Cells Flashcards
What is a cell?
A cell is the basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material ( DNA) and cytoplasm containing organelles.
What is an organelle?
A structurally and functionally distinct part of a cell.
E.g ribosomes , mitochondria
What is a eukaryote?
An organism whose cells contain a nucleus and other membrane-bound organelles
What is a prokaryote?
An organism whose cells do not contain a nucleus or any other membrane-bound organelles.
What is the cell surface membrane?
A very thin membrane (7.5-10nm) surrounding all cells.
Found in both eukaryotic and prokaryotic cells.
Describe the structure of the cell surface membrane.
At very high magnifications, it is seen to have a trilaminar appearance; two dark layers surrounding a pale interior.
Formed from a BILAYER OF PROTEINS AND PHOSPHOLIPIDS.
What are the functions of the cell surface membrane?
- It is SELECTIVELY PERMEABLE: controls the exchange of substances into and out of the cell.
•Forms a barrier across which all substances entering or exiting the cell must pass. - It retains the fluid cytosol.
What is the nucleus?
The largest organelle found in all eukaryotic cells, but absent from prokaryotic cells.
What is the nuclear envelope?
The two membranes , situated close together, that surround the nucleus. The envelope is perforated with NUCLEAR PORES.
The outer membrane of the nuclear envelope is continuous with the RER
What are nuclear pores?
Pores found on the nuclear envelope which control the exchange of materials between the nucleus and the cytoplasm.
What are example of substances entering the nucleus via nuclear pores?
- Proteins ( to make ribosomes)
- Nucleotides
- ATP
- Hormones (e.g thyroid hormone T3)
- Enzymes (e.g DNA polymerases)
- Signaling molecules.
What are examples of substances exiting the nucleus?
- mRNA
- tRNA
- ribosomes for protein synthesis
Describe the hierarchy of genetic material
Nucleus>chromosomes> DNA (genetic material)> organs used into functional units called genes> genes control cellular activities and inheritance
Hence, the nucleus controls cellular activities.
Why is the DNA folded and how is this achieved?
DNA molecules are very long and are hence folded into a more compact shape to prevent strands from becoming tangled.
This is achieved by combining HISTONE PROTEINS.
What is chromatin?
A complex of DNA and Histone proteins and some RNA; material from which chromosomes are made.
They are visible as patches or fibres within the nucleus when stained.
What are chromosomes?
Present in the nucleus of eukaryotic cells.
A structure made of tightly coiled chromatins visible during cell division.
What is circular DNA? Where is it present?
A circular strand of DNA.
Present in: prokaryotic cells, chloroplasts and mitochondria.
What is the nucleolus?
A small structure, one or more like of which is found Inside the nucleus; usually visible as a densely stained body.
What is the function of the nucleolus?
To manufacture ribosomes using the information in its OWN DNA.
Describe the structure of the nucleolus.
• contains a core of DNA, from one or more chromosomes, which contains the genes That code for RIBOSOMAL RNA (the form of RNA Used in the manufacture of ribosomes)
• also contains the genes for Making tRNA.
• Around the core are less densely stained regions where RIBOSOMAL SUBUNITS ARE ASSEMBLED:
Combining the rRNA with ribosomal proteins imported from the cytoplasm.
Note:
- The more ribosomes a cell makes, the larger it’s nucleolus.
- The different parts of the nucleolus only come together during the manufacture of ribosomes ( the nucleolus as a structure that disappears)
What are the functions of the nucleus?
- Controls the synthesis of enzymes and hence controls all metabolic activities.
- Contains the genetic material of the cell in the form of chromosomes.
- Controls cell division.
- Carries instructions on the nuclear DNA for the synthesis of proteins.
What is the endoplasmic reticulum describe its structure?
A network of flattened sacs running through the cytoplasm.
• bounded by a single membrane
• Continuous with the outer membrane of the nuclear envelope.
• the membranes of the ER form flattened compartments or sacs called CISTERNAE.
• Molecules particularly proteins can be transported through the cell inside these sacs, separated from the rest of the cytoplasm.
