AS/A2 Biology

AQA Specification A

Module 1

Rashpal Chana September 2004

Cell seen by Light Microscope Cell seen by Electron Microscope

Microscopy

Three types:

Transmission Electron – electrons transmitted through specimen can see internal organelles

Scanning Electron – electrons bounce off from surface of specimen can see external features

Light Microscope – Uses photons of white light

Electron Microscope Light Microscope

Advantages High Resolution;

of electron small

See Colour &

Living Cells

Disadvantages Dead Specimens &

B & W only

Low Resolution;

of photons too large

Structure & Function Organelles found in Cells (Eukaryotic)

Endoplasmic Reticulum (ER)

•An internal transport system made of cavities about 4nm

•All cavities interconnect, and interconnect with the Nuclear Membrane

There are two types:

Rough ER Smooth ER

•Flattened cavities * Tubular cavities

•Encrusted with Ribosomes on the * Not encrusted with cytoplasm side. Ribosomes

Function:

Isolates & Transports Proteins Synthesis & Transport of Lipids & Steroids

Golgi Body

Series of flattened cavities, lying parallel to one another, having vesicles pinching of the ends of the cavities.

Function:

•Synthesis of Glycoproteins

•Packages Glycoproteins in membrane-bound vesicles

Note: Cells actively involved in Glycoprotein Production (enzymes), or actively involved in secretory function have large prominent Golgi body

Mitochondrion

•In Animal Cells Rod Shaped, in Plant Cells,

•Round Shaped.

•Bound by double membrane

•1µm in , 2.5µm in length.

•Inner Membrane highly folded to form a number of projections called CRISTAE.

•Function: Sites of Energy (ATP) Production; Site of Respiration.

•Cells actively involved in functions requiring large amounts of energy will have many mitochondria.

Ribosomes•20nm in

•Made in the Nucleolus, migrate to cytoplasm via nuclear pores

•Made up of 2 subunits of RNA

•May be attached to ER or lie freely in cutoplasm.

•May be in small clusters (called Polysomes)

•Function: Sites of Protein Synthesis

Nucleus•Usually Largest Organelle

•Contains the Genetic Material, DNA which is attached to Proteins called Histones

•Surrounded by double membrane, called Nuclear Membrane.

•Nuclear Membrane has many Nuclear Pores 50-100nm in

•When the cell is not dividing, genetic material appears as tiny granules called Chromatin.

•Function: Contains the Genetic Material responsible for protein synthesis, cell division etc.

Lysosomes•Spherical Organelles 1 - 2.5µm in

•Contain enzymes (Hydrolases)

•Function: Involved in Extracellular or Intracellular digestion,

•can also cause Autolysis.

Plant Cell Wall•Consists of fibres of cellulose, which lie parallel to one another.

•Each fibre consists of repeated Glucose units

•Hydrogen Bonds between the fibres form Cross Bridges

•Width of about 10µm

•Function: Very Tensile & tough structure, which maintains cell shape, prevents plant cells from bursting or shrinking.

•Note Cell Wall in Fungi is NOT cellulose. Instead, Muco-polysaccaride layer

Chloroplasts•Found in Photo-autotrophic organisms only.

•Bi-Convex Lens shaped.

2-3 µm

4-10µm

•Originate from simple organelles called Proplastids.

•Inside the Cytoplasm (Stroma), is a system of layers of membranes called Grana.

•Grana made up of Thylakoid membranes, which contain light absorbing pigments.

•Stroma is an organic matrix of proteins, nucleic acids, DNA and enzymes.

•Following a period of illumination, Photosynthesis takes place, and Starch Grains & Lipid Globules appear.

•Function: Photosynthesis takes place in these organelles.

Cilia / Flagella

•Involved in movement

•About 0.1 µm in

•Cilia, up to 1 µm in length, flagella up to 10 µm

•Many cilia present, whereas, usually only 1 flagella

•Eukaryotic cells have ‘9+2’ microtuble structure

•Prokaryotes have ‘9+0’

Cell / Plasma Membrane

•Under the light microscope seen as two dark lines separated by a lighter layer.

•Middle lighter layer made up of a Phospholipid-Bilayer

•Each Phospholipid has hydrophilic head and hydrophobic tails.

•Heads point outwards (may have carbohydrates attached, Glycolipid), tails interact with each other in the middle

•Tails are long chains of hydrocarbons, and may have single or double bonds, between carbon atoms.

•Double bonds (Unsaturated), means tails ‘kink’, causing more gaps between phospholipids, membrane is ‘fluid’

•Proteins found throughout bilayer. Three types:

•Carrier Proteins – span right through bi-layer, involved in transport

•Extrinsic Proteins – on outer surface, may have carbohydrates attached (Glycoproteins)

•Intrinsic Proteins – on cytoplasm side

•Proteins can move in between the phospholipids.

•Cholesterol also found in between the bi-layer, causes rigidity.

•7.5nm width

Ultrastructure of a Eukaryotic cell