Cells as the basic units of living organisms
The cell is the basic unit of all living organisms.
The interrelationships between these cell structures show how cells function to transfer energy, produce biological molecules including proteins and exchange substances with their surroundings.
a) Electron micrographs of plant and animal cells
a) Plant cells
b) Animal cells
b) Eukaryotic cell structures and their functions
1) Cell surface membrane (phospholipid bilayer) (7 nm)
• selectively permeable membrane that allows for the exchange of certain substances
• barrier between cytoplasm and external environment
• cell recognition (surface antigens)
• selection of substances that enter/leave cells
2) Nucleus (7 μm)
Controls cell’s activities
• very dense, takes up colour the most when stained
• divides first during cell division
• surrounded by 2 membranes, known as the nuclear envelope which is continuous with the RER.
• contains:
a) nuclear pores: allow and control substances entering in (protein to make ribosomes, ATP, some hormones, nucleotides) and leaving (mRNA, ribosomes for protein synthesis) of nucleus
b) nucleolus (2.5 μm): contains loops DNA from several chromosomes and synthesises ribisomes
3) Ribosomes (25 nm)
• composed of 2 subunits
• carry out protein synthesis
• 80S – cytoplasm
• 70S – chloroplasts & mitochondria
4) Rough endoplasmic reticulum (RER)
• membranes that form an extended system of fluid- filled sacs (cisternae)
• single membraned organelle
• ribosomes are attached to the RER and are the site of protein synthesis
• proteins made by the ribosomes enter the sacs and are often modified as they go through them
• small sacs (vesicles) break off from the ER and join to form the golgi body
5) Golgi body / apparatus / complex
• stack of flattened sacs (cisternae) formed by the vesicles which bud off from the RER
• Single membraned organelle
• Packages substances into vesicles for transport
• glycosylation
• phosphorylating proteins
• assembly of polypeptides into proteins (4º structure)
• folding proteins
• removing the 1st amino acid methionine to activate proteins
6) Smooth endoplasmic reticulum (SER)
• synthesizes lipids and steroids such as cholesterol and the reproductive hormones estrogen and testosterone.
7) Lysosomes (0.1-1μm)
• spherical sacs surrounded by a single membrane
• not permanent structures
• no internal structure
• contain hydrolytic enzymes
• responsible for digestion/breakdown of unwanted structures e.g., old organelles
• can even digest whole cells e.g., in mammary glands after the period of lactation
8) Mitochondria (0.5-10μm)
• carries out aerobic respiration
• synthesizes ATP (adenosine triphosphate)
• more in cells that have a higher demand for energy e.g., muscle, liver, and root hair cells
• outer membrane contains a transport protein called porin
• energy released from energy-rich molecules e.g.,
sugars and fats during respiration is transferred to molecules of ATP
• ATP is the energy-carrying molecule in all living cells
• once made, ATP leaves the mitochondrion and can spread rapidly to all parts of the cell where energy is needed
• its energy is released by breaking ATP down to ADP (adenosine diphosphate) in a hydrolysis reaction
• see Chapter 12.2(i) for more details
9) Microtubules
• long, rigid, hollow tubes found in the cytoplasm
• made of a protein called tubulin
• tubulin has 2 forms – ⍺ & β tubulin
• ⍺ & β tubulin molecules combine to form dimers
• many dimers are joined end to end to form protofilaments
• 13 protofilaments are in a ring to form a cylinder with a hollow center
• this cylinder is the microtubule
TUBULIN → DIMERS → PROTOFILAMENTS → MICROTUBULE
(α & β)
• supports and gives shapes to the cell
• the assembly of microtubules from tubulin molecules is controlled by special locations in cells called microtubule organizing centers (MTOCs)
9.5) Centrioles (and centrosomes)
9 TRIPLETS OF → CENTRIOLE
MICROTUBULES
• outside the nucleus of animal cells, 2 centrioles are present close together at right angles in a region called the centrosome
• centrioles are hollow cylinders about 500 nm long
• produces spindle fibers
• organizes microtubules
10) Chloroplasts (3-10μm)
• diameter 3-10 um
• carries out photosynthesis
• contains starch grains, circular DNA, and 70S ribosomes
• ATP is also produced here
• see Chapter 13.3(a) for more details
11) Cell wall
• gives cell definite shape
• rigid as made of cellulose
• freely permeable
• prevents cell from bursting
12) Plasmodesmata
• plant cells are linked to neighboring cells by means of fine strands of cytoplasm called plasmodesmata which pass through pore-like structures in their walls
• allows the transport of water, sucrose, amino acids, ions, etc., between cells without crossing membranes
• this is called movement through the symplastic pathway
• allows communication/signaling between cells.
13) Vacuoles
• surrounded by a partially permeable tonoplast which controls exchange between the vacuole and cytoplasm
• helps regulate osmotic properties of cells
• fluid present in the vacuole consists of:
d) Structural features of prokaryotic cells
• organisms that lack nuclei or proper nuclear membranes are called prokaryotes
• unicellular
• 1-5um
• cell wall made of murein (peptidoglycan = protein + polysaccharides)
• no membranes around organelles
• 70S (smaller) ribosomes
• genetic material in the form of circular DNA
• have no ER
e) Differences between typical eukaryotic and prokaryotic cells
See Chapter 18.2 for more details
f) Viruses
• noncellular/acellular
• protein coat
• nucleic acid core; DNA/RNA strand
• replicate inside host cells only
• show no characteristics of living organism
• symmetrical shape
• the virus DNA/RNA takes over the protein synthesizing machinery of the host cell which helps to make new virus particles
• See Chapter 18.2(d) for more details