Module 3

Question 1

What are the basic characteristics of cell membranes?

Answer 1

Membranes are continuous, unbroken sheets enclosing compartments

Structure allows fusion without losing continuity

Cell (plasma) membrane → physically separates cells from external environment

Question 2

In eukaryotic cell, what internal membranes define organelles

Answer 2

Nuclear envelope

Mitcohondrial membranes

Chloroplast membranes

Lysosomal membrane

Endoplasmic recticulum

Question 3

What are the 7 functions of cell membranes?

Answer 3

1) Compartmentalization: compartments have different contents and activities

2) Provide a selectively permeable barrier: prevent unrestricted exchange of molecules

3) Transporting solutes: exchange of molecules across the membrane

4) Scaffold for biochemical activities

5) Responding to external signals: signal transduction

6) Intercellular interaction: mediates interaction between cell and neighbours

7) Energy transduction: conversion of one form of energy into another

Question 4

What makes up the cell membrane structure?

Answer 4

Lipids
Proteins
Carbohydrates

Question 5

What part of lipids are cell membranes composed of?

Answer 5

Composed of two layers of lipids and the major type → phospholipids

Question 6

Explain the makeup of phospholipids that make up cell membranes

Answer 6

Recall that phospholipds have both hydrophobic, nonpolar regions and hydrophilic, polar regions and are thus amphipathic

Hydrophillic region → head group
Contains glycerol, a phosphate group, and a polar group

Hydrophobic region → fatty acid tail

Question 7

Explain the phospholipid arrangement in cell membrane structure

Answer 7

• Phospholipids can arrange themselves spontaneously
• Arrangement depends on the size of the polar head group
• If the head is large and bulky with one hydrophobic tail that is buried → form spheres called micelles
• If they contain smaller heads and have two hydrophobic tails → form a bilayer with a closed structure and inner space

Question 8

What happens if phospholipids are added to a test tube of water?

Answer 8

If phospholipids are added to a test tube of water, they spontaneously form enclosed bilayers called → liposomes

Liposomes enclose an inner space → a bilayer make effective cell membranes

Because of the bilayer arrangement the membranes are self-healing → phospholipids spontaneously rearrange
Due to the tendency of water to exclude nonpolar molecules

Question 9

Why are cell membranes dynamic?

Answer 9

Membranes are dynamic because they are continually moving, forming and re-forming → membranes are said to be fluid

Question 10

How are cell membranes fluid?

Answer 10

Lipids move laterally within the membrane

Movement is affected by nature of the phospholipid tails

The following features of lipids can affect fluidity:
I. length of the fatty acid tails
Longer - less fluidity *more rigid, less flexible

II. Presence of carbon-carbon double bonds
Fewer - less fluid

III. cholesterol → inserts into the lipid bilayer of animal cell membranes

*Can increase or decrease fluidity depending on temperature

Question 11

What are lipid rafts?

Answer 11

Lipids, and other components such as proteins, may be found in the discrete regions of the membrane → lipid rafts

Lipid rafts are specialized microdomains that serve as platforms for various cell processes

Lipids move in the plane of the membrane

Rarely would they move between the layers of the bilayer → lipid flip-flop

This makes it possible for layers to differ in comparison

Question 12

What are the functions of membrane proteins?

Answer 12

1) Transporters that move ions and molecules

2) Receptors that allow the cell to receive signals

3) Enzymes that catalyze chemical reactions in the cell

4) Anchors that attach to other proteins

Question 13

What are the two types of membrane proteins:

Answer 13

I. integral → permanently associated with the membrane and can span the entire lipid bilayer (transmembrane)

II. peripheral → temporarily associated with the lipid bilayer or with integral proteins via noncovalent bonds
- Can be either cytoplasmic or extracellular

Question 14

What are membrane carbohydrates?

Answer 14

• All face outward to extracellular space
• Covalentely linked to either:
  Protein → glycoprotein (carb + protein)
  Lipid → glycolipid (carb + lipid)

Question 15

What is the fluid mosaic model?

Answer 15

Core of membrane is the lipid bilayer

The lipid bilayer is a fluid structure that allows molecules to move laterally within the membrane

Contains mosaic (mixture) of different types of molecules → lipids, proteins, and carbohydrates

Remember, membranes are dynamic structures in which components are mobile and capable of coming together to engage in various types of transient interactions

Question 16

What does it mean to say that the plasma membrane is selectively permeable?

Answer 16

Some molecules are more permeable than others → gases (O2, CO2) and nonpolar molecules (lipids) can move across the bilayer

Small uncharged polar molecules (H2O) can move across the bilayer

Macromolecules (protein and carbohydrates) are too large to move across the membrane → need another way

Question 17

In referral to diffusion and osmosis, what is influx?

