REVISION

🫀 1. Circulatory system

The big picture

Humans have a double circulatory system.

This means blood passes through the heart twice during one complete circuit:

1. Heart → lungs → heart

   • Pulmonary circulation

   • Blood picks up oxygen in the lungs.

2. Heart → body → heart

   • Systemic circulation

   • Oxygenated blood delivers oxygen to body cells.

⭐ Why double?
It allows blood to be pumped to the lungs to become oxygenated and then pumped around the body at high pressure.

The heart

The heart is a muscular organ that pumps blood around the body.

You need to know these chambers:

• Right atrium – receives deoxygenated blood from the body.

• Right ventricle – pumps deoxygenated blood to the lungs.

• Left atrium – receives oxygenated blood from the lungs.

• Left ventricle – pumps oxygenated blood to the rest of the body.

⭐ The left ventricle has a much thicker muscular wall than the right ventricle because it needs to pump blood around the entire body, whereas the right ventricle only pumps blood to the lungs.

Blood pathway

Memorise this:

Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body

That's a VERY common exam question.

Blood vessels connected to the heart

AQA specifically requires these names:

⭐ Trick:
An artery is defined by carrying blood away from the heart, NOT necessarily by carrying oxygenated blood.

Therefore:

• Pulmonary artery = deoxygenated

• Pulmonary vein = oxygenated

AQA explicitly limits the named heart vessels to these. You don't need to learn the names of individual heart valves. AQA

❤ Heart valves

Valves prevent the backflow of blood.

So blood is kept moving in one direction.

You don't need to memorise the names of the individual valves for AQA GCSE Biology. AQA

⚡ Pacemaker

The heart's natural resting rate is controlled by a group of cells in the right atrium.

These cells act as a pacemaker.

They produce electrical impulses that cause the heart muscles to contract.

If someone's heart rate is irregular, an artificial pacemaker can be implanted.

An artificial pacemaker is an electrical device that helps correct irregular heart rhythms. AQA

🩸 Blood vessels

There are three types:

Arteries

Carry blood away from the heart.

Adaptations:

• Thick muscular walls → withstand high blood pressure.

• Elastic walls → stretch and recoil as blood is pumped through.

• Small lumen → helps maintain high pressure.

Usually carry oxygenated blood, except pulmonary arteries.

Veins

Carry blood towards the heart.

Adaptations:

• Thin walls because blood is at lower pressure.

• Large lumen → reduces resistance to blood flow.

• Valves → prevent backflow.

Usually carry deoxygenated blood, except pulmonary veins.

Capillaries

Tiny vessels connecting arteries and veins.

Adaptations:

• Walls are one cell thick → very short diffusion distance.

• Very narrow → red blood cells pass close to the walls.

• Form extensive networks → large surface area for exchange.

They allow substances such as oxygen, glucose and carbon dioxide to be exchanged between blood and tissues.

🩸 Components of blood

Blood is a tissue made of plasma with cells and platelets suspended in it. AQA

Red blood cells

Function:

⭐ Transport oxygen.

Adaptations:

• Biconcave shape → large surface area for oxygen exchange.

• No nucleus → more room for haemoglobin.

• Contain haemoglobin → binds to oxygen.

• Small and flexible → can squeeze through narrow capillaries.

Haemoglobin

In the lungs:

haemoglobin + oxygen → oxyhaemoglobin

In body tissues, oxyhaemoglobin releases oxygen.

So:

Lungs: haemoglobin loads oxygen
Body tissues: haemoglobin unloads oxygen

⭐ Oxygen is therefore transported in the blood by haemoglobin in red blood cells.

White blood cells

Defend against pathogens.

They can:

• Carry out phagocytosis → engulf and digest pathogens.

• Produce antibodies.

• Produce antitoxins.

Platelets

Small fragments involved in blood clotting.

They help form clots, preventing:

• excessive blood loss

• pathogens entering through wounds.

Plasma

The liquid part of blood.

