AQA GCSE biology - cell biology

What are the two types of cells?

• Eukaryotic (plant and animal)

• Prokaryotic (bacteria)

What are the differences between eukaryotic and prokaryotic cells?

• Prokaryotic cells are much smaller than eukaryotic cells.

• They have a cytoplasm and a cell membrane surrounded by a cell wall.

• Their genetic material is not enclosed in a nucleus (like eukaryotic cells) - it is a single DNA loop and there may be one or more small rings of DNA called plasmids

How is genetic information stored in a prokaryotic cell?

It is found free within the cytoplasm as:

• a single large loop of DNA

• Plasmid DNA

What are plasmids?

Small circular loops of DNA found free in the cytoplasm and separate from the main DNA. They carry gender that provide genetic advantages

What is order of magnitude?

A power to the base 10 used to quantify and compare size

What is a centimetre (cm)?

1 × 10^-2m

What is a millimetre (mm)?

1 × 10^-3m

What is a micrometre (µm)

1 × 10^-6m

What is a nanometre (nm)?

1 × 10^-9m

How to calculate the difference in order of magnitude

1. Convert the measurements into metre

2. Calculate the difference between the indice values

List the organelles found in both plant and animal cells (5)

• Nucleus

• Cytoplasm

• Cell membrane

• Mitochondria

• Ribosomes

List the additional organelles found in plant cells (3)

• Chloroplasts

• Permanent vacuole filled with cell sap

• Cell wall made of cellulose

What is the function of the nucleus?

To store genetic information (DNA) and control cellular activities

Describe the structure of the cytoplasm

• Fluid component of the cell

• Contains organelles, enzymes and dissolved ions and nutrients

What is the function of the cytoplasm?

It is the site of chemical reactions in the cell and a transport medium

What is the structure and function of the cell membrane?

Its structure is permeable to some substances but not to others. It therefore controls the movement of substances in and out of the cell

What is the function of the mitochondria?

It contains the enzymes for respiration, and where most energy is released in respiration

What is the function of ribosomes?

Joins amino acids in a specific order for protein synthesis

What is the plant cell wall made of?

Cellulose

What is the function of the plant cell wall?

To provide strength and support, and prevent the cell curating when water enters via osmosis

What does the permanent vacuole contain?

Cell sap (a solution of salts, sugars and organic acids)

What is the function of the permanent vacuole?

To support the cell and maintain its turgidity

What is the function of chloroplasts?

Chloroplasts contain the green pigment chlorophyll which absorbs light energy for photosynthesis. Contains the enzymes needed for photosynthesis.

Site of photosynthesis

Describe how sperm cells in animals are adapted to their function

• Haploid nucleus contains genetic information

• Tail/flagellum enables movement

• Mitochondria provide energy for tail movement

• Acrosome contains enzymes that digest the egg cell membrane, so the sperm can enter the egg

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

• Arrangement of protein filaments allows them to slide over each other to produce muscle contraction

• Mitochondria to provide energy for muscle contraction

• Merged cells in skeletal muscle allow muscle fibre contraction in unison

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 that do not restrict water absorption

Describe how xylem cells in plants are adapted to their function

• No upper or lower margins between cells to provide a continuous route for water to flow

• 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

Why is cell differentiation important?

It 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?

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? (2)

• Inexpensive

• Easy to use

What is the disadvantage of light microscopes?

Limited resolution

How does an electron microscope work?

It uses a beam of electrons wich 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 cub-cellular structures (e.g. mitochondria, ribosomes) to be observed in much finer detail

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

What are the disadvantages of electron microscopes? (2)

• Expensive

• Large so less portable

What is the magnification equation?

Magnification = size of image/size of real object

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

Which 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 the action of disinfectants and antibiotics

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 an autoclave

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

3. Sterilise the inoculation 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 of the Petri dish and secure it with tape. Label accordingly and store upside down

6. Incubate the culture at 25 degrees C

Why must Petri dished 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 dully 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 25 degrees C?

Harmful pathogens are less likely to grow t this temperature

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

Pi r²

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 bacteria:

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

Express the answer in standard from if possible

What are chromosomes made up of?

Chromosomes consist of DNA molecules combined with proteins. They contain gene which provoked 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 many 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

It is the longest stage - cells grow and increase in mass, DNA replicates to form two copies of each chromosomes and synthesise more organelles/sub-cellular structures (e.g. 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

Why is cell division by mitosis important?

