lecture 1

• CELS191 studies cell and molecular biology

Lecture 3 Objectives

• Describe the structure of the plasma membrane and its importance to cell function
• Outline the role of membrane proteins
• Outline mechanisms by which substances cross the plasma membrane, which include passive transport, active transport and co-transport
• Outline the importance of organelles and subcellular compartments
• Identify some of the key organelles in eukaryotic cells

Tree of Life

• Focus on eukarya animal cells

Cell Pop Quiz!

• Which of the following is closest to the number of human cells in the body?
  • a) 30 million
  • b) 30 billion
  • c) 30 trillion
  • d) 30 quadrillion
• How long was spent as a single cell?
• What are the largest single human cells in a) diameter and b) length?

Discussion Points

• A cell must manufacture cellular materials
• A cell must obtain raw materials from inside or outside the cell
• A cell must remove waste
• A cell must generate the required energy
• A cell must control all of the processes above

Separating Cell

• Cells need to separate from the outside world ever
• The inside of the cell is alive and the outside is not alive
• The plasma membrane is located at the boundary of each cell
• The plasma membrane provides special conditions within the cell and acts as a semi-permeable membrane barrier
• The plasma membrane acts like a door, allowing things to cross when appropriate

Cells Bounded by Plasma Membrane

• Cells are bounded by the plasma membrane
• The plasma membrane is a semi-permeable barrier
• The plasma membrane is crucial for normal functioning of a cell
• Allows passage of oxygen, nutrients and waste
• Controls movement of substances in and out of the cell
• Interaction with the environment limits the maximum size of a cell
• Small cells have a greater surface to volume ratio than larger cells

Plasma Membrane

• Consists of a double layer of phospholipids
• The double layer of phospholipids has various embedded or attached proteins
• Composition of fatty acids affects membrane fluidity
• Hydrophilic phosphate groups are on the outside of the cell membrane
• Hydrophilic phosphate groups are on the inside of the cell membrane

Plasma Membrane is Not Static

• Plasma membrane isn't static; it is dynamic
• Saturated tails means the tails are packed tightly together, and are less fluid
• Unsaturated tails means the tails prevent tight packing, and are more fluid
• High temperatures result in a more fluid membrane
• Low temperatures result in a less fluid membrane
• Cholesterol stabilises membrane fluidity
• Cholesterol allows the membrane to change fluidity, but not too much
• Cholesterol stops phospholipids from being too close together and provides space

Plasma Membrane Proteins

• Essential for cells
• Determines the function of the membrane; can be used for cell function
• Thousands of membrane proteins have been identified and classified
• Often specific to a cell type
• Each cell can have many different proteins
• Proteins can have multiple functions

Membrane Proteins

• Can be involved in signal transduction
• Signal transduction molecules travel from one cell to another
• Signal transduction messages have to be received
• Used to relay messages from the body (or environment) into the cell
• Signal transduction is involved in grow, divide, move, make something, die, etc

Membrane Proteins and Cell Recognition

• Membrane proteins can be involved in cell recognition
• Cell recognition is good and bad
• Often involves glycoproteins, which are proteins with added sugars

Intercellular Joining

• Some proteins form long lasting connections between cells

Linking Cytoskeleton

• Allows a cell to physically connect with protein structures outside the cell, which is called the extracellular matrix

Membrane Transport

• Allows small amounts of molecules to move across the membrane
• Can be passive or active
  • Passive requires no energy
  • Active requires energy

Question Time!

• Signal transduction uses membrane proteins to relay messages from outside the cell to inside the cell - is this true or false?
• A phospholipid bilayer forms an impermeable membrane - true or false?
• Cholesterol stabilises the fluidity of the membrane - true or false?

Substances Moving Across Membranes

• Movement across membranes depends on:
  • The molecules
  • How big the substances are
  • How much is moving at once
• Bulk transport involves large substances or volumes
• Small molecules in small volumes involves membrane transport

Different Transport Mechanisms

• Different molecules require different mechanisms
• No energy needed for passive mechanisms
• Molecules move down the concentration gradient
• Requires energy for active mechanisms
• Molecules move against the concentration gradient

Passive Transport – Diffusion

• Membranes are permeable to lipid soluble (hydrophobic) molecules, such as steroid hormones and gases
• Molecules move down their concentration gradient and do not require energy
• The membrane restricts movement of water soluble and charged molecules, such as glucose, ions and water

Passive Transport – Facilitated Diffusion

• Facilitated diffusion requires movement of hydrophilic molecules
• Facilitated diffusion requires membrane proteins called channels and carriers
• Facilitated diffusion aids the movement of specific substances down their concentration gradient
• No energy is required in facilitated diffusion, but some channels open or close in response to signals
• Carriers undergo a shape change to help guide the molecule

Passive Transport – Osmosis

• Aquaporins facilitates the movement of water across a cell membrane
• Osmosis is the movement of water from a high water (low solute) concentration to a low water (high solute) concentration
• Cells osmoregulate to prevent swelling or shrinking under varying conditions

Active Transport

• Requires transport proteins, which are carriers that use energy (ATP)
• Moves specific substances against their concentration gradient
• Active transport allows a cell to have an internal concentration of a substance that is different from its surroundings
• can be higher inside the cell versus outside the cell
• An example of this is the sodium-potassium pump

Co-Transport

• Is indirect active transport
• Pumps one substance across the membrane
• Its concentration gradient is used to power the movement of a second substance against its concentration gradient

Question Time!

• In passive diffusion, molecules move again their concentration gradient – true or false?
• Aquaporins are channels that allow water to cross the cell membrane – true or false?
• Co-transport is indirect active transport as it does not require energy – true or false?

Cell Compartments

• Different processes require different conditions and separate compartments
• Organelles allow:
  • Provide special conditions for specific processes.
  • Keep incompatible processes apart.
  • Allow specific substances to be concentrated.
  • Form concentration gradients.
  • Package substances for transport or export.

Cellular Organelles and Membranes

• Each organelle provides its own special conditions
• Endoplasmic reticulum making, packing proteins and releasing vesicles that carry proteins elsewhere
• Lysosomes function
• Mitochondria have two membranes
• Nucleus has the nuclear envelope and therefore has two membranes
• All cellular membranes are composed of a phospholipid bilayer

Key Organelles

• Organelles in blue are common to both cell types, animal and plant
• Nucleus is where DNA is
• Organelles include the endoplasmic reticulum and golgi apparatus
• Organelles in red are unique to each individual cell type
• Each has a membrane, which means separate environments for separate functions

Lecture Summary 3

• Cells are bounded with a semi-permeable and dynamic membrane that contains many proteins; they have key functions
• Membrane transport is necessary to move substances across the plasma membrane, and different molecules require different mechanisms
• Organelles are separate, specialised, and dynamic compartments within the cell

Objective Based Questions

• Why are organelles membrane-bound?
• What is the plasma membrane made of?
• What molecule stabilises membrane fluidity?
• What's the difference between passive and active transport?
• What sort of molecules can passively diffuse across the plasma membrane?
• What happens during co-transport of molecules?
• What are the key functions of membrane proteins?

Lecture 3 Practice Questions

• Relaying messages from the external environment into the cell can best be described as:
  • A. Cell recognition
  • B. Intercellular joining
  • C. Signal transduction
  • D. Adhesion to ECM
• Facilitated diffusion allows _____ molecules to cross the cell membrane via _____ , while _____ transport requires energy in the form of _____