One problem, solved three times

Unit 1 looks like four separate things to learn: cells, how substances move in and out of them, the organ systems of an animal, and how a plant moves things around. ⚠️ THEY ARE FOUR ANSWERS TO ONE QUESTION, AND THE QUESTION IS ALWAYS THE SAME: how does every cell in this living thing get what it needs? What changes is only how big the living thing is. A SINGLE CELL is small enough that substances can simply move in and out across its own surface. Nothing else is needed. Every part of it is close to the outside. AN ORGANISM TOO BIG FOR THAT has an inside that is nowhere near the outside. So it grows a specialised EXCHANGE SURFACE: lungs, or gills, or the lining of the small intestine, or root hairs. A place built for things to cross. AN ORGANISM TOO BIG EVEN FOR THAT has a new problem: things now cross at the surface, but the cells that need them are metres away. So it grows a TRANSPORT SYSTEM to carry them the rest of the way. ⚠️ THAT IS THE WHOLE UNIT. THREE LEVELS, EACH ONE FORCED BY BEING TOO BIG FOR THE ONE BELOW. And it matters for this specification specifically. Other boards teach these four things in four separate places. ⚠️ WJEC PUTS THEM IN ONE UNIT AND ONE EXAM PAPER, which means a single question can ask you to cross between them. An answer that moves up the three levels is already organised, and step 7 explains why that is worth marks on this paper in particular.

The words this unit runs on

Five terms, defined and nothing more. What each one looks like in a real organism comes next.

Match each case to the word for it

Oxygen moves from an air sac, where there is plenty of it, into the blood, where there is less
Water passes into a root cell from soil that is more dilute than the cell contents
A root takes in minerals from soil that already contains fewer of them than the root does, and uses energy to do it
The folded lining of the small intestine, built so that digested food can pass into the blood
The vessels that carry that food from the intestine onwards to cells all over the body

DIFFUSION, because particles are moving down a concentration difference and no energy is needed
OSMOSIS, because it is water crossing a membrane towards the more concentrated side
ACTIVE TRANSPORT, because the movement is up a concentration difference and costs energy
AN EXCHANGE SURFACE, because it is a place specialised for substances to cross
A TRANSPORT SYSTEM, because it carries substances onwards to distant cells

Which process, and how do you know

A root hair cell contains a higher concentration of nitrate ions than the soil around it, yet nitrate keeps entering the cell. Which process is at work, and what tells you?

Active transport, because the nitrate is moving from lower to higher concentration, which cannot happen without the cell spending energy
Diffusion, because particles always spread out until they are evenly spread
Osmosis, because the cell contents are more concentrated than the soil
None of them, because substances cannot move against a concentration difference at all

Three sizes, three solutions

The same three levels from step 1, now with what each one relies on and why it stops working. Read the last row of each column as the reason the next column has to exist.

True about exchange and transport

Select the TWO statements that are true.

Even in a large organism with a transport system, substances still have to cross the membrane of each individual cell
Exchange surfaces exist because an organism has grown too large for substances to reach its inner cells directly
Once an organism has a transport system it no longer needs an exchange surface
Plants do not need exchange surfaces, because they make their own food

The extended-writing question, and what it is really marked on

Your papers carry a longer question, the extended-writing or QER question, and it behaves differently from everything around it. ⚠️ IT IS MARKED ON HOW WELL YOU COMMUNICATE AS WELL AS ON THE SCIENCE. Organisation, clear sentences and correct spelling of scientific words all count. You can know the right biology and still lose marks by presenting it as a heap. Which makes the planning worth more here than anywhere else on the paper. ⚠️ AND THE THREE LEVELS GIVE YOU A PLAN FOR FREE. When a question spans this unit, work up the sizes: START AT THE CELL. What has to get in or out, and how does it cross? MOVE TO THE SURFACE. Where does it enter the organism, and what makes that place good at the job? FINISH AT THE SYSTEM. How does it travel from there to the cells that need it? Three paragraphs, in an order that is obvious to the marker, and you have already earned the organisation marks before the science is judged. Two more habits that pay on this paper. ⚠️ USE THE TECHNICAL WORD AND SPELL IT. Write the proper term rather than talking around it, because an approximation costs you both the knowledge mark and the communication mark at once. ⚠️ AND ANSWER "WHY", NOT JUST "WHAT". Saying a structure is thin is a description. Saying it is thin SO THAT substances have a shorter distance to cross is an explanation, and on a question like this the explanation is the thing being paid for.

