BIOLOGY OVERVIEW

What does MRS HGREN stand for?

Movement, Respiration, Sensitivity, Homeostasis, Growth, Reproduction, Excretion, Nutrition — the 8 characteristics of living organisms.

What is homeostasis?

The maintenance of a constant internal environment, e.g. controlling body temperature and water content.

What is excretion?

The removal of waste products of cellular processes, e.g. CO₂ from respiration.

What is a eukaryotic organism?

An organism with cells that contain a true nucleus and membrane-bound organelles (e.g. mitochondria). Includes plants, animals, fungi and protoctists.

What are the key features of fungi?

Multicellular or unicellular; chitin cell walls; mycelium of hyphae; saprophytic nutrition (extracellular enzymes on dead material); store glycogen. Examples: yeast, Mucor.

What is a prokaryote? Give an example.

A cell without a true nucleus. DNA exists as a large circle + smaller plasmids in the cytoplasm. Cell wall made of murein. Example: bacteria (Lactobacillus, Pneumococcus).

What is a pathogen?

A microorganism that causes infectious disease. Can be fungi, bacteria, protoctists or viruses.

Why are viruses NOT classified as living?

They do not fulfil many of the 8 characteristics of life. They are particles containing DNA/RNA inside a protein coat (capsid). They can only replicate inside a host cell.

Which protoctist causes malaria?

Plasmodium (transmitted by Anopheles mosquito).

What are the 3 key features of protoctists?

Simple organisms with nuclei; cannot be classified as fungi, animals or plants; mostly unicellular. Examples: Amoeba, Chlorella, Plasmodium.

What is the hierarchy of organisation in a multicellular organism?

Organelles → Cells → Tissues → Organs → Organ Systems → Organism.

What are the 3 organelles found ONLY in plant cells?

Chloroplast, Cell Wall (cellulose), and Vacuole.

What is the function of mitochondria?

Site of aerobic respiration — produces ATP for the cell.

What is the function of ribosomes?

Site of protein synthesis.

What is the function of the cell membrane?

Controls the entry and exit of substances (partially permeable).

What is the function of the nucleus?

Contains genetic material (DNA) and controls the activity of the cell.

What is the function of the chloroplast?

Site of photosynthesis; contains chlorophyll which absorbs light energy.

What is the function of the vacuole in plant cells?

Contains cell sap (solution of sugars and salts); helps provide support to the cell.

Which biological molecule contains nitrogen?

Proteins (made of amino acids). Carbohydrates and lipids only contain C, H and O.

What are the subunits of: carbohydrates, proteins, and lipids?

Carbohydrates → simple sugars (e.g. glucose). Proteins → amino acids. Lipids → 1 glycerol + 3 fatty acids.

What is the difference between starch and glycogen?

Starch stores glucose in plant cells. Glycogen stores glucose in animal cells. Both are complex carbohydrates (polysaccharides).

What is the Benedict's test and what does a positive result look like?

Add Benedict's solution to the sample and heat at 80°C for 5 minutes. Positive = colour changes from blue → green → orange → brick red (glucose present).

How do you test for starch and what is a positive result?

Add iodine solution. Positive = changes from orange/brown to blue/black.

How do you test for protein and what is a positive result?

Add Biuret solution (NaOH + CuSO₄). Positive = blue changes to lilac/purple.

How do you test for lipids (emulsion test)?

Add ethanol to food sample, shake. Add equal volume of water. Positive = cloudy white emulsion forms.

What is the deficiency disorder for Vitamin C?

Scurvy (affects skin and gums). Sources: fruit and vegetables.

What is Vitamin D needed for, and what is its deficiency disease?

Needed for absorbing calcium from the diet. Deficiency causes rickets. Sources: eggs, sunlight.

Why is iron important in the diet?

Iron is needed to make haemoglobin in red blood cells. Deficiency causes anaemia.

What is an enzyme?

A biological catalyst — a protein that speeds up metabolic reactions without being used up. Each enzyme is specific to one reaction (lock and key model).

Explain the lock and key model.

The enzyme has an active site with a specific shape. Only a complementary-shaped substrate can bind to it, forming an enzyme–substrate complex. After reaction, product is released.

What happens to an enzyme above its optimum temperature?

The active site changes shape — the enzyme is DENATURED. It can no longer form enzyme–substrate complexes and no product is made. This is permanent.

What happens to enzyme activity below optimum temperature?

Fewer collisions between enzyme and substrate (less kinetic energy) → slower rate of reaction.

How does pH affect enzyme activity?

Each enzyme has an optimum pH. Deviation from optimum alters the active site shape → enzyme becomes denatured → reaction rate falls.

What enzyme digests starch → maltose?

Amylase (produced in salivary glands and pancreas).

What enzyme digests maltose → glucose?

Maltase (in the ileum).

