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: Encloses the cell, regulates transport, and facilitates communication.

• Cytoplasm: Contains cytosol (intracellular fluid), organelles (specialized structures), and the cytoskeleton (structural framework).

• Nucleus: Houses DNA and is the site of RNA production, controlling cellular activities.

Cell Size and Diversity

Cells vary greatly in size and shape, reflecting their specialized functions in the human body.

• Examples: Red blood cells (transport oxygen), nerve cells (transmit signals), epithelial cells (form barriers), skeletal muscle cells (enable movement).

The Plasma Membrane

The Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, forming a selective barrier between the extracellular fluid (ECF) and the cytosol.

• Hydrophilic (polar) heads: Face water on both sides of the membrane.

• Hydrophobic (nonpolar) tails: Face inward, away from water.

The Fluid Mosaic Model

The plasma membrane is a dynamic structure with proteins, lipids, and carbohydrates embedded within the bilayer, allowing lateral movement and flexibility.

• Integral proteins: Span the membrane; some are transmembrane proteins.

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

• Functions: Channels, carriers, receptors, enzymes, structural support, and cell linking.

Other Membrane Components

• Cholesterol: Stabilizes membrane structure during temperature changes.

• Glycolipids and Glycoproteins: Involved in cell recognition and signaling.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

• Diffusion: Movement of solute from high to low concentration.

• Simple Diffusion: Nonpolar molecules (e.g., O2, CO2) pass directly through the bilayer.

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

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from low to high solute concentration.

• Aquaporins: Water channels facilitating rapid water movement.

• Osmotic Pressure: Pressure needed to prevent water movement by osmosis.

• Hydrostatic Pressure: Force exerted by water on container walls.

Tonicity

Tonicity describes the ability of a solution to cause water movement into or out of a cell.

• Isotonic: No net water movement; cell volume remains constant.

• Hypertonic: Cell loses water and shrivels (crenates).

• Hypotonic: Cell gains water, swells, and may lyse.

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).

• Secondary Active Transport: Uses the energy from a concentration gradient established by primary active transport.

Vesicular Transport

Large particles are transported via vesicles in processes requiring ATP.

• Endocytosis: Bringing substances into the cell (phagocytosis for solids, pinocytosis for fluids).

• Exocytosis: Releasing substances from the cell.

• Transcytosis: Transport across the cell via vesicles.

Summary Table: Plasma Membrane Transport

The following table summarizes the main types of plasma membrane transport, their energy requirements, and examples.

Cytoplasmic Organelles

Membrane-Bound Organelles

• Mitochondria: Site of ATP production via oxidative catabolism; contains its own DNA.

• Peroxisomes: Detoxify substances, break down fatty acids, and synthesize certain phospholipids.

• Endoplasmic Reticulum (ER): Rough ER (protein synthesis and folding), Smooth ER (lipid synthesis, detoxification, calcium storage).

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

• Lysosomes: Digest macromolecules and worn-out organelles; function in autophagy and immune defense.

Non-Membrane-Bound Organelles

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

• Centrosome and Centrioles: Organize microtubules and are important in cell division.

The Cytoskeleton

Types of Cytoskeletal Filaments

• Actin Filaments (Microfilaments): Support cell shape, involved in movement and muscle contraction.

• Intermediate Filaments: Provide mechanical strength and structural support.

• Microtubules: Maintain cell architecture, facilitate organelle movement, and form cilia and flagella.

Cellular Extensions

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

• Cilia: Short, motile projections that move substances across cell surfaces.

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

The Nucleus

Structure and Function

The nucleus directs cellular activities and houses genetic material (DNA).

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

• Chromatin: DNA and associated proteins; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosome assembly.

Protein Synthesis

Gene Expression

• Transcription: DNA code is copied into messenger RNA (mRNA) in the nucleus.

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

• Posttranslational Modification: Folding and processing of the polypeptide into a functional protein.

The Cell Cycle

Phases of the Cell Cycle

• Interphase: Cell grows, replicates DNA, and prepares for division (includes G1, S, and G2 phases).

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

Mitosis Stages

• Prophase: Chromatin condenses, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase and Cytokinesis: Nuclear envelope reforms, cell splits.

Cell Cycle Control and Cancer

• Checkpoints: Ensure proper division; failure can lead to apoptosis or uncontrolled growth (cancer).

• Benign Tumor: Non-invasive growth.

• Malignant Tumor (Cancer): Invades tissues and may metastasize.

Additional info: For further details, refer to the full textbook or instructor-provided resources. This guide covers the essential concepts and structures for understanding the cell in Anatomy & Physiology.