Biology (9610) | Transport into and out of cells

Welcome to Transport into and out of Cells!

Hi there, aspiring Biologist! This chapter, "Transport into and out of cells," is absolutely fundamental. Think of the cell membrane as the ultimate bouncer or security guard, controlling who gets in and out of the party (the cytoplasm).

By the end of these notes, you will understand how cells manage this complex traffic, using different mechanisms—some that require zero energy (passive) and others that demand a lot of power (active).

Getting this right is crucial, as cell transport determines everything from nerve impulses to how plants absorb water. Let's dive in!

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3.1.4.1 The Plasma Membrane: The Gatekeeper

The boundary of the cell is the plasma membrane (or cell-surface membrane). Its structure is explained by the Fluid-Mosaic Model.

Key Features of the Fluid-Mosaic Model

Imagine the cell membrane as a vast, moving sea of fat (lipids) where different proteins are floating around like icebergs or ships. This 'sea' is fluid, and the components can move laterally (sideways).

1. The Phospholipid Bilayer

The backbone of the membrane is a double layer of phospholipids.

• The molecules have a hydrophilic head (attracted to water, pointing outwards and inwards towards the cytoplasm).
• They also have two hydrophobic tails (repelled by water, pointing towards the centre of the membrane).
• This arrangement means the center of the membrane is non-polar/fatty. This characteristic limits what can pass through easily.

2. Proteins

Proteins are scattered throughout the bilayer and are essential for controlled transport.

• Channel proteins: Provide tunnels (channels) for water-soluble ions to pass through.
• Carrier proteins: Bind to larger molecules (like glucose or amino acids) and change shape to ferry them across the membrane.
• Proteins are key to facilitated diffusion and active transport.

3. Cholesterol

Cholesterol molecules may be present within the bilayer. Their function is crucial for stability:

• They provide mechanical strength.
• They restrict the movement of other molecules, helping to maintain the membrane's optimum fluidity.

4. Carbohydrates

Carbohydrates (sugars) attach to lipids (forming glycolipids) or proteins (forming glycoproteins) on the outer surface of the membrane.

• Their main role is cell recognition and adhesion, acting like antennae or ID tags for the cell.

The Role of Microvilli

Some cells, especially those involved in absorption (like epithelial cells in the small intestine), have extensions called microvilli. These are tiny finger-like projections of the cell-surface membrane.

The structure of microvilli is an important adaptation because they dramatically increase the surface area of the membrane, leading to a much faster rate of absorption or exchange.

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3.1.4.2 Passive Transport Mechanisms

Passive transport is movement that does not require metabolic energy (ATP). Substances move naturally "downhill" from where they are highly concentrated to where they are less concentrated.

1. Diffusion

Diffusion is the passive movement of substances down a concentration gradient.

• It results from the random kinetic energy of the particles.
• Movement continues until the substance is evenly distributed (equilibrium).

Simple Diffusion Across Plasma Membranes

For a substance to diffuse directly through the phospholipid bilayer (simple diffusion), it must usually be:

• Small (e.g., water, though water also uses specific channels).
• Non-polar (e.g., oxygen, carbon dioxide, since they dissolve in the fatty core).

The Limitation: The hydrophobic nature of the bilayer limits diffusion of polar, charged, or larger molecules. These substances need help!

Factors Affecting the Rate of Diffusion

The speed at which a substance moves across an exchange surface (like the alveoli in the lung) depends on three major factors:

1. Surface Area (SA): The larger the surface area, the more space for molecules to cross, increasing the rate. (Think of a wide door instead of a small window.)
2. Difference in Concentration (The Gradient): The steeper the gradient (bigger difference between inside and outside), the faster the rate of movement.
3. Thickness of the Exchange Surface: A thinner membrane/surface (shorter distance) means a faster rate.

2. Facilitated Diffusion

When substances are too large or too polar to pass directly through the fatty core, they rely on proteins embedded in the membrane—this is facilitated diffusion.

• It is still passive (down the concentration gradient).
• It uses Carrier Proteins or Channel Proteins.

Channel Proteins: Are fixed shapes that form pores filled with water. Charged ions move through these channels rapidly.

Carrier Proteins: Bind specifically to the molecule (like glucose). When the molecule binds, the protein changes its tertiary structure (shape), releasing the molecule on the other side. This is slower than channels.

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3. Osmosis: The Diffusion of Water

Osmosis is a special, essential case of diffusion. It is defined as the movement of water across a partially permeable membrane.

Water Potential (\(\Psi\))

Water movement is governed by water potential (\(\Psi\)), measured in pressure units (e.g., kPa).

• Definition: Water potential is the tendency of water molecules to move out of a solution.
• Pure water has the highest water potential (0 kPa).
• Adding solutes (like salt or sugar) lowers the water potential, making it a negative value.

The Rule of Osmosis: Water always moves from a solution of higher water potential (less negative, less solute) to a solution of lower water potential (more negative, more solute) through a partially permeable membrane.

Analogy: Think of a crowded room (low water potential, high solute). Water moves from the uncrowded room (high water potential, low solute) into the crowded room to try and dilute it and balance the crowd.

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3.1.4.3 Active Transport

Sometimes, a cell needs to accumulate a substance, even if it is already highly concentrated inside. This requires a process that moves molecules against their concentration gradient—this is Active Transport.

Key Characteristics of Active Transport

• It requires ATP (Adenosine Triphosphate), the immediate source of energy for biological processes.
• It uses Carrier Proteins (often called 'pumps') embedded in the plasma membrane.
• It moves substances from a region of lower concentration to a region of higher concentration.

The Role of Carrier Proteins and ATP

The carrier proteins involved in active transport work differently than those in facilitated diffusion:

1. The substance binds to the specific carrier protein on the side where it is currently at a low concentration.
2. ATP is hydrolysed (broken down), releasing energy.
3. This energy causes the carrier protein to undergo a specific conformational change (change in shape).
4. The substance is released on the opposite side, where its concentration is now higher.

Did you know? The Sodium-Potassium pump in your nerve cells is a famous example of active transport, constantly using ATP to push sodium ions out and potassium ions in, maintaining the electrical potential necessary for signalling!

ATP Synthesis and Energy Supply

Where does the ATP come from?

ATP is constantly generated inside the cell (usually in the mitochondria) through respiration. It is synthesized by joining ADP (Adenosine Diphosphate) and inorganic phosphate (P), a process that stores energy:

\[ \text{ADP} + \text{P} + \text{Energy} \rightarrow \text{ATP} \]

When the cell needs to power active transport, the ATP is broken down (hydrolysed), releasing that immediate energy:

\[ \text{ATP} \rightarrow \text{ADP} + \text{P} + \text{Energy} \]

Good job! You've successfully navigated the complexities of cell transport. Remember to practice applying these concepts, especially when dealing with water potential and calculating diffusion rates!