Cell Structure and Support

Cytoskeleton: Support and Motility

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement within and by the cell.

• Microtubules, Microfilaments, and Intermediate Filaments: These three types of protein filaments support the cell’s shape and facilitate intracellular transport and cell division.

• Centrioles and Microtubules: Centrioles are composed of microtubules and help organize the mitotic spindle during cell division by forming centrosomes.

• Cilia and Flagella: These are motile structures made of microtubules that enable movement of cells or movement of substances along the cell surface.

Cell Walls and Extracellular Structures

Outside of Plasma Membrane

Many cells possess structures outside the plasma membrane that provide additional support and protection.

• Prokaryotic Cell Walls: Most prokaryotes are surrounded by cell walls, which protect the cell and maintain its shape.

• Plant Cell Walls: Plant cells have cell walls made of cellulose, a polysaccharide that provides rigidity.

• Fungal Cell Walls: Fungi have cell walls strengthened by chitin, another type of polysaccharide.

• Animal Cells: Animal cells lack cell walls but secrete an extracellular matrix (ECM) composed of proteins and carbohydrates for structural support.

• Cell Size: Prokaryotic cells tend to be smaller than most eukaryotic cells.

Cell Junctions

Cells in multicellular organisms are connected by specialized junctions that facilitate communication and adhesion.

• Plasmodesmata: Channels in plant cell walls that allow water and small solutes to pass from cell to cell, functioning like bridges.

• Tight Junctions: Membranes of neighboring animal cells are pressed together, preventing leakage of extracellular fluid.

• Desmosomes: Anchoring junctions where intermediate filaments made of keratin proteins anchor cells together, providing mechanical stability.

Plasma Membrane Structure

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts the membrane as a dynamic structure with proteins floating in or on a fluid lipid bilayer.

• Most lipids and some proteins can move laterally within the membrane.

• Membrane fluidity is influenced by temperature and the composition of fatty acids (unsaturated hydrocarbon tails increase fluidity).

• Proteins are distributed in a mosaic pattern; some are anchored in place, while others move more freely.

• Cholesterol: Acts as a fluidity buffer, stabilizing membrane fluidity across temperature changes.

Membrane Proteins and Carbohydrates

• Integral Proteins: Span the hydrophobic interior of the membrane and are involved in transport and signaling.

• Peripheral Proteins: Loosely bound to the membrane surface.

• Glycolipids and Glycoproteins: Carbohydrates attached to lipids or proteins, important for cell recognition and signaling.

Membrane Permeability

• Selective Permeability: The membrane allows some substances to cross more easily than others. Small, nonpolar molecules (e.g., O2, CO2) pass easily; polar or charged molecules require transport proteins.

• Diffusion: The tendency for molecules to spread out evenly into available space. Movement is from high to low concentration (down the concentration gradient).

Transport Across Membranes

Passive Transport

Passive transport does not require energy and relies on the concentration gradient.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Simple Diffusion: Movement of molecules from high to low concentration without assistance.

• Facilitated Diffusion: Transport proteins help move specific molecules across the membrane down their concentration gradient. No energy is required.

Tonicity

• Isotonic Solution: Solute concentration is the same inside and outside the cell; no net water movement.

• Hypertonic Solution: Solute concentration is greater outside the cell; cell loses water and shrinks.

• Hypotonic Solution: Solute concentration is less outside the cell; cell gains water and may burst.

Osmoregulation

• The control of water and solute balance in an organism to maintain homeostasis.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Transport Proteins: Move specific molecules across the membrane from low to high concentration.

• Cotransport: A transport protein couples the downhill diffusion of one solute to the uphill transport of another solute against its gradient.

Bulk Transport by Vesicles

Large molecules and particles are transported across the membrane via vesicles, a process that requires energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release their contents outside the cell.

• Endocytosis: The cell takes in materials by forming vesicles from the plasma membrane. Includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

All forms of bulk transport require energy.

Summary Table: Types of Membrane Transport