passive transport and osmosis

Cell Size Upper Limits

Set by the metabolic requirements of the cell.

Cell Growth Discrepancy

As a cell increases in size, its volume grows proportionately more than its surface area.

Sphere Surface Area Formula

SA = 4πr²

Sphere Volume Formula

V = (4/3)πr³

Purpose of High SA:V Ratio

Allows cells to efficiently remove waste products and exchange gases and heat.

Cell Size and Efficiency

Smaller cells are most efficient because they have the highest ratio.

Cell Size vs. SA:V Ratio

The larger the cell, the lower the SA:V ratio.

Concept 7.3 Core Idea

Passive transport is diffusion of a substance across a membrane with no energy investment.

Diffusion Definition

The tendency for molecules to spread out evenly into the available space.

Molecular vs. Population Movement

Individual molecules move randomly, but a population's diffusion may be directional.

Dynamic Equilibrium

As many molecules cross the membrane in one direction as in the other.

Concentration Gradient

The region along which the density of a chemical substance increases or decreases.

Movement Down a Gradient

Requires no work to be done to move substances.

Passive Transport Definition

The diffusion of a substance across a biological membrane without energy expenditure by the cell.

Osmosis Definition

The diffusion of water across a selectively permeable membrane.

Direction of Water in Osmosis

Water diffuses from lower solute concentration to higher solute concentration until equal on both sides.

Tonicity Definition

The ability of a surrounding solution to cause a cell to gain or lose water.

Isotonic Solution (No Cell Wall)

Solute concentration is the same as inside the cell; no net water movement occurs.

Hypertonic Solution (No Cell Wall)

Solute concentration is greater than inside the cell; the cell loses water.

Hypotonic Solution (No Cell Wall)

Solute concentration is less than inside the cell; the cell gains water.

Osmoregulation Definition

The control of solute concentrations and water balance, necessary for life in extreme environments.

Paramecium Osmoregulation

A protist hypertonic to its pond environment that uses a contractile vacuole as a water pump.

Human Osmoregulation Organ

Primarily performed by the kidneys.

ADH Function (High Fluid Balance)

Regulates the number of aquaporins in the cell membranes of kidney cells.

ADH Function (Low Fluid Balance)

Upregulates aquaporins to allow cells to reabsorb more water from urine.

Cell Wall Utility

Helps plant cells maintain water balance.

Plant Cell in Hypotonic Solution

Swells until the wall opposes uptake, making the cell turgid (firm).

Plant Cell in Isotonic Solution

No net movement of water into the cell occurs, making the cell flaccid (limp).

Plant Cell in Hypertonic Solution

Loses water to its surroundings, causing the membrane to pull away from the cell wall.

Plasmolysis Definition

A usually lethal effect where the plant cell membrane pulls away from the wall, causing wilting.