top of page
Search

GCSE/iGCSE Biology Week 8

Getting In and Getting Out


How Does Anything Actually Cross a Cell Membrane?

The Big Question This Week


"If a cell membrane is a barrier, how does anything ever get in or out of a cell at all?"


Write this in your learning journal as a title before you begin. Don't look anything up yet — just think about it. You already met the cell membrane back in Week 4 when we looked at cell structure. This week we find out what that membrane is actually doing all day.


Before You Begin — Predict!


Write down whether you think these are true or false, and why:


  1. "Substances move across cell membranes randomly; there's no pattern to it."

  2. "Cells never have to spend energy to move things across their membrane; it all just happens on its own."


Hold onto your predictions. We will return to them at the end of the week.


Story of the Week


Picture a drop of ink falling into a glass of still water. At first it's a small, dark cloud. Leave it alone, and without any stirring, without anyone helping it along, the ink slowly spreads out until the whole glass is a uniform, pale colour. Nobody pushed it. It just happened, because that is what particles do: they move from where they are crowded to where they are not, until they're spread evenly. This is diffusion, and it is one of the quiet, constant processes keeping you alive right now; oxygen diffusing into your blood in your lungs, carbon dioxide diffusing out, glucose diffusing from your gut into your bloodstream.


But cells are not just open glasses of water. They are wrapped in a membrane, and that membrane is fussy about what it lets through. Small, uncharged molecules like oxygen and carbon dioxide slip across easily. Water has its own special version of diffusion, called osmosis, that depends on how much "stuff" is dissolved on each side of the membrane. And some substances, the ones a cell genuinely needs, even when there's already plenty of it inside the cell, have to be dragged in against the natural flow, using energy the cell has made itself. That's active transport, and it's the reason a root hair cell can pull in minerals from soil that is actually more dilute than the cell itself.


This week we look at all three: the free ride of diffusion, the special case of osmosis, and the energy-costing push of active transport. Three different ways of solving the same problem — how do you get the right things across a barrier that's trying to keep most things out?


Video Session One


While you watch, look out for:


  1. Why the cell membrane is described as behaving like a "gatekeeper," and what the phospholipid bilayer is made of

  2. The difference between passive transport (diffusion, facilitated diffusion) and active transport, and why one needs ATP and the other doesn't

  3. How large molecules get in and out of cells via endocytosis and exocytosis, when diffusion alone isn't enough


Workthrough the presentation below.


Fill in the worksheet and add to your journal.



A good place to stop is once the video ends, if you need to.


Video Session Two



While you watch, look out for:

  1. The definition of osmosis: the movement of water across a partially permeable membrane, from a region of higher water potential to lower water potential

  2. What happens to an animal cell (like a red blood cell) and a plant cell when placed in a hypertonic, hypotonic, or isotonic solution

  3. Why plant cells don't burst in very dilute (hypotonic) solutions the way animal cells do — think back to Week 6 and what plant cells have that animal cells don't


⚠️ Common Misconception Alert

Students often say water moves "from low concentration to high concentration" in osmosis and get muddled, because it depends whether you're talking about the concentration of water or the concentration of solute (the dissolved substance). Water moves from a dilute solution (lots of water, few solute particles = high water potential) to a concentrated solution (less water, more solute particles = low water potential). If you find yourself confusing "concentrated" and "dilute," always translate the question into water potential language first, it removes the ambiguity.


Fill in the worksheet and add to your journal.



Reading Assignment



Hands-On Activity — Required Practical: Osmosis in Plant Tissue


What you need: A raw potato (or another firm vegetable like courgette), a sharp knife or cork borer (ask an adult to help with cutting), a ruler, kitchen scales if available, several small cups or jars, water, salt, and cling film.


What to do:


  1. Cut 4–5 potato cylinders or cubes of a similar size. Measure and record the mass (and length, if using cylinders) of each one before you start.

  2. Make up a range of salt solutions of different concentrations — for example 0%, 5%, 10%, 15%, and 20% salt by mass in water (roughly: dissolve 0 g, 5 g, 10 g, 15 g, and 20 g of salt into 100 ml of water for each cup).

  3. Place one potato piece into each solution, cover with cling film to reduce evaporation, and leave for at least an hour (overnight is even better).

  4. Remove each piece, gently pat dry, and re-measure the mass (and length).

  5. Calculate the percentage change in mass for each piece: (final mass − initial mass) ÷ initial mass × 100


Here is Mr Exhams video of this practical.


In your journal, plot a simple graph of percentage change in mass (y-axis) against salt concentration (x-axis). Find roughly where your line crosses zero — this is the point where the salt solution has the same water potential as the potato cells themselves. Write two sentences explaining, in terms of water potential, why the potato pieces gained mass in the weaker solutions and lost mass in the stronger ones.


