CIE As Level Biology: Cell Structure

Magnification

Magnification is how many times bigger the image of a specimen observed is in comparison to the actual (real-life) size of the specimen

How to calculate magnification

Image size
Magnification= ----------
Actual size

Units for magnification

The size of cells is typically measured using the micrometre (μm) scale, with cellular structures measured in either micrometers (μm) or nanometers (nm)
When doing calculations all measurements must be in the same units. It is best to use the smallest unit of measurement shown in the question
To convert units, multiply or divide depending if the units are increasing or decreasing
Magnification does not have units

Units for magnification calculations

There are 1000 nanometers (nm) in a micrometre (µm)
There are 1000 micrometres (µm) in a millimetre (mm)
There are 1000 millimetres (mm) in a metre (m)

What is an eyepiece graticule and stage micrometer

An eyepiece graticule and stage micrometer are used to measure the size of the object when viewed under a microscope

How is calibration done

Each microscope can vary slightly so needs to be calibrated when used
The calibration is done with a stage micrometer, this is a slide with a very accurate scale in micrometres (µm), it is usually in 10 µm divisions, so 1 mm divided into 100 divisions
The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale
To know what the divisions equal at each magnification the eyepiece graticule is calibrated to the stage micrometer at each magnification

How many lenses does a light microscope

A light microscope has two types of lens:
An eyepiece lens, which often has a magnification of x10A series of (usually 3)
objective lenses, each with a different magnification

How to calculate the magnification of the eyepiece lens

To calculate the total magnification the magnification of the eyepiece lens and the objective lens are multiplied together:

eyepiece lens magnification x objective lens magnification

= total magnification

What is resolution

Resolution is the ability to distinguish between two separate points,If two separate points cannot be resolved, they will be observed as one point

Compare the resolution of light and electron microscopes

The resolution of a light microscope is limited by the wavelength of light As light passes through the specimen, it will be diffracted The longer the wavelength of light, the more it is diffracted and the more that this diffraction will overlap as the points get closer together

Electron microscopes have a much higher resolution and magnification than a light microscope as electrons have a much smaller wavelength than visible light This means that they can be much closer before the diffracted beams overlap

Can the phospholipid bilayer structure of a cell be seen under a light microscope

The concept of resolution is why the phospholipid bilayer structure of the cell membrane cannot be observed under a light microscope The width of the phospholipid bilayer is about 10nm
The maximum resolution of a light microscope is 200nm (half the smallest wavelength of visible light, 400nm)
Any points that are separated by a distance less than 200nm (such as the 10nm phospholipid bilayer) cannot be resolved by a light microscope and therefore will not be distinguishable as "separate"

Features of a light microscope

Light microscopes are used for specimens above 200 nm
Light microscopes shine light through the specimen, this light is then passed through an objective lens (which can be changed) and an eyepiece lens (x10) which magnify the specimen to give an image that can be seen by the naked eye
The specimens can be living (and therefore can be moving), or deadLight microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs such as in leaves or skin

Features of electron microscopes

Electron microscopes, both scanning and transmission, are used for specimens above 0.5 nm
Electron microscopes fire a beam of electrons at the specimen either a broad static beam (transmission) or a small beam that moves across the specimen (scanning)
The electrons are picked up by an electromagnetic lens which then shows the image
Due to the higher frequency of electron waves (a much shorter wavelength) compared to visible light, the magnification and resolution of an electron microscope is much better than a light microscope
Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail
Electron microscopy requires the specimen to be dead however this can provide a snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and chromosome position within the stages of mitosis are visible

What is a photomicrograph

Photomicrographs are images obtained from a light microscope, these are used for specimens above 200 nm (a bacteria cell is about 1000 nm)

What is electron micrograph

Electron micrographs are images obtained from electron microscopes, both scanning and transmission, these are used for specimens above 0.5 nm
Electron microscopes are useful for looking at organelles and biological molecules, eg. DNA can be seen replicating

How to calculate actual size

Actual Size= Image size
------------
Magnification

Structures in plant cell that are absent in animal cells

Plant cells also have additional structures: the cellulose cell wall, large permanent vacuoles and chloroplasts

Processes that require energy

All organisms require a constant supply of energy to maintain their cells and stay alive
This energy is required:
In anabolic reactions - building larger molecules from smaller molecules
To move substances across the cell membrane (active transport) or to move substances within the cell
In animals, energy is required:
For muscle contraction - to coordinate movement at the whole-organism level
In the conduction of nerve impulses, as well as many other cellular processes

