Cell and Tissues

Every living thing you can name, from a banyan tree to the bacteria in your curd, is built from cells. A cell is the smallest unit that can carry out all the activities of life: take in food, release energy, grow, respond and reproduce. That is why the cell is called the structural and functional unit of life. Once you see the cell as a tiny, self-running factory, every later topic in Biology (tissues, organs, diseases, genetics) becomes easier.

Cell Theory in three lines: (1) All living organisms are made of one or more cells. (2) The cell is the basic structural and functional unit of life. (3) All new cells come from pre-existing cells (Virchow). Exception: viruses are not made of cells, so the theory does not apply to them. Note the split of credit: Schleiden and Schwann gave the first two points, Virchow gave the third.

Some organisms are a single cell (unicellular): Amoeba, Paramecium, Euglena, Chlamydomonas, yeast and all bacteria. This one cell does everything, so a single-celled organism needs no separate organs: its whole surface touches the surroundings, and food, gases and wastes pass directly through it. Amoeba changes shape constantly and pushes out finger-like pseudopodia (false feet) to move and to engulf food into a food vacuole. Higher organisms are multicellular: cells divide the work among themselves, so we get tissues, organs and organ systems.

Cells differ greatly in size and shape, and the shape fits the job. The nerve cell is long and branched to carry messages; the muscle cell is long and contractile; the RBC is a small biconcave disc with no nucleus (more room for haemoglobin); the WBC can change its shape to squeeze out of vessels and engulf germs.

Based on the type of nucleus, all living cells fall into two groups. Prokaryotic cells (Greek pro = before, karyon = nucleus) have no true nucleus; eukaryotic cells (eu = true) have a nucleus wrapped in a double membrane. This single difference decides almost everything else in the table below.

A cell has to be both closed (to keep its own chemistry safe) and open (to let food in and waste out). Two outer layers solve this problem: the plasma membrane, present in every cell, and the cell wall, present only in some. Inside lies the jelly-like cytoplasm and, in eukaryotes, the nucleus that controls everything.

Why can a plant cell sit in pure water and not burst, while an animal cell bursts? Because water enters both, but the plant cell has a cell wall that pushes back. The cell becomes firm (turgid) and the wall holds it. An animal cell has only a thin membrane, so it swells and bursts. That is the whole reason plants stand upright without a skeleton: turgid cells.

Plant cell walls have layers. The middle lamella is a glue-like layer of pectin (calcium and magnesium pectate) that cements two neighbouring cells together. Inside it come the primary wall (cellulose) and, in some cells, a thick secondary wall strengthened by lignin. Thin channels called plasmodesmata run through the walls and connect the cytoplasm of neighbouring plant cells, so that water, food and signals can pass between them.

How things cross the membrane. Small gases such as oxygen and carbon dioxide simply move from where they are more crowded to where they are less crowded: this is diffusion. Water moves the same way, but across a selectively permeable membrane, and this special diffusion of water is osmosis. When the cell needs a substance against the crowd, from low to high concentration, it must spend ATP: this is active transport. For big particles the membrane wraps around them (endocytosis): phagocytosis means 'cell eating' of solids and pinocytosis means 'cell drinking' of liquid droplets.

Cytoplasm is the jelly-like fluid between the plasma membrane and the nucleus. Its liquid part is the cytosol, which holds the organelles and is where glycolysis (breaking glucose into pyruvate) takes place. In eukaryotes the nucleus is the control room. It is wrapped in a double membrane punched with nuclear pores through which RNA and proteins travel. Inside are the nucleolus (makes ribosomal RNA and assembles ribosomes) and chromatin threads of DNA and protein, which condense into chromosomes when the cell divides. The DNA carries the genes, so the nucleus decides the cell's structure and activities and is called the 'brain' of the cell.

Think of a eukaryotic cell as a small town. The nucleus is the town hall, the mitochondria are the power stations, the ribosomes are the workshops, the ER is the road network, the Golgi is the post office and the lysosomes are the garbage and recycling unit. Each of these structures is called an organelle (cell organ). Most are wrapped in their own membrane, which is called compartmentalisation: it lets many different reactions go on in the same cell without mixing.

Plastids exist only in plants and algae. They are of three kinds. Chloroplasts are green because of chlorophyll; they have a double membrane, stacks of discs (grana made of thylakoids) and a fluid stroma, and they carry out photosynthesis. Chromoplasts carry yellow, orange and red pigments (carotenoids) and colour flowers and fruits. Leucoplasts are colourless and store food: amyloplasts store starch, elaioplasts store oils and fats, aleuroplasts store proteins. Like mitochondria, plastids have their own DNA and 70S ribosomes.

Mitochondria and chloroplasts are called semi-autonomous organelles: both have a double membrane, their own circular DNA and 70S ribosomes, so they can make some of their own proteins and multiply on their own. This supports the endosymbiosis idea (Lynn Margulis) that they were once free-living bacteria that moved inside bigger cells.

The cytoskeleton is a network of protein fibres (microtubules, microfilaments and intermediate filaments) that gives the cell its shape, anchors organelles and helps in movement. It is found in all eukaryotic cells. Cilia and flagella, the hair-like projections used for movement, are built from microtubules. Vacuoles also do special jobs in some organisms: the contractile vacuole of Amoeba and Paramecium pumps out excess water, and the food vacuole digests the engulfed food.

Cells are not permanent: they wear out and must be replaced, and living things must grow. So cells keep dividing. A cell does not simply split; it first copies its DNA, then shares the copies equally. The whole life of a dividing cell from one division to the next is the cell cycle. Remember the principle of Virchow here: every new cell comes from an old cell.

