Date: Lecture 3 | Source: BIO 203 Lecture 3 Objectives, Ch. 3 Tags: cell differentiation, cell structure, plasma membrane, phospholipid bilayer, organelles, cell components, lipid rafts, fluid mosaic model
Difficulty: Introductory to Intermediate
Prerequisites: Basic chemistry (atoms, molecules, bonds). Familiarity with the four major macromolecules (proteins, lipids, carbohydrates, nucleic acids) is helpful but not essential.
This lecture introduces the cell as the fundamental unit of life, covering how cells differ from one another in shape, size, and function, and then walking through every major structural component. You will need to know the parts of the plasma membrane at a molecular level, the layout of the cytoplasm and its organelles, and the structure of the nucleus. If you have missed earlier material on macromolecules, revisit that first, because membrane and organelle structure relies on understanding lipids and proteins. This content underpins every later topic in the course, from tissues to organ systems.
Cells vary in shape and function (muscle cells contract, neurons transmit signals, osteoblasts build bone). Every cell shares a common toolkit of components: a phospholipid-bilayer plasma membrane studded with proteins and cholesterol, a cytoplasm packed with organelles, a nucleus housing DNA, and junctions that connect cells to their neighbours. Understanding what each part does and where it sits is the foundation for everything else in anatomy and physiology.
Plasma membrane (cell membrane)
The outer boundary of the cell, composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It controls what enters and leaves the cell.
In simple terms, this is the cell's gatekeeper: a flexible, oily envelope that decides what gets in and what stays out.
Phospholipid bilayer
Two layers of phospholipid molecules arranged with their hydrophilic (water-loving) polar head groups facing outward and their hydrophobic (water-fearing) fatty acid tails facing inward.
Think of it as two rows of lollipops arranged tail-to-tail, with the sticks (tails) hidden in the middle and the candy (heads) facing the watery environment on each side.
Integral proteins (transmembrane proteins)
Proteins that span the entire thickness of the plasma membrane. They serve as channels, carriers, receptors, and enzymes.
Think of it as a bridge built through the membrane wall, connecting the outside world to the inside of the cell.
Peripheral proteins
Proteins attached to the inner or outer surface of the membrane but not embedded through it. They often function in cell signalling and maintaining the cell's shape.
In simple terms, these sit on the membrane surface like sticky notes rather than being built into the wall.
Lipid rafts
Specialised patches within the plasma membrane that are enriched in cholesterol and sphingolipids. They are more rigid than the surrounding membrane and serve as platforms for signalling molecules and membrane trafficking.
Think of it as a thicker, stiffer raft floating in the otherwise fluid sea of the membrane.
Cholesterol
A lipid molecule wedged between phospholipids in the membrane. It stabilises membrane fluidity: preventing the membrane from becoming too rigid at low temperatures or too fluid at high temperatures.
In simple terms, cholesterol is the membrane's thermostat, keeping it at just the right consistency.
Glycocalyx (cell coat)
A carbohydrate-rich layer on the outer surface of the plasma membrane, formed by sugar chains attached to membrane lipids (glycolipids) and proteins (glycoproteins). It plays roles in cell recognition, protection, and adhesion.
Think of it as the cell's fingerprint: a sugar coating that helps neighbouring cells and immune cells identify it.
Cytoplasm
Everything between the plasma membrane and the nuclear envelope, including the cytosol (gel-like fluid), organelles, and inclusions.
In simple terms, the cytoplasm is the cell's interior workspace, where most of the cell's activities take place.
Nucleus
The membrane-bound organelle that houses the cell's DNA (chromatin/chromosomes). Bounded by a double-membrane nuclear envelope perforated with nuclear pores.
Think of it as the cell's control centre, holding the master blueprint (DNA) and issuing instructions for protein synthesis.
Nucleolus
A dense, non-membrane-bound structure within the nucleus where ribosomal RNA (rRNA) is synthesised and ribosomal subunits are assembled.
In simple terms, the nucleolus is the ribosome factory inside the nucleus.
Endoplasmic reticulum (ER)
A network of membrane-enclosed channels and sacs extending from the nuclear envelope. Rough ER (studded with ribosomes) synthesises proteins for export; smooth ER (no ribosomes) synthesises lipids and detoxifies drugs.
Think of it as the cell's highway system: rough ER is the manufacturing lane (making proteins), and smooth ER handles lipid production and cleanup.
