Biology Unit 5 Essay Mark Schemes For Ib

Previous IB Exam Essay Questions: Unit 12

Use these model essay questions and responses to prepare for essay questions on your in-class tests, as well as the IB Examination, Paper 2. The questions below have appeared on IB HL Examinations over the past several years. The answers following the questions are the mark-scheme ideal responses used to evaluate student examination responses.
1. Draw a labelled diagram to show the internal structure of the heart.6 marksSL

Award one mark for each of the following structures clearly drawn and labelled correctly in a diagram of the heart

  • left and right ventricles
  • left and right atria
  • atrioventricular valves / bicuspid / mitral and tricuspid valves
  • semilunar valves
  • aorta and vena cava
  • pulmonary artery and pulmonary vein
  • ventricle thicker than atria
  • left ventricle wall thicker than right ventricle wall
Do not award marks for a diagram with only the ventricles or atria. However, it is not necessary to show the cordae tendinae.

2. Draw a labelled diagram of the heart showing all four chambers, associated blood vessels and valves.5 marksSL

Award one mark for any two of the following clearly drawn and correctly labelled.

  • vena cava
  • inferior and superior vena cava distinguished
  • aorta
  • pulmonary artery
  • pulmonary vein
  • left ventricle
  • right ventricle
  • left ventricle shown with thicker wall than right ventricle
  • septum
  • left atrium
  • right atrium
  • coronary artery
  • two semi-lunar valves
  • AV valves
  • bicuspid and tricuspid valves distinguished

3. Outline the events that occur within the heart, which cause blood to move around the body.6 marksSL

  • blood is collectred in the atria
  • blood is pumped from the atria to the ventricles
  • opened atrio-ventricular valves allow flow from the atria to the ventricles
  • closed semi-lunar valves prevent backflow from the arteries to the ventricles
  • blood is pumped out from the ventricles to the arteries
  • open semi-lunar valves allow flow from ventricles to arteries
  • closed atrio-ventricular valves prevent backflow to the atria
  • pressure generated by the heart causes blood to move around the body
  • pacemaker (SAN) initiates each heartbeat

4. Explain the relationship between the structure and function of arteries, capillaries and veins.9 marksSL

(3 marks maximum for information on arteries.)

  • carry blood away from the heart
  • have thick walls to withstand high pressure / prevent bursting
  • have muslce fibers to generate the pulse / help pump blood / even out blood flow
  • have elastic fibers to help generate pulse / allow artery wall to stretch / recoil
(3 marks maximum for information on capillaries.)
  • allow exchange of oxygen/carbon dioxide/ nutrients/waste products from tissues/cells
  • have a thin wall to allow (rapid) diffusion / movement in / out
  • have pores / porous walls to allow phagocytes / tissue fluid to leave
  • are narrow so can penetrate all parts of tissues / bigger total surface area
(3 marks maximum for information on veins.)
  • carry blood back to the heart / from the tissues
  • have thinner walls because the pressure is low / to allow them to be squeezed
  • have fewer muscle / elastic fibers because there is no pulse / because pressure is low
  • have valves to prevent backflow

5. Draw a simple diagram of the gas exchange system in humans.5 marksSL

For a diagram of the whole gas exchange system, award 1 mark for each of the following structures clearly drawn and labeled correctly.

  • trachea
  • lungs
  • bronchi
  • bronchioles
  • lungs (2 must be shown)
  • intercostal muscles between ribs
  • diaphragm
For a diagram of an alveolus only, award 1 mark for each of the following structures clearly drawn and labeled correctly.

6. Describe the mechanism of ventilation in the human lung.5 marksSL

  • consists of inhaling and exhaling air / exchanging stale air with fresh air (with the environment)
  • external intercostal muscles contract moving the rib cage up/out
  • diaphragm contracts
  • increaes volume of thorax / lowers lung pressure relative to air pressure / pulls air in
  • diaphragm relaxes
  • abdominal muscles contract
  • internal intercostal muscles contract moving the rib cage down/in
  • force air out / decreases volume of thorax / raise lung pressure relative to air pressure

