2.1 Cell Structure

Organisms can be prokaryotes or eukaryotes.

Prokaryotic organisms:

• They are single-celled organisms
• They are smaller and simpler
• e.g: bacteria

Eukaryotic organisma:

• Have membrane-bound organelles
• More complex
• E.g: all animal & plant cells

Both contain organelles.

• Organelle - a part of a cell that has a specific function

These contain:

• plasma membrane (cell surface membrane)
• rough endoplasmic reticulum
• nucleus
• smooth endoplasmic reticulum
• lysosome
• ribosome
• golgi apparatus (golgi body)
• cytoplasm
• mitochondrion
• vesicle
• cilia
• flagellum

These contain:

• chloroplast
• cell wall
• vacuole
• plasmodesma
• plasma membrane (cell surface membrane)
• rough endoplasmic reticulum
• nucleus
• smooth endoplasmic reticulum
• lysosome
• ribosome
• golgi apparatus (golgi body)
• cytoplasm
• mitochondrion
• vesicle
• cilia
• flagellum

Nucleus:

• nuclear envelope
• nuclear pore
• nucleolus
• chromatin

Ribosome:

• large subunit
• small subunit

Rough Endoplasmic Reticulum:

• ribosome
• fluid

Vesicle:

• cell plasma membrane
• vesicle

Golgi Apparatus:

• golgi body
• vesicle

Mitochondrion:

• outer membrane
• inner membrane
• cristae
• matrix

Chloroplast:

• stroma
• two membranes
• granum
• lamella

Centriole:

• microtubule

Cilia:

• side
• cross-section

Cell Wall:

• cell wall
• plasma membrane
• cytoplasm

Plasma membrane:

• cell surface membrane
• cytoplasm

Cell membrane - regulates the movement of substances into and out of the cell, it has receptor molecules which respond to chemicals (e.g: hormones)

Nucleolus - makes ribosomes

Nuclear envelope - double membrane

Nuclear pores - allow substances (e.g RNA) to move between the nucleus and the cytoplasm

Mitochondrion - site of aerobic respiration where ATP is produced, found in large number of active cells, require lots of energy

RER - folds and processes proteins that have been at the ribosomes

SER - synthesises and processes lipids

Golgi apparatus - processes and packages new lipids and proteins, also makes lysosomes

Vesicles - transports substances in and out of the cell and between organelles

Lysosome - contains digestive enzymes,kept separate from the cytoplasm by the surrounding membrane and can be used to digest invading cells or to break down worn out components of the cell

Ribosomes - site where proteins are made

Centrioles - involved with the separation of chromosomes during cell division

Chloroplasts - site where photosynthesis takes place

Vacuole - contains cell sap, sugars and solutes

Cell wall - supports plant cell, has channels called plasmodesmata

Cilia - the microtubules allow the cilia to move, this movement is used  by the cell to move substances along the cell structure

Flagellum - for movement, the microtubules contract to make the flagellum move

• mRNA copy of protein of the gene is made in the nucleus

• mRNA leaves the nucleus through the nuclear pore

• mRNA attaches to a ribosome, (could be RER)

• ribosome reads instructions to assemble the protein

• protein molecules are ‘pinched off’ in vesicles and travel towards golgi body

• vesicle fuses with golgi body

• golgi body ‘processes and packages’ protein molecules ready for release

• packaged protein molecules are ‘pinched off’ in vesicles from golgi body and move towards plasma membrane

• vesicles fuses with plasma membrane

• plasma membrane opens to release protein molecules outside

Cytoskeleton: the network of protein threads

Uses:

• provide mechanical strength to cells

• aiding transport within cells

• enabling cell movement

Structure:

• microtubules - tiny protein cylinders

• microfilaments - small solid strands

Main functions:

• support cell

• keep them in position

• strengthen the cell

• maintain its shape

• movement of materials (components) within the cell (e.g: movement of chromosomes during separation of cell division, this depends on microtubules in the spindle)

• proteins of cytoskeleton can also cause the cell to move (e.g: movement of cilia and flagella is caused by the cytoskeletal protein filaments that run through them)

Similarities & Differences:

Prokaryotes:

Eukaryotes:

no true nucleus, naked DNA

nucleus present - genetic material in chromosomes

circular DNA

linear DNA

average size - 0.5-5 um

average size - 10-100 um

ribosomes about 10-20 nm (small)

ribosomes about 22nm (large)

cell wall, not made of cellulose

no cell wall in animals, cellulose cell wall in plants

flagellum in helix

flagellum arranged in a ‘9+2’ formation

not membrane-bound organelles

membrane-bound organelles

Bacteria cells are prokaryotes:

• Prokaryotes are roughly a tenth the size of eukaryotic cells
• This means tge normal microscopes aren't really powerful enough to look at their internal structure.

They have:

• flaggelum - tail used to propel the cell
• circular DNA  - bacterial chromosome
• plasma mebrane
• cell wall
• plasmid - ring of DNA

• magnification: number of times an image has been enlarged
• resolution: the ability to distinguish two points as separate points (resolution of human eye is approx. 0.3mm / 3/10th mm
• organelles: structures within a cell, e.g: nucleus
• ultrastructure: inner detail organelles within a cell

Formula:

magnification = image size

                       __________

                        object size

Units:

Symbol:

Conversion factor to the next:

meter

m

x100

centimeter

cm

x10

millimeter

mm

x1000

micrometer

um

x1000

nanometer

nm

x1000

picometer

pm

x1000

• magnification: number of times an image has been enlarged

• resolution: the ability to distinguish two points as separate points

light microscope

transmission electron microscope

scanning electron microscope

max. resolution

0.2 um

0.00002 um

0.002 um

max. magnification

x1500

x1,000,000

x500,000

Light Microscopes:

• use light

• max. resolution = 0.2 um

• max. magnification = x1500

• lower resolution than electron microscopes

Laser Scanning Confocal Microscopes:

• use intense beams of light (laser beams) to scan a specimen which is usually tagged with a fluorescent dye

• laser causes the dye to fluoresce (give off light), the light is then focused through a pinhole onto a detector

• the detector is hooked up to a computer, which generates an image

• adv: the image can be 3D

• adv: they have a pinhole which means that any out-of-focus  light is blocked so these microscopes produce a much clearer image (than a light microscope)

• black and white, with false colour

• they can be used to look at objects at different depths in thick specimens

Transmission Electron Microscopes - TEM:

• uses electromagnets to focus a beam of electrons, which is then transmitted through their specimen

• max. resolution = 0.0002 um

• max. magnification = x1,000,000

• images are darker because denser parts of the specimen absorb more electrons

• black and white, with false colour

• adv: they provide high resolution images because they can distinguish between the smallest objects so they can be used to look at a range of organelles

• dis adv: they can only be used on thin specimens

Scanning Electron Microscope - SEM:

• scan a beam of electrons across the specimen, this knocks of electrons from the specimen which are gathered in a cathode ray tube to form an image

• max. resolution = 0.002 um

• max. magnification = x500,000

• black and white, with false colour

• adv: the image produced shows the surface of the specimen

• adv: the image produced can be 3D

• dis adv: they have a lower resolution images than TEM

Name of microscope

Magnification

Resolution

2D or 3D

Black, white or colour

optical/light microscope

x1500 → x2000

0.2 um

2D

colour

laser scanning

3D

colour

transmission electron microscope

x2,000,000

50 pm

2D

black + white but adds false colour

scanning electron

x15 → x200,000

50 nm

3D

colour