Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus

Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids

Most animal cells have: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes

In addition to the parts found in animal cells, plant cells often have: chloroplasts, a permanent vacuole filled with cell sap

Plant and algal cells also have a cell wall made of cellulose which strengthens the cell

Nucleus - Contains genetic material that controls the activities of the cell

Cytoplasm - Gel like substance where most of the chemical reactions happen. It contains enzymes that control these reactions

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

Mitochondria - These are where most of the reactions for aerobic respirations take place. Respiration transfers energy that the cell needs to work

Ribosomes - Where proteins are made in the cell / the site of protein synthesis

Cell wall - Made of cellulose. It supports the cell and strengthens it

Vacuole - Contains cell sap, a weak solution of sugar and salts

Chloroplasts - Site of photosynthesis which makes food for the plant. They contain a green substance called chlorophyll which absorbs the light needed for photosynthesis

Plasmids - Small rings of DNA

Required Practical:

If you want to look at a specimen under a light microscope, you need to put it on a microscope slide first. A slide is a strip of clear glass or plastic onto which the specimen is mounted

How to prepare a slide to view onion cells:

Add a drop of water to the middle of 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 the water on the slide

Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them

Place a cover slip on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there as they will obstruct the view of the specimen

To view the slide under a light microscope

Clip the slide prepared onto the stage

Select the lowest powered objective lens

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

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

Adjust the focus with the fine adjustment knob until you get a clear image of what’s on the slide

If you need to see the slide with greater magnification, swap to a higher powered objective lens and refocus

Drawing observations

Draw what you see under the microscope using a sharp pencil

Make sure the drawing takes up at least half of the space available and that it is drawn with clear, unbroken lines

Your drawing should not include any shading or colouring

If you are drawing cells, the subcellular structures should be drawn in proportion

Remember to include a title of what you were observing and write down the magnification that it was observed under

Label the important features of your drawing using straight uncrossed lines

Cells may be specialised to carry out a particular function:

Sperm cells, nerve cells and muscle cells in animals

Root hair cells, xylem and phloem cells in plants

Sperm cells are specialised for reproduction

The function of a sperm is to get the male DNA to the female DNA. It has a long tail and a streamlined head to help it swim to the egg. There are also a lot of mitochondria 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

The function of nerve cells is to carry electrical signals from one part of the body to another. These cells are long to cover more distance and have branched connections at their ends to connect to other nerve cells and form a network through the body

Muscle cells are specialised for contraction

The function of a muscle cell is to contract quickly. These cells are long so that they have space to contract and contain lots of mitochondria to generate the energy needed for contraction

Root hair cells are specialised for absorbing water and minerals

Root hair cells are cells on the surface of plant roots, which grow into long hairs that stick out into the soil. This gives the plant a large surface area for absorbing water and mineral ions from the soil

Phloem and Xylem cells are specialised for transporting substances

Phloem and xylem cells from phloem and xylem tubes, which transport substances such as food and water around plants. To form the tubes, the cells are long and joined 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

As an organism develops, cells differentiate to form different types of cells

Most types of animal cell differentiate at an early stage

Many types of plant cells retain the ability to differentiate throughout life

In mature animals, cell division is mainly restricted to repair and replacement. As a cell differentiates it acquires different sub cellular structures to enable it to carry out a certain function. It has become a specialised cell

Cells differentiate to become specialised and carry out a specific function. Differentiation is the process by which a cell changes to become specialised for its job.

Microscopy techniques have developed over time as technology and knowledge have improved. Light microscopes use light and lenses to form an image of a specimen and magnify it. They let us see individual cells and large sub cellular structures like nuclei. Electron microscopes use electrons instead of light to form an image. They have a much higher magnification than light microscopes

The electron microscope has increased the understanding of sub cellular structures because they have a higher resolution. They allow us to see smaller things in more detail like the internal structure of mitochondria and chloroplasts or even tinier things like ribosomes and plasmids

An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enable biologists to see and understand many more sub cellular structures

Magnification = size of image/size of real object

The nucleus of a cell contains chromosomes made of DNA molecules. Each chromosome carries a large number of genes

In body cells the chromosomes are normally found in pairs

Cells divide in a series of stages called the cell cycle.

