Cell structure

• A population of bacteria divides every 15 minutes
• 15 minutes is equal to 0.25 hours
• How many divisions will bacteria undergo in 3 hours if they divide every 15 minutes?

  12
• A bacterial population starting with 5 bacteria and dividing 12 times will result in 20,480 bacteria.
• Steps to calculate the number of bacteria in a population after a certain time

  1️⃣ Calculate the number of times the bacteria will divide in the given time period.

  2️⃣ Use the formula: Number of bacteria at the end = Number of bacteria at the beginning × 2number of divisions.

  3️⃣ Express the answer in standard form if possible.
• The formula to calculate the cross-sectional area of a bacterial colony is πr$^{2}$
• What is the approximate value of π used in the formula for cross-sectional area?

  3.14
• Harmful pathogens are less likely to grow at 25°C.
• The Petri dish lid must be secured with adhesive tape
• Why is the Petri dish lid not fully sealed during incubation?

  To prevent anaerobic growth
• Storing Petri dishes upside down prevents condensation from dripping onto the colonies.
• Inoculating loops are sterilised by passing them through a Bunsen burner flame
• Why must Petri dishes and culture media be sterilised before use?

  To kill bacteria
• Steps to prepare an uncontaminated culture using aseptic technique

  1️⃣ Use pre-sterilised Petri dishes or sterilise glass dishes and agar gel in an autoclave.

  2️⃣ Pour the sterile agar gel into the Petri dish and allow it to set.

  3️⃣ Sterilise the inoculating loop by passing it through a Bunsen burner flame.

  4️⃣ Dip the inoculating loop into the solution of microorganisms and make streaks on the agar surface.

  5️⃣ Put the lid on the Petri dish, secure it with tape, and store upside down.

  6️⃣ Incubate the culture at 25°C in school laboratories.
• Uncontaminated cultures are needed to investigate disinfectant and antibiotic action.
• How do bacteria multiply?

  Binary fission
• Standard form is a way of expressing numbers as a figure between 1 and 10 multiplied by a power of 10.
• The formula to calculate magnification is: magnification = image size / real object size
• What are four disadvantages of electron microscopes?

  Expensive, large, require training, dead specimens only
• What is the main advantage of electron microscopes over light microscopes?

  Greater magnification and resolution
• A light microscope passes a beam of light through a specimen to produce an image.
• What is resolution in microscopy?

  The smallest distance between objects
• In mature animals, cell division is primarily for repair and replacement of cells.
• For how long do plant cells retain the ability to differentiate?

  Throughout their life cycle
• Why is cell differentiation important in multicellular organisms?

  To create tissues and organs
• Cell differentiation is the process by which cells become specialized.
• Phloem cells have sieve plates to allow transport of dissolved amino acids and sugars.
• For how long do plant cells retain the ability to differentiate?

  Throughout their entire life cycle
• Most animal cells differentiate early in their life cycle
• What is cell differentiation?

  Becoming specialised
• Sieve plates in phloem cells allow the transport of dissolved amino acids and sugars
• Companion cells in phloem provide energy for active transport of substances.
• What adaptation allows water to flow continuously in xylem cells?

  No upper or lower margins
• Thick, woody side walls in xylem cells strengthen their structure and prevent collapse
• Why do root hair cells have a large surface area?

  To absorb nutrients and water
• The thin walls of root hair cells do not restrict water absorption.
• Mitochondria in muscle cells provide energy for muscle contraction
• What allows skeletal muscle fibres to contract in unison?

  Merged cells
• Arrange the adaptations of nerve cells in order of their function:

  1️⃣ Long axon for impulse transmission

  2️⃣ Dendrites to receive impulses

  3️⃣ Myelin sheath to speed up impulses
• What is the function of chloroplasts?

  Site of photosynthesis