Module B2 Topic 2: Osmosis and Plant Transport

Comprehensive vocabulary flashcards covering the definitions, cell states, calculations, environmental factors, and experimental setups for Year 12 Biology Module B2: Osmosis and Plant Transport.

Selectively Permeable Membrane

A membrane that allows certain small particles and water molecules to pass through while preventing larger molecules from crossing.

Cell Wall (in Plants)

A completely permeable outer layer made of cellulose that does not regulate substance entry or exit, but provides structural strength to prevent the plant cell from bursting when water enters.

Diffusion

The passive movement of particles within a gas or liquid from an area of high concentration to an area of lower concentration (down a concentration gradient) until equilibrium is reached, powered by the particles' own kinetic energy.

Diffusion Model Across a Membrane

The net movement of solute particles across a selectively permeable membrane from an area of higher solute concentration to an area of lower solute concentration over time until evenly spaced.

Concentration Gradient

The difference in the concentration of a substance between two adjacent areas, determining the direction and rate of passive net movement.

Equilibrium

A state of balance reached when the concentration of particles or water molecules is equal throughout a space or on both sides of a membrane, resulting in no further net movement.

Osmosis

The movement of water from an area of greater concentration of water to an area of lower concentration of water through a selectively permeable membrane.

Osmosis Solution Model

The process whereby water moves across a partially permeable membrane from a dilute sugar solution to a concentrated sugar solution, increasing the volume on the concentrated side because solute molecules are too large to cross.

Dilute Solution

A weak solution that contains a high concentration of water molecules and a low concentration of dissolved solute particles.

Concentrated Solution

A strong solution that contains fewer water molecules and a high concentration of dissolved solute particles.

Visking Tubing Model Cell

An artificial dialysis membrane tied at the ends to replicate a cell; used in osmosis experiments to demonstrate the uptake of water when surrounded by a more dilute solution.

Lysis

The bursting of an animal cell caused by excessive intake of water by osmosis, which increases internal volume and stretches the flexible cell membrane until it splits.

Turgid

The firm state of a plant cell when it has taken in enough water by osmosis to expand the vacuole and push the cell membrane firmly against the cellulose cell wall.

Turgor Pressure

The outward hydrostatic force exerted by the swollen cell contents and cell membrane against the cell wall, providing structural support to keep non-woody plants upright.

Flaccid

The limp condition of a plant cell that has lost water by osmosis, resulting in low turgor pressure where the cell membrane is barely touching the cell wall, leading to wilting.

Plasmolysis

The process occurring when a plant cell loses too much water by osmosis in a concentrated external solution, causing the vacuole and cytoplasm to shrink and pull the cell membrane away from the cell wall.

Plant Cells in Solutions of Differing Concentrations

The morphological states of plant cells across external conditions: becoming turgid in dilute solution, maintaining volume in isotonic solution, becoming flaccid in concentrated solution, and undergoing plasmolysis in very concentrated solution.

Plasmolysed Onion Tissue

Strips of plant epidermis placed in a strong sugar solution where water has left the cells by osmosis, visibly pulling the protoplast away from the rigid cell wall boundaries.

Isotonic Point

The sugar or solute concentration where the external solution has the same water concentration as the inside of the plant tissue, identified on a percentage mass change graph as the point where the line crosses the $x$-axis (zero mass change).

Percentage Change in Mass

A standardized measure used to evaluate osmosis across tissue samples with differing initial masses, calculated as: $\frac{\text{Change in mass (g)}}{\text{Initial mass (g)}} \times 100$.

Uses of Water in Plants

The four critical functions of water in plant physiology: support (turgor), transport (carrying dissolved mineral ions), transpiration, and photosynthesis (acting as an essential raw material).

Transpiration

The evaporation of water from mesophyll cells followed by diffusion of water vapour through intercellular air spaces and out through the stomata.

Transpiration Pathway in Leaves

The internal route of water movement where water exits leaf xylem vessels, moves across mesophyll cells, evaporates into intercellular air spaces, and diffuses through stomatal pores.

Stomata

Microscopic pores located predominantly on the lower epidermis of leaves that regulate gas exchange ($CO_2$ and $O_2$) and permit the exit of water vapour during transpiration.

Spongy Mesophyll

A loosely packed leaf tissue layer containing large intercellular air spaces that facilitate gas exchange and provide surfaces from which water evaporates.

Waxy Cuticle

A waterproof, non-cellular lipid layer covering the leaf epidermis that restricts uncontrolled water loss via evaporation through the leaf surface.

Bubble Potometer

A piece of laboratory apparatus that measures the rate of water uptake of a cut shoot by tracking the movement of an air bubble along a graduated capillary tube over time: $\text{Rate} = \frac{\text{Distance moved by air bubble}}{\text{Time}}$.

Weight Potometer

An apparatus used to directly determine the rate of transpiration by measuring the decrease in mass of a potted plant or cutting on a balance over time, with the soil or water surface sealed to prevent non-transpirational evaporation.

Washing Line Method

A comparative technique where detached leaves are weighed and suspended along a line under different environmental conditions or surface coatings (e.g., petroleum jelly) to determine rates of water loss and wilting.

Effect of Wind Speed on Transpiration

An increase in wind speed increases the transpiration rate because moving air blows away water vapour surrounding the leaf, maintaining a steep water concentration gradient.

Effect of Temperature on Transpiration

An increase in temperature increases the transpiration rate because the added thermal energy gives water molecules more kinetic energy, accelerating evaporation and diffusion.

Effect of Humidity on Transpiration

An increase in humidity decreases the transpiration rate because high external atmospheric water vapour reduces the concentration gradient between the inside of the leaf and the outside air.

Effect of Leaf Surface Area on Transpiration

An increase in leaf surface area increases the transpiration rate because larger leaves have more stomata through which water can evaporate and diffuse.

Effect of Light Intensity on Transpiration

An increase in light intensity increases the rate of transpiration and water uptake because light stimulates stomata to open wider and drives photosynthesis.