The Brilliant Mistake — Stories Science Gets Wrong

This Week's Story: The Membrane That Was Supposed to Be a Sandwich

In 1935, two scientists named Hugh Davson and James Danielli proposed a model for the structure of the cell membrane. Based on the evidence available at the time, they suggested the membrane was built like a sandwich: a layer of fat (lipid) in the middle, with a solid layer of protein coating each side, like bread around a filling. For decades, this "Davson–Danielli model" was taught as fact in textbooks around the world, because it fitted the data scientists had, and because, at the time, nobody had a way of actually seeing individual molecules moving within a membrane.


The trouble was, the sandwich model made the membrane sound rigid and static, a fixed structure, always in the same arrangement. But newer evidence kept turning up that didn't fit: some membrane proteins seemed to move around, some stuck out unevenly on one side, and the numbers for how much protein and lipid were present didn't quite add up to a neat, solid sandwich.


In 1972, scientists Jonathan Singer and Garth Nicolson proposed something different: the fluid mosaic model. Instead of a rigid sandwich, they described the membrane as a fluid layer of lipids with proteins floating and drifting within it, like icebergs bobbing in a slow-moving sea, some sitting on the surface, some spanning all the way through. New tools, particularly a technique called freeze-fracture electron microscopy that let scientists see membranes split open and photographed at huge magnification, gave them the evidence to prove it.


The fluid mosaic model is still the accepted picture of the cell membrane today.

Journal Prompt:


  • The Davson–Danielli model wasn't a careless guess, it was a reasonable model built from the best evidence available in 1935. Why is it still fair to call it a "mistake" if the scientists did nothing wrong?

  • What role did better tools (freeze-fracture microscopy) play in overturning the old model, compared to just "thinking harder" about the problem?

  • Can you think of another idea from this course so far where an old model was replaced not because scientists were foolish, but because the technology to test it properly didn't exist yet?


Scientist Portrait

Jonathan Singer (1924–2017)


Add Jonathan Singer to your Scientist journal. Draw or print a portrait. Beneath it, write: "He and Garth Nicolson turned a static sandwich into a moving sea — and gave the cell membrane back its motion."


Return to Your Predictions


Go back to the two statements from the start of the week:


  1. "Substances move across cell membranes randomly, there's no pattern to it."

  2. "Cells never have to spend energy to move things across their membrane; it all just happens on its own."


Rewrite each one now, using what you've learned this week.


The Big Question — Final Answer


Return to this week's big question:


"If a cell membrane is a barrier, how does anything ever get in or out of a cell at all?"


Write a paragraph in your learning journal. A strong answer will mention diffusion, osmosis, and active transport by name, note which ones require energy, and give at least one real example of each.


Practice Questions — 10 Question Set

  1. Define diffusion.

  2. Define osmosis, including reference to a "partially permeable membrane."

  3. What is meant by active transport, and how is it different from diffusion?

  4. Name two factors that affect the rate of diffusion.

  5. A student places a red blood cell into pure water. Using the term water potential, explain what will happen to the cell and why.

  6. A plant cell placed in a concentrated salt solution loses water and its cell membrane pulls away from the cell wall. What is the name of this process?

  7. Give one example in the human body where active transport is essential, and explain why diffusion alone would not be enough.

  8. What is the source of energy used in active transport?

  9. In the potato practical, a potato piece placed in distilled water gained mass. Explain why, in terms of water potential.

  10. Explain why large molecules such as proteins cannot cross the cell membrane by simple diffusion, and name the process cells use instead to move them.


Week 8 Checklist


Before moving to Week 9, tick off each item:

  •  Watched Crash Course Biology #24: Cell Membranes

  •  Completed the passive/active transport table in your journal

  •  Watched Amoeba Sisters: Osmosis and Water Potential

  •  Completed the potato osmosis practical and plotted your results

  •  Read the Davson–Danielli/fluid mosaic story and answered the journal prompts

  •  Added Jonathan Singer to your Scientist journal

  •  Rewritten your two predictions from the start of the week

  •  Answered the 10 practice questions

  •  Written a final answer to the Big Question


Looking Ahead — Week 9 Preview


Next week we turn to the process that captures the sun's energy and feeds almost every food chain on Earth: photosynthesis. We'll look at the word equation, what affects the rate of photosynthesis, and run a required practical using pondweed to measure it directly.


Prepare by writing this question in your journal: "Where does a plant actually get its mass from, the soil, the air, or somewhere else?"


Made with love, hot chocolate, and an unreasonable number of browser tabs. Happy Home Education

💬 Spotted a broken link or an error? Message me on Facebook — I'll fix it when I can. ☕ Finding this useful? Consider clicking Buy Me a Hot Chocolate — it means the world.

bottom of page