Cell Structure and Function

Overview of Cellular Organization

Cells are the fundamental units of structure and function in living organisms. Their organization and subcellular components determine their specific roles within tissues and organs.

• Cell Theory:

  • All organisms are composed of one or more cells.

  • Cells are the basic units of structure and function in living things.

  • Cells arise only from pre-existing cells.

  • Organismal function depends on the function of its cells.

• Basic Cell Components: Each cell contains a plasma membrane, nucleus, and cytoplasm.

Subcellular Structures

• Nucleolus: Produces ribosomes and is the site where ribosomal RNA genes are transcribed. It appears as a spherical structure within the nucleus.

• Chromatin: Contains DNA and proteins called histones. Chromatin condenses to form chromosomes during cell division.

• Golgi Apparatus: Packages proteins and fats and ships them to other parts of the cell.

• Mitochondria: Known as the "powerhouse" of the cell, mitochondria generate ATP (adenosine triphosphate), the cell's main energy currency.

• Endoplasmic Reticulum (ER):

  • Rough ER: Site of protein synthesis due to ribosomes attached to its surface.

  • Smooth ER: Site of fat (lipid) production.

Cell Membrane Proteins

• Integral Proteins: Firmly inserted into the lipid bilayer; serve as transport channels, carriers, enzymes, and receptors.

• Peripheral Proteins: Located on the membrane surface; act as enzymes, motor proteins, and are involved in cell-to-cell links and support.

Plasma Membrane Properties

• Selective Permeability: The plasma membrane allows some substances to pass more easily than others. Fats and lipids pass through easily, while water-soluble molecules have difficulty.

Cell Junctions

Cell junctions are specialized structures that connect adjacent cells and regulate the passage of materials.

• Tight Junctions: Prevent fluids from moving between cells, keeping the cell layer watertight.

• Gap Junctions: Allow small molecules to pass from cell to cell, facilitating communication between cells.

• Desmosomes: Spot rivets between cells that resist shearing forces and keep cells together.

Cellular Extensions

• Cilia: Small, mobile, whip-like extensions on the surface of some cells; move substances across the cell surface.

• Microvilli: Small, finger-like extensions of the plasma membrane; increase surface area for absorption (e.g., in the intestines).

• Flagella: Long, mobile whip-like extensions that propel the cell through its environment; in humans, only sperm cells have flagella.

Membrane Transport

Passive Transport

Passive transport involves the movement of substances down their concentration gradients without the use of cellular energy (ATP).

• Simple Diffusion: Movement of nonpolar lipid-soluble substances directly through the lipid bilayer.

• Facilitated Diffusion: Transport of polar water-soluble substances through protein channels or carriers.

• Osmosis: Movement of water molecules through water channels called aquaporins.

• Filtration: Passage of substances along a pressure gradient (e.g., in kidneys).

Leakage Channels

• Sodium Leakage Channels: Always open, allowing Na+ to move across the membrane.

• Potassium Leakage Channels: Always open, allowing K+ to move across the membrane.

Gated Channels

• Voltage-Gated Channels: Open in response to changes in membrane potential.

• Chemically-Gated Channels: Open in response to specific chemical signals.

• Mechanically-Gated Channels: Open in response to physical deformation of the receptor.

Active Transport

Active transport requires ATP to move substances against their concentration gradients.

• Primary Active Transport: Uses ATP and carrier proteins to pump substances across membranes (e.g., sodium/potassium ATPase pump).

• Secondary Active Transport: Uses gradients established by primary active transport to move other substances (symport/antiport).

• Vesicular Transport: Moves larger molecules across membranes using vacuoles.

  • Exocytosis: Expels substances from the cell.

  • Endocytosis: Engulfs substances into the cell (includes phagocytosis and pinocytosis).

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the amount of electrical energy needed to move a charge across the membrane. It is essential for nerve impulse transmission and muscle contraction.

• There is a higher concentration of potassium (K+) inside the cell.

• There is a higher concentration of sodium (Na+) outside the cell.

• Potassium leakage channels are always open, allowing potassium to leak out of the cell constantly.

Graded Potentials

Graded potentials are small, localized changes in the resting membrane that occur due to a stimulus.

• Can depolarize (less negative) or hyperpolarize (more negative) the membrane.

Action Potentials

Action potentials are rapid, long-distance reversals of membrane potential that are essential for nerve impulse transmission.

• All-or-none phenomenon and are unidirectional.

• Serve as signals along membranes to allow nerve impulses and muscular contractions.

Summary Table: Cell Membrane Transport Mechanisms

Key Equations

• Nernst Equation (for membrane potential):

• Sodium-Potassium Pump: For every ATP hydrolyzed, 3 Na+ are pumped out and 2 K+ are pumped in.

Additional info: Academic context and definitions have been expanded for clarity and completeness.