Cell: MRCEM Primary questions with answers

Physiology carries 60 of the 180 questions in MRCEM Primary, and the Cell topic covers its basic cellular component. Lessons include cell organisation, homeostasis, membrane transport, osmolality and the osmolal gap, vessel fluid dynamics, nerve and muscle function, the action potential, the neuromuscular junction, and tetanus and rigor mortis. These ideas explain fluid shifts, electrolyte disturbances and neuromuscular blockade, all met during resuscitation.

5 free questions below, each with the answer and the full explanation · 136 in the full bank · Last updated after the 27 October 2026 MRCEM Primary sitting.

1. Question 1 · Organisation of Cell

   Which one of the following organelles has an essential role in achieving uterine involution?

   1. ALysosome
   2. BMitochondria
   3. CSmooth endoplasmic reticulum
   4. DGolgi body
   5. ERough endoplasmic reticulum
   Show answer and explanation

   Answer: A. Lysosomal enzymes break down uterine tissue during involution.

   • B: Mitochondria produce ATP.
   • C: Smooth ER synthesises lipids and detoxifies; not involved in involution.
   • D: Golgi body packages and modifies proteins.
   • E: Rough ER synthesises proteins.

   Full explanation: Organisation of Cell

   Organization of cell

   A typical cell is made up of four basic structures, which from outside to inside are: 

   • Cell membrane 
   • Cytoplasm 
   • Nuclear membrane and nucleus 

    Cell membrane: 

   Outer covering of cell that isolate individual cell from its Neighbours, composed of phospholipid bilayer with embedded proteins which provide signaling. 

   • The membrane is selectively permeable; lipid-soluble molecules such as O2 and CO2, and small molecules such as water and urea can readily pass through the lipid bilayer, but larger molecules such as glucose, and polar molecules such as ions cannot, and their transport is mediated by transporter and ion channel membrane proteins. 
   • Cell membrane consists of thin layer called glycocalyx which attaches one cell to the adjacent cells.  

   Cell organelles quick revision: 

   • 1: Lysosomes contains hydrolases responsible for digestion and breakdown of defective proteins 
   • Uterus and breast after pregnancy regress by lysosomes  
   • 2: Mitochondria: Powerhouse responsible for production of chemical energy in the form of ATP 
   • Unique feature in mitochondria is of own DNA.  
   • 3: Endoplasmic reticulum:  
   • Rough endoplasmic reticulum most likely synthesizes proteins. 
   • Detoxification of drug occurs in smooth endoplasmic reticulum. 
   • The smooth endoplasmic reticulum serves as a store for intracellular Ca2+ and is the major site of lipid production. 
   • 4: Golgi body: Secretory vesicles found 
   • The Golgi apparatus packages proteins for delivery to specific intracellular destinations or into vesicles which can then be secreted from the cell for extracellular action. 
   • 5: Nucleus contains genetic material and nucleolus responsible for ribosomes production.
   • 6: Ribosomes: Ribosomes are the cellular structures where protein synthesis takes place. They can be found in the cytoplasm (free ribosomes) or attached to the endoplasmic reticulum (bound ribosomes).

2. Question 2 · Cell

   Where does haemopoiesis mainly take place in normal adult life:

   1. ALiver
   2. BSpleen
   3. CBone marrow
   4. DLymph nodes
   5. EThymus
   Show answer and explanation

   Answer: C. In adults, haemopoiesis occurs in the bone marrow.

   • A: Liver is a foetal site.
   • B: Spleen is a foetal site.
   • D: Lymph nodes are not the primary haemopoietic site.
   • E: Thymus matures T cells, not the main blood cell source.

   Full explanation: Haemopoiesis

   Haemopoiesis Sites

   Haemopoiesis in the foetus occurs firstly in the yolk sac, and later in the liver and spleen, which are the major haemopoietic organs from about 6 weeks until 6 - 7 months gestation, at which point the bone marrow becomes the most important site. In normal childhood and adult life, haemopoiesis is restricted to the bone marrow.

   Haemopoietic Stem Cells

   All blood cells develop from haemopoietic stem cells (HSCs). Haemopoiesis occurs when HSCs bind, via cell surface receptors, to adhesion molecules and to fixed or secreted cytokines and growth factors. This binding triggers a cascade of phosphorylation reactions which transmits a signal to the nucleus to upregulate gene transcription. This signal promotes haemopoiesis by activating pathways that lead to cell proliferation, differentiation, maturation, inhibition of apoptosis (programmed cell death) or activation of mature cells.

