Biology

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Cell Biology

Cells

Organisms can be prokaryotes or eukaryotes.

All living things are made up of cells.

Eukaryotic cells = complex and include all animal and plant cells.

Prokaryotic cells = smaller and simpler. For example: bacteria.

Eukaryotes - organisms made of eukaryotic cells.

A prokaryote = prokaryotic cell (single-celled organism).

Plant and animal cells have similarities and differences. The different parts of a cell are called subcellular structures.

Most animal cells have...

Nucleus - contains genetic material that controls the activities of the cell.

Cytoplasm - gel-like substance where most chemical reactions happen. Contains enzymes that control the chemical reactions.

Cell membrane - holds the cell together and controls what goes in and out.

Mitochondria - these are where most of the reactions for aerobic respiration takes place. Respiration transfers energy that the cells need to work.

Ribosomes - where proteins are made in the cell.

Plant cells usually have everything animal cells have plus a few extra...

Cell wall - made of cellulose. Supports the cell and strengthens it.

Vacuole - contains cell sap (weak solution of sugar and salts).

Chloroplasts - where photosynthesis occurs, makes food for plant, contains a green substance called chlorophyll (absorbs light needed for photosynthesis).

Bacteria are prokaryotes. Bacterial cells don't have a true nucleus, they just have a single circular strand of DNA that floats freely in the cytoplasm, may contain plasmids (rings of DNA), don't have chloroplasts or mitochondria. Have a cell membrane, cell wall and cytoplasm.

Microscopy

Cells are studied using microscopes.

Microscopes let us see things that cannot be seen with a naked eye. The microscopy techniques have improved.

Light microscopes - light and lenses to form an image of a specimen, magnify it. Let us see individual cells and large subcellular structure. For example: nuclei.

Electron microscopes - electrons, higher magnification, higher resolution. Let us see smaller things in more detail. For example: internal structure of mitochondria, chloroplasts, ribosomes and plasmids.

Resolution - ability to distinguish between two points (sharper image).

Formula for magnification:

magnification = image size/real size

Image and real size should both have the SAME units.

Microscopy Practical

Preparing the slide to view onion cells...

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

Cut up onion, separate into layers. Use tweezers to peel off epidermal tissue from the bottom of a layer.

Using the tweezers, place the epidermal tissue into the water on the slide.

Add a drop of iodine solution - stain used to highlight objects in a cell by adding colour.

Place a cover slip on top - stand the cover slip upright on the slide, next to water droplet, then carefully tilt and lower it to cover specimen. Try not to get air bubbles under there - obstruct view.

How to use a light microscope...

Clip prepared slide onto the stage.

Select the lowest-powered objective lens.

Use coarse adjustment knob to move the stage up to just below the objective lens.

Look down the eyepiece. Use coarse adjustment knob to move stage downwards until the image is roughly in focus.

Adjust focus with fine adjustment knob, until you get clear image of what is on the slide.

If you need to see the slide with higher magnification - swap to higher-powered objective lens and refocus.

How to draw observations neatly...

Draw what you see under the microscope using a sharp pencil.

Drawing should take up at least half of the space available. Drawn with clear, unbroken lines.

No colouring or shading.

If drawing cells - subcellular structures should be drawn in proportion.

Title and magnification.

Label important features.

Cell Differentiation and Specialisation

Cells differentiate to become specialised.

Differentiation - process by which cell changes to become specialised for its job.

As cells change - develop different subcellular structures and turn into different type of cells. Allows to carry out specific functions.

Most differentiation occurs as an organism develops. In most animal cells, the ability to differentiate is lost at an early stage after they become specialised but a lot of plant cells do not lose this ability.

Cells that differentiate in mature animals - repairing and replacing cells. For example: skin and blood.

Stem cells - undifferentiated cells.

Examples of specialised cells:

Sperm cells - reproduction - get male DNA to female DNA. Long tail and streamlined head to help it swim to the egg. Lots of mitochondria in the cell to provide energy. Enzymes in its head to digest through the egg cell membrane.

Nerve cells - rapid signalling - carry electrical signals from one part of the body to another. Long to cover more distance and have branched connections at ends to connect to other nerve cells and form a network.

