Transport in living things. 25.1 Diffusion How are you able to smell various aromas from food? oThe...

Transport in living things

25.1 Diffusion

• How are you able to smell various aromas from food?o The aromas come from vapour from the food.o Molecules in the vapour move about randomly.o They bump into one another and spread out quickly.o As a result, some molecules enter your nostrils.o The spreading out of molecules is brought about by

diffusion.

• Diffusion in the overall movement of molecules from a region of higher concentration to a region of lower concentration.

Movement of sucrose molecules from A to BMovement of water molecules from A to B

Diffusion in non-living systems

Solutions A and B are separated by a permeable membrane, which allows both small and large molecules to pass through. Both the solvent (water) and the solutes (the sucrose molecules) can pass through it.

A B

permeable membranepermeable membrane

10 % sucrose solution

2 % sucrose solution

permeable membrane

sucrose molecules

water molecules

Movement of sucrose molecules from A to BMovement of water molecules from A to B

Both types of molecules are free to move across the membrane as the particles are in constant random motion.

Since solution A has a higher concentration of sugar, more sugar molecules will diffuse from solution A to solution B.

Also, solution B has a higher concentration of water molecules. Thus, more water molecules will diffuse from solution B to solution A.

Hence, there is a net movement of sugar molecules from solution A to solution B and a net movement of water molecules from solution B to solution A. Eventually, both solutions will have the same concentration of sugar and water molecules.

Diffusion in living organisms

• Diffusion in living organisms occurs continuously and it does not always take place across a membrane (e.g. cell membranes).

• Diffusion is important in moving materials in and out of the cells.


• Diffusion in the absence of membranes

cros

s-se

ctio

n of

par

t of

leaf air spaces

high concentration of oxygen

low concentration of carbon dioxide

carbon dioxide diffuses in through stomata

oxygen diffuses out through stomata

Diffusion occurs during photosynthesis in the absence of membranes.

Carbon dioxide enters the leaves through the stomata, moving into air spaces in the leaves.

This is because the carbon dioxide concentration is higher outside the leaves than in the air spaces inside the leaves.

Similarly, oxygen given off during photosynthesis diffuses out of leaf air spaces as there is a lower concentration of oxygen outside the leaves.


• Diffusion through a partially permeable membraneo A partially permeable membrane is a membrane that

allows some molecules to pass through but not others.o The cell surface membrane in plants is an example of a

partially permeable membrane. • Gases (e.g. oxygen) and smaller molecules (e.g. sugar) diffuse into

the plants cells through the membrane.

o In humans, gaseous exchange occurs in the lungs by diffusion.• When we breathe in, oxygen enters the air sacs in our lungs.• The oxygen concentration is higher in the air sacs than in the blood

surrounding them. • Thus, oxygen diffuses out of the air sacs into the bloodstream.


• Diffusion through a partially permeable membraneo In humans, gaseous exchange occurs in the

lungs by diffusion.• During respiration in the cells, carbon dioxide is

produced as waste.• Carbon dioxide is carried in the bloodstream leading

to the lungs.• It then diffuses out into the air sacs, and leaves the

body when we breathe out.


• Diffusion through a partially permeable membrane

air sac

tube

cluster of air sacs

lungsBlood entering blood vessel has high concentration of carbon dioxide and low concentration of oxygen.

Carbon dioxide diffuses out of the bloodstream and into the air sac. Oxygen diffuses from

surrounding air into the air sac and bloodstream.

Blood leaving blood vessel has high concentration of oxygen and low concentration of carbon dioxide.

25.2 Osmosis

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

5% sucrose solution (higher concentration of water molecules; higher water potential)

Osmosis in non-living systems

A

Partially permeable membrane

Solutions A and B are separated by a partially permeable membrane. The membrane allows smaller water molecules to pass through but not the larger sugar molecules.

Water molecules can pass freely through the membrane

10% sucrose solution (lower concentration of water molecules; lower water potential)

B

Sucrose molecules are too big to pass through its pores.

During osmosis, the water molecules will flow from solution B with higher concentration of water molecules (higher water potential) to solution A with lower concentration of water molecules (lower water potential).

As water molecules flow, the volume of solution A rises and the volume of solution B drops. When the concentrations of both solutions A and B are the same, there will be no net movement of water molecules.