It is important in the growth and development of multicellular organisms

What is a stem cell?

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

What is the function of stem cells in embryos?

Embryonic stem cells can replicate themselves. And differentiate into many other types of cells.

Embryonic stem cells may be able to treat conditions such as paralysis and diabetes by dividing to replace damaged cells

What is the function of stem cells in adult bone marrow?

Adult stem cells can differentiate into several cell types to replace dead or damaged tissues. For example, bone marrow stem cells can form many different types of blood cell

What is the function of stem cells in plant meristems?

Meristem stem cells retain the ability to differentiate into any type of plat cell throughout their lifespan - they can differentiate into any cell which is required by the plant

What conditions can stem cell treatment help with?

Diabetes and paralysis

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

• Crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers

• Can produce identical plants for research

• Can produce clones of plants quickly and economically

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

• Many embryonic stem cells are sourced from aborted embryos - some people have as 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 onor 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 nay substance in solution, or particles of a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration

What three main factors affects the rate of diffusion?

• Concentration gradient - larger gradient, faster diffusion

• Temperature - higher temperature, faster diffusion

• Surface area - larger surface area, faster diffusion

Give examples of substances transported by diffusion in the lungs

Oxygen diffuses into the blood from the lungs and carbon dioxide diffuses into the lungs from the blood, both down their concentration gradient

Give examples of substances transported by diffusion in the kidney

Urea diffuses from cells into blood plasma so it can be excreted in urine

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

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 the process of diffusion?

Diffusion is the spreading out of the particles of any substances in solution, or particles in a gas, resulting in a net movement of particles from an area of higher concentration to an area of lower concentration

How are multi-celled organisms adapted for diffusion?

• Large multi-cellular organisms have relatively small surface area to volume ratios

• This is why larger organisms usually have exchange surfaces and transport systems, as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organisms needs otherwise

How is the small intestine adapted for diffusion/exchanging materials?

• It has a highly folded surface which is lined with specialised intestinal epithelial cells (which themselves have a highly folded cell membrane) – this increases SA

• Only one layer of epithelial cells covers the surface of each villus – this decreases diffusion distance

• Each villus has a good blood supply – this maintains a concentration gradient

How are the lungs adapted for gas exchange?

• Millions of alveol which collectively provide a huge surface area - this increases SA

• The wall of each alveolus is one cell thick - this decreases diffusion distance

• There is an excellent blood supply - this maintains a concentration gradient

How are the gills in fish adapted for diffusion?

• Each gill is made from lots of smaller plates called filaments, which are covered i projections called lamellar where gas exchange takes place (this increases SA)

• The stacks of gill filaments are kept apart by water, exposing a large surface area for diffusion

• A dense capillary network ensures a good blood supply

• The blood flows in the opposite direction to water passing through the gills - this maintains a concentration gradient

How are root cells adapted for exchanging materials?

• The root hair cells are extremely thin - decreases diffusion distance

• There are lots of root hair cells - increases SA

How are leaves adapted for diffusion?

• Stomata allow air to circulate inside the leaf - this decreases diffusion distance for carbon dioxide and oxygen

• The lower layer of the leaf is made from spongy mesophyll cells which allow air to circulate inside the leaf

What is osmosis?

Osmosis is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane

What happens if a potato piece gains mass after being placed in a solution?

• Water has moved into the plant tissue from the solution surrounding it by osmosis

• The solution surrounding the tissue is more dilute than the plat tissue (which is more concentrated)

What happens if a potato piece loses mass after being placed in a solution?

• Water must have moved out of the plant tissue into the solution surrounding it by osmosis

• The solution surrounding the plant tissue is more concentrated than the plant tissue (which is more dilute)

What happens if there is no overall change in mass after a potato piece is placed in solution?

There has been no net movement of water as the concentration in both the plat tissue and the solution surrounding it must be equal

What is the equation to calculate the percentage change in mass for each potato piece (osmosis)

(Final mass - Initial mass)/Initial mass x 100

How to calculate the rate of water uptake?

Change in mass x time (e.g per hour) / period of time where water uptake was measured

What is active transport?

Active transport moves substances from a more dilute solution to a more concentrated solution.

It requires energy released by respiration because active transport travels against/up the concentration gradient.

Active transport in plants: root hair cells

• Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)

• Mineral ions are needed by plants to function healthily

  • Magnesium ions are needed to make chlorophyll

  • Nitrate ions are needed to make amino acids and therefore for protein synthesis (and subsequently growth)