OCR GCSE Biology Paper 3

B1 - What is the main structural advantage of a transmission electron microscope over a light microscope?

Higher resolution and higher magnification

B1 - What is the equation to calculate image magnification?

Magnification = Image size ÷ Actual size

B1 - Which sub-cellular structure is the site of aerobic respiration?

Mitochondria

B1 - Which sub-cellular structure controls the movement of substances into and out of a cell?

Cell membrane

B1 - What are plasmids in a prokaryotic cell?

Small loops of DNA found in the cytoplasm

B1 - What polymer is made of two strands forming a double helix?

DNA

B1 - What three components make up a single DNA nucleotide?

A common sugar, a phosphate group, and one of four bases

B1 - Which DNA bases pair complementary to each other?

Adenine (A) with Thymine (T), and Cytosine (C) with Guanine (G)

B1 - What is the process of copying a gene's DNA code into mRNA in the nucleus called?

Transcription

B1 - Where in the cell does translation of the mRNA sequence occur?

In the cytoplasm on ribosomes

B1 - What molecule carries specific amino acids to the ribosome during translation?

tRNA (transfer RNA)

B1 - How does the DNA code determine the primary structure of a protein?

The sequence of bases in triplets determines the sequence of amino acids

B1 - What hypothesis describes enzyme specificity based on active site shape?

Lock and Key hypothesis

B1 - What happens to an enzyme's active site when it is denatured by high temperature or extreme pH?

The active site changes shape so the substrate can no longer fit

B1 - Why does increasing substrate concentration eventually plateau the rate of an enzyme-controlled reaction?

All enzyme active sites become saturated/occupied

B1 - Is cellular respiration an endothermic or exothermic process?

Exothermic (releases energy to the surroundings)

B1 - What are the products of anaerobic respiration in yeast cells?

Ethanol and carbon dioxide

B1 - What product accumulates in human muscle cells during anaerobic respiration?

Lactic acid

B1 - Which organelle is the site of photosynthesis in plant cells?

Chloroplast

B1 - What pigment inside chloroplasts absorbs light energy for photosynthesis?

Chlorophyll

B1 - Is photosynthesis an endothermic or exothermic reaction?

Endothermic (takes in energy from surroundings/light)

B1 - What are the two main reactants needed for photosynthesis?

Carbon dioxide and water

B1 - State the inverse square law relationship between light intensity (I) and distance (d)

Light intensity is inversely proportional to distance squared (I = 1 / d²).

B1 - What happens to the rate of photosynthesis when a limiting factor (e.g., CO₂ concentration) reaches saturation?

The rate levels off/plateaus as another factor becomes limiting

B2 - By what process do water molecules move across a selectively permeable membrane?

Osmosis

B2 - Define active transport

Movement of substances against a concentration gradient using energy/ATP

B2 - How does water move into a root hair cell?

By osmosis down a water potential gradient (from higher to lower water potential)

B2 - What happens to an animal cell placed in a hypotonic (higher water potential) solution?

It absorbs water by osmosis, swells, and undergoes lysis/bursts

B2 - What occurs during the S phase of the cell cycle?

DNA synthesis / DNA replication

B2 - What type of cell division produces two genetically identical diploid daughter cells?

Mitosis

B2 - What is a stem cell?

An undifferentiated cell capable of dividing to produce specialized cells

B2 - What is the key difference between embryonic and adult stem cells in animals?

Embryonic stem cells can differentiate into any cell type; adult stem cells are limited

B2 - Where are stem cells located in plants?

Meristems

B2 - Why do large multicellular organisms require specialised exchange surfaces and transport systems?

They have a small surface area to volume ratio and long diffusion distances

B2 - How does the biconcave shape of a red blood cell adapt it to its function?

Provides a large surface area to volume ratio for rapid oxygen diffusion

B2 - Why do mature mammalian red blood cells lack a nucleus?

