The Cell

Basic Processes of Cells

Cells are the fundamental units of life, carrying out essential processes to maintain homeostasis and support the organism.

• Cell Metabolism: Chemical reactions within the cell, including anabolic (building), catabolic (breaking down), and oxidation-reduction reactions.

• Substance Transport: Movement of compounds into, out of, or within the cell.

• Communication: Cells interact with their environment and other cells via signaling mechanisms.

• Cell Reproduction: Many cells divide to produce new cells, essential for growth and repair.

Overview of Cell Structure

Most animal cells share three basic components: the plasma membrane, cytoplasm (including cytosol, organelles, and cytoskeleton), and nucleus.

• Plasma Membrane: The outer boundary that separates the cell from its environment.

• Cytoplasm: The region between the plasma membrane and nucleus, containing cytosol (intracellular fluid), organelles, and the cytoskeleton.

• Nucleus: The control center housing DNA and the site of RNA production.

Functions of the Plasma Membrane

The plasma membrane physically isolates the cell, provides structural support, communicates with other cells, regulates transport, and identifies the cell. It defines intracellular (cytosol) and extracellular (ECF) fluid compartments.

Components of the Cytoplasm

• Cytosol: Watery gel with proteins, solutes, and RNA; site of many cellular processes.

• Organelles: Specialized structures performing specific functions.

• Cytoskeleton: Protein filament network supporting cell shape, organelle positioning, and intracellular transport.

Nucleus

The nucleus is surrounded by a double membrane (nuclear envelope), contains most of the cell’s DNA, and is the site for RNA production. DNA and RNA direct cellular functions by coding for proteins.

Cell Size and Diversity

Cells vary greatly in size and appearance, allowing for specialized functions. Examples include red blood cells, nerve cells, epithelial cells, and skeletal muscle cells.

The Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, forming a barrier between the ECF and cytosol. Phospholipids have hydrophilic (polar) heads and hydrophobic (nonpolar) tails, causing them to arrange into two layers in aqueous environments.

The Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which highlights its dynamic nature and the presence of proteins, lipids, and carbohydrates. Membrane components move laterally, contributing to membrane fluidity.

Membrane Proteins

Membrane proteins are essential for various functions and are classified by location and function:

• Integral (Transmembrane) Proteins: Span the membrane.

• Peripheral Proteins: Located on one side of the membrane.

• Functional Types: Channels, carriers, receptors, enzymes, structural support, and linker proteins.

Other Membrane Components

• Cholesterol: Stabilizes membrane structure during temperature changes.

• Glycolipids and Glycoproteins: Carbohydrate chains attached to lipids or proteins; function in cell recognition.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require energy and includes diffusion and osmosis.

• Diffusion: Movement of solute from high to low concentration, driven by a concentration gradient.

• Simple Diffusion: Nonpolar solutes and gases pass directly through the bilayer.

• Facilitated Diffusion: Polar or charged solutes cross via channel or carrier proteins.

• Osmosis: Movement of water across a selectively permeable membrane from low to high solute concentration.

• Tonicity: Comparison of solute concentrations between solutions.

• Isotonic: Equal solute concentration; no net water movement.

• Hypertonic: Higher solute concentration outside; cell loses water and shrivels.

• Hypotonic: Lower solute concentration outside; cell gains water and may burst.

Active Transport

Active transport requires ATP to move substances against their concentration gradients via carrier proteins (pumps).

• Primary Active Transport: Direct use of ATP, e.g., sodium-potassium pump ( out, in).

• Secondary Active Transport: Uses the energy from a concentration gradient created by primary active transport to move another substance.

Vesicular Transport

Large particles are transported via vesicles in processes requiring ATP:

• Endocytosis: Bringing substances into the cell (phagocytosis for particles, pinocytosis for fluids, receptor-mediated for specific molecules).

• Exocytosis: Releasing substances from the cell.

• Transcytosis: Transport across the cell.

Summary Table: Plasma Membrane Transport

The following table summarizes the main types of plasma membrane transport:

Cytoplasmic Organelles

Membrane-Bound Organelles

• Mitochondria: Site of ATP production; contains its own DNA and ribosomes; inner membrane forms cristae.

• Peroxisomes: Use oxygen to detoxify substances, break down fatty acids, and synthesize certain phospholipids.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) synthesizes/folds proteins; Smooth ER synthesizes lipids, detoxifies, and stores calcium.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and old organelles; function in autophagy and immune defense.

Non-Membrane-Bound Organelles

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins; can be free or bound to ER.

The Cytoskeleton

Types of Filaments

• Actin Filaments (Microfilaments): Support cell shape, bear tension, and enable movement (with myosin).

• Intermediate Filaments: Provide mechanical strength and support nuclear envelope.

• Microtubules: Hollow tubes for organelle movement, cell division, and forming cilia/flagella.

Cellular Extensions

• Microvilli: Increase surface area for absorption (e.g., intestines, kidneys).

• Cilia: Short, motile projections for moving substances over cell surfaces.

• Flagella: Long, whip-like structures for cell movement (e.g., sperm).

The Nucleus

Structure and Function

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Chromatin: DNA wrapped around histones; condenses into chromosomes during cell division.

• Nucleolus: Site of rRNA synthesis and ribosome assembly.

Protein Synthesis

Gene Expression

• Transcription: DNA code is copied into mRNA in the nucleus.

• Translation: Ribosomes read mRNA and assemble amino acids into a polypeptide chain in the cytoplasm.

Genetic Code

• DNA triplets are transcribed into mRNA codons, which specify amino acids.

• Mutations in DNA can lead to diseases, including cancer.

The Cell Cycle

Phases of the Cell Cycle

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• M Phase: Mitosis (division of genetic material) and cytokinesis (division of cytoplasm and organelles).

• Mitosis Stages: Prophase, metaphase, anaphase, telophase, and cytokinesis.

Cell Cycle Control and Cancer

• Checkpoints regulate cell division; failure can lead to uncontrolled growth (tumors).

• Benign tumors remain localized; malignant tumors (cancer) invade other tissues (metastasis).

Additional info: This guide covers the essential concepts of cell structure, membrane transport, organelles, cytoskeleton, nucleus, protein synthesis, and the cell cycle, as outlined in a typical Anatomy & Physiology curriculum.