B2 Notes

B2.1 Part 1: Membranes

Theme: Form and Function

Level of Organisation: Cells

Driving Question: How do molecules of lipid and protein assemble into biological membranes?

City Planning

• Scenario Explanation: Imagine constructing a small city in a rainforest and the importance of controlling what enters and exits this city.

  • Discussion Points:

  • What criteria or rules would be established to manage access to the city?

  • Importance of controlling entry/exit in a rainforest environment:

    • Implications of unregulated access could lead to ecological imbalance, resource depletion, and potential harm to the city and its inhabitants.

• Task for Students: Collaboratively design a protective model for the city to be presented to the class.

B2.1.1-2.1.2: Lipid Bilayers as Barriers

Basis of Cell Membranes

Phospholipids

• Structure:

  • Composed of two fatty acid chains and a phosphate group bonded to a glycerol molecule.

  • The fatty acid tails are nonpolar and hydrophobic, while the phosphate head is charged and hydrophilic.

• Amphipathic Nature: Phospholipids exhibit both hydrophobic and hydrophilic characteristics.

Phospholipid Bilayers

• Formation: Phospholipids spontaneously arrange into bilayers when introduced to water.

  • Orientation: Hydrophilic phosphate heads face the aqueous environment, whereas hydrophobic fatty acid tails are positioned internally.

• Function: All cellular membranes consist of a phospholipid bilayer, acting as a barrier against environmental substances.

Drawing Phospholipids

• Guidelines for Drawing:

  • Start with one labeled phospholipid:

  • Circle for the phosphate head.

  • Two lines for the fatty acid tails.

Drawing Phospholipid Bilayers

• Diagram Directions:

  • Illustrate two rows of phospholipids, ensuring the fatty acid tails are within the bilayer.

Lipid Bilayers

• Barrier Function: The phospholipid bilayer separates cellular contents from the environment.

  • Permeability: Only hydrophobic (uncharged) molecules can diffuse through the bilayer independently; large and hydrophilic/charged particles cannot.

B2.1.4: Integral and Peripheral Proteins in Membranes

Integral Proteins

• Description: Integral proteins are permanently anchored within the plasma membrane, penetrating its structure.

  • Composition: Contains one hydrophobic domain within the fatty acid region and two hydrophilic domains on each surface.

  • Types: Can be transmembrane proteins (spanning the entire membrane) or partially inserted.

  • Functions: Can act as glycoproteins, channels, or transport pumps for materials across membranes.

Peripheral Proteins

• Description: Peripheral proteins are loosely attached to one side of the membrane and usually associate with integral proteins.

  • Characteristics: Hydrophilic and do not cross the phospholipid bilayer.

  • Functions: Often act as receptors or enzymes that catalyze reactions.

Drawing Proteins

• Drawing Instructions:

  • Peripheral proteins are to be depicted on the surface of phospholipids.

  • Integral proteins should be illustrated penetrating through the bilayer.

B2.1.9: Structure and Function of Glycoproteins and Glycolipids

Glycoproteins and Glycolipids

• Composition: Carbohydrate chains can attach to phospholipids or membrane proteins via glycosylation.

  • Glycoproteins: Membrane proteins with attached carbohydrates.

  • Glycolipids: Phospholipids with carbohydrate chains.

  • Location: Carbohydrates project outward from the cell's surface.

Functions of Glycoproteins and Glycolipids

• Roles:

  • Receptors: Glycoproteins function as receptors for hormones, triggering metabolic changes upon binding.

  • Cell-to-Cell Communication: Glycoproteins assist neurotransmitter binding, facilitating cell interactions.

  • Immune Response: Serve as markers for immune recognition, distinguishing self-cells from non-self.

  • Cell Adhesion: Interact with other glycoproteins on adjacent cells to form tissues.

Cell Recognition

• Functionality: Glycoproteins and glycolipids possess specific shapes necessary for immune recognition.

  • Antigens: If carbohydrate chains are unrecognized, they stimulate an immune response by prompting antibody production.

Blood Types

• Description: Blood types are defined by the presence of distinct glycoproteins on red blood cells (RBCs).

  • Groups: Group A, B, AB, O each with specific antigens and corresponding antibodies in plasma:

  • Group A: A antigen, anti-B antibodies.

  • Group B: B antigen, anti-A antibodies.

  • Group AB: A and B antigens, no antibodies.

  • Group O: No antigens, anti-A and anti-B antibodies.

Drawing Glycoproteins and Glycolipids

• Specifications for Diagramming:

  • For glycoproteins: Illustrate at least one protein with labeled carbohydrate chains.

