IB BIOLOGY- B2

Organelles

• Membrane bound structures in cells that are adapted to perform one or more vital functions. 

• Efficient → specialized for limited range of functions.

what structures are organelles?

Nuclei

Vesicles

Ribosomes

Plasma Membrane

what structures are NOT organelles?

Cell Wall 

Cytoskeletons

Cytoplasm

Transcription

Uses DNA to make mRNA

Translation

Uses mRNA to make Proteins

Advantage of the separation of the nucleus

-Keeps DNA safe

-It facilitates efficient post-transcriptional modification of mRNA.

Advantage of compartmentalization in the cytoplasm of cells.

Helps prevent incompatible biochemical processes

Background Information for #2: 

mRNA

carries protein information from the DNA in a cell's nucleus to the cell's cytoplasm.

What’s the range of cell size in humans?

Human cells range in size from 7.5µm to 150µm

White blood cells (fight infection)

-Monocyte

-Eosinophil

-Basophil

-Lymphocytes

-Neutrophil

Sperm

-Size (diameter): 50µm

-Adaption to Function: Narrowness and small volume reduce resistance and make it easier to swim.

Egg

-Size (diameter): 110µm

-Adaption to Function: large quantities of food reserves to be stored in the cytoplasm

Red blood cells

-Size (diameter): 6µm to 8µm

-allow passage along narrow capillaries and gives a large surface area to volume ratio, so loading and unloading of oxygen is faster.

Why is a high surface area-to-volume ratio beneficial for cells?

It allows for more efficient nutrient intake and waste elimination.

Membrane structure

Membranes contain

• Lipids (phospholipids, glycolipids and sterols)

• Proteins

• Carbohydrates (glycolipids & glycoproteins)

Lipid Bilayers

-Basis of cell membranes

-Contain 3 major classes of lipids - phospholipids, glycolipids and sterols like cholesterol.

Phospholipids

• Most abundant in plasma membrane

  • Glycerol

  • Phosphate 

  • 2 Fatty acids

• Naturally form bilayers

• 

Why do phospholipids form bilayers?

Because they are amphipathic

Lipid Bilyarers as barriers

Hydrophobic hydrocarbon chains that form the core of a membrane have low permeability to large molecules and hydrophilic particles. Including ions and polar molecules, so membrane’s function as effective barriers between aqueous solutions.

Integral protein

• Embedded in the lipid bilayer

• Amphipathic → interact with phospholipids in bilayer

• Transmembrane = goes all the way across/through membrane

Peripheral protein

• Found on outer surface of bilayer

• Hydrophilic

What function do proteins serve in the plasma membrane?

1- Transport (channel, carrier)

2- Recognition

3-Receptors

4-Enzymes

Transport proteins.

Membrane proteins facilitate the movement of molecules in and out of the cell. These include both channel proteins and carrier proteins.

 Channel proteins

transmembrane proteins that form channels or pores for the passage of molecules

 Carrier proteins

undergoes a conformational change to transfer the molecules from one side of the membrane to the other.

Recognition

Membrane proteins help in cell–cell recognition acting as ‘name tags’ for the cells. This is essential, especially in the functioning of the immune system, as it helps to distinguish between self and non-self cells.

Receptors

Membrane proteins act as receptors for chemical signals and are binding sites for molecules like hormones and neurotransmitters. Often, binding of these molecules triggers a chain of intracellular reactions.

Enzymes

Membrane proteins show enzymatic activity and catalyse reactions. For example, glucose-6-phosphatase is a membrane-bound enzyme found in the endoplasmic reticulum. They can help in cell adhesion to other cells or to the environment and play a role in cell motility.

Glycolipids

carbohydrate + lipid, Extends to extracellular space

Glycoprotein

carbohydrate + protein, Extends to extracellular space.

Cell recognition

 ‘markers’ on the cell surface help cells of the body recognize each other; also, immune system.

Cell adhesion

attach and bind to other cells to form tissues. 

Cell signalling

receptors for enzymes and other molecules helping in cell signaling, i.e. receiving and transmitting chemical signals.

Fluid mosaic model of membrane structure

Simple Diffusion

-Particles move from high to low concentration (down concentration gradient)

No energy needed for this to happen.

concentration gradient

occurs when the concentration of particles is higher in one area than another.

plasma membrane

selectively permeable and allows some molecules like O₂  or CO₂ to pass through via simple diffusion.

Osmosis

net movement of water in or out of a cell through the plasma membrane

1. Directly through membrane bilayer

2. With help of water selective pore called an aquaporin

3. Water moves toward higher concentration of solute.

   Movement stops when solute concentration inside the cell equals solute concentration outside the cell.

Hypertonic solution

greater solute concentration

Isotonic solution

same solute concentration

Hypotonic solution

lower solute concentration than cell

Hypertonic, Isotonic, Hypotonic

Facilitated Diffusion

-Molecules move from high to low concentration (down the concentration gradient)

-facilitated by transport proteins.

Channel Protein

• Highly selective

  • Size

  • Chemically specific binding sites

• Open and close in response chemical signals

  • Ions can’t move through unless open

Carrier Proteins

binds to the solute molecules

↓

undergoes a conformational change 

↓

transfers the molecules to the other side of the membrane

Channels are highly selective and only open/close in response to stimuli like

1. changes in voltage across the membrane or voltage-gated channels

2. binding of small molecules to the channel proteins or ligand-gated channels

3. mechanical forces like pressure.

Active Transport

• molecules transported from low to high concentration (i.e. against their concentration gradient.)

• Requires energy in the form of ATP

Utilizes protein (transport) pump

Simple diffusion

-Dependent on size and affinity for water (hydrophobic or hydrophilic)

-Anything good or bad can pass through if it fits this criteria!

Active transport & facilitated diffusion

- Active transport and facilitated diffusion are more selective than simple diffusion.

-Involve proteins that recognize certain molecules

Differentiation

when cells develop from unspecialized to specialized cells.

Morphogen

A chemical in the cell; the concentration gradient of which determines the fate of surrounding cells.

Stem cell

Cell that has the ability to differentiate into any cell type

Types of Stem Cells

1. Totipotent

2. Pluripotent

3. Multipotent

4. Unipotent

Totipotent

• Can differentiated into any type of cell

• Can give rise to a complete organism

Pluripotent

• Can differentiate into any type of body cell

• Can’t give rise to new organism

Multipotent

Can differentiate into a few closely related types of body cell

Unipotent

• Can only differentiate into their associated cell type. 

• Ex. liver stem cells can only make liver cells

Stem cell niche

microenvironment in organism that influences the stem cells and how they differentiate

Examples of stem cell niche

-Blood stem cells are found in the bone marro

-Hair follicle stem cells