Organisation

What you'll learn

Organisation in biology describes how cells form tissues, tissues form organs, and organs form organ systems. This topic explores the levels of organisation in both animals and plants, with detailed focus on the human digestive and circulatory systems. You'll learn how enzymes catalyse reactions, how nutrients are absorbed, and how the heart and blood vessels transport substances around the body.

Key terms and definitions

Tissue — a group of similar cells working together to perform a specific function

Organ — a structure made of different tissues working together to perform a particular function

Organ system — a group of organs working together to perform a specific physiological function

Enzyme — a biological catalyst that speeds up chemical reactions without being changed or used up

Active site — the region on an enzyme where substrate molecules bind

Catalyst — a substance that increases the rate of a chemical reaction without being changed or used up in the process

Diffusion — the spreading out of particles from an area of high concentration to an area of low concentration

Villi — small finger-like projections in the small intestine that increase surface area for absorption

Core concepts

Levels of organisation in animals

The human body is organised in a hierarchical structure, from smallest to largest:

Cells are the basic building blocks. Examples include muscle cells, nerve cells, and epithelial cells. Each cell type is adapted to its specific function.

Tissues form when similar cells group together. The four main tissue types in animals are:

• Muscular tissue — contracts to cause movement
• Glandular tissue — produces and secretes substances such as enzymes or hormones
• Epithelial tissue — covers internal and external body surfaces
• Nervous tissue — transmits electrical impulses

Organs contain multiple tissue types working together. The stomach, for example, contains:

• Muscular tissue to churn food
• Glandular tissue to produce digestive enzymes and acid
• Epithelial tissue to line the stomach wall

Organ systems are groups of organs with related functions. The digestive system includes the mouth, oesophagus, stomach, small intestine, large intestine, liver, and pancreas, all working together to digest food and absorb nutrients.

The human digestive system

The digestive system breaks down large insoluble food molecules into small soluble molecules that can be absorbed into the bloodstream.

Structure and function:

• Mouth — mechanical digestion (chewing) and chemical digestion (amylase in saliva breaks down starch)
• Oesophagus — muscular tube that moves food to the stomach using peristalsis
• Stomach — muscular organ that churns food, produces hydrochloric acid (pH 2) to kill bacteria and provide optimum pH for pepsin, and produces pepsin enzyme to digest proteins
• Small intestine — where most digestion and absorption occurs; enzymes complete the breakdown of proteins, fats, and carbohydrates; villi provide large surface area for absorption
• Large intestine — absorbs excess water from undigested food
• Pancreas — produces digestive enzymes (amylase, protease, lipase) that are released into the small intestine
• Liver — produces bile, which is stored in the gallbladder and emulsifies fats, increasing surface area for lipase action; bile also neutralises stomach acid

Enzymes and digestion

Enzyme specificity: Each enzyme catalyses a specific reaction due to the unique shape of its active site. The substrate molecule fits into the active site like a key in a lock (lock and key theory).

Factors affecting enzyme activity:

Temperature — as temperature increases, enzyme activity increases because molecules have more kinetic energy and collide more frequently. Above the optimum temperature (usually around 37°C for human enzymes), the enzyme denatures. The active site changes shape permanently, and the substrate no longer fits.

pH — each enzyme has an optimum pH. Changes in pH alter the shape of the active site. Pepsin works best at pH 2 (acidic), while amylase and lipase work best at pH 7-8 (neutral to slightly alkaline).

The three main digestive enzymes:

Carbohydrases (e.g., amylase) break down carbohydrates:

• Starch → maltose and other sugars
• Produced in salivary glands, pancreas, and small intestine

Proteases (e.g., pepsin) break down proteins:

• Proteins → amino acids
• Produced in stomach, pancreas, and small intestine

Lipases break down lipids (fats):

• Lipids → fatty acids + glycerol
• Produced in pancreas and small intestine

The heart and blood vessels

The heart is a muscular organ that pumps blood around the body in a double circulatory system.

Heart structure:

The heart has four chambers:

• Two atria (upper chambers) receive blood
• Two ventricles (lower chambers) pump blood out

The right side pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body. The left ventricle wall is thicker and more muscular because it must pump blood at higher pressure around the entire body.

Valves prevent backflow of blood. Atrioventricular valves separate atria from ventricles; semilunar valves are found at the exits of the ventricles.

Blood vessels:

Arteries:

• Carry blood away from the heart
• Thick muscular and elastic walls to withstand high pressure
• Small lumen (central cavity)
• Carry oxygenated blood (except pulmonary artery)

Veins:

• Carry blood back to the heart
• Thinner walls with less muscle and elastic tissue (lower pressure)
• Larger lumen
• Contain valves to prevent backflow
• Carry deoxygenated blood (except pulmonary vein)

Capillaries:

• Connect arteries to veins
• Walls one cell thick for efficient diffusion of substances
• Very small lumen (approximately the width of a red blood cell)
• Form networks at tissues and organs for exchange of materials

Coronary arteries supply the heart muscle with oxygenated blood. Blockage of these arteries leads to heart attack.

Blood composition and function

Blood is a tissue consisting of a liquid plasma carrying cells and dissolved substances.