Golgi apparatus (Golgi complex)
A stack of flattened membrane sacs (cisternae) that modifies, sorts, and packages proteins and lipids received from the ER for transport to their final destinations.
In simple terms, this is the cell's post office: it processes, labels, and ships molecular packages.
Mitochondria (singular: mitochondrion)
Double-membrane organelles responsible for producing ATP through aerobic cellular respiration. They have their own DNA and are bounded by an outer membrane, an inner membrane folded into cristae, and an internal matrix.
Think of it as the cell's power station, converting nutrients into usable energy (ATP).
Microvilli
Tiny, finger-like projections of the plasma membrane that increase the cell's surface area for absorption. Found on cells lining the small intestine, for example. Supported internally by actin (microfilaments).
In simple terms, microvilli are like the bristles on a brush, vastly increasing the area available for absorbing nutrients.
Cells vary in shape: some are elongated, some spherical, some flattened. Certain cells (e.g. white blood cells) change shape over time.
Cells vary in function:
Muscle cells contract.
Nerve cells (neurons) transmit and process information.
Osteoblasts manufacture bone.
White blood cells combat invading bacteria.
Cells vary in behaviour: some are stationary; others (e.g. certain white blood cells) move around the body.
Cell division varies: some cells do not routinely divide (e.g. nerve cells in most parts of the brain), while others divide regularly (e.g. cells in the stratum germinativum of the skin), providing new cells for growth, development, and tissue repair.
1. Plasma membrane
Composed of lipids and glycolipids (including phospholipids and cholesterol), proteins and glycoproteins (integral and peripheral), and carbohydrates.
Can have microvilli on its surface.
2. Cytoplasm
Contains the following organelles and structures:
Endoplasmic reticulum (rough and smooth)
Ribosomes
Golgi apparatus
Mitochondria
Lysosomes
Peroxisomes
Centrosomes
Cilia and flagella
Vesicles
Microfilaments
Microtubules
Inclusions (e.g. glycogen granules, lipid droplets)
3. Nucleus
Nuclear envelope (double membrane)
Nucleolus
Chromatin (DNA and protein)
Nuclear matrix
4. Intercellular junctions
Tight junctions
Desmosomes
Gap junctions
Adherens junctions
The phospholipid bilayer foundation
The membrane begins as a phospholipid bilayer: two layers of phospholipids with polar head groups (hydrophilic) on the outside and fatty acid tails (hydrophobic) on the inside.
Under the electron microscope, this appears as two dark lines (the head groups) with a lighter region between (the tails).
Additional components embedded in the bilayer
Cholesterol molecules are wedged between the phospholipids, modulating membrane fluidity.
Integral (transmembrane) proteins span the membrane and serve as channels, carriers, receptors, or enzymes.
Peripheral proteins attach to the inner or outer surface without spanning the membrane.
Carbohydrates (as glycolipids and glycoproteins) extend from the outer surface, forming the glycocalyx.
Lipid rafts
Specialised, more rigid regions of the membrane enriched in cholesterol and sphingolipids.
They organise signalling molecules and facilitate membrane trafficking.
They float within the more fluid surrounding membrane, concentrating certain proteins for efficient signal transduction.
The plasma membrane's selective permeability is the basis for how drugs enter cells. Lipid-soluble drugs cross the membrane easily; water-soluble drugs often need protein channels or carriers. This is why pharmaceutical design pays close attention to a drug's lipophilicity.
Lipid rafts are implicated in viral entry. Several viruses, including influenza, hijack lipid raft domains to enter host cells, making rafts a target for antiviral research.
Students often think the plasma membrane is a rigid wall. It is not. The membrane is fluid and flexible, with components moving laterally within the bilayer (the fluid mosaic model).
Students frequently confuse integral and peripheral proteins. Integral proteins span the membrane; peripheral proteins sit on the surface. If the protein passes all the way through, it is integral.
Students sometimes assume all cells look the same. In reality, cell shape is tightly linked to function: a flat epithelial cell, a branching neuron, and a round red blood cell look nothing alike, and their shapes reflect their jobs.
Students often forget that cholesterol is a normal, essential component of the membrane, not just something found in blood tests. Without it, the membrane would not function properly at varying temperatures.