Answer 17

Movement of substance into the cell

Question 18

In referral to diffusion and osmosis, what is efflux?

Answer 18

Movement of substances out of the cells

Question 19

When either influx or efflux exceeds the other, what is this process called in referral to diffusion and osmosis?

Answer 19

net flux

Question 20

What is diffusion?

Answer 20

Diffusion is a spontaneous process

Net movement of molecules from an area of high concentration to an area of low concentration

Net movement stops when no concentration gradient → random movement in both directions

Question 21

What is passive transport?

Answer 21

Occurs across a membrane via diffusion → no energy required

Driven by concentration gradient across the membrane

Question 22

What are the two mechanisms for passive transport?

Answer 22

Simple diffusion → through the cell membrane

• Small molecules → O2, CO2
• Hydrophobic molecules → triglycerols

Facilitated Diffusion → molecules move down the concentration gradient through a protein transporter

Question 23

Regarding passive transport, what are the two types of membrane transporters involved in facilitated diffusion?

Answer 23

1) Channel Proteins
Channel formed by integral membrane proteins → surround aqueous pore

Permeable to specific molecules → selective

Some exist in either open or closed confirmation → gated channels ‘

2) Carrier Proteins
Specific molecules bind to transporter and moves down concentration gradient

Mechanism:
Molecule binds to carrier protein on one side of membrane → this triggers a conformational change in the carrier protein

Molecule is now exposed to other surface of membrane → diffuses down concentration gradient

*movement of molecules down the concentration gradient
*simple = directly across membrane
*facilliated = move through a channel or carrier protein

Question 24

What is osmosis?

Answer 24

The diffusion of water across a semi-permeable membrane

Due to the concentration gradient of H2O → moves from a higher H2O to lower H2O

The osmotic pressure is key → important to consider the tonicity of the solution a cell is surrounded by

*If equal on either side of the membrane → isotonic

Question 25

What are hypertonic solutions?

Answer 25

If a solution is hypertonic relative to cell (higher [solute] than the cell) → water molecules will move out of the cell, through the semipermeable membrane, and into the surrounding solution As water moves out of the cell → cell will shrink Animal cells will shrivel and die → crenation Plant cells will pull away from surrounding cell wall → plasmolysis

Question 26

What are hypotonic solutions?

Answer 26

If a solution is hypotonic relative to cell (lower [solute] than the cell) → water molecules will move from the solution, through the semipermeable membrane, and into the cell As water molecules move into the cell → the cell will swell Animal cells will swell and eventually burst *if red blood cell → hemolysis Plant cells will push against surrounding cell wall → turgid (turgor pressure)

Question 27

What is active transport?

Answer 27

Movement of substances against the concentration gradient → active transport Active energy coupled transport → requires energy Uses protein carrier → active transport Subtance binds to transporter and moves against the [gradient] Active transporter changes conformation upon binding to substance → movement is in one direction

Question 28

What are the two types of active transport?

Answer 28

primary active transport & secondary active transport

Question 29

What is primary active transport?

Answer 29

The sodium potassium pump → Na+ / K+ - ATPase Only present in animal cells → moves Na+ ions out of the cell and K+ ions in Na+ / K+ pump uses energy released from hydrolosis of ATP to move 3 Na+ ions and 2 K+ ions against the concentration gradient The sodium and potassium ions move in opposite directions → Na+ / K+ ATPase is an antiporter

Question 30

What are the steps of the Na+ / K+ -ATPase transport cycle (primary active transport)

Answer 30

1) Na+ ions bind to the transport protein on the inside of the membrane 2) ATP is hydrolyzed → Pi, transferred to the transport protein 3) Conformation of Na+ / K+ ATPase is changed 4)Na+ ions are released to external space 5) K+ ions bind on outside of membrane Phosphate group on the transport protein is removed Na+/K+ -ATPase transport protein snaps back to its original conformation K+ ions move into inside of cell

Question 31

What are electrochemical gradients?

Answer 31

Energy stored in the concentration gradient that is established through primary active transport can produce an electrochemical gradient This is a gradient that has both an electrical (charge) component & a chemical component Can drive the movement of other molecules through secondary active transport

Question 32

What is secondary active transport?

Answer 32

Primary active transport is necessary to begin SAT Cells use transport proteins to move ions across the membrane → this drives the movement of other substances against their concentration gradient For example, cells may actively pump protons against the membrane using ATP → the pump creates a proton concentration gradient, also called a chemical gradient Because the protons are charged, the proton concentraton gradient → is also an electrical gradient The movement of protons down their [gradient] drives the movement of other molecules against their [gradients] → this is secondary active transport (co-transport)

Question 33

What are the two types of secondary active transport, (co-transport)?