  • For glycolipids: Show a carbohydrate chain attached to a phospholipid.

B2.1.10: Fluid Mosaic Model of Membrane Structure

Fluid Mosaic Membrane

• Definition: The fluid mosaic model describes membrane architecture as dynamic and flexible.

  • Composition: Contains a mixture of phospholipids, proteins, and cholesterol, which maintains membrane fluidity.

Labelled Diagram of Fluid Mosaic Model

• Key Components to Include:

  • Glycolipid

  • Phospholipid

  • Hydrophobic fatty acid tails

  • Hydrophilic phosphate head

  • Carbohydrate chain

  • Channel Protein

  • Integral Proteins

  • Glycoproteins

  • Peripheral Protein

  • Cholesterol

  • Phospholipid bilayer

Drawing a Membrane

• Instructions: Create a scale drawing of a cell membrane including:

  • Labels and definitions for:

  • Phospholipids

  • Hydrophobic tails

  • Hydrophilic heads

  • Glycolipids

  • Glycoproteins

  • Peripheral and integral proteins

  • Cholesterol

  • Ensure the inclusion of the bilayer and labeling of two cellular environments (inside and outside).

B2.1 Part 2: Membrane Transport

Theme: Form and Function

Level of Organisation: Cells

Driving Question: What determines whether a substance can pass through a biological membrane?

Learning Objective: I can outline the types of transport that happen across a membrane.

Bubble Frame Membrane Activity

• Activity Instructions:

  • Construct a bubble frame using four straws bent at elbows.

  • Form a bubble “cell membrane” using bubble solution. Tasks:

  • Pass a pencil through the membrane without breaking it.

  • Pass a pencil through without getting it wet (using fishing wire).

B2.1.3, B2.1.5-2.1.6: Diffusion and Osmosis Across Membranes

Kinetic Theory

• Definition: Kinetic theory posits that particles are in constant motion within gases, liquids, and solutes in aqueous solutions.

  • Relevance: This random movement underpins processes like diffusion and osmosis.

Simple Diffusion

• Definition: Passive transport mechanism where particles move from areas of high concentration to areas of low concentration without cellular energy use.

  • Concentration Gradient: Describes movement along a gradient, from high to low concentration.

• Examples of Diffusion: Small uncharged molecules (e.g., O2 and CO2) and fat-soluble molecules diffuse across membranes directly.

  • Case Studies:

  • Oxygen moves from alveoli into blood.

  • Carbon dioxide flows from blood into alveoli.

Osmosis

• Definition: Passive transport of water from a region of low solute concentration to a region of high solute concentration through a semipermeable membrane.

  • Polar Nature: Water is polar but small enough to traverse the phospholipid bilayer.

  • Impediment of Solutes: Charged or polar solutes are unable to directly cross the bilayer.

Solutions

• Definition: A homogeneous mix of two or more substances, consisting of a solvent (majority) and solute(s) (minor).

Significance of IV Saline Solutions

• Reason for Use: Saline solutions help maintain osmotic pressure around cells, preventing excessive loss or gain of water.

Osmosis and Aquaporins

• Definition: While direct osmosis is slow, aquaporins are integral proteins that facilitate rapid water transport across membranes.

  • Role: Aquaporins enhance membrane permeability to water, exemplifying facilitated diffusion.

Facilitated Diffusion

• Definition: Passive transport process facilitating molecules moving from high to low concentration through channel proteins.

• Importance: Channel proteins are selective, allowing only specific molecules to permeate, thus maintaining selective permeability.

Channel Proteins

• Structure: Composed of integral proteins featuring a central pore that permits specific particles.

  • Composition of Pores: Hydrophilic R groups facilitate the passage of designated molecules through the channel.

  • Gated Mechanisms: Some channels open in response to changes in membrane potential (e.g., sodium and potassium channels).

Drawing Channel Proteins

• Diagram Guidelines:

  • Illustrate channel proteins as transmembrane structures with a defined pore.

B2.1.7-2.1.8: Active Transport

Active Transport

• Definition: Process wherein particles move from low to high concentration requiring energy (ATP) and specific protein pumps.

Mechanisms of Active Transport

1. Binding Phase: A specific molecule binds to a designated site on the protein pump.

2. Energy Requirement: ATP attaches to the pump, hydrolyzing to form ADP.

3. Shape Change: The phosphate remains bound, altering the pump's configuration.

4. Transport Phase: The particle is transported against its concentration gradient and discharged.

5. Restoration Phase: The phosphate detaches, allowing the pump to revert to its initial state.

Passive vs Active Transport

Passive Transport

• Types: Simple and facilitated diffusion.