Plasma — the liquid component (about 55% of blood volume) that transports:

• Carbon dioxide from tissues to lungs
• Soluble products of digestion (glucose, amino acids) from small intestine to tissues
• Urea from liver to kidneys
• Hormones
• Heat energy

Red blood cells:

• Transport oxygen from lungs to tissues
• Contain haemoglobin, which binds to oxygen forming oxyhaemoglobin
• Biconcave disc shape increases surface area for oxygen diffusion
• No nucleus — more space for haemoglobin

White blood cells:

• Part of immune system
• Phagocytes engulf and digest pathogens
• Lymphocytes produce antibodies

Platelets:

• Cell fragments that help blood clot at wounds
• Prevent excessive bleeding and entry of pathogens

Plant tissues and organs

Plants also show levels of organisation, though their organs differ from animals.

Plant tissues:

Epidermal tissue — covers surfaces of leaves, stems, and roots; waxy cuticle on leaves reduces water loss

Palisade mesophyll — found near upper surface of leaves; contains many chloroplasts for photosynthesis

Spongy mesophyll — has air spaces for gas diffusion; found below palisade layer

Xylem — transports water and mineral ions from roots to leaves; made of dead cells forming hollow tubes; provides structural support

Phloem — transports dissolved sugars from leaves to rest of plant in both directions (translocation); made of living cells

Meristem tissue — found at growing tips of shoots and roots; cells can differentiate into any type of plant cell

Plant organs:

Leaves — the main site of photosynthesis; adapted with large surface area, many chloroplasts in palisade cells, stomata for gas exchange

Stems — transport water, nutrients, and sugars; provide support; may store food

Roots — anchor plant; absorb water and mineral ions; may store food; root hair cells increase surface area for absorption

Factors affecting transpiration

Transpiration is the loss of water vapour from leaves through stomata. The rate is affected by:

Temperature — higher temperature increases kinetic energy of water molecules, increasing evaporation rate

Humidity — high humidity reduces the concentration gradient between leaf and air, decreasing transpiration rate

Wind speed — wind removes water vapour from leaf surface, maintaining concentration gradient and increasing transpiration rate

Light intensity — light causes stomata to open for gas exchange during photosynthesis, increasing transpiration rate

Worked examples

Question 1: A student investigated the effect of pH on amylase activity. They added amylase solution and starch solution to wells in a spotting tile containing iodine solution at different pH values. They recorded the time taken for the iodine to stop turning blue-black.

(a) Explain why the iodine eventually stops turning blue-black. [2 marks]

(b) The student found that at pH 7, the reaction took 60 seconds, but at pH 3 it took 240 seconds. Explain why. [3 marks]

Mark scheme answers:

(a)

• Amylase breaks down/digests all the starch [1 mark]
• So there is no starch present to react with iodine [1 mark]

(b)

• pH 7 is closer to the optimum pH for amylase [1 mark]
• At pH 3, the active site shape is altered/enzyme activity is reduced [1 mark]
• So there are fewer successful collisions between enzyme and substrate / the rate of reaction is slower [1 mark]

Question 2: Describe how the structure of an artery is related to its function. [4 marks]

Mark scheme answer:

• Thick muscular wall to withstand high blood pressure [1 mark]
• Elastic tissue allows artery to stretch and recoil, maintaining blood pressure [1 mark]
• Thick wall to prevent artery bursting [1 mark]
• Small lumen to maintain high pressure [1 mark]

Question 3: Explain why the left ventricle has a thicker wall than the right ventricle. [3 marks]

Mark scheme answer:

• The left ventricle pumps blood around the entire body / to all organs [1 mark]
• The right ventricle only pumps blood to the lungs [1 mark]
• The left ventricle needs to generate higher pressure, so requires more muscle [1 mark]

Common mistakes and how to avoid them

• Confusing organs with tissues. Remember: organs contain multiple tissue types. The stomach is an organ containing muscular, glandular, and epithelial tissues.

• Saying enzymes are "killed" by high temperature. Enzymes are not alive. Use the term "denatured" — the active site changes shape permanently so the substrate no longer fits.

• Thinking all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood from the heart to the lungs. Define arteries as vessels that carry blood away from the heart.

• Confusing the products of digestion. Carbohydrases break down carbohydrates to sugars; proteases break down proteins to amino acids; lipases break down lipids to fatty acids and glycerol. Learn these precisely.

• Reversing the functions of xylem and phloem. Xylem transports water up from roots; phloem transports dissolved sugars from leaves. Remember: phloem = food.

• Not explaining adaptations fully. Don't just describe a feature — explain how it helps the function. For example: "Villi have a large surface area, which increases the rate of absorption of nutrients into the blood."

Exam technique for "Organisation"

• "Describe" questions require you to state features or characteristics. Use clear, concise statements without explanation. For 4 marks, give 4 distinct points.

• "Explain" questions require reasoning. Use "because," "so," or "therefore" to link cause and effect. For 3 marks, typically give three linked points or three separate explanations.

• Structure-function questions are common. Always link structural features to their functional advantage. Use phrases like "which allows/enables" or "this means that."

• Extended response questions (6 marks) require detailed, logically structured answers covering multiple aspects. Include relevant detail about processes, adaptations, and mechanisms. Quality of written communication is assessed.

Quick revision summary

Organisation describes hierarchy: cells → tissues → organs → organ systems. The digestive system uses enzymes (carbohydrases, proteases, lipases) to break down food, with absorption in the small intestine via villi. Enzymes are affected by temperature and pH; denaturation occurs outside optimal conditions. The circulatory system includes the heart (four chambers, valves) and blood vessels (arteries, veins, capillaries) transporting blood components. Plants have specialised tissues (xylem, phloem, mesophyll) organised into organs (leaves, stems, roots). Transpiration rate varies with environmental factors.