It transports dissolved substances around the body, including:

• glucose

• amino acids

• urea

• carbon dioxide

• hormones

• heat.

🫁 How oxygen gets to your cells

This chain is worth memorising:

Oxygen breathed into lungs → alveoli → diffuses into blood → haemoglobin in red blood cells binds oxygen → oxygenated blood travels to heart → heart pumps it around body → oxygen leaves blood at tissues → oxygen enters cells → used in aerobic respiration.

At the muscles:

glucose + oxygen → carbon dioxide + water + energy

That energy is used for things such as muscle contraction.

🫁 Alveoli

Gas exchange occurs in the alveoli.

They are adapted for rapid diffusion because they have:

• ⭐ Large surface area

• ⭐ Very thin walls – one cell thick

• ⭐ Good blood supply

• ⭐ Moist surface

Therefore oxygen has a short diffusion distance and a steep concentration gradient can be maintained.

🏃 2. Effects of exercise

During exercise, muscles require more energy.

Therefore the rate of respiration increases.

Your body responds by increasing:

• ⭐ heart rate

• ⭐ breathing rate

• ⭐ breath volume

This supplies muscles with more oxygenated blood. AQA

Why does heart rate increase?

More blood needs to reach the muscles per minute.

More blood = more oxygen and glucose delivered = more respiration = more energy available.

Why does breathing rate increase?

More oxygen needs to enter the lungs and more carbon dioxide needs to be removed.

Why does breath volume increase?

Each breath brings more air into the lungs, increasing gas exchange.

😵 Anaerobic respiration and oxygen debt

During very vigorous exercise, muscles may not receive enough oxygen for their energy requirements.

They begin using anaerobic respiration.

In muscles:

glucose → lactic acid + energy

⭐ Anaerobic respiration releases much less energy than aerobic respiration.

Lactic acid builds up in muscles.

This causes muscle fatigue and eventually muscles stop contracting efficiently.

Oxygen debt

After exercise, the body needs extra oxygen to deal with the lactic acid that has accumulated.

This is called oxygen debt.

⭐ HT: Blood transports lactic acid to the liver, where it is converted back into glucose. The extra oxygen required afterwards is the oxygen debt. AQA

🌱 3. Organisation — Plants

AQA calls this Plant tissues, organs and systems.

The key plant tissues you need are:

• Epidermal tissue

• Palisade mesophyll

• Spongy mesophyll

• Xylem

• Phloem

• Meristem tissue

The leaf is a plant organ. AQA

🍃 Leaf tissues

Epidermis

Forms the outer layer of the leaf.

Functions:

• Protects the leaf.

• Helps reduce water loss.

• Allows light through to photosynthetic cells.

The upper epidermis often has a waxy cuticle, which reduces evaporation.

Palisade mesophyll

Found near the top of the leaf.

Adaptations:

• Many chloroplasts → absorbs lots of light.

• Cells are packed closely together.

• Positioned near the upper surface → receives lots of light.

⭐ Main function = photosynthesis.

Spongy mesophyll

Found underneath the palisade layer.

Contains lots of air spaces.

These allow gases to move through the leaf.

⭐ Main function = facilitate gas exchange and also carry out some photosynthesis.

🌬 Stomata and guard cells

Stomata are tiny pores, usually found in the epidermis.

They allow:

• carbon dioxide to enter

• oxygen to leave

• water vapour to leave.

Each stoma is surrounded by guard cells.

Guard cells control whether the stomata are open or closed.

⭐ Stomata therefore control gas exchange and water loss. AQA

💧 Root hair cells

Root hair cells absorb substances from the soil.

Water

Water enters by osmosis.

Root hair cells have a large surface area because of their long hair-like extension.

This increases the rate of water uptake.

Mineral ions

Mineral ions are absorbed by active transport.

Active transport:

• moves substances from low concentration → high concentration

• requires energy from respiration.

