Biology (9700) | Movement into and out of cells

Welcome to Topic 4: Movement Into and Out of Cells!

Hello future biologists! This chapter is absolutely fundamental because it explains how cells—the basic units of life—interact with their environment. Everything a cell needs (glucose, oxygen, water) and everything it needs to get rid of (waste, hormones) must cross the cell membrane.

If you understand these transport mechanisms, you will understand topics throughout the entire course, from gas exchange in the lungs to nutrient absorption in the gut. Don't worry if the concepts seem tricky at first; we'll break them down using clear steps and relatable analogies!

4.1 Prerequisite Review: The Cell Membrane

Remember the structure of the cell membrane (Topic 4.1)? It's a selective barrier, controlling what enters and leaves, thanks to the Fluid Mosaic Model.

• Phospholipids: Form the main bilayer. They have a hydrophilic (water-loving) head and two hydrophobic (water-hating) fatty acid tails. This structure makes the membrane highly permeable to small, non-polar molecules (like oxygen).
• Proteins: These are scattered throughout the bilayer. They are essential for transport, cell signalling, and cell recognition.
• Cholesterol: Helps maintain membrane fluidity and stability across different temperatures.

Quick Memory Aid: The Cell Membrane Analogy

Think of the cell membrane like a security checkpoint at an airport:
Phospholipid Bilayer: The main wall. Only very small, sneaky passengers (O₂ and CO₂) can slip through easily.
Proteins: The actual gates and security personnel, checking ID and moving specific molecules through special tunnels.

4.2 Passive Transport Mechanisms

Passive transport is the movement of substances down their concentration gradient (from an area of high concentration to an area of low concentration). Crucially, this process does not require metabolic energy (ATP).

Memory Trick: P.A.S.S. = Passive Always Starts Slowly (as molecules spread out naturally).

A. Simple Diffusion

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, due to their random kinetic energy.

• Molecules Moved: Typically small, non-polar, lipid-soluble molecules (e.g., O₂, CO₂).
• Pathway: Directly through the phospholipid bilayer.
• Effect of Gradient: The steeper the concentration gradient, the faster the rate of diffusion.

B. Facilitated Diffusion

This mechanism helps substances that cannot pass easily through the phospholipid bilayer (e.g., large molecules like glucose, or polar/charged ions). It is still passive and moves substances down the gradient, but it requires help from proteins.

• Channel Proteins: These form pores or tunnels through the membrane, allowing specific ions (like Cl⁻ or Na⁺) or water to pass rapidly. They are often gated (can be opened or closed).
• Carrier Proteins: These bind to a specific molecule (like glucose). When the molecule binds, the carrier protein changes its shape, carrying the molecule across the membrane before releasing it on the other side.

Did you know? Facilitated diffusion shows saturation. If all the carrier proteins are busy, increasing the external concentration won't increase the transport rate—it has reached its V_max!

C. Osmosis (The Movement of Water)

Defining Osmosis and Water Potential ($\Psi$)

Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

The concept used to predict water movement is Water Potential ($\Psi$).

• Water Potential is the tendency of water molecules to move freely from one region to another.
• Pure water has the highest water potential, defined as zero (0 kPa).
• Adding solutes (like sugar or salt) lowers the water potential, making it a negative value.

Rule of Thumb: Water always moves from less negative $\Psi$ towards more negative $\Psi$.

Effect of Osmosis on Cells (4.2.6)

The outcome of osmosis depends heavily on whether the cell has a rigid cell wall (plant) or not (animal).

1. Plant Cells (with a rigid cell wall)

Plant cells are surrounded by a strong, inelastic cell wall.

• In pure water (High $\Psi$ external): Water enters the cell by osmosis. The cytoplasm and vacuole swell, pushing the cell membrane against the cell wall. The cell becomes turgid. This is the healthy state for a plant cell, providing support.
• In concentrated solution (Low $\Psi$ external): Water leaves the cell by osmosis. The cytoplasm shrinks, pulling the cell membrane away from the cell wall. This state is called plasmolysis, and the cell is flaccid (limp).

2. Animal Cells (no cell wall)

Animal cells (like red blood cells) are only protected by the flexible cell membrane.

• In pure water (High $\Psi$ external): Water enters the cell by osmosis. The cell swells and bursts because there is no rigid wall to prevent expansion. This is called lysis (specifically haemolysis in red blood cells).
• In concentrated solution (Low $\Psi$ external): Water leaves the cell by osmosis. The cell shrinks and develops a spiked appearance. This is called crenation.

Common Mistake Alert: Never say a plant cell "bursts"! The cell wall prevents this. The correct term is plasmolysis or flaccid.

4.3 Active Transport Mechanisms (4.2.1)

Active transport is the movement of molecules or ions against their concentration gradient (from an area of low concentration to an area of high concentration).