Be able to draw and label a cell with the plasma membrane, nucleus, nucleolus, rough ER, smooth ER, Golgi apparatus, and mitochondria. This is a classic exam question.
Know the difference between integral and peripheral proteins and give an example function of each.
Understand what lipid rafts are and why they matter for cell signalling.
Be ready to explain how cell shape relates to cell function, with specific examples (muscle, neuron, osteoblast, white blood cell).
Know that some cells divide regularly (stratum germinativum) and others rarely or never (most neurons). This distinction comes up in tissue repair and cancer biology.
True or False: The plasma membrane is a rigid structure that does not allow lateral movement of its components.
False. The membrane is fluid; phospholipids and proteins move laterally (fluid mosaic model).
Fill in the blank: Proteins that span the entire thickness of the plasma membrane are called ______ proteins.
Integral (or transmembrane) proteins.
True or False: Cholesterol makes the plasma membrane more rigid at all temperatures.
False. Cholesterol buffers fluidity: it prevents excess fluidity at high temperatures and excess rigidity at low temperatures.
Fill in the blank: The carbohydrate-rich layer on the outer surface of the plasma membrane is called the ______.
Glycocalyx.
True or False: Nerve cells in most parts of the brain routinely divide to replace themselves.
False. Most brain neurons do not routinely divide.
Q: Name three ways in which cells differ from each other, and give a specific example of each.
A: Cells differ in shape (e.g. elongated muscle cells vs spherical white blood cells), function (e.g. osteoblasts build bone, neurons transmit signals), and behaviour regarding division (e.g. cells of the stratum germinativum divide regularly, while most neurons do not).
Q: List the four major categories of cell components.
A: Plasma membrane, cytoplasm (with organelles), nucleus, and intercellular junctions.
Q: Describe the basic structure of the phospholipid bilayer. Why do the phospholipids arrange themselves this way in an aqueous environment?
A: Two layers of phospholipids with hydrophilic polar head groups facing the aqueous environment (extracellular fluid and cytosol) and hydrophobic fatty acid tails facing inward, away from water. This arrangement is energetically favourable because the hydrophobic tails are shielded from water.
Q: Compare and contrast integral and peripheral membrane proteins.
A: Integral proteins span the entire membrane (transmembrane) and function as channels, carriers, receptors, or enzymes. Peripheral proteins attach to the membrane surface (inner or outer) without spanning it and are involved in signalling and structural support.
Q: What are lipid rafts, and why are they functionally important?
A: Lipid rafts are cholesterol- and sphingolipid-enriched regions of the plasma membrane that are more rigid than the surrounding bilayer. They serve as platforms that concentrate signalling molecules and facilitate efficient signal transduction and membrane trafficking.
Q: Draw and label a generalised cell, including the plasma membrane, nucleus, nucleolus, rough ER, smooth ER, Golgi apparatus, and mitochondria.
A: Your drawing should show: the plasma membrane as the outer boundary; the nucleus (with nuclear envelope and pores) containing the nucleolus and chromatin; rough ER (with ribosomes on the surface) near the nucleus; smooth ER (no ribosomes) nearby; the Golgi apparatus as a stack of flattened sacs; and mitochondria with an outer membrane, inner membrane folded into cristae, and a matrix.
The plasma membrane's structure connects directly to membrane transport (Ch. 3 continued), where you will learn how substances cross the bilayer via diffusion, osmosis, and active transport. Without understanding the membrane's architecture, transport mechanisms will not make sense.
Cell differentiation ties into histology (the study of tissues). Each tissue type (epithelial, connective, muscle, nervous) is defined by its cells' shapes, functions, and arrangements.
The organelles covered here reappear throughout the course: mitochondria in cellular respiration, the ER and Golgi in protein secretion, and the nucleus in cell division (mitosis and meiosis).
Cell structure, cell organelles, plasma membrane, cell membrane, phospholipid bilayer, fluid mosaic model, integral proteins, transmembrane proteins, peripheral proteins, lipid rafts, cholesterol in membranes, glycocalyx, cell coat, glycoproteins, glycolipids, cytoplasm, cytosol, nucleus, nucleolus, rough ER, smooth ER, endoplasmic reticulum, Golgi apparatus, Golgi complex, mitochondria, powerhouse of the cell, cell differentiation, cell shape and function, osteoblasts, neurons, stratum germinativum, BIO 203, anatomy and physiology, cell biology basics