Unit 4 IB HL Biology

Cell Membrane

Selective permeability - Only certain substances can cross the cell membrane without assistance

  • Permeability is determined by substance size, charge, and polarity

    • Permeable - The hydrophobic bilayer allows small non-polar substances through easily “liquid soluble”

      • O2

      • CO2

      • N2

      • Steroids

    • Mostly Permeable - Small polar molecules without a charge

      • Water (H2O)

      • Glycerol

    • Mostly impermeable - Large, polar molecules (with no charge) rarely cross the membrane

      • Glucose

      • Sucrose

    • Impermeable - Anything with a charge/ ions

      • K+

      • Na+

      • Cl-

      • H+

      • Ca2+

Membrane proteins

  • There are 2 types of membrane proteins

    • Integral proteins

      • Must be amphipathic

      • Most integral proteins are considered transmembrane because they span across the entire bilayer

      • “All transmembrane proteins are integral proteins but not all integral proteins are transmembrane”

    • Peripheral Proteins

      • Located on the surface of the bilayer (the hydrophilic heads) on either side

Membrane protein functions

  • Transport - transport proteins transport materials across the bilayer

    • Channel proteins

    • Pump proteins

    • Carrier protein

  • Recognition - Membrane proteins on the outside of the membrane serve as “ID tags” for cell-cell recognition

  • Receptors - Membrane proteins receive chemical signals

  • Enzymes - Enzymes can be embedded in or associated with the bilayer

    • Purpose: Catalyse a wide variety of reactions

Glycoproteins and Glycolipids

Typically found on the outer side of the cell membrane

  • Glycoproteins - carbohydrate chains that link to proteins

    • Considered conjugated proteins because they contain a protein + non-protein parts

  • Glycolipids - carbohydrate chains that link to lipids

Glycoprotein and Glycolipid functions

  • Cell recognition - carbohydrate chains serve as ID tags

  • Cell adhesion - carbohydrate chains bing to chains on other cells

  • Cell signaling - receptors for chemical messengers

  • Cell protection - Carbohydrate chains create a “sticky” layer called the glycocalyx on the outer side surface. Glycocalyx forms a protective layer for the cell

Cholesterol

  • Steroids found in animal cell membranes

  • Amphipathic

Membrane fluidity

  • Fluidity increases as temperature increases

  • Cholesterol increases fluidity at low temperatures and decreases fluidity at high temperatures

  • Phospholipid structure - types of fatty acids

    • Phospholipid structure - Fatty acid composition

      • Length: Shorter fatty acid tails increase fluidity

      • Saturated vs Unsaturated: unsaturated fatty acids contain double bonds that create kinked hydrocarbon tails- harder to pack together and have lower melting points compared to saturated fatty acids

Membrane fluidity adaptions

  • Ectothermic “cold-blooded” organisms - adapt to lower temperatures by increasing the amount of unsaturated fatty acids in their membranes

  • Hibernating mammals - as body temperature drops, the proportion of unsaturated fatty acids in the membrane phospholipids increase

Membrane transport

Concentration gradient

A difference in concentration

  • A bigger gradient = a steeper hill (if you think of it as a hill)

  • “Down the concentration gradient” = moving from high concentration to low concentration

Types of membrane transport

  • Passive transport - does not require an additional input of energy (ATP)

    • Simple diffusion

    • facilitated diffusion

    • Osmosis

  • Active transport - Does require an additional input of energy (ATP)

    • Pump proteins

    • Bulk transport

      • Exocytosis

      • Endocytosis

        • Pinocytosis

        • Phagocytosis

Passive transport

Diffusion

Diffusion - the movement of a substance down a concentration gradient

  • Passive transport (no energy needed)

Factors that impact diffusion rate

  • Temperature - higher temperatures = more molecular movement, therefore faster diffusion

  • Size - smaller molecules will diffuse faster than larger molecules

  • Concentration gradient - the ‘steeper’ the concentration gradient, the faster diffusion will occur

  • Distance - the shorter the distance, the more efficient the diffusion

  • Polarity - more polar means slower diffusion through a bilayer (charged ions cannot do simple diffusion)

Simple diffusion (Biological)

The movement of a substance down a concentration gradient across a phospholipid bilayer

  • Passive transport

  • Eventually, a dynamic equilibrium will be reached

  • No NET movement (no gain from the movement)

