B1 - 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 of prokaryotes 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 and plant cells have the following parts:

  • a nucleus

  • cytoplasm

  • a cell membrane

  • mitochondria

  • ribosomes
• In addition to the parts found in eukaryotic cells, plant cells often also have:

  • chloroplasts

  • a permanent vacuole filled with cell sap
• 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.
• A sperm cell is specialised to carry genetic material to an egg cell, it has features such as:

  • Enzymes in the head, to digest coating of the egg
  • Many mitochondria so it has the energy to swim
  • Head contains genetic material for fertilisation
• A muscle cell is specialised to contract in order to produce movement, it has features such as:

  • Mitochondria, which provide energy for contraction
  • Protein fibres which shorten to make muscle cells contract - shortening the whole muscle
• A nerve cell (neurone) is specialised to carry electrical impulses around the body, it has features such as:

  • Long axons to carry impulses around the body
  • Dendrites that allow connections to other nerve cells
  • Fatty myelin sheath which speeds up the impulse
• A root hair cell is specialised to absorb water and mineral ions from soil, it has features such as:

  • A long thin projection which increases surface area - increased absorption
  • Many mitochondria which provide energy for active transport
• A phloem cell is specialised to transport sugars made in leaves to the rest of the plant, it has features such as:

  • Elongated cells to transport sugars over long distances
  • Pores at end of the cell so sugars can pass from cell to cell easily
  • Companion cells have many mitochondria for active transport
• A xylem cell is specialised to transport water and mineral ions through the plant, it has features such as:

  • Hollow tubes with no end walls to allow water to flow easily
  • Lignin in walls to provide strength and support
• 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.
• Cell Differentiation is important to us because it assists in the repair of damaged tissues and the replacement of worn out cells
• 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.
• 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 = $\dfrac{ImageSize}{RealSize}$
• • 1590s - Dutch spectacle makers Janssen experimented with putting lenses in tubes. They made the first compound microscope. None of their microscopes have survived, but they are thought to have magnified from ×3 to ×9.
• • 1650 - British scientist, Robert Hooke 1650 – also famous for his law of elasticity in Physics – observed and drew cells using a compound microscope.
• • Late 1600s – Dutch scientist Antonie van Leeuwenhoek constructed a microscope with a single spherical lens. It magnified up to ×275.
• • 1800s - the optical quality of lenses increased and the microscopes are similar to the ones we use today.
• Throughout their development, the magnification of light microscopes has increased, but very high magnifications are not possible. The maximum magnification with a light microscope is around ×1500.
• 1. The scanning electron microscope (SEM) has a large field of view so can be used to examine the surface structure of specimens. SEMs are often used at lower magnifications.
• 1. The transmission electron microscope (TEM) is used to examine thin slices or sections of cells or tissues.
• TEMs have a maximum magnification of around ×1 000 000, but images can be enlarged beyond that photographically. The limit of resolution of the transmission electron microscope is now less than 1 nm.

  The TEM has revealed structures in cells that are not visible with the light microscope.
• Electron microscopes use a beam of electrons instead of light rays.
• The resolution of a light microscope is around 0.2 μm, or 200 nm. This means that it cannot distinguish two points closer than 200 nm. One nm, or nanometre, is one billionth of a metre. This is written as:
• Resolution is the fineness of detail that can be seen in an image - the higher the resolution of an image, the more detail it holds. In computing terms, resolution is measured in dots per inch (dpi).
• 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
• 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. (STEP 1 OF THE CELL CYCLE)
• In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides. (STAGE 2 OF THE CELL CYCLE)
• Finally the cytoplasm and cell membranes divide to form two identical cells. (STAGE 3 OF THE CELL CYCLE)
• 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 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.
• 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.