G1 + S + G2 together form interphase, the long 'preparation' period (the cell looks resting but is very busy). The actual division, M phase, is short. Some cells leave the cycle and enter G0, a quiet resting stage: they stay alive and work but no longer copy DNA or divide. Mature neurons and cardiac muscle cells stay in G0 permanently; liver cells can come back into the cycle when needed. Cancer cells are the ones that refuse to stop dividing.

Mitosis (equational division) takes place in body (somatic) cells. One diploid parent cell gives two genetically identical diploid daughter cells. It is used for growth, repair, replacement of worn cells and, in many organisms, asexual reproduction. Its four stages are remembered as PMAT.

Meiosis (reductional division) happens only in the reproductive (germ) cells of the gonads: the testes and ovaries. The chromosome number is halved, from diploid (2n) to haploid (n), so that after the sperm and egg fuse, the zygote gets back the normal number. Without it, the chromosome number would double every generation. Meiosis has two divisions (Meiosis I and II) and gives four haploid cells. In prophase I the paired chromosomes exchange pieces: this is crossing over, the main source of genetic variation.

Stem cells are unspecialised cells that can divide again and again and can turn into specialised cells (differentiation). Embryonic stem cells can become almost any cell type; adult stem cells, such as those in bone marrow, make only some types (blood cells). Stem cell therapy and regenerative medicine aim to repair or replace damaged tissues; bone marrow transplant is the oldest example. Plants show the same idea as totipotency: even a single plant cell can grow into a whole plant, which is the basis of plant tissue culture, where cells or small pieces (explants) are grown on a sterile nutrient medium for rapid clonal propagation (micropropagation) of orchids, bananas, etc.

In a multicellular organism, cells of the same kind and doing the same job form a tissue (Histology is the study of tissues). Plants stand still, so they do not need much movement but they need support, storage, transport and continuous growth. This is why plant tissues are of two big types: meristematic (the dividing, growing ones) and permanent (the ones that have stopped dividing and taken up a job).

Meristematic tissue has young, actively dividing cells with thin walls, dense cytoplasm, a large nucleus and little or no vacuole, packed tightly without gaps. Based on position there are three types: apical meristem at the tips of roots and shoots (makes the plant grow longer, primary growth); lateral meristem (cambium) along the sides, which makes the stem and root grow thicker (secondary growth, wood and bark); and intercalary meristem at the base of leaves or near the nodes (internodes), which is why grass grows back quickly after being cut or grazed.

When meristem cells stop dividing and differentiate to take up a specific job, they become permanent tissue. Simple permanent tissues are made of one type of cell; complex permanent tissues are made of more than one type working together.

Complex permanent tissues are the plumbing of the plant, together called the vascular tissue (conducting tissue). Xylem carries water and minerals from roots upwards to leaves. Phloem carries the food made in leaves (mainly sucrose) to every part, upward and downward; this movement is translocation and it needs energy (ATP). Both are essential for plants living on land, which is why vascular tissue is the sign of the more advanced plant groups (pteridophytes onwards).

Protective tissues cover the plant. The epidermis is the outermost single layer of cells; it is covered by a waxy cuticle (made of cutin) that cuts water loss, and it has stomata (pores for gas exchange and transpiration, each guarded by two bean-shaped guard cells) and in roots root hairs to absorb water. In old stems, the epidermis is replaced by cork, dead cells filled with suberin, which is waterproof and gives bark, and the cork used in bottle stoppers. Desert plants have a thick cuticle, and in trees the cork layer forms bark.

Animals move, hunt, digest and think, so their tissues are built for action rather than for standing still. There are four basic animal tissues: epithelial (covering), connective (binding and supporting), muscular (movement) and nervous (control). Nearly every organ is a team of all four. For example, the stomach has an epithelial lining, connective tissue holding it, muscle to churn food and nerves to control it.

Plant tissues vs animal tissues: plants have dividing (meristematic) tissue in specific growth regions and many dead, supporting tissues. Animals grow all over the body and have no meristematic tissue; 'meristematic tissue' is therefore not a type of animal tissue. Animal tissues are more specialised for movement and sensing, hence muscle and nerve tissue.

Epithelial tissue forms the covering of the body and the lining of organs and cavities. Its cells sit tightly side by side with almost no space (intercellular matrix) between them and rest on a thin, non-living basement membrane. It has no blood vessels of its own. It protects, absorbs, secretes and filters. Skin, the lining of the mouth, blood vessels, lung alveoli, kidney tubules, the gut and the windpipe are all epithelium.

Connective tissue is the 'glue and scaffolding' of the body. Unlike epithelium, its cells are scattered in a large amount of matrix (a jelly, fibres, or a hard or liquid material). It binds organs together, supports them, stores fat and carries materials. Blood, bone, cartilage, tendons, ligaments, fat and areolar tissue all belong here.

Muscular tissue is made of long cells called muscle fibres, which contain the contractile proteins actin and myosin. When they contract and relax, we get movement. There are three types. Note that muscle cells are long, not cuboidal.

Nervous tissue is made of neurons (nerve cells) and supporting cells called neuroglia. A neuron has a cell body (cyton) with the nucleus, short branched dendrites that receive a signal, and a long axon that carries it away. A nerve impulse is electrical-chemical: it travels as an electrical signal along the neuron and crosses the gap between two neurons (synapse) with chemicals (neurotransmitters). Brain, spinal cord and nerves are made of nervous tissue. The neuron is the longest cell of the human body.