Why every exchange surface looks the same

Four structures, from four different places in this unit, usually learned as four separate sets of facts. THE LUNGS: millions of tiny air sacs rather than two smooth bags. THE SMALL INTESTINE: a lining thrown into folds and covered in projections. A FISH'S GILLS: stacked filaments, each carrying rows of finer branches. A PLANT ROOT: cells drawn out into long thin hairs, pushing out between the soil particles. ⚠️ THEY LOOK NOTHING ALIKE AND THEY ARE BUILT THE SAME WAY, BECAUSE THEY ARE ALL ANSWERING ONE QUESTION: how do you get as much crossing as possible, as fast as possible? There are only three ways to do that, and all four structures use all three. MAKE THE AREA ENORMOUS. That is what every one of those shapes is for. Folding, branching and projecting are all ways of packing a huge area into a small space. A smooth surface of the same overall size would do a fraction of the work. MAKE IT THIN. The shorter the distance something has to cross, the faster it gets across. Each of these surfaces is often a single cell thick. KEEP BOTH SIDES SUPPLIED. Whatever crosses has to be taken away on the far side and replaced on the near side, or the difference between the two sides disappears and the crossing slows to nothing. That is why lungs are wrapped in blood vessels and why you keep breathing rather than filling your lungs once. ⚠️ SO IN AN EXAM, ANY STRUCTURE DESCRIBED AS FOLDED, BRANCHED, THIN OR RICHLY SUPPLIED IS TELLING YOU WHAT IT IS FOR. You do not need to have met that particular organ before. Three features, one purpose, and it works on a structure you have never seen.

Work out the magnification

A student measures a plant cell on a photograph as 60 mm across. The real cell is 0.05 mm across. Magnification is image size divided by real size. What is the magnification?

Order the journey of one oxygen molecule

Put the stages in order, from the air outside to the inside of a muscle cell.

Oxygen in the air reaches the air sacs deep inside the lungs
It crosses the thin wall of an air sac into the blood
The heart pumps the blood carrying it out towards the body
It reaches a narrow vessel running close to a muscle cell
It crosses the membrane of the muscle cell itself

The exchange and transport run

Five questions across the three levels. Three lives.

Complete the cell transport facts

The net movement of particles from where they are more concentrated to where they are less concentrated is _____. The movement of water only, across a partially permeable membrane, is _____. Moving a substance from a lower to a higher concentration requires energy from respiration and is called _____. A part of an organism specialised for substances to cross into or out of it is called an _____.

diffusion osmosis active transport exchange surface respiration transpiration photosynthesis transport system

Spot the true cell and organ system facts

Tap the TWO statements that are true.

Exchange surfaces are typically large in area and thin, because both features speed up substances crossing them
Substances still cross the membrane of each individual cell even in an organism that has a transport system
Diffusion and osmosis both require the cell to use energy from respiration
A transport system removes the need for an exchange surface

Three judgements to make

Three cases. Each answer has to carry the reasoning, not just the label.

You are shown an organ you have never studied. Its lining is thrown into deep folds, it is a single cell thick, and it is packed with blood vessels. What can you say about its job, and why?
A cell is placed in a solution and water moves OUT of it, while at the same time the cell takes IN potassium ions from a solution containing very few. A student says both movements are passive. Are they right?
In the extended-writing question a student writes: "The alveoli are very thin and there are lots of them." The content is correct but the marks are low. Why?

Explain how oxygen reaches a muscle cell

An extended-writing question of the kind this unit sets. Work up the three levels, and give a reason for every structure you name rather than only describing it.

Explain why a large animal cannot rely on substances crossing its outer surface, as a single-celled organism does
Describe the features that make an exchange surface good at its job, and for each one say what it achieves
Explain how the transport system carries oxygen onwards from that surface to the cells
Name the process by which oxygen finally enters a muscle cell, and say why it needs no energy from the cell
Use the correct technical terms throughout and spell them accurately, since this question also assesses how clearly you communicate

Cells and Organ Systems

Get the method right under pressure

What you'll cover