L: Intro to cells and organelles

16.09.25

What do subcellular organelle enable cells to do?

Compartmentalise biochemical processes and maintain different conditions (for example pH levels.

What are some of the properties of cells?

• they act as independent units

• they replicate (originate from existing cells)

• they have a finite lifespan - die due to design, age or disease

• they can change, adapt and respond

What is the plasma membrane?

A semipermeable bilayer of phospholipids with integrated proteins that regulates the exchange of molecules between the intracellular fluid and the extracellular fluid.

Are all organelles bound by a membrane?

No, some structures do not have a membrane, such as ribosomes.

In conditions such as diabetes mellitus, what accumulates in the cytoplasm?

Excess fat.

What does it mean for a cell to be polarised?

Different structures perform different functions at different ends of the cell.

What do plants and bacteria have in addition to the plasma membrane?

The cell wall.

What is a cell wall made up of?

Peptidoglycan.

What is the function of the cell membrane?

• compartmentalisation 

• maintaining concentration gradients and transmembrane potential (difference in electrical potential.

• help to maintain cell structure and provide scaffolding for some proteins

• involved in signal transmission

What does the plasma membrane contain?

Phospholipids, sphingolipids, cholesterol and embedded proteins.

What types of phospholipids does the outer leaflet of the plasma membrane contain? (2)

Phosphatidylchloline and sphingomyelin.

What types of phospholipids does the inner leaflet contain?

phosphatidylethanolamine, phosphatidylserine and phospophatidylinositol.

Is the membrane always tightly packed?

No, there can be ordered structure (tightly packed) and disordered structure (more loose - caused by unsaturated fatty acids in the membrane.)

What molecules can pass through the bilayer without facilitating proteins?

lipid soluble hydrophobic molecules and small molecules such as oxygen and carbon dioxide. Also water.

How does water cross the cell membrane?

Through a process called solubility-diffusion or through specific pores called aquaporins.

How do hydrophilic molecules cross the membrane?

Through a variety of transport systems embedded in the membrane.

What does amphipathic mean?

having both hydrophilic and hydrophobic parts.

What molecule reduces the fluidity of the membrane and what group does it have?

Cholesterol and it has a polar hydroxyl group.

Where can glycolipids and glycoproteins be found in the plasma membrane?

The outer leaflet.

Is the ratio between proteins and lipids the same in every cell type?

No.

Where are transmembrane proteins synthesised?

The endoplasmic reticulum.

What are the two types of membrane transport proteins?

Channel proteins and carrier proteins.

What is the function of channel proteins?

Allows the passage of appropriately sized and charged molecules through the membrane by passive transport. (along a concentration or electrochemical gradient).

What are carrier proteins?

Integral membrane proteins that transport molecules across the membrane by selectively binding to molecules and changing conformation to move the molecule across the membrane. This is active transport powered by ATP hydrolysis.

What are some other types of membrane protein?

• receptor proteins

• enzymes

• linking proteins

• cell identity markers

Why are receptors important in disease?

Antibodies and antigens (SARS-CoV-2 and ACE-2 receptor).

Name some enzymes that drive biochemical reactions at cell surface level.

Beta-secretase and gamma-secretase enzymes.

What does the cytoskeleton consist of?

Filaments and tubules made from proteins.

What are microfilaments made of?

Double stands of actin.

What is the function of filaments?

Structural support and form the cores of membrane projections like microvilli (immobile).

What are microtubules made up of?

Two globular proteins polymers (alpha and beta tubulin).

What is the function of microtubules?

They have a role in intracellular movement of organelles and vesicles and also form the spindle fibres during mitosis. They form mobile membrane projections like cilia and flagella.

Where do microtubules extend from?

The centrosome, located near the nucleus.

What are cilia and flagella moved by?

Dyneins.

What is the only human cell to have flagella?

Sperm cells.

What is the nuclear envelope?

A double layer of membrane.

Where are ribosomes assembled?

Inside the nucleus at nucleoli.

What is the difference between constant and regulated secretion?

Constant secretion is direct and fast, whereas in regulated secretion, the secretory vesicles may be stored before being secreted in response to a stimulus.

Where is most of the cellular ATP generated?

At the mitochondria.

What is the name of the process by which mitochondria create ATP?

Oxidative phosphorylation.

Do mitochondria have a single or double layer membrane?

A double layer membrane.

What do lysosomes contain?

Acid hydrolyses.

Do lipids have a single or double layer membrane?

Single.

What is the function of lysosomes?

Degradation of biological molecules.

What is the approximate pH of the inside of a lysosome?

5.

What happens if many lysosomes are ruptured?

The cell self digests, known as autolysis.

Is autolysis physiological or pathological?

Physiological.

What are proteasomes?

Multi-subunit enzyme complexes involved in the degradation of cytosolic proteins.

What are proteins destined for destruction by proteasomes tagged with?

Copies of a protein called ubiquitin.

How are proteasomes arranged?

The protein complexes are arranged in 4 rings around a central core.

What is one of the functions of peroxisomes?

Reactions producing hydrogen peroxide.

What do peroxisomes contain and what is the function of them?

Oxidases which oxidise substrates (such as some acids). Other oxidases are involved in the biosynthesis of cholesterol.

What are prokaryotes?

Single-celled organisms that lack a nucleus and membrane bound organelle.

What are the three shapes of prokaryotes?

Spherical, rod-shaped or spiral.

Where is the genetic material of a prokaryote found?

In a region of the cell called the nucleoid and in the form of a single circular chromosome of DNA.

What are eukaryotes?

Cells that contain a membrane bound nucleus and complex internal structures.

Where is the genetic material found in eukaryotes?

In the nucleus, in the form of multiple chromosomes. (linear DNA)

Which evolved more recently eukaryotes or prokaryotes?

Eukaryotes.

What does protein synthesis enable?

Growth and differentiation.

What are the steps to proteins being made?

DNA is transcribed to form mRNA, the mRNA passes from the nucleoplasm through the nuclear envelope through holes called nuclear pores, the mRNA is translated at ribosomes and proteins are formed from an amino acid chain.

What are nuclear pores?

Selective aqueous channels that allow transport between the nucleoplasm and the cytosol.

What is chromatin made up of?

DNA and histones.

What are the two types of chromatin?

Euchromatin and heterochromatin (more dense).

Where are most of the active genes found and where are most of the inactive genes found?

Euchromatin has most of the active genes, while heterochromatin has most of the inactive ones.

Where does all protein synthesis begin?

The cytosol.

What are the functions of the RER?

• Site of membrane synthesis (membrane proteins)

• modifies proteins

• monitors correct folding of proteins

• signals stress (e.g. when secretion is blocked or poorly folded proteins)

What are the functions of the SER?

• lipid synthesis (membrane lipids)

• steroid synthesis

• detoxification

No protein synthesis occurs at the SER

What are three key locations of the secretory pathway?

The rough endoplasmic reticulum, the Golgi apparatus and the plasma membrane.

What happens to the vesicles containing secretory proteins?

• Some vesicles go to the PM (constant secretion = constitutive vesicles)

• Some vesicles leave the golgi containing packaged (stored) secretory vesicles

Is there a signal required for constitutive secretion?

No.

Is there a signal required for regulated secretion?

Yes.

How are proteins destined for lysosomes modified?

Mannose phosphorylation.

How does proinsulin turn into insulin?

The connecting polypeptide is removed.

What turns into a lysosome?

A late endosome.

A defective … causes cystic fibrosis?

The cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel protein.