  • Small, non-polar molecules

    • ‘Permeable’

    • Some ‘Mostly permeable’

Facilitated diffusion

The movement of the molecules down a concentration gradient is assisted (facilitated) by transport proteins

  • Passive transport

  • Requires the assistance of a transmembrane integral protein

    • Channel proteins

    • Carrier proteins

Examples:

  • Glucose

  • Ions

Control of facilitated diffusion

  • Channel proteins and carrier proteins are selective

    • Selectivity is maintained by:

      • hydrophilic/hydrophobic side chains lining the channel

      • size of the channel

  • Channel proteins can be opened and closed

Gated channels

  • Channel proteins can sometimes be always open but often, their opening and closing are carefully controlled

    • Ligand-gated channels

    • Voltage-gated channels

Carrier proteins

  • When the molecule/ion binds to the carrier protein, the carrier protein undergoes a conformational change and transfers the molecules to the other side of the membrane

Key point:

  • Molecules that are mostly impermeable and impermeable cannot pass through the cell membrane without:

    • 1. A transmembrane integral protein specifically for them

    • 2. The protein being “open”

Osmosis

Osmosis - The diffusion of water across a semipermeable membrane

  • The cell membrane is mostly permeable to water

    • Water is a small, uncharged, polar molecule - only a little bit of water can cross the cell membrane (simple diffusion)

    • Aquaporins are channel proteins that are specific to water - they allow for greater volumes of osmosis to occur (facilitated diffusion)

  • Passive transport

Solutions

Solution = solvent + solute

  • The solvent does the dissolving - typically water

  • The solute is what is being dissolved

    • Sugar

    • Salt

  • Higher concentrations of solute means a lower concentration of solvent (water)

Osmosis and concentrations

  • water moves from a low concentration of solute to a high concentration of solute

Tonicity

  • Tonicity - The ability of a solution to make water move into or out of a cell by osmosis

    • Hypertonic, Isotonic, Hypotonic (comparison terms)

  • Isotonic

    • Solution A is isotonic to solution B if:

      • They both have the same solute concentration

    • Implications:

      • Solution B is also isotonic to solution A

      • No NET movement of water molecules (no gain of water in either solution)

  • Hypertonic

    • Solution A is hypertonic to solution B if:

      • Solution A has a higher solute concentration compared to solution B

    • Implication:

      • Solution B is hypotonic to solution A

      • NET movement of water molecules into solution A

    • Water always moves into the hypertonic solution in an attempt to create an equilibrium in the solute concentration

  • Hypotonic

    • Solution A is hypotonic to solution B if:

      • Solution A has a lower solute concentration compared to solution B

    • Implications:

      • Solution B is hypertonic to solution A

      • NET movement of water molecules into solution B

      • The hypotonic solution always loses water

Animal cells

  • Normal - In an isotonic solution the cells will be normal

    • There is no NET movement of water, so the cell stays the same

  • Lyse - When a cell is in a hypotonic solution, the water moves into the cell causing the cell to swell and burst

    • There will be a NET movement of water into the cell (the cell is hypertonic compared to the solution)

  • Crenate - When a cell is in a hypertonic solution, the water moves out of the cell causing the cell to shrivel and shrink

    • There will be a NET movement of water out of the cell (the cell is hypotonic compared to the solution)

Plant cells

  • Turgid - Plant cells are considered normal (turgid) in hypotonic solutions because water diffuses into the cell

    • Turgor pressure - the pressure of the water inside the cell keeps it standing upright and prevents it from wilting

    • The cell does not burst because the cell wall prevents too much water from entering

    • Plant cells thrive in hypotonic solutions

  • Flaccid - When a plant cell is in an isotonic solution the cell is flaccid

    • There is no NET movement of water - without the net diffusion of water into the cell, the plant will begin to wilt

  • Plasmolyze - When plant cells are in hypertonic solutions the cells will plasmolyze

    • The cell membrane shrivels up and pulls away from the cell wall as the cell loses water

Amoeba and Paramecium

  • Amoeba and paramecium are unicellular organisms that live in freshwater (hypotonic solution)

  • Water diffuses into the cells from the freshwater

  • Contractile vacuole - To expel the water, the organisms have a contractile vacuole which collects the water and expels it to maintain internal osmotic balance

