Cell biology

• 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 the following parts:

  • a nucleus

  • cytoplasm

  • a 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
• 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 enabled biologists to see and understand many more sub-cellular structures.
• magnification = size of image/ size of real object
• Bacteria multiply by simple cell division (binary fission) as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
• Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.
• Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.
• Students should be able to describe how to prepare an uncontaminated culture using aseptic technique.

  They should be able to explain why:

  • Petri dishes and culture media must be sterilised before use

  • inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame

  • the lid of the Petri dish should be secured with adhesive tape and stored upside down

  • in school laboratories, cultures should generally be incubated at 25°C.
• 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. Students should be able to describe the stages of the cell cycle, including mitosis
• During the cell cycle the genetic material is doubled and then divided into two identical cells.

  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.
• In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
• 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 from differentiation.
• 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.
• 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 via 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 higher 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 temperature

  • the surface area of the membrane.
• 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
• 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 via osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• 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.