The cell cycle & mitosis

A human body cell has 46 chromosomes (23 pairs). New body cells are made by the cell cycle, which includes mitosis:

Key: before dividing, the cell grows and copies its DNA. In mitosis the copied chromosomes line up and separate into two new nuclei, each with the full 46 chromosomes. Edexcel names the stages: interphase, then mitosis (prophase, metaphase, anaphase, telophase), then cytokinesis, when the cytoplasm and cell membrane divide. Cancer is the result of changes in cells that lead to uncontrolled cell division.

What does mitosis make?

Your turn — counting cells

Growth in animals and plants

Growth is an increase in size. It involves cell division, cell enlargement (elongation in plants) and differentiation:

• Animals grow mostly by cell division (mitosis) followed by differentiation; most cells differentiate early and growth stops once the animal is mature (cell division continues for repair and replacement, and some stem cells remain).
• Plants grow throughout life. Cell division happens in meristems (root and shoot tips), then cells elongate to make the plant taller, and finally differentiate.

Percentile growth charts: a baby's mass or length is plotted against age and compared with percentile curves (e.g. the 50th percentile is the median). A child on the 75th percentile is heavier than 75% of children of that age. Doctors watch the trend over time, not a single point.

Reading a percentile chart

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. in bone marrow; form a limited range (mainly blood cells).
• Plant meristem cells — in root and shoot tips; can make any plant cell and can clone whole plants.

Uses: treating conditions such as diabetes and paralysis, and growing replacement tissue that won't be rejected if it is made from the patient's own cells. Cloned meristem cells let growers mass-produce identical plants, e.g. copies of a disease-resistant plant.

Risks & issues: ethical objections to using embryos; the risk of rejection or of passing on viral infection; and the small risk that stem cells could grow uncontrollably (tumours). You should be able to discuss both sides.

Where do the new cells in a cut come from?

How can you move before you think?

Which neurone?

Tap a description, then tap the neurone it belongs to.

The nervous system & neurones

The nervous system lets you detect and respond to changes (stimuli). It has the central nervous system (CNS) — the brain and spinal cord — and the nerves that connect to it. Signals travel as tiny electrical impulses along neurones.

• Sensory neurone — carries impulses from receptors (e.g. in the skin) to the CNS; it has a long dendron carrying impulses to the cell body.
• Relay neurone — carries impulses within the CNS.
• Motor neurone — carries impulses from the CNS to an effector (a muscle or gland).

The reflex arc

A reflex is a fast, automatic response that does not involve conscious thought, protecting the body from harm. Its pathway is the reflex arc:

Where two neurones meet there is a tiny gap called a synapse. The impulse cannot jump the gap electrically, so the first neurone releases a chemical neurotransmitter that diffuses across and starts an impulse in the next neurone.

Why it's fast: the reflex arc bypasses the conscious brain — the relay neurone in the spinal cord passes the signal straight to a motor neurone, so you pull your hand away before you feel the pain.

Crossing the synapse

How can you move before you think?

Regions of the brain

The brain is part of the CNS and coordinates the body. Key regions:

• Cerebral hemispheres (cerebrum) — the two large halves of the brain, with a folded outer layer (the cerebral cortex); control consciousness, intelligence, memory and language.
• Cerebellum — at the back; controls balance, posture and coordinated (fine) movement.
• Medulla oblongata — controls unconscious actions such as heart rate and breathing rate.

Studying the brain (higher tier): brain tissue is hard to reach inside the skull, so scientists use CT scans (X-ray images showing the brain's structure) and PET scans (a radioactive tracer shows which areas are active) to investigate brain function. Treating damage and disease in the brain and spinal cord is difficult: the tissue is delicate, damaged neurones are hard to repair, and tumours may be hard to reach without harming nearby tissue.

Your turn — a fraction of the cycle

Structure of the eye

The eye is a sense organ containing light receptors. Light is focused onto the retina, which sends impulses along the optic nerve to the brain.

• Cornea — transparent front; does most of the focusing (refraction).
• Iris — controls the size of the pupil, so it controls how much light enters.
• Lens — fine-focuses light onto the retina; its shape is changed by the ciliary muscles and suspensory ligaments.
• Retina — contains light receptors: rods (work in dim light) and cones (detect colour, in bright light).

Accommodation & correcting vision

Accommodation is how the eye changes the lens shape to focus on near or distant objects:

• Near object: ciliary muscles contract, suspensory ligaments slacken, the lens becomes fatter (more curved) — more refraction.
• Distant object: ciliary muscles relax, suspensory ligaments pull tight, the lens becomes thinner — less refraction.

Vision defects are corrected with lenses:

• Long-sightedness (hyperopia) — can't focus on near objects; corrected with a convex (converging) lens.
• Short-sightedness (myopia) — can't focus on distant objects; corrected with a concave (diverging) lens.

Also treatable by: contact lenses, laser eye surgery (reshaping the cornea) or a replacement lens. Cataracts (a cloudy lens) are corrected by replacing the lens with an artificial one. Colour blindness (faulty or missing cones) cannot be corrected.

Match each part to its job

Tap a part on the left, then its function on the right.

Your turn — chromosomes after mitosis

The big ideas to know

You've covered the core of Edexcel Topic 2 — Cells and control. Press Finish to see your score.