Cell Biology

1. All living things are made of cells.

• Cells can be either prokaryotic or eukaryotic.
• Eukaryotic cells are complex and include all animal and plant cells.
• Prokaryotic cells are smaller and simpler.

2. Eukaryotes are organisms that are made up out of eukaryotic cells.

3. A prokaryote is a prokaryotic cell (single-celled organism).

Animal cells have:

• Nucleus - contains genetic material that controls the activities of the cell.
• Mitrochondria - these are where most of the reactions for aerobic respiration take place. Respiration transfers energy that the cell needs to work.
• Cytroplasm - gel-like substance where most of the chemical reactions happen.It contains enzymes that control these chemical reactions.
• Cell membrane - holds the cell together and controls what goes in and out.
• Ribosomes - these are where proteins are made in the cell.

Plant Cells have:

• Cell Wall - made out of cellulose and are rigid. It supports the cell and strengthens it.
• Chloroplasts - these are where photosythesis occurs, which makes food for the plant. They contain a green substance called chlorophyll, which absorbs light needed for photosynthesis.
• Permanent Vacuole - contains cell sap, a weak solution of sugar and salts.

Bacterial Cells are much smaller:

• Bacteria are prokaryotes.
• They don't have a nucleus, but a single strand of DNA which floats freely in the cytoplasm.
• They contain small rings of DNA called plasmids.
• They don't have chloroplasts or mitrochondria.

Microscopes let us see things that can't be seen with the naked eye.

• Light microscopes use light and lense to form an image and magnify it.
• Electron microscopes use electrons instead of light to form an image. They have a much higher magnification than light microscopes - they also have a higher resolution.

Formula for magnification:

Magnification = image size ÷ real size

You also need to work out numbers in standard form:

http://www.bbc.co.uk/schools/gcsebitesize/maths/number/powersrootshirev1.shtml

First you have to prepare a slide:

• Add a drop of water onto a clean slide
• Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
• Using the tweezers, place the epidermal tissue into water on the slide.
• Add a drop of iodine solution creating a stain. Stains are used to highlight objects.
• Place a cover slip on top. To do this, carefully tilt and lower it to cover. Make sure there aren't any bubbles there.

Viewing the slide:

• Clip the slide onto the stage.
• Select the lowest-powered objective lense.
• Use the coarse ajdjustment knob to move the stage up to just below the objective lense.
• Look down the eyepiece. Make sure it's a clear image using adjustment knobs.
• If you need a clearer image, use a higher-powered objective lense and refocus.

Differentiation is the process by which a cell changes to become specialised for its job.

• As cells change, they develop different subcellular structures and turn into different types of cells. This allows them to carry out specific functions.
• Most differentiation occurs as an ****** develops. In most animal cells, the ability to differentiate is then lost at an early stage, after they become specialised. However, lots of plant cells don't ever loose this ability.
• The cells that differentiate in mature animals are mainly used for repairing and replacing cells, such as skin or blood cells.
• Some cells are undifferentiated cells - they're called stem cells.

These examples of Specialised Cells have to be learned.

Sperm Cells are specialised for reproduction:

• They get the male DNA to the female DNA and has a long tail and a streamline head to help it swim to the egg.
• There are lots of mitrochondria in the cell to provide the energy needed.
• It also carries enzymes in its head to digest through the egg cell membrane.

Nerve Cells are specialised for rapid signalling:

• They carry electrical signals from one part of the body to another.
• They are long, and have branched connections at their ends to connect to other nerve cells and form a network throughout the body.

Muscle Cells are specialised for contraction:

• They are meant to contact quickly.
• They are long and contain lots of mitrochondria to generate energy needed for contraction.

Root Hair Cells are specialised for absorbing water and minerals:

• They are on the surface of plant roots, which grow into long 'hairs' that stick out in the soil.
• They give the plant a big surface area for absorbing water and mineral ions from the soil.

Phloem and Xylem cells are specialised for transporting substances:

• They form phloem and xylem tubes, which transport substances such as food and water around a plant.
• To form - long cells join end to end.
• Xylem cells are hollow in the centre and phloem cells have very few subcellular structures, so that things can flow through them.

Embryonic Stem Cells can turn into any kind of cell.

• Undifferentiated cells, called stem cells, can divide and produce more stem cells. These can change into different types of cell.
• Stem cells are found in human embryos. All the different types of cell found in a human have come from a few cells in a early embryo.
• Adults also have stem cells, but they're only found in places such as the bone marrow.
• Adult stem cells can't turn into any cell type - apart from blood cells.
• Stem cells can be grown and cloned in labs; these can be used in medication.

Stem Cell may be able to cure disease.

• Medicine uses adult stem cells to cure disease. They can replace faulty blood cells in a patient.
• Embryonic cells could also be used. They could become insulin-producing cells for people.
• During therapuetic cloning, an embryo could be made to have identical gentic information to a patient. This means they won't be rejected by the body.
• There are risks involved. These stem cells could contain a virus which makes the patient sicker.

Some People Are Against Stem Cell Research.

