Source: Cell Organelle Chart Key (Michigan State University)
Tags: cell membrane, plasma membrane, phospholipid bilayer, cell wall, cellulose, nucleus, nuclear envelope, nuclear pores, chromatin, chromosome, nucleolus, eukaryotic cell, prokaryotic cell
Difficulty: Introductory | Prerequisites: None, though a basic understanding of prokaryotic vs. eukaryotic cells is helpful.
Every cell, whether it belongs to a bacterium or a blue whale, needs to separate its internal chemistry from the outside world and store the instructions that keep the whole operation running. This set of notes covers the structures responsible for both: the cell membrane and cell wall that form the boundary, and the nucleus with its internal components that house and organise DNA. These are the first organelles most biology courses cover because nothing else in the cell makes sense until you understand where the instructions live and how the cell controls what crosses its borders. If you are coming in cold, start here.
Cells are wrapped by a selectively permeable membrane (and, in plants and prokaryotes, a rigid cell wall outside that). Inside eukaryotic cells, the nucleus acts as the control centre, holding DNA in the form of chromatin that condenses into chromosomes during division. Surrounding the nucleus is a double-membrane envelope studded with pores that regulate traffic in and out.
Cell membrane (plasma membrane)
A flexible boundary made of a phospholipid bilayer with embedded proteins that surrounds every cell. It is selectively permeable, meaning it controls which substances enter and leave.
In simple terms, think of it as a security gate that decides what gets in and what stays out.
Cell wall
A rigid structure found outside the plasma membrane in plant cells and prokaryotes. In plants it is made of cellulose; in bacteria, peptidoglycan.
Think of it as the sturdy outer casing that gives the cell its fixed shape and extra protection.
Nucleus
A membrane-bound organelle found in eukaryotic cells (animal and plant) that contains the cell's DNA. It directs cell activities by controlling which proteins are made.
In simple terms, it is the control room of the cell.
Nuclear membrane / nuclear envelope
A double-membrane layer that surrounds the nucleus, separating nuclear contents from the cytoplasm. Continuous with the endoplasmic reticulum.
Think of it as the wall around the control room, with doors (pores) built in.
Nuclear pores
Thousands of small openings in the nuclear envelope that regulate the movement of molecules (especially RNA and proteins) between the nucleus and cytoplasm.
In simple terms, these are the doorways that let messengers carry instructions out of the nucleus.
Chromatin
Granular material inside the nucleus consisting of DNA wound around histone proteins. It is the loosely organised form of genetic material during normal cell activity.
Think of it as DNA in its "working" state, spread out so genes can be read.
Chromosome
A distinct, thread-like structure formed when chromatin condenses tightly during cell division. Each chromosome carries a portion of the organism's genetic information.
In simple terms, chromosomes are the tightly packed version of chromatin, visible under a microscope only when a cell is dividing.
Nucleolus
A small, dense region within the nucleus where ribosomal RNA (rRNA) is produced and ribosome subunits are assembled.
Think of it as the ribosome factory inside the control room.
The cell membrane is found in all cell types: prokaryotic, animal, and plant.
Structure: phospholipid bilayer with embedded proteins and lipids.
Function: selectively permeable barrier. Controls what enters and exits the cell.
It is flexible, not rigid, which allows animal cells to change shape.
The cell wall is found in prokaryotes and plant cells, but not in animal cells.
Sits outside the plasma membrane.
Made of cellulose (in plants) or peptidoglycan (in bacteria).
Provides structural support, protection, and gives the cell a fixed shape.
Because it is rigid, plant cells do not burst when they take in water (unlike animal cells).
Found only in eukaryotic cells (animal and plant), not in prokaryotes.
Contains the cell's DNA, which carries the instructions for making proteins.
The nucleus directs all cell activities by regulating gene expression.
The nuclear envelope is a double membrane surrounding the nucleus.
Separates nuclear contents from the cytoplasm.
Continuous with the rough endoplasmic reticulum.
Nuclear pores (thousands per envelope) allow selective transport of molecules.
mRNA passes out through pores to reach ribosomes for translation.