   HSCs give rise to two major lineages, the lymphoid lineage in which a common lymphoid progenitor gives rise to B-cells, T-cells and natural killer (NK) cells, and a myeloid lineage in which a common myeloid progenitor gives rise to erythrocytes, platelets, granulocytes and monocytes.

   Haemopoiesis. (Image by unknown [CC-BY-SA-3.0 , via Wikimedia Commons)

3. Question 3 · Cell Membrane Transport

   You are discussing possible treatments for cerebral oedema. How does hypertonic saline act (what is its mechanism of action):

   1. ANo net movement of water
   2. BNa+ moves out of the cells
   3. CWater moves out of the cells
   4. DNa+ moves into the cells
   5. EWater moves into the cells
   Show answer and explanation

   Answer: C. Hypertonic saline makes the ECF more concentrated, drawing water out of cells.

   • A: Net water movement occurs out of cells.
   • B: The shift is of water, not Na+ leaving cells.
   • D: Na+ does not move into cells; it stays extracellular.
   • E: Water moves out, not into, the cells.

   Full explanation: Cell Membrane Transport

   Cell membrane transport

   Cell membrane transport: Occurs through large pores, career proteins and ion channels. 

   Diffusion	Facilitated diffusion.
   Passive movement of ions across cell membrane No carrier mediated. Follow Fick’s law of diffusion. Example: 02 and c02 changes in alveoli	Passive movement of ions across cell membrane Carrier mediated (specific) More rapid than simple diffusion Example: Glucose transported to placenta by facilitated diffusion and transport of glucose into rbcs

    

    Fick’s law of diffusion: (Net rate of diffusion) J=- DA ∆C/ X  

   Where:

   •  J = Net rate of diffusion 
   • D = Diffusion coefficient 
   •  A = Area 
   • ∆C = Concentration difference 
   •  X = thickness of the membrane Negative sign: direction of diffusion   

   Factors increasing diffusion: 

   •  Decrease size of solute.  
   •  Decrease membrane thickness. 
   • Increase oil/ water partition co- efficient. 

    

   Active transport 

   Primary Active transport	Secondary Active transport
   Against concentration/Electrochemical gradient Source of energy: ATP Example: The Na+/K+ -ATPase antiporter pump uses metabolic energy to move 3 Na+ ions out of the cell for every 2 K+ ions in, against their respective electrochemical gradients. H-K Pump in stomach	One substance along concentration/Electrochemical gradient other substance against the gradient Source of energy: Concentration gradient produced by primary active transport. Example: Amino acids are absorbed from kidney to blood by secondary active transport Sodium/Calcium exchanger

    

    Filtration, osmosis and tonicity:

   Filtration	Osmosis	Tonicity
   Process by which fluid is forced through membrane mainly because of difference in hydrostatic and oncotic pressure on both sides. Example: Water enters interstitium through filtration	Passive movement of water across a semipermeable membrane from regions of low solute concentration to high concentration. Example: Biological membranes are semipermeable in that they usually allow the free movement of water but restrict the movement of solutes.	Tonicity is a measure of the relative effective osmotic potentials of two solutions separated by a semipermeable membrane.

    

   Note: 

   Hypertonic fluid	Extracellular fluid becomes more concentrated relative to intracellular fluid, osmotic potential draws water out of cells, cells lose water and shrink
   Isotonic fluid	Extracellular and intracellular fluid are isotonic, no osmotic potential generated, no net movement of water
   Hypotonic fluid	Extracellular fluid diluted relative to intracellular fluid, osmotic potential draws water into cells, cells swell and may burst

   •  Example of hypotonic fluid: Pure water we drink is hypotonic to body fluids.
   •  Example of Isotonic fluid: NS 0.9% , Ringer’s lactate
   • Example of hypertonic fluid: 3% Hypertonic saline

    

    

4. Question 4 · Homeostasis

   Concerning homeostasis, which one of these statements is CORRECT:

   1. AThe set point is a narrow range of values within which normal physiological function occurs.
   2. BNegative feedback can lead to uncontrolled amplification of a response.
   3. CAn effector receives input from a detector and determines the need for a response.
   4. DHomeostasis must always be restored using negative feedback mechanisms.
   5. EPositive feedback is typically used to restore disturbed homeostasis.
   Show answer and explanation

   Answer: A. The set point is a narrow range of values within which normal physiological function occurs.

   • B: Uncontrolled amplification is a feature of positive feedback.
   • C: The comparator, not the effector, determines the need for a response.
   • D: Homeostasis may be restored by negative or positive feedback.
   • E: Negative feedback typically restores homeostasis.

