A cell structure  The microscope in cell studies  Cells as basic units of living organisms  Detailed structure of typical animal and plant cells, as seen using the electron microscope.  Outline functions of organelles in plant and animal cells.  Characteristics of Prokaryotic and eukaryotic cells. Learning Objective a) Use an eyepiece graticule and stage micrometer scale to measure cells and be familiar with units (millimetre, micrometre, nanometre) used in cell studies. Cells and organells can be measured with a microscope by means of an eyepiece graticule. This is a transparent scale. It usually has 100 divisions. The eyepiece graticule is placed in the microscope eyepiece so that it can be seen at the same time as the object to be measured. We will not know the actual size of the eyepiece units until the eyepiece graticule scale is calibrated. To calibrate the eyepiece graticule scale a miniature transparent rules called a stage micrometre. Scale is placed on the microscope stage and is brought into focus. This scale may be etched onto a glass slide or printed on the transparent film. It commonly has subdivisions of 0.1 and 0.0 eyepice graticule scale (arbitrary units)

Eyepiece graticule stage micrometer scale (marked in 0.01 mm and 0.1 mm divisions

Fraction of a metre One thousandth= 0.01= 1/1000= 10^-3 One millionth= 0.000001= 1/1000000= 10^-6 one thousand millionth= 0.0000000001= 1/1000000000= 10^-9

Unit Millimeter Micrometer Nanometer

Symbol mm μm

nm

b) Explain and distinguish between resolution and magnification with reference to light microscopy and electron microscopy. Resolution – The ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the detail that can be seen. Magnification – The number of times greater than an image is than the actual object; Magnification = image size / actual (real size) Light microscopy – Light rays pass through the specimen on a side and are focused by an objective lens and an eyepiece lens. This produces a magnified image of the specimen on the retina of your eye. Alternatively, the image can be projected onto a screen, or recorded by a camera. Electron microscope – An electron microscope uses beams of electrons rather than light rays. The specimen has to be very thin and must be placed in a vacuum, to allow electrons to pass through it. The electrons are focused onto a screen, or on to photographic film, where they form a magnified image of the specimen. Types of electron microscope There are two types of electron microscope: Transmission electron microscope (TEM):  The electron bean passes through a very thin prepared sample  Electrons pass through the denser parts of the sample less easily, so giving some contrast  The final image produced is two-dimensional  The magnification possible with a TEM is x500,000 Scanning electron microscope (SEM):  The electron beam is directed onto a sample. The electrons don’t pass through the specimen  They are ‘bounced off’ the sample  The final image produced is a 3D view of the surface of the sample



The magnification possible with an SEM is about x100,000

Light microscopy Transmission Electron Microscope Scanning Electron Microscope

Resolution 200nm 0.1nm

Magnification X1,500 X500,000

0.1nm

X100,000

c) Describe and interpret drawings and photographs of of typical animals and plant cells, as seen using the electron microscope recognising following: rough endoplasmic reticulum, and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, lysosomes, chloroplasts, cell surface membrane, nuclear envelope, centrioles, nucleus and nucleolus.

d) Outline the functions of the structure listed in ( C )

Rough Endoplasmic Reticulum is so called because it is covered with many tiny organelles called ribosomes. These are just visible as black dots. At high magnification they can be seen to consist of two subunits: a large and small subunits. Ribosomes are the sites of protein synthesis. They can be found free in the cytoplasm as well as on the rough ER. They are very small, only about 25nm in diameter. They are made of RNA (ribonucleic acid) and protein. The rough ER forms an extensive system of flattened sacs spreading in sheets throughout the cell. Protein made by the ribosomes on the rough ER enter the sacs and move through them. The proteins are often processed in same way on their journey. Small sacs called vesicles can break

off from the ER and these joins together to form the Golgi apparatus. Proteins can be exported from the cell via the Golgi apparatus.

Smooth Endoplasmic Reticulum so called because it lacks ribosomes has completely different functions. It makes lipids and steroids, such as cholesterol and the reproductive hormones oestrogen and testosterone.

Golgi apparatus the Golgi apparatus is a stack of flattened sacs. This stack of sacs is sometimes referred to as the Golgi body. More than one may be present in a cell. The stack is constantly being formed at one end from vesicle which bud off from the ER, and broken down again at the other end to form Golgi vesicles. The stack of sacs with the associated vesicles is referred to as the Golgi apparatus as Golgi complex. The Golgi apparatus collects, processes and sorts molecules (particular proteins from the Rough ER) ready for transport in Golgi vesicles either to other parts of the cell or out of the cell (secretion). Two examples of protein processing in the golgi apparatus are the addition of sugar to protein to make molecules known as glycoproteins, and removal of the first amino acid, methionine, from newly formed proteins to make a functioning proteins. In plants, enzymes in the Golgi apparatus convert sugars into cell wall components. Golgi vesicles are also used to make lysosomes.

