1. Cell Biology & Biochemistry Series:Set 3
Version: 1.0

2. Objectives Learn the structure of eukaryotic cells.
Structure to function of:
Cell surface membrane
Nucleus
Mitochondria
Chloroplasts
Golgi apparatus
Lysosomes
Ribosomes
Rough Endoplasmic reticulum
Smooth Endoplasmic reticulum
Cell Wall
Vacuole

3. Features Shared by Plant and Animal Cells
Organelles and structures found in both plant and animal cells include:
Nucleus, chromosomes and nuclear envelope
Plasma membrane
Ribosomes
Mitochondria
Golgi apparatus
Endoplasmic reticulum (rough and smooth)
Vacuoles and Vesicles, although these differ in size and function in plants and animal cells.

4. Animal & Plant Cells Animal Cell Plant Cell
Animal and plant cells have many organelles in common, as well as several features specific to each. Specialized features of each are labelled on the diagrams of a animal cell and an plant cell below.
Lysosome
Centrioles
Cell wall
Chloroplast
Starch granule
Animal Cell
Plant Cell

5. Nucleus
Located: Variable location; not necessarily near the center of the cell.
Structure: Surrounded by a double nuclear envelope and encloses the genetic material (chromatin). Contains nucleoplasm (a granular, jelly-like material)
Function:
Contains most of the cell’s genetic material as DNA,
Regulates all the activities of the cell and involved in protein synthesis through production of RNA.
Size: 5 µm diameter.
Nuclear pores
Nucleolus
Nuclear membrane
Chromatin

6. Nucleolus
Located: Within the nucleus. Depending on the organism, there may be more than one.
Structure: A prominent structure which appears under EM as a mass of darkly stained granules and fibers adjoining part of the chromatin.
Function:
Synthesis of ribosomal RNA
Assembly of ribosomal subunits.
Size: 1-2 µm diameter.
Nucleolus

7. The Nuclear Membrane/Envelope
Nuclear pores
The nuclear membrane (or nuclear envelope) is a double membrane, similar to the cell plasma membrane.
It encloses the nucleus to separate its contents from the cell cytoplasm.
The nuclear membrane has many holes in it called nuclear pores (3000 each nm).
The nuclear pores allow the selective passage of materials between the nucleus and the cytoplasm.
This includes the movement of mRNA into the cytoplasm.
Outer membrane layer
Inner
membrane layer
Cell cytoplasm

8. Folded inner membrane forms cristae
Mitochondria
Folded inner membrane forms cristae
Located: Cytoplasm
Structure: Rod shaped organelles occurring in large numbers, especially in metabolically very active cells.
Bounded by a double membrane; the inner layer is extensively folded to form partitions called cristae.
Cristae provide large surface area for the attachment of enzymes and other proteins.
Mitochondria contain some DNA.
Matrix contains all the required substances for reactions.
Function: The site of cellular aerobic respiration (the production of ATP).
Size: Variable but 1-10µm
Smooth outer membrane
Matrix
A single mitochondrion in cross section

9. The plasma membranes of two adjacent cells joined with desmosomes
Located: Surrounds the cell forming a boundary between the cell contents and the extracellular environment.
Structure: Semi-fluid phospholipid bilayer in which proteins are embedded. Some of the proteins fully span the membrane.
Function:
Forms the boundary between the cell and the extracellular environment.
Regulates movement of substances in and out of the cell.
Size: 3–10 nm thick.
Phospholipid bilayer
Protein
The plasma membranes of two adjacent cells joined with desmosomes

10. Polypeptides being produced on a polyribosome system
Small subunit
Located: Free in the cytoplasm or bound to rough endoplasmic reticulum.
Structure: Made up of ribosomal RNA (mRNA) and protein and composed of two subunits, a larger and a smaller one.
Function: Site of protein (polypeptide) synthesis
Size: 20 nm.
Large subunit
Polypeptide chain
Ribosomes
Polypeptides being produced on a polyribosome system

11. Rough Endoplasmic Reticulum
Membranous tubules
Ribosome
Located: Continuous with the nuclear membrane and extending to the cytoplasm as part of the endomembrane system.
Structure: A complex system of membranous tubules studded with ribosomes. Connected to the smooth ER but structurally and functionally distinct from it.
Function:
Synthesis, folding, and modification of proteins.
Transport of proteins through the cell.
Membrane production.
Size: Variable according to cell size.
Transport vesicle budding off

12. Smooth Endoplasmic Reticulum
Membranous tubules lacking ribosomes
Located: In the cytoplasm as part of the endomembrane system.
Structure: A system of membranous tubules similar in appearance to the rough ER but lacking ribosomes.
Function:
Synthesis of lipids, including oils, phospholipids, and steroids.
Carbohydrate metabolism.
Transport of these materials through the cell.
Detoxification of drugs and poisons.
Size: Variable according to cell size.
Transport vesicle budding off

13. Transfer vesicle from the ER
Golgi Apparatus
Transfer vesicle from the ER
Cisternae
Located: Cytoplasm, associated with the ER.
Structure: Stack of flattened, membranous sacs called cisternae.
Function:
Modification of proteins and lipids received from the ER adding non-protein component (eg carbohydrates).
Sorting, packaging, and storage of proteins and lipids.
Transport of these materials in vesicles through the cell.
Manufacture of some certain macromolecules, e.g. hyaluronic acid.
Size: 1-3 µm diameter
Vesicle from the ‘shipping’ side of the Golgi

