Topic 2 Movement of Substances (CHS Year 3 Chronological Mastery)

Define diffusion in terms of particle movement and concentration gradient.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. (⚠️ Trap: Passive process requiring NO cellular ATP energy; relies on kinetic energy of particles).

Describe the role of diffusion in human gaseous exchange at the alveolar surface.

Oxygen diffuses down its concentration gradient from the alveolar air space into the blood capillaries, while carbon dioxide diffuses down its concentration gradient from the blood capillaries into the alveoli for exhalation.

Describe the role of diffusion in plant gaseous exchange during photosynthesis in leaf mesophyll cells.

Carbon dioxide diffuses down its concentration gradient from the atmosphere through open stomata into the intercellular air spaces, then dissolves in the thin film of moisture and diffuses into mesophyll cells for photosynthesis.

State how surface area-to-volume ratio affects the rate of diffusion across a cellular membrane.

A larger surface area-to-volume ratio provides a greater area relative to volume, which increases the rate of diffusion of substances into or out of the cell.

Explain the effect of temperature on the rate of passive diffusion across a plasma membrane.

Increasing temperature increases the kinetic energy of particles, causing them to move faster and collide more frequently, which increases the rate of diffusion.

Explain how diffusion distance influences the rate of passive transport across biological tissues.

A shorter diffusion distance decreases the distance particles must travel, which increases the rate of diffusion (e.g., alveolar and capillary walls are each one cell thick to minimize diffusion distance).

Describe how large polar molecules or charged ions cross the hydrophobic core of the plasma membrane down a concentration gradient.

They cross via facilitated diffusion, where specific transport proteins (channel or carrier proteins) provide a passage across the membrane down a concentration gradient without requiring ATP energy.

Compare simple diffusion and facilitated diffusion in terms of energy requirements, direction of movement, and membrane structures involved.

Both are passive processes moving particles down a concentration gradient without ATP energy. However, simple diffusion occurs directly across the phospholipid bilayer, whereas facilitated diffusion requires specific transport proteins (channel or carrier proteins).

State the mechanism of channel proteins in facilitating passive movement across the cell membrane.

Channel proteins provide a hydrophilic pore/passage through the membrane that allows specific small polar molecules or charged ions (e.g., Na⁺, Ca²⁺) to flow rapidly across by shielding them from the hydrophobic phospholipid tails.

Describe how carrier proteins move specific solute molecules across the plasma membrane during facilitated diffusion.

The specific solute binds to a binding site on the carrier protein, inducing a conformational change in the protein that transfers the solute to the opposite side of the membrane where it is released.

Explain why the rate of facilitated diffusion plateaus at high solute concentrations, unlike simple diffusion.

At high solute concentrations, all available carrier or channel proteins become saturated (all binding sites are occupied), making the transport proteins the limiting factor, so the rate reaches a maximum plateau.

Define osmosis using exact CHS terms.

Osmosis is the net movement of water molecules from a solution of higher water potential to a solution of lower water potential, down a water potential gradient, across a partially permeable membrane. (⚠️ Trap: R: 'high to low water concentration' — MUST use 'water potential').

Define water potential and state the reference value assigned to pure water at atmospheric pressure.

Water potential is a measure of the tendency of water molecules to move from one region to another via osmosis. Pure water has the highest water potential, which is set at zero (0 kPa); adding solutes lowers the water potential into negative values.

Describe how water moves from the soil into a root hair cell in terms of water potential.

The cell sap inside the root hair cell central vacuole has a lower water potential than the soil water. Water enters the root hair cell by osmosis across the partially permeable cell surface membrane down a water potential gradient.

Describe the appearance and state of a plant cell when placed in a solution with a higher water potential than its cell sap.

Water enters the cell by osmosis across the partially permeable membrane. The central vacuole expands, pushing the cytoplasm against the rigid cellulose cell wall, which exerts turgor pressure that prevents further water entry, making the cell turgid. (⚠️ Trap: R: 'cell wall prevents water from entering' — the cell wall resists expansion and exerts turgor pressure).

Describe what happens to an animal cell (such as a red blood cell) when placed in a solution with a higher water potential (hypotonic solution).

Water enters the red blood cell by osmosis across the partially permeable cell surface membrane. Because animal cells lack a cell wall, the cell expands and bursts, undergoing lysis (or hemolysis).

Describe the appearance and state of a plant cell when placed in a solution with a lower water potential than its cell sap (hypertonic solution).

Water leaves the central vacuole by osmosis across the partially permeable cell surface membrane. The vacuole and cytoplasm shrink, causing the cell surface membrane to pull away from the cell wall, resulting in plasmolysis and making the cell flaccid.

Describe what happens to an animal cell (such as a red blood cell) when placed in a solution with a lower water potential (hypertonic solution).

Water leaves the cell by osmosis across the partially permeable cell surface membrane. The cell shrinks in volume and forms small spikes on its surface, causing the cell to become crenated (undergoing crenation).

State the significance of turgor pressure in non-woody (herbaceous) plant tissues.

Turgor pressure provides mechanical support to non-woody plant tissues, keeping stems upright and leaf blades flat to maximize light absorption for photosynthesis.

Explain why plant cells do not burst when placed in distilled water, whereas animal cells burst.

Plant cells possess a rigid cellulose cell wall surrounding the plasma membrane that resists expansion and exerts turgor pressure to prevent rupture, whereas animal cells lack a cell wall and undergo lysis when water enters.

Define active transport using exact CHS keywords.

