The Cellular Level of Organization

The Cellular Level of Organization

OpenStax Anatomy and Physiology - Chapter 3

The Cell Membrane: Objectives

  • Compare and contrast different types of passive transport with active transport, providing examples of each.

  • Differentiate between materials that can and cannot diffuse through the lipid bilayer.

  • Explain the major features and properties of the cell membrane.

  • Describe the molecular components that make up the cell membrane.

What is a Cell?

  • Smallest Unit of Life: The cell is the most basic unit of life.

  • Highly Organized for Metabolism: Cells exhibit complex organization that supports metabolic processes.

  • Senses and Responds to Environment: Cells can detect changes in their environment and respond accordingly.

  • Has Potential to Reproduce: Cells have the ability to replicate, ensuring the continuation of genetic material.

Membrane Structure

  • Phospholipids: Amphipathic molecules that form the structure of the cell membrane.

    • Lipid Bilayer: Comprised of phospholipids arranged with hydrophilic heads facing the extracellular and intracellular fluids and hydrophobic tails facing inward.

    • Heads: Polar and Hydrophilic; attracted to water in both intracellular and extracellular fluids.

    • Tails: Non-Polar and Hydrophobic; repelled by water, ensuring the membrane's barrier properties to polar substances.

Membrane Proteins

  • Glycolipids: Located on the surface, serve as cellular markers.

  • Cholesterol: Interspersed within the membrane, it serves to stiffen the membrane by inserting itself between fatty acid tails.

  • Types of Membrane Proteins:

    • Integral Proteins: Transmembrane channels that extend across the entire bilayer, allowing passage of substances.

    • Peripheral Proteins: Attached to one side of the membrane, playing specific roles in cellular functions; account for approximately 50% of the membrane's weight.

    • Channels: Facilitate ion passage (can be open or gated).

    • Carriers: Transport solutes across the membrane using ATP, for example, the Na+/K+ pump.

Cellular Differentiation

  • Stem Cell: An unspecialized cell that can divide indefinitely, having several categories including:

    • Totipotent: Can differentiate into any cell type.

    • Pluripotent: Can become any cell type within the human body.

    • Multipotent: Can differentiate into a limited number of cell types specific to a certain lineage (ectoderm, mesoderm, endoderm).

    • Oligopotent: Can differentiate into a few cell types.

    • Unipotent: Can only generate its own cell type.

Cell Connections

  • Desmosomes: Anchoring junctions found in tissues like skin and the heart.

  • Gap Junctions: Allow ions to pass between adjacent cells, particularly important in heart cells for synchronized contraction.

  • Tight Junctions: Prevent leakage of fluids across cell layers, found in tissues like the stomach and skin, ensuring that cells remain closely packed together.

Membrane Transport

  • Selective Permeability: The cell membrane selectively allows certain substances to pass while blocking others.

  • Passive Transport: Does not require energy (ATP) to move substances across the membrane.

  • Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

Types of Membrane Transport

Passive Transport

  • Concentration Gradient: Movement of particles from areas of high concentration to areas of low concentration.

  • Diffusion: Movement of particles through a semipermeable membrane.

  • Facilitated Diffusion: Substances that cannot cross the lipid bilayer due to size, charge, or polarity use specific channels or carriers.

  • Filtration: Movement of water and solutes through a membrane by hydrostatic pressure.

  • Osmosis: Diffusion of water across a semipermeable membrane, affected by the tonicity of the solution (isotonic, hypertonic, hypotonic):

    • Isotonic: Equal concentration of solute inside and outside the cell.

    • Hypertonic: Higher concentration of solute outside the cell, causing the cell to lose water and potentially shrink.

    • Hypotonic: Lower concentration of solute outside the cell, causing water to move into the cell, potentially leading to cell lysis.

Active Transport

  • Involves processes such as:

    • Exocytosis: Process by which large substances are expelled from a cell.

    • Endocytosis: Process in which cells engulf external substances. Types of endocytosis:

    • Phagocytosis: Engulfing large particles.

    • Pinocytosis: Engulfing liquids or small particles.

    • Receptor-mediated Endocytosis: Specific molecules are ingested into the cell after binding to a receptor.

Fluid Balance

  • Intercellular Fluid (ICF): The cytosol found inside the cell, typically maintaining a pH of 7.0.

  • Extracellular Fluid (ECF): The fluid found surrounding cells, generally maintaining a pH of 7.4.

  • Tonicity Terms:

    • Isotonic: Equilibrium in solute concentration.

    • Hypertonic: Higher solute concentration outside the cell; water movement leads to cell drying.

    • Hypotonic: Lower solute concentration outside; water moves into the cell possibly leading to bursting.

The Cytoplasm and Cellular Organelles: Objectives

  • Describe the structure and function of cellular organelles related to the endomembrane system (endoplasmic reticulum, Golgi apparatus, lysosomes).

  • Describe the structure and function of mitochondria and peroxisomes.

  • Explain the three components of the cytoskeleton, including their composition and functions.

Overview of the Cell

  • Cytoplasm: Composed of cytosol and organelles.

    • Key organelles include:

    • Rough Endoplasmic Reticulum (RER): Studded with ribosomes; involved in protein synthesis and modification.

    • Nucleus: Contains genetic material.