What enzyme digests proteins → amino acids?

Protease (pepsin in stomach; also produced by pancreas).

What enzyme digests lipids → fatty acids + glycerol?

Lipase (produced by pancreas, acts in duodenum).

Define diffusion.

The net movement of particles from a region of higher concentration to a region of lower concentration. Requires no energy.

What 4 factors increase the rate of diffusion?

1. Higher temperature 2. Steeper concentration gradient 3. Shorter diffusion distance 4. Larger surface area : volume ratio

Define osmosis.

The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

What happens to an animal cell placed in distilled water?

Water moves in by osmosis (higher water conc outside) → cell swells and bursts.

What happens to a plant cell placed in distilled water?

Water moves in by osmosis → cell becomes turgid (rigid, pushes against cell wall). Does NOT burst because of the cell wall.

What is plasmolysis?

When a plant cell loses water by osmosis (placed in concentrated solution) → cytoplasm shrinks and pulls away from the cell wall.

Define active transport.

Movement of molecules from low → high concentration (against the concentration gradient). Requires energy (ATP) from respiration.

Give an example of active transport in plants.

Mineral ions (e.g. nitrates, magnesium) are actively transported into root hair cells from the soil, even though their concentration in the soil is lower.

Write the word equation for photosynthesis.

carbon dioxide + water → glucose + oxygen (requires light energy and chlorophyll)

Write the balanced symbol equation for photosynthesis.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What is a limiting factor?

A factor that limits the rate of a process controlled by more than one variable. For photosynthesis: light intensity, CO₂ concentration, or temperature.

Why do palisade cells sit near the upper surface of a leaf?

To maximise exposure to light intensity for photosynthesis — they contain most of the chloroplasts.

What is the role of stomata in leaves?

Allow CO₂ to diffuse in and O₂ to diffuse out of the leaf.

What mineral ion is needed for chlorophyll production?

Magnesium (Mg²⁺). Deficiency → yellow leaves (chlorosis).

What mineral ion is needed to make amino acids in plants?

Nitrate (NO₃⁻). Deficiency → stunted growth and yellow leaves.

What is the role of the stomach in digestion?

HCl kills bacteria; pepsin (protease) digests proteins to amino acids; muscular wall churns food (mechanical digestion).

What is peristalsis?

A wave of muscular contraction of the gut wall that moves food along the alimentary canal.

Where is bile produced and stored?

Produced in the liver; stored in the gall bladder; released into the duodenum.

What are the two functions of bile?

1. Neutralises stomach acid (provides optimum pH for duodenum enzymes) 2. Emulsifies lipids (breaks large fat globules into tiny droplets, increasing SA for lipase)

How is the ileum adapted for absorption?

Villi (+ microvilli) = large surface area; only 2 cells thick = short diffusion distance; rich blood supply = high concentration gradient maintained.

What is the difference between digestion and absorption?

Digestion = breaking large insoluble molecules into small soluble ones. Absorption = food molecules passing from the gut into the bloodstream (in ileum).

What do lacteals absorb?

Fatty acids (lipid digestion products) — lacteals are branches of the lymphatic system inside the villi.

Write the word equation for aerobic respiration.

glucose + oxygen → carbon dioxide + water (+energy released as ATP)

Write the balanced symbol equation for aerobic respiration.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

How many ATPs are produced per glucose in aerobic vs anaerobic respiration?

Aerobic = 32 ATPs. Anaerobic = 2 ATPs. Aerobic is far more efficient.

Write the word equation for anaerobic respiration in animals.

glucose → lactic acid

Write the word equation for anaerobic respiration in plants/yeast.

glucose → ethanol + carbon dioxide

What is oxygen debt?

After anaerobic exercise, lactic acid is transported to the liver and broken down using oxygen. This extra oxygen needed is the oxygen debt — it's why you breathe rapidly after sprinting.

Name 3 processes that use ATP in cells.

1. Movement (muscle contraction) 2. Active transport 3. Biosynthesis (making large molecules from small ones, e.g. proteins)

Describe breathing in (inhalation).

Intercostal muscles AND diaphragm both CONTRACT → ribs move up & out, diaphragm flattens → volume increases → pressure decreases → air flows in.

Describe breathing out (exhalation).

Intercostal muscles AND diaphragm RELAX → ribs move down & in → volume decreases → pressure increases → air flows out.

How are alveoli adapted for gas exchange?

Millions → huge SA. Walls one cell thick → short diffusion distance. Ventilation + blood flow → maintained steep concentration gradient.

How does smoking cause bronchitis?

Cigarette smoke stops cilia beating → mucus builds up → bacteria not removed → infection → bronchitis.

What is emphysema?

White blood cells digest the walls of alveoli to fight infection → loss of surface area for gas exchange → shortness of breath.

How does nicotine cause coronary heart disease?

