Cambridge International AS & A Level Biology: every topic, by paper

What is the function of the rough endoplasmic reticulum?

Synthesis and transport of proteins, via ribosomes on its surface.

What is the function of the smooth endoplasmic reticulum?

Synthesis of lipids and steroids.

What does the Golgi apparatus do?

Modifies, packages and sorts proteins and lipids into vesicles, e.g. for secretion.

What type of bond joins two monosaccharides?

A glycosidic bond, formed by a condensation reaction.

Distinguish α-glucose and β-glucose.

They differ in the position of the –OH group on carbon 1: below the ring in α-glucose, above it in β-glucose.

Which disaccharides form from glucose + glucose, glucose + fructose and glucose + galactose?

Maltose, sucrose and lactose.

What is an enzyme?

A globular protein that acts as a biological catalyst by lowering the activation energy of a reaction.

Compare the lock-and-key and induced-fit models.

Lock and key: the active site is a rigid, exact fit. Induced fit: the active site changes shape slightly as the substrate binds, forming a tighter fit.

Distinguish intracellular and extracellular enzymes.

Intracellular enzymes work inside cells (e.g. catalase); extracellular enzymes are secreted to work outside (e.g. amylase).

What is the fluid mosaic model?

A phospholipid bilayer with proteins scattered in it ("mosaic") that can move around ("fluid").

What is the role of cholesterol in membranes?

It regulates fluidity and stability of the membrane.

What are glycoproteins and glycolipids for?

Cell recognition and signalling (e.g. as antigens and receptors).

Name the stages of the cell cycle.

Interphase (G1, S, G2), mitosis, then cytokinesis.

What happens during the S phase?

DNA replication.

Describe prophase.

Chromosomes condense and become visible as two chromatids joined at a centromere; the nuclear envelope breaks down; spindle forms.

What are the components of a DNA nucleotide?

Deoxyribose sugar, a phosphate group and a nitrogenous base (A, T, C or G).

Distinguish purines and pyrimidines.

Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one.

How many hydrogen bonds are between A–T and C–G?

Two between A and T; three between C and G.

What does xylem transport?

Water and mineral ions from roots to leaves.

What does phloem transport?

Assimilates such as sucrose and amino acids, from sources to sinks.

How is xylem adapted?

Dead, hollow vessel elements with no end walls form continuous tubes; lignin strengthens and waterproofs them; pits allow sideways movement.

Why do mammals need a double circulation?

Blood returns to the heart after the lungs so it can be pumped at high pressure to the body.

How are arteries adapted to their function?

Thick walls with elastic tissue and smooth muscle withstand and maintain high pressure; narrow lumen.

How are veins adapted?

Thin walls, a wide lumen and valves prevent backflow of low-pressure blood.

Give three features of an efficient gas exchange surface.

Large surface area, thin (short diffusion distance) and a steep concentration gradient maintained.

What is the role of cartilage in the trachea?

C-shaped rings support the airway and stop it collapsing.

What do goblet cells and ciliated cells do?

Goblet cells secrete mucus to trap particles; cilia beat to move the mucus up to the throat.

What is a pathogen?

A microorganism that causes disease.

Name the pathogens causing cholera, malaria, TB and HIV/AIDS.

Cholera: Vibrio cholerae (bacterium). Malaria: Plasmodium (protoctist). TB: Mycobacterium tuberculosis (bacterium). HIV/AIDS: human immunodeficiency virus.

How is malaria transmitted?

By the female Anopheles mosquito, which acts as a vector.

Why do organisms need ATP?

ATP is the immediate energy source; hydrolysis to ADP and Pi releases energy in small, manageable amounts.

Name the four stages of aerobic respiration and where they occur.

Glycolysis (cytoplasm), link reaction and Krebs cycle (mitochondrial matrix), oxidative phosphorylation (inner mitochondrial membrane).

Summarise glycolysis.

Glucose is phosphorylated using 2 ATP, split into two triose phosphates, then oxidised to two pyruvate, giving a net 2 ATP and 2 reduced NAD.

Where do the light-dependent and light-independent reactions occur?

Light-dependent: thylakoid membranes. Light-independent (Calvin cycle): stroma.

What are the products of the light-dependent reaction?

ATP, reduced NADP and oxygen.

What is photolysis?

Splitting of water by light energy into H⁺, electrons and oxygen.

Define homeostasis.

Maintaining a constant internal environment within narrow limits.

What is negative feedback?

A change from the set point triggers a response that reverses the change.

Describe the structure of a nephron in order.

Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct.

Name the three types of neurone.

Sensory, relay (intermediate) and motor.

What is the resting potential and how is it maintained?

About −70 mV; the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in, and the membrane is more permeable to K⁺, which leaks out.

Describe depolarisation and repolarisation.

Voltage-gated Na⁺ channels open and Na⁺ floods in (depolarisation to about +40 mV); they close, K⁺ channels open and K⁺ leaves (repolarisation), overshooting (hyperpolarisation).

Define allele.

A different version of the same gene.

Distinguish genotype and phenotype.

Genotype: the alleles an organism has. Phenotype: its observable characteristics.

What is codominance?

Both alleles are expressed in the heterozygote, e.g. blood group AB.

Name the stages of the cell cycle.

Interphase (G1, S, G2), mitosis, then cytokinesis.

What happens during the S phase?

DNA replication.

Describe prophase.

Chromosomes condense and become visible as two chromatids joined at a centromere; the nuclear envelope breaks down; spindle forms.

Distinguish continuous and discontinuous variation.

Continuous: a range of values (e.g. height), often polygenic. Discontinuous: distinct categories (e.g. blood group), often one gene.

Give two sources of genetic variation.

Mutation and meiosis (crossing over and independent assortment), plus random fertilisation.

Describe natural selection.

Variation exists; individuals with advantageous alleles survive and reproduce more, passing on alleles, so their frequency increases.

List the taxonomic ranks from largest to smallest.

Domain, kingdom, phylum, class, order, family, genus, species.

Name the three domains.

Bacteria, Archaea and Eukarya.

Why are viruses not in any domain?

They are acellular, cannot reproduce or metabolise outside a host cell, and are not usually classed as living organisms.

What is recombinant DNA?

DNA combined from two different organisms.

What do restriction enzymes do?

Cut DNA at specific recognition sequences, often leaving sticky ends.

What does DNA ligase do?

Joins sugar–phosphate backbones of DNA fragments, e.g. sealing a gene into a plasmid.