Muscle cells - contraction - long and contain lots of mitochondria to generate energy needed for contraction.

Root hair cells - absorbing water and minerals - big surface area for absorbing water and mineral ions from soil.

Phloem and xylem cells - transporting substances - long and joined end to end. Xylem cells are hollow in the centre and phloem cells have very few subcellular structures so that stuff can flow through them.

Chromosomes and Mitosis

Chromosomes contain genetic information.

Most cells have a nucleus. The nucleus contains your genetic material - chromosomes.

Chromosomes - coiled up lengths of DNA molecules.

Each chromosome carries a large number of genes. Different genes - development of different characteristics. For example: hair colour.

Body cells - normally have two copies of each chromosome, one from organism's mother and one from father.

The cell cycle makes new cells for growth, development and repair.

Body cells in multicellular organisms divide - produce new cells, part of cell cycle.

Stage where cell divides - mitosis.

End of cell cycle results in two new cells identical to original cell, same number of chromosomes.

There are two main stages of the cell cycle - growth and DNA replication, and mitosis.

Growth and DNA Replication:

In a cell that is not dividing - DNA all spread out in long strings.

Before it divides, cell has to grow and increase the number of subcellular structures. For example: mitochondria and ribosomes.

It then duplicates its DNA - one copy for each new cell. DNA is copied and formed X-shaped chromosomes. Each 'arm' of chromosome is duplicate of the other.

Once its contents and DNA have been copied, the cell is ready for mitosis...

Chromosomes line up at centre of cell and cell fibres pull them apart - two arms of each chromosome go to opposite ends of cell.

Membranes form around each of the sets of chromosomes - nuclei of the two new cells, nucleus has been divided.

Cytoplasm and cell membrane divide - cell has produced two new daughter cells. The daughter cells contain exactly the same DNA. Also identical to the parent cell.

Binary Fission

Prokaryotic cells replicate by binary fission. In binary fission the cell splits into two.

Circular DNA and plasmids replicate.

Cells gets bigger, circular DNA strands move to opposite 'poles' (ends) of the cell.

Cytoplasm starts to divide, new cells walls begin to form.

Cytoplasm divides and two daughter cells produced. Each daughter cell has one copy of the circular DNA but can have a variable number of copies of the plasmid(s).

Bacteria can divide very quickly if given the right conditions. However if conditions become unfavourable, the cells will stop dividing and start to die.

Culturing Microorganisms (Practical)

You can grow bacteria in the lab.

Bacteria are grown in a 'culture medium' which has carbohydrates, minerals, proteins and vitamins they need to grow.

The culture medium used can be a nutrient broth solution or solid agar jelly.

Bacteria grown on agar plates will make visible 'colones' on the surface of the jelly or can spread out to give an even covering of the bacteria.

In school labs, cultures of the microorganisms are not kept above 25 degrees as harmful pathogens are likely to grow above this temperature.

In industrial conditions, cultures are incubated at higher temperatures.

To make an agar plate...

Hot agar jelly poured into shallow round plastic dishes called 'Petri dishes'.

When the jelly is cool and set, inoculating loops can be used to transfer microbes to the culture medium. A sterile dropping pipette and spreader can be used to get an even covering of the bacteria.

The microorganisms then multiply.

You can test the action of antibiotics on cultures of bacteria:

Place paper discs soaked in different types or concentrations of antibiotics on an agar plate - even covering of bacteria. Leave space between discs.

Antibiotic should diffuse into agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the paper discs but non-resistant strains will die. Clear area will be left where the bacteria have died - inhibition zone.

Make sure to use control - paper disk that has not been soaked in an antibiotic. Soak it in sterile water - can be sure that the bacteria have died only because of the antibiotics and nothing else.

Leave the plate at 25 degrees for 48 hours.

More effective the antibiotic against bacteria = larger inhibition zone.

Avoid contamination of unwanted pathogens:

Petri dish and culture medium must be sterilised to kill any unwanted microorganisms.

If inoculating loop is used, sterilise it by passing it through a flame.

The lid of the petri dish should be taped on to stop microorganisms from the air getting in.

Petri dish should be stored upside down, stop drops of condensation falling onto agar surface.

