Cell Biology

There are two types of cells

• Eukaryotic - Comlex cells including animal and plant cells.
• Prokaryotic - Smaller and simpler, for example bacteria. Bacteria dont have chloroplasts, mitochondria or a nucleus, but a single circular strand of DNA that floats freely in the cytoplasm and sometimes one or more small rings of DNA called plasmids.

Most animal cells have the following sub-cellular structures -

• Nucleus - contaisn genetic material that countrols the activity of the cell.
• Cytoplasm - gel-like substance where most of the chemical reactions happen. Contains enzymes to control them.
• Cell membrane - holds the cell together and controls what goes in and out.
• Mitochondria - where most of the reactions for aerobic respiration takes place, providing energy.
• Ribosomes - where protein synthesis occurs.

Plants have all these and some extra structures:

• Cell wall - made of cellulose to support the cell and strengthen it.
• Permanent vacuole - contains cell sap, a weak solution of sugar and salts.
• Chloroplasts - where photosythemsis occurs. Contain chlorophyll which absobs light.

• Light microscopes use light and lenses to form an image of a specimen and magnify it , alloing us to see individual cells and large subcellular structures like nuclei.
• Electron microscopes allow us to see higher magnifications as they use electrons to form an image. They have a higher resolution and so we can see small things in detail, for example mitochodria.

magnification = image size / real size

Preparing a slide:

• Add a drop of water to the middle of a clean slide.
• Cut up an onion and seperate it out into layer, using tweezers to peel off some epidermal tissue. Place this into the water on the slide.
• Use a drop of iodine solution  as a stain. This will highlight objects in a cell by adding colour to them.
• Place a cover slip on top by standing it upright on the slide next to the water droplet and then carefully tilting it until it covers the specimen. Try not to get any air bubbles as this will obstruct the view.

Using a microscope:

• Clip the prepared slide onto the stage.
• Select the lowest-powered objective lens and use the coarse adjustment knob to move the stage just below the lens.
• Look down the eyepiece and use the coase adjustment knob to move the stage until it is mostly in focus.
• Adjust the focus with the fine adjustment knob, until the image is clear.
• If you need a higher magnificatin, swap to a higher-powered objective lense and refocus.

Drawings must be in pencil with clear, unbroken lines, taking up at least half of the space. They should not include colour or shading and the subcellular structures should be in proportion. Include a title and write down the magnification. Label important feactures using straight, uncrossed lines.

Differenation is the process by which a cell changes to become specialised for its job, developing different subcellular strctures and turning into different types of cells. This allows them to carry out specific functions.

Most differentation occurs when an organism develops, therfore most animal cells lose this ability at an early age, after they have become specialised. However, many plant cells never lose this ability.

The cells that differentiate in mature animals are mainly used for repairing and replacing cells. Some cells are undifferatiated, these are called stem cells.

A nucleus contains genetic information in the form of chromosomes. These are coiled up lengths of DNA molecules containing large numbers of genes. Different genes control the development of different characteristics, e.g. hair colour. Body cells normally have 2 copies of each chromosome (exept y and z). There are 23 pairs of chromosomes in the human body.

Cell cycle

The stage of the cell cycle when the cell divides is called mitosis, this is used to grow or replace cells that have been damaged. The end of the cell cycle results in two new cells identical to the original cell, with the same number of chromosomes.

Stage 1, Growth and DNA Replication - The cell hrows and increases the amount of subcellular structures such as mitochondria and ribosomes. It the duplicates its DNAand so each one forms x-shaped chromosomes. Each "arm" is an exact duplicate, one for each new cell.

Stage 2, Mitosis - The chromosomes line up in the centre of the cell and cell fibres pull them apart, the two arms of each chromosome go to opposite ends of the cell. Membranes form around each of the sets of chromosomes which become the nuclei of the two new cells. Finally, the cytoplasm and cell membranes divide.

The cell has now produced two new daughter cells with exactly the same DNA, identical to each other and their parent cell.

Undifferntiated cells can divide to produce more and can differentiate into different types of cell, depending on the instructions they are given. Stem cells are found in early human embryos, these have the potential to turn into any cell. Adults have stem cells in places like bone marrow, however they can not turn into any cell, just specific ones like blood cells. Stem cells frm embyos and bone marrow can be grown in a lab to produce clones and to made to differentait into specialised cells to use in medicine or research.

Stem cells transferred from the bone marrow of a healthy person are being used to replace faulty blood cells in patients. However, embryonic cells could be used to replace any faulty cells, for example insulin-producing cells to cure diabetes or nerve cells to cure people paralysed by spinal injuries. In theraputic cloning, an embryo could be made to have the same genetic information as the patient, meaning it wuld not be rejected by the patient's body. However, there are risk, for example cells grwon in a lab could become contaminated by a virus, thus making the patient sicker.

