Chapter 6: Model of cells – the basic units of life

Science vocabulary

CellBasic building blocks of organisms
Cell membraneExternal boundary of a cell that separates the cell from its external environment
Cell wallThick layer surrounding the cell membrane of a plant cell
ChloroplastsOrganelles in green plant cells that contain the pigment chlorophyll which are responsible in absorbing light energy to make food through photosynthesis
CytoplasmContents of the cell, excluding the nucleus, that is bound by the cell membrane
NucleusStructure in the cytoplasm that controls the functions of the cell. It contains genetic material that is transmitted from generation to generation. This genetic material determines the characteristics of the organism.
VacuoleFluid-filled structure in the cytoplasm that is surrounded by a membrane

Tissue

A group of specialised cells performing a similar function
OrganelleA membrane-bound compartment in the cytoplasm that has a specialised function

The differences between a typical plant cell and a typical animal cell.

FeatureAnimalPlant
ShapeRelatively spherical, no specific shapeAngular (e.g oblong, polygonal)
Cell wallAbsentPresent outside the cell
NucleusOften centrally locatedLocated towards the side
ChloroplastsNoneMany
VacuolesMany small ones within the cytoplasm (often called vesicles)One large vacuole in the central position

Common misconception

  • Misconception: only plant cell has a cell wall. Actual fact: Bacteria and Chlamydomonas (a unicellular organism) have a cell wall. However, unlike plant cells, which have a cellulose cell wall, bacteria and Chlamydomonas have a cell wall that is not made of cellulose.
  • Misconception: only plant cells have chloroplasts. Actual fact: The Euglena is a unicellular organism that is not a plant but contains chloroplasts. Thus, it produces its own food but also consumes food from its environment.

Plant leaf

  • Beside the leaf tissue, plants also contain epidermal and vascular tissues. In epidermal tissue, cells are closely arranged to form a surface covering. In land plants, the outer surface of the epidermis is covered with a waxy layer called cuticle that helps to reduce water loss from the leaves. Some epidermal cells may be specialised in pairs to form guard cells that control the size of the stoma (open = guard cells become turgid; close = guard cells become flaccid) which controls the flow of carbon dioxide and oxygen. Inevitably, water is loss when stomata are open for release of oxygen. [Hint: read up on transpiration]. The vascular tissues are made up of xylem and phloem that responsible for transportation of water, nutrients and other dissolved substances within the plant.

Science bites

  • Without a nucleus, the lifespan of red blood cells in blood plasma is about 120 days.
  • Red blood cells contain haemoglobin which gives the cells their red colour and oxygen-carrying capability.
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