Culturing Microorganisms (Practical)

Calculate the sizes of the inhibition zone to compare results.

Compare the effectiveness of different antibiotics or antiseptics on bacteria by looking at the relative sizes of the inhibition zones. Larger the inhibition zone around a disk - the more effective the antibiotic is against the bacteria.

Can measure by eye, but for accurate results calculate the area of the inhibition zones using the distance across - diameter. Same as area of a circle equation.

Stem Cells

Embryonic stem cells can turn into any type of cell.

Differentiation - cell changes to become specialised for its job.

Undifferentiated cells = stem cells, can divide to produce lots of stem cells. They can differentiate into different types of cells, depending on instructions given.

Stem cells - found in early human embryos, interesting for doctors and medical researchers because it has the potential to turn into any type of cell they want.

Adults have stem cells in certain places. For example: bone marrow. They can't turn into any type of cell only certain ones. For example: blood cells.

Stem cells from embryos and bone marrows can be grown in a lab to produce clones (genetically identical cells) and made to differentiate into specialised cells - used in medicine or research.

Stem cells may be able to cure many diseases.

Medicine already uses adult stem cells to cure disease. For example: stem cells transferred from the bone marrow of a healthy person can replace faulty blood cells in the patient who receives them.

Embryonic stem cells could also be used to replace faulty cells in sick people - could make insulin-producing cells for people with diabetes, nerve cells and for people paralysed by spinal injuries.

Therapeutic cloning - an embryo could be made to have the same genetic information as the patient. Wouldn't be rejected by patient's body.

Risks in using stem cells in medicine. For example: stem cells grown in a lab can be contaminated with a virus which could be passed onto the patient resulting in more sickness.

Some people are against stem cell research.

They feel human embryos shouldn't be used in experiments - potential human life.

Others think that curing existing patients who are suffering is more important than the rights of an embryo. Embryos used in research are usually unwanted ones from fertility clinics which would have been destroyed.

Campaigners think that scientists should concentrate more on finding and developing other sources of stem cells, so people can be helped without having to use embryos.

In some countries stem cell research is banned. Allowed in the UK under strict guidelines.

Stem cells can produce identical plants.

In plants, stem cells are found in the meristems.

Throughout the plants's life, cells in the meristem tissues can differentiate into any type of plant cell - can be used to produce clones of whole plants quickly and cheaply, can be used to grow more plants of rare species.

Stem cells can also be used to grow crops of identical plants that have desired features for farmers, for example: disease resistance.

Diffusion

Particles move about randomly and they end up evenly spaced.

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

Happens in both solutions and gases, because particles in these substances are free to move about randomly.

The bigger the concentration gradient, the faster the diffusion rate.

Higher temperature = faster diffusion rate because the particles have more energy so they move around faster.

Cell membranes hold the cell together and let stuff in and out as well.

Dissolved substances can move in and out through diffusion.

Only small molecules can diffuse through the cell membrane. For example: oxygen, glucose, amino acids and water.

Osmosis

Osmosis - movement of water particles across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

Partially permeable membrane - very small holes which let only tiny particles pass through them.

Water actually pass both ways through osmosis because water molecules move around randomly all the time.

Because there are more water molecules on one side than the other, there is a steady net flow of water into the region with fewer water molecules.

Sugar solution gets more dilute.

Osmosis (Practical)

Cut up a potato into identical cylinders and get some beakers with different sugar solutions in them. One should be pure water and another should be a very concentrated sugar solution. For example: 1mol/dm^3. Then you can have others with concentrations in between these two.

Measure the mass of the cylinders, then leave one cylinder in each beaker for around 24 hours.

Take them out, dry them with a paper towel and measure masses again.

If the cylinders have drawn in water by osmosis - increased in mass. If water has been drawn out - decrease in mass. Calculate percentage change in mass then plot a graph.

Dependant variable - chip mass. Independent variable - concentration of the sugar solution. All other variables must be kept the same.

Errors - if some cylinders were not fully dried, excess water would give higher mass. Water evaporated from beakers, concentrations of sugar solutions change. Reduce the effect of these errors by repeating experiment and calculating mean percentage change at each concentration.

