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The chemical nature of cells

VCE Biology Unit 3

ContentsCell TheoryCell StructurePlant CellAnimal CellInorganic MoleculesOrganic Molecules

MembranesDiffusionOsmosisProtein-mediated movement

Cell Theory

There are three very important scientists associated with the Cell Theory. They are: Theodor Schwann, Matthias Schleiden and Rudolf Virchow. They proposed that;All organisms are composed of cellsAll cells come from pre-existing cellsThe cell is the smallest living organisational unit

Cell StructureCells are comprised of many organelles

Plasma membraneNucleusCytosol/CytoplasmMitochondriaEndoplasmic ReticulumRibosomesGolgi Apparatus

CentriolesLysosomesPlastids*VacuolesVesiclesChloroplast*Large Vacuoles*Cell Wall*Tonoplast**Found in plant cells

Ancient and Modern Cells

The earliest cells lacked membranes around their organelles. All the materials currently organised in more recent cells simply floated about in the cytosol, including the DNA.The earliest cells are known as prokaryotesand more modern cells are known as eukaryotes.

Generalised Plant Cell

Cells Alive

Generalised Animal Cell

Cells Alive

Inorganic MoleculesThe following inorganic molecules (not

produced by organisms) are important to cells:WaterOxygenCarbon DioxideNitrogenMineralsAcid/Bases and pH

WaterLife began in waterOrganisms 70 – 90% waterWater is biologically important due to its properties: acts as a solvent, high heat capacity, high heat of vaporisation, highly cohesive, with high degree of surface tension.These properties are related to the hydrogen bonds in water molecules which are polar.

Oxygen

Oxygen is required to release the energy bound up in organic molecules, such as glucose, in cellular respirationThe atmosphere is approximately 21% oxygen.

Carbon DioxideIn all organisms carbon dioxide is released as a by-product of cellular respiration and is therefore a waste.However, autotrophs use carbon dioxide to synthesise simple organic molecules such as glucose in the process known as photosynthesis.The atmosphere contains less than 1% carbon dioxide.

NitrogenNitrogen is a component of all protein structures.Animals are highly dependent on many types of proteins (plants less so) – including hormones and enzymes.The atmosphere is approximately 78% nitrogen, but most organisms are unable to fix atmospheric nitrogen.

MineralsMost of the other nutrients required by organisms are found in the soil as mineral salts, from where they are taken up by plants and consumed by heterotrophs.These include: phophorous, potassium, calcium, magnesium, iron, sodium, iodine, sulfur which are needed in reasonable quantities and boron, manganese, zinc, molybdenum, copper and chlorine which are required in smaller amounts.

pHpH is the concentration of H+ and OH- ions in solution.The greater the concentration of H+ ions the more acidic a substance.The greater the concentration of OH- ions the more alkaline a substanceThis occurs in solutions as water acts as a solvent ionising the substances it dissolves.

Organic Compounds

Organic molecules are made by organisms and are often long chain molecules known as polymers.CarbohydratesProteinsNucleic AcidsLipids

CarbohydratesCarbohydrates are the most abundant source of chemical energy for living organismsAre used as energy reserves in plants and animalsForm structural components such as cell wallsForm part of both DNA and RNACombine with proteins and lipids to form glycoprotein and glycolipids as found in cell membranes

ProteinsProteins are highly complex molecules made up of chains of amino acids joined together by peptide bonds.Enzymes, hormones, catalysts, structural components of cells, carrier molecules such as haemoglobin, and protein channels in membranes are all made from proteins.

Nucleic Acids

Nucleic Acid are made of long chains of nucleotides.They contain a sugar, a phosphate and a proteinThey contain all the genetic information of the organism.

LipidsLipids (we used to call these fats and oils) are non-polar hydrophobic molecules.They are used to store energy (adipose tissue), as structural components (cell membranes) and steroids (such as hormones and vitamins).

MembranesThe membrane of cells is comprised of two layers of phospholipids with protein channels implanted to join the extra and intra cellular environments and with glycoprotein molecules with act as markers to identify the cell.They control what enters and leaves the cell and communicate with other cells in the same tissue or those that may pass by.

Movement across MembranesDiffusion - is the movement of molecules of a substance from an area of high concentration of that substance to an area of low concentration. This is easily achieved by gases not contained within a vessel and for solids such as salts and sugars dissolved in water. In organisms this takes place across partially permeable membranes.

Movement across MembranesOsmosis - refers to the movement of water molecules. They will move from an area of low concentration of a dissolved substance such as sugar to an area of high concentration of that dissolved substance. This is important to organisms for the movement of water and other substances into and out of cells. There are three conditions to consider:

IsotonicHypotonicHypertonic

Isotonic

The surrounding fluid and the cellular fluid are of equal concentration. Molecules will move in both directions, but there will be no change in overall concentration either side of the cellular membrane.

Hypertonic

If the cell is surrounded by a solution with a higher concentration than the cellular fluid, then water will leave the cell by diffusion. This can lead to cells drying out. We say they have become desiccated.

Hypotonic

When cells are surrounded by a solution of lower concentration to the cellular fluid then water will diffuse into the cell. This can be a problem as the cell will swell and possible burst. Plant cells have a cell wall that partially protects the cell from this and plant cells have mechanisms which regulate and control the concentration of cell fluids.

Movement across MembranesProtein mediated Transport

Facilitated diffusion – no expenditure of energy can’t move against the concentration gradient.Active transport – expenditure of energy can therefore move against the concentration gradient.Both are faster than simple diffusion, the channels are specific, but one substance will inhibit the movement of another and the channels do become saturated.

Interactive Animation

Switch to the Heinemann Biology 2 CD and play the Movement across Membranes Interactive animation.