Cell Organelles

# Cell Organelles

Definition of Organelles

• Organelle: A part of a cell that has one or more specific functions to perform for the cell and/or organism.

  • Cells combine to form tissues.

  • Tissues form organs.

  • Organs make up organ systems.

  • Organ systems combine to create an organism.

Organization of All Cells

Composition of Cells

• According to recent additions to cell theory:

  • All cells share a basic composition:

  • Cell membrane

  • Cytoplasm

  • Ribosomes

  • DNA

Cell Membrane

• Also known as the plasma membrane.

• Structure:

  • Comprised of phospholipids arranged in a bilayer.

• Characteristics:

  • Outermost part of animal cells.

  • Present in plant cells but not as the outermost structure.

  • Controls the movement of materials, crucial for maintaining homeostasis (stable internal conditions).

  • Is described as selectively permeable (allows only certain substances to enter and exit the cell).

Cytoplasm

• Defined as:

  • The fluid-filled space inside the cell that cushions the organelles.

  • The jelly-like fluid within this space is called cytosol.

Ribosomes

• Description:

  • Small structures composed of RNA and protein.

  • Found scattered throughout the cytoplasm and attached to the rough endoplasmic reticulum (rough ER).

• Function:

  • Create proteins by joining amino acids in chains.

  • Amino acids: The building blocks of proteins.

DNA (Deoxyribonucleic Acid)

• Role:

  • The hereditary material of all organisms.

• Location:

  • Found within cells and passed on through reproduction.

• Storage Differences:

  • Prokaryotes: DNA stored as a loop in the cytoplasm in an area referred to as the nucleoid region.

  • Eukaryotes: DNA located within the nucleus, organized into linear strands called chromosomes.

Organization of Prokaryotic Cells

Prokaryotic Cell Characteristics

• Smallest and simplest cell types, always single-celled.

• Found in the domains/kingdoms:

  • Archaea

  • Bacteria

Key Distinction from Eukaryotic Cells

• Prokaryotic cells possess organelles without internal membranes, including a nucleus.

Unique Features of Prokaryotic Cells

• Prokaryotic cells have:

  • A cell membrane.

  • Cytoplasm.

  • Ribosomes.

  • DNA (in nucleoid region).

• Diversity in bacterial structures; however, not all bacteria are identical.

  • Example: Not all bacteria contain the same additional parts beyond aforementioned structures.

• Cell Wall:

  • All possess a cell wall, but composition varies:

  • Eubacteria have cell walls made of peptidoglycan:

    • Bacteria with thick layers (gram-positive).

    • Bacteria with thin layers (gram-negative).

  • Archaebacteria have cell walls without peptidoglycan.

Additional Features

• Capsule:

  • Sticky outside structure aiding bacteria in attachment to surfaces.

• Pili:

  • Structures found in some bacteria.

  • Functions:

  • Attachment to surfaces.

  • DNA transfer between cells.

• Flagella:

  • Tail-like structures aiding in cell movement.

  • Some prokaryotic cells may possess multiple flagella.

Organization of Eukaryotic Cells

Eukaryotic Cell Characteristics

• Contain numerous smaller structures known as organelles.

Cell Wall

• Outermost structure in:

  • Plant cells

  • Fungal cells

  • Protist cells (plant-like)

• Function:

  • Provides protection, support, and shape to the cell.

• Composition:

  • Cell wall of plants (and plant-like protists): Cellulose.

  • Cell wall of fungi (and fungus-like protists): Chitin.

Nucleus

• Central component of the cell containing DNA.

• Structure:

  • Surrounded by a double membrane known as the nuclear envelope or nuclear membrane with nuclear pores.

• Function:

  • Acts as the “command center,” controlling cell activities.

• Nucleoplasm: Fluid-filled space inside the nucleus.

Nucleolus

• Description:

  • Small sphere located at the nucleus center.

• Function:

  • Responsible for creating ribosomes.

