AQA BIOLOGY TOPIC 1 CELL BIOLOGY

What are the two types of cells?

● Eukaryotic (plant and animal)

● Prokaryotic (bacteria)

What are the differences between eukaryotic and prokaryotic cells?

- Eukaryotic cells have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.

- Prokaryotic cells are much smaller in comparison and they have a cytoplasm and a cell membrane surrounded by a cell wall.

- The genetic material is not enclosed in a nucleus, it is a single DNA loop and there may be one or more small rings of DNA called plasmids.

List the components of animal cells (5)

● Nucleus

● Cytoplasm

● Cell membrane

● Mitochondria

● Ribosomes

List the components of plant cells (8)

● Nucleus

● Cytoplasm

● Cell membrane

● Mitochondria

● Ribosomes

● Cell wall

● Chloroplasts

● Permanent vacuole

List the components of bacterial cells (5)

● Cell membrane

● Cytoplasm

● Cell wall

● Single circular strand of DNA

● Plasmids

What is the function of the nucleus?

Contains genetic material that controls the activities of the cell

What is the function of the cytoplasm?

Gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.

What is the function of the cell membrane?

The cell membrane regulates what enters and leaves the cell and also protects and supports the cell.

What is the function of the mitochondria?

Where the reactions for aerobic respiration take place to provide energy for the cell

What is the function of the ribosomes?

Where proteins are made in the cell

What is the function of the cell wall?

Supports and strengthens the cell

What is the function of chloroplasts?

The site of photosynthesis

What is the function of the permanent vacuole?

Contains cell sap, which supports the cell, maintaining its turgidity.

What is the single circular strand of DNA?

Contains genetic material and floats freely in the cytoplasm

What is the function of plasmids?

Contain genetic material in small rings

Describe how sperm cells in animals are adapted to their function

- Long tail and streamlined head to help it swim to the egg

- Lots of mitochondria in the middle of its body to provide energy

- Carries enzymes in its head to digest through the egg membrane

Describe how nerve cells in animals are adapted to their function

● Long axon allows electrical impulses to be transmitted all over the body from the central nervous system.

● Dendrites from the cell body connect to and receive impulses from other nerve cells, muscles and glands.

● Myelin sheath insulates the axon and speeds up the transmission of impulses along the nerve cell.

Describe how muscle cells in animals are adapted to their function

- Cells are long so that they have space to contract

- Lots of mitochondria to generate energy for the contraction

Describe how root hair cells in plants are adapted to their function

- Large surface area to absorb nutrients and water from surrounding soil

- Thin walls to minimise water absorption distance

- They also contain lots of mitochondria , which release energy from glucose during respiration in order to provide the energy needed for active transport.

Describe how xylem cells in plants are adapted to their function

- Hollow and have very few subcellular structures.

- Thick, woody side walls strengthen their structure and prevent collapse.

Describe how phloem cells in plants are adapted to their function

● Sieve plates let dissolved amino acids and sugars be transported up and down the stem.

● Companion cells provide energy needed for active transport of substances along the phloem.

What is cell differentiation?

The process by which cells become specialised for different functions.

Why is cell differentiation important?

Allows production of different tissues and organs that perform various vital functions in the human body.

At what point in their life cycle do most animal cells differentiate?

Early in their life cycle

For how long do plant cells retain the ability to differentiate?

Throughout their entire life cycle

What is the purpose of cell division in mature animals?

Growth, repair and replacement of cells

What changes does a cell go through as it differentiates?

Becomes specialised through acquisition of different sub-cellular structures to enable a specific function to be performed by the cell

Define magnification

The number of times bigger an image appears compared to the size of the real object.

Define resolution

The smallest distance between two objects that can be distinguished

How does a light microscope work?

Passes a beam of light through a specimen which travels through the eyepiece lens, allowing the specimen to be observed.

What are the advantages of light microscopes? (4)

● Inexpensive

● Easy to use

● Portable

● Observe both dead and living specimens

What is the disadvantage of light microscopes?

Limited resolution

Limited magnification

Cannot produce 3d images

How does an electron microscope work?

It uses a beam of electrons which are focused using magnets. The electrons hit a fluorescent screen which emits visible light, producing an image.

Name the two types of electron microscope

- Transmission electron microscope (TEM)

- Scanning electron microscope (SEM)

What is the advantage of electron microscopes?

Greater magnification and resolution.

Why do electron microscopes have a greater magnification and resolution?

They use a beam of electrons which has a shorter wavelength than photons of light.

How have electron microscopes enabled scientists to develop their understanding of cells?

● Allow small sub-cellular structures (e.g. mitochondria, ribosomes) to be observed in detail.

● Enable scientists to develop more accurate explanations about how cell structure relates to function.

