BIOL 201 Chapter 3

Overview of Cells

  • The average human body has over 100 trillion cells, which are the basic structural and functional units of life.

  • Cell biology (cytology) is the study of cells.

Structure of a Cell

  1. Plasma Membrane

    • Forms the outer boundary of the cell.

    • Functions as a selective barrier regulating material flow in and out of the cell.

    • Crucial for cellular communication.

  2. Cytoplasm

    • Contains all cellular contents between the plasma membrane and nucleus.

    • Components include:

      • Cytosol: The fluid portion containing water, solutes, and suspended particles.

      • Organelles: Subcellular structures with specific functions, e.g., ribosomes, endoplasmic reticulum (ER), Golgi complex, lysosomes, peroxisomes, and mitochondria.

  3. Nucleus

    • The largest organelle, housing DNA.

    • Contains chromosomes (DNA and proteins) and genes that control cellular activity.

Plasma Membrane

  • Described by the Fluid Mosaic Model:

    • Composed mainly of a phospholipid bilayer with embedded proteins.

    • Phospholipids:

      • Polar heads (hydrophilic) and fatty acid tails (hydrophobic).

    • Proteins in the membrane serve various functions, such as:

      • Acting as channels for ions.

      • Serving as receptors for signaling molecules.

      • Cell-identity markers.

Membrane Proteins

  • Integral Proteins: Span the membrane, can be glycoproteins.

  • Peripheral Proteins: Located on the inner or outer membrane surface.

  • Glycocalyx: A sugar coating made of glycoproteins and glycolipids, aiding in cell recognition and protection.

Membrane Transport Mechanisms

Passive Transport

  • Simple Diffusion: Movement from high to low concentration without energy.

  • Facilitated Diffusion: Uses channel and carrier proteins for polar/charged substances.

  • Osmosis: Diffusion of water across a semi-permeable membrane.

    • Can occur through phospholipid bilayer or through aquaporins.

Active Transport

  • Requires energy (ATP) to move substances against their gradient.

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

  • Secondary Active Transport: Uses the energy from concentration gradients.

  • Vesicular Transport (e.g., endocytosis and exocytosis): Involves vesicles for large particles or fluids.

Types of Vesicular Transport

  • Endocytosis: Incorporation of materials into the cell (e.g., phagocytosis and receptor-mediated endocytosis).

  • Exocytosis: Expulsion of materials from the cell via vesicle fusion with the plasma membrane.

  • Transcytosis: Combination of endocytosis and exocytosis for transporting substances across the cell.

Cytoplasmic Organelles

  1. Ribosomes: Sites of protein synthesis; can be free in cytoplasm or bound to RER.

  2. Endoplasmic Reticulum (ER):

    • Rough ER: Studded with ribosomes, synthesizes and processes proteins.

    • Smooth ER: Synthesizes lipids and detoxifies harmful substances.

  3. Golgi Complex: Modifies, sorts, and packages proteins and lipids for distribution.

  4. Lysosomes: Contain enzymes for digestion; involved in autophagy and autolysis.

  5. Peroxisomes: Contain enzymes for detoxification, breakdown of fatty acids, and hydrogen peroxide.

  6. Mitochondria: The powerhouse of the cell, generating ATP through aerobic respiration.

Cell Division

  1. Somatic Cell Division (Mitosis):

    • Involves duplication and division of the cell nucleus and cytoplasm.

    • Phases: Prophase, Metaphase, Anaphase, and Telophase.

  2. Meiosis: Division that produces gametes (sperm and egg);

    • Meiosis I reduces chromosome number by half, resulting in haploid cells.

    • Meiosis II separates sister chromatids into individual chromosomes.

Aging and Cellular Functions

  • Cellular functions decline with age; telomeres shorten with each division contributing to aging.

  • Dysfunction in cellular processes can lead to age-related diseases, including cancer.

Summary

  • Understanding cell structure and function is crucial in biology and medical science.

  • Different cellular mechanisms enable complex life processes and adaptation to various environmental conditions.