The cell (ll)

Chapter 5: The Cell - An Overview

Mitochondria

  • General Information

    • Membrane-bound organelles where cellular respiration occurs.

    • Responsible for breaking down energy-rich food molecules into water and carbon dioxide, capturing energy in ATP.

    • Require oxygen for cellular respiration; thus, breathing supplies essential oxygen for mitochondrial reactions.

  • Structure

    • Enclosed by two lipid bilayer membranes:

      • Outer mitochondrial membrane: covers the organelle.

      • Inner mitochondrial membrane: expanded by folds known as cristae.

    • Surrounds the mitochondrial matrix, which contains DNA, ribosomes, and enzymes necessary for ATP generation.

  • Function

    • ATP-generating reactions take place in both the cristae and matrix.

Endosymbiotic Theory

  • Mitochondrial DNA and ribosomes resemble those found in bacteria.

  • The theory suggests that mitochondria originated from a mutualistic relationship between an ingested prokaryotic cell and the cell that ingested it.

Microbodies

  • Definition

    • Small, simple membrane-bound organelles with diverse functions specific to cell types.

  • Characteristics

    • Enclose a collection of enzymes and proteins within a single boundary membrane.

  • Types of Microbodies

    • Peroxisomes: produce hydrogen peroxide (H2O2) as a byproduct.

    • Glyoxysomes or Glycosomes: plant microbodies involved in producing sugars used by mitochondria in seeds.

The Cytoskeleton

  • An interconnected system of protein fibers and tubes extending throughout the cytoplasm.

  • Functions:

    • Maintains cell shape and internal organization.

    • Facilitates cell movement.

  • Contains three main components:

    • Microtubules

    • Intermediate filaments

    • Microfilaments

Microtubules

  • Composed of 13 protein filaments (tubulin dimers) arranged side by side, exhibiting polarity (+ and – ends).

  • Dynamic structures capable of changing length by adding or removing tubulin dimers.

  • Radiate from centrosomes, anchoring organelles in place and facilitating movement of vesicles and chromosomes during cell division.

Motor Proteins

  • Generate eukaryotic cell movements.

  • Types: dyneins (move along microtubules) and myosins (move along microfilaments).

  • Work by attaching to cell structures or protein filaments and walk along them through a movement mechanism.

Intermediate Filaments

  • Intermediate in size between microtubules and microfilaments, offering structural support across various cells and tissues.

  • Exist in different configurations: singly, in bundles, or interlinked networks.

Microfilaments

  • Thin protein fibers made from actin subunits, possessing polarity.

  • Key roles include:

    • Contractile elements in muscle fibers.

    • Facilitating cytoplasmic streaming (transport of nutrients and organelles).

    • Responsible for dividing the cytoplasm during cell division.

Flagella and Cilia

  • Structure: Comprise a circle of nine double microtubules surrounding a central pair (9 + 2 complex).

  • Functions:

    • Flagella: propel cells through fluids (e.g., sperm cells).

    • Cilia: move fluids over cell surfaces (e.g., in lungs for mucus clearance).

  • Dynein motor proteins enable microtubule sliding, producing movement.

Eukaryotic Flagellum

  • Base consists of centrioles; the structure includes plasma membranes and dynein arms.

Specialized Structures of Plant Cells

  • Distinct structures not present in animal cells include:

    • Chloroplasts (sites of photosynthesis).

    • A large central vacuole.

    • Rigid cell walls.

Chloroplasts

  • Yellow-green plastids, crucial for photosynthesis.

  • Types include:

    • Amyloplasts: store starch.

    • Chromoplasts: contain pigments for color in fruits/leaves.

  • Surrounded by inner and outer membranes enclosing stroma, with thylakoids stacked into grana containing chlorophyll.

Central Vacuoles

  • Large vesicles making up to 90% of mature plant cell volume.

  • Functions include:

    • Storing various substances (salts, organic acids, sugars).

    • Enzymatic breakdown of biological molecules.

    • Providing chemical defenses against pathogens.

Cell Walls

  • Structure:

    • Composed of cellulose fibers; primary wall (flexible) and secondary wall (reinforced with lignin in woody plants).

  • Functions:

    • Support and protection against invading organisms.

    • Pectin layers (middle lamella) hold adjacent cells together.

    • Perforated by plasmodesmata allowing communication between cells.

Cell Junctions

  • Various forms found in animal tissues, such as anchoring junctions and gap junctions, facilitating adhesion and communication between cells.

Extracellular Matrix (ECM)

  • Many animal cells exist within an ECM composed of proteins and polysaccharides.

  • Plays a role in cell division, adhesion, motility, and responds to injury:

    • Main component: glycoproteins (especially collagen).

    • Proteoglycans maintain matrix consistency.