From organelle to organism

Living things are organised in a clear hierarchy. Each level is built from the one before it:

• Organelles — tiny structures inside a cell (e.g. mitochondria, ribosomes).
• Cells — the smallest structural and functional unit of life.
• Tissues — groups of similar cells working together (e.g. muscle tissue).
• Organs — several tissues working together for a function (e.g. the heart).
• Organ systems — organs working together (e.g. the circulatory system).

Key idea: a specialised cell has a shape and contents suited to its job — e.g. a red blood cell has no nucleus, leaving more room for haemoglobin.

Which level?

The animal cell

A typical animal cell contains these organelles:

• Nucleus — controls the cell; contains DNA (the genetic material).
• Cytoplasm — jelly where most chemical reactions happen.
• Cell-surface membrane — controls what enters and leaves.
• Mitochondria — site of aerobic respiration (release energy).
• Ribosomes — where proteins are made.

The plant cell

A plant cell has everything an animal cell has, plus three extra features:

• Cell wall — made of cellulose; gives support and shape.
• Chloroplasts — contain chlorophyll; site of photosynthesis.
• Large permanent vacuole — filled with cell sap; keeps the cell turgid.

Also on 4BI1: a bacterial cell has a cell wall (not cellulose), cell-surface membrane, cytoplasm, plasmids and a circular chromosome of DNA — but no nucleus, mitochondria or chloroplasts.

Organelle ↔ function

Tap an organelle on the left, then its correct function on the right.

Magnification

4BI1 expects: magnification = image size ÷ real (actual) size.

Carbohydrates, proteins & lipids

The three main food groups are built from smaller units and specific elements:

• Carbohydrates — made of simple sugars (e.g. glucose); large ones include starch and glycogen. Elements: C, H, O.
• Proteins — chains of amino acids. Elements: C, H, O, N (and sometimes S).
• Lipids (fats & oils) — made of fatty acids and glycerol. Elements: C, H, O.

Watch out: the difference between carbohydrates/lipids and proteins is the nitrogen (N) — only proteins contain it.

Testing for food groups

• Benedict's (heat) → reducing sugar → brick-red precipitate.
• Iodine solution → starch → blue-black.
• Biuret → protein → purple/lilac.
• Emulsion test (ethanol then water) → lipid → milky-white emulsion.

Reading a food test

Enzymes: biological catalysts

Enzymes are proteins that speed up reactions (act as biological catalysts) without being used up. Each enzyme has a specific active site that fits only its substrate — the lock-and-key model.

Temperature & pH

Enzymes work best at an optimum temperature and pH:

• As temperature rises, molecules collide more → faster rate — up to the optimum (~37 °C in humans).
• Too hot: the active site changes shape and the enzyme is denatured — the substrate no longer fits.
• Too far from the optimum pH also denatures the enzyme.

Misconception buster: enzymes are not alive, so they do not "die" when heated — the active site simply denatures (changes shape permanently).

Name that word

Diffusion, osmosis & active transport

• Diffusion — net movement of particles from a region of high to low concentration, down a gradient. Passive (no energy).
• Osmosis — net movement of water molecules from a region of higher water potential to lower water potential, across a partially permeable membrane. Passive.
• Active transport — movement against the concentration gradient, using energy from respiration.

Misconception buster: osmosis moves only water — not dissolved solutes. Water moves from high water potential (dilute) to low water potential (concentrated).

Which way does water go?

Photosynthesis

Green plants make their own food by photosynthesis, using light energy trapped by chlorophyll:

The rate is controlled by limiting factors: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply caps the rate.

Plants also need mineral ions from the soil: nitrate (for amino acids/proteins) and magnesium (to make chlorophyll).

Inside a leaf

Diet & the alimentary canal

A balanced diet supplies carbohydrates, proteins, lipids, vitamins, minerals, water and fibre in the right amounts.

Food travels through the alimentary canal in order:

mouth → oesophagus → stomach → small intestine → large intestine → rectum

• Digestion breaks large insoluble molecules into small soluble ones using enzymes.
• Carbohydrases (e.g. amylase) → sugars; proteases → amino acids; lipases → fatty acids + glycerol.
• Bile (from the liver) emulsifies fats (bigger surface area) and neutralises stomach acid.
• Peristalsis — waves of muscle contraction push food along the gut.

Absorption & the villus

Digested food is absorbed in the small intestine, which is lined with millions of villi:

• Large surface area — many villi (and microvilli) for fast absorption.
• Thin wall (one cell thick) — short diffusion distance.
• Good blood supply — keeps the concentration gradient steep.

Why villi?

Releasing energy

Respiration releases energy from glucose in every living cell (plants and animals).

Key contrast: aerobic respiration releases much more energy per glucose than anaerobic. Anaerobic in muscle produces lactic acid; in yeast it produces ethanol + CO₂ (used in brewing and baking).

Which respiration?

Lungs & alveoli

In humans, gas exchange happens in the alveoli — tiny air sacs at the ends of the airways.

• Ventilation (breathing): the diaphragm and intercostal muscles change chest volume to move air in and out.
• Effect of exercise: breathing rate and depth increase to supply more O₂ and remove more CO₂.
• Plants exchange gases through the stomata in the leaf.

Where does gas exchange happen?

Tap the correct exchange surface for each organism.

Xylem, phloem & transpiration

• Xylem — carries water and mineral ions from roots to leaves (one direction, upward). Dead, hollow tubes.
• Phloem — carries dissolved sugars (sucrose) made in the leaves to the rest of the plant (translocation).
• Transpiration — evaporation of water from the leaves (through stomata) pulls the water column up the xylem.

Faster transpiration when: it is hotter, drier, windier or brighter. (A "TIME" way: Temperature, air movement, humidity, light.)

Name the tissue

The heart & circulation

Humans have a double circulatory system: blood passes through the heart twice per full circuit (once to the lungs, once to the body).

Vessels & blood

• Arteries — carry blood away from the heart, at high pressure; thick, muscular, elastic walls.
• Veins — carry blood back to the heart, at low pressure; have valves to stop backflow.
• Capillaries — one-cell-thick walls; where exchange with cells happens.

Blood contains: red blood cells (carry O₂ using haemoglobin), white blood cells (defence), platelets (clotting) and plasma (carries dissolved substances).

Misconception buster: arteries carry blood AWAY from the heart — not "always oxygenated". The pulmonary artery carries deoxygenated blood to the lungs.

Toward or away from the heart?

Tap a term, then the box it belongs in.

Getting rid of waste

Excretion = removing the waste products of metabolism.

• Plants — excrete oxygen (from photosynthesis) and carbon dioxide (from respiration) through the stomata.
• Humans — lungs: remove carbon dioxide.
• Humans — kidneys: remove urea (from breaking down excess amino acids in the liver) and excess water and salts, as urine.

The reflex arc

The nervous system detects stimuli with receptors and responds via effectors (muscles or glands). A reflex is a fast, automatic response:

Order of the reflex

Hormones & homeostasis

Hormones are chemical messengers made by glands and carried in the blood — slower but longer-lasting than nerves. e.g. adrenaline, insulin.

Homeostasis = keeping a stable internal environment:

• Body temperature — sweating and vasodilation cool you; shivering and vasoconstriction warm you.
• Blood glucose — insulin (from the pancreas) lowers blood glucose; when it is too high, glucose is stored as glycogen.

Plants respond too: tropisms are growth responses. Phototropism — shoots grow towards light; gravitropism — roots grow down. These are controlled by the hormone auxin.

Name the hormone

The big ideas to know

You've covered all ten sub-topics of Edexcel 4BI1 Section 2. Press Finish to see your score.