Topic 3 - Cellular Level of Organization

Question 1

describe the lipid, protein and carbohydrate components of the cell membrane and their functions

Answer 1

lipid:

a) phospholipid bilayer
i) phosphate head group (hydrophobic)
ii) FA tails (hydrophilic)
iii) divides ICF + ECF
- > intracellular fluid (ICF) = inside cell
- > extracellular fluid (ECF) = outside cell

b) cholesterol
i) scattered throughout membrane
ii) adds stability to the cell membrane

protein:
2 types:
i) integral proteins
- integrated within the membrane ∴ have a hydrophobic region
- some extend across entire membrane = transmembrane proteins
ii) peripheral proteins - attached to extracellular or intracellular region of integral proteins i.e. on the periphery of the cell membrane
Functions: 
- enzymes
- transporters
- channels
- receptors
- anchors
- identity markers (for immune system)

carbohydrate:
- only on outer surface
- bound to proteins (glycoproteins) or lipids (glycolipids)
Functions:
i) cell recognition e.g. how sperm recognizes egg cell
ii) anchor cells together

Question 2

explain why the cell membrane is described by the fluid mosaic model

Answer 2

• fluid -> constituents can move around (phospholipids + some proteins)
• mosaic -> proteins dot the surface like tiles in a mosaic

Question 3

describe the two divisions of the cytoplasm

Answer 3

division 1: cytosol = gel-like intracellular fluid

• contains water and a suspension of carbohydrates, proteins ad lipids
• may contain inclusions e.g. melanin (pigment), glycogen (stored glucose)

division 2: organelles

• structures that perform a specific function and are essential for life
  i) non-membranous
• in direct contact with cytosol
  ii) membranous
• surrounded by membrane that isolates them from they cytosol

Question 4

list the non-membranous organelles and list their characteristics and functions

Answer 4

a) ribosomes
- sites for protein synthesis
- contain ribosomal RNA (rRNA) + proteins which may be:
i) free in cytosol - make proteins destined for cytosol
ii) attached to endoplasmic reticulum (ER) - make proteins destined for all other sites

b) centrosomes
- dense area of cytoplasm, usually contains 2 centrioles (function uncertain - some cells lack them)
- organizes microtubules of cytoskeleton & spindle apparatus (for cell division)

c) cytoskeleton
- determines cell shape (cell support)
- formed from proteins
- important in cell movement, cell division, movement/anchoring of organelles + proteins (e.g. receptors, enzymes)

3 components:
i) microfilaments 
- made of actin
Function: 
- muscle contraction (with myosin)
- cell locomotion
- maintenance of cell shape and projections (e.g. microvilli)
- cytokinesis 

ii) intermediate filaments
- composition is tissue specific e.g. keratin
Function:
- support the cytoplasm (scaffolding for the cell)

iii) microtubules
- hollow tubes made of tubulin
Function / form:
a) centrioles
b) spindle apparatus
c) cilia (short)
d) fagella
- structural
- move or secure organelles in place

Question 5

describe membranous organelles and list their characteristics and functions

Answer 5

a) mitochondria
- site of ATP
- contain own DNA, RNA + proteins
- have a double membrane

b) endoplasmic reticulum
- membranous network throughout cytoplasm, 2 types:
i) rough er (RER)
- ribosomes attached
- synthesis of secretory, lysosomal and membrane proteins
ii) smooth er (SER)
- lacks attached ribosomes
- continuous with RER
- synthesizes lipids + steroid hormones

c) golgi apparatus
- stacks of membrane discs
- modifies (e.g. trims or adds CHO groups), sorts, packages and delivers proteins/lipids to cell membrane, lysosomes, or for (cellular post office)
- secretion

d) lysosomes
- filled with digestive enzymes

Question 6

describe function and structure of the nucleus of a cell

Answer 6

• largest structure within the cell
• cell control centre
• cells may have 1 or more nuclei:
• nuclear envelop, nucleolus
• chromosomes/chromatids

Question 7

describe the two forms in which chromosomes may be found, and in which stage of the cell cycle is each form found

Answer 7

Form 1:
dispersed = chromatin
- DNA uncoiled, chromosomes not individually visible
- in this form MOST of the time (when cell is not dividing)

Form 2:

• condensed + chromosomes individually visible
• found in dividing cells (during mitosis/meiosis)

Question 8

define cell

Answer 8

• basic structural and functional unit of the body

- can perform all basic life functions