How can xylem vessels be identified when viewed under a light microscope?

By their thick lignified walls, large empty lumen, and lack of cell contents.

How can sclerenchyma fibres be identified when viewed under a light microscope?

By their very thick walls, small lumen, and elongated shape often seen in bundles.

How can phloem sieve tubes be identified when viewed under a light microscope?

By their thin walls, sieve plates between cells, and companion cells alongside them.

Edexcel A-Level Biology A 9BN0

Lifestyle, Health and Risk

Topic overview Water and the carbohydrates Condensation, hydrolysis and glycosidic bonds Lipids, food tests and units The cardiac cycle and the structure of the heart Arteries, veins and capillaries Core practical: caffeine and the heart rate of Daphnia The blood clotting cascade Thrombus, embolus and their consequences Atherosclerosis and the risk factors for CVD Treatments for CVD, and cholesterol transport BMI, waist-to-hip ratio and energy balance Core practical 2: the vitamin C content of food and drink Correlation, causation and the design of health studies Risk perception, and handling quantitative data

Genes and Health

Topic overview Gas exchange surfaces and Fick's Law The fluid mosaic model of the cell membrane Osmosis, diffusion and active transport Surface area and volume calculations The structure of DNA and RNA Transcription Translation and the genetic code Amino acids and the levels of protein structure Globular and fibrous proteins: haemoglobin and collagen Alternative splicing and tissue-specific proteins Enzyme specificity, the active site and activation energy Rates of reaction, Vmax and Core Practical 4 Semi-conservative replication and Meselson-Stahl Types of mutation and the CFTR gene Alleles, genotype and monohybrid inheritance Pedigree diagrams and the inheritance of cystic fibrosis The effects of cystic fibrosis, and the chi-squared test Prenatal genetic testing: PGD, amniocentesis and CVS The ethics of genetic screening

Voice of the Genome

Topic overview Prokaryotic cells Eukaryotic organelles and their functions Identifying organelles in an electron micrograph Protein secretion, and levels of organisation Microscopy: the graticule, magnification and standard form The cell cycle and the stages of mitosis Core practical: the root tip squash Meiosis, linkage and sex linkage The structure of sperm and egg cells Fertilisation: the acrosome and cortical reactions Totipotency, pluripotency and sources of stem cells The ethics and risks of stem cell therapy Differential gene expression and specialisation The lac operon Epigenetics: DNA methylation and histone modification Polygenic inheritance, continuous variation and statistics

Biodiversity and Natural Resources

Topic overview Biodiversity, species richness and the index of diversity Niches and the three kinds of adaptation Natural selection and evolution The Hardy-Weinberg equation and its assumptions Reproductive isolation and speciation Classification The plant cell wall, chloroplast and amyloplast The vacuole, plasmodesmata, pits and middle lamella Plant cells in the electron microscope Starch and cellulose Lignin, xylem vessels and sclerenchyma fibres Identifying plant tissues under the microscope The structure of xylem, phloem and sclerenchyma Water and the mineral nutrition of plants Core practical: investigating mineral deficiency Core practical: the tensile strength of plant fibres Drug development from Withering to modern trials Growing bacteria and aseptic technique Core practical: the antimicrobial properties of plants Sustainability and Conservation

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Immunity, Infection and Forensics

Topic overview Decomposition, succession and forensic entomology Body cooling and rigor mortis DNA profiling and the polymerase chain reaction Gel electrophoresis The structure of bacteria and viruses Decomposition, and how pathogens enter the body The body's barriers, and modelling bacterial growth Tuberculosis and HIV The non-specific immune response Antigens, antibodies and B cells T cells and cell-mediated immunity Active and passive immunity, natural and artificial The evolutionary race and pathogen evasion Antibiotics and testing their effectiveness Hospital acquired infections and their control

Run for Your Life

Topic overview Muscles and Movement The myofibril and the sarcomere The sliding filament theory, calcium and ATP Slow-twitch and fast-twitch muscle fibres Glycolysis and the link reaction The Krebs cycle, oxidative phosphorylation and oxygen debt The conduction system: SAN, AVN and Purkyne fibres The sequence of cardiac excitation The electrocardiogram, cardiac output and stroke volume The cardiovascular and ventilation control centres Spirometry and ventilation during exercise Homeostasis, feedback and gene control by hormones Thermoregulation and the hypothalamus The effects of too much and too little exercise Medical technology and performance-enhancing substances

Grey Matter

Topic overview The structure of neurones and myelination The action potential and saltatory conduction The structure of a synapse and calcium entry Neurotransmitter release and acetylcholine Rod cells and the detection of light Effectors, the neuromuscular junction and the pupil reflex Phytochrome and the response to light IAA, phototropism and gravitropism Nervous against hormonal coordination The regions of the brain and their functions Brain imaging: MRI, fMRI, PET and CT Critical periods and the Hubel and Wiesel experiments Nature, nurture, habituation and research ethics Dopamine, serotonin and their imbalance L-Dopa and MDMA Genome sequencing, personalised medicine and its ethics Genetically modified organisms: benefits and risks

Biodiversity and Natural Resources

Identifying plant tissues under the microscope

3 things to know
Papers 1, 2 and 3

Overview

Recall questions on recognising xylem, phloem and sclerenchyma, and on the arrangement of vascular bundles.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe the arrangement of vascular bundles in a dicotyledonous stem.

Key term

Compare the type of glucose and molecular orientation in starch versus cellulose.

Key term

Within a vascular bundle of a dicotyledonous stem, where is the xylem located relative to the phloem?

Questions from this subtopic

Each one with the model answer underneath.

1
Compare the type of glucose and molecular orientation in starch versus cellulose.
Model answer
Starch contains α-glucose with all molecules oriented the same way, forming coiled chains; cellulose contains β-glucose with alternating orientation, forming straight chains.
2
How can xylem vessels be identified when viewed under a light microscope?
Model answer
By their thick lignified walls, large empty lumen, and lack of cell contents.
3
How can sclerenchyma fibres be identified when viewed under a light microscope?
Model answer
By their very thick walls, small lumen, and elongated shape often seen in bundles.
4
How can phloem sieve tubes be identified when viewed under a light microscope?
Model answer
By their thin walls, sieve plates between cells, and companion cells alongside them.
5
Describe the arrangement of vascular bundles in a dicotyledonous stem.
Model answer
Vascular bundles are arranged in a ring.
6
Within a vascular bundle of a dicotyledonous stem, where is the xylem located relative to the phloem?
Model answer
Xylem is towards the inside and phloem is towards the outside of each bundle.

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