Lecture 3: The Cell (Part 1) Flashcards
Plasma membrane
- primary function:
- why it’s important
- 2 other functions:
Plasma Membrane:
-primary function: semipermeable barrier that separates the intracellular from extracellular environment (and regulates what goes in/out of cell)
- channels and transporters in the membrane enable the import and export of small molecules
-
important for homeostasis:
- Hydrophobic portions of membrane phospholipids - barrier to most hydrophilic molecules
- Cells must also have a way to allow molecules to enter and exit the cytosol in a controlled fashion
-
important for homeostasis:
-also involved in: cell communication, cell growth and motility
- Receptor proteins in the plasma membrane enable the cell to receive signals from the environment
- flexibility of the membrane and its capacity for expansion allow the cell to grow, change shape, and move.
Nucleus
- contains:
- stains what color in H&E
Nucleus:
- Contains genetic information
- stains purple in H&E (because of nucleic acids that attract hemotoxylin)
Cytoplasm contains: (4)
- color when stained with H&E
1) organelles
2) cytoskeleton
3) cytosol (or cytoplasmic matrix)
4) inclusions
* Cytoplasm is pink when stained with H&E because of protein content (attracts eosin)
Organelles
- what type of function
- location
- 2 formats
Organelles:
- metabolic function
- located in cytoplasm
- can be membranous (membrane bound) or non-membranous
Cytoskeleton
- 3 main functions
- location
Cytoskeleton:
- proteins for transportation, movement, structural support
- located in cytoplasm
Cytosol or cytoplasmic matrix
- definition
- location
Cytosol or cytoplasmic matrix
- aqueous gel containing molecules
- located in cytoplasm
Inclusions
- definition
- location
Inclusions:
- products of metabolic activity (they do not have a metabolic function)
- located in cytoplasm
Homeostasis of plasma membrane
- hydrophobic portions of membrane phospholipids = barrier to most hydrophilic molecules
- cells must also have a way to allow molecutes to enter and exit the cytosol in a controlled fashion
In addition to a plasma membrane, eukaryotic cells also have
-
internal membranes that enclose individual organelles.
- All cell membranes prevent molecules on one side from freely mixing with those on the other
Phospholipid bilayer
- 2 parts & location of each
- overall structure and how it forms
- Hydrophpobic tail = inside
- Hydrophilic head = outside
- Phospholipid bilayers spontaneously close in on themselves to form sealed compartments (circles)
- The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable.
How does cholesterol effect cell membranes?
Cholesterol tends to stiffen cell membranes
- cholesterol fits into the gaps between phospholipid molecules in a lipid bilayer
- non-polar hydrophobic tail of cholesterol is chemically equivalent to the hydrophobic tails of the phospholipids
The fluidity of lipid bilayer depends on (2)
Compositon:
1) cholesterol
2) membrane proteins
Types of membrane proteins (4)
1) Channels and Transporters: allow for movement/tight control of cells homeostasis
2) Anchors: anchor to extracellular matrix or cytoskeletal proteins
3) Receptors: involved in cell signaling
4) Enzymes: involved in cell signaling
* Membrane proteins form a mosaic of particles penetrating the lipids*
The lateral mobility of plasma membrane proteins can be restricted in several ways
a)
b)
c)
d)
Proteins can be tethered to:
(A) intracellular cytoskeleton inside the cell
(B) to extracellular matrix molecules outside the cell
(C) to proteins on the surface of another cell (connecting one cell to another)
(D) Diffusion barriers (shown as black bars) can restrict proteins to a particular membrane domain.
Some of the ___ and ___ in the plasma membrane have ___ attached ____ to them
Some of the lipids and proteins in the plasma membrane have sugars (aka carbohydrates) covalently attached to them
- (aka - the cell surface is coated with carbohydrates)*
-
this carbohydrate-rich layer is made of:
- oligosaccharide side chains attached to membrane glycolipids and glycoproteins,
- polysaccharide chains on membrane proteoglycans
- As shown, glycoproteins that have been secreted by the cell and then adsorbed back onto its surface can also contribute.
- Note that all the carbohydrate is on the external (noncytosolic) surface of the plasma membrane.
What is the distance between the 2 phosphate heads in the plasma membrane bilayer?
- can you see that with a light microscope?
8-10 nm
- cannot see with a light microscope because its resolution is 200 nm (not strong enough)
- need TEM at least to see
Resolution of light microscope
200 nm
TEM appearance of plasma membrane
- Question marks?
- MV?