• Some people are against stem cell reseach because they feel that human embryos shouldn't be used for experiments since each one has potential human life.
• Others think that curing patients who already exist and who are suffering is more important than the rights of embryos.
• Embryos used in research are usually unwanted ones from fertility clinics which, if they weren't used for research, would probably be destroyed.
• Some believe scientists should concentrate more on finding and developing other sources of stem cells, so people could be helped without using embryos.
• In some countries stem cell research is banned but is allowed in the UK if following scrict rules.

Stem Cells Can Produce Identical Plants.

• In plants, stem cells are in the meristems; in the plant's life they can change to any type of cell.
• These stem cells can be used to produce clones and can be used to grow plants of rare types.
• Stem cells can also be used to grow crops of identical plants that have desired features for farmers, for example, disease resistance.

In order to survive, our cells have to divide.

Chromosomes have Gentic Information.

• Most cells have a nucleus. This carries your genteic material in the form of chromosomes.
• They are coiled up lengths of DNA molecules.
• Each one has a large number of genes; they control characterists.
• Body cells usually have two copies. One from the mother and one from the father.

The Cell Cycle

• Body cells in multicellular organisms divide to produce new cells.
• When they divide, it is known as mitosis.
• Multicellular organisms use mitosis to grow, develop and replace cells.
• In the end, you end up with two cells that are identical to the original cell, with the same amount of chromosomes.

There are two main stages of the Cell Cycle.

Growth and DNA Replication

• In a cell that isn't dividing, DNA is all spread out in long strings.
• Before it divides, the cell must grow and increase amounts of subcellular structures such as mitrochondria and ribosomes.
• Then it dupicates DNA - one copy for each new cell. These DNA form X-shaped chromosomes.

Mitosis

• The chromosomes line up in the centre of the cell and cell fibres seperate them. The two arms of each chromosome go to opposite ends of the cell.
• Membranes form around the sets of chromosomes. These become the nuclei as the nucleus divides.
• The cytroplasm and cell membrane divide.
• The cell has produced two new daughter cells. They have the exact same DNA and identical to the parent cell.

Meioses:

• Meioses is a process of cell divison that only hapens in gamets, which are sex cells
• in order to devide by meioses, the chromosomes in the cell devide in half so in a human gamete each cell would have 23 chromosomes
• the last chromosome ina sperm cell has a 50% chance of either being X or Y, where as in and egg the last chromosome will be an X.

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

• Diffusion is the gradual movement of particles from places where there are lots of them to few of them.
• Diffusion happens in solutions and gases are they are free to move about freely and randomly.
• The bigger the concentration gradient the faster the diffusion rate. A higher temperature will also give a faster reaction because the particles have more energy.

Cell Membranes

• They hold the cell together but let stuff in and out such as dissolved substances by diffusion.
• Only very small molecules can diffuse through cell membranes, such as ones for respiration.
• Big molecules such as starch and protiens can't fit through.
• Particles flow where there is a higher concentration to a lower concentration.
• The larger the surface area the faster the diffusion rate.

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

• A partically permeanble membrane just is a membrane with small holes in it.
• Water can pass both ways through the membrane during osmosis.
• There's a steady net flow to the region of water with fewer water molecules. E.g. stronger suger solution.
• Water makes the solution more dilute; water tries to even up each side. Osmosis is a type of diffusion, that only happens with water.

Observe the Effect of Sugar Solutions on Plant Tissue.

1) Cut up pieces into identical cylinders and get beakers with different sugar solutions in them. One should have pure water and another very concentrated sugar solution. Have some others inbetween the two.

2) You can measure the mass of the cylinders, then leave one cylinder in each beaker for 24 hours

3) Then take them out, dry them with a paper towel and measure their masses again.

4) If the cylinders have drawn in water by osmosis, they'll have increased in mass. If water has been drawn out, they'll decreased in mass. You can calculate the percentage change in mass, then plot a few graphs.

• The dependent variable is the chip mass and the independant variable is the concentration of the sugar solution. All other variables must stay the same in each case or it won't be a fair test.
• There can be issues. Some potato cylinders may not be fully dried, so excess water may give it a higher mass and concentrations of sugar solutions may change.

Some substances need to be absorbed against a concentration.

Root Hairs take in Minerals and Water

• Each root is covered in millions of these microscopic hairs.
• Gives the plant a large surface area for absorbing water and mineral ions from soil.
• Plants need these mineral ions for healthy growth.
• Concentration of minerals is uaually higher in root hair cells than in soil.
• Root hair cells can't use diffusion to take up minerals.

We need active transport to stop us starving

• Active transport is used in the gut where there is a lower concentration of nutrients in the gut, but a higher concentration of nutrients in the blood.
• It allows nutrients to be taken into blood.
• This means glucose can be taken into the bloodstream where its concentration in the blood is already higher than the gut.

Organisms exchange substances with their environment.

• Cells can use diffusion to take in substances they need and get rid of waste products.
• We see this in when oxygen and carbon dioxide are transferred between cells and the environment during gas exchange.
• In humans, urea diffueses from cells into the blood plasma for removal from the body by the kidneys.
• How easy it is depends on the organisms surface area to volume ratio.

See how to calculculate this on page 33 of the Biology revision guide.

• For more detail see Biology revision guide.