  • Occurs naturally without energy; relies on concentration gradients.

  • Involves small, uncharged particles or specific channels for larger, charged particles.

Active Transport

• Characteristics: Requires ATP to move substances contrary to their concentration gradients.

  • Specificity is dictated by the protein structure of pumps/channels for designated solutes.

Membrane Selectivity

• Process Selection:

  • Facilitated diffusion uses specific protein channels for targeted particles.

  • Active transport utilizes specific protein pumps.

  • Simple diffusion allows unimpeded passage for small or hydrophobic molecules.

B2.3 Cell Specialization

Theme: Form and Function

Level of Organisation: Cells

IB Guiding Questions:

• What are the roles of stem cells in multicellular organisms?

• How are differentiated cells adapted to their specialized functions?

Stem Cell Debate

• Instructions: Discuss interesting facts as assigned, evaluate personal positions on stem cell issues, and explore statements and their impacts on perspectives.

B2.3.1-2.3.4: From Gametes to Specialized Cells

Fertilization

• Definition: The fusion of sperm and ovum resulting in the formation of a zygote.

  • The zygote is classified as a totipotent stem cell, capable of developing into any cell type.

Embryonic Stem Cells

• Development Process: The zygote develops into a blastocyst via cell division over 5 days.

  • The blastocyst contains pluripotent embryonic stem cells which can differentiate into nearly every cell type, but not into a complete organism.

Cell Differentiation

• General Principle: All multicellular organism cells possess identical genomes.

  • Differentiation occurs through selective gene expression, leading to specialized cells.

Adult Stem Cells

• Function: Some adult stem cells persist post-embryonic development for tissue replenishment and repair.

  • Most adult stem cells are multipotent, capable of forming closely related cell types (e.g., hematopoietic stem cells can become various blood cells).

Embryonic Development

• Definition: Morphogens are gene-regulating chemicals influencing cellular specialization.

  • Morphogens diffuse through tissues establishing concentration gradients that dictate gene expression patterns.

Mechanism of Morphogen Action

• Process Understanding: Morphogens bind to cell receptors activating or suppressing gene expression.

  • The concentration gradient influences the types of cells that will differentiate within the embryo.

Stem Cells

• Definition: Undifferentiated cells that retain the capacity for unlimited division and differentiation into specialized types.

Stem Cell Niches

• Definition: Microenvironments facilitating the maintenance and differentiation of adult stem cells.

  • Examples of Niches:

  • Bone Marrow: Serves as a niche for hematopoietic stem cells.

  • Hair Follicles: Home to various stem cell types with roles in skin homeostasis, hair growth, and renewal.

B2.3.5: Cell Size and Specialization

Variability in Cell Size

• Description: Human cells exhibit a wide range of sizes influenced by their specialized functions.

  • Noteworthy Sizes of Specialized Cells:

  • Sperm: Smallest human cells, 50-70 µm long, 2-3 µm wide.

  • Ova: Largest in volume, approximately 100 µm diameter.

  • Neurons: Width ranges from 4-100 µm; length can exceed 1 meter.

Notable Cell Sizes

• Examples:

  • Red Blood Cells: Diameter 6-8 µm, facilitating movement through capillaries.

  • White Blood Cells: Sizes range from 6-20 µm based on type.

  • Striated Muscle Fibers: Atypical multinucleate cells, lengths from millimeters to centimeters, diameters of 10-100 µm.

Scientific Models

Nature of Science

• Definition: Scientific models simplify complex systems to facilitate understanding in a pedagogical context.

  • Example Activity: Using cubes as model cells to explore surface area to volume relationships.

Surface Area to Volume Ratios

• Dimensions: As a cell increases in size, its volume grows faster than surface area, causing a declining surface area to volume ratio.

• Example Computation:

  • Edge length of 1, 2, and 3:

    • 1: Surface Area = 6, Volume = 1; Ratio = 6:1

    • 2: Surface Area = 24, Volume = 8; Ratio = 3:1

    • 3: Surface Area = 54, Volume = 27; Ratio = 2:1

Implications of Surface Area to Volume Ratio

• Functionality: Cell membrane (surface area) regulates material exchange; cytoplasm (volume) is where metabolism occurs.

  • Growth Response: Cells tend to divide if the surface area cannot efficiently support metabolic exchange as they grow.

Assignment Reminder

• Task: Complete the "Modeling Specialized Cells" assignment on Canvas, collaboration in groups is encouraged.