GCSE Biology Cell Biology

Cells

Smallest unit of life that can replicate independently

Animal Cell Structure

Cell Membrane

Nucleus

Cytoplasm

Mitochondria

Ribosomes

Plant Cell structure

Cell Membrane

Nucleus

Cytoplasm

Mitochondria

Ribosomes

Cell wall

Permanent Vacuole

Chloroplasts

Cell membrane use

Control which substances can pass in and out of the cell

Nucleus use

Contains genetic material / DNA

Cytoplasm use

Where Chemical Reactions take place

Mitochondria use

Provide cells with the energy they need to function

Ribosomes

Site of protein synthesis

Cell wall

Rigid cell wall made of cellulose for plants which provide support and structure

Permanent Vacuole

Plants

Contains cell sap (mixture of sugars, salts and water)

Chloroplasts

Contains Chlorophyll (green dye which absorbs light energy) needed for photosynthesis

Bacterial Cells (Prokaryotes)

Unicellular

Cell wall - peptidoglycan cell wall for bacteria

Cytoplasm

Cell membrane

Ribosomes

Nucleoid - circular strand of DNA

Plasmids - Small free flowing rings of DNA

Flagella - allow Bacteria to move around

Differences between Eukaryotes and Prokaryotes

Prokaryotic cells are much smaller than Eukaryotic (10-100x smaller)

Prokaryotic cells have no membrane bound organelles

DNA is free flowing in the cytoplasm in Prokaryotic cells

Prokaryotic cells have no nucleus but Eukaryotic cells do

Light Microscope parts

Base

Arm

Light source (Lamp or mirror)

Stage above light source - Microscope slide goes on top

3 Objective lenses with different magnifications

Eyepiece lens at the top with fixed magnification

Body Tube between two lenses

Coarse focusing knob (bigger)

Fine focusing knob (smaller)

Object

The real object or sample that is being looked at

Image

The image that we see when we look down the microscope

How Light Microscope works

Light from room or lamp passes through microscope slide and through objective and eyepiece lenses into eyes

Lenses spread out light rays so that image is larger than object

Magnification

How many times larger the image is than the object
Magnification = Image size/Object size

Resolution

The shortest distance between two points on an object that can still be distinguished as two separate entities - how detailed an image is

Light microscopes advantages

Easy to use

Relatively cheap

Light microscope disadvantages

Rely on light (wavelength of 0.2 micro meters), Resolution is limited to 0.2 micro meters

Not good enough to see sub cellular structures

Electron microscopes Advantages

Use electrons instead of light - Have a wave length of 0.1 nm which means their resolution is much higher

Can be used to study sub cellular structures

Electron microscope Disadvantages

Very expensive

Very hard to use

Units of measurement of length

micrometer (um) - 1000x smaller than mm

nanometer (nm) - 1000x smaller than um

Why do Multicellular Eukaryotic Organisms need a constant supply of new cells

Growth, Development and Repair (especially for younger and older organisms)

Cell cycle

Growth
DNA replication and Mitosis
Division - Cytokinesis

Cell cycle 1 - Growth process

Cell grows in size

Increases number of sub cellular structures (mitochondria and ribosomes)

Cell Cycle 2 - DNA Replication and Mitosis

DNA Duplicated - So new cells have full sets of DNA

• Chromosomes duplicate but stay attached to original chromosome (forming X shape) (46 chromosomes in each cell, 23 from each parent, found in pairs)

• All 46 of cell chromosomes line up in centre of the cell

• Nucleus divides and Cell Fibres from either side of the cell attach to respective half of chromosome (X Shaped)

• Cell fibres pull half of each chromosome to opposite poles of the cell which breaks the chromosomes in half

• Happens to all 46 of the chromosomes

Cell Cycle 3 - Division/Cytokinesis

Cell membrane and cytoplasm pull apart forming 2 daughter cells - Genetically identical to each other and parent cell

