Cell Structure and Diversity

• This lecture focuses on cell structure and diversity, specifically the plasma membrane and organelles
• Dr. Rebecca Bird from the Department of Anatomy presents this lecture

Lecture 3 Objectives

• 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
• Outline the importance of organelles and subcellular compartments
• Identify the key organelles in eukaryotic cells

Cell Pop Quiz

• Consider how many cells you think are in the human body
• Consider how long you spent as one cell
• What are the largest human cells in terms of diameter and length?

Cell Function

• Cells must manufacture cellular materials
• Cells need to obtain raw building block materials from either inside or outside the cell
• Waste materials must be removed from the cell
• Cells must generate the required energy to function
• Cells control all of the above functions

Plasma Membrane

• Cells need to be separated from the outside world
• The inside of cells are alive, while the outside is not
• The plasma membrane is at the boundary of each cell
• The membrane provides special conditions within the cell
• The membrane acts as a semi-permeable barrier
• The plasma membrane is a semi-permeable barrier that bounds cells
• It allows the passage of oxygen, nutrients, and waste
• It controls the movement of substances in and out of the cell
• Interaction with the environment limits the maximum size of a cell
• Smaller cells have a greater surface area to volume ratio than larger cells

Phospholipid Bilayer

• The plasma membrane is a double layer of phospholipids
• It has various embedded or attached proteins
• The outside of the cell has hydrophilic phosphate groups, and so does the inside of the cell
• The inside of the membrane is composed of fatty acids, whose composition affects membrane fluidity

Membrane Dynamics

• The plasma membrane isn't static, its structure is dependent on saturation, temperature, and cholesterol
• Saturated lipids pack tightly together, reducing fluidity, whereas unsaturated lipids tails prevent tight packing, which increases fluidity
• High temperatures increase fluidity, while low temperatures decrease it
• Cholesterol stabilizes membrane fluidity, preventing it from responding too much to extremes

Membrane Proteins

• They are essential for cells to function
• Proteins determine the function of the membrane
• Thousands of membrane proteins have been identified and classified
• Proteins are often specific to a given cell type
• Each cell can have many different proteins
• Proteins can have multiple functions

Signal Transduction

• Membrane proteins are involved in this process
• Signal transduction relays messages from the body or environment into the cell for processes like cell growth, division, movement, etc.

Cell Recognition

• Membrane proteins are involved in this process
• This often involves glycoproteins

Intercellular Joining

• Membrane proteins are involved in this process
• Some proteins form long-lasting connections between cells

Linking Cytoskeleton and Extracellular Matrix

• Membrane proteins are involved in this process
• Membrane proteins allow a cell to physically connect with protein structures outside the cell via the extracellular matrix

Membrane Transport

• Membrane proteins are involved in this process
• Allows small amounts of molecules to move across the membrane
• Transport can be either passive or active

Movement Across Membranes

• How substances move across membranes depends on what the molecules are, how big they are, and how much is moving at once
• Bulk transport is for large substances or large volumes
• Small molecules in small volumes use membrane transport

Transport Types

• Different molecules require different mechanisms
• No energy needed in passive transport, energy is needed for active transport
• Molecules move down the concentration gradient with passive transport
• Molecules move against the concentration gradient with active transport
• There is diffusion, facilitated diffusion, active transport and co-transport

Passive Transport

• In passive transport, no energy is required
• Membranes are permeable to lipid-soluble and hydrophobic molecules like steroid hormones and gasses
• The molecules move down the concentration gradient and do not require energy
• The membrane restricts movement of water-soluble and charged molecules, like glucose, ions and water

Facilitated Diffusion

• This requires membrane proteins known as channels and carriers
• Channels and carriers aid 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

• Osmosis is the movement of water across a cell membrane
• The process 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

• It requires transport proteins which are carriers that use ATP energy
• Moves specific substances against their concentration gradient
• Allows a cell to have an internal concentration of a substance that is different from its surroundings, even if higher inside than outside
• An example of Active Transport is the sodium-potassium pump

Co-Transport

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

Organelles

• Different processes occur in different conditions, so there are separate compartments
• The existence of organelles allows for creation of different compartments
• These compartments can be used to provide special conditions for processes, keep incompatible processes apart, allow concentration, form concentration gradients, and package substances for transport

Organelle Features

• Cellular organelles are bounded by membranes
• Each organelle provides its own special conditions
• The endoplasmic reticulum is responsible for making and packing proteins, and releasing vesicles that carry the proteins elsewhere
• Lysosomes are organelles
• Mitochondria have two membranes
• The nucleus, bound by the nuclear envelope, has two membranes
• All cellular membranes are composed of a phospholipid bilayer

Eukaryotic Cells

• Organelles can be either unique depending on the cell type, or common to all cell types
• Animal and plant cells share some common organelles, these include the nucleus, the golgi apparatus, cell membranes, and endoplasmic reticulum.

Lecture 3 Summary

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