallest structural and functional unit of an organism. It can perform all life processes, such as metabolism, growth, reproduction, and response to environmental stimuli. Cells can exist as independent entities (as in unicellular organisms) or as part of multicellular organisms, where they often differentiate into specialized types to perform specific functions.

Basic Characteristics of Cells

• Life Functions: Cells carry out essential life functions:
  • Metabolism: Chemical reactions that provide energy and materials for growth and maintenance.
  • Reproduction: The ability to divide and produce new cells.
  • Homeostasis: Maintaining a stable internal environment.
  • Response to Stimuli: Ability to react to changes in the environment.
• Cellular Organization: All living organisms are made up of cells. The structure and complexity of cells vary greatly depending on their functions.

Basic Cell Structure

Cell Membrane:

• Structure: Composed primarily of a phospholipid bilayer with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward. Proteins (integral and peripheral), cholesterol molecules, and carbohydrate chains are embedded within.
• Functions:
  • Selective Permeability: Regulates what enters and exits the cell, allowing for nutrient uptake and waste elimination.
  • Signal Reception: Membrane proteins act as receptors for signalling molecules, initiating cellular responses.
  • Cell Recognition: Glycoproteins and glycolipids on the surface help in cell recognition and immune response.

Cytoplasm:

• Structure: A viscous fluid that fills the cell, containing organelles, cytosol, and various inclusions (e.g., nutrients, pigments).
• Functions:
  • Site of Metabolic Reactions: Many cellular processes occur here, including glycolysis.
  • Support for Organelles: Provides a medium for organelles to remain suspended and function effectively.

Nucleus:

• Structure: Surrounded by a double membrane (nuclear envelope) with nuclear pores that regulate the movement of molecules. Contains chromatin (DNA and proteins) and the nucleolus.
• Functions:
  • Genetic Information Storage: Houses DNA, which contains the instructions for protein synthesis.
  • Regulation of Cellular Activity: The nucleus controls the cell’s growth, metabolism, and reproduction.
  • Ribosome Production: The nucleolus synthesizes ribosomal RNA (rRNA), crucial for ribosome assembly.

Organelles and Their Functions

 

Mitochondria:

• Structure: Double-membrane organelles with an inner membrane folded into cristae, increasing surface area for energy production.
• Functions:
  • ATP Production: Sites of aerobic respiration, converting glucose and oxygen into ATP, carbon dioxide, and water.
  • Metabolism Regulation: Involved in the metabolism of carbohydrates and fats.

Endoplasmic Reticulum (ER):

• Rough ER:
  • Structure: Studded with ribosomes, giving it a “rough” appearance.
  • Functions: Synthesizes and processes proteins for secretion or use in the cell membrane.
• Smooth ER:
  • Structure: Lacks ribosomes, appearing smooth.
  • Functions: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons, and stores calcium ions.

Ribosomes:

• Structure: Composed of rRNA and proteins; can be free-floating in the cytoplasm or attached to the rough ER.
• Functions: Sites of protein synthesis, translating messenger RNA (mRNA) into polypeptide chains.

Golgi Apparatus:

• Structure: Stacked, flattened membrane sacs (cisternae).
• Functions:
  • Modification of Proteins and Lipids: Receives proteins from the rough ER, modifies them (e.g., glycosylation), and sorts them for transport.
  • Secretion: Packages proteins into vesicles for secretion outside the cell.

Lysosomes:

• Structure: Membrane-bound organelles containing digestive enzymes (acid hydrolases).
• Functions:
  • Digestion: Break down macromolecules (proteins, lipids, carbohydrates) and cellular debris through hydrolysis.
  • Autophagy: Degrades worn-out organelles and recycles cellular components.

Peroxisomes:

• Structure: Membrane-bound organelles containing enzymes that catalyze reactions.
• Functions:
  • Fatty Acid Oxidation: Breaks down fatty acids to produce energy.
  • Detoxification: The enzyme catalase converts toxic hydrogen peroxide into water and oxygen.

Cytoskeleton:

• Structure: A dynamic network of protein filaments, including microfilaments (actin), intermediate filaments, and microtubules (tubulin).
• Functions:
  • Structural Support: Maintains cell shape and organization.
  • Cell Motility: Facilitates movement through muscle contraction and cell crawling.
  • Intracellular Transport: Motor proteins (e.g., kinesin, dynein) transport organelles along microtubules.

Cell Types and Specializations

• Prokaryotic Cells:

  • Characteristics: Simpler structure, lack a true nucleus, and have fewer organelles. Examples include bacteria and archaea.
  • Functions: Carry out all life processes, including metabolism and reproduction, despite their simplicity.

• Eukaryotic Cells:

  • Characteristics: More complex, with a defined nucleus and membrane-bound organelles. Examples include animal cells, plant cells, fungi, and protists.
  • Functions: Specialized for various functions, including nutrient absorption, photosynthesis (in plants), and immune response.

• Specialized Cells:

  • Muscle Cells: Contain contractile proteins (actin and myosin) for movement; specialized types include skeletal (voluntary), cardiac (involuntary), and smooth muscle (involuntary).
  • Nerve Cells (Neurons): Specialized for signal transmission; consist of dendrites (receive signals), axons (send signals), and synapses (communication points).
  • Epithelial Cells: Form protective layers, involved in secretion and absorption (e.g., intestinal epithelial cells).
  • Blood Cells: Include erythrocytes (red blood cells for oxygen transport), leukocytes (white blood cells for immune defence), and platelets (involved in clotting).

Cell Function and Processes

Metabolism:

• Catabolism:
  • Breakdown of complex molecules into simpler ones, releasing energy (e.g., glycolysis, Krebs cycle).
• Anabolism:
  • Building up complex molecules from simpler ones requires energy (e.g., protein synthesis, DNA replication).

Cell Division:

• Mitosis: Process by which a eukaryotic cell divides to produce two identical daughter cells for growth and repair. Phases include prophase, metaphase, anaphase, and telophase.
• Meiosis: A special form of cell division that produces gametes (sperm and eggs) with half the genetic material. Involves two rounds of division: meiosis I and meiosis II.

Cell Signaling:

• Endocrine Signaling: Hormones are secreted into the bloodstream to reach distant targets (e.g., insulin).
• Paracrine Signaling: Signals act on nearby cells (e.g., growth factors).
• Autocrine Signaling: Cells respond to signals they produce themselves (e.g., certain cancer cells).
• Signal Transduction Pathways: Involves receptor activation, relay of signals through cascades, and cellular response (e.g., gene expression changes).

Transport Mechanisms:

• Passive Transport:

  • Diffusion: Movement of molecules from high to low concentration (e.g., oxygen).
  • Facilitated Diffusion: Requires transport proteins for specific molecules (e.g., glucose).
  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport:

  • Requires energy (ATP) to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Endocytosis/Exocytosis:

  • Endocytosis engulfs materials into the cell (e.g., phagocytosis for large particles, pinocytosis for liquids).
  • Exocytosis is expelling materials from the cell, often used to produce hormones and neurotransmitters.

Homeostasis and Cell Interaction

• Homeostasis: The ability of cells to maintain a stable internal environment despite external changes. This includes regulating pH, temperature, ion concentrations, and nutrient levels.
• Intercellular Communication:
  • Gap Junctions: Direct communication between neighbouring cells enables rapid signal transmission.
  • Chemical Signals: Hormones and neurotransmitters facilitate communication across distances, coordinating physiological responses.