Cells

The head of the sperm contains the genetic material for fertilisation. The acrosome in the head contains enzymes so that the sperm can penetrate an egg. The middle piece is packed with mitochondria to release energy needed to swim and fertilise the egg. The tail enables the sperm to swim.

The nerve cell is extended, so that nerves can run to and from different parts of the body to the central nervous system. The cell has extensions and branches, so that it can communicate with other nerve cells, muscles and glands. The nerve cell is covered with a fatty sheath, which insulates the nerve cell and speeds up the nerve impulse.

Muscle cells contain filaments of protein that slide over each other to cause muscle contraction. The arrangement of these filaments causes the banded appearance of heart muscle and skeletal muscle. They contain many well-developed mitochondria to provide the energy for muscle contraction. In skeletal muscle, the cells merge so that the muscle fibres contract in unison.

The root hair cell has a large surface area to provide contact with soil water. It has thin walls so as not to restrict the movement of water.

There are no top and bottom walls between xylem vessels, so there is a continuous column of water running through them. Their walls become thickened and woody. They therefore support the plant.

Dissolved sugars and amino acids can be transported both up and down the stem. Companion cells, adjacent to the sieve tubes provide energy required to transport substances in the phloem.

Mitosis

Cells divide when:

• an organism grows
• an organism becomes damaged and needs to produce new cells. It is essential that any new cells produced contain genetic information that is identical to the parent cell.

Stem cells can divide to produce new cells, which can then divide into different cell types. They therefore have the potential to be transplanted into patients to treat medical conditions and disease. They could be used to replace cells that have been damaged or destroyed, eg:

• in type 1 diabetes
• in cases of multiple sclerosis, which can lead to paralysis
• in cases of spinal cord or brain injury, that have led to paralysis

The stem cells used could be:

• embryonic stem cells
• adult stem cells

Embryonic stem cells can differentiate into a wider range of cell types, but are difficult to obtain and their use raises ethical challenges. The best source is the five-day-old embryo.

Adult stem cells will differentiate into a narrower range of cell types. Bone marrow transplants are an example of adult stem cell transplant. Bone marrow cells will differentiate into different types of blood cell. Bone marrow transplants are carried out:

• in cases of blood cell cancer such as leukaemia and lymphoma
• when blood cells have been destroyed by cancer treatment

Clinical issues

• There is no guarantee how successful these therapies will be, for example the use of stem cells in replacing nerve cells lost in Parkinson’s disease patients.
• The current difficulty in finding suitable stem cell donors.
• The difficulty in obtaining and storing a patient’s embryonic stem cells. These would have to be collected before birth - some clinics offer to store blood from the umbilical cord when a person is born.
• Mutations have been observed in stem cells cultured for a number of generations, and some mutated stem cells have been observed to behave like cancer cells.
• Cultured stem cells could be contaminated with viruses which would be transferred to a patient.

Ethical issues

• A source of embryonic stem cells is unused embryos produced by in vitro fertilisation (IVF)
• For therapeutic cloning is it right to create embryos for therapy, and destroy them in the process?
• Embryos could come to be viewed as a commodity, and not as an embryo that could develop into a person.
• At what stage of its development should an embryo be regarded as, and treated as a person?

Social issues

• Educating the public about what stem cells can, and can't do, is important.
• Whether the benefits of stem cell use outweigh the objections.
• Much of the research is being carried out by commercial clinics, so reported successes are not subject to peer review. Patients could be exploited by paying for expensive treatments and being given false hope of a cure as stem cell therapies are only in their developmental stages.

Particles (molecules and ions) in a liquid and a gas move continuously. Because of this movement, particles will spread themselves evenly throughout a liquid or a gas.

If there is a situation where particles of a substance are in a higher concentration, they will move from this region to where they are in a lower concentration. This is called diffusion.

It is important to remember that the particles:

• will move in both directions, but there will be a net movement from high to low concentration
• will end up evenly spread throughout the liquid or gas, but will continue to move

If a crystal of a coloured chemical, eg potassium manganate (VII), is placed in water, the particles spread out and mix with the water particles.

• The potassium manganate (VII) is the solute.
• The water is the solvent.
• The potassium manganate (VII) has dissolved.
• The mixture that results is the solution.

The particles have moved from a region of high concentration in the crystal to a low concentration in the water. This difference in concentration is called a concentration gradient. Particles will move down a concentration gradient, from a high concentration to a low concentration.

As well as diffusion occurring between different regions, it also occurs across membranes, between the outside and inside of cells.

The rate of diffusion

The rate of diffusion can be affected by a number of factors:

FactorReason The concentration gradient The greater the difference in concentration, the quicker the rate of diffusion. The temperature The higher the temperature, the more kinetic energy the particles will have, so they will move and mix more quickly. The surface area of the cell membrane separating the different regions The greater the surface area, the faster the rate of diffusion.

Osmosis is the diffusion of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane.

A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.

Osmosis refers to the movement of water molecules only.

Osmosis across living cells

Cells contain dilute solutions of ions, sugars and amino acids.

The cell membrane is partially permeable.

Water will move into and out of cells by osmosis.

Substances are transported passively down concentration gradients. Often, substances have to be moved from a low to a high concentration - against a concentration gradient.

Active transport is a process that is required to move molecules against a concentration gradient.The process requires energy.

For plants to take up mineral ions, ions are moved into root hairs, where they are in a higher concentration than in the dilute solutions in the soil. Active transport then occurs across the root so that the plant takes in the ions it needs from the soil around it.

In animals, glucose molecules have to be moved across the gut wall into the blood. The glucose molecules in the intestine might be in a higher concentration than in the intestinal cells and blood – for instance, after a sugary meal – but there will be times when glucose concentration in the intestine might be lower.

All the glucose in the gut needs to be absorbed. When the glucose concentration in the intestine is lower than in the intestinal cells, movement of glucose involves active transport. The process requires energy produced by respiration.

Process

Descriptions

Substances moved

Energy required

Diffusion Substances move from a high to a low concentration down a concentration gradient Carbon dioxide, oxygen, water, food substances, wastes, eg urea No

Osmosis Water moves from a high to a low concentration across a partially permeable membrane and down a concentration gradient

Water No Active transport Substances move against a concentration gradient Mineral ions into plant roots, glucose from the gut into intestinal cells, from where it moves into the blood Yes