Nicotine raises blood pressure → damages artery lining → fatty plaques deposit in coronary arteries → arteries narrow → reduced blood flow → heart attack.

What is the function of rings of cartilage in the trachea?

They hold the trachea open, preventing it from collapsing when you breathe in.

What is transported in blood plasma?

CO₂, digested food (glucose, amino acids), urea, hormones, and heat energy.

What adaptations make red blood cells ideal for oxygen transport?

Biconcave shape (large SA); no nucleus (more space for haemoglobin); haemoglobin binds O₂ to form oxyhaemoglobin in lungs, releases O₂ at respiring tissues.

What is the difference between phagocytes and lymphocytes?

Phagocytes: engulf and digest pathogens (non-specific). Lymphocytes: produce antibodies specific to antigens on pathogens (specific immune response).

Why is the left ventricle wall thicker than the right?

The left ventricle pumps blood around the whole body (higher pressure needed). The right ventricle only pumps to the nearby lungs.

What do valves in the heart do?

Prevent backflow of blood, ensuring blood flows in one direction.

How does exercise increase heart rate?

Muscle uses O₂ → produces CO₂ → detected by receptors in carotid artery/aorta → nerve signal to brain → brain signals heart → faster, stronger contractions.

Compare the structure of arteries, veins, and capillaries.

Arteries: thick elastic walls, narrow lumen, high pressure. Veins: thin walls, wide lumen, valves, low pressure. Capillaries: one cell thick walls, permeable, short diffusion distance.

What is the path of a reflex arc?

Receptor → Sensory neurone → Relay neurone (spinal cord) → Motor neurone → Effector (muscle). Bypasses conscious brain for speed.

How does a synapse work?

Electrical impulse reaches end of neurone → releases neurotransmitter into gap → diffuses across → binds to receptor on next neurone → triggers new electrical impulse.

How does the eye focus on a near object?

Ciliary muscle contracts → suspensory ligaments slacken → lens becomes fat/round → more refraction.

How does the eye focus on a distant object?

Ciliary muscle relaxes → suspensory ligaments taut → lens becomes thin/flat → less refraction.

What is the pupil reflex in bright light?

Circular muscles of iris CONTRACT; radial muscles relax → pupil constricts → less light enters retina.

What are rod and cone cells?

Rods: detect low light levels, work in greyscale (peripheral retina). Cones: detect bright light, give colour vision — 3 types (Red, Green, Blue), concentrated in fovea.

How does auxin cause phototropism in stems?

Auxin accumulates on the shaded side → more elongation on shaded side → stem bends towards light (positive phototropism).

Compare nervous and hormonal control.

Nervous: electrical impulses, rapid, short-lived, localised. Hormonal: chemical messengers in blood, slow, long-lasting, widespread effect.

What is negative feedback?

A mechanism where any change in internal conditions triggers a response to oppose that change, restoring the original state. Underpins homeostasis.

How does the body cool down when too hot?

Sweating (evaporation removes heat) + vasodilation (blood vessels widen → more blood near skin surface → more heat lost by conduction/radiation).

How does the body warm up when too cold?

Vasoconstriction (vessels narrow → less blood to skin → less heat loss) + shivering (muscle contraction generates heat) + piloerection (hairs trap insulating air).

What does insulin do?

Released by pancreas; increases uptake of glucose into liver and muscle cells; stimulates conversion of glucose → glycogen (glycogenesis). Lowers blood glucose.

What is the role of adrenaline?

Released from adrenal glands ('fight or flight'): increases heart rate, increases blood glucose, increases blood flow to muscles.

What controls the menstrual cycle?

Days 1–14: Oestrogen (repairs uterine lining). Day 14: Ovulation. Days 14–28: Progesterone (maintains lining). If no pregnancy, progesterone falls → menstruation.

What is the difference between sexual and asexual reproduction?

Sexual: 2 parents, gametes (meiosis), fertilisation, genetically unique offspring, variation. Asexual: 1 parent, mitosis, genetically identical offspring (clones), faster.

What conditions are needed for seed germination?

Warmth (enzyme activity), water (reactions in solution), oxygen (aerobic respiration for energy). Light is NOT required.

What is the role of the placenta?

Exchange surface between mother's and foetus's blood (via umbilical cord). Mother → foetus: O₂, glucose, amino acids, antibodies. Foetus → mother: CO₂, urea.

What protects the developing embryo?

Amniotic fluid (produced by amnion membrane) — cushions embryo from physical damage.

What is the difference between insect-pollinated and wind-pollinated flowers?

Insect: large colourful petals, sticky stigma inside flower. Wind: small drab petals, long filaments so anther hangs out, large feathery stigma.

What happens after pollination?

Pollen tube grows down through style → ovary → into ovule through micropyle → male nucleus fuses with female nucleus (fertilisation) → zygote → embryo → seed.