Active transport is an energy-consuming process (using ATP released during cellular respiration) that moves substances from a region of lower concentration to a region of higher concentration, against a concentration gradient, across a partially permeable membrane. (⚠️ Trap: R: 'produces/creates energy' — MUST state 'releases energy via cellular respiration in the form of ATP').

Explain how root hair cells take up dissolved mineral ions from soil water when soil ion concentration is lower than cell sap.

Specific carrier proteins in the root hair cell surface membrane use energy in the form of ATP from aerobic respiration to pump mineral ions against their concentration gradient into the cell sap.

Explain how intestinal epithelial cells absorb glucose from the ileum lumen even when lumen glucose concentration drops below blood concentration.

Glucose is absorbed by active transport using carrier proteins in the plasma membrane that consume ATP energy from cellular respiration to transport glucose against its concentration gradient into the epithelial cells.

Contrast facilitated diffusion and active transport in terms of concentration gradient, energy requirement, and carrier protein action.

Facilitated diffusion moves substances down a concentration gradient without ATP energy and solute can bind on either side. Active transport moves substances against a concentration gradient, requires hydrolysis of ATP energy from cellular respiration, and solute binds on only one specific side.

Define exocytosis and endocytosis as forms of bulk transport across the plasma membrane.

Exocytosis is a form of energy-requiring bulk transport where a secretory vesicle fuses with the plasma membrane to expel large molecules out of the cell. Endocytosis is where the plasma membrane invaginates to engulf large particles or fluids into a vesicle within the cell.

Describe the arrangement of phospholipids in the fluid mosaic model of membrane structure.

Phospholipids are arranged in a bilayer with their hydrophilic phosphate heads facing outwards towards the aqueous medium and their hydrophobic fatty acid tails facing inwards, forming a hydrophobic core barrier that prevents free passage of water-soluble/charged particles.

Explain why the plasma membrane model is described as "fluid" and "mosaic".

It is "fluid" because phospholipids and proteins can move laterally within the bilayer. It is "mosaic" because various protein molecules are randomly embedded in or attached to the phospholipid bilayer in an irregular pattern.

State the experimental evidence that supported the fluid mosaic model over earlier membrane models.

The freeze-fracture technique, where frozen membranes were split along the hydrophobic core and viewed under an electron microscope, revealed protein "bumps" embedded within both lipid layers.

Distinguish between intrinsic (integral) proteins and extrinsic (peripheral) proteins in terms of membrane arrangement.

Intrinsic proteins are tightly bound within the bilyaer. The hydrophobic amino acids will be in contact with the hydrophobic lipid bilayer, while the hydrophilic portions will be exposed to the aqueous medium on either side of the membrane.. Extrinsic proteins are loosely attached to the outer or inner surface of the membrane.

List all six major functions of membrane proteins listed in the CHS Year 3 syllabus.

1. Transport proteins(channels and carriers) | 2. Enzymatic reactions (membrane-bound enzymes) | 3. Signal transduction (receptors) | 4. Cell-to-cell recognition (glycoproteins/glycolipids as markers) | 5. Intercellular joining/adhesion | 6. Active transport pumps (e.g., Na⁺/K⁺ pump).

Describe the structure and function of channel proteins in the plasma membrane.

Channel proteins are intrinsic transport proteins with a hydrophilic pore that allows rapid passive movement of small polar molecules or charged ions (e.g., Na⁺, Ca²⁺) across the membrane by shielding them from the hydrophobic core.

Describe the mechanism of the Sodium-Potassium (Na⁺/K⁺) pump in animal cell membranes.

The Na⁺/K⁺ pump is an active transport carrier protein that uses ATP energy from respiration to undergo conformational changes, pumping 3 Na⁺ ions OUT of the cell and 2 K⁺ ions IN against their respective concentration gradients.

Explain the role of membrane-bound enzymes in metabolic reaction sequences.

Membrane-bound enzymes have their active sites exposed to substances on one side of the membrane and are often organized sequentially along the membrane to catalyze step-by-step metabolic pathways.

Describe the role of membrane receptor proteins in signal transduction.

Receptor proteins have specific binding sites for chemical messengers (e.g., hormones); binding causes a conformational change in the receptor that relays a signal into the cytoplasm to activate a cellular response.

State the function of glycoproteins and glycolipids in cell-to-cell recognition and cell adhesion.

They possess carbohydrate chains extending outwards that act as specific identification tags/surface antigens for recognition by other cells (e.g., immune cells) and form intercellular connections for cell adhesion.

State the structural adaptation of the cell surface membrane in small intestine epithelial cells that optimizes absorption, and explain its significance.

The plasma membrane is folded into microvilli to increase the surface area-to-volume ratio for rapid absorption of digested nutrients. (⚠️ Trap: Microvilli are subcellular folds of the plasma membrane, whereas the epithelium tissue layer is one cell thick to reduce diffusion distance).

State the structural adaptation of red blood cells regarding nuclear presence, and explain its exact biological significance.

The red blood cell lacks a nucleus (enucleate), which provides more internal volume/space to pack more haemoglobin (⚠️ Trap: R: 'packs more oxygen' — you pack haemoglobin, which increases oxygen-carrying capacity).

State the adaptation of the red blood cell's plasma membrane that allows it to pass through narrow capillaries.

The red blood cell has a flexible/elastic plasma membrane that allows the cell to fold into a bell shape to squeeze through narrow capillaries single-file, reducing diffusion distance.

Explain how the biconcave disc shape of a human red blood cell adapts it for gas exchange.

The biconcave disc shape increases the surface area-to-volume ratio of the cell, increasing the rate of oxygen uptake/diffusion and release.