Why is a cover slip lowered gently at an angle onto a specimen rather than dropped flat?

So that air bubbles are not trapped underneath, because bubbles obscure the cells being viewed.

Why must many biological specimens be stained before they are viewed under a light microscope?

Most cells are colourless and almost transparent, so a stain adds contrast and makes the structures visible.

How can stains be used to tell different structures or tissues apart within one section?

Different stains are taken up by different structures, so each one takes on a different colour.

OCR GCSE Biology A J247

Cell Level Systems

Topic overview Microscopy, and sub-cellular structures DNA structure: polymer, double helix and nucleotides Enzyme action, and investigating enzymes Cellular respiration as an exothermic process Aerobic and anaerobic respiration compared Synthesis and breakdown of carbohydrates, proteins and lipids Photosynthesis: producers and the process Photosynthesis as endothermic, and investigating it Limiting factors in photosynthesis

Community Level Systems

Topic overview Cycling materials, and the role of microorganisms The carbon and water cycles, and decomposition Levels of organisation, and abiotic and biotic factors Interdependence, competition and trophic levels Pyramids of biomass and the efficiency of transfer

Genes, Inheritance and Selection

Topic overview Genetic terms, and the genome Genome, environment and mutations Asexual and sexual reproduction compared Haploid, diploid and meiosis Single gene inheritance and Punnett squares Sex determination, multiple genes and Mendel's work Variation within a population, and classification Natural selection and evolution Evidence for evolution, and the work of Darwin and Wallace

Global Challenges

Topic overview Field investigations, and human impact on ecosystems Maintaining biodiversity, monitoring conservation and ecotourism Food security, agricultural and biotechnological solutions Selective breeding and genetic engineering Health, disease, and types of disease Interactions between diseases, and how they spread Reducing the spread of disease, and named examples Physical and chemical plant defences White blood cells, antibodies and platelets Non-specific defences and the immune system Vaccines, medicines and aseptic technique Developing new medicines, and non-communicable disease Treating cardiovascular disease, and lifestyle factors Cancer, and the use of stem cells in medicine Gene technology in medicine, and the human genome

Organism Level Systems

Topic overview The nervous system and coordinated responses The reflex arc The eye and its common defects The structure and function of the brain Hormones and the endocrine glands Hormones in reproduction and the menstrual cycle Methods of contraception Tropisms, auxin and the roles of plant hormones Homeostasis, and the skin in temperature control Insulin and the control of blood glucose Type 1 and type 2 diabetes Osmotic changes in body fluids, and the kidneys Kidney structure and the parts of the tubule

Scaling Up

Topic overview Diffusion, osmosis and active transport Mitosis, the cell cycle and cell differentiation Stem cells in animals and meristems in plants Surface area to volume ratio and exchange surfaces The circulatory system, the heart and blood vessels Red blood cells and plasma Root hair cells, transpiration and translocation Xylem and phloem Factors affecting water uptake, and the potometer

Cell Level Systems

Microscopy, and sub-cellular structures

3 things to know
Paper 1

Overview

The parts of a light microscope and preparing and staining slides, calculating magnification, where genetic material and plasmids sit in plant, animal and bacterial cells, the jobs of mitochondria, chloroplasts, the membrane and its receptors, and ribosomes, resolution and what electron microscopes revealed, converting between millimetres, micrometres and nanometres in standard form, estimating cell sizes, and significant figures.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Which part of a light microscope supplies the light that passes up through a specimen?

Key term

What is the flat platform called that a prepared slide rests on, and how is the slide held still?

Key term

Through which two lenses does light travel after leaving the specimen and before reaching the eye?

Questions from this subtopic

Each one with the model answer underneath.

1
Which part of a light microscope supplies the light that passes up through a specimen?
Model answer
The lamp, positioned beneath the stage.
2
What is the flat platform called that a prepared slide rests on, and how is the slide held still?
Model answer
The stage, where the slide is held in place by clips.
3
Through which two lenses does light travel after leaving the specimen and before reaching the eye?
Model answer
First the objective lens, then the eyepiece lens.
4
Why is a cover slip lowered gently at an angle onto a specimen rather than dropped flat?
Model answer
So that air bubbles are not trapped underneath, because bubbles obscure the cells being viewed.
5
Why must many biological specimens be stained before they are viewed under a light microscope?
Model answer
Most cells are colourless and almost transparent, so a stain adds contrast and makes the structures visible.
6
How can stains be used to tell different structures or tissues apart within one section?
Model answer
Different stains are taken up by different structures, so each one takes on a different colour.

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Cell Level Systems

Community Level Systems

Genes, Inheritance and Selection

Global Challenges

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Algae and .

phytoplanktonfungianimals

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AA diet high in sugar and fat leads to obesity.

BBody cells then respond poorly to insulin.

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Egestion means passing out of undigested food as faeces.

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