    • Golgi Apparatus: Processes and distributes proteins and lipids.

    • Mitochondria: Energy production via cellular respiration.

    • Lysosomes: Break down waste materials and cellular debris.

    • Cytoskeleton: Composed of microtubules, microfilaments, and intermediate filaments, crucial for structure and transport.

The Endomembrane System

  • Endoplasmic Reticulum (ER): System of membranes continuous with the nuclear membrane, involved in synthesis and storage of proteins and lipids.

    • Rough ER: Ribosomes present; involved in the synthesis and modification of proteins.

    • Smooth ER: Lacks ribosomes; involved in lipid synthesis.

  • Golgi Apparatus: Often referred to as the "Post Office"; it sorts, modifies, and ships products synthesized at the Rough ER.

  • Vesicles and Lysosomes:

    • Vesicles transport materials within the cell.

    • Autophagy: The process of lysosomes digesting worn-out organelles.

    • Autolysis: Self-digestion of cells typically after cell death.

Energy Production and Detox

  • Mitochondria: Referred to as the powerhouses of the cell, involved in cellular respiration, converting nutrients into ATP.

  • Peroxisomes: Organelles that contain enzymes involved in the detoxification of harmful substances, such as hydrogen peroxide.

The Cytoskeleton

  • Composed of three types of fibers:

    • Microfilaments: Fine, thread-like structures providing structural support.

    • Intermediate Filaments: Provide mechanical support.

    • Microtubules: Hollow tubes that assist in cell movement (e.g., cilia and flagella) and serve as tracks for intracellular transport.

The Nucleus and DNA Replication: Objectives

  • Describe the structure and features of the nuclear membrane.

  • List the contents of the nucleus.

  • Explain the organization of the DNA molecule within the nucleus.

  • Describe the process of DNA replication.

The Nucleus

  • Described as the "Control Center" of the cell; it is the largest organelle.

  • Some cells, such as muscle cells, can contain multiple nuclei.

  • Key structures:

    • Nuclear Envelope: Double membrane that encloses the nucleus.

    • Nuclear Pores: Allow molecules to enter and exit the nucleus.

    • Chromatin: DNA in its relaxed form when not dividing.

    • Nucleolus: Site of ribosome synthesis.

    • Cisternae: Membrane-bound spaces within the nucleus.

Organization of the Nucleus and DNA

  • Nuclear Envelope and Pores: Regulates molecular transport between the nucleus and cytoplasm.

  • Nucleolus: Site for ribosomal RNA (rRNA) synthesis and ribosome assembly.

  • DNA Packaging: DNA is organized with the aid of proteins called histones, forming structures called nucleosomes. Chromosomes are tightly packed DNA structures during cell division.

Cell Growth and Division

  • Interphase: The period when a cell is not dividing; it constitutes the majority of the cell's life cycle.

  • Mitosis: The process of nuclear division resulting in two nuclei.

  • Cytokinesis: The division of the cytoplasm resulting in two distinct daughter cells.

Sister Chromatids

  • Following the S phase of the cell cycle, each chromosome consists of two identical sister chromatids, which are connected at a structure called the centromere.

  • During cell division, chromatids will separate, with each becoming an individual chromosome.

Phases of Mitosis

  • Prophase: Sister chromatids condense and become visible; the nucleolus and nuclear envelope disintegrate; centrosomes move apart, and microtubules attach to kinetochores.

  • Metaphase: Chromatids align at the cell equator, preparing for separation.

  • Anaphase: Sister chromatids are pulled toward opposite poles of the cell.

  • Telophase: Two new daughter nuclei form around the separated chromosomes; nucleoli reappear, and the mitotic spindle disassembles.

  • Cleavage Furrow: A band of microfilaments that forms during cytokinesis, squeezing the two daughter cells apart.

Timing of Mitosis

  • There are critical checkpoints in the cell cycle that regulate cell division, preventing unregulated growth which can lead to abnormalities or cancer.

  • The immune system plays a role in identifying and destroying abnormal cells.

DNA Structure

  • DNA is composed of two complementary strands arranged in a double helix, resembling a twisted ladder.

  • Four DNA Bases:

    • Adenine (A)

    • Thymine (T)

    • Cytosine (C)

    • Guanine (G)

  • The bases pair specifically: A with T and C with G.

DNA Replication and Protein Synthesis

  • During cell division, DNA is replicated, ensuring each daughter cell receives the same genetic material.

  • Transcription: The process where DNA is transcribed into messenger RNA (mRNA).

  • Translation: The process where mRNA is translated into a polypeptide chain (protein).

  • Replication process involves correcting mistakes to maintain genetic integrity.

Protein Synthesis

  • The sequence of bases in DNA dictates which proteins are produced.

  • DNA triplets correspond to specific amino acids in proteins.

  • Protein synthesis occurs on ribosomes, where mRNA provides the coding sequence.

  • tRNA carries amino acids; its anticodon matches with mRNA codons, linking amino acids together to form a growing polypeptide chain.

  • Most polypeptide chains consist of 200-300 amino acids forming functional proteins.

Review Questions

  • What are the types of membrane transport?

  • What are the phases of mitosis?

  • What are the four DNA bases?

What's Up Next?

  • Topic 3 Quiz

  • Topic 4