Eukaryotic Cells

Main Parts of a Eukaryotic Cell

  • Cell: The basic unit of life.

  • Plasma Membrane:

    • Surrounds the entire cell and regulates what enters and exits.

  • Nucleus:

    • Contains DNA and is responsible for storing genetic information and assembling ribosome sub-units.

    • Nuclear Envelope: Consists of two membranes surrounding the nucleus.

    • Nuclear Pores: Holes in the nuclear envelope that allow proteins and RNAs to move in and out of the nucleus.

    • Nucleolus: Inside the nucleus, it is the site of ribosome assembly.

  • Cytoplasm:

    • The material within a cell, excluding the nucleus. It consists of cytosol and organelles.

    • Cytosol: Thick liquid (gel-like) that contains various components.

Intracellular Structures

  • Organelles:

    • Membrane-bound compartments with specific functions.

    • Two Membranes: Found in the nucleus, mitochondria, and chloroplasts.

    • One Membrane: Includes the Golgi apparatus, endoplasmic reticulum (ER), vacuoles, lysosomes, peroxisomes, and vesicles.

  • Intracellular Structures without a Surrounding Membrane:

    • Ribosome: Composed of RNA molecules and proteins. It synthesizes proteins from mRNA directions (this process is called translation).

    • Free Ribosomes: Located in cytosol.

    • Bound Ribosomes: Attached to the rough ER (RER).

The Endomembrane System

  • A series of organelles functionally connected by vesicles that facilitate the synthesis of lipids and proteins.

  • Vesicle: Spherical membrane sac.

  • Components:

    • Nuclear Envelope: Specifically the outer membrane.

    • Endoplasmic Reticulum (ER): An extensive network of membranous structures.

    • Rough ER: Flattened tubes with attached ribosomes; responsible for protein synthesis.

    • Smooth ER: Round tubes without attached ribosomes; responsible for lipid synthesis.

    • Transport Vesicles: Move materials between ER and Golgi apparatus.

    • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids into new vesicles.

    • Lysosomes:

    • Digestive organelles that contain enzymes to break down large molecules.

    • Secretory Vesicles:

    • Store materials for later release from the cell or transport materials immediately from the Golgi to the plasma membrane.

    • Plasma Membrane: Also known as the cell membrane.

Vacuoles

  • Vacuoles: Large (spherical) membrane sacs that serve different purposes depending on the type of cell.

    • Central Vacuole: Present in plants; filled with water to maintain cell turgidity and firmness. Also stores selected materials.

    • Food Vacuoles: Found in animals; contain extracellular materials, often containing nutrients brought into cells as "food." Fuse with lysosomes to digest materials.

    • Contractile Vacuoles:

    • Expel excess water; specifically found in animal-like freshwater protists.

    • Storage Vacuoles: Store materials for later use.

Mitochondria and Chloroplasts

  • Mitochondrion:

    • Site of aerobic respiration. More detail later with respiration topics.

  • Chloroplasts:

    • Site of photosynthesis. More detail later regarding photosynthesis.

  • Endosymbiont Theory:

    • Mitochondria and chloroplasts evolved when a pre-eukaryotic cell engulfed prokaryotic cells without digesting them, creating a symbiotic relationship.

    • Evidence for this theory includes:

    • Presence of circular DNA molecules in mitochondria and chloroplasts.

    • Prokaryotic-type ribosomes found within them.

    • Reproduce through binary fission similar to prokaryotes.

Peroxisomes

  • Peroxisomes:

    • Contain two membranes and facilitate dangerous oxidative reactions under controlled conditions.

Cytoskeleton

  • Cytoskeleton:

    • An internal support network made up of protein-based structures.

    • Microtubules:

    • Small tubes formed from tubulin protein.

    • Cilia and Flagella: Long structures that create movement.

    • Kinesin and Dynein: Motor proteins that use energy to drive movement.

    • Microfilaments:

    • Small rods made from actin, a major structural protein.

    • Myosin: A motor protein that uses energy to drive movement.

    • Intermediate Filaments:

    • Rods that are slightly larger than microfilaments, made of various proteins.

Cell Junctions

  • Cell Junctions:

    • Connections between cells that are crucial for communication and adhesion.

    • Types of Junctions:

    • Plasmodesmata: Channels that pass through the cell walls of two adjacent plant cells, allowing cytoplasmic communication.

    • Tight Junctions: Prevent fluid movement between cells by sealing them together.

    • Desmosomes: Reinforcing junctions that hold cells together.

    • Gap Junctions: Cytoplasmic channels between cells that facilitate communication.

Extracellular Structures

  • Extracellular Structures:

    • Structures located outside the cell membranes, composed primarily of carbohydrates and proteins.

    • Cell Wall:

    • Present in plants and fungi. Provides structural support, protection against external pressures, and prevents cell rupture due to osmotic pressure.

    • Extracellular Matrix:

    • Found in animal cells, this structure allows cells to attach to one another and provides protection against external mechanical forces.