movement of substances

two types of transport

passive transport - movement of substances across the cell membrane with no utilization of energy from cellular respiration (eg. diffusion & osmosis)

active transport - movement of substances from a region of lower water potential to higher water potential (against a concentration gradient), requiring energy from cellular respiration

definition of diffusion

diffusion is the net movement (overall movement) of particles (molecules, atoms, ions) from a region of higher concentration to a region of lower concentration, that is, down a concentration gradient until equilibrium is reached.

define concentration gradient

concentration gradient is the difference in concentration between two regions

factors that affect diffusion

concentration gradient — the steeper (bigger) the concentration gradient, the higher the rate of diffusion for that substance

diffusion distance — the shorter the diffusion distance, the less time is needed for the substance to travel, the higher the rate of diffusion for that substance

surface area to volume ratio — the greater the surface area to volume ratio, the higher the rate of diffusion for that substance

temperature — higher temperatures increase kinetic energy of particles, thus increasing the rate of diffusion for that substance.

size of molecules — smaller molecules tend to diffuse faster than larger molecules and they can move through spaces and barriers more easily because they have less mass and require less energy to move.

medium — diffusion happens the fastest in gas, this is because the molecules in gases are farther apart and can move more freely. in liquids, molecules are more tightly packed, and in solids, the molecules are almost locked in place.

application of diffusion

gaseous exchange — exchange of water and carbon dioxide between alveoli in the lungs and blood and between intercellular spaces in the leaf and its surroundings

removal of excretory products (eg. carbon dioxide and urea)

absorption of digested food products (eg. glucose and amino acids) into blood through the walls of small intestines

absorption of mineral salts from soil solution into root hair cells

define osmosis

osmosis is the net movement (overall movement) of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

define water potential

water potential is the measure of the tendency of water molecules to move from one place to another

examples of small molecules / large molecules which can / can’t past through partially permeable membrane

smaller molecules : water, glucose

large molecules : sucrose, protein, starch

factors that affect the rate of osmosis

water potential gradient — a larger difference in water potential speeds up osmosis

distance over which the molecules need to move

surface area to volume ratio — more surface area allows for more water molecules to pass through, speeding up osmosis

what happens when an animal cell is placed in a dilute solution

there’s a higher water potential in the solution compared to the cell’s cytoplasm

water will enter cell by osmosis across a partially permeable membrane

cell will swell and may eventually burst (cell lysis) as the cell membrane is unable to withstand the pressure (without a cell wall)

what happens when animal cell is placed in a concentrated solution

there’s a higher water potential in the cell’s cytoplasm compared to the solution

water will leave cell by osmosis across a partially permeable membrane

cell shrinks and spikes appear (crenation)

cell becomes dehydrated and eventually die / become crenated

what happens when a plant cell is placed in a dilute solution

plant cells will expand or swell. this is because the plant cell is protected by an inelastic cell wall and supported by the pressure of the water inside the cells pressing on the cell wall.

there’s a higher water potential in the solution compared to the cell sap

water enters the cell by osmosis across the partially permeable membrane

the vacuole increases in size and pushes the cytoplasm against the cell wall

the rigid, inelastic cell prevents the plant cell from bursting. the pressure exerted by the water in the vacuole on the cell wall is known as turgor pressure.

a cell in this state is said to be turgid plant cells will expand or swell, this is because plant cells are protected by an inelastic cell wall and supported by the pressure of water inside the cells pressing outside on the cell wall.

what happens when a plant cell is placed in a concentrated solution

plant cell will decrease in size and become limp

there’s a higher water potential in the cell sap compared to the solution

water leaves the cell by osmosis across the partially permeable membrane

the vacuole decreases in size and the cytoplasm and cell membrane pulls away from the cell wall, the plant loses turgor pressure

the shrinkage of cytoplasm and cell membrane away from the cell wall is called plasmolysis

a cell in this state is known to be plasmolysed

define active transport

active transport is the process in which energy is used to move substances across a membrane against its concentration gradient, that is, from a region of lower concentration to higher concentration

where does active transport occur?

active transport only occurs in living cells, because living cells respire. it is during respiration that energy is released, and part of this energy is used in active transport

examples of active transport in human body

the uptake of glucose by microvilli in epithelial cells in small intestines (ileum).

movements of glucose and amino acids from the kidney tubules to the bloodstream during selective reabsorption.

examples of active transport in plants

root hair cells take in mineral salts from the soil solution by diffusion or active transport, depending on the concentration of mineral salts in the solution