Medical application of osmosis

  • IV fluids are used to deliver fluids to cells

    • The fluids must be isotonic to prevent damage to human cells and organs

      • If the solution was hypertonic the cells would create

      • If the solution was hypotonic the cells would lyse

Active transport

Pump proteins

  • Pump proteins - Transport proteins that move molecules/substances across a phospholipid bilayer against their concentration gradient

    • Moves molecules from low concentrations to high concentrations

    • Must be a transmembrane integral protein

  • Active Transport

  • Examples:

    • Proton pumps

    • Na+ / K+ pump

Bulk transport - Exocytosis

  • Bulk transport of material to be secreted or excreted out of the cell via vesicles

  • Process:

    • Vesicle (secretory or excretory) containing secretory/excretory material fuses with the cell membrane

    • Contents of the vesicle are discharged to the extracellular side

  • Examples:

    • Secretion of glycolipids

    • Excretion of wastes

    • Secretion of neurotransmitters

Bulk transport - Endocytosis

  • Bulk transport mechanism by which particles are moved into the cell

  • Process:

    • Cell membrane progressively invaginates and eventually engulfs the particles

    • Creates a vesicle surrounding the engulfed particles

Endocytosis: Phagocytosis

  • Ingestion of large solid particles

  • “Cellular eating”

  • Example: White blood cells

    • Engulf pathogenic bacteria when fighting an infection

    • Bacterium contained within a phagosome (food vacuole)

    • Lysosome fuses with phagosome to break down the bacterium

Endocytosis: Pinocytosis

  • Ingestion of liquids

  • “Cellular drinking”

  • Vesicle engulfs extracellular fluids and solutes

  • Vesicles that are created are much smaller than the ones in phagocytosis

Vesicles

Types of vesicles

  • There are several types of vesicles within cells

    • Transport vesicles -moves materials within the cell

    • Secretory vesicles - store and transport materials to be secreted out of the cell

    • Lysosomes - contain hydrolytic enzymes

    • Peroxisomes - contain enzymes involved in:

      • 1. Detoxification

      • 2. lipid metabolism

Formation of Vesicles

  • Clathrin - a protein that plays an important role in vesicle formation

    • Recruits elements required for the budding and scission of vesicles

    • Vesicles that are created are clathrin-coated vesicles (CCV)

  • Examples:

    • Endocytosis

    • phagocytosis

    • lysosome formation

Formation of CCV’s

  • Forms a cage-like structure at the vesicle formation site

  • As the membrane begins to invaginate, a clathrin-coated pit (CCP) is formed

  • Clathrin framework acts as a scaffold for vesicle creation

  • Facilitates scission

  • Disassembles after vesicle formation

Cell size and Compartmentalization

Organelle Membrane Fluidity

  • All internal membranes (membrane-bound organelles) are made of the phospholipid bilayer

  • Allows vesicles to seamlessly form from, move between, and fuse with the membrane-bound organelles

Vesicles and cell size

  • During exocytosis, vesicles fuse with the cell membrane - increase the size of the cell membrane

  • During endocytosis, vesicles are created from the cell membrane - decreases the size of the cell membrane

  • Membrane fluidity allows for this seamless transition into/out of the cell membrane

Cell size

  • Large cells require large amounts of nutrients and energy in order to survive

  • Large cells also generate a lot of waste that needs to be excreted

  • The transport of materials is most effective/efficient over short distances, so if a cell is too large, it will not be efficient

Exchange of materials

  • The cell membrane controls what enters and exits the cell

  • In order for a cell to be efficient, there needs to be enough cell membrane compared to the size of the cell

  • The surface area to volume ratio (SA:V) must be large in order for the cell to be efficient

SA:V Ratio

  • Cells require a high surface area to volume ratio

  • High SA:V = high efficiency

  • SA:V increases if:

    • SA increases

    • V increases

  • High SA:V ratio also equates to a shorter diffusion pathway

*Long, skinny, or flat shapes are ways to increase surface area

Compartmentalization

  • The presence of membrane-bound organelles allows for different functions and processes to occur in the different organelles

  • The presence of phospholipid bilayers (membranes) - allows for the separation, and thus different conditions within different organelles in the same cell

  • Allows for specialization of different organelles, and thus, more complex cellular functions (and faster reactions)

  • ONLY PRESENT IN EUKARYOTIC CELLS

  • Example:

