(2d) Movement of substances into and out of cells

What is diffusion?

The net movement of molecules from a region of higher concentration to a region of lower concentration

Name 2 examples of diffusion in plants

• oxygen and co2 diffuse in and out of leaves through the stomata

• water molecules move out of leaves through stomata during transpiration

Name 2 examples of diffusion in humans

• oxygen diffuses from air in alveoli to blood in lungs

• dissolved substances such as sugars and amino acids dissolve from blood into cells

What is simple diffusion?

Small particles are able to pass through the cell membrane without help to diffuse inside the cell

What is facilitated diffusion?

Particles that can’t pass through the cell membrane on their own need help to get through, so there are special ‘channels’ to facilitate them passing through and diffusing.

What is osmosis?

• the movement of water molecules from a region with a

• higher concentration of water molecules (more dilute solution/high water potential)

• to a region with a lower concentration of water molecules (less watery, more concentrated solution/low water potential) through a partially permeable membrane

What is active transport?

The movement of molecules against the concentration gradient, requiring energy in the form of ATP

Examples of active transport

• plant: mineral ions taken up by active transport from soil to root hair cells

• human: glucose taken back into blood by active transport in the kidney

Name 5 factors affecting the movement of substances

• concentration gradient

• temperature

• surface area

• distance

• sa:v ratio

How does concentration gradient affect diffusion?

The greater the difference in concentration (steeper concentration gradient), the quicker the rate of diffusion

How does temperature affect diffusion?

• The higher the temperature, the more kinetic energy the particles will have

• so they will move faster resulting in more frequent collisions with the cell membrane

How does surface area affect diffusion?

The greater the surface area, the faster the rate of diffusion

How does distance affect diffusion?

The greater length of the journey travelled by diffusing molecules, the slower the rate of diffusion

How does SA : V ratio affect diffusion?

Greater surface area to volume ratio results in faster diffusion

Why do the potatoes’ mass change percentage increase as the concentration of sucrose increased?

• there is a lower concentration of free water molecules than inside the potato

• so water travels out of the potato into the solution

• as the sucrose solution becomes more concentrated, it results in a steeper concentration gradient of water molecules

• so more osmosis occurs and the potato loses more mass