2.4 Movement of Substances Into and Out of Cells

REVISION NOTES

IGCSE Edexcel Biology

2.4 Movement of Substances Into and Out of Cells

2.4.1 Understand the processes of diffusion, osmosis and active transport by which substances move into and out of cells

Diffusion:

Movement of molecules from an area of high concentration to an area of low concentration
Affected by partially permeable cell membrane
- Restricts the movement of molecules down the concentration gradient
- Restriction is based on molecular size
- Smaller molecules like water can diffuse
- Larger molecules like salts cannot
Passive process

Factors affecting diffusion:

- Temperature
  - Increase kinetic energy

Concentration gradient

- Surface area
  - Larger surface area Increases the space available for diffusion

Distance

- Longer the distance, lower the rate of diffusion

Osmosis:

Net movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane
Water moves down the concentration gradient
Dilute solutions have a high water potential
Concentrated solutions have a low water potential
Passive process

Osmosis in animal cells:

Causes loss or gain of water
In concentrated solutions:
- Cells have a higher water potential
- Cells lose water and shrivel up
In a dilute solution:
- Cells have a lower water potential
- Cells gain water creating turgor pressure
- Cells may bursts

Types of solutions:

Isotonic solution
- Concentration of solute in external environment is the same as internal
- No net movement
Hypertonic solution
- Concentration of solute in external environment is higher than internal
- Net movement of water out of the cell
Hypotonic solution
- Concentration of solute in external environment is lower than internal
- Net movement of water into the cell

Osmosis in plant cells:

Causes loss or gain of water
Cell wall protects from cell lysis
In concentrated solutions:
- Cells have a higher water potential
- Cells lose water
- Cell membrane shrivels up and detaches from cell wall
- Cells become flaccid or plasmolysed
In a dilute solution:
- Cells have a lower water potential
- Cells gain water creating turgor pressure
- Cells become turgid

Active transport:

Movement of particles through a cell membrane against the concentration gradient using energy
Movement from an area of low concentration to an area of higher concentration

Requires protein carrier molecules

Active transport in animals:

Food molecules diffuse across the small intestinal wall
- Only if concentration gradient between lumen and the bloodstream exists
- Active transport used for food molecules if concentration gradient does not exist

Active transport in plants:

Root hair cells absorb mineral ions from the soil
Mineral ions are in high concentration within cells
- They travel against the concentration gradient

2.4.2 Understand how factors affect the rate of movement of substances into and out of cells, including the effects of surface area to volume ratio, distance, temperature and concentration gradient

Surface area to volume ratio:

The larger a cell, the smaller its surface area (SA) to volume (V) ratio
- Volume of the cell increases
Smaller SA:V ratio:
- Decrease the rate of movement of substances
- Smaller surface available for movement
Cells are adapted for diffusion by:
- Presence of root hairs in root hair cells
- Presence of villi in cells lining the ileum

Distance:

The smaller the distance, the faster the rate of diffusion
Necessary for gas exchange during respiration
Blood capillaries and alveoli have one cell thick walls
- Decreases distance of diffusion

Temperature:

Increasing temperatures increases kinetic energy
Increase in speed of molecules increases collisions with cell wall
Increases rate of movement of molecules

2.4.3 Practical: investigate diffusion and osmosis using living and non-living systems

Diffusion practical (non-living):

Cut 3 equal sized cubes of beetroot
1. Keep the surface area and volume of each sample the same
2. SA:V ratio can affect the rate of diffusion
Rinse the beetroot with water
Add 10cm3 of water into two test tubes
Label the test tubes as “A”, “B” and “C”
Keep test tube A at room temperature
Transfer test tube B to a water bath at 65oC
Transfer test tube C to an ice bath
Add a piece of beetroot into each test tube
Observe the colour of the water in each test tube after 20 mins

Results:

Test tube B should be the darkest in colour as:
- At high temperatures, the cell membrane becomes damaged
- More pigment leaves the cells
- Pigment particles would also have the most kinetic energy
- Faster movement of molecules across the membrane

Osmosis practical (living):

Prepare 6 different concentrations of sucrose solutions ranging from distilled water 0M to 1M
Add 5 cm3 of the respective concentrations of sucrose to 6 test tubes
Cut 6 equal size pieces of potato
Blot each piece with a paper towel
Weigh each piece on a balance and record the starting weight
Put a piece of potato into each test tube
After 2 hours, remove the pieces of potato from the test tubes
Blot them with paper to remove excess water
Measure the final weight of the potato pieces
Calculate the change in weight for each sucrose concentration

Results:

The piece of potato in distilled water will have the largest increase in mass
- Largest concentration gradient between the water and potato
- The potato has a lower water potential than the water
- Force water molecules to enter the potato cells by osmosis
- Increased water retention in potato cells
- Increases turgor pressure making them turgid

The piece of potato in 1M sucrose solution will show the largest decrease in mass
- Larger concentration gradient between the sucrose solution and the potato
- Potato has a higher water potential than the sucrose solution
- Water molecules move out of the potato cells through osmosis
- Decreased water retention in potato cells
- Cells become flaccid and plasmolysed