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Cell Structure and Organelles

Sep 5, 2025

Overview

This lecture covers the structure and functions of the cytoplasm and its components (cytosol, organelles, inclusions), details both membrane-bound and non-membrane-bound organelles, and explains the organization of the nucleus and genetic material. It also discusses the central dogma of molecular biology and the flow of genetic information within the cell.

Cytoplasm & Its Components

  • The cell consists of three main parts: the plasma membrane, cytoplasm, and nucleus.
  • The cytoplasm is everything inside the cell except the nucleus.
  • The three main components of the cytoplasm are:
    • Cytosol: The jelly-like fluid inside the cell, containing water, ions, and enzymes. It makes up about half the volume of the cytoplasm and suspends all cellular elements, including organelles and inclusions.
    • Organelles: Specialized structures that perform specific cellular functions. They are considered the "organs" of the cell and can be either membrane-bound or non-membrane-bound.
    • Inclusions: Temporary storage structures not found in all cell types. Examples include pigments (like melanin), lipid droplets, and glycogen granules. Inclusions are not always present and can appear or disappear depending on the cell’s needs and environment.

Organelles: Types and Functions

  • Organelles are divided into membrane-bound (surrounded by a phospholipid bilayer) and non-membrane-bound types.
  • The number and type of organelles vary by cell type and function. For example, muscle cells have more mitochondria, while liver cells have more peroxisomes.
  • Ribosomes:
    • Sites of protein synthesis (translation).
    • Composed of two subunits: large (60S) and small (40S), each made of ribosomal RNA (rRNA) and proteins.
    • Ribosomal subunits are produced in the nucleolus and assembled in the cytoplasm.
    • Two types:
      • Free ribosomes: Float in the cytosol and synthesize proteins that function within the cytosol.
      • Attached ribosomes: Bind to the rough ER and synthesize proteins destined for secretion, insertion into membranes, or storage in vesicles.
    • Translation always begins on free ribosomes. If the protein is to be secreted, embedded in the membrane, or stored in a vesicle, the ribosome attaches to the rough ER to complete translation.
    • The process of translation involves reading messenger RNA (mRNA) and assembling amino acids into proteins. The sequence of amino acids is determined by the mRNA, and the growing protein is held together by covalent (peptide) bonds.
    • Ribosomes are not membrane-bound organelles.
  • Endoplasmic Reticulum (ER):
    • Rough ER: Studded with ribosomes. Synthesizes proteins that will be secreted, embedded in membranes, or stored in vesicles. Proteins enter the rough ER’s cisterna, fold into their proper shape, and are packaged into vesicles for transport.
    • Smooth ER: Lacks ribosomes. Involved in lipid metabolism (synthesis and breakdown of lipids) and calcium storage. Not involved in protein synthesis.
  • Golgi Apparatus:
    • A membrane-bound organelle made of flattened disk-like structures (cisternae).
    • Functions as the packaging and shipping center of the cell, similar to a delivery service.
    • Receives proteins from the rough ER at the cis (receiving) face, modifies and sorts them, and ships them from the trans (shipping) face.
    • Proteins processed by the Golgi can be exocytosed (released from the cell), embedded in the plasma membrane, or stored in vesicles for later use.
  • Mitochondria:
    • Known as the "powerhouse" of the cell; site of ATP production via cellular respiration.
    • Double-membrane structure: an outer membrane and a highly folded inner membrane (cristae) that increases surface area for enzymes involved in ATP production.
    • Contains its own circular DNA (mitochondrial DNA), which is inherited maternally.
    • The inner membrane folds (cristae) allow for more enzymes and greater ATP production.
  • Lysosomes:
    • Spherical, membrane-bound organelles containing acid hydrolase enzymes.
    • Responsible for intracellular digestion; break down phagocytosed material, worn-out organelles, and cellular debris.
    • Formed by packaging of digestive enzymes into vesicles (pathway three).
    • The enzymes inside lysosomes are called acid hydrolases.
  • Peroxisomes:
    • Larger than lysosomes, spherical, and membrane-bound.
    • Contain oxidase and catalase enzymes for detoxification of harmful substances and breakdown of free radicals.
    • Convert free radicals to hydrogen peroxide, then to water and oxygen using catalase.
    • Abundant in liver and kidney cells, which are heavily involved in detoxification.
  • Cytoskeleton:
    • Provides structural support, maintains cell shape, and enables movement.
    • Composed of three types of protein filaments:
      • Microfilaments (actin): Thinnest, dynamic, involved in cell movement, endocytosis, and exocytosis. Can grow or shrink by adding or removing actin.
      • Intermediate filaments: Medium thickness, provide tensile (pulling) strength, stable and not dynamic. Help resist pulling forces and prevent the cell from being torn apart.
      • Microtubules (tubulin): Thickest, hollow, dynamic (can grow or shrink), organize cell contents, and form the core of cilia and flagella. Organelles can move along microtubules, which act like tracks.
  • Centrosomes:
    • Non-membrane-bound organelles consisting of two perpendicular centrioles surrounded by the centrosome matrix.
    • Act as microtubule organizing centers, especially important during cell division and in the formation of cilia and flagella.
    • Each centriole is made of 27 short microtubules arranged in nine groups of three.
    • The centrosome matrix contains proteins that help microtubules grow from the centrioles.

