HO - 03.2 Membranes

Cells and Cell Membranes

  • Living organisms are composed of one or more cells.

  • Categories of cells:

    • Unicellular: Single-celled organisms (e.g., single-celled protists).

    • Multicellular: Organisms composed of multiple cells.

Overview of Cell Membranes

  • Plasma Membrane: Defining boundary of the cell.

    • Structure and types: animal cell plasma membrane and human RBC plasma membrane.

Number of Cells

  • Unicellular Organisms: Example of a single-celled protist.

  • Colonial Forms: Few cells forming a colony (e.g., Anabaena sp., a filamentous photosynthetic bacteria).

  • Multicellular Organisms: Composed of many cells.

    • Example: Human body contains approximately 70 trillion cells.

Cell Size and Multicellularity

  • Large organisms are made of many small cells instead of a few large cells.

  • Limitations of cell size: as cells grow, surface area to volume ratio decreases, impacting nutrient transport and waste removal.

Surface Area to Volume Ratio

  • Surface-area-to-volume problem as a reason for multicellularity.

  • Calculating Surface Area and Volume:

    • For a cubic structure:

      • Surface area increases with dimensions.

      • Volume increases cubically.

  • Effects on Transport: Importance of adequate surface area for nutrient and waste transport.

  • Examples of cell specialization:

    • Plants have multiple cell types (about 30 types).

    • Humans contain close to 200 different types of cells.

Properties of the Plasma Membrane

  • Basic requirements for a cell membrane:

    1. Must contain the cell's contents.

    2. Must be flexible.

    3. Must allow transport in and out of the cell.

  • Plasma Membrane Structure:

    • Composed of phospholipid bilayers.

    • Membrane thickness: approximately 100 nm.

    • Spontaneity of vesicle formation in water.

Phospholipid Bilayer

  • Characteristics:

    • Flexible and not held together by covalent bonds.

    • Fluid nature allowing lateral movement and rotation of components.

    • Rare flip-flop motion of lipids.

Membrane Fluidity

  • Factors affecting membrane fluidity:

    • Presence of unsaturated fatty acids enhances fluidity.

    • Saturated fatty acids make membranes less fluid.

    • Cholesterol in animals regulates fluidity: stiffens at higher temperatures, prevents solidifying at lower temperatures.

Membrane Proteins

  • Types of membrane proteins:

    • Integral Proteins: Span the phospholipid bilayer.

    • Peripheral Proteins: Attached to integral proteins.

  • Functions:

    1. Transporters: Assist molecules through the membrane (active or passive transport).

    2. Anchors: Facilitate attachment of other molecules to the membrane.

    3. Additional functions include enzymatic activity and signaling.

Carbohydrates in Membranes

  • Glycolipids and glycoproteins play protective roles.

  • Surface molecules can have antigenic properties.

Antigen-Antibody Interactions

  • Antibodies target specific antigens on cell surfaces.

  • Example with RBCs: contain protein hemoglobin (transports O2 and CO2).

  • RBC surface sugars identify cells and can include antigens (A, B, H).

Blood Types and Antigens

  • Blood group classification based on membrane sugars.

    • Alleles:

      • O: recessive, encodes non-functional enzyme.

      • A/B: co-dominant, encode specific transferase enzymes.

  • Glycolipids serve as antigens for blood groups (e.g., H, A, B).

Prokaryotic Cell Membranes

  • Structure of prokaryotic plasma membranes:

    • Comprised of a phospholipid bilayer with transport proteins.

    • Peptidoglycan layer and outer membrane analysis in future lectures.

    • More membrane correlates with increased import, export, respiration, photosynthesis, etc.

Internal Membranes and Infoldings

  • Infolds of plasma membranes (e.g., thylakoids in cyanobacteria) may be:

    • Continuous with the plasma membrane (e.g., aerobic bacteria respiratory membranes).

  • Importance of surface area in eukaryotic internal membranes (rough/smooth ER, Golgi apparatus).

Increased Surface Area for Absorption

  • Outfolds of the plasma membrane enhance absorption capacity (e.g., intestinal epithelial cells).

Review Questions (Examinable Content)

  • Define unicellular and multicellular organisms.

  • Estimate the number of cells and cell types in the human body.

  • Discuss the size limitations of cells.

  • Identify major molecules constituting the plasma membrane and their functions.

  • Explain the basis of blood types and the differences between animal and plant cell membranes.

  • Describe the mechanisms enabling cells to have extra membranes.