Anatomy & Physiology: Biology of the Cell Notes
Anatomy & Physiology: Biology of the Cell
The Range of Cell Sizes
Cell sizes vary greatly, necessitating different observation tools:
Unaided eye: Can discern objects from human height (10 \, m) down to about (0.1 \, m).
Light microscope: Essential for viewing cells, effective from (1 \, mm) down to (1 \, \mu m).
Electron microscope: Required for the smallest cellular components and macromolecules, effective from (100 \, μm) down to (0.1 \, nm).
Specific examples of cell and biological structure sizes:
Human height: (10 \, m)
Some skeletal muscle cells: (0.1 \, m)
Ostrich egg: (1 \, cm)
Human oocyte: (1 \, mm)
Most plant and animal cells: (100 \, \mu m) (average \sim 30 \, \mu m)
Red blood cell: (10 \, \mu m)
Mitochondrion: (1 \, \mu m)
Most bacteria: (100 \, nm)
Viruses: (100 \, nm)
Ribosomes: (10 \, nm)
Large biological macromolecules (proteins): (10 \, nm)
Small molecules (amino acids): (1 \, nm)
Atom: (0.1 \, nm)
The Variety of Cell Shapes
Cell shape is often specialized for its function:
Irregular-shaped: Nerve cells (for complex connections).
Biconcave disc: Red blood cells (for efficient oxygen transport and flexibility).
Cube-shaped: Kidney tubule cells (for secretion and absorption).
Column-shaped: Intestinal lining cells (for absorption and protection).
Spherical: Cartilage cells (often found in extracellular matrix, less need for distinct shape).
Cylindrical: Skeletal muscle cells (for contraction).
The Structure of a Prototypical Cell
A generalized animal cell consists of:
Plasma Membrane: The outer boundary of the cell.
Modifications: Microvilli, Cilia, Flagellum (note: not all cells have a flagellum).
Cytoplasm: Encompasses all materials between the plasma membrane and the nucleus.
Cytosol: The intracellular fluid (ICF), where many metabolic reactions occur.
Organelles: Cellular structures that perform specific functions.
Membrane-bound organelles (indicated by dots in diagrams):
Nucleus: Contains genetic material.
Nuclear envelope: Double membrane surrounding the nucleus.
Nucleoplasm: Fluid inside the nucleus.
Nucleolus: Site of ribosomal RNA synthesis.
Rough Endoplasmic Reticulum (RER).
Smooth Endoplasmic Reticulum (SER).
Golgi Apparatus.
Lysosome.
Peroxisome.
Mitochondrion.
Vesicle: Small membrane-bound sac for transport or storage.
Non-membrane-bound organelles:
Ribosomes (free and bound).
Centrosome.
Proteasomes.
Cytoskeleton.
Inclusions: Temporary storage of lipids, carbohydrates, pigments, etc.
Interstitial fluid: The extracellular fluid (ECF) surrounding cells.
The Cell Cycle
The cell cycle represents the life of a cell from its formation until it divides into two new cells. It consists of two major phases:
Interphase (\sim 23 \, hours): A period of growth and preparation for cell division.
G\mathbf{1} phase: (Growth and preparation for DNA replication) — Cells perform normal metabolic functions and grow. Differentiation occurs here, where a cell specializes.
S phase: (DNA replication) — The cell synthesizes a complete copy of its DNA, resulting in replicated chromosomes with two sister chromatids.
G\mathbf{2} phase: (Growth and preparation for division of DNA) — A brief phase where the cell grows further, centrosome replication is completed (resulting in two pairs of centrioles), and enzymes for cell division are synthesized.
Mitotic (M) phase (\sim 1 \, hour): The period of cell division.
Mitosis: Division of the nucleus, resulting in two identical nuclei. This process includes several stages:
Prophase: The first stage of mitosis.
Chromatin supercoils and condenses into visible, replicated chromosomes, each composed of two sister chromatids joined at a centromere.
The nucleolus breaks down.
Spindle fibers (microtubules) begin to grow from the centrioles.
Centriole pairs move apart, migrating to opposite poles of the cell.
