1.1 Cell Structure

What is the definition of a eukaryotic cell?

• Found in plants and animals

• Contains nucleus

What is the definition of a prokaryotic cell?

• Found in bacteria

• No nucleus

What do plant and animal cells (eukaryotic cells) have?

• Cell membrane

• Cytoplasm

• Genetic material — enclosed in nucleus

What do bacterial cells (prokaryotic cells) have?

• Cytoplasm

• Cell membrane

  • Surrounded by cell wall

• Genetic material — not enclosed in nucleus

  • Single loop of DNA

  • Plasmids — small rings of DNA

Compare the size of eukaryotic cells and prokaryotic cells.

prokaryotic cells < eukaryotic cells

How much is a centimetre (cm) in metres?

1 × 10^-2 m

How much is a millimetre in metres?

1 × 10^-3 m

How much is a micrometre in metres?

1 × 10^-6 m

How much is a nanometre in metres?

1 × 10^-9 m

What parts do animal cells have?

• Nucleus

• Cytoplasm

• Cell membrane

• Mitochondria

• Ribosomes

What parts do plant cells have?

• Nucleus

• Cytoplasm

• Cell membrane

• Mitochondria

• Ribosomes

• Chloroplasts

• Cell wall — cellulose

• Permanent vacuole — filled with cell sap

Plant and ________ cells have a cell wall made of ___________, which _____________ the cell.

• algal

• cellulose

• strengthens

Label the animal cell.

• A — Cytoplasm

• B — Nucleus

• C — Ribosomes

• D — Mitochondria

• E — Cell membrane

Label the plant cell.

• A — Cytoplasm

• B — Nucleus

• C — Ribosomes

• D — Vacuole

• E — Mitochondria

• F — Cell membrane

• G — Cell wall

• H — Chloroplasts

What is the function of the nucleus?

• Controls activities of cell

• Contains DNA

What is the function of the cell membrane?

Controls movement and transport of substances — in and out of cell

What is the function of the mitochondria?

Energy is released and transferred → aerobic respiration

What is the function of ribosomes?

Proteins are synthesised

What is the function of the cell wall?

• Strengthens cell

• Prevents bursting — too much water enters cell by osmosis

What is the function of chloroplasts?

Traps light → photosynthesis

What is the function of the permanent vacuole?

Contains cell sap — keeps cell turgid

What is the function of the cytoplasm?

Site of chemical reactions

Label the microscope.

• A — Eyepiece

• B — Objective lenses

• C — Stage clip

• D — Condenser

• E — Mirror

• F — Coarse focus

• G — Fine focus

• H — Arm

• I — Stage

Describe a method for Required Practical 1 — Microscopy.

• Peel off thin epidermal layer on onion — forceps

• Place sample flat onto drop of water on slide

• Add few drops of iodine solution — stains cells

• Slowly lower cover slip at an angle (mounting needle) — no air bubbles

• Place slide on stage — select lowest power objective

• Use coarse focus — brings cells into focus

• Use fine focus — makes image clearer

A student prepared some animal cells to view using a microscope.

Figure 2 shows the student’s light microscope.

The student tried to look at the cells using the microscope.

Suggest one reason why the student could not see any cells when looking through part A. [1 mark]

No cells in the field of view

A student prepared some onion cells.

The student viewed the onion cells using a light microscope.

Figure 1 shows what the student saw under the microscope at a magnification of ×400.

Figure 2 shows the student’s drawing of Figure 1.

Give two ways the student could improve the drawing in Figure 2.

• Include magnification

• Use continuous lines

Why do you use a thin tissue layer in RP1?

• Helps see individual cells clearly

• Allows light to penetrate sample

Why do you use iodine in RP1?

Stains cells — makes sub-cellular structures visible

Why do you lower the cover slip at an angle in RP1?

Prevents air bubbles from getting trapped under slip

What are some hazards/risks in RP1 — how can they be minimised?

• Iodine (irritant) — may cause rash/allergic reaction

  • Wear gloves

  • Wash hands after contact

  • Clean spills quickly

• Sharp knives — can cut skin

  • Use chopping board

  • Cut away from body

  • Keep fingers clear of blade

What are sperm cells specialised to do?

Carry male’s DNA to egg cell — reproduction

How does the structure of a sperm cell help it carry out its function?

• Streamlined head and long tail — helps it swim to egg

• Many mitochondria — site of respiration → energy for movement

• Acrosome has digestive enzymes — breaks down outer layers of membrane of egg cell

What are nerve cells specialised to do?