Proteins needed inside the nucleus pass in through pores.
Chromatin is the "relaxed" form of DNA inside the nucleus during interphase.
Made of DNA wrapped around histone proteins.
Condenses into visible chromosomes only when the cell is about to divide.
Chromosomes are the condensed, thread-like structures visible during mitosis and meiosis.
Carry genetic information passed to the next generation.
Function as the cell's blueprint.
The nucleolus is a dense sub-region within the nucleus.
Site of ribosomal RNA synthesis and ribosome subunit assembly.
Not membrane-bound; it is simply a concentrated area of activity.
Antibiotics like penicillin work by targeting the bacterial cell wall. Because animal cells lack a cell wall, the drug harms bacteria without damaging your own cells. This is why understanding the difference between cell membranes and cell walls matters in pharmacology.
The selective permeability of the cell membrane is the principle behind dialysis machines, which filter waste from blood using a semi-permeable membrane that mimics what your kidney cells do naturally.
Students often think the cell wall replaces the cell membrane in plant cells. It does not. Plant cells have both: the membrane sits inside the wall.
Students often confuse chromatin and chromosomes as two different things. They are the same DNA, just in different states of packaging. Chromatin is loose; chromosomes are condensed.
Students sometimes believe the nucleus "does everything" in the cell. The nucleus stores instructions and regulates gene expression, but the actual work (protein synthesis, energy production) happens outside it.
Students often think nuclear pores are wide open holes. They are gated channels with selective transport, not passive gaps.
⚠️ Know which organelles are found in prokaryotes vs. eukaryotes. The cell membrane and ribosomes appear in both; the nucleus, nuclear envelope, and nucleolus are eukaryotic only.
⚠️ Be able to distinguish between chromatin and chromosomes and state when each form is present.
⚠️ Understand selective permeability: the membrane does not let everything through. Expect a question asking you to explain what "selectively permeable" means.
⚠️ Know the function of nuclear pores: they regulate the passage of RNA and proteins between nucleus and cytoplasm.
True or False: Animal cells have a cell wall made of cellulose. (False: only plant cells have a cellulose cell wall; animal cells have no cell wall.)
Fill in the blank: The nucleus is surrounded by a double membrane called the ______. (nuclear envelope)
True or False: Chromatin and chromosomes are made of different material. (False: both are DNA + histone protein, just packaged differently.)
Fill in the blank: The ______ is the site within the nucleus where ribosome subunits are assembled. (nucleolus)
True or False: Prokaryotic cells contain a nucleus. (False: prokaryotes lack a membrane-bound nucleus.)
Q: What is the primary structural difference between a cell membrane and a cell wall?
A: The cell membrane is a flexible phospholipid bilayer, while the cell wall is a rigid structure (made of cellulose in plants) located outside the membrane.
Q: Name two functions of nuclear pores.
A: Nuclear pores allow mRNA to exit the nucleus for protein synthesis and allow proteins (such as enzymes and transcription factors) to enter the nucleus from the cytoplasm.
Q: Explain the relationship between chromatin and chromosomes.
A: Chromatin is the loosely organised form of DNA bound to histone proteins, present during interphase. When a cell prepares to divide, chromatin condenses and coils tightly into distinct, visible structures called chromosomes.
Q: Which cell types have a cell membrane? Which have a cell wall?
A: All cells (prokaryotic, animal, plant) have a cell membrane. Only prokaryotic and plant cells have a cell wall. Animal cells do not.
Q: What is the function of the nucleolus?
A: The nucleolus is the site where ribosomal RNA is synthesised and ribosome subunits are assembled before being exported to the cytoplasm.
The cell membrane connects directly to the study of transport mechanisms (osmosis, diffusion, active transport), which most courses cover next. Understanding selective permeability here is essential for those topics.
The nucleus and DNA storage connect to genetics and cell division (mitosis and meiosis). Knowing how chromatin condenses into chromosomes is foundational for understanding how genetic information is passed to daughter cells.
The nucleolus links forward to protein synthesis: ribosome assembly starts here, and ribosomes are the machinery that translates mRNA into proteins in the cytoplasm.
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