4.1.1 Cell structure

4.1.1.1 Eukaryotes and prokaryotes

Eukaryotic Cells

• A eukaryotic cell contains a nucleus
• Plant and animal cells are eukaryotic
• Plant and animal cells have common organelles

• A eukaryotic cell contains a nucleus
• Plant and animal cells are eukaryotic
• Plant and animal cells have common organelles

Prokaryotic Cells

• A prokaryotic cell does not contain a nucleus
• Bacterial cells are prokaryotic cells
• Bacterial cells have similarities to plant and animal cells
• Bacterial cells have a cell wall
• Bacterial cells do not have a nucleus

Eukaryotes and Prokaryotes

• Prokaryotes: Single celled organisms which lack a true nucleus, i.e bacteria.
• Eukaryotes: Single celled and multicellular organisms which have a true, membrane bound nucleus.

Size and Scale

There are prefixes that stand for certain orders of magnitude:

• Centi – 10-2
• Milli – 10-3
• Micro – 10-
• Nano – 10-9

4.1.1.2 Animal and plant cells

Animal Cell Structure

Cell Membrane

• Controls movement in and out of cell
• Encapsulates the cell
• It is found in animal, plant and bacterial cells

Cytoplasm

• The solvent where chemical reactions occur
• Contains enzymes
• It is found in animal, plant and bacterial cells

Nucleus

• Genetic material contained in circular nucleus
• It is vital to the life of an organism
• They are found in animal and plant cells

Mitochondria

• Powerhouse of the cell
• Site of aerobic respiration – provides energy for survival and function
• They are found in animal and plant cells

Ribosomes

• Site of protein synthesis
• They are the smallest organelle in cells (not seen with a light microscope)
• They are found in animal, plant and bacterial cells

Cell Wall

• Made of the carbohydrate cellulose
• It provides rigidity to cell
• It is also found in cells of algae

Plant Cells

Remember that plant cells also have all of the organelles covered in animal cells

Permanent Vacuole

• The vacuole is permanent and contains cell sap
• Contains a mixture of salt and sugars
• Also helps to keep the cell turgid

Chloroplasts

• Site of photosynthesis to make glucose
• Chlorophyll absorbs light energy for photosynthesis
• Chlorophyll gives the cell a green colour

Cell Wall

• Made of carbohydrate peptidoglycan (might also be written as murein in some texts)
• Helps to provide rigidity to cell

Bacterial Cells

Bacterial cells have the same organelles as plant cells, except for a true nucleus, chloroplasts or mitochondria

Free DNA

• DNA is free in cytoplasm
• Single-stranded (unlike double-stranded DNA
• found in eukaryotic nucleus)

Plasmids

• Circular loops of DNA
• Contain additional genes

Cell Sizes

• Plant and animal cells are larger than bacterial cells
• Plant cells – 10 to 100 micrometres long
• Animal cells – 10 to 30 micrometres long
• Bacterial cell – 0.5 to 5 micrometres long

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4.1.1.3 Cell specialisation

Specialisation

Sperm Cells

Functions

• Used in reproduction
• Transfer genetic material from male parent to female parent
• Are adapted for fertilisation

Sperm Cells: Adaptations

• Acrosome – enzymes digest outer layers of egg cell
• Long tail – swim through the female reproductive system

• Mitochondria – provides energy for movement
• Big nucleus – holds genetic information

Nerve Cells

Functions

• Involved in the transmission of electrical impulses
• Aids sensation and movement of organism
• Are adapted to carry impulses

Nerve Cells: Adaptations

• Long axon – move impulses in the body
• Dendrites – contact other nerves at synapses using neurotransmitters
• Mitochondria – provides energy to make neurotransmitters
• Myelin sheath – provides insulation

Muscle Cells

Function

• Used for movement
• Cells contract and relax
• Are adapted for contraction

Muscle Cells: Function

• Striated muscle – striped, used for voluntary
• movement
• Biceps or skeletal muscle
• Smooth muscle – vessels and digestive system,
• used for involuntary movement
• Gastrointestinal tract moves food via peristalsis

Muscle Cells: Adaptations

• Many mitochondria are found in the striated muscle
• Provides energy
• Provides protein to aid movement
• Provides glycogen for respiration

Root Hair Cells: Function

• Absorption of water and mineral ions from soil
• Found on the surface of roots
• Are adapted for absorption for photosynthesis

Root Hair Cells: Adaptation

• Surface area increased – more water moves into cell
• Permanent vacuole – speed of osmosis increased
• Mitochondria – energy for active transport of mineral ions

Xylem

Function

• Provides support for the plant
• Movement of mineral ions and water from roots to the leaves and the stem
• Is adapted for support and transport

Xylem: Adaptation

• Spiral shape – kills tissue to form hollow tubes, for water and mineral ions to move through
• Lignin – strengthens cell for support

Phloem

Function

• Transports products of photosynthesis from the leaves to the stem and roots
• Is adapted for transport

Phloem: Adaptation

• Sieve plate – formed between the walls of cells, to allow movement of food
• Companion cells – contain mitochondria for energy transfer and moving food through phloem

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4.1.1.4 Cell differentiation

Cell Differentiation

• Cell differentiation is vital for the survival of the organism
• It is required for cells to specialise to a particular function
• During specialisation, the number of organelles or the size of the cell may change

Timing of Differentiation

• An embryo has stem cells, which are undifferentiated
• Clusters of cells become specialised to different tissues
• Animal cells differentiate early
• Plant cells can remain undifferentiated for longer

Differentiation > Specialisation

• Cell division is used to replace damaged cells
• Loss of skin cells causes surface cells to multiply to replace these lost cells