What are the disadvantages of electron microscopes? (4)

● Expensive

● Large so less portable

● Require training to use

● Only dead specimens can be observed

What are the different parts of a light microscope?

- Eyepiece lens

- Objective lens

- Coarse adjustment knob

- Fine adjustment knob

- Stage

- Slide

- Light

What is the function of the eyepiece lens?

The part of the microscope that you look through, it magnifies the image that is produced by the objective lens.

What is the function of the objective lens?

Magnifies the object on the slide.

What is the function of the coarse adjustment knob?

To move the stage up and down to bring the object into focus.

What is the function of the fine adjustment knob?

This is used to make small adjustments to the focus.

What is the function of the stage?

The platform on which a slide is placed.

What is the function of the light?

To illuminate the specimen on the stage

Describe how you would prepare a microscope slide using onion tissue

● Peel off epidermal layer using forceps

● Place sample on drop of water on microscope slide

● Add drops of iodine solution to sample

● Lower cover slip onto sample using mounting needle

Why is a thin sample of tissue required?

Allows light to pass through so internal structures can be observed

Why is water added to the slide before the sample is mounted?

Allows the sample to be suspended between the slide and the cover slip, and ensures the cover slip stays in place

Why is iodine solution added to stain the cells?

Iodine solution adds contrast - reacts with starch in plant cells and turns blue-black; allows internal structures (e.g. nucleus) to become more visible

Why is the cover slip lowered using a mounting needle?

Ensures that no air bubbles are trapped

How would you measure the length of a cell viewed under the microscope?

Measure using eyepiece graticule and calibrate with stage micrometer

How would you calculate the magnification of an image?

Magnification = observed cell length / actual cell length

Magnification = size of image / size of real object

What safety precautions should be taken during this experiment?

Wear safety goggles when handling iodine solution

How do bacteria multiply?

Binary fission (simple cell division)

How often do bacteria multiply?

Once every 20 minutes if enough nutrients are available and the temperature is suitable.

State 2 ways in which bacteria can be grown

● Nutrient broth solution

● Colonies on an agar gel plate

What nutrients make up a nutrient broth solution?

All nutrients required for bacteria to grow including nitrogen for protein synthesis, carbohydrates for energy and other minerals.

What are uncontaminated cultures of microorganisms needed for?

Investigating disinfectant and antibiotic action.

Describe the preparation of an uncontaminated culture using aseptic technique

1. Use pre-sterilised plastic Petri dishes or sterilise glass Petri dishes and agar gel before using with an autoclave.

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

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

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

5. Put the lid on the Petri dish and secure it with tape. Label accordingly then turn and store upside down.

6. Incubate the culture at 25oC in school laboratories.

Why must Petri dishes and culture media be sterilised before use?

To kill any bacteria already present.

Why must inoculating loops be sterilised by passing them through a Bunsen burner flame?

To kill any bacteria present on the inoculating loop.

Why must the Petri dish lid be secured with tape and the whole dish stored upside down?

- Stops bacteria in the air contaminating the culture.

- The lid is not fully sealed to prevent the growth of anaerobic bacteria in a lack of oxygen.

- Upside down to prevent condensation from forming and dripping down onto the colonies

Why are cultures incubated at 25oC in school laboratories?

Harmful pathogens are less likely to to grow at this temperature

What is the formula used to calculate cross-sectional area of a bacterial colony or clear area around a bacterial colony?

πr2

How is the number of bacteria in a population after a certain time calculated from the mean division time?

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

2. Use the following equation to calculate the number of

Number of bacteria in population at end of time period = number of bacteria at the beginning of the time period x 2^number of divisions in the time period .

How to test the effectiveness of antibiotics?

1. Soak the paper discs in different types/concentrations of antibiotics and place on an agar plate evenly spread with bacteria. One disc should be a control, soaked in sterile water.

2. Leave the plate at 25 degrees for 2 days.

3. The zone of inhibition can be measured- the bigger it is, the more bacteria are killed and therefore the more effective the antibiotic is

What are chromosomes made up of?

Chromosomes consist of DNA molecules combined with proteins. They contain genes which provide the instructions for protein synthesis.

What is a gene?

A section of DNA which codes for a protein.

How many chromosomes are found in the nucleus of human body cells?

46

How many chromosomes are found in the nucleus of gametes?

23

How are chromosomes arranged in the nucleus of a body cell?

Arranged in pairs - 46 chromosomes form 23 pairs.

What are the three main stages of the cell cycle?

1. Replication of DNA and synthesis of organelles

2. Mitosis

3. Division of cell

Why is mitosis important?

● Produces identical cells which all have the same genetic information.

● Produces additional cells for growth and repair.