TEM appearance of plasma membrane = trilaminar structure; two dark bands (e- density) at phosphate heads, sandwiching a light band.
- Question marks: Glycocalyx (sugar chains off phospholipid bilayer)
- “MV” = microvilli - curvy, short finger link projections from the edge of an epithelial cell
(cannot see with plasma membrane light microscope, must use TEM or stronger)
Can you see the plasma membrane indicated by the arrow?
No - this is a light microscope, the resolution isn’t powerful enough
What you are seeing is the proteins associated with the plasma membrane (so can see where the plasma membrane is without actually seeing it itself)
nuclear envelope
-surrounded by what?
- houses nucleus
- surrounded by 2 membranes
-
the outer membrane continuous with the endoplasmic reticulum (ER) membrane
-
The space inside the endoplasmic reticulum (the ER lumen) is colored yellow; it is continuous with the space between the two nuclear membranes.
- The lipid bilayers of the inner and outer nuclear membranes are connected at each nuclear pore.
-
The space inside the endoplasmic reticulum (the ER lumen) is colored yellow; it is continuous with the space between the two nuclear membranes.
nuclear lamina
- definition
- made of what?
- purpose
•Nuclear lamina – meshwork of proteins (brown in image)
- made of intermediate filaments called lamins
- Stabilizes nuclear envelope
- A sheetlike network of intermediate filaments (brown) inside the nucleus forms the nuclear lamina (brown), providing mechanical support for the nuclear envelope.
strategies to protect DNA in nucleus (5)
strategies to protect DNA in nucleus (5)
1) nuclear envelope
* surrounds nucleus, has 2 membranes (inner and outer)
2) nuclear lamina
* meshwork of proteins, layer underneath nuclear envelope (next to its inner membrane), stabilizes nuclear envelope
3) nuclear pore complexes
* forms a gate through which selected macromolecules and larger complexes enter or exit the nucleus
4) chromatin
* DNA packing occurs on several levels in chromosomes
5) RNA copies of DNA sequences (transcription)
* RNA is what leaves nucleus so proteins can be made, DNA stays in nucleus
Nuclear pore complexes
- purpose
- structure
- how it works
nuclear pore complexes: control what can enter and exit the nucleus (keeps DNA protected inside nucleus)
- have fibrils that extend into cytosol and into nucleus (dark red)
- in nucleus: form basket shape
- Proteins with a nuclear localization signal (NLS) will bind to an NLS receptor/nuclear import receptor (ex. importin)
- Help chaperone protein of interest through the pore complex and into the nucleus
- Conformation of the nuclear pore complex changes as the protein passes through
- similar process happens in reverse to chaperone proteins out of nucleus to the cytosol
-
there are many different types of NLS signals and they will bind to and need help from different types of NLS receptors, will somtimes also need help from a third adaptor protein
- Overall: tightly regulated process that has many moving pieces
Nucleus Microscopy (TEM)
A)
B)
C)
-dashed line:
Nucleus TEM Appearance:
-
A - Heterochromatin = Electron dense areas representing tightly packed chromatin fibers. No transcription happening.
- Called “peripheral heterochromatin” if it’s associated with the nuclear membrane.
- B - Euchromatin = Lighter areas with dispersed granular material. Represents loosely arranged chromatin fibers. Transcription occuring.
-
C - Nucleolus = Round area containing both light and dark areas. Represents the site of rRNA synthesis and initial ribosome assembly.
- There may be more than one if the cell produces a lot of proteins
- dashed line: nuclear envelope
nucleolus microscopy (TEM)
A)
B)
C)
A) Fibrillar Center = Most pale areas
- DNA that is very unwound (increase transcription)
- contains genes for rRNA
B) Pars Fibrosa = darker areas
- rRNA genes that are actually undergoing transcription
- Can start to see rRNA (why more electron density than fibrillar centers)
C) Pars granulosa = dark but looks like grains of sand
- site of initial ribosome assembly
- *nucleolus is dark but it has little light patches (unlike heterochromatin, which is just dark)’*
- -size of nucleolus: 1-2 um*
Nucleus microscopy under light micropscope
-where is each of the 3 terms located
-
heterochromatin: the most darkly staining material inside the nucleus
- notice heterochromatin tends to aggregate on the nuclear envelope (called peripheral heterochromatin)
- euchromatin: pale area inside the nucelus
-
nucleolus: large dark area, that looks like a lot of heterochromatin
- may not see it if cell is not making a lot of rRNA at the time or if it’s not in the plane of cut
Can you see the nuclear envelope indicated by the arrows?