Describe the structure of the RER?
Surface covered with ribosomes which are the sites for protein synthesis.
Note: ribosomes are found free in the cytoplasm or on the surface of the RER
What is the function of the RER?
Processes, packages and transports proteins made by ribosomes.
What are the functions of the smooth ER?
Lacks ribosomes
Functions:
- Synthesis of lipids, phospholipids and steroids ( cholesterol, œstrogen and testosterone)
- A major storage site for calcium ions. Hence, smooth ER are abundant in muscle cells as calcium ions are required for muscle contraction.
- Participates in carb metabolism.
- Involved in drug metabolism (in the liver)
What are ribosomes? And state the sizes of prokaryotic and eukaryotic ribosomes
A tiny organelle found in large numbers in all cells
- not bounded by any membranes.
- found freely in the cytoplasm of ALL CELLS and the RER of EUKARYOTIC CELLS.
- prokaryotic: 20 Nm
- eukaryotic: 25 Nm
Describe the structure of ribosomes.
- Made of two subunits: a large subunit and a small subunit.
* composed of almost equal amounts of rRNA and protein.
What are the types of ribosomes and where are they found?
80s: eukaryotic ribosomes
70s: prokaryotes, mitochondria and chloroplasts
What are the functions of ribosomes?
Allow all interacting molecules involved in protein synthesis to gather up in one place
What are examples of molecules involved in protein synthesis?
mRNA, tRNA, aminos acids and regulatory proteins.
What are polyribosomes?
Clusters of ribosomes formed to synthesise multiple copies of the same protein.
What is the Golgi apparatus/ Golgi body?
A stack of flattened membrane-bound sacs called CISTERNAE AND associated small, hollow spheres of membrane called VESICLES.
Describe the protein pathway across the cell.
- Vesicles containing protein bud off form the ER
- Vesicles fuse together to form flattened membrane-bound CISTERNAE of the Golgi body at its CIS/forming face.
- Inside the CISTERNAE, carbohydrates may be added to proteins to form GLYCOPROTEINS.
(Golgi body contains enzymes) - After processing, secretory vesicles called Golgi versicles in turn bud off from the Golgi CISTERNAE at its maturing/ TRANS face.
- These vesicles move towards the the plasma membrane and fuse with it, releasing the contents out of the cell- the secretory pathway.
Note; the process of releasing a material by a secretory vesicle out of the cell is called EXOCYTOSIS.
What are the functions of the Golgi apparatus?
- Formation of primary lysosomes
- Involved in the modification and final processing of proteins, glycoproteins and phospholipids from the ER:
- sugars are added to proteins to form GLYCOPROTEIN molecules.
- sugars are added to lipids to from glycolipids. - During plant cell division, the Golgi enzymes are involved in the synthesis of new cell walls.
- In the gut and the respiratory system, goblet cells release a substance called MUCIN from the Golgi apparatus.
Mucin- main component of mucus.
What are the importance of glycoproteins and glycolipids?
They are important components of membranes and are important molecules in cell signaling.
What is the mitochondria?
The organelle in eukaryotes in which aerobic respiration takes place.
Most numerous organelles seen with light microscope.
• can move about, divide and change shape.
Describe the structure of the mitochondria.
- 1 micrometer In diameter
- often sausage-shaped ( circular in cross-section)
- surrounded by a double membrane (an envelope)
- The inner membrane is folded to form finger-like cristae, which project into the interior of the mitochondrion which is called the MATRIX.
- The space between the two membranes Is called the intermembrane space
What is the matrix and state it’s function.
The matrix is formed by the cristae
Function: contains the enzymes needed for aerobic respiration, producing ATP.
• small circular DNA (mitochondrial DNA) + 70s ribosomes are present in the matrix; needed for replication.
What are cristae and state it’s function.
Cristae are folds of the inner membrane of the mitochondrial envelope.
•stalked proteins of ATP Synthase and electron transport chains associated with aerobic respiration are found on the cristae.
What is the function of the mitochondria?
Main: to carry out aerobic respiration (powerhouse of the cell)
Secondary: synthesis of lipids