Inclusions

  • Inclusions are temporary, non-membrane-bound storage structures found in some cells.
  • Examples include:
    • Pigments: Such as melanin in skin, hair, and eyes. Not present in all cell types (e.g., not in liver cells).
    • Lipid droplets: Store fat; present in cells storing large amounts of fat. Not always present—appear when fat is stored.
    • Glycogen granules (glycosomes): Store glycogen, especially in liver and muscle cells. Appear when glycogen is stored and disappear when it is used.
  • Inclusions are not present in all cells and may appear or disappear depending on cell activity and environment.

Nucleus & Genetic Material

  • The nucleus is the largest organelle and acts as the control center of the cell, storing DNA and directing cell activities.
  • Nuclear Envelope:
    • Double membrane structure separating the nucleus from the cytoplasm.
    • Contains nuclear pores that allow transport of ribosomal subunits and mRNA between the nucleus and cytoplasm.
    • The nuclear envelope is a double membrane, with an outer and inner membrane. At certain points, these membranes fuse to form nuclear pores.
  • Nucleolus:
    • Dense region within the nucleus; site of ribosomal subunit (60S and 40S) production.
    • Some nuclei have one nucleolus, others have two.
  • Chromatin:
    • DNA wrapped around histone proteins, forming a "beads on a string" structure (nucleosomes).
    • Chromatin is a combination of DNA and histones. Each "bead" is a nucleosome (DNA wrapped around histones), and the "string" is linker DNA.
    • Chromatin can be extended (euchromatin, active) or condensed (heterochromatin, inactive).
    • During cell division, chromatin condenses further to form chromosomes (the highest level of DNA packing).
  • DNA Structure:
    • Double-stranded, helical, and antiparallel (strands run in opposite directions: one 5' to 3', the other 3' to 5').
    • Base pairing: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds; Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.
    • DNA is supercoiled and highly organized within the nucleus.
    • The two DNA strands are complementary, meaning the sequence of one strand determines the sequence of the other.
  • Chromosomes:
    • Humans have 46 chromosomes (23 pairs): 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females, XY for males).
    • Homologous chromosomes are pairs (one from each parent) that have the same genes but may have different forms (alleles).
    • Chromosomes are only visible during cell division; otherwise, DNA exists as chromatin.
    • Homologous chromosomes are similar but not identical; they have the same genes but may have different versions (e.g., blue eyes vs. brown eyes).
  • Karyotype:
    • A pictorial representation of all chromosomes in a cell.
    • The 23rd pair determines sex (XX = female, XY = male); the other 22 pairs are autosomes.
    • Each pair consists of one chromosome from the mother and one from the father.

Central Dogma of Molecular Biology

  • The central dogma describes the flow of genetic information:
    • DNA → RNA → Protein
    • Transcription: DNA is copied into RNA in the nucleus.
    • Post-transcriptional modification: RNA is processed into messenger RNA (mRNA) in the nucleus.
    • Translation: mRNA is translated into protein in the cytoplasm by ribosomes.
  • The process of translation is essential for cell survival. Without translation, proteins cannot be made, leading to cell death and failure of tissues, organs, and the organism.
  • Certain antibiotics target bacterial translation without affecting human translation, exploiting differences in ribosomal subunits (bacteria: 50S/30S; humans: 60S/40S).

Key Terms & Definitions

  • Cytosol: Fluid part of cytoplasm with water, ions, and enzymes.
  • Organelle: Specialized cell structure with a specific function.
  • Ribosome: Site of protein synthesis (translation); made of rRNA and proteins.
  • Rough ER: Endoplasmic reticulum with ribosomes; synthesizes proteins for membranes, secretion, or vesicles.
  • Smooth ER: Endoplasmic reticulum without ribosomes; involved in lipid metabolism and calcium storage.
  • Golgi apparatus: Modifies, sorts, and ships proteins to their destinations.
  • Mitochondrion: Organelle that produces ATP; has a double membrane and its own DNA.
  • Lysosome: Organelle with digestive enzymes for breaking down materials.
  • Peroxisome: Organelle for detoxification; contains oxidase and catalase enzymes.
  • Microfilament: Thin actin filament in the cytoskeleton; dynamic and involved in movement.
  • Intermediate filament: Medium-thickness cytoskeletal filament; provides tensile strength.
  • Microtubule: Hollow tubulin structure; organizes cell contents and movement.
  • Centrosome: Microtubule organizing center with centrioles; important for cell division.
  • Inclusion: Temporary cytoplasmic storage structure (e.g., pigment, lipid, glycogen).
  • Nucleus: Organelle containing DNA; control center of the cell.
  • Nuclear envelope: Double membrane with pores around the nucleus.
  • Nucleolus: Site of ribosomal subunit production within the nucleus.
  • Chromatin: DNA-histone complex in non-dividing cells.
  • Chromosome: Condensed, coiled chromatin visible during cell division.
  • Homologous chromosomes: Chromosome pairs, one from each parent, similar but not identical.
  • Euchromatin: Extended, active form of chromatin.
  • Heterochromatin: Condensed, inactive form of chromatin.
  • Karyotype: Visual representation of all chromosomes in a cell.

Action Items / Next Steps

  • Review diagrams of cell organelles, cytoskeleton, and nuclear structure.
  • Study the central dogma (DNA → RNA → Protein), including where each step occurs (transcription in the nucleus, translation in the cytoplasm).
  • Complete assigned readings on cell structure, organelle function, and genetic material organization.
  • Practice identifying organelles and their functions in cell diagrams and micrographs.
  • Review the differences between membrane-bound and non-membrane-bound organelles, and understand the significance of inclusions and chromatin organization.
  • Familiarize yourself with the structure and function of chromosomes, homologous pairs, and the karyotype.

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