7. Describe the need for a ventilation system.6 marksSL

  • (small) animals obtain oxygen (by diffusion) through skin / in humans (large) animals skin is ineffective for ventilation
  • humans are large / have a small ratio of surface area:volume
  • so need ventilation system to increase surface area
  • to maintain a concentration gradient in alveoli
  • as oxygen is used in respiration (and carbon dioxide is produced)
  • gaseous exchange occurs between air in alveoli and blood capillaries
  • alveoli have high ratio of surface area:volume (to facilitate ventilation)
  • to bring in fresh air (and remove stale air)

8. Explain the need for, and mechanism of, ventillation of the lungs in humans.8 marksSL

  • draws fresh air / oxygen into the lungs
  • removal / excretion of carbon dioxide
  • maintains concentration gradient of oxygen / carbon dioxide / respiratory gases
  • diaphragm contracts
  • (external) intercostal muscles contract
  • increased bolume (of thorax / thoracic cavity)
  • decreasing air pressure in lungs
  • air rushes in down air pressure gradient
  • converse of the above causes exhalation
  • abdominal muscles contract during active exhalation
  • elastic recoil of lungs helps exhalation

9. Many processes in living organisms, including ventilation and gas exchange, involve moving materials. State the differences between ventilation and gas exchange in humans.4 marksSL

ventiallation:2 max

  • movement of air
  • movement in and out of the lungs
  • caused by muscles
  • an active process
  • involves mass flow / involves flow along air passages

gas exchange2 max

  • movement of carbon dioxide and oxygen
  • (occurs when) oxygen moves from lungs / alveoli to red blood cells / carbon dioxide moves to lungs / alveoli from red blood cells
  • (occurs when) oxygen moves from red blood cells to tissues / carbon cioxide moves to red blood cells from tissues
  • a passive process / diffusion
  • takes place across a surface

Previous IB Exam Essay Questions: Unit 1

Use these model essay question responses to prepare for essay questions on your in class tests, as well as the IB Examination, Paper 2. These questions have appeared on recent IB examinations, exactly as shown below. Following each question is the markscheme answer which was used to evaluate student answers on the examination paper.
1. Discuss possible exceptions to cell theory.4 marks
  • skeletal muscle fibers are larger/have many nuclei/are not typical cells
  • fungal hyphae are (sometimes) not divided up into individual cells
  • unicellular organisms can be considered acellular
  • because they are larger than a typical cell/carry out all functions of life
  • some tissues/organs contain large amounts of extracellular material
  • e.g. vitreous humor of eye/ mineral deposits in bone/ xylem in trees/other example
  • statement of cell theory/all living things/most tissues are composed entirely of true cells

2. Eukaryotic cells have intracellular and extracellular components. State the functions of one named extracellular component.4 marks

name of component: 1 max

  • e.g. plant cell wall/cellulose/interstitial
  • matrix/basement membrane/glycoprotein/bone matrix;
functions: 3 max


  • e.g. (plant cell wall) strengthens/supports the cell/plant (against gravity);
  • prevents the entry of pathogens;
  • maintains the shape of plant cells;
  • allows turgor pressure/high pressure to develop inside the cell;
  • prevents excessive entry of water to the cell;
  • helps cells to stick together/adhere;
  • needed to hold cells/tissues together / example of cells/tissues holding together;
  • forms interstitial matrix / forms basement membrane to support single layers of cells;
  • e.g. around a blood capillary;
  • forms (part of the) filtration membrane in the glomerulus;

3. Explain how the surface are to volume ratio influences cell sizes.3 marks

  • small cells have larger ratio (than larger cells)/ratio decreases as size increases
  • surface area/membrane must be large enough to absorb nutrients/oxygen/substances needed
  • surface area/membrane must be large enough to excrete/pass out waste products
  • need for materials is determined by (cell) volume
  • cell size is limited (by SA/Volume ratio)/cells divide when they reach a certain size
  • reference to diffusion across/through membrane/surface area

4. Outline differentiation of cells in a multicellular organism.4 marks

  • differentiation is development in different/specific ways
  • cells carry out specialized functions/become specialized
  • example of a differentiated cell in a multicelluar organism
  • cells have all genes/could develop in any way
  • some genes are switched on/expressed but not others
  • position/hormones/cell-to-cell signals/chemicals determine how a cell develops
  • a group of differentiated cells is a tissue

5. Describe the importance of stem cells in differentiation.3 marks

  • stem cells are undifferentiated cells;
  • embryo cells are stem cells;
  • stem cells can differentiate in many/all ways / are pluripotent/totipotent;
  • differentiation involves expressing some genes but not others;
  • stem cells can be used to repair/replace tissues/heal wounds;

6. Draw a labelled diagram to show the ultrastructure of Escherichia coli.6 marks

Award 1 for each structure clearly drawn and correctly labelled.