Osmosis in living organisms

• Recall some properties of cells:o Cell membrane of both plant and animal cells

is partially permeable.o The plant cell wall is made of cellulose. It is

permeable.o The plant cell has a relatively rigid (firm) cell

wall.


• What happens to animal cells when they are placed in distilled water?

• Animal cells o Animal cells have elastic membranes. When the

water molecules flow in, the animal cells will swell and eventually burst.

water moves in

Water potential outside the cell is higher than that in the cytoplasm.

Cell expands and eventually bursts.


• What happens to plant cells when they are placed in distilled water?

• Plant cells o Plant cells have strong, rigid cells walls which prevent the cells

from expanding too much.o When water molecules flow in, the contents in the cell press the

cell wall.o The water creates a pressure on the cell wall of the plant cell.

This is called turgor pressure and keeps the plant tissues turgid.

o Soft tissues in plants depend on turgor for support. If plants lose too much water, they will wilt.


• Plant cells

Water potential outside the cell is higher than that in the cell sap.

Cell expands and becomes turgid. The rigid cellulose cell wall expands slightly only. This prevents the cell from bursting.

water moves in


• What happens to animal cells when they are placed in concentrated solution?

• Animal cells o Animal cells will shrivel up as they lose water.

Water leaves the cell by osmosis

Concentration of water molecules outside the cell is lower than that in the cytoplasm.

Cell shrinks and becomes soft. It is dehydrated.


• What happens to plant cells when they are placed in concentrated solution?

• Plant cells o When the concentration of water molecules of the

cytoplasm and cell sap is higher than that of the surrounding solution, water leaves the plant cells by osmosis.

o The vacuoles shrink and the cell contents pulls away from the cellulose cell walls. The plant cells lose turgor pressure and become flaccid.


• Plant cells

Concentration of the water molecules outside the cell is lower than that in the cell sap.

Cell contents pull away from the cell walls and the cell becomes flaccid.

Water leaves the cell by osmosis.

25.3 The Need for a Transport System

• A multicellular organism like a plant or a human being is made up of billions of cells.

• Most of these cells lie deep inside the body of the organism.

• Nutrients and oxygen need to reach each cell in the body to support the life of the cells.

• A transport system ensures that substances are transported fast enough throughout the organism.

25.4 Absorption of Water and Mineral Salts by Roots of Plants

• Water and mineral salts are absorbed by the root hairs from the surrounding soil.

• Water enters the roots by osmosis.

• Mineral salts enter by active transport.


• How is water absorbed by the roots?o There are numerous root hairs at the ends of

young roots.o A root hair is an outgrowth of a root hair cell.o It has a cell membrane, cytoplasm and a large

central vacuole. o The vacuole contains cell sap, which contains

mainly water and dissolved substances such as mineral salts.


• How is water absorbed by the roots?

root hair cell

soil particles

film of water

water entering root hair

lower concentration of water molecules

higher concentration of water molecules


• How is water absorbed by the roots?o The soil solution consists of water with

dissolved mineral salts. o It is less concentrated (higher concentration of

water molecules) than the cell sap and the cytoplasm.

o As the cell membrane is partially permeable, water enters the root hairs by osmosis.


• How are dissolved mineral salts absorbed by the roots?o The concentration of mineral salts in the root

hairs is often higher than in the soil solution.o Thus, mineral salts should diffuse out of the

root hairs.o However, this does not occur as the root hairs

not only prevent the outflow of the salts, but absorb salts from the soil solution.


• How are dissolved mineral salts absorbed by the roots?

xylem

soil particles

root hair absorbing mineral salts from the soil

Flow of mineral salts upwards

The root hairs absorbed dissolved mineral salts from a region of lower concentration to a region of higher salt concentration. This occurs because the root hairs are using energy during the absorption process.

The movement of substances against a concentration gradient using energy is called active transport.


• Why is energy required in active transport? o The higher concentration of

molecules is at the top of the slope.

o The lower concentration of molecules is at the bottom of the slope.

o In diffusion, molecules can roll down the slope without using much energy.

Diffusion


• Why is energy required in active transport? o If the molecules were to

move up the slope, energy is needed.

o This is what happens in active transport.

Active transport

Transport in living things. 25.1 Diffusion How are you able to smell various aromas from food? oThe...

Documents

Transcript of Transport in living things. 25.1 Diffusion How are you able to smell various aromas from food? oThe...