Cell: The Unit Of Life – Structure And Function

Cell Biology Fundamentals

Cells are the basic building blocks of all living things. Understanding them helps us learn how life works. Let's explore the exciting journey of cell discovery and the main ideas about cells.

Who Found Cells First?

Many scientists helped us learn about cells. Robert Hooke first saw "cells" in cork in 1665. He saw tiny, empty box-like structures.

Later, Anton von Leeuwenhoek saw living cells for the first time. He used his own powerful microscopes. In 1831, Robert Brown discovered the nucleus, a key part inside many cells.

The Cell Theory: Core Ideas

The Cell Theory explains what cells are and how they work. It was developed by different scientists:

• Matthias Schleiden (1838) said all plants are made of cells.
• Theodor Schwann (1839) said all animals are also made of cells. He also noted that plant cells have a cell wall.
• Rudolf Virchow (1855) added a very important idea: "Omnis cellula-e-cellula." This means all new cells come from cells that already exist.

So, the Cell Theory has two main points:

1. All living things are made of cells and their products.
2. All cells come from pre-existing cells.

Cells Come in Many Sizes!

Cells are incredibly diverse in size. Some are tiny, and some are quite large!

• Mycoplasmas are the smallest cells, about 0.3 micrometers (µm) across.
• The egg of an ostrich is the largest single cell you can see without a microscope.
• A nerve cell can be very long, stretching from your spine to your toe!
• Bacteria are usually 3-5 µm long.
• Your red blood cells (RBCs) are about 7.0 µm in diameter.

Cell Boundaries and Transport

Cells are amazing! They have special coverings that protect them and control what goes in and out. These coverings are the cell wall and the cell membrane. They also have clever ways to move things around, called transport mechanisms.

Let's explore these important parts of a cell. (Based on pages 6-8)

The Cell Wall: A Strong Outer Layer

The cell wall is a tough, non-living outer layer found in plants, algae, and fungi. It gives the cell its shape and protects it. Think of it like a strong fence around a garden.

• Plants have cell walls made of cellulose, hemicellulose, and pectin.
• Algae use cellulose, galactans, mannans, and even calcium carbonate.
• Fungi have cell walls made of chitin.

Cell Wall Layers and Connections

Plant cells first form a thin primary cell wall. Later, a thicker secondary cell wall grows inside it.

The middle lamella is like a glue, made of calcium pectate, that holds neighboring plant cells together. Imagine it as the mortar between bricks.

Tiny channels called plasmodesmata act as bridges, allowing materials to pass directly between plant cells. This helps cells communicate and share resources.

The Cell Membrane: A Smart Gatekeeper

The cell membrane, also called the plasma membrane, is the inner boundary of the cell. It's like a smart gatekeeper that decides what enters and leaves. It is semi-permeable, meaning it only lets certain things pass through.

It's made of lipids (fats) and proteins. For example, in human red blood cells, it's about 52% protein and 40% lipids.

Fluid Mosaic Model of the Cell Membrane

Scientists Singer and Nicolson proposed the Fluid Mosaic Model in 1972. It describes the cell membrane as a 'quasi-fluid' or 'semi-fluid' structure. Imagine a sea of lipids with proteins floating and moving within it.

Phospholipids are key components. They have a hydrophilic head (loves water) and hydrophobic tails (hates water). This structure forms a double layer, the lipid bilayer.

Proteins are also important:

• Integral proteins are partly or fully buried in the membrane.
• Peripheral proteins sit on the surface.

Cell Membrane Functions

The cell membrane does many vital jobs:

• It transports molecules in and out.
• It helps in cell growth and cell division.
• It forms connections between cells.
• It's involved in secretion (releasing substances) and endocytosis (taking things in).

Cell Transport: Moving Things Around

Cells constantly need to move substances like nutrients, water, and waste. This movement is called cell transport. There are two main ways this happens: Active Transport and Passive Transport.

Active Transport: Using Energy to Move

Active transport is like pushing a ball uphill. It moves molecules from an area of low concentration to an area of high concentration. This requires energy.

Cells use ATP (Adenosine Triphosphate), their main energy currency, to power active transport. It's an 'uphill' journey for molecules.

Passive Transport: Moving Without Energy

Passive transport is like a ball rolling downhill. It moves molecules from an area of high concentration to an area of low concentration, so it does not need energy.

Two common types of passive transport are:

• Diffusion: Molecules spread out evenly. For example, when you spray perfume, its scent diffuses across the room.
• Osmosis: This is the special diffusion of water across a semi-permeable membrane. Water moves to balance the concentration of dissolved substances.