Active Transport

Active transport - substances absorbed against a concentration gradient, from a lower to a higher concentration.

Root hairs take in minerals and water.

Cells on plant roots grow into hairs, stick out into soil.

Each branch of a root covered in these.

Gives plant a large surface area, absorbing water and mineral ions from soil.

Plants need mineral ions for healthy growth.

Concentration of minerals - usually higher in root hair cells than soil.

Root hair cells cannot use diffusion to take up mineral ions from soil.

Root hairs take in minerals using active transport.

Active transport allows the plant to absorb minerals from very dilute solution, against concentration gradient, essential for growth. Needs energy from respiration.

Also happens in humans - taking glucose from the gut and the kidney tubules.

Active transport is used in the gut when there is a lower concentration of nutrients in the gut, but a higher concentration of nutrients in the blood.

Exchange Surfaces

Organisms exchange substances with their environment. Cells can use diffusion to take in substances they need and get rid of waste products:

Oxygen and carbon dioxide transferred between cells and the environment during gas exchange.

In humans, urea diffuses from cells into blood plasma for removal from the body by the kidneys.

Urea - waste product produces from breakdown of proteins.

How easy it is for an organism to exchange substances with the environment depends on the organism's surface area to volume ratio (SA:V).

The larger an organism is, the smaller its surface area is compared to its volume. Show this by calculating surface area to volume ratio.

Work out surface area.

Work out volume. (Length * Width * Height)

Multicellular organisms need exchange surfaces.

Single-celled organisms - gasses and dissolved substances can diffuse directly into or out of the cell across cell membrane, they have a large surface area compared to their volume, enough substances can be exchanged across membrane to supply volume of cell.

Multicellular organisms - smaller surface area compared to volume, not enough substances can diffuse from their outside surface to supply their entire volume. Need some sort of exchange surface for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through.

Exchange surfaces are adapted to maximise effectiveness:

Thin membrane - substances only have short distance to diffuse.

Large surface area - lots of substances can diffuse at once.

In animals, lots of blood vessels - to get stuff in and out more quickly.

Gas exchange surfaces in animals are often ventilated - air moves in and out.

Exchanging Substances (Animals)

Gas exchange happens in the lungs.

Job of lungs - transfer oxygen to blood, remove carbon dioxide from it.

Lungs have millions of air sacs - alveoli, where gas exchange takes place.

Alveoli specialised to maximise diffusion of oxygen and carbon dioxide:

Enormous surface area.

Moist lining - dissolving gases.

Thin walls.

Good blood supply.

The inside of the small intestine is covered in millions of tiny little projections called villi. Increase surface area, digested food is absorbed much more quickly into the blood.

Single layer of surface cells.

Good blood supply - quick absorption.

Exchanging Substances (Plants and Fish)

The structure of leaves lets gases diffuse in and out of cells.

Carbon dioxide diffuses into the air spaces within leaf, then diffuses into cells - photosynthesis occurs. Leaf's structure adapted so that this can happen easily:

Underneath the leaf is an exchange surface - covered in tiny little holes called stomata which carbon dioxide diffuses in through.

Oxygen and water vapour diffuses out of stomata.

Size of stomata controlled by guard cells - close the stomata if plant is losing water faster than it is being replaced by the roots.

Flattened shape of leaf increases area of this exchange surface - more effective.

Walls of the cells inside the leaf form another exchange surface - air spaces inside leaf increase area of this surface, more chance for carbon dioxide to get into cells.

Gills have a large surface area for gas exchange.

Water (containing oxygen) enters fish through mouth and passes out through gills. Oxygen diffuses from water into blood in the gills and carbon dioxide diffuses from blood into water.

Each gill made of thin plates - gill filaments, give big surface area for exchange of gases.

Gill filaments covered in lots of tiny structures - lamellae, increase surface area even more.

Lamellae - lots of blood capillaries, speed up diffusion. Also have a thin surface layer - minimise distance that gases have to diffuse.

Blood flows through lamellae in one direction, water in the opposite direction. Maintains large concentration gradient between water and blood.

Concentration of oxygen in water is always higher than in blood, so as much oxygen as possible diffuses from water to blood.