GCSE AQA Biology - Cell Biology

Eukaryotic cell

A cell that’s genetic material is enclosed in a nucleus

Prokaryotic cell

A cell that’s genetic material is not enclosed in a nucleus

Subcellular structures in an animal cell

Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes

Nucleus

Contains genetic material that controls cell activities

Cell membrane

Holds the cell together and controls what goes in and out.

Mitochondria

The site of reactions for aerobic respiration, transferring energy that the cell needs to work.

Ribosomes

Produce proteins in the cell.

Features plant cells have that animal cells do not

Cell wall, permanent vacuole, chloroplasts

Cell wall

Supports the cell and strengthens it.

Permanent vacuole

Contains cell sap, a weak solution of sugars and salts.

Chloroplasts

The site of photosynthesis, containing chlorophyll which absorbs light for photosynthesis.

Subcellular structures in a bacterial cell

cytoplasm, cell membrane, cell wall, single strand of DNA, plasmids, flagella

plasmids

small rings of DNA

Light microscopes

Use light and lenses to form a magnified image of the specimen.

Electron microscopes

Use electrons to form a magnified image.

Electron microscope advantages (3)

• Higher magnification

• Higher resolution

• Allows us to see much smaller things in more detail

Resolution

The ability to distinguish between two points.

magnification =

image size / actual size

Steps to prepare a microscope slide (4)

• Add a drop of water to the middle of a clean slide

• Using tweezers, place the tissue into the water

• Add a drop of iodine to stain, highlighting objects in a cell by adding colour to them

• Place a cover slip on top

What is the first step to using a microscope?

Clip the slide onto stage

What is the first lens to select when using a microscope?

The lowest-powered objective lens (produces the lowest magnification).

How should you position a slide correctly under the lens?

Use the coarse focus knob to position the stage just below the objective lens.

What is the next step after adjusting the stage?

Look down the eyepiece, and adjust the coarse focus knob until the image is roughly in focus.

How do you achieve a clear image after rough focusing?

Adjust the focus using the fine focus knob.

Differentiation

The process by which a cell becomes specialised.

When do most animal cells differentiate?

At an early stage

What is the importance of cell differentiation?

Allows cells to carry out specific functions.

What happens to the ability to differentiate in most animal cells?

It is lost at an early stage.

What happens to the ability to differentiate in most plant cells?

It is retained.

What are cells that differentiate in mature animals used for?

Repairing and replacing cells.

Sperm cells

Specialised for reproduction.

Sperm cell adaptations

• Long tail and streamlined head

• Lots of mitochondria

• Enzymes - allow it to digest through egg cell membrane

Nucleus

Contains the genetic material.

Chromosomes

A long molecule of DNA.

How are chromosomes normally found?

In pairs.

3 stages of the cell cycle

• Growth and DNA replication

• Mitosis

• Cytokinesis

Describe the growth and DNA replication stage of the cell cycle (2)

• The cell grows and increases the amount of sub-cellular structures

• The DNA is duplicated

Describe the mitosis stage of the cell cycle (4)

• Chromosomes line up along the centre

• Cell fibres pull them apart to opposite ends

• The nucleus divides into two

• The cytoplasm and cell membrane divide, forming two daughter cells

Binary fission

The type of cell division carried out by prokaryotic cells in which the cell makes copies of it’s genetic material before splitting in two.

Conditions required for binary fission (2)

• warm environment

• Lots of nutrients

Culture medium

Allows growth of bacteria (and other microorganisms) by providing the nutrients they need to grow.

Why are cultures of microorganisms not incubated above 25°C in schools?

To prevent the growth of more harmful pathogens

Examples of culture mediums

Nutrient broth solution and solid agar jelly.

How can we use sterilisation to prevent contamination?

• Sterilise all petri dishes and culture media

• Sterilise inoculating loop by passing it through

How can we prevent microorganisms from the air getting in?

Attach the lid of the petri dish using adhesive tape

Why should the petri dish be stored upside down?

To stop drops of condensation falling onto the agar surface

How can we calculate the area of inhibition?

a=πr²

Stem cells

Undifferentiated cells that have the ability to become one of many different types of cells.

Embryonic stem cells

Stem cells found in early human embryos which can be cloned and made to differentiate into most different types of human cells.

Adult stem cells

Stem cells found in certain tissue (for example bone marrow) that can form many types of cells.

Meristem tissue

Tissue found at the tips of plant shoots and roots that is able to differentiate into any type of plant cell.

What conditions can be treated using stem cells?

Diabetes and paralysis

Therapeutic cloning

An embryo is made to have the same genetic information as the patient.

Advantage of therapeutic cloning

Stem cells produced from the embryo will contain the same genes so are less likely to be rejected by the patient’s body.

Risk of using stem cells in medicine

Stem cells may be contaminated with a virus which could be passed on to the patient.

Ethical issue with stem cell research

Each human embryos can be considered a potential human life, so potentially shouldn’t be used in experiments.

Uses of meristem tissue

• Produce clones of whole plants

• Grow more plants of a rare species

• Grow crops of identical plants that have desired features

Diffusion

The spreading out of particles from an area of higher concentration to an area of lower concentration.

Factors affecting the rate of diffusion

• Concentration gradient

• Temperature

• Surface area of the membrane

Why do single-celled organisms have a relatively large surface area to volume ratio?

To provide sufficient transport of molecules into and out of the cell.

Osmosis

The movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

Active transport

The movement of substances from a more dilute solution to a more concentrated solution (against the concentration gradient).

What does active transport require?

Energy from respiration

What do root hair cells use active transport for?

Absorbing mineral ions from very dilute solutions in the soil.

What do plants require mineral ions for?

Healthy growth

How is active transport used in the gut?

To allow nutrients to be taken into the blood when there’s a lower concentration of nutrients in the blood.

How should potatoes be prepared in the osmosis RPA?

Cut into identical cylinders.

Independent variable in osmosis RPA

Sugar concentration of solution

Why are the potato cylinders dried with a paper towel?

To remove any surface moisture which may affect the mass measurement

What does an increase in mass of the potato cylinder show?

Water has been drawn into the cylinder by osmosis