CELLS

AS OCR Biology - Module 1: Cells, Exchange and Transport

Unit 1: Cells Revision Notes

Magnification and Resolution

Cells are too small to be seen with the naked eye; the light microscope was developed to produce enlarged and detailed images of cells.

  • Magnification: The magnification of an image is the factor by which it appears larger than in real life.
      - Formula: extmagnification=racextimagesizeextactualsizeext{magnification} = rac{ ext{image size}}{ ext{actual size}}

  • Resolution: Refers to the ability to distinguish two distinct points as separate.
      - Example: If the resolution of a light microscope is 200 nm (0.2 μm), it can differentiate points that are 200 nm apart or more.

The Light Microscope

  • Components and Function:
      - Utilizes lenses to magnify images.
      - Light source from a bulb passes through a condenser lens and the specimen, then through an objective lens to the eyepiece lens.
      - Magnification levels provided by objective lenses are x4, x10, x40, and x100, with an eyepiece lens magnifying by x10.
      - Final magnifications include x40, x100, x400, and x1000.
      - Overall magnification formula:
        extoverallmagnification=extobjectivelensmagnificationimesexteyepiecelensmagnificationext{overall magnification} = ext{objective lens magnification} imes ext{eyepiece lens magnification}

Specimen Preparation

Some specimens require preparation due to issues such as lack of color or distortion upon sectioning:

  1. Staining: Utilizes colored stains that bind to chemicals in the specimen for visualization.

  2. Sectioning: Involves embedding specimens in wax and cutting thin sections to avoid distortion.

The Electron Microscope

Light microscopes have a resolution limitation of about 200 nm.

  • Electron microscopes use electron beams to achieve a higher resolution, distinguishing objects that are 0.2 nm apart.
      - Types:
        - Transmission Electron Microscope (TEM): Electrons pass through a thin sample, creating a 2D image; maximum magnification of x500,000.
        - Scanning Electron Microscope (SEM): Electrons bounce off a surface, producing a 3D image; maximum magnification of x100,000.

Advantages and Limitations of Electron Microscopes

  • Advantages:
      - High resolution of 0.1 nm (2000 times better than light microscopes).
      - Capable of producing detailed images of internal cell structures.

  • Limitations:
      - Requires a vacuum to prevent deflection by air molecules.
      - Extremely expensive.
      - Requires skilled preparation and operation; final images are in greyscale and may be colored using computer software.

Cell Ultrastructure and Cytoskeleton

  • Cytoskeleton: A network of protein fibers that maintains cell shape and allows movement within the cell.
      - Fibers:
        - Actin Filaments: Produce movement in white blood cells and transport organelles.
        - Microtubules: Cylindrical structures made from tubulin, facilitating movement of microorganisms and organelles via motor proteins.

Undulipodia and Cilia

  • Structure:
      - Both undulipodia (flagella) and cilia consist of microtubules arranged in a specific configuration (9+2 arrangement).
      - Differences: Undulipodia are longer and facilitate locomotion (e.g., sperm tail).

  • Function: Movement requires energy (ATP).

Organelles in Animal and Plant Cells

Organelles are structures within cells that serve specific functions, and most are membrane-bound, facilitating compartmentalization.

Key Organelles and Their Functions

  1. Nucleus:
       - Largest organelle; houses chromatin (DNA and proteins).
       - Nuclear envelope with pores allows selective passage of large molecules.
       - Nucleolus: Produces RNA and ribosomes.

  2. Endoplasmic Reticulum (ER):
       - Series of flattened sacs (cisternae); continuous with the nuclear membrane.
       - Rough ER: Studded with ribosomes (protein synthesis).
       - Smooth ER: Synthesizes lipids.

  3. Golgi Apparatus:
       - Stack of membrane sacs; modifies and packages proteins received from ER.
       - Forms vesicles to transport proteins.

  4. Mitochondria:
       - Site of ATP production; double membrane with extensive folding (cristae).
       - Matrix contains enzymes for respiration.

  5. Chloroplasts (plant cells only):
       - Double membrane; contains thylakoids for photosynthesis.
       - Converts light energy into chemical energy.

  6. Lysosomes:
       - Membrane-bound sacs containing digestive enzymes to break down materials (pathogens, damaged cell parts).

Ribosomes and Centrioles

  • Ribosomes: Sites for protein synthesis, can be free in cytoplasm or attached to ER.

  • Centrioles: PAired structures involved in cell division; produce spindle fibers that separate chromosomes.

Prokaryotic vs. Eukaryotic Cells

  • Eukaryotic Cells:
      - Have membrane-bound organelles including nucleus.
      - Larger and more complex.
      - Examples: animal and plant cells.

  • Prokaryotic Cells:
      - Lack membrane-bound organelles; smaller in size.
      - Circular DNA in cytoplasm.

Cell Membranes

  • Function: Separate contents within the cell; involved in recognition, signaling, and transport.

  • Structure: Phospholipid bilayer with embedded proteins; hydrophilic heads and hydrophobic tails create a barrier.

  • Fluid Mosaic Model: Describes the structure and dynamic nature of cell membranes.

Cell Signaling

Cells communicate via receptors on the membrane that bind signaling molecules (e.g., hormones). These receptors facilitate responses to external signals, crucial for metabolic regulation and interaction.

Transport Mechanisms
Passive Transport

  1. Diffusion: Movement of molecules from high to low concentration (e.g., O2, CO2).

  2. Osmosis: Movement of water molecules across a partially permeable membrane, driven by water potential (ψ).

  3. Facilitated Diffusion: Specific transport via protein channels.

Active Transport

  • Involves energy (ATP) to move substances against their concentration gradient (low to high).

  • Processes include ion pumps and bulk transport (endocytosis and exocytosis).

Cell Cycle and Division

  • Mitosis: Process where one parent cell divides to form two genetically identical daughter cells.
      - Stages include interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.

  • Meiosis: Generates gametes (haploid cells) for sexual reproduction; involves two rounds of cell division leading to genetic variation.

Cell Differentiation and Specialized Cells

  • Stems cells are undifferentiated and can become any cell type. Differentiation leads to specialized cells with distinct structures and functions.

Examples of Specialized Cells:

  1. Erythrocytes (Red Blood Cells): Transport oxygen; lack nucleus for more hemoglobin.

  2. Neutrophils: Phagocytic cells with lobed nuclei and lysosomes for fighting infection.

  3. Sperm Cells: Flagellum for movement, enzyme-rich acrosome for fertilization.

  4. Palisade Cells (in leaves): Loaded with chloroplasts for photosynthesis; elongated for optimal light capture.

  5. Root Hair Cells: Increase surface area for water absorption.

Tissues and Organs

  • Epithelial Tissues: Protective layers covering surfaces.

  • Connective Tissues: Support structures.

  • Muscle Tissues: Facilitating movement.

  • Nervous Tissues: Relaying signals and response to stimuli.

  • Plant Transport Tissues: Phloem and xylem for nutrient and water transport.

Each of these themes and points encapsulates crucial aspects of cellular biology, illustrating the complexity of life at the cellular level. Understanding these elements provides the foundation for further studies in biological sciences.