Cell biology: Class 11 Biology

Key points

Cell theory says that all living things are made of one or more cells, the cell is the basic unit of structure and function, and new cells come only from existing cells. Schleiden and Schwann proposed the first two ideas and Virchow added the third. Cells are small, so they need a microscope to be seen.

A light microscope uses visible light and glass lenses. It can magnify up to about 1500 times and has a resolving power of about 0.2 µm. An electron microscope uses a beam of electrons and magnets, and can resolve details down to about 0.1 nm, so it shows the internal structure of organelles. Living cells can be seen with a light microscope, but specimens for an electron microscope must be dead and in a vacuum.

Magnification is how many times larger the image is than the object. Resolution is the ability to see two close points as separate. Total magnification of a compound microscope is the eyepiece magnification multiplied by the objective magnification. The unit conversions are 1 mm = 1000 µm and 1 µm = 1000 nm.

Main organelles of a eukaryotic cell and their functions:

• Nucleus: surrounded by a double membrane with pores. It holds the DNA as chromatin and controls the activities of the cell. The nucleolus makes ribosomal RNA.
• Rough endoplasmic reticulum: has ribosomes on its surface and makes and transports proteins. Smooth endoplasmic reticulum makes lipids and steroids and helps detoxify drugs.
• Golgi apparatus: a stack of flattened sacs that modifies, packs and sends out proteins in vesicles. It also forms lysosomes.
• Mitochondria: have a double membrane and an inner folded membrane called cristae. They carry out aerobic respiration and make ATP.
• Ribosomes: made of RNA and protein, they make proteins. Free ribosomes make proteins for use in the cell, and bound ribosomes make proteins for export.
• Lysosomes: contain digestive enzymes that break down worn out organelles, food and germs.
• Centrioles: found in animal cells, they form the spindle during cell division.

Plant cells also have a cellulose cell wall, chloroplasts and a large central vacuole. The cell wall gives strength and shape. Chloroplasts hold chlorophyll and carry out photosynthesis. The vacuole is surrounded by a membrane called the tonoplast and stores water, sugars and pigments, and helps keep the cell firm. Animal cells have no cell wall or chloroplasts and have small vacuoles.

Prokaryotic cells such as bacteria are much smaller, usually 1 to 10 µm. They have no true nucleus, and their DNA lies free in the cytoplasm in a region called the nucleoid. They have no membrane bound organelles and have smaller ribosomes (70S). Eukaryotic cells are larger, have a true nucleus and membrane bound organelles, and have 80S ribosomes in the cytoplasm.

The cytoskeleton is a network of protein fibres in the cytoplasm. It supports the cell, holds organelles in place and helps in movement. Microtubules, microfilaments and intermediate filaments are its three types. Cilia and flagella are made of microtubules and move cells or the fluid around them.

The plasma membrane is described by the fluid mosaic model. It is a double layer of phospholipids with proteins scattered in it. Each phospholipid has a hydrophilic head that faces water and two hydrophobic tails that face inward. Cholesterol in animal membranes keeps the membrane stable, and carbohydrate chains on the outside act as recognition sites.

Movement of substances across the membrane:

• Diffusion: net movement of particles from high to low concentration. It needs no energy.
• Facilitated diffusion: movement down a gradient through channel or carrier proteins. It needs no energy.
• Osmosis: diffusion of water through a partially permeable membrane from high water potential to low water potential.
• Active transport: movement against the gradient using carrier proteins and energy from ATP, as in the sodium potassium pump.
• Endocytosis and exocytosis: bulk movement of material into or out of the cell in vesicles.

Cells in different solutions behave differently. In a hypotonic solution an animal cell takes in water and may burst, while a plant cell becomes turgid because the wall resists. In a hypertonic solution water leaves, an animal cell shrinks and a plant cell becomes flaccid, then plasmolysed. Cells in an isotonic solution show no net movement of water.

Study tip: Draw and label a plant cell, an animal cell and a mitochondrion until you can do each in two minutes, because diagrams are asked often. Learn each organelle as structure plus function in one sentence. For magnification questions always write the formula, convert units first and give the unit in the answer.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Mixing up magnification and resolution. A bigger image is not always clearer.
• Saying that osmosis is the movement of any substance. It is movement of water only, across a partially permeable membrane.
• Writing that prokaryotes have no DNA. They do have DNA, but it is not inside a nucleus.
• Saying active transport needs no energy, or that facilitated diffusion needs ATP.
• Forgetting to convert units (mm, µm, nm) before calculating actual size from magnification.

Practise this chapter

Practice questions and timed test Printable worksheet