Binary Fission

Process by which prokaryotic organisms, like bacteria, divide and reproduce

Bacterial cells grow and replicated Genetic material (nucleotides and plasmids)

Nucleotides move to opposite sides of the cell (daughter cells get one each) and plasmids move randomly

New cell wall grows in the middle of the cell which allows the two halves to pull apart

Binary Fission - How quickly bacterial populations can grow

Simple process so can happen very fast: Some can divide once every 20 minutes

Total time/mean division time = number of rounds of divisions
Amount of cells made in that time = starting number of bacterial cells x 2 to the power of number of rounds of divisions

Optimal conditions for bacterial growth

Warm

Moist

Plenty of nutrients

Oxygen

Nutrient Broth (liquid medium) and Agar (gel medium)

Nutrient rich substances used to grow bacteria

Bacteria need a mixture of nutrients in order to reproduce - carbohydrates (energy), nitrogen compounds (protein synthesis), vitamins and minerals

Preventing contamination

Contamination - presence of unwanted microorganisms

Aseptic techniques

Cleaning surfaces with disinfectant

Washing hands with antiseptic

Sterilising all instruments, solutions and mediums - Heating objects to a temperature at which all contaminating microorganisms are destroyed

Creating a sterile field using Bunsen Burner - created by updraft of flame + minimising time that cultures and growth media are open to environment

Growing bacteria in incubators set to a max of 25 C to prevent growth of harmful pathogens

Inoculation

Process of transferring bacteria from a broth to an agar plate

Stem Cells

Divide by mitosis to form more cells

Stem cells are undifferentiated cells of organisms which are capable of differentiating into other cells

Embryonic stem cells

Can differentiate into any type of cell

Adult stem cells

Found in bone marrow and Can divide by mitosis

Can only differentiate into different types of blood cells

Replace damaged cells however don’t form any new tissues

Plant stem cells

Found in Meristems - areas of the plant that are continually growing (roots and shoots)

Can differentiate into any cell - phloem/xylem cells, palisade cells, root hair cells

Persist for the plant’s entire life

Use of meristems

Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.

• Rare species can be cloned to protect from extinction.

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

Specialised cells

Cells that have specific role in body

Have specific shape or structure or amount of different organelles to help them fulfil that role

Red blood cell

• Biconcave shape for a large surface area for faster diffusion of gases

• Small + Flexibility to pass through capillaries which are narrow

• Absence of nucleus to maximise space for haemoglobin allowing them to carry more oxygen

• Thing membrane for faster diffusion of gases

Sperm cell

Role: Deliver Genetic material to an egg in order to fertilise it

Specialisation:

• Has half as much genetic material as a normal adult cell to combine with egg’s genetic material

• Flagellum allows sperm cell to swim through the uterus and fallopian tube to reach the egg

• Cell is streamline - helps with swimming

• Lots of mitochondria which provides energy required for swimming

• Lots of digestive enzymes at the tip to break hole into egg when the sperm reaches it

Differentiation

The process by which a cell changes to become specialised

Zygote (Fertilised egg cell) divide by mitosis until it forms a ball of cells that implants in the uterus wall called an embryo which contains embryotic stem cells

Stem cells in embryo differentiate into different specialised cells

Stem cells in medicine

Lots of conditions caused by faulty cells (cells that are damaged/don’t work properly)

Stem cells can be used to replace faulty cells with working cells to treat the condition

How Stem cells are used in Medicine to replace faulty cells

Scientists can extract embryonic stem cells from early embryos

Grow them in a laboratory

Stimulate them to differentiate into whichever type of specialised cell that is needed

Then give them to the patient to replace faulty cells

How stem cells are used to treat Type 1 Diabetes

Type 1 Diabetes - Damage to the pancreas cells that normally produce insulin

Scientists can extract embryonic stem cells and grow them in laboratory then stimulate them to differentiate into pancreas cells that can be injected into the patient where they continue to divide and provide patient with insulin that is needed