⭐ Don't mix this up:

Water → osmosis

Mineral ions → active transport

🚰 Xylem

Xylem transports:

⭐ water + mineral ions

Direction:

Roots → stem → leaves

Xylem vessels are:

• hollow tubes

• strengthened with lignin

• adapted to transport water in the transpiration stream.

Lignin also helps prevent the vessels collapsing.

🍬 Phloem

Phloem transports:

⭐ dissolved sugars

Sugars are transported from the leaves to other parts of the plant for:

• immediate use

• storage.

The movement of food molecules through phloem is called:

⭐ TRANSLOCATION

Phloem consists of tubes of elongated cells with pores between cells.

Don't confuse:

• Xylem → water + mineral ions

• Phloem → dissolved sugars

💨 Transpiration

Transpiration = loss of water vapour from the plant, mainly through the stomata in leaves.

Basic sequence:

Water absorbed by roots → travels up xylem → reaches leaves → evaporates from cells → water vapour diffuses out through stomata.

This creates a transpiration stream pulling water up through the xylem.

🌡 Factors affecting transpiration

You need to know four:

Higher temperature → higher transpiration

Water molecules have more kinetic energy → evaporation happens faster.

Higher light intensity → higher transpiration

Stomata tend to open for photosynthesis → more water vapour can escape.

Higher air movement → higher transpiration

Moving air removes water vapour around the leaf → maintains a concentration gradient → faster diffusion.

Higher humidity → lower transpiration

The air already contains lots of water vapour → smaller concentration gradient → slower diffusion.

⭐ Memorise this table

AQA specifically requires these four factors. AQA

🌱 Meristem tissue

Found at the growing tips of shoots and roots.

Meristem cells can divide and differentiate to produce different types of plant cells.

⭐ Plants retain the ability to produce new specialised cells throughout their lives.

🦠 4. Antibiotics

Antibiotics are medicines used to treat bacterial diseases.

Example:

penicillin

⭐ Antibiotics kill bacteria, NOT viruses.

This is because viruses reproduce inside cells and it is difficult to destroy viruses without damaging the body's own cells. AQA

Antibiotics vs painkillers

Antibiotics: kill bacteria.

Painkillers: reduce symptoms such as pain but do not kill pathogens.

🧬 Antibiotic resistance

This is extremely important.

Some bacteria have mutations that make them resistant to an antibiotic.

When an antibiotic is used:

1. Most non-resistant bacteria are killed.

2. Resistant bacteria survive.

3. Resistant bacteria reproduce.

4. They pass on the resistance genes.

5. The population becomes more resistant.

⭐ Important wording:
The antibiotic does not cause the useful mutation.

The resistant bacteria were already present due to random mutation. The antibiotic provides a selection pressure.

🧫 5. Culturing microorganisms

Bacteria reproduce by binary fission.

Under suitable conditions, some bacteria can divide approximately every 20 minutes. AQA

Bacteria can be grown:

• in a nutrient broth

• as colonies on an agar plate.

A bacterial colony is a visible group of bacteria produced by repeated cell division.

🧪 Aseptic technique

Aseptic technique is used to prevent contamination by unwanted microorganisms.

You need to know why these things are done:

Petri dishes and culture medium are sterilised

→ kills microorganisms already present.

Inoculating loop is sterilised

→ prevents unwanted microorganisms being transferred.

AQA specifies sterilising the loop by passing it through a flame.

Petri dish lid is secured with adhesive tape

→ reduces contamination from microorganisms in the air.

Dish stored upside down

→ reduces condensation dripping onto the culture.

Incubation at maximum 25°C in school/college laboratories

→ allows bacterial growth while reducing the risk of growing harmful human pathogens. AQA

⭐ Safety point: Don't say "incubate at body temperature" in a school practical. AQA specifies maximum 25°C.

