CELS191 Lecture 3: Plasma Membrane & Organelles

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Dr Rebecca Bird

• Dr Bird works in Anatomy
• Her email is [email protected]
• Her research interests include Anatomical Science Education, First Year Experience, Student Success, and Educational Technology

Lecture 3: Plasma Membrane & Organelles

• Describe the structure of the plasma membrane and its importance to cell function
• Outline the role of membrane proteins
• Explain the mechanisms by which substances cross the plasma membrane, including passive transport, active transport, and co-transport
• Explain the importance of organelles and sub-cellular compartments
• Be able to identify some of the key organelles in eukaryotic cells

Tree of Life Focus

• The tree of life focus of this lecture is on Eukarya Animal Cells

Cell Pop Quiz

• This quiz includes questions to test the students knowledge
• Asks to estimate the number of human cells in the body
• Asks about the duration spent spent as a single cell
• Asks about the largest single human cells in diameter and length

Cell functions

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

Cell Separation

• Cells need to be separated from the outside world
• The inside of the cell is alive, while the outside is not
• The plasma membrane at the boundary of each cell provides special conditions and acts as a semi-permeable barrier

Plasma Membrane

• Cells are bounded by a plasma membrane that is a semi-permeable barrier
• It allows passage of oxygen, nutrients, and waste
• The membrane controls movement of substances in and out of the cell
• Interaction with the environment limits the maximum size of a cell
• A small cell has a greater surface to volume ratio than a larger cell

Phospholipid Bilayer

• Plasma membrane is a double layer of phospholipids with various embedded or attached proteins

Plasma Membrane Dynamics

• The plasma membrane isn't static
• Saturation affects fluidity
• Saturated lipids pack tightly together, resulting in less fluidity
• Unsaturated lipids have tails that prevent tight packing, allowing for more fluidity
• High temperatures increase fluidity, while low temperatures decrease fluidity
• Cholesterol stabilizes membrane fluidity

Plasma Membrane Proteins

• Proteins determine the function of the membrane
• 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 Involvement

• Signal Transduction: relay messages from the body (or environment) into the cell for processes like growth, division, movement, and cell death
• Cell Recognition: often involves glycoproteins (proteins with added sugars)
• Intercellular Joining: some proteins form long-lasting connections between cells
• Linking Cytoskeleton & Extracellular Matrix: allows a cell to physically connect with protein structures outside the cell
• Membrane Transport: allows small amounts of molecules to move across the membrane, can be passive or active

Substance Movement Across Membranes

• Movement depends on what the molecules are, how big the substances are and how much is moving at once
• Bulk transport is for large substances/large volumes
• Membrane transport is for small molecules in small volumes

Molecules Mechanisms

• Different molecules require different mechanisms
• No energy is needed for passive transport, and molecules move down the concentration gradient
• Energy is needed for active transport, and molecules move against the concentration gradient

Passive Transport

• Diffusion occurs in membranes that are permeable to lipid-soluble (hydrophobic) molecules such as steroid hormones and gasses
• These 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

Facilitated Diffusion

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

Osmosis

• This occurs when movement of water across a cell membrane requires channels called aquaporins
• Water moves 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

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

Co-Transport

• This is indirect active transport
• One substance is pumped across the membrane
• Its concentration gradient is used to power the movement of a second substance against its concentration gradient

Organelles

• Different processes require different conditions in separate compartments
• 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

• They are bounded by membranes, with each organelle providing its own special conditions
• Endoplasmic reticulum makes and packs proteins, releasing vesicles that carry proteins elsewhere
• Includes lysosomes
• Mitochondria have two membranes
• The nucleus (the nuclear envelope) has two membranes
• All cellular membranes are composed of a phospholipid bilayer

Eukaryotic Cells Key Organelles

• Common organelles in both animal and plant cells are in blue
• Unique organelles in each individual cell type are in red

Lecture 3 Summary

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