Cellular Level of Organization Lecture

Cellular Level of Organization Lecture by Chasity O’Malley

Presented by: Lori Garrett, Palm Beach State College
Copyright: © 2018 Pearson Education, Inc.

Major Components of a Cell

  • Cytoskeleton

  • Plasma Membrane

  • Mitochondrion

  • Ribosomes

  • Rough Endoplasmic Reticulum (RER)

  • Smooth Endoplasmic Reticulum (SER)

  • Golgi Apparatus

  • Centrosome

  • Nucleus

  • Microvilli

  • Lysosome

  • Peroxisome

Organization of Cells

General Overview

  • Extracellular Fluid:

    • A plasma membrane (cell membrane) separates the cytoplasm from the extracellular fluid.

  • Cytoplasm:

    • Defined as the material located between the plasma membrane and the nuclear membrane.

    • Consistency:

    • Varies between that of maple syrup and almost-set gelatin.

    • Comprised of a higher concentration of proteins compared to the extracellular fluid.

Subdivisions of Cytoplasm

  • Cytosol:

    • The fluid component of the cytoplasm, also referred to as intracellular fluid, and may contain inclusions of insoluble materials.

  • Organelles:

    • Defined as intracellular structures with specific functions.

    • Nonmembranous Organelles: Not completely enclosed by membranes; directly contact the cytosol. Examples include:

    • Cytoskeleton

    • Microvilli

    • Centrioles

    • Cilia

    • Ribosomes

    • Membranous Organelles: Enclosed in phospholipid membranes, isolating them from the cytosol. Examples include:

    • Mitochondria

    • Nucleus

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Lysosomes

    • Peroxisomes

Organelles

Nonmembranous Organelles

  • Characteristics:

    • Not completely enclosed by membranes.

    • In direct contact with cytosol.

  • Examples:

    • Cytoskeleton

    • Microvilli

    • Centrioles

    • Cilia

    • Ribosomes

Membranous Organelles

  • Characteristics:

    • Enclosed in a phospholipid membrane.

    • Isolated from cytosol.

  • Examples:

    • Mitochondria

    • Nucleus

    • Endoplasmic Reticulum

    • Golgi Apparatus (Golgi Complex)

    • Lysosomes

    • Peroxisomes

Ribosomes

Structure

  • Comprised of two subunits:

    • Large Ribosomal Subunit

    • Small Ribosomal Subunit

    • Contains special proteins and ribosomal RNA (rRNA).

Types of Ribosomes

  • Fixed Ribosomes:

    • Attached to the endoplasmic reticulum.

  • Free Ribosomes:

    • Scattered throughout the cytoplasm.

Function

  • Protein Synthesis:

    • Ribosomes are essential for the synthesis of proteins.

Endoplasmic Reticulum (ER)

Structure

  • A network of intracellular membranes attached to the nucleus.

  • Forms hollow tubes, sheets, and chambers known as cisternae (singular: cisterna), which acts as a reservoir for water.

Function

  • Involved in the synthesis of secretory products, storage, and transport.

Types of Endoplasmic Reticulum

  • Smooth Endoplasmic Reticulum (SER):

    • Lacks attached ribosomes.

    • Synthesizes lipids and carbohydrates.

  • Rough Endoplasmic Reticulum (RER):

    • Has attached (fixed) ribosomes.

    • Modifies and packages newly synthesized proteins.

    • Exports these proteins to the Golgi apparatus.

Variability in RER/SER Ratio

  • The ratio of SER to RER depends on the type of cell and its function.

    • Example: Pancreatic cells, which manufacture digestive enzymes, have extensive RER but smaller SER.

    • Example: Reproductive system cells that synthesize steroid hormones have a significant amount of SER relative to RER.

Endoplasmic Reticulum Functions

  • SER Functions:

    • Synthesizes phospholipids and cholesterol needed for maintenance and growth of:

    • Plasma membrane

    • ER

    • Nuclear envelope

    • Golgi apparatus

    • Synthesizes steroid hormones (e.g., testosterone, estrogen) in reproductive organs.

    • Synthesizes and stores glycerides, particularly triglycerides in liver and fat cells.

    • Synthesizes and stores glycogen in skeletal muscle and liver cells.

Protein Formation in Rough ER

  • As a polypeptide is synthesized on a ribosome:

    • The growing chain enters the cisterna of the RER.

    • Assumes its secondary and tertiary structure.

    • The completed protein may become an enzyme or a glycoprotein.

    • Products not destined for RER are packaged into transport vesicles for delivery to the Golgi apparatus.

Golgi Apparatus

Structure

  • Typically consists of 5-6 flattened discs called cisternae.

  • May be more than one Golgi apparatus in a cell.

  • Usually located near the nucleus.

Function

  • Storage:

    • Stores synthesized products.

  • Modification:

    • Alters and packages synthesized products.

    • Renews or modifies the plasma membrane.