🧫 Antibiotic practical

This is an AQA required practical:

Investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition. AQA

Method idea

1. Spread bacteria evenly across an agar plate.

2. Place antibiotic discs onto the agar.

3. Incubate the plate.

4. Bacteria grow around the discs.

5. If the antibiotic kills/inhibits bacteria, a clear area appears.

6. Measure the zone of inhibition.

Zone of inhibition

The clear area around an antibiotic disc where bacteria have not grown.

⭐ Larger zone = antibiotic more effective against that bacterial strain.

📏 Calculating the area of a zone

If you're given the radius:

Area = πr²

If you're given the diameter:

radius = diameter ÷ 2

then:

Area = π(d/2)²

AQA specifically expects you to be able to calculate the cross-sectional area of colonies/clear zones using πr². AQA

🧪 Making the antibiotic experiment good

Independent variable

What you change.

Example:

Type/concentration of antibiotic

Dependent variable

What you measure.

Example:

Area of zone of inhibition

Control variables

Keep the same:

• amount/concentration of bacteria

• agar

• temperature

• incubation time

• size of antibiotic discs

• volume/concentration of antibiotic where appropriate.

Why repeat?

To:

• identify anomalous results

• calculate a mean

• improve reliability.

Why use an uncontaminated culture?

Because other microorganisms could grow and affect the results, making it impossible to know whether the antibiotic affected the intended bacteria. AQA

🧬 6. Monoclonal antibodies ⭐ HT

Important: In the AQA specification, monoclonal antibodies are Higher Tier only. AQA

What are they?

Monoclonal antibodies are antibodies produced from a single clone of cells.

They are specific to one binding site on one protein antigen.

Therefore, they can target a specific chemical or specific cell.

🧪 How monoclonal antibodies are made

This sequence is THE thing to memorise:

1. Stimulate mouse lymphocytes

A mouse is stimulated so its lymphocytes produce the desired antibody.

2. Obtain the lymphocytes

These lymphocytes produce the specific antibody.

3. Fuse lymphocyte + tumour cell

They are combined to form a:

⭐ HYBRIDOMA

4. Why a hybridoma?

It has two useful properties:

• the lymphocyte produces the desired antibody

• the tumour cell allows it to divide repeatedly

So the hybridoma can:

divide + produce the antibody

5. Clone the hybridoma

A single hybridoma cell is cloned.

This produces many genetically identical cells.

6. Collect and purify antibodies

Large quantities of identical monoclonal antibodies are produced, collected and purified.

⭐ Sequence:

Mouse lymphocyte → fuse with tumour cell → hybridoma → clone → lots of identical cells → collect/purify antibodies

That sequence is worth learning almost word-for-word. AQA

🎯 Uses of monoclonal antibodies

You need to know examples, but AQA says you don't need to memorise specific named tests/treatments. AQA

1. Pregnancy tests

Monoclonal antibodies can detect a particular hormone associated with pregnancy.

If the hormone is present, the antibody binds to it and produces a visible result.

2. Measuring substances

They can be used in laboratories to measure levels of:

• hormones

• chemicals in blood.

They can also detect pathogens.

3. Research

A monoclonal antibody can be attached to a fluorescent dye.

It binds to a specific molecule.

Scientists can then locate that molecule in a cell/tissue.

4. Treating cancer

A monoclonal antibody can be attached to:

• a radioactive substance

• a toxic drug

• a chemical that stops cells growing/dividing.

Because the antibody is specific, it targets the cancer cells and delivers the substance to them.

⭐ The advantage is that it can reduce damage to healthy cells compared with targeting the whole body.

⚠ Disadvantages of monoclonal antibodies

AQA specifically says monoclonal antibodies have caused more side effects than expected and haven't been as widely used as initially hoped. AQA

So if asked to evaluate them:

Advantages

• Very specific.

• Can target particular cells/chemicals.

• Useful for diagnosis.

• Useful in research.

• Can deliver treatments directly to particular cells.

Disadvantages

• Can cause side effects.

• Some treatments may damage healthy cells.

• Their use hasn't been as widespread as originally expected.

• Ethical issues can arise from producing antibodies using animals.