unit 2 - movement of substances

purpose of movement of substances across plasma membrane

1. to obtain nutrients → for energy and raw materials

2. to excrete waste products

3. to generate ionic gradients → essential for nervous and muscular activities

4. to maintain suitable pH and ionic concentration within cell for enzyme activity

what are the 2 types of processes that substances can be transported across plasma membrane

1. passive transport → movement of substances without expenditure of energy (diffusion and osmosis)

2. active transport → movement of substances involving expenditure of energy

what is diffusion

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

when does diffusion stop

down a concentration gradient → until all particles are evenly distributed → dynamic equilibrium (no net movement) → is reached

• dynamic equilibrium reached → particles are still in continual movement

what is concentration gradient

difference in concentration between two regions

• each type of particles moves down its own concentration gradient → independent of other concentration gradients

where does diffusion take place in

gas → fastest diffusion

liquid

solid → slowest diffusion

what are the 5 factors affected the rate of diffusion

1. molecular size

2. concentration gradient for molecule

3. kinetic energy of particles

4. diffusion distance

5. surface area-to-volume ratio

how does molecular size affect rate of diffusion

• different shapes → chains folded differently → different molecular size

smaller particle → higher rate of diffusion

• O2 (smaller) diffuse faster than CO2 (larger)

how does concentration gradient for molecule affect diffusion

greater/steeper the concentration gradient → higher rate of diffusion

how does kinetic energy of particles affect diffusion

higher temperature → particles have more energy and move faster → higher rate of diffusion

• very high temperature → can disrupt structure of membrane → lose its selectivity (allow all particles to go through)

how does diffusion distance affect diffusion

shorter diffusion distance → less time required for substance to travel → higher rate of diffusion

• diffusion → only effective over very short distances

how does surface area-to-volume ratio affect diffusion

larger surface area-to-volume ration → higher rate of diffusion

• object increase in size → surface area relative to volume/surface area-to-volume ration → gets smaller (it is a ratio!!!!!!!)

• some cells → specifically adapted → for absorption of substances → long narrow protrusions → increase surface area-to-volume ratio (root hair cell, epithelial cells of small intestine → microvilli)

what is the biological importance of diffusion in animals

• movement of oxygen in lungs to bloodstream and movement of carbon dioxide from bloodstream to lungs in human

• movement or absorption of soluble products of digestion → glucose and amino acids into villi and microvilli of small intestine

what is the biological importance of diffusion in plants

• gaseous exchange in leaves

  • in daylight → stomata open → allow carbon dioxide to diffuse into leaf → for photosynthesis → oxygen and water vapour → more concentrated in air spaces → diffuse out to drier air → outside stomata

  • diffusion → take place across cell membranes → gaseous exchange of oxygen and carbon dioxide at root hair cells

• movement of mineral salts from soil solution into root hair cells of plants

what is osmosis

osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

what is a partially/selected permeable membrane

allows particles which are smaller than pores of membrane to pass through

• passage of large particles prevented (starch, sucrose, proteins)

• e.g plasma membrane, visking tubing, cellophane bag → particles e.g. glucose, iodine, salt, dissolved gases)

what is concentrations of solutions

• solute → dissolved in solvent → solution formed

• concentration of solution → amount of solute dissolved in a given volume of solvent

what is the water potential of a solution

measure of tendency of water molecules to move from one place to another

• of solution → cell sap of plant cell, cytoplasm of animal cell, soil solution

• H2O molecules → move from solution of higher water potential → to another of lower water potential → down water potential gradient → across partially permeable membrane

what is a dilute solution

• more H2O molecules → per unit volume —> tend to move/leave more readily → than a concentrated solution = fewer H2O molecules per unit volume

• dilute solution → higher water potential → than concentrated solution

what are the different types of solutions in terms of difference in concentrations

1. hypotonic solution → solution with higher water potential than another → move into cell from solution

2. isotonic solution → solution with same water potential as another → no net movement

3. hypertonic solution → solution has lower water potential than another → move out of cell into solution

what are the factors affecting rate of osmosis

1. water potential gradient

2. distance over which water molecules need to move

3. surface area-to-volume ratio

how does water potential gradient affect osmosis

greater/steeper water potential gradient → higher rate of osmosis

how does the distance over which water molecules need to move affect osmosis

shorter distance over which water molecules need to move → less time required for water molecules to travel → higher rate of osmosis

how does surface area-to-volume ratio affect osmosis

larger surface area-to-volume ration → higher rate of osmosis

what is turgor and turgor pressure

• turgor -→ turgidity of plant cell with water

• turgor pressure → pressure exerted by the water in the vacuole → pressing outwards on the cell wall

what is the biological importance of osmosis to plants

• turgor → maintains the shape of soft tissues in plants

• young stems and leaves → of herbaceous and non-woody plants → able to remain firm and erect

