exam prep

Vesicles and Vacuoles

  • Definition: Vesicles and vacuoles are both membranous sacs within cells; however, vacuoles are generally larger and more specialized than vesicles.

    • Contractile Vacuoles: In aquatic protists, contractile vacuoles assist in expelling excess water from the cell.

    • Digestive Vacuoles: Some protists possess large digestive vacuoles that are essential for the breakdown of nutrients.

    • Plant Vacuoles: In plants, vacuoles serve as storage units for various substances, including:

    • Nutrients

    • Ions

    • Water

    • Sugars

    • Salts

    • Pigments (responsible for the colors of flowers and leaves)

    • Toxic molecules that deter herbivory.

  • Figures:

    • Figure 4.13a: Illustrates contractile vacuoles in protists.

    • Figure 4.13b: Shows a large central vacuole in a plant cell.

Energy-Related Organelles

Chloroplasts

  • Function: Chloroplasts use solar energy to synthesize carbohydrates through photosynthesis.

  • Structure:

    • Larger than mitochondria, with dimensions approximately twice the width and up to five times the length.

    • Comprised of a double membrane with:

    • Outer Membrane

    • Inner Membrane

    • The internal area includes:

    • Stroma: Contains enzymes and is the site for carbohydrate synthesis.

    • Thylakoids: Disc-shaped sacs, where light energy is absorbed, organized in stacks known as granum.

    • Thylakoid Space: The lumen of thylakoids forms a significant internal compartment.

  • Significance of Chloroplasts: The carbohydrates created here serve as organic nutrients for plants and all living organisms.

  • Endosymbiotic Theory: The presence of own DNA and ribosomes in chloroplasts supports the theory that they evolved from photosynthetic bacteria incorporated into an ancient eukaryotic cell.

Mitochondria

  • Function: Mitochondria break down carbohydrates to produce adenosine triphosphate (ATP), essential for cellular processes.

  • Structure:

    • Smaller than chloroplasts, typically only visible with an electron microscope.

    • Shape can vary (lengthen or shorten) and can form chains or remain static where energy is needed (e.g., between cardiac cells and in sperm).

    • Double Membrane: Similar to chloroplasts, containing:

    • Outer Membrane

    • Inner Membrane: Highly folded into structures called cristae, increasing surface area significantly (about one-third of total membrane in liver cells).

    • Matrix: Enzymatic reactions for carbohydrate breakdown occur here for ATP synthesis.

  • Cellular Respiration: A complete carbohydrate breakdown process requiring oxygen (and producing carbon dioxide).

  • Endosymbiotic Theory: Like chloroplasts, mitochondria contain their own DNA, further supporting a common bacterial ancestry.

Cytoskeleton and Motor Proteins

Cytoskeleton

  • Definition: A dynamic network of protein filaments and tubules running from the nucleus to the plasma membrane, crucial for cell shape and movement.

  • Components:

    • Microtubules: Hollow cylinders composed of tubulin dimers, providing structural support and serving as tracks for organelle movement.

    • Intermediate Filaments: Ropelike proteins that provide mechanical strength to cells, differing in composition based on the cell type.

    • Actin Filaments: Networks beneath the plasma membrane, involved in shape support and microvilli structure.

Motor Proteins

  • Function: Essential for cellular movement, utilizing ATP to power their motions.

  • Types:

    • Myosin: Interacts with actin filaments, facilitating muscle contraction, cell division, and amoeboid movement.

    • Kinesin and Dynein: Move along microtubules carrying organelles and vesicles, functioning similarly to cars on a highway.

Centrioles and Cell Movement

  • Centrioles: Barrel-shaped structures located within centrosomes, organizing microtubules during cell division; not all eukaryotes have centrioles, showing variations in cell structure.

  • Cilia and Flagella: Hair-like structures that facilitate movement; cilia move stiffly (like oars), while flagella move undulating (like snakes).

Ciliary Function

  • Example Functions:

    • Cilia in bronchial walls help clear mucus and debris from the lungs.

    • Cilia lining the uterine tubes assist egg transport to fertilization sites.

Extracellular Structures

Cell Walls

  • Definition: Provide structure and support to plant cells, fungi, and most protists; absent in animal cells.

  • Composition in Plants:

    • Primary Cell Wall: Made of cellulose fibrils and other substances allowing stretch during growth.

    • Middle Lamella: Adhesive layer holding two plant cells together.

    • Secondary Cell Wall: Provides extra strength with more cellulose laid at right angles, often includes lignin.

Extracellular Matrix (ECM)

  • Definition: A meshwork outside animal cells, comprised of fibrous proteins such as collagen and elastin, serving as a medium for communication and support.

  • Functions of ECM:

    • Direction of cell migration during development.

    • Communication through binding to plasma membrane receptors.

    • Variability in rigidity and flexibility between tissues depending on protein fiber composition and arrangement.

Cell Junctions

  • Types:

    • Adhesion Junctions: Flexible connections between cells, important in tissues needing to stretch (such as heart and bladder).

    • Tight Junctions: Seal cells to prevent leakage between them.

    • Gap Junctions: Allow direct communication and transfer of materials between adjacent cells.