• Cell differentiation leads to different sub-cellular structures and characteristics
• Cell differentiation helps cells carry out a specific function, known as cell specialisation

4.1.1.5 Microscopy

Magnification and Resolution

Magnification and Resolution

Magnification – size of image compared to real size of object

Resolution – ability to tell the difference between two points

Higher resolution = more detailed image

Higher magnification = smaller the object seen through it

Calculating Magnification

• Magnification = Image Size ÷ Actual Size
• Standard form makes calculations easier
• Power is the number of places the decimal place moves to reach the key number
• E.g. 0.005 is represented as 5 x 10-3

Light Microscopes

• Created first, magnify things not usually seen with naked eye
• Magnification power up to x2000
• Beams of light and lenses magnify living object
• Used to see cells and large organelles (such as nuclei)
• Cheap, portable

Electron Microscopes

• Changed the field of biology
• Higher magnifying power, around x2,000,000
• Beams of electrons magnify non-living cells
• Used to see more sub-cellular structures (such as ribosomes) – not previously seen with a light microscope
• Large, expensive (specific operating conditions)

4.1.1.6 Culturing microorganisms

Bacterial Growth

Bacteria (and all prokaryotic cells) replicate themselves in a process called binary fission:

1. Plasmids and DNA are copied

First, all the genetic material in the cells are duplicated. This includes the plasmids and circular DNA.

2. DNA strands move to opposite poles of the cells

Next the cells grow in size and the circular DNA move to the two ends (poles) of the cells.

3. The cytoplasm of each cells splits

The cytoplasm of each cell starts to be split in two and new cells wall start being formed.

4. Daughter cells are formed

Two new cells called daughter cells are formed and have one copy of the circular DNA. The two new cells formed will each have one copy of circular DNA and are called daughter cells.

5. For some bacteria, this process occurs every 20 minutes

In the right environment, E. coli are able to duplicate themselves in just 20 minutes.

Culture Mediums

• Like all living organisms, bacteria need the right conditions to grow, which means providing carbohydrates, minerals, proteins and vitamins.
• Therefore, for bacteria to grow, you need to keep them in a culture medium.
• There are two culture mediums you need to know for AQA exams: agar jelly and nutrient broth solution.
• Agar jelly and nutrient broth solution are both ways to provide the required nutrients for

bacteria.

Agar Jelly

The steps below show how to set up an agar jelly culture medium:

1. Agar jellies are prepared in petri dishes to form plates

Hot agar jelly is poured into petri dishes which are plastic circular dishes and allowed to solidify and cool.

2. Microorganisms are spread on the plates

Microorganisms are then spread on the plates using an inoculating loop. Using an inoculating loop ensures that the microorganisms are spread evenly.

3. Keep the plates at below 25C0

The cultures should be kept at a temperature below 25oC to minimise the risk of harmful pathogens growing. In an industrial lab the temperature would be kept higher to increase the speed of growth.

Investigating Disinfectants + Antibiotics

• We can carry out simple investigations to find out the effectiveness of disinfectants or

antibiotics.

• We first need to decide whether to compare strength or type of antibiotic.
• This method can be used to assess either the effect of different concentrations of the

same antibiotic or compare different antibiotics.

1. Soak paper disks in the disinfectant or antibiotic

Soaking the papers ensures that the paper discs are saturated with the disinfectant or antibiotic.

2. Place these disks evenly around an inoculated agar jelly

The discs should be placed with gaps in between them around the cultured agar jelly.

3. To ensure that the test is valid, use a control

We want to ensure that it is actually the disinfectant or antibiotic causing an inhibition zone, as opposed to another factor. Therefore set up a separate experiment and the paper disk in distilled water instead of the disinfectant / antibiotic.

4. Leave the plate at 25oC for 48 hours

The plate should be kept at 25oC for two days to give the disinfectant/antibiotic time to take effect.

5. Measure the size of the clear area around the paper disk

This is known as the zone of inhibition and can be calculated by finding the radius of the circle and using the equation Area = π x r2

6. The larger the zone, the more effective the disinfectant/antibiotic

A more effective antibiotic will kill more bacteria and so create a larger zone of inhibition

around the paper disk soaked in it.

Aseptic Technique

• To make sure that our results are valid, we need to ensure that the environment is sterile.
• Unwanted microorganisms could cause random errors and even cause pathogens to be

incubated.

• We can keep things sterile by:

Sterilising equipment – heat the culture mediums and Petri dishes to a high temperature to ensure any microorganisms on them are killed and pass the inoculating loop through a hot blue flame.

Taping the lid on the petri dish – to ensure that any airborne microorganisms don’t get into

the petri dish, tape the lid on the petri dish after inoculating it with bacteria.

Placing the petri dish in the fridge upside down – placing the plates upside down will

prevent condensation falling onto the culture.

Calculating Colony Size

• The colony size is the number of bacteria in a specific population.
• To work out the colony size you need to know the mean division time, which is the average time taken for each bacteria to divide to form two daughter bacteria.
• This can be used to find out how many times the bacteria have divided in a time period.
• If both the time period and the mean division time are in the same units (e.g. minutes) then you can find the number of divisions.
• Every time division that occurs leads to a doubling in the population of bacteria.
• Therefore, to find the new number of bacteria, just multiply the original number of bacteria
• by 2 for each division.
• For example for 3 divisions it would be 2x2x2 so a multiplication by 8.

Using Standard Form

• For large numbers, we can express the answer in standard form.
• This involves expressing a single digit number before a decimal point raised to a power of 10.
• For example 1,850,000 can be expressed as 1.85 x106. The value 1.85 is multiplied by 10 six times to make 1,850,000.