To create maximum space for haemoglobin

B2 - Which blood vessel has thick muscular and elastic walls to withstand high pressure?

Artery

B2 - What feature in veins prevents the backflow of blood under low pressure?

Valves

B2 - Why are capillary walls only one cell thick?

To provide a short diffusion distance for exchange of substances

B2 - Trace the path of blood returning from the body through the heart to the lungs

Vena cava → Right atrium → Right ventricle → Pulmonary artery

B2 - What tissue transports water and mineral ions upwards in a plant?

Xylem

B2 - What process describes the transport of dissolved sugars through phloem tissue?

Translocation

B2 - How do guard cells open stomata when light is present?

They take in water by osmosis, become turgid, and curve open due to unevenly thickened inner walls

B2 - Name three environmental factors that affect the rate of transpiration

Temperature, light intensity, and air movement (wind)

B2 - What piece of apparatus is used to estimate the rate of transpiration by measuring water uptake?

Potometer

B3 - Name the components of the central nervous system (CNS)

Brain and spinal cord

B3 - What is the sequence of structures in a reflex arc?

Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector

B3 - What is a synapse?

A junction/gap between two neurones

B3 - How is an electrical impulse transmitted across a synaptic gap?

By diffusion of chemical neurotransmitters

B3 - Which part of the eye controls the amount of light entering the pupil?

Iris

B3 - How does the eye focus on a distant object (accommodation)?

Ciliary muscles relax, suspensory ligaments pull tight, making the lens thin and flat

B3 - What type of lens is used to correct short-sightedness (myopia)?

Concave (divergent) lens - Short people live in Caves

B3 - Which region of the brain is responsible for consciousness, intelligence, memory, and language?

Cerebral cortex / Cerebrum

B3 - Which region of the brain controls unconscious activities like heart rate and breathing rate?

Medulla

B3 - How do hormones travel from endocrine glands to their target organs?

In the blood / blood plasma

B3 - What is the primary role of adrenaline released by the adrenal glands?

Prepares the body for 'fight or flight' by increasing heart rate and blood glucose

B3 - What negative feedback mechanism maintains metabolic rate via the thyroid gland?

Release of thyroxine

B3 - Which hormone stimulates egg maturation in the follicles of the ovary?

FSH (Follicle Stimulating Hormone)

B3 - Which hormone triggers ovulation on around day 14 of the menstrual cycle?

LH (Luteinising Hormone)

B3 - What is the function of progesterone in the menstrual cycle?

Maintains the thickened lining of the uterus and inhibits FSH and LH release

B3 - How do auxins cause phototropism in plant shoots?

Auxins accumulate on the shaded side, causing cell elongation on that side so the shoot bends towards light

B3 - Which plant hormone is responsible for breaking seed dormancy and shoot elongation?

Gibberellins

B3 - Which gas hormone stimulates fruit ripening?

Ethene

B3 - Define homeostasis

The maintenance of a constant internal environment

B3 - How does vasodilation help lower body temperature when overheated?

Arterioles dilate, increasing blood flow through skin capillaries so more heat is radiated to surroundings

B3 - Which hormone is released by the pancreas when blood glucose levels are too high?

Insulin

B3 - How does glucagon increase blood glucose levels when they fall too low?

Stimulates liver and muscle cells to convert stored glycogen into glucose

B3 - What is the underlying cause of Type 1 diabetes?

The pancreas fails to produce sufficient insulin

B3 - What is the cause of Type 2 diabetes?

Body cells no longer respond/are resistant to insulin

B3 - What process occurs in the Bowman's capsule during urine formation in the kidney?

Ultrafiltration

B3 - Which part of the nephron reabsorbs all filtered glucose back into the blood?

Proximal convoluted tubule (by active transport)

B3 - Which part of the brain detects changes in blood water potential?

Hypothalamus

B3 - How does ADH affect the permeability of the kidney collecting ducts?

ADH increases permeability, causing more water to be reabsorbed into the blood