Chapter 2Membranes and cell organelles

The plasma membrane

7-9nm thick (nucleus & ER membranes = 5-7nm)

All biological membranes are the same : fluid-mosaic model phospholipid bilayer, proteins, carbohydrates and cholesterol

Membranes are fluid structures - individual lipid molecules & some proteins move about within the layers

Phospholipids Hydrophobic tails will face away from water to form a monolayer or micelle (if tails

are short) Therefore membrane is impermeable to water soluble (polar) molecules

Proteins carry out most other membrane functions, they are located throughout the membrane (hence the term mosaic) can cross both layers OR be confined to one Provide channels for water-soluble molecules & ions to pass

Cholesterol (between the phospholipid molecules) makes the membrane less fluid & more stable

Carbohydrates are usually on the outer surface & linked to protruding proteins

Membranes

Diffusionthe passive, net (overall) movement of

particles from a region of relatively high concentration to a region where they are at a relatively low concentration. O2 CO2 N2 H2O Ethanol Glycerol

Concentration gradient: (diffusion gradient) the steeper the gradient, that more rapid the rate of diffusion. As the gradient declines, diffusion slows down until it

reaches equilibrium (particles move to and fro at the same rate in all directions)

Osmosis (special kind of diffusion)

the diffusion of water molecules from one

solution to another of different concentration

through a selectively permeable membrane

OR

the net overall movement of water molecules

across a selectively permeable membrane from

a dilute solution (high concentration of water

molecules + low concentration of solute

particles) to a concentrated solution

Solute

Solvent(eg. Water)

Isotonic: (iso - same) surrounding fluid and cells internal fluid are of equal concentration

Hypotonic: (hypo - lower) surrounding solution has a lower concentration than the cells.Water will diffuse through the membrane into the cells

• Hypertonic: (hyper - higher) surrounding solution has a higher concentration than the cells

Channel/Carrier mediatedChannel mediated: free passage across the

membrane with the assistance of protein channel

Carrier mediated: Larger polar & non-polar molecules like glucose need proteins to carry them through: facilitated diffusion

Active transportThe net movement of dissolved substances

into or out of cells against a concentration gradient

This process requires energy

Mitochondrion

Golgi apparatus

Centriole

smooth ER

Lysosome

Cytoplasm/Cytosol

Plasma membrane Cell Wall

Ribosome

rough Endoplasmic Reticulum (ER)

Nucleus

nuclear pore

nucleolus

nuclear membrane

Cytoplasm / CytosolCytoplasm

contents of the cell (without nucleus) 90% water + ions, salts, enzymes, food

molecules & organelles

Cytosol fluid component of cytoplasm in which

organelles are located

Cytoplasm/Cytosol

Nucleusdouble membrane = nuclear envelope = eukaryote

lacking nuclear envelope = prokaryote

granular DNA until mitosis, then DNA becomes

organised into chromosomes

nucleolus/ nucleoli = aggregation of RNA

controls cellular activities

nuclear pores to allow movement between

Nucleus

nucleus and cytoplasm

Mitochondrionproduces chemical energy in the form of adenosine

triphosphate (ATP) through the process of cellular

respiration

outer & inner membrane

ATP produced by reactions on inner membrane

only in eukaryotes

Mitochondrion

Ribosome

protein factories = synthesis of polypeptides

in the cytosol or associated with ER

are made of protein and rRNA (ribosomal

RNA)Ribosome

Endoplasmic reticulum (ER)network of intracellular membranes

production, processing, transport & storage of

materials within a cell

links with plasma membrane and other

membranous organelles rough ER

smooth ER

Golgi apparatus

stack of flat membrane sacks

final synthesis & packaging of proteins

into membrane-bound vesicles before

secretion

Nature of Bio p. 47

Golgi apparatus

Lysosomes

contain powerful enzymes to break down

debris & foreign material

membrane bound & in most animal cells

Lysosomes

Plant cell

c

c

c

c

Cell Wall (not an organelle)plant & bacterial cells

non-living cellulose (chitin for bacteria)

provides support, strength & prevents

expansion

freely allows water & dissolved substances

through

cc

Chloroplastsgreen organelle (due to lots of chlorophyll)

found in most plant cells

many folded membranes

site of photosynthesis

c

Vacuolesmembrane bound, liquid filled spaces

food vacuoles (intracellular digestion)

contractile vacuoles (water balance)

plant cells typically have large ones filled with sap

(turgidity & storage)

tonoplast vacuole membrane in plant cellc

Other OrganellesPeroxisomes

detoxify various toxic materials that enter the bloodstream (ie: alcohol)

Endosomes pass newly ingested material to lysosomes for digestion

Centrioles small pair of cylindrical structures composed of microtubules involved in separating chromosomes during mitosis in animal and

protist cells (not in plants)

Cilia & Flagella used for movement in many prokaryotic cells & some eukaryotes covered by an extension of the cell membrane made up of 9 doublet microtubules & 2 single, central microtubules

Apoptosis / Programmed cell death

Neat cell death - unlike Necrosis

1. Many different caspases are

activated

2. Cell begins to shrink and

develop small bumps

3. Caspases cause DNA and

proteins to degrade and

mitochondria to break down

4. Phagocytosis of parts,

components recycled

The cell skeletonMaintain the shape of the cellProvide a support structure for other components in cellMovement of materials within cellMovement of the cell itself - if required

Occluding (Tight) junctionsCell membranes “come

together”Function:

hold cell togetherhelp to maintain polarity

of cellprevent passage of

molecules

NO movement of material between cells

Communicating (gap) junctionsProtein-lined pores in

the membranes of adjoining cells

Permit the passage of: amino acids, sugars, salt ions, and other small molecules

Anchoring junctions (Desmosomes)Dense plaques of protein with fine fibrils

Serve as a bridge connecting the actin cytoskeleton of neighbouring cells

Connections between plant cells: plasmodesmata

Chapter Review QuestionsChapter Review Questions: 2, 3, 4, 5, 7, 8 &

10 due on Monday 15th February

Biozone .... due on