Diffusion — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 17What diffusion is

Diffusion

Diffusion the movement of molecules from a region of their higher concentration to a region of their lower concentration.

• Molecules move down a concentration gradient as a result of their random movement.

Passive

• Passive diffusion needs no energy from respiration.

Section 2 of 17Source of energy

Brownian motion

• Brownian motion all particles move randomly at all times.

Energy source

• Energy source the energy for diffusion comes from the kinetic energy of the random movement of molecules and ions.

Section 3 of 17Diffusion across the cell membrane

Diffusion across the cell membrane

Cell membrane

• Cell membrane the principle is the same in a living cell, but the membrane can restrict the free movement of molecules.

Partially permeable

• Partially permeable the membrane lets some molecules cross easily and others with difficulty or not at all.
• The simplest selection is by the size of the molecules.

Section 4 of 17What diffusion does for organisms

Obtain requirements

• Obtain requirements organisms take in many substances, such as oxygen and nutrients.

Remove waste

• Remove waste many waste products leave the cell by diffusion.

Gas exchange

• Gas exchange carbon dioxide and oxygen move for respiration and photosynthesis.

Section 5 of 17Examples of diffusion in organisms

Site	Molecules moving	From	To
Small intestine	Digested food products (glucose, amino acids, fatty acids, glycerol)	Lumen of the small intestine	Blood or lymph in the villi
Leaf	Oxygen	Air spaces between mesophyll cells	Mitochondria in all cells
Leaf	Carbon dioxide	Air spaces between mesophyll cells	Chloroplasts in mesophyll cells
Leaf	Water vapour	Stomatal pores	Air outside the stomata
Lungs	Oxygen	Alveolar air space	Blood in the capillaries
Lungs	Carbon dioxide	Blood in the capillaries	Alveolar air space

Section 6 of 17Plants and gases

Oxygen

• Oxygen plants need it for respiration at all times.

Carbon dioxide

• Carbon dioxide plants need it for photosynthesis when light and temperature are suitable.

Section 7 of 17When diffusion stops

• Equal concentrations
  • there is no net movement once the concentration is the same everywhere.
  • Molecules still move randomly but equally in each direction.

Section 8 of 17More from the specification

More from the specification

Passive Solute Movement Down Concentration Gradients

• Diffusion involves the net thermal movement of solute particles from regions of higher concentration to regions of lower concentration until uniform equilibrium is established.

Membrane Surface Area and Fick's Law

• The rate of molecular diffusion is directly proportional to available membrane surface area and concentration gradient,
  • and inversely proportional to diffusion pathway thickness.

Dialysis Tubing as an Artificial Membrane Model

• Semi-permeable cellulose dialysis tubing allows selective diffusion of small solvent molecules and monosaccharides while retarding the passage of large colloidal polysaccharides like starch.

Active transport

• Movement of particles against a concentration gradient using energy from respiration.

Water potential

• A measure of the tendency of water to move, highest in dilute solutions.

Dialysis tubing

• A non-living partially permeable membrane.

Osmosis

• The diffusion of water through a partially permeable membrane from dilute to concentrated solution.

Tissue

• A group of similar cells working together to do one job.

Solvent

• Liquid in which a solute dissolves.

Section 9 of 17Factors that affect the rate of diffusion

Factor	Effect on the rate	Reason
Surface area to volume ratio	A larger ratio gives faster diffusion	More surface for substances to cross
Distance	A shorter distance gives faster diffusion	Molecules have less far to travel
Temperature	Higher temperature gives faster diffusion	Molecules have more kinetic energy
Concentration gradient	A greater difference gives faster diffusion	More molecules move from high to low than the reverse

Section 10 of 17Surface area to volume ratio

Bigger cells

• Bigger cells the bigger a cell or structure is, the smaller its surface area to volume ratio.
• This slows the rate at which substances can cross its surface.

Adaptation

• Adaptation many cells that exchange substances have an increased surface area.
• Root hair cells absorb water and mineral ions.
• Cells lining the ileum absorb the products of digestion.

Folding

• Folding the highly folded surface of the small intestine increases its surface area.

Section 11 of 17Distance

Short distance

• Short distance the smaller the distance molecules travel, the faster the diffusion.