Transmit electrical signals quickly — one place in body to another

How does the structure of a nerve cell help it carry out its function?

• Long axon — impulses carried long distances

• Dendrites — branched connections form with other nerve cells

• Many mitochondria (nerve endings) — site of respiration → energy to make neurotransmitters

  • allows impulses to be passed from cell to cell

What are muscle cells specialised to do?

Contract and relax → causes movement in the body

How does the structure of a muscle cell help it carry out its function?⁵

• Proteins slide over each other → causes contraction

• Many mitochondria — site of respiration → energy for contraction

• Stores glycogen — used in respiration by mitochondria

What are root hair cells specialised to do?

• Take up:

  • water → osmosis

  • mineral ions → active transport — soil → cell

How does the structure of a root hair cell help it carry out its function?

• Root hairs = large SA → more water moves in

• Large permanent vacuole:

  • affects speed of movement of water — soil → cell

• Mitochondria — site of respiration → energy for active transport

What are xylem cells specialised to do?

Transport water and mineral ions up plant — root → shoot

How does the structure of a xylem cell help it carry out its function?

• Lignin = cell death — cells become hollow and join end-to-end → forms continuous tube

  • water and mineral ions move through

• Lignin deposited in spirals — cells withstand pressure ← movement of water

What are phloem cells specialised to do?

Carry products of photosynthesis to all parts of plant

How does the structure of a phloem cell help it carry out its function?

• Cell walls of each cell form sieve plates when broken down → movement of substances — cell → cell

• Mitochondria in companion cells — energy for cells to stay alive

As an organism develops, cells _____________ to form different types of cells:

• Most types of ___________ cell ______________ at an _______ stage.

• Many types of ________ cells retain the ability to ______________ throughout life.

• differentiate

• animal

• differentiate

• early

• plant

• differentiate

What is cell division mainly restricted to in mature animals?

• Repair

• Replacement

What happens as a cell differentiates?

• Acquires different sub-cellular structures — enables it to carry out specific function

  • → Specialised cell

Why is cell differentiation important?

Allows production of different tissues and organs — perform vital functions

What is cell differentiation?

Process by which cells become specialised

• An electron microscope has much higher _______________ and ___________ _________ than a light microscope.

• This means that it can be used to study cells in much _________ _______.

• magnification

• resolving power

• finer detail

What is the formula for magnification?

magnification = size of image ÷ size of real object

What process of cell division do bacteria multiply by?

Binary fission

Bacteria multiply by simple cell division (__________ _________) as often as once every ____ minutes if they have enough ___________ and a suitable _______________.

• binary fission

• 20

• nutrients

• temperature

How can bacteria be grown?

• Nutrient broth solution

• Colonies on agar gel plate

What is required for investigating the action of disinfectants and antibiotics?

Uncontaminated cultures of microorganisms

• You are preparing an uncontaminated culture using aseptic technique.

Explain why petri dishes and culture media must be sterilised before use.

• Kills unwanted microorganisms already present — they could:

  • contaminate culture

  • compete with your bacteria for nutrients

• You are preparing an uncontaminated culture using aseptic technique.

Explain why inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame.

Kills bacteria on loop — prevents transfer to agar

• You are preparing an uncontaminated culture using aseptic technique.

Explain why the lid of the Petri dish should be secured with adhesive tape and stored upside down.

• Loosely taping lid:

  • stops airborne microbes from entering — stills allows oxygen in → prevents growth of harmful anaerobic bacteria

• Storing upside down:

  • stops condensation dripping onto colonies → would disrupt growth

• You are preparing an uncontaminated culture using aseptic technique.

Explain why in school laboratories, cultures should generally be incubated at 25°C.

Reduces risk of growing human pathogens (37°C)

Required Practical 2 — Microbiology:

Describe a method for investigating the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition.

• Sterilise petri dish and agar → kills unwanted bacteria

• Pass inoculating loop through flame → sterilises it

• Use loop to spread bacteria onto agar

• Open petri dish lid slightly → prevents microbes from air entering

• Place filter paper discs soaked in different antiseptics onto agar — forceps

• Seal lid with tape → prevents airborne microbes from entering

• Incubate plate at 25°C → prevents growth of human pathogens

• Measure zones of inhibition — ruler

  • compare effectiveness of each antiseptic

Explain why scientists use 37°C but students must use 25°C to incubate bacteria. [3 marks]

• 37°C is body temperature — bacteria grows better at 37°C

• 25°C prevents growth of bacteria that are harmful to humans