The cell cycle & mitosis

Body cells divide by mitosis during a sequence called the cell cycle:

• Stage 1 (interphase): the cell grows, makes more sub-cellular structures and copies its DNA so each chromosome becomes two identical copies.
• Stage 2 (mitosis): chromosomes line up and are pulled apart into two new nuclei.
• Stage 3: the cytoplasm and membrane split, giving two identical cells.

Result: mitosis makes two genetically identical daughter cells with the same chromosome number — used for growth, repair and asexual reproduction.

What does mitosis make?

Stem cells & their uses

A stem cell is an undifferentiated cell that can keep dividing and can differentiate into other cell types.

• Embryonic stem cells — from early embryos; can become almost any cell type.
• Adult stem cells — e.g. bone marrow; form a limited range (mostly blood cells).
• Plant meristems — in root and shoot tips; make any plant cell throughout life, so plants can be cloned.

Evaluate: stem cells could treat diabetes or paralysis, but there are ethical concerns about using embryos and a risk of viral contamination.

Where in a plant?

Diffusion, osmosis & active transport

• Diffusion — net movement of particles from high to low concentration. No energy needed.
• Osmosis — movement of water across a partially permeable membrane, dilute → concentrated. No energy needed.
• Active transport — moves substances against the gradient, so it needs energy from respiration.

Rate of transport increases with a steeper concentration gradient, higher temperature and a larger surface area. As organisms get bigger their surface area : volume ratio falls, so they need specialised exchange surfaces.

Osmosis practical: potato cylinders in different sugar concentrations — measure the % change in mass. Gains mass in dilute solutions, loses mass in concentrated ones.

Match each statement to its process

Tap a statement, then its matching process.

Your turn — osmosis % change

The heart & circulatory system

Humans have a double circulatory system: the heart pumps blood twice per circuit — once to the lungs and once to the rest of the body.

• Arteries — carry blood away from the heart; thick, muscular, elastic walls; high pressure.
• Veins — carry blood back to the heart; thinner walls, wide lumen and valves to stop backflow.
• Capillaries — one-cell-thick walls for fast exchange of substances with cells.

Blood: red blood cells (carry oxygen as oxyhaemoglobin), white blood cells (defence), platelets (clotting) and plasma (carries CO₂, glucose, urea).

Artery or vein?

Transport in plants

• Xylem — dead, hollow tubes that carry water and mineral ions from roots to leaves in the transpiration stream (one direction, up).
• Phloem — living tubes that carry dissolved sugars (made in the leaves) around the plant — this is translocation (both directions).
• Root hair cells have a large surface area to absorb water (osmosis) and mineral ions (active transport).

Transpiration — loss of water vapour from leaves through stomata. It is faster in higher temperature, more air movement, lower humidity and brighter light.

Xylem = water up; phloem = food both ways. Guard cells open and close the stomata to balance water loss and gas exchange.

Which tissue carries sugar?

Sort each into its group

Tap a feature, then tap the vessel or tissue it belongs to.

The big ideas to know

Press Finish to see your score.