Why is ATP a universal energy currency

In all known forms of life, ATP from respiration is used to transfer energy in all energy-requiring processes in cells,This is why ATP is known as the universal energy currency,Adenosine Triphosphate (ATP) is a nucleotide.The monomers of DNA and RNA are also nucleotides

Distinct features of prokaryotes

Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote
Prokaryotes have a cellular structure distinct from eukaryotes:
Their genetic material is not packaged within a membrane-bound nucleus and is usually circular (eukaryotic genetic material is packaged as linear chromosomes)
Prokaryotes lack membrane-bound organelles
They are many (100s/1000s) of times smaller than eukaryotic cells
Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S)

What is a virus

Viruses are non-cellular infectious particles that straddle the boundary between 'living' and 'non-living' They are relatively simple in structure; much smaller than prokaryotic cells (with diameters between 20 and 300 nm)

Structure of a virus

Structurally they have:
A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)
A protein coat called a 'capsid'

Some viruses have an outer layer called an envelope formed usually from the membrane-phospholipids of a cell they were made in
All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles

What are all cells surrounded by

All cells are surrounded by a cell surface membrane which controls the exchange of materials between the internal cell environment and the external environment,The membrane is described as being 'partially permeable'

The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a diameter of around 10 nm

Cell walls

Cell walls are formed outside of the cell membrane and offer structural support to cell
Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan in most bacterial cells
Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata connect the cytoplasm of neighbouring plant cells

Nucleus

Present in all eukaryotic cells, the nucleus is relatively large and separated from the cytoplasm by a double membrane (the nuclear envelope) which has many pores
Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes (eg. DNA polymerases) and signalling molecules to travel in
The nucleus contains chromatin (the material from which chromosomes are made)
Usually, at least one or more darkly stained regions can be observed - these regions are individually termed 'nucleolus' and are the sites of ribosome production

Mitochondria

The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a light microscope
Surrounded by double-membrane with the inner membrane folded to form cristae
The matrix formed by the cristae contains enzymes needed for aerobic respiration, producing ATP
Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the matrix (needed for replication)

Chloroplast

Larger than mitochondria, also surrounded by a double-membrane
Membrane-bound compartments called thylakoids containing chlorophyll stack to form structures called grana
Grana are joined together by lamellae (thin and flat thylakoid membranes)
Chloroplasts are the site of photosynthesis:The light-dependent stage takes place in the thylakoids
The light-independent stage (Calvin Cycle) takes place in the stroma

Also contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis

Ribosomes

Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells
Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
70S (composed of 50S and 30S subunits) ribosomes in prokaryotes, mitochondria and chloroplasts
Site of translation (protein synthesis)

Endoplasmic Reticulum

Rough Endoplasmic Reticulum (RER)
Surface covered in ribosomes
Formed from continuous folds of membrane continuous with the nuclear envelope
Processes proteins made by the ribosomes

Smooth Endoplasmic Reticulum (ER)

Does not have ribosomes on the surface, its function is distinct to the RER
Involved in the production, processing and storage of lipids, carbohydrates and steroids

Golgi complex (Golgi apparatus)

Flattened sacs of membrane similar to the smooth endoplasmic reticulum
Modifies proteins and packages them into vesicles or lysosomes

Large permanent vacuole

Sac in plant cells surrounded by the tonoplast, selectively permeable membrane
Vacuoles in animal cells are not permanent and small

Vesicles

Membrane-bound sac for transport and storage

Lysosome

Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break biological molecules down)
Break down waste materials such as worn-out organelles, used extensively by cells of the immune system and in apoptosis (programmed cell death)

Centriole

Hollow fibres made of microtubules, two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
Not found in flowering plants and fungi

Microtubules

Makes up the cytoskeleton of the cell about 25 nm in diameter
Made of α and β tubulin combined to form dimers, the dimers are then joined into protofilaments. Thirteen protofilaments in a cylinder make a microtubule
The cytoskeleton is used to provide support and movement of the cell

Microvili

Cell membrane projections that increase the surface area for absorption

Cilia

Hair-like projections made from microtubules
Allows the movement of substances over the cell surface

Flagella

Similar in structure to cilia, made of longer microtubules
Contract to provide cell movement for example in sperm cells

Structures present in animal cells that are absent in plant cells

The only structures found in animal cells but not plant cells are the centrioles and microvilli