The dissolution of the nuclear envelope marks the end of prophase.
Metaphase: The second stage of mitosis.
Replicated chromosomes are aligned in a single file along the equatorial plate (or metaphase plate) at the center of the cell.
Spindle fibers extend from each centriole and attach to the centromere of each chromosome.
Anaphase: The third stage of mitosis.
Begins as spindle fibers pull sister chromatids apart, moving them toward opposite poles of the cell.
Each separated chromatid is now considered an individual chromosome (single-stranded) with its own centromere.
Cytokinesis (division of the cytoplasm) typically begins during anaphase, with the formation of a cleavage furrow.
Telophase: The final stage of mitosis (often described as the reverse of prophase).
The single-stranded chromosomes arrive at opposite poles and begin to uncoil into chromatin.
A new nuclear envelope reforms around each set of chromosomes.
The nucleolus re-forms within each new nucleus.
Spindle fibers disassemble.
Cytokinesis: Division of the cytoplasm, distinct from nuclear division.
Overlaps with late anaphase and telophase.
Microfilament proteins at the cell periphery form a cleavage furrow.
The cleavage furrow deepens, pinching the mother cell into two genetically identical daughter cells.
Cell division is complete after cytokinesis.
Cell Division: Mitosis vs. Meiosis
Mitosis:
Occurs in somatic cells (all body cells other than sex cells).
One cell divides to produce two genetically identical daughter cells.
Essential for development, tissue growth, replacement of old cells, and tissue repair.
Meiosis:
Occurs in sex cells (cells that give rise to sperm or oocytes).
Involves two rounds of division, producing four genetically distinct haploid cells.
Clinical View: Tumors
Regulatory mechanisms normally signal cells to divide or stop dividing to maintain tissue homeostasis.
Cell signaling disruption: If these regulatory mechanisms are disrupted, uncontrolled cell division can occur, leading to the formation of tumors.
Impact: Tumors interfere with the function of normal surrounding cells.
Metastasis: Tumor cells can enter the blood or lymphatics and spread (metastasize) to other areas of the body, forming secondary tumors.
Structure and Functions of the Plasma Membrane
The plasma membrane is a selectively permeable barrier that surrounds the cell.
Lipid Components: Form the basic framework of the membrane.
Phospholipid Bilayer: The primary structural component.
Composed of two layers of phospholipids.
Each phospholipid has a polar (hydrophilic) head (facing interstitial fluid and cytosol) and two nonpolar (hydrophobic) tails (forming the inner core).
This arrangement creates a barrier to water-soluble substances.
Cholesterol: A four-ring lipid molecule scattered within the phospholipid bilayer.
Strengthens the membrane.
Stabilizes the membrane against temperature extremes, making it less fluid at high temperatures and preventing excessive rigidity at low temperatures.
Glycolipids: Lipids with attached carbohydrate groups.
Located exclusively on the outer phospholipid region (facing the interstitial fluid).
Contribute to the glycocalyx.
Membrane Proteins: Perform most of the membrane's functions and float/move within the fluid bilayer (fluid mosaic model).
Structural Types:
Integral proteins: Embedded within or extend across the entire lipid bilayer (transmembrane proteins). Many are glycoproteins.
Peripheral proteins: Not embedded in the bilayer; loosely attached to the external or internal surface of the membrane or to integral proteins.
Functional Types:
Transport proteins: Regulate movement of substances across the membrane.
Channels: Provide a pore through which specific ions or small polar molecules can pass.
Carriers: Bind to specific substances and change shape to move them across.
Pumps: Use energy (e.g., ATP) to move substances against their concentration gradient.
Receptors: Bind specific molecules (ligands) to trigger a cellular response.
Identity markers (Glycoproteins): Cell-surface carbohydrates (often part of glycoproteins or glycolipids) that allow cells to recognize each other.
Enzymes: Catalyze chemical reactions at the membrane surface.
Anchoring sites: Secure cytoskeleton to the plasma membrane, maintaining cell shape and stability.
Cell-adhesion proteins: Used for cell-to-cell attachment, forming junctions between cells.
Glycocalyx: A