Chapter 3: Movement into and out of cells

diffusion

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

energy for diffusion comes from

the kinetic energy of random movement of molecules and ions

importance of diffusion of gases and solutes in living organisms

without it, molecules which are needed for life, for example glucose and oxygen for respiration, would not be able to get to the places they are needed, water is needed as a solvent

factors that influence diffusion

• surface area

• temperature

• concentration gradient

• distance

limiting factor diffusion; surface area

the bigger a cell, the smaller its surface area:volume ratio is, this slows down the rate at which substances can move across its surface. cells adapted for diffusion have larger surface areas, e.g. root hair cells

limiting factor diffusion; temperature

the higher the temperature, the faster molecules move, meaning there will be more collisions against the cell membrane

limiting factor diffusion; concentration gradient

the greater the difference in concentration on either side, the faster movements will occur, because on the higher concentration side, there’ll be more collisions.

limiting factor diffusion; distance

the smaller the distance the molecules have to travel,the faster transportation will happen.

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

role of water as a solvent in organisms (osmosis)

• digested food molecules in the alimentary canal need to be moved to cells, which can’t be done without water

• toxic substances (e.g. urea) in excess of requirements (e.g. salt) can be dissolved, making them easier to excrete

• dissolved substances can be easily transported around the organism e.g. xylem of plants or dissolved food molecules in the blood

how does water moves into and out of cells?

diffuses through partially permeable membranes by osmosis

osmosis using materials such as dialysis tubing

dialysis tubing: non- living partially permeable membrane (aka visking tubing)

• notice whether H2O levels go up or down, shows whether osmosis happened or not

the effects on plant tissues of immersing them in solutions of different concentrations

• GAINS MASS: H2O moved into the cell via osmosis, solution is more dilute than tissue

• LOSES MASS: H2O moved out of the the cell via osmosis, tissue is more dilute than solution

• NO CHANGE: no net movement of H2O, concentration must be equal

plants are supported by (osmosis)

the pressure of water inside the cells pressing outwards on the cell wall

effects on plant cells of immersing them in solutions of different concentrations

• HYPOTONIC SOLUTION: cells have lower H2O potential than solution, net movement into cells of H2O into cell, cell may burst, no cell wall for turgor pressure

• HYPERTONIC SOLUTION: cells have higher H2O potential than solution, net movement out of the cell, cell is shrivelled

• ISOTONIC SOLUTION: H2O potential is equal, no net movement in or out of the cell

importance of water potential and osmosis in the uptake and loss of water by organisms

• high H2O potential: H2O moves into the cell via osmosis, H2O molecules push the cell membrane against the cell wall, increasing turgor pressure, cell becomes turgid

• low H2O potential: H2O molecules move out of the cell via osmosis, making the cell flaccid, cell may become plasmolysed, cell membrane pulls away from the cell wall

active transport

the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration (i.e. against a concentration gradient), using energy from respiration

importance of active transport

a process for movement of molecules or ions across membranes, including ion uptake by root hairs

who moves molecules or ions across what during active transport?

protein carriers, across a membrane