B.b 1, 2, 3

What is the definition of Diffusion?

The passive net movement of particles from a region of their higher concentration to a region of their lower concentration (down a concentration gradient), as a result of their random movement.

What is a Concentration gradient?

The difference in concentration on either side of a cell membrane

State the sites and directions of diffusion in animals (\text{O}_2, \text{CO}_2, digested food)

\text{O}_2: Alveoli \rightarrow blood capillaries (lungs); \text{CO}_2: Blood capillaries \rightarrow alveoli (lungs); Digested food: Small intestine (villi) \rightarrow blood capillaries.

State the sites and directions of diffusion in plants (\text{CO}_2, \text{O}_2, water vapour).

\text{CO}_2: Air spaces \rightarrow leaf cells (via stomata); \text{O}_2 & water vapour: Leaf cells \rightarrow atmosphere (via stomata).

What 4 factors affect the rate of diffusion?

1. Higher temperature (more kinetic energy)

2. Steeper concentration gradient

3. Larger surface area

4. Shorter diffusion distance.

What is the definition of Osmosis?

The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially-permeable membrane.

What is the function of a Partially-permeable membrane?

It has tiny holes that permit small water molecules to pass through, while being too small to allow larger solute molecules through. /

What is the water potential of pure water vs. a solution?

Pure water has the highest water potential (0\text{ kPa}). Adding solute lowers the water potential, so solutions have a lower (negative) water potential than pure water.

Define Tonicity and distinguish Hypertonic, Isotonic, and Hypotonic solutions. / 定义渗透压（Tonicity）并区分高渗、等渗和低渗溶液。 

Tonicity is the relative solute concentration across a semi-permeable membrane.

Hypertonic: more solute outside

Isotonic: equal solute concentration

Hypotonic: less solute (more water) outside. / 渗透压指半透膜两侧的相对溶质浓度。高渗：外部溶质更多；等渗：内外溶质浓度相等；低渗：外部溶质更少（水更多）。

What happens to animal cells in hypotonic, isotonic, and hypertonic solutions?

Hypotonic: water enters, cell swells and bursts (lysis) due to no cell wall. Isotonic: no net water movement (normal). Hypertonic: water leaves, cell shrinks (crenation)

Define Turgid, Flaccid, and Plasmolyzed in plant cells

Turgid (in hypotonic): water enters, cytoplasm presses against the strong cellulose cell wall (turgor pressure), preventing bursting.

Flaccid (in isotonic): cell loses firmness and wilts.

Plasmolyzed (in hypertonic): water leaves vacuole, cytoplasm shrinks and pulls away from the cell wall. / 硬挺（低渗中）：水进入，细胞质紧压纤维素细胞壁（产生膨压），防止破裂。松弛（等渗中）：细胞失去硬度并萎蔫。质壁分离（高渗中）：液泡失水，细胞质收缩并与细胞壁分离

What is the definition of Active transport?