    • Modifies and packages secretions (e.g., hormones or enzymes) for release outside the cell (exocytosis).

    • Packages special enzymes within vesicles for cytosolic use.

Membrane Flow

  • Indicates that, excluding mitochondria, all membranous organelles in the cell are interconnected through the movement of vesicles.

  • Continuous movement and exchange of materials between organelles using vesicles is termed membrane flow.

    • In actively secreting cells, an area equal to the entire membrane surface may be replaced each hour.

    • Represents the dynamic nature of cells, enabling them to adjust plasma membranes and their components in response to environmental stimuli.

Golgi Apparatus Products

  • Membrane Renewal Vesicles:

    • Add to plasma membrane and can result in functional changes.

  • Secretory Vesicles:

    • Contain products (hormones or enzymes) for use outside the cell.

    • Fuse with plasma membrane to release contents.

  • Lysosomes:

    • Contain digestive enzymes for intracellular digestion.

    • Isolate potentially hazardous reactions from the rest of the cell.

Peroxisomes and Lysosomes

Peroxisomes

  • Structure:

    • Vesicles containing hydrolytic enzymes.

  • Function:

    • Breakdown organic compounds.

    • Neutralize toxic compounds produced during metabolic processes.

Lysosomes

  • Structure:

    • Vesicles containing digestive enzymes.

  • Function:

    • Breakdown organic compounds, damaged organelles, or pathogens.

    • Three Basic Functions:

    • Eject waste products and debris from the cell via exocytosis when vesicles fuse with the plasma membrane.

    • Reabsorb nutrients during digestion for recycling.

    • Initially contain inactive enzymes which become active when fusion occurs with another vesicle or organelle.

Mitochondria

General Overview

  • Required for energy production in all living cells; produces ATP.

  • Vary in shape, ranging from long and slender to short and fat.

  • Number of mitochondria in a cell depends on energy needs:

    • May account for 30% of the volume of heart muscle cells; absent in red blood cells due to their unique functions.

  • Contains its own DNA (mtDNA) and ribosomes.

mtDNA Functions

  • Codes for a small set of RNA and polypeptide molecules, essential for functioning in energy production.

  • Although possessing its own genetic code, mitochondrial functions still rely on proteins encoded by nuclear DNA.

Membrane Structure

  • Comprises a unique double membrane:

    • Outer Membrane: Surrounds the organelle.

    • Inner Membrane: Contains numerous folds (cristae).

    • Increases surface area in contact with the matrix, allowing for more protein complexes and enzymes associated with ATP production.

  • Matrix: The fluid enclosed by the inner membrane, where metabolic enzymes catalyze reactions releasing CO2 and providing energy for cellular functions.

Mitochondrial Function

  • ATP Production:

    • Produces 95% of the cellular ATP.

Nucleus

General Overview

  • Acts as the control center for cellular operations and maintenance of homeostasis.

  • Typically the largest cellular structure.

    • Directs synthesis of over 100,000 different proteins.

    • Genetic information encoded in nucleotide sequences determines cell structure and function.

  • Usually contains only one nucleus per cell:

    • Exceptions:

    • Skeletal muscle cells may have multiple nuclei (multinucleate).

    • Mature red blood cells lack a nucleus (anuclear) and disintegrate within 3-4 months.

Structure

  • Composition:

    • Fluid nucleoplasm containing enzymes, proteins, DNA, and nucleotides.

  • Nuclear Envelope:

    • A double membrane surrounding the nucleus.

Function

  • Control of Cell Metabolism: Regulates cellular activities including growth and response to stimuli.

  • Storage/Processing of Genetic Information: Houses DNA for coding protein synthesis.

  • Control of Protein Synthesis: Directs the production of proteins within the cell.

Nucleoplasm and DNA

Nucleoplasm

  • Similar to the cytoplasm; it supports DNA and the nucleolus.

  • Nucleolus:

    • Typically the largest structure in the nucleus and highly prominent in cells producing large amounts of proteins (liver, nerve, muscle cells).

    • Composed of DNA, RNA, and proteins, including enzymes; serves as the site for RNA subunit assembly found in ribosomes.

DNA Structure and Function

  • Storage:

    • Contains instructions for protein synthesis.

  • Coiling Structure:

    • DNA strands coiled around histone molecules to form nucleosomes:

    • Loosely coiled (chromatin) in non-dividing cells.

    • Tightly coiled to form chromosomes during cell division.

  • Chromosome Composition:

    • In human somatic cells, nuclei contain 23 paired chromosomes.

    • One chromosome from the mother and another from the father carries instructions for proteins and RNA, along with regulatory and unknown functions.

Coiled Structure of DNA

Non-Dividing Cells

  • Display tightly packed DNA known as chromatin, composed of nucleosomes and histones.

Dividing Cells

  • Tightly coiled DNA forms chromosomes during cell division, holding copies at a central region termed the centromere.

    • Involves supercoiling of DNA into visible chromosomes that facilitate accurate distribution during cell division.