Movement in and out of Cells

Particles in liquids and gases move randomly. Over time the random movement results in particles spreading from areas of HIGH concentration to LOW concentration. This is diffusion.

Diffusion is passive — needs no energy. It happens because particles are always moving (random thermal motion).

Examples in living things:

• O₂ in the lungs diffuses from air (high concentration) into blood (low).
• CO₂ diffuses the opposite way (high in blood, low in air).
• Glucose and amino acids diffuse from the small intestine into capillaries.
• CO₂ diffuses out of all body cells into blood.

Factors that speed up diffusion:

• Bigger concentration gradient (high to low difference).
• Larger surface area.
• Shorter distance.
• Higher temperature.

The combination 'large SA, thin walls, big gradient' is why villi and alveoli are so effective — they engineer ALL the factors at once.

Osmosis is the net movement of WATER molecules through a PARTIALLY-PERMEABLE MEMBRANE from a DILUTE solution to a more CONCENTRATED solution.

Cell membranes are partially permeable — they let water through but block larger molecules like glucose or proteins. So if cells are in different solutions, water moves to even out the concentration.

Three scenarios for a cell:

1. Hypotonic (cell is in pure water or very dilute solution). Water rushes IN. Animal cells SWELL and may BURST (lysis). Plant cells become TURGID (firm) but don't burst because the cell wall resists.
2. Isotonic (cell solution = surroundings). No net movement. Cells stay the same.
3. Hypertonic (surroundings more concentrated than cell). Water leaves cell. Animal cells SHRIVEL (crenation). Plant cells PLASMOLYSE (cytoplasm pulls away from cell wall).

Examples:

• Salting meat draws water out → preserved (bacteria can't grow in low water).
• Red blood cells burst if put in distilled water; shrivel in concentrated salt water.
• Wilted plants — cells have lost water → not turgid. Watering restores turgor.
• Plant roots take up water from soil by osmosis.

A classic MYP investigation: weigh potato chips, put each in different sugar solutions for an hour, weigh again. Chips in pure water gain mass; chips in concentrated sugar lose mass. The point of zero change identifies the sugar concentration inside the potato cells.

Sometimes cells need to move substances AGAINST the concentration gradient (low to high concentration). Like pushing water uphill — needs energy.

Active transport uses ATP (energy from respiration) and a SPECIFIC CARRIER PROTEIN in the membrane. The carrier protein binds the substance and 'flips' it across the membrane, then resets.

Examples:

• Plant roots take up MINERAL IONS (nitrate, potassium, phosphate) from soil. The soil concentration is often LOWER than the inside of the root cell, so it requires active transport.
• Glucose absorption from gut into intestinal cells — even when blood glucose is high, the cell uses active transport to get all available glucose.
• Sodium-potassium pump in nerve cells (essential for nerve impulses).

Feature	Diffusion	Osmosis	Active transport
What moves	Any solute	Water	Any solute
Direction	High → low	Dilute → concentrated	Low → high (AGAINST)
Energy needed	No	No	YES (ATP)
Carrier protein needed	Sometimes	No	YES

Because active transport needs ATP, cells doing a lot of it have many MITOCHONDRIA (e.g. root hair cells, cells lining the small intestine).