How stem cells are used to treat Paralysis

Paralysis: Sometimes caused by damaged nerve cells

Growth of Stem cells that differentiate in laboratory to create healthy nerve cells

Drawbacks of using stem cells in medicine

Requires embryonic stem cells - limited supply of them

Rejection - embryo and patient may have different genomes which may cause patient’s immune system to reject the stem cells (Risk can be reduced via medication but doesn’t always work and has side effects)

Alternative to embryonic stem cells

Adult stem cells from bone marrow

Won’t cause rejection as they are taken from patient

Can only differentiate into different types of blood cells - can be used to treat sickle cell anaemia

Risks of using stem cells in medicine

Virus transmission - If donor stem cells are infected with virus, patient can receive virus and become infected

Tumour development - stem cells divide so quickly, there is a chance they will get out of control once they have been transplanted, forming tumour/cancer

Ethical objection + solutions to using embryonic stem cells

Human embryos used have potential for human life

• some people object to their use in research

Other people think that the benefits of curing existing people who are suffering is more important than the rights of embryos

Embryos used are usually unwanted ones from fertility clinics which would be destroyed anyway + Governments heavily regulate stem cell research (UK legal but tightly controlled)

Diffusion

Molecules left alone will move about randomly until they are spread out

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration

Happens in both gases and liquids

Can take place through materials like cell membranes (partially permeable - only some molecules can diffuse through like water, glucose and amino acids)

Diffusion is a passive process - doesn’t require any energy from the cell

3 Factors than affect rate of Diffusion

Concentration gradient - Difference in concentration between 2 places. The larger the concentration gradient, the higher the rate of diffusion.

Temperature - Higher temperature gives the particles more energy so they will move around faster and therefore diffuse quicker

Surface area - Higher the surface area, the higher the rate of diffusion as there are more particles available at any time to diffuse

Osmosis

The diffusion of water molecules

Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water concentration. to a region of lower water concentration

Water concentration - amount of water molecules as compared to other molecules (solutes) that are dissolved in the water

Active Transport

Movement of molecules across a cell membrane, against their concentration gradient, from a region of lower concentration to a region of higher concentration and requiring energy from cellular respiration (active process)

Requires special proteins to transfer molecules from one side to the other

Energy from active transport comes from cellular respiration that happens in mitochondria - energy is stored in ATP

Example of Active Transport - Root hair cells adaptations

Plants need to absorb water and mineral ions from the soil to survive through the roots

Root hair cells absorb water and mineral ions

Adaptations

• Have long hair like protrusions to give cell large surface area for diffusion

• Lots of mitochondria for energy for active transport

• Large vacuole to help maintain gradient

• Partially permeable membrane

Minerals that plants need can’t be absorbed by diffusion as the concentration of them in the root hair cells are higher than the soil - therefore absorbed by active transport

Surface area to volume ratio

Larger organisms have a less SA:V

Bacteria have high SA:V therefore can rely on diffusion to exchange everything they need

Humans have low SA:V therefore cannot rely on diffusion and require specialised exchange surfaces

Specialisation of specialised exchange surfaces (villi, root hair cells, leaves, alveoli)

• Millions of alveoli in lungs which increase Surface area - absorb carbon dioxide and oxygen

• Villi in intestines provide large surface area for absorption - absorb nutrients

• Thin exchange surfaces therefore short distance from substances to diffuse across

• Exchange surfaces are permeable

• Good supply of the external medium (e.g: good supply of air in lungs - maintain concentration gradient)

• Good blood supply

Diffusion distances

As organisms get larger, the distance that molecules would have to diffuse to get from the outside of their body to the inside of their body increases - therefore diffusion much slower and cannot be relied on

Larger organisms often have transport system - Circulatory system

Benefits of good blood supply in animals

Helps maintain concentration gradient

Glucose is absorbed into blood

Blood will be taken away

Replaced with blood that doesn’t yet have lots of glucose