Some are gainst stem cell research as they feel that human embros should not be experimented on as they have the potential to become a human life and they think that scientists should concentrate on finding and developing other sources of stem cells, however others argue that it is more imprtant to help patients who are suffering especially as the embryos used are usually unwanted ones from a fertility clinics which would be destroyed anyway. In the UK stem cell research is legal as long as it follows stict guidlines.

In plants, stem cells are found in the meristems and can differentiate into any type of plant cell throughout its whole life. These stem cells can be used to produce clones of plants quickly and cheaply or to grow rare species. They can also be used to grow plants with desired feactures, such as disease ressistant.

Cells can diffuse to take in substances they need and get rid of waste products, for example:

• Oxygen and carbon dioxide are tranferred between calls and the enviroment during gas exchange.
• In humans, urea diffuses from cells into the blood plasma for remoal from he body by the kidneys.

A larger surface area to ratio volume makes it easier for organisms to diffuse. The larger an organism is, the smaller its surface area is compared to its volume.

In single-celled organisms, gasses and dissolved substances can diffuse directly into or out of the cell as they have a large surface area to volume ratio. Multicellular orgaisms have a smaller ratio so not enough substances can diffuse from their outside surface to supply their entire volume, therfore they need some exchange surfaces for efficient diffusion. These substances have adapted to provide maxiumim efficietcy when gasses diffuse:

• They have a thn membrane to provide a short diffusion pathway
• They have a large surface area so lots of gasses ca diffuse at once.
• Exchange substances in animals have lots of blood vessels, mostly cappilaies, to get the gasses into and out of the blood quickly.
• Gas exchnge substance in animals, e.g. alveoli, are well ventilated so air can ove in and out.

Gas Exchange in the Lungs.

The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it. To do this they contain millions of little air sacs called alveoli where gas exchange takes place. The alveoli have adapted in four main ways:

• An enormous surface area.
• A moist lining for dissolving gasses.
• Very thin walls.
• A good blood supply.

Gas Exchange in the Small Intestine.

The inside of the small intestine is covered in millions of tiny projections called villi inceasing the surface are to help digested food be absorbed much more quickly into the blood. They have two main adaptations:

• A single layer of surface cells.
• A very good blood supply for quick absorbtion.

Carbon dioxide diffuses into the air spaces within a leaf, then diffuse into the cells where photosynthesis happens.

• The underneath of the leaf is covered in little holes called stomata to allow cabon dioxide to diffuse into and oxygen and water vapour out diffuse out of the leaf. 
• The size of the stomata are controlled by guard calles which close the stomata if the plant is losing water faster than it's being replaced by the roots. Without these the plant would wilt.
• The flattened shape of the leaf increases the area of this exchage so its more efficient.
• The air spaces also increase the aea of carbon dioxide to diffuse into specific cells

The gills are the gas exchange surface for fish. Water, containing oxygen, enters the fish through its mouth and diffuses into the blood at the gills and carbon dioxide back into the water.

• Each gill is made of thin filaments which give it a big surface area for gas exchange.
• These are covered in tiny structures called lamellae, which increase the surface even more.
• These have lots of capillaries to speed up diffusion and a thin layer of cells for a short diffusion pathway.
• Blood flows through the lamellae in one direction and water flows over in the opposite to mainatin a large concetration gradient between the water and blood. The concentration of oxygen in the water is always higher than in the blood, so as much oxygen as possible diffuses from the water into the blood.

Diffusion is the spreading out of particles from an area of higher concentration to an area of lower concentration. Thus happens in solutions and gasses as particles are able to move about.
The bigger the concentration gradient, the faster the diffusion rate. A higher temperature will also increase the rate as the particles have more energy. When diffusing across a membrane, the larger the surface area, the faster the diffusion rate as more oarticles can diffuse at once.
Dissolved substances can move in and out of cells by diffusion, however only very small particles can fit through the pores in the cell membrane and other, larger, particles are left behind. There will be a net movement from the area of higher concentration to lower concentration, but the moving is random

Osmosis is the movement if water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
A partially permeable membrane is only that only lets across tiny particles.
There will be a net flow from high to low concentration, even though they flow in both directions, e.g. into the stronger sugar solution. Thus means the sugar solution would become more dilute as the water tries to 'even up' the concentration either side of the membrane.
Osmosis is a type of diffusion.
Practical:
Cut up a potato into identical cylinders and measure the mass of each one, then get some beakers with different sugar solutions ranging from pure to very concentrated and leave one cylinder in each beaker for about 24 hours.
Take out the potatoes, dry with a paper towel, and md record the new mass.
If the cylinders have drawn in wate by osmosis, theyll have increased in mass. Calculate the percentage change in mass and plot on a graph.

Active transport takes place in root hair to take in minerals and water. Each branch of root is covered in millions of root hairs, giving the root a large surface area of absorb mineralions and water. The concentration of these substances is usually greater in the soil, therfore active transport allows uses energy from respiration to transport these substances from an area of high to low concentration.
Active transport is also used in the gut to take nutrients into the blood. When there is a higher concentration if glucose and amino acids, active transport takes it into the blood stream to be transported into the blood where its used for respiration.