Cell Structure and Organization

Cell Structure and Organisation

• All living things are composed of one or more cells.
• Cells are the basic units of life.
• Cells vary in shape, size and function.
• Examples include nerve cells and kidney cells, differing in function, shape and size.

Cell Shapes and Sizes

• Cell shapes can be rounded, oval, flat, cubical, columnar, spindle, or fusiform.
• Lymphocytes are the smallest cells (6µm).
• Fat and ovum (ova) are the largest cells (160µm).

Cell Types

• Cells are of two types: prokaryotic and eukaryotic.
• Prokaryotic cells lack a nucleus and other membrane-bound organelles.
  • They are surrounded by cell walls.
  • Examples are bacteria and blue-green algae.
• Eukaryotic cells contain a nucleus and membrane-bound organelles.
  • They are surrounded by a cell membrane (plasma membrane).
  • Examples are plant and animal cells.

Eukaryotic Cell Components

• Eukaryotic cells possess:
  • Cell (plasma) membrane: Forms a barrier enclosing the cell and regulating transport.
  • Protoplasm: A fluid-like material.
    • Cytoplasm: Surrounds the nucleus.
    • Nucleoplasm: Inside the nucleus.
    • Organelles: Living structures essential for cell functions.
    • Inclusions: Non-living substances like metabolites or cell products.
    • Substances such as water, proteins, carbohydrates, lipids, nucleic acids, and minerals.
  • Nucleus: Contains the genetic material (DNA).

Cell Membrane Structure

• Cell membrane (or plasma membrane): A thin membrane (7.5-10 nanometers thick) separating the interior of the cell from its surroundings.
• The membrane is composed of:
  • Bilayer of phospholipids
  • Proteins
  • Cholesterol
  • Short chain of sugars (glycoproteins and glycolipids)
• The structure is a mosaic model, meaning the components can move laterally.
• Phospholipids have a hydrophilic head (attracted to water) and hydrophobic tails (repelled by water). The heads face outward; tails inwards.

Cell Membrane Lipids

• Phospholipids form a bilayer in the cell membrane.
• Cholesterol molecules are embedded within the phospholipid bilayer, regulating membrane fluidity.

Cell Membrane Proteins

• Proteins are globular masses floating in the phospholipid bilayer.
  • Integral proteins: Span the entire membrane, include ion channels (transmembrane). Some proteins are only embedded in one side of the membrane e.g., enzymes(monotopic).
  • Peripheral proteins: Loosely attached to the surface of the membrane.

Membrane Protein Functions

• Structural support.
• Transport molecules (channels or carriers).
• Cell-cell recognition (e.g., antibodies).
• Hormone and antigen receptors.
• Enzymatic control of chemical reactions.

Cell Membrane Carbohydrates

• Carbohydrates are attached to proteins (glycoproteins) and lipids (glycolipids) and form a glycocalyx on the exterior of the membrane.
• These carbohydrates act as cell-adhesion molecules and in cell-cell recognition.

Transport Across Cell Membranes

• Passive transport: Movement of substances across the membrane without energy input, down the concentration gradient. Types include:
  • Simple diffusion: Movement of small molecules (e.g., oxygen, carbon dioxide) across the membrane, until evenly distributed.
  • Osmosis: Movement of water across a semipermeable membrane from a region of high water concentration to low.
  • Facilitated diffusion: Movement of large molecules (e.g., glucose) through protein channels or carriers.
• Active transport: Movement of substances across the membrane against the concentration gradient, requiring energy (ATP).
• Bulk transport: Movement of large particles into or out of the cell via endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) or exocytosis.

Solutions Regarding Osmosis

• Isotonic solutions: Similar concentration of solutes to the cell's cytoplasm; no net water movement.
• Hypertonic solutions: Higher concentration of solutes than the cell's cytoplasm; water moves out of the cell, causing it to shrink.
• Hypotonic solutions: Lower concentration of solutes than the cell's cytoplasm; water moves into the cell, causing it to swell or burst.

Active Transport

• Active transport moves molecules against the concentration gradient, requiring energy (ATP). This involves protein channels and carriers.

Bulk Transport

• Bulk transport involves endocytosis (taking substances into the cell) and exocytosis(releasing substances from the cell).
  • Phagocytosis: Cellular "eating," engulfing large particles.
  • Pinocytosis: Cellular "drinking," engulfing fluids and dissolved substances.
  • Receptor-mediated endocytosis: Specific molecules bind to receptors on the cell membrane, triggering uptake.

This quiz explores the fundamental concepts of cell structure and organization. It covers the types of cells, their shapes and sizes, and the components of eukaryotic cells. Perfect for students learning about biology and the basics of life.