Cell Structure

• All living things are made of...

  Cells
• Cells can either be...

  Prokaryotic or eukaryotic.
• Features of eukaryotic cells

  Animal and plant cells are eukaryotic. They have a:

  Cell membrane.

  Cytoplasm.

  Nucleus containing DNA.
• Features of prokaryotic cells

  Bacterial cells are prokaryotic. They have a:

  Cell wall.

  Cell membrane.

  Cytoplasm.

  Single circular strand of DNA.

  Plasmids. (small rings of DNA)
• Organelles

  Sub-cellular structures.
• Units of measurement

  Centi- = Multiply by 0.01

  Milli- = Multiply by 0.001

  Micro- = Multiply by 0.000001

  Nano- = Multiply by 0.000000001
• Subcellular structures in animal and plant cells

  In animal AND plant cells:

  Nucleus.

  Cytoplasm.

  Cell membrane.

  Mitochondria.

  Ribosomes.

  ONLY in plant cells:

  Cell wall.

  Permanent vacuole.

  Chloroplasts.
• Nucleus

  Contains the genetic material (DNA) which controls the activities of the cell.
• Cytoplasm

  Gel-like substance where chemical reactions take place.
• Cell membrane

  Controls what enters and leaves the cell.
• Mitochondria

  Where aerobic respiration takes place, providing energy for the cell.
• Ribosomes

  Where protein synthesis occurs.
• Chloroplasts

  Where photosynthesis takes place, providing glucose for the plant.

  Contains green pigment chlorophyll which absorbs light needed for photosynthesis.
• Permanent vacuole

  Contains cell sap.

  Improves cell rigidity.
• Cell wall

  Gives strength and rigidity to the cell.

  Made of cellulose in plants, and peptidoglycan in bacteria.
• Single circular strand of DNA

  Controls cell activities.

  Floats in the cytoplasm as prokaryotic cells lack a nucleus.
• Plasmid

  Small circular rings of DNA in the cytoplasm.
• How are sperm cells specialised for reproduction?

  Streamlined head and long tail to aid swimming.

  Many mitochondria to provide energy for swimming.

  Acrosome (head) has digestive enzymes to break through egg cell's membrane.
• How are nerve cells specialised for conduction of impulses?

  Axon is covered with a fatty myelin sheath which speeds up nerve impulses.

  Axon is long, enabling impulses to be carried across long distances.

  Lots of dendrites to form branched connections with other nerve cells.
• How are muscle cells specialised for contraction for movement?

  Have layers of proteins that slide over each other causing muscle contraction.

  Lots of mitochondria to provide energy for contraction.
• How are root hair cells specialised for absorption of water and mineral ions from soil?

  Root hairs increase surface area so rate of water uptake by osmosis is greater.

  Thin walls so water moves through easily due to shorter diffusion distance.

  Mitochondria for active transport of mineral ions.

  No chloroplasts as photosynthesis doesn't occur underground making room.
• How are xylem cells specialised for transport of water and dissolved ions?

  No top and bottom walls between cells creating continuous hollow tubes in which water is drawn upwards towards the leaves.

  Cells lack organelles or cytoplasm allowing free movement of water.

  Outer walls thickened with lignin, strengthening tubes and supporting plant.
• How are phloem cells specialised for transport of dissolved sugars and amino acids?

  Cells are joined end to end and contain sieve plates forming tubes which allow sugar and amino acids to easily flow through by translocation.

  Few subcellular structures to aid flow of materials.
• How do stem cells differentiate to become specialised?

  When a cell differentiates, it develops a structure and composition of subcellular structures which enables it to carry out a certain function e.g. elongated membrane and cytoplasm to form a nerve cell.

  It does this by only using some of its genes to control its development.
• Differentiation in animal cells

  Most animal cells differentiate in early development.

  Therefore cell division is mainly restricted to cell repair or replacement.

  Animal cells therefore lose the ability to differentiate early on in life.

  Some cells in certain locations e.g. bone marrow retain ability to differentiate throughout life. These are adult stem cells.
• Differentiation in plant cells

  Most plant cells retain the ability to differentiate throughout life.
• Light vs Electron microscopes

  Light microscopes use light and lenses to form images; electron microscopes use a beam of electrons to form images.

  Light microscopes usually require the specimen to be stained.

  Light microscopes can only see large subcellular structures e.g. nuclei and vacuoles.

  Electron microscopes have higher magnification and resolution and therefore see more subcellular structures e.g. mitochondria.
• Magnification

  Magnification = Image size/Actual size

  Actual size = Image size/Magnification

  Image size = Actual size X Magnification
• Microscopy PRACTICAL: Preparing slide

  Peel epidermal tissue off of onion with tweezers.

  Add drop of water to a clean slide then place tissue onto water.

  Stain tissue with iodine solution then place cover slip on top.
• Microscopy PRACTICAL: Using microscope

  Clip prepared slide onto stage and ensure the lowest-powered objective lens is over the slide.

  Use coarse adjustment knob to bring stage just below lens then look down eyepiece and adjust stage placement until image is roughly visible.

  Use fine adjustment knob until image is clearly visible.

  To change magnification power, change objective lens to desired magnification then readjust stage using coarse and fine adjustment knobs.
• How do bacterial cells divide?

  Binary fission.

  Cell contents double; cell then divides into two, forming two genetically identical daughter cells.
• Ideal conditions for growth of bacteria

  Supply of nutrients. (carbohydrates, proteins, minerals and vitamins)

  Appropriate temperature. (warmer temperatures = faster growth)

  Bacteria are grown in a nutrient broth solution or an agar gel plate.
• Aseptic techniques to keep bacteria cultures uncontaminated

  Work should be carried out in front a yellow-flame Bunsen burner.

  Hot agar jelly is poured into a sterilised Petri dish then left to cool.

  Inoculating loops must be passed through flame before used to transfer bacteria.

  Lid of petri dish should be secured with tape and stored upside down.
• Inhibition zone area

  Pi X Radius ^2.

  
• Calculating bacteria in a population

  No. of Divisions = Time Spent Dividing/Mean Division Time.

  No. of Cells = 2^No. of Divisions.