Answer 33

Antiport → two solutes move in opposite direction (Na+ and H+) Symport → two solutes move in same direction (Na+ and glucose)

Question 34

True or false: Eukaryotic cells have an internal set of membranes that define organelles

Answer 34

true

Question 35

What are the typical organelles found in animal cells?

Answer 35

Recall that eukaryotic cells have many internal organelles that are surrounded by membranes These are either physically connected by membrane “bridges” or transiently by the vesicles → form by budding off an organelle → Endoplasmic reticulum: involved in the synthesis of proteins and lipids → Golgi apparatus: modifies proteins and lipids produced by the endoplasmic reticulum *acts as a sorting station → Lysosomes: contain enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates → Mitochondria: specialized organelles that harness energy for the cell → Cytoskeleton: a protein scaffold that provides cell structure

Question 36

What are the typical organelles found in plant cells?

Answer 36

*many of the same as animal cells, but also: → Cell wall: provides additional support to the cell → Vacuoles: contribute to the structure by maintaining turgor pressure → Chloroplasts: convert the energy from sunlight into chemical energy that the cell can use

Question 37

What is the endomembrane system?

Answer 37

The system of interconnected organelles within the cell The endomembrane system allows specific functions to take place within defined spaces of the cell

Question 38

The entire contents of the cell minus the nucleus make up the...

Answer 38

cytoplasm

Question 39

The region outside the organelles but inside the plasma membrane is the...

Answer 39

cytosol

Question 40

What does the endomembrane system include?

Answer 40

Plasma membrane & nuclear envelope ER & golgi apparatus Lysosomes Vesicles that move between the above

Question 41

What is bulk transport?

Answer 41

Cells also need to bring in and remove larger molecules and particles across the membrane Need to do this without breaking continuity of the cell membrane

Question 42

What is exocytosis?

Answer 42

The process where a vesicle fuses with the cell membrane is known as → exocytosis A vesicle that has budded off from the endomembrane system can fuse with the plasma membrane and deliver its contents into the extracellular space

Question 43

What is endocytosis?

Answer 43

The process where the material from outside the cell is brought into a vesicle that can then fuse with other organelles is known as → endocytosis All the variations of endocytosis have common characteristics: → cell membrane invaginates → forms a pocket around the target material → the pocket pinches off inside the cell → results in the material surrounded by a newly created intercellular vesicle formed inside the cell membrane

Question 44

What is phagocytosis?

Answer 44

A variation of endocytosis: uptake of particulate matter Delivery to a lysosome via a phagosome Phagosome fuses with primary lysosome to form secondary lysosome → acitivates lysosomal hydrolytic enzymes Contents are then digested into the secondary lysosome

Question 45

What does phagocytosis provide?

Answer 45

I. nutrition → products move into cytoplasm II. defense → kill microorganisms Done by professional phagocytes in animals → macrophages and neutrophils Lysosome contains lysozymes → enzyme that degrades bacterial cell walls Acidic pH in lysosome kills some microorganisms Oxidases on inner surface of phagosome produce H2O2 → kills some bacteria

Question 46

What is pinocytosis?

Answer 46

A variation of endocytosis: means “cell drinking” Extracellular fluids taken up without recognition by plasma membrane surface Results in smaller vesicles and the vesicle does not need to merge with a lysosome Any molecules present in the enclosed fluid enters the cell

Question 47

What is receptor mediated endocytosis?

Answer 47

A variation of endocytosis: cell membrane receptor proteins have binding specificity for certain substances Coated pit is the site where receptors on the extracellular side are concentrated → indented surface Clathrin is attached to the cytoplasmic side → dynamic changes bring the vesicle into the cell Material is then delivered to a network of tubules and vesicles → endosomes Early endosomes are located near the peripheral region of cell Late endsomes are in the interior part of cell → receives material from early endosomes and Golgi apparatus

Question 48

What is the nucleus in the endomembrane system?

Answer 48

The nucleus is the innermost organelle of the endomembrane system The nucleus stores the cells DNA

Question 49

In the nucleus in the endomembrane system, what is the nuclear envelope?

Answer 49

a double membrane that surrounds the nucleus and consists of two membranes → inner and outer The nuclear membrane is perforated by protein openings called nuclear pores → allow molecules to move into and out of the nucleus The pores are essential for the nucleus to communicate with the rest of the cell

Question 50

What is the endoplasmic reticulum in the endomembrane system?