No, too small for light microscope (at most nuclear envelope is 50 nm)
What we are seeing aggregating on the nuclear envelope is peripheral heterochromatin
Judging by the nucleus (dashed line), which cell looks to be the most active in terms of protein synthesis?
A
- Cells undergoing protein synthesis typically have a pale nucleus and prominent nucleolus. The pale nucleus is due to the prevalence of euchromatin, chromatin that has become unwound to allow for transcription. A prominent nucleolus is the site of rRNA transcription and initial assembly of the ribosome.
- Nucleus B has a mix of heterochromatin and euchromatin and nucleus C is almost entirely heterochromatic.
Specialized cell: Muscle and other contractile cells
- General cellular activity:
Specialized cell: Muscle and other contractile cells
- General cellular activity: Movement
Specialized cell: Epithelial cells
- General Cellular activity:
Specialized cell: Epithelial cells
- General Cellular activity: form junctions between cells
Specialized cell: Fibroblasts, cells of cartilage and bone
- General Cellular activity:
Specialized cell: Fibroblasts, cells of cartilage and bone
- General Cellular activity: Synthesize and secrete extracellular matrix
Specialized cell: Neurons and sensory cells
- General Cellular activity:
Specialized cell: Neurons and sensory cells
- General Cellular activity: Convert physical and chemical stimuli into action potentials
Specialized cell: Cells of digestive glands
- General Cellular activity:
Specialized cell: Cells of digestive glands
- General Cellular activity: Synthesis and secretion of degradative enzymes
Specialized cell: Cells of mucous glands
- General Cellular activity:
Specialized cell: Cells of mucous glands
- General Cellular activity: Synthesis and secretion of glycoproteins
Specialized cells: Cells in the adrenal gland, testes, ovary
- General cellular activity:
Specialized cells: Cells in the adrenal gland, testes, ovary
- _General cellular activity: s_ynthesis and secretion of steroids
Specialized cells: Cells of kidney and salivary gland ducts
- General cellular activity:
Specialized cells: Cells of kidney and salivary gland ducts
- General cellular activity: Ion transport
Specialized cells: Neutrophils and macrophages
- General cellular activity:
Specialized cells: Neutrophils and macrophages
- General cellular activity: Intracellular digestion
Specialized cells: Adipocytes
- General cellular activity:
Specialized cells: Adipocytes
- General cellular activity: Lipid storage
Specialized cells: Cells lining intestines
- General cellular activity:
Specialized cells: Cells lining intestines
- General cellular activity: Metabolite absorption
8 distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane:
distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane:
1) the cytosol (gray)
2) endoplasmic reticulum
3) Golgi apparatus
4) nucleus
5) mitochondrion
6) endosome
7) lysosome
8) peroxisome
How do membrane phospholipids move within the lipid bilayer?
Membrane phospholipids move within the lipid bilayer:
- Because of these motions (flexion, rotation, lateral diffusion), the bilayer behaves as a two-dimensional fluid, in which the individual lipid molecules are able to move in their own monolayer
- Note that lipid molecules do not move spontaneously from one monolayer to the other (flip-flopping)
3 main roles of carbohydrates on cell surface
- carbohydrates/sugars covalently attached to some proteins and lipids in the plasma membane*
1) Help protect cell from mechanical damage
2) Help with cell motility
3) Important role in cell-cell recognition & adhesion - The recognition of cell-surface carbohydrates on neutrophils allows these immune cells to begin to migrate out of the blood and into infected tissues.
- Specialized transmembrane proteins (called lectins) are made by the endothelial cells lining the blood vessel in response to chemical signals emanating from a site of infection.
- These proteins recognize particular sugar groups carried by glycolipids and glycoproteins on the surface of neutrophils (a type of white blood cell, also called a leukocyte) circulating in the blood.
- The neutrophils consequently stick to the endothelial cells that line the blood vessel wall.
- This association is not very strong, but it leads to another, much stronger protein–protein interaction (not shown) that helps the neutrophil slip between the endothelial cells, so it can migrate out of the bloodstream and into the tissue at the site of infection
A =
B =
C =
A = microvilli
B = plasma membrane
C = glycocalyx
Can you see the plasma membrane indicated by the arrows?
(keep in mind we are using a light microscope)
No! What we are seeing is proteins that are located within the plasma membrane
(a light microscope is not strong enough to see plasma membrane because it only has a resolution of 200 nm - and plasma membrane diameter is 8-10 nm)