    • Lysosomes contain hydrolytic enzymes that need to be packaged to prevent cellular damage

Organelles

  • Endomembrane System

    • Function: regulates protein traffic

    • Organelles:

      • Endoplasmic reticulum

      • Golgi apparatus

      • Lysosomes

      • Plasma membrane

      • Vesicles

    • Vesicles connect all of the organelles

    • All organelles of the endomembrane system are membrane-bound

Endoplasmic reticulum

  • There are two kinds of endoplasmic reticulum:

    • Smooth endoplasmic reticulum (SER)

      • Smooth ER looks smooth because it lacks ribosomes

    • Rough endoplasmic reticulum (RER)

      • Looks rough because ribosomes are attached to the outside

      • Functions:

        ~ The ribosomes on the RER assemble amino acids together to make the protein polymer

        ~ the synthesized protein then enters the RER and is chemically modified - folds

  • Function:

    • Detoxifies poison

    • Processes lipids

    • membrane factory - where phospholipids are made

Golgi/Golgi bodies/Golgi apparatus

  • The golgi apparatus consists of flattened membranous sacs

  • Functions of golgi apparatus:

    • Receives folded proteins from the RER

    • Sorts and packages them based on their final destination

    • Final destinations include:

      • Various organelles - think lysosome

      • Export from the cell - think plasma membrane

Lysosome

  • The lysosome is a membrane-bound sac of hydrolytic enzymes (formed from the golgi)

    • Hydrolytic enzymes perform hydrolysis

  • Lysosomes can fuse with food vacuoles, formed when a food item is brought into the cell

    • As polymers are digested, their monomers pass out to the cytoplasm to become available for use by the cell

  • Lysosomes are also involved in breaking down organelles that have become too old to function

Vesicles

  • How proteins move in between different cell structures

  • How proteins move from the Golgi to their final destination - either in the lysosome (the vesicle is now a separate organelle) or to the plasma membrane for excretion

  • Vesicles are membrane-bound, this allows them to fuse with the other membrane-bound structures of the endomembrane system

Cell Membrane

  • Mostly made out of phospholipids

  • Functions as a selective barrier that allows passage of oxygen, nutrients, and wastes for the cell

Cell Structures

Cell wall

  • Found in prokaryotes, plants, fungi, and some protists

  • In plants, the cell wall protects the cell, maintains its shape, and prevents excessive uptake of water

  • It also supports the plant against the force of gravity

  • The thickness and chemical composition of cell walls differs from species to species and among cell types

Centrosome

  • Microtubules that make up the centrosome from spindle fibers and move chromosomes during cell division

  • Only in Animals

Chloroplast

  • Energy transformer in a plant cell

  • The sites of photosynthesis - make food that can be used by the plant in the form of glucose

    • Eventually the glucose that the chloroplasts make is broken down and used as fuel in cellular respiration, this is why plants have both chloroplasts and mitochondria

Cytoskeleton

  • The cytoskeleton is a filamentous scaffolding within the cytoplasm.

  • Gives the cell shape

  • Acts as a transport system around the cell

  • Function: Provides internal structure and mediates intracellular transport

Mitochondria

  • Energy transformer of a cell

  • Mitochondria are the sites of cellular respiration which generates ATP (energy) from the breakdown of sugars in the presence of oxygen

Nucleolus

  • Where ribosome subunits are made

  • The subunits pass through the nuclear envelop into the cytoplasm where they combine together to form ribosomes

Nucleus

  • The nucleus contains most of the DNA in a eukaryotic cell - the instructions for synthesizing proteins

  • The nucleus is separated from the cytoplasm by a double membrane - called the nuclear envelope

Plasma membrane

  • Made out of phospholipids

  • functions as a selective barrier that allows the passage of oxygen, nutrients, and wastes for the cell

Structure of a membrane

  • All organelles in the endomembrane system are made out of the membrane

  • Membranes in the cell are composed of phospholipids

Ribosomes

  • Made out of RNA

  • A ribosome is composed of two subunits that combine to carry out protein synthesis (reading instructions indirectly from DNA)

Vacuoles

  • Membrane-bound

  • There are many different types of vacuoles, depending on the cell

    • Food vacuoles - temporary storage of food (can fuse with lysosomes)

    • Central vacuoles are found in many plant cells - they store water, inorganic molecules (potassium), and some macromolecule