• lose too much water from cells → loss turgidity → wilt

how does some plants react to changes in turgor

• changes in turgor of guard cells -→ cause opening and closing of stomata in leaves

• change in turgidity of cells on opposite surfaces of petals -→ bending movement of petals → control opening and closing of flower

• change in turgor of cells at the bases of leaflets → leaflets fold when touched (e.g. mimosa)

how to answer osmosis questions

1. contrast of water potential → concentration is higher where and lower where?

2. movement of water molecules → where would the water molecules move from to where… via osmosis

3. what happens to the cell

what is active transport

process whereby energy is used → to move the particles of a substance → across a membrane → against its concentration gradient, that is, from a region of where particles are of lower concentration to a region where they are of higher concentration

how does active transport work (characteristics)

1. occurs against a concentration gradient

2. requires energy in the form of AP (adenosine trisphosphate)

3. active uptake → highly selective process

4. involves protein “pumps” at the membrane → to convert ATP to ADP (adenosine diphosphate) and P (phosphate group) → change their shape to do work (energy)

what is the energy needed for active transport

• adenosine trisphosphate (ATP) molecules → supply energy

• every cell → own store of ATP by respiration in mitochondria

• active transport → only in living cells

what are the proteins involved in active transport

• transport proteins → carry out active transport → control content of the cytoplasm in the cell sap

what is the biological importance of active transport

1. uptake of dissolved mineral salts → from soil solution → by root hair cells

2. absorption of soluble products of digestion → by villi of small intestine in humans

3. reabsorption of amino acids, glucose, mineral salts and water → back into the network of blood capillaries → surrounding coiled parts of kidney tubules

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

1. cell sap → lower water potential than external solution → water molecules enter the vacuole → through the partially permeable cell surface membrane → via osmosis

2. vacuole → expands in volume → pushing the cell surface membrane and cytoplasm → against the cellulose cell wall

3. cell → become turgid

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

1. cytoplasm → lower water potential than the external solution → water molecules enter cytoplasm through partially permeable membrane of cell surface membrane → via osmosis

2. cell → expands in volume

3. cell surface membrane → delicate → unable to prevent further expansion → bursts

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

1. cell sap → higher water potential than the external solution → water molecules leave through the partially permeable membrane of the cell surface membrane → via osmosis

2. vacuole → shrinks → cell surface membrane and cytoplasm → shrinks from the cellulose cell wall which is flaccid

3. cell → plasmolysed

   • outside solution in plasmolysed cell → because cellulose cell wall → fully permeable and allows solution to pass through

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

1. cytoplasm → higher water potential than external solution → water molecules leave through the partially permeable membrane of the cell surface membrane → via osmois

2. cell → shrinks in volume

3. cell → crenated

compare between osmosis and active transport in living cells

4 Transport proteins (carrier proteins) are ALWAYS involved in

active transport BUT osmosis may require transport proteins

(water channels / aquaporins).

1. osmosis involves movement of water molecules → but active transport involves all types of particles [D]

2. osmosis is passive (does not require the expenditure of energy) → but active transport requires energy for movement of molecules [D]

3. osmosis involves movement of substances from solution o higher water potential to one of a lower water potential down a water potential gradient → but active transport moves particles from a region of lower concentration to one of higher concentration against concentration gradient [D]

4. transport proteins are always involved in active transport → but osmosis may require transport proteins [D]

5. both osmosis and active transport in living cells → involves particles passing through a partially permeable membrane [S]

describe and explain the changes that would be observed when red blood cells are places in distilled water

When the red blood cell is placed in distilled water, the cytoplasm

of the red blood cell has a lower water potential than the distilled

water.

2 Therefore, water MOLECULES enter the cytoplasm of the red

blood cell by osmosis.

3 The cell expands in volume. As the plasma membrane is too

delicate to prevent further expansion, red blood cells will

eventually burst.

1. when red blood cells are placed in distilled water → cytoplasm of the rbc has lower water potential than the distilled water

2. water molecules enter the cytoplasm of the red blood cell → through the partially permeable membrane of the cell surface membrane of the red blood cell → via osmosis

3. cell → expands in volume

4. cell surface membrane → too delicate to prevent further expansion → red blood cells will eventually burst