Cytoskeleton

• Function:

  • Provides structural support to the plasma membrane and overall cell shape.

  • Positions organelles effectively and aids in vesicle movement.

  • Found in different types of protein fibers (filaments).

Endoplasmic Reticulum (ER)

• Description:

  • A system of interconnected tubes and sacs made from membrane.

• Types:

  • Rough ER:

  • Ribosome presence on surface.

  • Used for synthesizing proteins.

  • Smooth ER:

  • No ribosomes on surface.

  • Used for synthesizing and storing lipids.

Golgi Apparatus/Body/Complex

• Works alongside the ER as part of the cell’s endomembrane system.

• Process:

  • Molecules created in the ER are transported in vesicles to the Golgi for modification and processing.

  • Modified molecules are repackaged into vesicles for intra- or extracellular transport.

Vesicles

• Definition:

  • Small spheres of phospholipid bilayer designed for molecule transport around the cell via the cytoskeleton.

• Characteristics:

  • They are transient, formed and deformed as needed within a cell.

  • Can originate from the cell membrane, ER, or Golgi.

Lysosomes

• Structure:

  • Similar to vesicles and temporary vacuoles, both being small spheres made of the phospholipid bilayer.

• Function:

  • Contain digestive enzymes to break down worn-out organelles and foreign substances.

Temporary Vacuoles

• Description:

  • Spheres made from the phospholipid bilayer, similar in structure to vesicles and lysosomes.

• Function:

  • Store a variety of materials (food and waste).

  • Often small and temporary in animal cells.

Central Vacuole in Plant Cells

• Function:

  • Stores materials (notably water, essential for photosynthesis).

• Structure:

  • Typically large and centrally located within plant cells.

Mitochondria

• Often termed the “powerhouse” of the cell.

• Function:

  • Convert chemical energy from food into usable chemical energy in the form of ATP through a process called aerobic cell respiration.

• Structure:

  • Double membrane-bound.

  • Contains its own DNA and ribosomes.

  • Certain cells (e.g., muscle cells) have more mitochondria due to higher energy needs.

Chloroplasts

• Unique to plants and plant-like protists for their role in photosynthesis.

• Function:

  • Capture light energy via chlorophyll, converting it into chemical energy (e.g., glucose).

• Structure:

  • Double membrane-bound.

  • Contains its own DNA and ribosomes.

  • Certain cells have increased chloroplast numbers due to higher photosynthesis rates.

Centrioles

• Non-membrane bound organelles occurring in pairs within animal cells.

• Function:

  • Organize chromosomes during cell division (mitosis).

  • Located within the centrosome, near the nucleus when the cell is not dividing.

Cilia

• Description:

  • Hair-like structures extending from the surface of specific motile cells (e.g., certain protists).

• Function:

  • Movement (cell movement or material movement around the cell).

  • Composed of protein filaments similar to those in the cytoskeleton.

Flagella

• Tail-like structures that promote motility in specific cells, including prokaryotic cells.

• Similar to cilia but fewer in number on a given cell.

• Function:

  • Propel the cell by whipping motion.

Comparison of Plant vs. Animal Cells

Organelles found in Plant Cells but not in Animal Cells:

• Cell Wall

• Chloroplasts

• Central Vacuole

Organelles found in Animal Cells but not in Plant Cells:

• Centrioles

• Lysosomes

Summary of Structures

Animal Cells:

• Mitochondria

• Peroxisome

• Rough Endoplasmic Reticulum

• Ribosomes

• Vacuole

• Centrosome

• Lysosome

• Golgi Apparatus

• Smooth Endoplasmic Reticulum

• Cell (Plasma) Membrane

• Nucleus

• Nucleolus

Plant Cells:

• Intermediate Filaments

• Chloroplasts

• Central Vacuole

• Cell Wall

• Others (similar to animal cells): Mitochondria, Ribosomes, Golgi Apparatus, etc.