Because this movement is "uphill," it requires:

1. Energy supplied by ATP.
2. Specialised membrane proteins called carrier proteins (often referred to as pumps).

Active Transport Analogy: The Pump

Think of active transport like using a water pump to move water uphill. You are moving the substance against the natural flow, which requires energy (fuel) to run the pump (the carrier protein).

Process Steps (e.g., pumping ions):

1. The ion/molecule binds to the specific carrier protein on the low concentration side.
2. ATP hydrolysis provides the energy (often causing phosphorylation of the carrier protein).
3. The energy causes the carrier protein to change its tertiary structure (shape).
4. The ion/molecule is released on the high concentration side of the membrane.

4.4 Bulk Transport (Vesicular Transport) (4.2.1)

Sometimes, cells need to transport very large molecules (like proteins) or even entire foreign particles (like bacteria). Diffusion and active transport are too slow or impossible for these large loads. This is achieved using vesicles in processes called Endocytosis and Exocytosis. This process requires significant energy (ATP).

A. Endocytosis (Moving IN)

This is the process where the cell takes in materials by enclosing them in a vesicle formed from the cell surface membrane.

• The membrane folds inwards, trapping the material inside.
• The membrane pinches off to form an internal vesicle.
• Phagocytosis: Cell eating (taking in solid particles, e.g., white blood cells engulfing bacteria).
• Pinocytosis: Cell drinking (taking in liquid droplets).

B. Exocytosis (Moving OUT)

This is the process where the cell releases materials by fusing a vesicle with the cell surface membrane.

• Vesicles containing cell products (like hormones or enzymes) or waste move toward the membrane.
• The vesicle membrane fuses with the cell membrane.
• The contents are expelled outside the cell.

Example: Neurons release neurotransmitters into the synapse via exocytosis. Plant cells use exocytosis to move materials needed to build the cell wall.

4.5 Surface Area to Volume Ratio (SA:V) (4.2.3, 4.2.4)

For transport processes like diffusion to be effective, cells must maintain a suitable surface area relative to their volume.

The Principle

As an organism (or cell) increases in size, its volume increases faster than its surface area. This means the Surface Area to Volume ratio decreases as size increases.

Imagine a small cube (1x1x1 unit). SA = 6, V = 1. SA:V = 6:1.
Imagine a large cube (10x10x10 units). SA = 600, V = 1000. SA:V = 0.6:1.

Why the Ratio Matters for Transport

The surface area is where substances enter and leave the cell (diffusion/transport). The volume represents the amount of material inside that needs nutrients supplied or waste removed.

A high SA:V ratio is crucial for efficient transport because:

• It ensures a short diffusion distance to all parts of the cell (or organism).
• There is a large membrane area available relative to metabolic needs (volume).

Adaptations for High SA:V

Many cells and organs are adapted to maximise surface area without drastically increasing volume:

• Microvilli: Finger-like projections on epithelial cells in the small intestine, dramatically increasing absorption surface.
• Alveoli: Tiny air sacs in the lungs, providing huge surface area for gas exchange.
• Flattened Shape: Many single-celled organisms (and red blood cells) are flattened to maximise the ratio.

Practical Investigation Link (4.2.4)

You might investigate this using agar blocks (often coloured with indicator) of different sizes placed in an acid solution. The smaller blocks (high SA:V) will change colour fastest because the substance diffuses throughout the volume more quickly relative to their size.

4.6 Practical Investigations into Water Potential (4.2.5)

A common practical skill is determining the water potential of plant tissue (like potato or carrot strips).

Step-by-Step Procedure Outline

1. Prepare a series of salt or sugar solutions with known, different water potentials (e.g., 0.0 M, 0.1 M, 0.2 M...).
2. Cut pieces of plant tissue (e.g., potato cylinders) to a standard size and record their initial mass.
3. Place one cylinder in each solution for a fixed time (e.g., 30 minutes).
4. Remove, carefully blot dry (important step!), and record the final mass.
5. Calculate the percentage change in mass for each sample.
6. Plot a graph of Percentage Change in Mass (Y-axis) against External Solution Concentration (X-axis).

Estimating Tissue Water Potential

The water potential of the plant tissue is estimated by finding the point on the graph where the percentage change in mass is zero.

• At 0% change, there was no net movement of water.
• This means the water potential of the external solution was equal to the water potential of the plant tissue cytoplasm.

The concentration corresponding to this point of zero change is then used to find the estimated $\Psi$ of the tissue (usually by looking up the known $\Psi$ of that specific concentration).

Key Takeaways: Movement into and out of cells

Movement across membranes is determined by two main factors: the type of molecule (size, polarity) and the energy available (ATP). Diffusion and facilitated diffusion are passive, driven by kinetic energy down the gradient. Active transport and bulk transport require metabolic energy (ATP) to move substances against the gradient or move large particles. Remember that maintaining a high SA:V ratio is vital for ensuring these exchange processes occur rapidly enough to support cellular metabolism.