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OCR A-Level Biology H420

Cell Structure

Topic overview Microscopy, slide preparation and staining Biological drawings, magnification and resolution Eukaryotic cell ultrastructure The secretory pathway and the cytoskeleton Prokaryotic and eukaryotic cells compared

Biological Molecules

Topic overview The properties of water Monomers, polymers and the elements of life Monosaccharides and disaccharides Polysaccharides and their functions Lipids Amino acids and protein structure Globular and fibrous proteins Inorganic ions The biochemical tests Colorimetry and chromatography

Nucleotides and Nucleic Acids

Topic overview Nucleotide structure and the bases Polynucleotides, ADP and ATP The structure of DNA Semi-conservative DNA replication The genetic code, transcription and translation

Enzymes

Topic overview Enzyme action: lock and key and induced fit Factors affecting enzyme activity Cofactors, coenzymes and enzyme inhibition

Biological Membranes

Topic overview Membrane structure and the fluid mosaic model Factors affecting membrane permeability Diffusion, active transport, endocytosis and exocytosis Osmosis and water potential

Cell Division, Cell Diversity and Cellular Organisation

Topic overview The cell cycle Mitosis and observing it in root tips Meiosis Specialised cells Tissues, organs and organ systems Stem cells and differentiation

Exchange Surfaces

Topic overview Surface area to volume and the features of an exchange surface The mammalian gas exchange system Ventilation and lung volumes Gas exchange in fish and insects Dissecting and examining gas exchange tissue

Transport in Animals

Topic overview Why animals need transport systems Arteries, veins and capillaries Tissue fluid and lymph The structure of the mammalian heart The cardiac cycle and control of the heartbeat The electrocardiogram Transporting oxygen and carbon dioxide Oxygen dissociation curves and the Bohr effect

Transport in Plants

Topic overview Xylem, phloem and vascular tissue Transpiration and the potometer Water uptake and movement up the xylem Xerophytes and hydrophytes Translocation

Communicable Diseases, Disease Prevention and the Immune System

Topic overview Pathogens and how disease is transmitted Plant defences and non-specific animal defences Phagocytes and phagocytosis The specific immune response Antibodies Types of immunity Vaccination Medicines and antibiotic resistance

Biodiversity

Topic overview The three levels of biodiversity Sampling methods and equipment Simpson's Index of Diversity Measuring genetic biodiversity Threats to biodiversity and reasons to maintain it Conservation: in situ and ex situ

Classification and Evolution

Topic overview The taxonomic hierarchy and binomial naming The five kingdoms and the three domains Phylogeny and the evidence for evolution Variation and adaptation Natural selection and resistance

Communication and Homeostasis

Topic overview Cell signalling and communication systems Homeostasis, negative and positive feedback Thermoregulation in endotherms and ectotherms

Excretion as an Example of Homeostatic Control

Topic overview Excretion and nitrogenous waste The liver: structure and function The structure of the kidney The nephron Ultrafiltration Selective reabsorption and the loop of Henle Osmoregulation and ADH Kidney failure, dialysis and transplant Urine testing and monoclonal antibodies

Neuronal Communication

Topic overview Sensory receptors and the Pacinian corpuscle Types of neurone and myelination Resting potential and the action potential Transmission across a cholinergic synapse Summation and inhibitory synapses

Hormonal Communication

Topic overview Hormones and the adrenal glands The pancreas and the islets of Langerhans Controlling blood glucose Diabetes and its treatment

Plant and Animal Responses

Topic overview Tropisms and plant responses to stress and herbivory Plant hormones and their commercial uses The nervous system and the brain Reflex actions and the knee jerk reflex Fight or flight and the control of heart rate The structure of skeletal muscle The neuromuscular junction and the sliding filament model

Photosynthesis

Topic overview Photosynthesis and chloroplast structure Photosynthetic pigments and chromatography The light-dependent stage The Calvin cycle Limiting factors

Respiration

Topic overview Respiration, ATP and the mitochondrion Glycolysis The link reaction and the Krebs cycle Oxidative phosphorylation and chemiosmosis Anaerobic respiration Respiratory substrates and the respiratory quotient

Cellular Control

Topic overview Gene mutations and their effects The lac operon Transcription factors and post-transcriptional control Homeobox and Hox genes Mitosis and apoptosis Oncogenes, tumour suppressor genes and cancer

Patterns of Inheritance

Topic overview Genetic and environmental variation Monogenic and dihybrid inheritance, sex linkage and epistasis The chi-squared test Selection, genetic drift and the founder effect The Hardy-Weinberg principle Speciation and isolating mechanisms Artificial selection

Manipulating Genomes

Topic overview DNA sequencing and bioinformatics DNA profiling, PCR and gel electrophoresis Genetic engineering: restriction enzymes and vectors Genetic modification, its ethics, and gene therapy

Cloning and Biotechnology

Topic overview Natural plant cloning and taking cuttings Micropropagation Advantages and disadvantages of plant cloning Animal cloning: embryo twinning and nuclear transfer Advantages and disadvantages of animal cloning Microorganisms in food production Aseptic technique Fermentation and its conditions The microbial growth curve Immobilised enzymes

Ecosystems

Topic overview Ecosystems, biotic and abiotic factors Biomass transfer between trophic levels The nitrogen and carbon cycles Succession, distribution and abundance

Populations and Sustainability

Topic overview Population size and limiting factors Predator-prey cycles and competition Conservation and preservation Sustainable timber production and fishing Human impacts and how they are controlled

Practical Skills and Mathematical Requirements

Topic overview Planning investigations and choosing apparatus Recording and presenting data Gradients, intercepts and logarithmic scales Averages, spread and statistical tests Evaluating experiments and uncertainty Estimates, symbols and biological calculations

Transport in Plants

Xylem, phloem and vascular tissue

1 thing to know
Components 01 and 03

Overview

Xylem vessels and lignin, sieve tube elements, sieve plates and companion cells, and how the two tissues are arranged in root, stem and leaf.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What are companion cells and why are they important for phloem function?

Questions from this subtopic

Each one with the model answer underneath.

1
Why do multicellular plants require specialised transport systems?
Model answer
Their large size means diffusion distances are too great for efficient exchange between cells and the external environment.
2
What happens to the surface area to volume ratio as organisms increase in size, and why does this matter for transport?
Model answer
The surface area to volume ratio decreases, making simple diffusion insufficient to meet metabolic demands.
3
Why do plants with higher metabolic rates require more developed transport systems?
Model answer
They have greater demands for nutrient supply and waste removal, necessitating specialised transport systems.
4
What are xylem vessels and what do they transport?
Model answer
Xylem vessels are dead, hollow, lignified cells that transport water and dissolved mineral ions from roots to leaves.
5
What two functions does lignin serve in xylem vessel walls?
Model answer
Lignin provides mechanical strength and waterproofs the vessels, preventing water loss to surrounding cells.
6
Why can water flow uninterrupted through xylem vessels?
Model answer
Xylem vessels have no end walls between adjacent cells, forming continuous tubes.

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Cell Structure

Biological Molecules

Nucleotides and Nucleic Acids

Enzymes

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Centrioles consist of .

microtubuleschromosomesribosomes

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AA phospholipid contains one glycerol molecule.

BTwo fatty acids bond to the glycerol molecule.

COne phosphate group replaces the third fatty acid.

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Habitat means the physical place where an organism lives.

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