Mitochondria have an envelope (two membrane) surrounding them. The inner one is folded to form cristae. This is where aerobic respiration takes place, producing ATP. This first stage of this process, called the Krebs cycle, takes place in the matrix. The final stage, oxidative phosphorylation, takes place on the membrane of the cristae.

Ribosomes are small structures made of RNA and proteins. They are found free in the cytoplasm, and attached to rough ER. Proteins are made on the ribosomes, by linking them together amino acid.

Lysosomes are little membrane- bound packages of hydrolytic (digestive) enzymes. They form by breaking off from the Golgi apparatus. They are used to digest bacteria or other cells taken into the cell by phagocytosis, or to breakdown unwanted, or damaged organelles within the cell.

Chloroplasts are found in the same plant cells. Like mitochondria, they are surrounded by an envelope made up of two membranes. Their background materials is called the stroma and it contains many paired membranes called thylakoids. In places, these forms stacks called grana. The grana contains chlorophyll, which absorbs energy from sunlight. The first reactions in the in photosynthesis, called the light-dependent reactions

and photophosphorylation, takes place on the membranes. The final stages called, the Calvin cycle, takes place in the stroma. Chloroplasts often contain starch grains, which are storage materials formed from the sugars that are produced in photosynthesis.

Cell surface membrane controls what enters and leaves the cell. These are many membranes within the cell, which help to make different compartments in which different chemical reactions can take place without interfering with another.

Nucleus

A relatively large organelle found in eukaryotic cells, but absent from prokaryotic cells; the nucleus contains the cell’s DNA and therefore controls the activities of the cell. Nuclear envelope – The two membranes, situated closely together, that surround the nucleus; the envelope is performed with nuclear pores. Nucleolus – A small structure, one or more of which is found inside the nucleus; the nucleolus is usually visible as a densely stained body; its function is to manufacture ribosomes using the information in its own DNA.

Centrioles one of two small, cylindrical structures found just outside the nucleus in animal cells, but absent from plant cells; centrioles help control spindle formation during nuclear division. e) Draw and label low power plan diagram of tissues and organs ( including a transverse section of stem, roots and leaves and calculate the linear magnification of drawings.

Magnification = measured length of the image /measured length of the specimen Length of the actual specimen = length on the image/ image

magnification ( e.g. rose leaf =

length 4.2cm/ magnification 0.82 = 5cm real length

f) Compare the structure of typical animal and plant cells.

Plant cell Nucleus Golgi apparatus Golgi vesicles Mitochondria Ribosomes Cell surface membrane SER RER Choroplast – grana and envelope Cytoplasm Nuclear Pore Nucleolus Chromatin Nuclear envelope Cell wall Vacuole Plasmodesma Middle lamella

Animal cell Nucleus Golgi apparatus Golgi vesicles Mitochondria Ribosomes Cell surface membrane SER RER Cytoplasm Nuclear Pore Nucleolus Nuclear envelope Lysosomes Centrioles Chromatin microvilli

g) Calculate linear magnification of drawing and photographs; h) Calculate actual size of specimens from drawing and photographs;

i) Describe the structure of a prokaryotic cell and compare and contrast the structure of prokaryotic cells with Eukaryotic cells. Features

Prokaryotic cells

Cell surface membrane

Always present

Eukaryotic Animal cells Always present

Eukaryotic plant cells Always present

Cell wall

Always present; made up of peptidoglycans Never present

Nucleus and nuclear envelope Chromosomes Contain so called ‘bacteria chromosomes’ - a circular molecule of DNA not associated with histones, bacteria may also contain small circles of DNA called plasmid Mitochondria Never present Chloroplast Never present, though some do contain chlorophy II or other photosynthesis pigment RER & SER Never present &Golgi apparatus Ribosomes Present, about 18nm diameter Centrioles Never present

Never present Always present

Always present; made up of cellulose Always present

Contains several chromosomes, each made up of a linear DNA molecule associated with histone

Contains several chromosomes, each made up of a linear DNA molecule associated with histone

Usually present Never present

Usually present Sometimes present

Usually present

Usually present

Present about 22nm diameter Usually present

Present about 22nm diameter Never present