Diffusion, Osmosis, Active Transport and Respiration

What is diffusion?

Net movement of particles from high to low concentration, down a concentration gradient, due to random motion. Passive (no energy). Happens in liquids/gases.

What factors increase the rate of diffusion?

Higher temperature, steeper concentration gradient, larger surface area, higher surface-area:volume ratio, thinner exchange surface.

What is osmosis?

Net movement of particles from an area of high water potential to an area of low water potential through a partially permeable membrane

What is water potential?

Measure of free water molecules; water moves from higher → lower water potential.

What happens to animal cells in a dilute solution?

Water enters → cell swells → may burst (lysis) because no cell wall.

What happens to animal cells in a concentrated solution?

Water leaves → cell shrinks → becomes crenated.

What happens to plant cells in a dilute solution?

Water enters → vacuole swells → cell pushes on wall → becomes turgid.
Cell wall prevents bursting.

Why is turgidity important for plants?

Provides structural support and keeps stems/leaves upright.

What happens to plant cells in a concentrated solution?

Water leaves → cytoplasm and membrane pull away from cell wall → plasmolysis.

What is a flaccid plant cell?

Water leaves the cell but not enough for plasmolysis; cell becomes limp and soft.

Factors affecting rate of diffusion?

Temperature ↑, concentration gradient ↑, surface area ↑, SA:V ↑, membrane thinner.

Why does a large organism need specialised exchange surfaces?

Low surface‑area:volume ratio → diffusion too slow → requires lungs/gills/circulation.

What substances diffuse across the placenta from mother → fetus and fetus → mother?

Mother → fetus: oxygen, glucose, amino acids, fatty acids, minerals.
Fetus → mother: carbon dioxide, urea, other metabolic wastes.

Adaptations of the placenta that increase diffusion rate?

Large surface area (villi), thin walls (short diffusion distance), rich blood supply maintaining steep concentration gradients, maternal blood close to fetal capillaries. steep gradient mantained by blood flow

Why is diffusion essential for fetal development?

Fetus cannot breathe, digest, or excrete; relies entirely on diffusion to receive oxygen and nutrients and remove waste.

Why don’t maternal and fetal blood mix?

They are separated by thin membranes; prevents pressure damage and prevents immune rejection

Which gases diffuse in and out of leaves?

CO₂ diffuses in for photosynthesis.
O₂ diffuses out (from photosynthesis).
CO₂ diffuses out and O₂ in during respiration.

What adaptations make diffusion fast in leaves?

Large surface area; thin leaf; air spaces in spongy mesophyll; stomata provide direct openings to cells; moist surfaces for dissolved gases.

How do stomata affect diffusion?

Guard cells open stomata → greater gas exchange.
Closed stomata → reduced diffusion to conserve water.

How does photosynthesis maintain diffusion gradients

CO₂ is constantly used inside cells → low internal concentration → CO₂ diffuses in.
O₂ is produced → high internal concentration → O₂ diffuses out.

What substances diffuse from the small intestine into the blood?

Glucose, amino acids, fatty acids, glycerol, vitamins, minerals.

How is the small intestine adapted for rapid diffusion?

Villi & microvilli → massive surface area.
Very thin epithelial layer → short diffusion distance.
Large blood supply → maintains steep gradients.
Constant movement of blood → absorbed nutrients removed quickly.

Difference between villi and microvilli?

Villi = large finger-like projections.
Microvilli = microscopic projections on epithelial cells.
Both massively increase surface area for diffusion & absorption.

Why isn’t diffusion alone enough in the small intestine?

After a meal, concentration gradient is high → diffusion works.
Later, glucose concentration in intestine becomes lower → active transport absorbs remaining glucose into blood.

How does blood circulation help diffusion in the intestine?

Blood continuously removes absorbed nutrients → keeps concentration low in blood → maintains steep gradient.

What is respiration?

Chemical process releasing energy from glucose to produce ATP.

What is ATP used for?

Muscle contraction, active transport, protein synthesis, cell division.

Differences between aerobic and anaerobic respiration?

Aerobic: needs O₂, lots of ATP, complete breakdown of glucose.
Anaerobic: no O₂, little ATP, lactic acid or ethanol + CO₂.

Word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water (+ energy).

Balanced equation for aerobic respiration?

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ATP).

Word equation for anaerobic respiration in animals?

Glucose → lactic acid (+ energy).

Word equation for anaerobic respiration in plants/yeast?

Glucose → ethanol + carbon dioxide (+ energy).

What is oxygen debt?

Extra oxygen needed to remove lactic acid & restore ATP/oxygen levels after exercise.

Where does aerobic vs anaerobic respiration occur?

Aerobic: mitochondria.
Anaerobic: cytoplasm.

Why does anaerobic respiration occur during intense exercise?

Oxygen supply can’t keep up → anaerobic respiration provides rapid ATP.

Why do mitochondria have folded inner membranes (cristae)?

increase surface area for enzymes → more ATP made.

Why does anaerobic respiration release much less energy?

Glucose is only partially broken down. Most energy remains in lactic acid/ethanol.

Where in the cell do aerobic and anaerobic respiration take place?

Aerobic — mitochondria.
Anaerobic — cytoplasm.

Which cells have the highest mitochondria counts and why?

Muscle cells (need lots of ATP), liver cells (high metabolic activity), sperm cells (movement).

Why does respiration depend on enzymes?

Each step of respiration is enzyme‑controlled → temperature and pH affect rate.

How does the body deal with lactic acid after exercise?

Transported to liver and broken down into carbon dioxide + water (requires oxygen). Some is converted back into glucose or glycogen

How do photosynthesis and respiration interact?

Photosynthesis makes glucose; respiration breaks glucose down to release energy.
At night → only respiration occurs.

Why do breathing and heart rate increase during exercise?

To deliver more oxygen and glucose to muscles and remove carbon dioxide faster.

What is the energy from respiration used for?

Muscle contraction, active transport, maintaining body temperature, cell division, protein synthesis.

How does temperature affect respiration rate?

Higher temperature → faster enzyme activity → more respiration.
Above optimum → enzymes denature → respiration decreases.

Limiting factors for respiration in living cells?

Availability of glucose, oxygen (for aerobic respiration), enzyme activity, temperature.