14. Vacuoles and Vesicles Located: In the cytoplasm; often numerous.
Structure: Vacuoles and vesicles are both membrane- bound sacs, but vacuoles are larger.
Function:
food vacuoles in animal cells are formed by phagocytosis of food particles.
central vacuole of plants provides cell volume and stores inorganic ions and metabolic wastes.
Support plant cells through turgor pressure on cell walls
Vesicles transport substances out of the cell through exocytosis
Size: varies according to cell type and size.
Food vacuole in a human lymphocyte

15. Specialist Plant Cell Features
A small number of cellular organelles are typically found in plant cells but not in animal cells.
Organelles and structures found in plant cells are:
cellulose cell wall
Chloroplasts
Amyloplasts
iStock
Cross section through a buttercup stem showing the individual cells

16. Diagrammatic representation of plant cell wall structure
Cellulose Cell Wall
Located: Surrounds the plant cell and lies outside the plasma membrane.
Structure: microfibrils of Cellulose fibers, with associated within a matrix. Between the walls of adjacent cells, is a sticky substance called the middle lamella.
Function:
mechanical strength to prevent cell bursting
maintains cell shape
Size: 0.1 µm to several µm thick.
Middle lamella
Pectins
Hemicelluloses
Cellulose fibers
Diagrammatic representation of plant cell wall structure

17. Chloroplasts
Located: Within the cytoplasm of plant leaf (and sometimes stem) cells.
Structure:
Specialized plastids containing the green pigment chlorophyll.
Chloroplast envelope – selective double outer membrane which are separated by a narrow inter-membrane space.
Inside the chloroplasts are stacks of flattened sacs or thylakoids which are stacked together as grana. Chlorophyll pigments inside thylakoid absorb light.
Stroma is a fluid filled matrix where the second stage of photosynthesis takes place.
Chloroplasts contain some DNA
Function: The site of photosynthesis
Size: µm.
Stroma
Grana
Thylakoids

18. Specialist Animal Cell Features
A small number of cellular organelles are typically found in animal cells but not in plant cells.
Organelles and structures found in animal cells are:
lysosomes
cilia
flagella
TEM of a human lymphocyte

19. Lysosomes Located: Free in the cytoplasm.
Membrane proteins
Located: Free in the cytoplasm.
Structure: Single-membrane-bound vesicle of hydrolytic enzymes (proteases and lipases). Lysosomes bud off the Golgi apparatus.
Function:
Hydrolyse material ingested by phagocytic cells such as white blood cells and bacteria
Release enzymes to outside of cell (exocytosis) in order to destroy material around cell
Digest worn out organelles and recycling of cellular components (autophagy)
Completely break down cells after they have dies (autolysis)
Size: varies according to cell size
Hydrolytic enzymes break down compounds by adding water
Lysosomes in a lymphocyte. Note how they are budding from the Golgi

20. Plant Cell

21. Animal Cell

22. Homework Chloroplast Job: Photosynthesis
Read pages 67 – 71 and make a top trump card for each organelle If you have any more information you want to add then you can put it on the back of the card There is a Top Trump Template on Moodle if you wish to print them off or do it by computer. Deadline for H/W Thursday Lesson. Extension: Look at ‘Division of Labour’ article on Moodle
Chloroplast
Grana
Stroma
Thylakoids
Job: Photosynthesis
Number of Membranes: 2
Special Structures: Thylakoids and Grana
Size: µm

23. Relative Sizes
The following scale shows the size range of some representative cellular organelles, cells, and multicellular structures.
The scale is logarithmic to accommodate the range of sizes shown.
From left to right, each reference measurement marks a tenfold increase in diameter or length.
Leaf tissue
Plasma membrane
Animal cell
DNA
Plant cell
Leaf
Golgi
Ribosome
Nucleus

24. Cell Fractionation
Cell fractionation is the process where cells are broken up and the different organelles are separated out.
Before cell fractionation begins cells are placed in a cold buffered solutions of the same water potential (isotonic).
Cold – to reduce enzyme activity that might break down organelles
Isotonic – to prevent organelles bursting or shrinking as a result of water intake/loss
Buffered – so that pH does not fluctuate. Any change in pH may alter protein/enzyme/organelle structure

25. Cell Fractionation There are two stages to Cell Fractionation
Homogenation:
Cells are broken up by a homogeniser (blender). This breaks the cell membrane and releases the organelles. The resultant fluid is known as the homogenate and can be filtered to remove large debris and unbroken cells.
Ultracentrifugation
The filtered homogenate is separated out in a machine called a centrifuge. This spins very fast in order to create a centrifugal force.

26. Cell Fractionation The process is as follows:
The tube of homogenate is placed in a centrifuge and spun at slow speed.
The harvested organelles, the nuclei, are forced to the bottom of the tube, where they form a sediment or pellet
The fluid at the top of the tube (supernatant) is removed, leaving just the sediment of nuclei.
The supernatant is transferred to another tube and spun in the centrifuge at a faster speed than before
The next heaviest organelles, the mitochondria, are forced to the bottom of the tube.
The process is continued in this way so that at each increase in speed the next heaviest organelle is sedimented and separated out.

27. Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. [3 marks]
The suspension is filtered before spinning to remove debris / intact cells which would contaminate sediment / interfere with the results.
Tissue is homogenenised to break open the cell and release the cell contents.
The solution is ice-cold because it slows / prevents enzymes being denatured.
It is isotonic to stop osmotic effects on cells / organelles.
It contains a buffer to prevent damage to proteins / enzymes due to changes in pH.
The second pellet collected is Chloroplasts
Organelles can be separated by centrifugation because they have different density.
Centrifuge at low speed to obtain nuclei as they are the largest / densest organelle.
Order of density from highest to lowest: nuclei, mitochondria, ER, golgi, ribosomes