Describe the first stage of the cell cycle

Longest stage - cells grow and increase in mass, replicate DNA and synthesise more organelles (eg. mitochondria, ribosomes).

Describe the second stage of the cell cycle

Each chromosome in a pair is pulled to opposite poles of the cell, then the nucleus divides.

Describe the third stage of the cell cycle

The cytoplasm and cell membrane divide - two identical daughter cells are produced.

What is a stem cell?

An unspecialised cell which is capable of differentiating into other cell types and of self-renewal.

What are the 3 main types of stem cells?

- Embryonic stem cells

- Adult stem cells

- Meristems in plants

What are embryonic stem cells?

● Form when an egg and sperm cell fuse to form a zygote

● They can differentiate into any type of cell in the body

● Could be used to replace insulin-producing cells, create new neural cells or nerve cells for those paralysed

What are adult stem cells?

- Adult stem cells are found in differentiated tissue, such as bone or skin - they divide to replace damaged cells.

- Stem cells from adult bone marrow can form many types of cells including blood cells.

What are meristem cells?

● Found in root and shoot tips

● They can differentiate into any type of plant, and have this ability throughout the life of the plant

● They can be used to make clones of the plant

What is therapeutic cloning?

A cloning method where an embryo is produced with the same genetic makeup as the patient. The stem cells which originate from the embryo will not be rejected by the patient's immune system, so can be used to treat certain medical conditions

What are the advantages of cloning plants using meristem stem cells?

● Can prevent rare plants from becoming extinct.

● Can produce large numbers of plants with a favourable characteristic.

● Can produce identical plants for research.

What are issues are associated with the use of stem cells?

● Many embryonic stem cells are sourced from aborted embryos - some people have an ethical/religious objection.

● Development of stem cell therapies is slow, expensive and difficult.

● Adult stem cells infected with viruses could transfer infections to patients.

● If donor stem cells do not have a similar genetic makeup to the patient, an immune response could be triggered.

What is diffusion?

The spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

Give examples of substances transported by diffusion in the lungs and the kidney

● Lungs: oxygen diffuses into the blood from the lungs and carbon dioxide diffuses into the lungs from the blood, both down their concentration gradient.

● Kidney: urea diffuses from cells into blood plasma so it can be excreted in urine.

What three main factors affect the rate of diffusion?

● Concentration gradient - larger gradient, faster diffusion.

● Temperature - higher temperature, faster diffusion.

● Surface area - larger surface area, faster diffusion.

How are single-celled organisms adapted for diffusion?

They have a large surface area to volume ratio - maximises the rate of diffusion of molecules to meet the organism's needs

How is surface area to volume calculated?

Surface Area = Number of Sides x (Side Length x Side Width)

Volume = Length x Width x Depth

Ratio = Surface Area:Volume

How the small intestine is adapted for exchanging materials?

- Villi: Finger-like projections that increase surface area

- Microvilli: Hair-like projections on each villus that further increase surface area for absorption.

- Thin walls: Allow nutrients to pass through easily.

- Long length: Provides ample time for nutrient absorption to occur.

- Blood supply: Richly supplied with capillaries that carry absorbed nutrients into the bloodstream.

- Enzymes: Secretes digestive enzymes

How are the gills in fish adapted for exchanging materials?

- Large surface area: Many filaments increase oxygen absorption.

- Thin walls: Efficient gas exchange due to thin walls.

- Rich blood supply: Many capillaries in contact with water for oxygen and carbon dioxide exchange.

How are the roots in plants adapted for exchanging materials?

- Root hairs: Thin projections increase water and mineral absorption.

- Xylem: Transports water and minerals from roots to plant.

How are the leaves in plants adapted for exchanging materials?

- Stomata: Openings for gas exchange (CO2 and O2).

- Mesophyll cells: Contain chloroplasts for photosynthesis and optimize light absorption and gas exchange.

What four factors increase the effectiveness of a gas exchange surface?

● Large surface area

● Thin membrane (short diffusion path)

● Efficient blood supply (animals)

● Ventilation (animals)

What is osmosis?

The movement of water from a dilute solution to a concentrated solution through a partially permeable membrane.

What is meant when a solution is isotonic to a cell?

The concentrations of the external and internal (inside cell) solutions are the same.

What is meant when a solution is hypertonic to a cell?

The concentration of the external solution is higher than that of the internal solution (inside cell).

What is meant when a solution is hypotonic to a cell?

The concentration of the external solution is lower than that of internal solution (inside cell).

What may happen when an animal cell is placed in a hypotonic solution?

Water moves into the cell, causing it to burst.

What may happen when an animal cell is placed in a very hypertonic solution?

Water moves out of the cell, causing it to shrivel up.