   Full explanation: Homeostasis

   Normal functioning of proteins is essential for life. Seemingly small changes in the external environment (particularly in temperature and pH) can irreversibly denature proteins that are essential for normal physiological function. As long as conditions are maintained within the normal physiological range within the internal environment, the cells of the body continue to live and function properly.

   Definition

   Homeostasis is defined as 'the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant'. Homeostasis preserves protein functionality and maintains most physiological systems in the body.

   Set Point

   The 'set point' is a narrow range of values within which normal physiological function occurs. The set point can under certain circumstances be reset to meet physiological requirements e.g. acclimatisation at high altitude.

   Negative Feedback

   The most common type of regulation is by negative feedback e.g. control of body temperature, acid-base balance and blood pressure.

   A negative feedback system comprises three components:

   • A detector (often neural receptor cells) to measure the variable in question and to provide input to the comparator
   • A comparator (usually a neural assembly in the central nervous system) to receive input from the detector, to compare the variable against the set point and to determine the need for a response
   • An effector (usually muscular or glandular tissue) that is activated by the comparator to enact the appropriate response to restore the variable to its set point

   The term 'negative feedback' refers to the fact that effectors always act to move the variable in the opposite direction to the change that was originally detected.

   Due to the inherent time delay between detecting a change in a variable and effecting a response, negative feedback mechanisms cause oscillations in the variable they control. This delay means that feedback control always causes the variable to overshoot the set point slightly activating the opposite restorative mechanism to induce a smaller overshoot in that direction, until the oscillations fall within the range of values that are optimal for physiological function.

   Negative Feedback Loop. (Image modified by FRCEM Success. Original by OpenStax [CC BY 4.0 , via Wikimedia Commons)

   Positive Feedback

   Some physiological systems use positive feedback mechanisms e.g. hormonal control of childbirth (where pressure on the cervix causes increased release of oxytocin increasing uterine contraction) or initiation of an action potential (where a sodium influx causes depolarisation which causes further sodium channel opening).

   Positive feedback systems are less common in the body due to their inherent instability and risk of uncontrolled amplification. Positive feedback mechanisms require a mechanism to break the feedback loop ( such as by birth of the child in the first example above and by inactivation of sodium channels in the second).

   Positive Feedback Loop. (Image by OpenStax [CC BY 4.0 , via Wikimedia Commons)

5. Question 5 · Vessel Fluid Dynamics

   Which of these does NOT affect the rate of flow of a liquid through a tube:

   1. APressure differences across the ends of the tube
   2. BLength of the tube
   3. CDiameter of the tube
   4. DViscosity of the liquid
   5. ESurface tension
   Show answer and explanation

   Answer: E. Surface tension does not affect flow through a tube; pressure difference, length, radius and viscosity do.

   • A: Pressure difference drives flow.
   • B: Longer tubes have higher resistance.
   • C: Radius strongly affects resistance.
   • D: Higher viscosity increases resistance.

   Full explanation: Vessel Fluid Dynamics

   Vessel fluid dynamics

   Flow rate(Q) = vA (where v is flow velocity and A cross sectional vector Area 

   Tube flow: 

   Flow is directly proportional to radius 4 (r4) 

   • Tubing diameter: An important and frequently cited relationship is that of the tubing’s radius. Doubling the radius of a catheter increases the flow rate by 16-fold (r4). The larger the IV catheter the greater the flow 
   • The larger the flow capacity (cv) the larger the flow 
   • In Coronary circulation if flow capacity(cv) is reduced to 50% or half flow will be decreased, or radius decreased to half flow will decrease by 16 times. 
   • Fluids with higher viscosity also have a slower rate of flow. Plasma has a similar viscosity to water, but blood contains cells which effectively increase the viscosity by three- to four-fold. Changes in cell number e.g., polycythemia, therefore, affect blood flow. 

   Turbulent vs Laminar Flow 

   • Turbulent blood flow is multidirectional and travels at different velocities leading to increased resistance and additional shear stress on the vessel wall. 
   • Laminar flow: Frictional forces at the sides of a vessel cause a drag force on the fluid touching them, creating a velocity gradient where the flow is greatest at the centre.  

   Wall Tention:

   • Pressure across the wall of a flexible tube (transmural pressure) tends to extend it and increases wall tension. 
   • This can be described by Laplace’s Law: Pt = (Tw)/r 

             where, 

   • Pt = Transmural Pressure 
   • T = Wall Tension 
   • w = Wall Thickness 
   • r = Radius 

   Laplace’s Law also means that a large, dilated heart (e.g., heart failure) has to develop more wall tension (contractile force) in order to obtain the same ventricular pressure, making it less efficient.