Answer 50

The outer membrane of the nuclear envelope is physically continous with the endoplasmic reticulum The ER is a large organelle in most eukaryotic cells Produces many of the lipids and proteins used inside and outside the cell and is a network of interconnected tubules and flattened sacs → the interior space is called the lumen Its highly folded nature allows for a large membrane surface area to fit inside the cell

Question 51

What is the rough endoplasmic reticulum in the endomembrane system?

Answer 51

Has ribosomes associated with the membrane and exposed to cytosol Ribosomes are sites of protein synthesis → where amino acids are assembled into polypeptides Ribosomes may also be free in the cytosol

Question 52

What is the smooth endoplasmic reticulum in the endomembrane system?

Answer 52

Lacks ribosomes → parts of the smooth ER bud off to produce vesicles that are free to move in the cytosol The smooth ER is the site of fatty acid and phospholipid synthesis There is a lot of smooth ER in cells that are specialized for lipid production

Question 53

Where is the golgi apparatus in the endomembrane system?

Answer 53

Not physically attached to the ER → it is the next stop for vesicles that bud from the ER

Question 54

What are the three functions of the golgi apparatus in the endomembrane system?

Answer 54

1) TO further modify proteins and lipids produced in the ER 2) TO sort proteins and lipids as they move to their final destination 3) TO add carbohydrates to proteins and lipids

Question 55

What does the golgi apparatus in the endomembrane system consist of?

Answer 55

The golgi apparatus consists of flattened sacs called → cisternae These are surrounded by small vesicles which transport proteins from the ER to the Golgi apparatus and between the Golgi apparatus and the cell membrane / other organelles Enzymes within the Golgi apparatus chemically modify proteins and lipids Each step is performed in a different region of the Golgi apparatus → different sets of enzymes catalyze specific reactions eg. glycosylation is the process which carbohydrates (glycans) are covalently linked to lipids or specific amino acids of proteins

Question 56

What are lysosomes in the endomembrane system?

Answer 56

Specialized vesicles derived from the Golgi apparatus Degrade damaged or unneeded macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates The Golgi apparatus can package these macromolecules into vesicles that fuse with lysosomes Enzymes found inside a lysosome are synthesized in the rough endoplasmic retitculum → sorted in the Golgi apparatus → then packaged into lysosomes Lysosomes contain specialized proteins in their membranes This includes a proton pump → keeps the internal pH of the lysosome at around 5

Question 57

Why is it important to keep the contents of the lysosomes separate from the rest of the cell in the endomembrane system?

Answer 57

Because the optimal pH of the cell is around 7 (neutral) → proteins and organelles within the cytosol would degradebif exposed to a pH of 5 found inside lysosomes

Question 58

What two membranous organelles are not a part of the endomembrane system?

Answer 58

mitochondria and chloroplasts

Question 59

What is common between mitochondria and chloroplasts?

Answer 59

→ specialized in harnessing energy for the cell → semi-autonomous: grow and multiply independently and contain their own circular genomes

Question 60

What is the mitochondria?

Answer 60

Mitochondria (singular, mitochondrion) harness energy from chemical compounds such as sugars and convert it into → ATP Rod-shaped organelles → present in almost all eukaryotic cells

Question 61

What are the four parts of mitochondria?

Answer 61

1) Outer membrane Integral proteins → form large, nonselective membrane channels → porins 2) Inter membrane space → aqueous compartment between outer and inner membrane 3) Inner membrane → highly convoluted: electron transport chain and ATP synthase 4) Matrix → citric acid cycle, ribosomes, DNA

Question 62

What are chloroplasts?

Answer 62

Where photosynthesis takes place Captures the sun's energy to synthesize simple sugars for use in the plant cell Surrounded by a double membrane and have an internal membrane-bound compartment → thykaloid The thykaloid membrane contains light-collecting molecules called pigments Chlorophyll is the most common → produces green colour seen in many plants

Question 63

What is the chloroplast structure?

Answer 63

Thykaloids → flattened membrane sacs within chloroplasts Space inside a thykaloid → lumen Orderly stacks of thykaloids → grana Space surrounding thykaloids → stroma

Question 64

What is the origin of mitochondria and chloroplasts?

Answer 64

- Chloroplasts resemble cyanobacteria (a photosynthetic bacteria) in organization and biochemistry - Organized as an endosymbiotic relationship between a cyanobacterial cell that had been engulfed by a eukaryotic cell - Mitochondria resemble free-living bacteria in organization and biochemistry - Like chloroplasts, mitochondria originated as endosymbiotic bacteria, in this case, proteobacteria