Cell Structure and Functions

Cell Membrane and Fluid Mosaic Model

  • Functions of the Cell Membrane:

    • Communication: Facilitates signaling between cells.

    • Homeostasis: Maintains a stable internal environment.

    • Recognition: Identifies cells to the immune system and other cells.

    • Transport: Regulates movement of materials in and out of the cell.

  • Structure:

    • Composed of a lipid bilayer (phospholipids) and proteins, creating a "fluid mosaic".

    • Phospholipids: Have hydrophilic (polar) heads and hydrophobic (non-polar) tails.

    • Proteins can be integral (embedded) or peripheral (attached to surface).

    • Cholesterol is present, providing stability.

  • Permeability:

    • Membranes are selectively permeable, allowing certain substances to pass while restricting others of similar size.

Cell Wall in Plant Cells

  • Structure:

    • Composed of cellulose fibers cemented together, making it strong yet semi-permeable.

    • Allows small molecules to easily penetrate, but larger molecules may not.

  • Composition:

    • Made of cellulose, hemicellulose, and pectin.

The Nucleus: The Cell's Control Center

  • Structure:

    • Double membrane (nuclear envelope) with nuclear pores for transport.

    • Contains the nucleolus, chromatin, and nucleoplasm.

  • Functions:

    • Houses DNA, which is packaged as chromosomes.

    • Site of RNA synthesis and gene expression control.

Endoplasmic Reticulum (ER)

  • Types of ER:

    • Rough ER: Has ribosomes; involved in protein synthesis.

    • Site where proteins are synthesized and folded.

    • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies drugs.

    • Involved in metabolism of carbohydrates and storage of calcium ions.

  • Functions:

    • Increased surface area for reactions, storage for synthesized products, transport to Golgi Apparatus.

Ribosomes

  • Structure:

    • Composed of rRNA and proteins; consists of two non-identical subunits.

  • Function:

    • Site of protein synthesis; can be free-floating or attached to the ER.

  • Polysomes:

    • Clusters of ribosomes synthesizing the same protein.

Golgi Apparatus

  • Structure:

    • Stacks of flattened membrane-bound sacs (saccuoles).

  • Functions:

    • Modifies, assembles, packages, and stores proteins.

    • Processes proteins from ER for secretion or delivery to various destinations.

Vacuoles and Vesicles

  • Vacuoles:

    • Large storage organelles in plant cells; store water, food, and waste.

    • Central vacuole is essential for cell support in plants.

  • Vesicles:

    • Smaller than vacuoles, formed by pinching off Golgi Apparatus, play roles in transport and storage.

Lysosomes: Cellular Stomachs

  • Structure:

    • Membrane-bound vesicles containing hydrolytic enzymes.

  • Functions:

    • Digestion of macromolecules; removal of damaged cell components; can release enzymes to digest entire cells.

Peroxisomes

  • Structure:

    • Single membrane-bound organelles.

  • Functions:

    • Metabolizes fats and detoxifies by converting hydrogen peroxide into water and oxygen.

Mitochondria: Powerhouses of the Cell

  • Structure:

    • Double-membraned organelles with an outer and highly folded inner membrane (cristae).

    • Size ranges from 0.5 to 7 micrometers.

  • Functions:

    • Site of cellular respiration, converting nutrients to ATP.

    • Contains its own DNA and ribosomes, capable of self-replication.

Chloroplasts: Photosynthesis Organelles

  • Structure:

    • Double-membraned organelles found only in plant cells; contain chlorophyll.

  • Functions:

    • Conducts photosynthesis: CO2 + H2O + light energy → Glucose + O2.

    • Contain thylakoids, where the light-dependent reactions occur.

Cytoskeleton

  • Structure:

    • Network of protein fibers consisting of microtubules, intermediate filaments, and microfilaments.

  • Functions:

    • Provides structural support, facilitates cell movement, and anchors organelles.

Comparison of Prokaryotic and Eukaryotic Cells

  • Prokaryotic Cells:

    • Lacks a true nucleus; generally smaller and simpler; circular DNA.

  • Eukaryotic Cells:

    • Contains a true nucleus and membrane-bound organelles; larger and more complex; linear DNA organized into chromosomes.

  • Differences:

    • Prokaryotes have a cell wall; eukaryotes may have a cell wall in plants only.

    • Eukaryotic cells have mitochondria and ER; prokaryotic cells do not.