Thin walls

• Thin walls blood capillaries and alveoli have walls only one cell thick to keep diffusion fast.

Section 12 of 17Temperature

Kinetic energy

• Kinetic energy at a higher temperature molecules have more kinetic energy and move faster.

Net rate

• Net rate the net rate of diffusion from high to low concentration is therefore faster.

Section 13 of 17Concentration gradient

Steeper gradient

• Steeper gradient the greater the difference in concentration across a membrane, the faster the diffusion.

Reason

• Reason more molecules on the high-concentration side move across by random motion than move back.

Section 14 of 17Planning an investigation

1

Choose one factor to change, such as temperature or the size of the diffusion surface.

2

Keep every other factor the same.

3

Measure how far or how fast a substance diffuses.

4

Repeat for each value of the factor.

5

Use the factors above to explain the results.

Section 15 of 17More from the specification

More from the specification

Osmosis in Plant Cells

• The net movement of water molecules from a solution of higher water potential to a lower water potential across a partially permeable cell membrane.

Turgidity and Plant Tissue Support

• When immersed in a hypotonic solution, water enters plant cells by osmosis, creating internal turgor pressure against the cell wall that supports the plant.

Active Transport across Membranes

• The selective movement of mineral ions against a concentration gradient into root hair cells, requiring carrier proteins and metabolic energy from respiration.

Plasmolysis in Hypertonic Solutions

• When plant tissue is placed in a concentrated solute solution, water leaves the cells by osmosis,
• causing the cytoplasm to shrink away from the cell wall.

Osmosis in Plant Cells

• The net movement of water molecules from a solution of higher water potential to a lower water potential across a partially permeable cell membrane.

Turgidity and Plant Tissue Support

• When immersed in a hypotonic solution, water enters plant cells by osmosis, creating internal turgor pressure against the cell wall that supports the plant.

Active Transport across Membranes

• The selective movement of mineral ions against a concentration gradient into root hair cells, requiring carrier proteins and metabolic energy from respiration.

Plasmolysis in Hypertonic Solutions

• When plant tissue is placed in a concentrated solute solution, water leaves the cells by osmosis,
• causing the cytoplasm to shrink away from the cell wall.

Solvent

• Liquid in which a solute dissolves.

Solute

• Substance that dissolves in a solvent.

Active transport

• Movement of particles against a concentration gradient using energy from respiration.

Water potential

• A measure of the tendency of water to move, highest in dilute solutions.

Carbon dioxide

• Gas that turns limewater cloudy.

Dialysis tubing

• A non-living partially permeable membrane.

Osmosis

• The diffusion of water through a partially permeable membrane from dilute to concentrated solution.

Tissue

• A group of similar cells working together to do one job.

Organism

• A whole living thing made of organ systems.

Section 16 of 17Key terms

Term	Meaning
Diffusion	Movement of molecules from higher to lower concentration.
Concentration gradient	The difference in concentration between two regions.
Partially permeable membrane	A membrane that lets some molecules through but not others.
Brownian motion	The random movement of particles.
Kinetic energy	The energy of movement.
Gas exchange	The movement of oxygen and carbon dioxide into and out of an organism.

Term	Meaning
Surface area to volume ratio	Surface area divided by volume.
Concentration gradient	A difference in concentration between two places.
Villi	Folds in the small intestine wall that increase its surface area.
Exchange surface	A surface across which substances diffuse into or out of an organism.
Kinetic energy	The energy of moving particles.
Ileum	The part of the small intestine where absorption takes place.

Section 17 of 17Exam technique

1

Saying diffusion needs energy from respiration — it uses the kinetic energy of the molecules.

2

Saying molecules stop moving when concentrations are equal — they keep moving but with no net movement.

3

Saying diffusion moves molecules from low to high concentration — it is from high to low.

4

Confusing diffusion with osmosis — osmosis is diffusion of water only, through a partially permeable membrane.

1

Saying a bigger cell diffuses faster — a bigger cell has a smaller surface area to volume ratio.

2

Forgetting that temperature increases speed of diffusion — particles have more kinetic energy.

3

Saying a long distance speeds diffusion — a short distance does.

4

Calling the cell membrane fully permeable — it is partially permeable.