Cell Structure and Microscopy

Suitable for Cambridge International Examinations (CIE) Biology at A Level

magnification

the number of times an image is larger than the original

magnification equation

magnification = image size/actual size; M = I/A

1 cm = ? micrometers (μm)

10,000

1 cm = ? nanometres (nm)

10,000,000

resolution

the smallest distance where two points can be distinguished as separate

scanning electron microscope

view images in 3D, scans surface of samples

transmission electron microscope

view images in 2D, can be used to view inside specimens as small as 5 nm

light microscope

uses natural/artificial light to view (living) specimens under a lens

difference(s) between electron and light microscopes

• light microscopes: use light rays; electron microscopes: use electron beams (with electromagnets)

• light microscopes: specimens can be alive; electron microscopes: specimens are dead (due to vacuum)

• light microscopes are cheaper and portable; electron microscopes are expensive and are not portable

• light microscopes have lower magnification and resolution; electron microscopes have higher magnification and higher resolution

cell

basic unit of all living organisms, consists of organelles with specific functions

organelle

a part of the cell with specific functions and structures

eukaryote

a cell containing a nucleus and membrane-bound organelles, large (up to 40 μm in diameter), genetic material is linear and associated with proteins (histones), ribosomes can be 80S or 70S

prokaryote

a cell without any membrane-bound organelles, small (up to 5 μm in diameter), consists of a nucleoid region (where genetic material is found as circular DNA) and ribosomes (70S only)

nucleus

largest cell structure, found in eukaryotic cells, holds DNA, consists of a nuclear envelope with pores and a nucleolus that is darkly stained; responsible for storing and replicating of genetic material, synthesis of ribosomes (in nucleolus), and production of RNA (specifically mRNA)

cell surface membrane

a thin membrane about 7 nm wide, partially permeable; responsible for controlling the exchange between the intracellular and extracellular environment via cell transport, cell signalling, and acts as a boundary between the intracellular and extracellular environment

cytoplasm

contents of the cell, excluding the nucleus

mitochondria

a double-membrane bound organelle with infoldings called cristae; responsible for aerobic respiration

cell wall

present in prokaryotes and plant cells, contains strengthening material such as peptidoglycan or lignin; responsible for preventing bursting by osmosis and keeping the cell's shape

plasmodesmata

pore-like structures found in plant cells connecting neighbouring plant cells; responsible for transporting materials from one plant cell to another

vacuole

usually found in plant cells, a large storage organelle; responsible for storing biochemicals, waste products, and temporary vacuoles such as phagocytic vacuoles

tonoplast

the partially permable membrane surrounding plant vacuoles

chloroplast

a double-membrane bound organelle containing ribosomes (70S), grana made up of thylakoids and connected by lamellae, lipid droplets/starch granules; responsible for carrying out photosynthesis

rough endoplasmic reticulum

contains ribosomes on its surface; responsible for transport and synthesis of proteins

smooth endoplasmic reticulum

does not have ribosomes on its surface; responsible for transport and synthesis of lipids and steroids

Golgi body

flattened discs made up of cisternae; responsible for modification and packaging materials such as proteins and lipids into vesicles, also produces lysosomes

lysosome

a vesicle containing hydrolytic enzymes (hydrolases); responsible for breaking down pathogens and/or old organelles, they also break down cells after cell death

ribosome

contains two subunits (depending on size), can be 80S (only in eukaryotes in cytoplasm) or 70S, made up of ribosomal RNA (rRNA) and protein; responsible for synthesis of proteins

centrioles

a cylindrical structure formed by nine 'triplets' of microtubules; responsible in replication during interphase and organise microtubules during mitosis

microtubules

long, rigid, hollow tubes about 25 nm in diameter, made from the protein tubulin; responsible for forming the cytoskeleton and allow transport within the cell

cilia

hair-like projection of a cell that may allow movement (motile) or not (non-motile), has a '9+2' arrangement of microtubules

flagella

tail-like projection of a cell that allows movement of a cell

microvilli

projections of a cell membrane (can be caused by infolding); responsible for increasing the surface area to volume ratio to increase efficiency of cell transport

bacteria

unicellular prokaryotic microorganism, typically associated with disease, cell wall contains peptidoglycan and may also have a capsule layer among other structural features

virus

a very small (20-300 nm) infectious particle that only replicates in living cells; contains genetic material (DNA/RNA) and surrounded by a protein coat, viruses do not contain any other structures