  • cell wall – with some thickness;
  • plasma membrane – shown as single line or very thin;
  • cytoplasm;
  • pilus/pili – shown as single lines;
  • flagellum/flagella – shown as thicker and longer structures than pili and embedded in cell wall;
  • 70S ribosomes;
  • nucleoid / naked DNA;
  • approximate width 0.5 μm / approximate length 2.0 μm;
Award 4 max if the bacterium drawn does not have the shape of a bacillum (rounded-corner rectangle with length approximately twice its width).Award 4 max if any eukaryotic structures included.

7. Draw a labelled diagram to show the organelles which are found in the cytoplasm of plant cells.6 marks

Award 1 mark for each of the following structures accurately drawn and labelled

  • rough endoplasmic reticulum
  • free ribosomes
  • Golgi apparatus
  • mitochondrion
  • chloroplast
  • vacuole
  • nucleus
  • lysosome
  • smooth endoplasmic reticulum

8. State one function of each of the following organelles: lysosome, Golgi apparatus, rough endoplasmic reticulum, nucleus, mitochondrion.5 marks

  • lysosome: hydrolysis/digestion/break down of materials (macromolecules)
  • Golgi apparatus: synthesis/sorting/transporting/secretion of cell products
  • rough endoplasmic reticulum: site of synthesis of proteins (to be secreted)/ intracellular transport of polypeptides to Golgi apparatus
  • nucleus: controls cells activities/mitosis/replication of DNA/transcription of DNA (to RNA)/directs protein synthesis
  • mitochondrion: (aerobic) respiration/generates ATP

9. Draw a labelled diagram showing the ultra-structure of a liver cell.4 marks

Award 1 for each structure clearly drawn and correctly labelled. Whole cells not necessary.

  • (plasma) membrane – single line surrounding cytoplasm;
  • nucleus – with a double membrane and pore(s) shown;
  • mitochondria(ion) – with a double membrane, the inner one folded into internal
  • projections, shown no larger than half the nucleus;
  • rough endoplasmic reticulum – multi-folded membrane with dots/small circles on surface;
  • Golgi apparatus – shown as a series of enclosed sacs with evidence of vesicle formation;
  • ribosomes – dots/small circles in cytoplasm/ribosomes on rER;
  • lysosome;
Award 0 if plant cell is drawn. Award 2 max if any plant cell structure (e.g. cell wall) is present.

10. Distinguish between the structure of plant and animal cells.6 marks

Award 1 mark per differenceplant cells

  • have cell walls, animals do not
  • have plastids/ chloroplasts, animals do not
  • have a large central vacuole, animals do not
  • store starch, animal cells store glycogen
  • have plasmodesmata, animal cells do not

animal cells

  • have centrioles, plant cells do not
  • have cholesterol in the cell membrane, plant cells do not
  • plant cells are generally have a fixed shape/ more regular whereas animal cells are more rounded

11. Using a table, compare the structures of prokaryotic and eukaryotic cells.5 marks

P: prokaryotic cells; E: eukaryotic cells

  • DNA: P: naked/loop of DNA; E: associated with protein/histones/nucleosomes/DNA in chromosomes
  • location of DNA: P: in cytoplasm/nuceloid/no nucleus; E: within a nucleus/nuclear membrane
  • membrane bound organelles: P: none; E: present
  • ribosomes: P: 70S ; E: 80S
  • plasma membrane: P & E: same structure within both groups
  • cell wall: P: peptidoglycan/not cellulose/not chitin; E: cellusose/chitin/not peptidoglycan
  • respiratory structures: P: no mitochondria; E: mitochondria
  • pili: P: pili present E: pili absent;
  • plasmids: P: plasmids (sometimes) present E:plasmids absent;
  • flagella: P: flagella solid E: flagella flexible/membrane-bound;

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