Module 2 - Foundations in biology

Magnification

• How many times larger the image is than the actual size of the object being viewed

• magnification = size of image / actual size of object

Resolution

The ability to see individual objects as separate entities

Light microscope

• living specimen can be viewed

• maximum Magnification: x2000

• maximum Resolution: 0.2 micrometers or 200nm

• inexpensive and easy to operate

• sample preparation does not usually lead to distortion

• small and portable

• simple sample preparation

• natural colour of sample is seen or stains are used

Electron microscopes

• use a focused beam of electrons in a vacuum to create highly magnified and detailed images

• max magnification - x 1,000,000 to 1,500,000

• max resolution - 0.0002 micrometers or 0.2 nm

• only view dead species due to the vacuum seal

• black and white images produced but can be coloured digitally

• sample preparation often distorts material

• expensive to buy and operate

• large and needs to be installed

Transmission electron microscope

• beam of electrons is transmitted through a specimen and is focused to produce an image

• best resolution with a resolving power of 0.5nm

Scanning electron microscope

• beam of electrons is sent across the surface of a specimen and the reflected electrons are collected

• resolving power of 3-10nm

• produces 3D images

Artefacts

A visible structural detail caused by processing the specimen and not a feature of the specimen (can appear in both light and electron microscopy)

Nucleus

• contains coded genetic information in the form of DNA molecules

• DNA directs the synthesis of all proteins required by the cell

• DNA controls the metabolic activities

• often the biggest single organelle

• DNA contained within a double membrane called a nuclear envelope to protect it from damage in the cytoplasm

• nuclear envelope contains nuclear pores that allows molecules to move into and out of the nucleus

• DNA is too large to leave the nucleus so is transcribed into smaller RNA molecules

• DNA associates with proteins called histones to form chromatin

• chromatin coils and condenses to form chromosomes - these only become visible when cells are preparing to divide

Nucleolus

• responsible for producing ribosomes

• composed of proteins and RNA

• RNA is used to produce rRNA which is then combined with proteins to form the ribosomes necessary for protein synthesis

Mitochondria

• site of cellular respiration

• very active cells usually have a lot of mitochondria

• have a double membrane - the inner membrane is highly folded to form structures called cristae and the fluid interior is called the matrix

• the membrane forming the cristae contains the enzymes used in aerobic respiration

• can produce their own enzymes and reproduce themselves

• contain a small amount of DNA

Vesicles and lysosomes

• vesicles are membranous sacs that have storage and transport roles - they consist of a single membrane with fluid inside (used to transport materials inside the cell)

• lysosomes are specialised forms of vesicles that contain hydrolytic enzymes - they are responsible for breaking down waste material in cells

• lysosomes are important for breaking down pathogens ingested by phagocytic cells and play an important role in programmed cell death (apoptosis)

Cytoskeleton

• network of fibres necessary for the shape and stability of a cell

• organelles are held in place by the cytoskeleton and it controls cell movement and the movement of organelles within cells

Components of cytoskeleton

• microfilaments - contractile fibres formed from the protein actin (responsible for cell movement and cell contraction during cytokinesis)

• microtubules - globular tubulin proteins polymerise to form tubes that are used to form scaffold like structures that determined the shape of a cell (also act as tracks for the movement of organelles)

• intermediate fibres - gives mechanical strength to cells and helps maintain their integrity

Centrioles

• component of the cytoskeleton present in most eukaryotic cells

• composed of microtubules

• two associated centrioles form the centrosome, which is involved in the assembly and organisation of the spindle fibres during cell division

Flagella and cilia

• extensions that protrude from some cell types

• flagella are longer than cilia but cilia are usually present in much greater numbers

• flagella are used primarily to enable cells motility (in some cells they are used as a sensory organelle detecting chemical changes)

• stationary cilia are present on the surface of many cells and have an important function in sensory organs

• mobile cilia beat in a rhythmic manner, creating a current, and cause fluid or objects adjacent to the cell to move (trachea to move mucus away or in fallopian tubes to move egg cells from the ovary to the uterus)

• pairs of parallel microtubules slide over each other causing the cilia to move in a beating motion

Endoplasmic Reticulum

• network of membranes enclosing flattened sacs called cisternae

• smooth endoplasmic reticulum - responsible for lipid and carbohydrate synthesis, and storage

• rough endoplasmic reticulum has ribosomes bound to the surface and is responsible for the synthesis and transport of proteins

• secretory cells, which release hormones or enzymes, have more rough endoplasmic reticulum than cells that do not release proteins

Ribosomes

• not surrounded by a membrane

• constructed of RNA molecules made in the nucleolus of the cell

• site of protein synthesis

Golgi apparatus

• formed of cisternae and does not contain ribosomes

• modifying proteins and packaging them into vesicles

• these may be secretory vesicles, if the proteins are destined to leave the cell, or lysosomes, which stay in the cell

Protein production

1. proteins are synthesised on the ribosomes bound to the endoplasmic reticulum

2. they then pass into its cisternae and are packaged into transport vesicles

3. vesicles containing the newly synthesised proteins move towards the golgi apparatus via the transport function of the cytoskeleton

4. the vesicles fuse with the cis face of the golgi apparatus and proteins enter

5. the proteins are structurally modified before leaving the golgi apparatus in vesicles from its trans face

6. secretory vesicles carry proteins that are to be released from the cell, the vesicles move towards and fuse with the cell-surface membrane, releasing their contents by exocytosis

7. some vesicles form lysosomes - these contain enzymes for use in the cell

Cellulose cell wall

• made of cellulose - complex carbohydrate

• freely permeable so substances can pass into and out of the cell through the cellulose

• gives it shape

• contents of the cell press against the cell wall making it rigid

• acts as a physical barrier against invading pathogens

Vacuoles

• membrane lined sacs in the cytoplasm containing cell sap

• important in the maintenance of turgor

• membrane of a vacuole in a plant cell is called the tonoplast

• selectively permeable

Chloroplasts

• responsible for photosynthesis

• found in the cells in the green parts of plants such as the leaves and the stems but not in the roots

• double membrane structure

• fluid enclosed in the chloroplast is called the stroma

• internal network of membranes, which form flattened sacs called thylakoids

• several thylakoids stacked together are called a granum

• grana are joined by membranes called lamellae

• grana contain the chlorophyll pigments

• starch produced by photosynthesis is present as starch grains

• contain their own DNA and ribosomes

• can make their own proteins

• the internal membranes provide the large surface area needed for enzymes, proteins and pigment molecules necessary in the process of photosynthesis