During the cell cycle the genetic material is doubled and then divided into two identical cells

Growth and DNA replication

Before a cell can divide it needs to grow and increase the number of sub cellular structures such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome

Mitosis

The chromosomes line up at the centre of the cell and cell fibres pull them apart

In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides

Finally the cytoplasm and cell membranes divide to form two identical cells

Cell division by mitosis is important in the growth and development of multicellular organisms

A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type and from which certain other cells can arise form differentiation

Stem cells transferred from the bone marrow of a healthy person can replace faulty blood cells in the patient who receives them

Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells

Stem cells from adult bone marrow can form many types of cells including blood cells

Meristem tissue in plants can differentiate into any type of plant cell throughout the life of the plant

Treatment with stem cells may be able to help conditions such as diabetes and paralysis

In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment

The use of stem cells has potential risks such as transfer of viral infection and some people have ethical or religious objections

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

Substances may move into and out of cells across the cell membranes by diffusion

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of high concentration to an area of lower concentration

Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney

Factors which affect the rate of diffusion are:

The difference in concentrations (concentration gradient): The bigger it is, the faster the diffusion rate

The temperature: The higher it is, the faster the diffusion rate because the particles have more energy so move around faster

The surface area of the membrane: The bigger it is, the faster the diffusion rate because more particles can pass through at once

A single celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism

Surface area = length x width of each surface

Volume = length x width x height

In single celled organisms, gases and dissolved substances can diffuse directly into or out of the cell across the cell membrane because they have a large surface area compared to their volume so enough substances can be exchanged across the membrane to supply the volume of the cell

Multicellular organisms have a smaller surface area compared to their volume meaning not enough substances can diffuse from their outside surface to supply their entire volume. This means that they need an exchange surface for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through

The small intestine adaptations:

The inside of the small intestine is covered in millions of villi. They increase the surface area by a lot so that digested food is absorbed quicker into the blood. They have a single layer of surface area cells and a very good blood supply to assist quick absorption

The lungs adaptations:

The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it. To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place. The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have a big surface area, a moist lining for dissolving gases, very thin walls and a good blood supply

Gills in fish adaptations:

Water (containing oxygen) enters the fish through its mouth and passes out through the gills. As this happens, oxygen diffuses from the water into the blood in the gills and carbon dioxide diffuses from the blood into the water. Each gill is made of lots of thin plates called gill filaments which give a big surface area for exchange of gases. The gill filaments are covered in lots of tiny structures called lamellae which increase the surface area even more. The lamellae have lots of blood capillaries to speed up diffusion. They also have a thin surface layer of cells to minimise the distance that the gases have to diffuse. Blood flows through the lamellae in one direction and water flows over in the opposite direction. This maintains a large concentration gradient between the water and the blood. The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood

Roots and leaves in plants adaptations:

Carbon dioxide diffuses into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens. The leaf’s structure is adapted so that this can happen easily. The underneath of the leaf is an exchange surface and is covered in tiny holes called stomata which the carbon dioxide diffuses in through. Oxygen (produced in photosynthesis) and water vapour also diffuse out through the stomata. The size of the stomata are controlled by guard cells. These close the stomata if the plant is losing water faster than it is being replaced by the roots. Without these guard cells the plant would soon wilt. The flattened shape of the leaf increases the area of this exchange surface so it’s more effective. The walls of the cells inside the leaf form another exchange surface. The air spaces inside the leaf increase this surface so there’s more chance for carbon dioxide to get into the cells. The water vapour evaporates from the leaf. Then it escapes by diffusion because there’s a lot of it inside the leaf and less of it in the air outside

In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by:

Having a large surface area

A membrane that is thin, to provide a short diffusion path

In animals having an efficient blood supply

In animals for gaseous exchange being ventilated

Water may move across cell membranes by osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane

Potato 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. Then you can have a few others with concentrations in between

Measure the mass of the cylinders then leave one cylinder in each beaker for about 24 hours

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

If the cylinders have drawn in water by osmosis, they’ll have increased in mass. If water has been drawn out they will have decreased in mass. You can calculate the percentage change in mass then plot a graph. By calculating percentage change you can compare the effect of sugar concentration on cylinders that didn’t have the same initial mass. An increase in mass will give a positive percentage change and a decrease will give a negative percentage change

The dependent variable is the chip mass and the independent variable is the concentration of the sugar solution. All other variables (volume of solution, temperature, time, type of sugar used…) need to be kept the same in each case or the experiment won’t be a fair test

Errors may occur when carrying out the method (if some potato cylinders were not fully dried the excess water would give a higher mass or if water evaporated from the beakers the concentrations of the sugar solutions would change). You can reduce the effect of these errors by repeating the experiment and calculating a mean percentage change at each concentration

Percentage change = final value - original value x 100/original value

Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration

Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth

It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration