Movement of Substances

What you need to know

1. What is diffusion?2. What is osmosis?3. How is active transport different from diffusion?4. What causes the movement of substances?5. How does the permeability of the membranes

affect the movement of substances?6. How does the movement of substances affect

cells?

Diffusion• Net (overall)movement of

substances from a region of higher concentration to a region of lower concentration (down a concentration gradient)

• Passive – no cellular energy required

• Osmosis – a special type of diffusion

Equilibrium

Molecules are equally spaced apart/equally distributed throughout the solution

Factors affecting the rate of diffusion1. Size of the particle: At a given temperature, a smaller particle

(molecule) moves/diffuses faster than a larger one. 2. Temperature: As the temperature increases, particles gain energy

and move faster, thus the rate of diffusion is increased.3. Concentration Difference: The greater the concentration

difference between the two regions, the faster the substance will diffuse

4. Diffusion Distance: At a given temperature, it takes longer for the particle to diffuse a farther distance, thus the slower the rate of diffusion.

5. Surface Area: The greater the surface area, the greater the rate of diffusion.

6. Permeability: The more permeable the separating surface is, the faster a substance can diffuse through it.

Life example: Absorption of nutrients by the intestine

• Diffusion of digested nutrients (food molecules) into fingerlike structures called villi in the small intestine

Life example 2: Exchange of gases in the lungs

• Movement of O2 and CO2 between the air sacs (alveoli) and the blood capillaries surrounding them during gaseous exchange in the lungs

Active transport• Movement of

substances from a region of lower concentration to a region of higher concentration (against a concentration gradient)

• Require cellular energy• Involve transport

proteins found on the cell surface membrane

Life example: Absorption of nutrients from the soil

• Some essential nutrients are present in low concentration in the soil

• They are taken up by the root hair cells via active transport

Osmosis• Net (overall) movement of

water molecules from a region of higher water potential (lower concentration) to a region of lower water potential (higher concentration)

• Occurs across a partially/semi- permeable membrane

• Passive – no cellular energy required

Comparison between the 3

What causes substances to move passively?

• No energy involved• A concentration gradient drives the movement

of substances• i.e. a DIFFERENCE in concentration between

two regions • E.g., between inside the cell and the cell’s

external environment

Permeability

Membranes can be

Permeable – let everything in and out e.g., cell wall

Semi Permeable - let some things in and out e.g., cell surface membrane

Impermeable – let nothing in and out e.g.,

Osmosis and its effects on cells

• Depending on the concentration of the environment, cells can undergo physical changes

• These changes differ slightly in animal and plant cells because of their structural differences

A cell’s external environment can be

(a) Dilute (hypotonic): water molecules > solute molecules

• concentrationE < concentrationi

(b) Similar to the cell’s internal environment (isotonic):• concentrationE = concentrationi

(c) Concentrated (hypertonic): water molecules < solute molecules

• concentrationE > concentrationi

Osmosis and animal cells

• Recap: Animal cells do not have a cell wall(a) Hypotonic: water enters the cell as water potential outside the

cell is > water potential inside the cell(b) Isotonic: no net movement of water molecules since water

potential inside and outside the cell is similar(c) Hypertonic : Water leaves the cell as water potential is inside

the cell is > water potential outside the cell

What happens to animal cells in different environments?

Many animals that live in the sea have cytoplasm with a similar concentration to sea water

It is the function of our kidneys to make sure the fluid in our bodies that surrounds our cells (plasma) has a stable concentration

Life example: Amoeba

Osmosis and plant cells

• Recap: Plant cells have a cell wall

• A cell wall keeps the cell in shape despite changes in movement of water

1. Hypotonic: water molecules enter the cells– cells become turgid due to pressure (turgor

pressure) exerted on cell wall

2. Isotonic: no net movement of water molecules

3. Hypertonic: water molecules leave the cells– Cells become flaccid/plasmolysed– Cytoplasm shrinks away from the cell wall– But cell retains its shape due to presence of a cell wall

Plasmolysed red onion cells

•

Importance of turgor pressure

• Turgor pressure gives plants their strength

• If plants did not have this they would be wilted

• Plants that don’t have wood such as lettuce and house plants rely on turgor pressure for strength

Osmosis and Food Preservation Bacteria and Fungi are the most common

causes of food spoilage

Both these cells are enclosed by a membrane

If a food is placed in a sugary or salty solution then any bacteria or fungi present will lose the water in their cells to the more concentrated solution outside

When this happens the cells will shrivel and die and the food will not become contaminated with loads of bacteria nad fungi

Examples

Fish and Meat (bacon) may be stored in a salty solution

Jams, marmalades and tinned fruits are stored in a sugary solution

Life example 1: Absorption of water by roots

• Water moves from the soil into the roots and finally into the xylem via osmosis

Life example 2: Ultrafiltration by the kidneys

Life example 3: Freshwater and marine fishes/animals

Saltwater fishes Freshwater fishes

Experimental point of view